Preparation and Mechanical Behavior of Ultra-High Strength Low-Carbon Steel

materials

Article Preparation and Mechanical Behavior of Ultra-High Strength Low-Carbon Steel

Zhiqing Lv 1,2,*, Lihua Qian 1, Shuai Liu 1, Le Zhan 1 and Siji Qin 1

1 Key Laboratory of Advanced Forging & Stamping Technology and Science, Ministry of Education of China Yanshan University, Qinhuangdao 066004, China; [email protected] (L.Q.); [email protected] (S.L.); [email protected] (L.Z.); [email protected] (S.Q.) 2 State Key Laboratory of Metastable Material Science and Technology, Yanshan University, Qinhuangdao 066004, China * Correspondence: [email protected]

Received: 16 December 2019; Accepted: 14 January 2020; Published: 18 January 2020

Abstract: The low-carbon steel (~0.12 wt%) with complete martensite structure, obtained by quenching, was cold rolled to get the high-strength steel sheets. Then, the mechanical properties of the sheets were measured at different angles to the rolling direction, and the microstructural evolution of low-carbon martensite with cold rolling reduction was observed. The results show that the hardness and the strength gradually increase with increasing rolling reduction, while the elongation and impact toughness obviously decrease. The strength of the sheets with the same rolling reduction are different at the angles of 0◦, 45◦, and 90◦ to the rolling direction. The tensile strength (elongation) along the rolling direction is higher than that in the other two directions, but the differences between them are not obvious. When the aging was performed at a low temperature, the strength of the initial martensite and deformed martensite increased with increasing aging time during the early stages of aging, followed by a gradual decrease with further aging. However, the elongation increases with increasing aging time. The change of hardness is consistent with that of strength for the cold-rolled martensite, while the hardness of the initial martensite decreases gradually with increasing aging time.

Keywords: lath martensite; cold deformation; aging; ultra-high strength steel

1. Introduction At present, the production technology of the traditional steel has matured. The research and development of a new generation of steel are the primary objectives of steel manufacturers in the 21st century. In recent years, numerous advanced technologies for preparing ultra-high strength steel have emerged. As is well known, grain refinement can significantly improve strength of the materials. Therefore, the preparation of ultra-fine grain steel has become the focus of research on high-strength materials. The methods of severe plastic deformation (SPD) are an effective means of grain refinement [1–4], which have been widely used in the preparation of ultra-high strength materials. The SPD methods mainly include cold rolling [5,6], equal channel angular pressing (ECAP) [7–9], high-pressure torsion (HPT) straining [10,11], accumulative roll-bonding (ARB) [12–14], multidirectional forging (MDF) [15–17], etc. Valiev et al. [18,19] reported the preparation and application of bulk nanostructured materials from SDP. Astafurova et al. [20] prepared high-strength steel with nanosized grain subgrain structure by HPT deformation of a low-alloy steel. Mousavi Anijdan et al. [21] treated Fe-28.5Ni steel by the ARB process and obtained the high-strength steel with a mean grain size of a few hundred nanometers after a 6-cycle process. Wang et al. [22] prepared ultra-fine-grained low carbon steel (Fe–0.15 wt% C–0.52 wt% Mn) with ultra-high strength by performing 10 times equal

Materials 2020, 13, 459; doi:10.3390/ma13020459 www.mdpi.com/journal/materials Materials 2020, 13, 459 2 of 14 channel angular pressing (ECAP) at room temperature. Generally, severe cold deformation can increase the strength and hardness of the material while obviously reducing plasticity and toughness [18–22]. Martensitic transformation is another effective method to refine the grains of the materials, and so the strength level of the material can be greatly improved when the martensite structure is obtained [23–26]. The phase transformation strengthening of low carbon martensitic steel is mainly used to form fine lath martensite structure during quenching [27], and introduces high dislocation density [28,29] which can obtain high strength while retaining a certain plasticity to facilitate subsequent rolling deformation. An IF steel with two different initial structural states, namely ferrite and lath martensite, was cold rolled. When the cold rolling deformation reached to 80%, the UTS of the IF steel sheet with the initial ferrite state (grain size 150 µm) was about 616 MPa, while the UTS of the IF steel sheet with the initial lath martensite state reached to about 770 MPa [30]. Li et al. [31] performed cold rolling deformation on an interstitial free (IF) steel with coarse grain ferrite (120 µm). It was then found that the strength increased with increasing reduction, and the UTS was 530 MPa when the cold rolled reduction reached to 80%. These findings indicate that an IF steel with initial lath martensite can acquire higher strength than the IF steel with the initial ferrite state after the same plastic deformation. Astafurova et al. [20] prepared high-strength steel by HPT deformation of a low-alloy steel with two different initial structural states (ferritic-pearlitic state and martensitic state). They found that HPT deformation leads to a considerable increase in the microhardness compared to its microhardness in the initial state. Additionally, the hardness of the steel with an initial martensitic state was obviously higher than that of the steel with an initial ferritic-pearlitic state after the same HPT deformation. At present, there are few studies on the preparation of ultra-high strength steel by small plastic deformation. The combination of martensitic transformation and plastic deformation can further enhance the grain refinement process while still maintaining good plasticity, which will be an effective way to prepare a high-strength low-carbon steel with higher strength grades. The ultra-high strength low-carbon steel will be obtained after plastic deformation at low strain when the initial structure of the steel is martensite. In the present study, a plain AISI 1010 low-carbon steel is quenched to obtain a martensitic structure. Then the martensite lath structure was refined by plastic deformation at low strain, followed by an aging treatment to prepare 1600 MPa grade steel. The microstructural evolution and mechanical properties of the low-carbon steel are further examined and analyzed during the cold rolling deformation and subsequent aging treatment. This will provide the theoretical basis and experimental support for the preparation of ultra-high strength low carbon steel sheet.

2. Materials and Methods A commercial plain low-carbon steel, namely AISI 1010 (0.12C-0.3Mn-0.15Cr-0.2Si, wt%), was used in this work. The sheets were 3 mm thick, 20 mm wide, and 50 mm long. The sheets were austenitized at 1200 ◦C for ﬁve min, followed by water quenching (at about 0 ◦C) to obtain the martensite structure. The sheets with martensitic microstructure were cold rolled to a reduction of 10% and 30% in thickness. The preparation process of the high-strength martensitic steel is depicted in Figure1. The as-transformed specimens and the cold rolled specimens were subsequently aged at 120Materials◦C. 2020, 13, x FOR PEER REVIEW 3 of 15

Figure 1.1. Preparation process of ultra-high strength lowlow carboncarbon steel.steel.

The microstructure of each specimen was examined by various techniques. The structures were characterized by optical microscopy (OM), with an Axiovert-200MAT (Carl Zeiss Microscopy GmbH, Jena, Germany) and scanning electron microscopy (SEM) using a Hitachi S-3400N scanning electron microscope (Hitachi Corp., Tokyo, Japan) that the accelerating voltage was 20 KV. All specimens were observed on the rolling direction (RD)-normal direction (ND) plane of the sheets. Thin foils for TEM observation were cut into 6 mm long and 0.4 mm thickness by wire cutting equipment, mechanically thinned to 30 μm thickness, and electropolished by a twin-jet polisher with 10% perchloric acid solution at 30 V. TEM observations were performed with a Talos f200x (Thermo scientific TM, Hillsboro, OR, USA) transmission electron microscope with an acceleration voltage of 200 KV. The dislocation density of the material was analyzed by X-ray diffraction (XRD) on a D/max- 2500PC X-ray diffractometer (Rigaku Corp, Tokyo, Japan), with Cu-Kα (λ = 0.15406 nm) radiation at a scan rate of 2°/min, and operation voltage and current of 40 kV and 100 mA, respectively. Tensile specimens at different angles 0°, 45°, and 90° to the RD were selected from the steel sheets. The sampling positions and dimensions of the tensile specimens are shown in Figure 2. The tensile tests were conducted on a table Inspekt 100 kN universal material testing machine (Hegewald & Peschke MPT GmbH, Nossen, Germany), and the hardness was measured on an FM-700 FM-ARS microhardness tester (Future-Tech Corp., Kawasaki City, Kanagawa, Japan). The impact toughness test was carried out on Zwick RKP450 pendulum impact tester (Zwick GmbH & Co., Ulm, Germany) at room temperature. The impact specimens (cross-sectional dimension of about 10 mm thickness, 10 mm width, and 50 mm length) and Charpy U-notch impact specimens were assembled in multiple layers of thin plates of equal thickness. The approximate impact toughness of the sheets was obtained from the actual cross-sectional area calculated according to the actual thickness of the experimental sample.

Figure 2. Sampling positions and dimensions of the tensile specimens.

3. Results

3.1. Microstructure After Cold Rolling The quenched martensite of AISI 1010 steel is shown in Figure 3, revealing the typical lath martensite structure in the original austenite grains of the low-carbon steel. The parallel direction of the laths shows various angles with respect to the axes of the sample. The lath bundles are randomly distributed in multiple orientations and there is no specific angle with the subsequent deformation direction.

Materials 2020, 13, x FOR PEER REVIEW 3 of 15

Figure 1. Preparation process of ultra-high strength low carbon steel. Materials 2020, 13, 459 3 of 14 The microstructure of each specimen was examined by various techniques. The structures were characterized by optical microscopy (OM), with an Axiovert-200MAT (Carl Zeiss Microscopy GmbH, The microstructure of each specimen was examined by various techniques. The structures were Jena, Germany) and scanning electron microscopy (SEM) using a Hitachi S-3400N scanning electron characterized by optical microscopy (OM), with an Axiovert-200MAT (Carl Zeiss Microscopy GmbH, microscope (Hitachi Corp., Tokyo, Japan) that the accelerating voltage was 20 KV. All specimens Jena, Germany) and scanning electron microscopy (SEM) using a Hitachi S-3400N scanning electron were observed on the rolling direction (RD)-normal direction (ND) plane of the sheets. Thin foils for microscope (Hitachi Corp., Tokyo, Japan) that the accelerating voltage was 20 KV. All specimens were TEM observation were cut into 6 mm long and 0.4 mm thickness by wire cutting equipment, observed on the rolling direction (RD)-normal direction (ND) plane of the sheets. Thin foils for TEM mechanically thinned to 30 μm thickness, and electropolished by a twin-jet polisher with 10% observation were cut into 6 mm long and 0.4 mm thickness by wire cutting equipment, mechanically perchloric acid solution at 30 V. TEM observations were performed with a Talos f200x (Thermo thinned to 30 µm thickness, and electropolished by a twin-jet polisher with 10% perchloric acid solution scientific TM, Hillsboro, OR, USA) transmission electron microscope with an acceleration voltage of at 30 V. TEM observations were performed with a Talos f200x (Thermo scientific TM, Hillsboro, OR, 200 KV. The dislocation density of the material was analyzed by X-ray diffraction (XRD) on a D/max- USA) transmission electron microscope with an acceleration voltage of 200 KV. The dislocation density 2500PC X-ray diffractometer (Rigaku Corp, Tokyo, Japan), with Cu-Kα (λ = 0.15406 nm) radiation at of the material was analyzed by X-ray diffraction (XRD) on a D/max-2500PC X-ray diffractometer a scan rate of 2°/min, and operation voltage and current of 40 kV and 100 mA, respectively. Tensile (Rigaku Corp, Tokyo, Japan), with Cu-Kα (λ = 0.15406 nm) radiation at a scan rate of 2 /min, and specimens at different angles 0°, 45°, and 90° to the RD were selected from the steel◦ sheets. The operation voltage and current of 40 kV and 100 mA, respectively. Tensile specimens at different angles sampling positions and dimensions of the tensile specimens are shown in Figure 2. The tensile tests 0 , 45 , and 90 to the RD were selected from the steel sheets. The sampling positions and dimensions ◦were◦ conducted◦ on a table Inspekt 100 kN universal material testing machine (Hegewald & Peschke of the tensile specimens are shown in Figure2. The tensile tests were conducted on a table Inspekt MPT GmbH, Nossen, Germany), and the hardness was measured on an FM-700 FM-ARS 100 kN universal material testing machine (Hegewald & Peschke MPT GmbH, Nossen, Germany), microhardness tester (Future-Tech Corp., Kawasaki City, Kanagawa, Japan). The impact toughness and the hardness was measured on an FM-700 FM-ARS microhardness tester (Future-Tech Corp., test was carried out on Zwick RKP450 pendulum impact tester (Zwick GmbH & Co., Ulm, Germany) Kawasaki City, Kanagawa, Japan). The impact toughness test was carried out on Zwick RKP450 at room temperature. The impact specimens (cross-sectional dimension of about 10 mm thickness, 10 pendulum impact tester (Zwick GmbH & Co., Ulm, Germany) at room temperature. The impact mm width, and 50 mm length) and Charpy U-notch impact specimens were assembled in multiple specimens (cross-sectional dimension of about 10 mm thickness, 10 mm width, and 50 mm length) and layers of thin plates of equal thickness. The approximate impact toughness of the sheets was obtained Charpy U-notch impact specimens were assembled in multiple layers of thin plates of equal thickness. from the actual cross-sectional area calculated according to the actual thickness of the experimental The approximate impact toughness of the sheets was obtained from the actual cross-sectional area sample. calculated according to the actual thickness of the experimental sample.

FigureFigure 2. 2.Sampling Sampling positions positions and and dimensions dimensions of of the the tensile tensile specimens. specimens.

3.3. Results Results 3.1. Microstructure after Cold Rolling 3.1. Microstructure After Cold Rolling The quenched martensite of AISI 1010 steel is shown in Figure3, revealing the typical lath The quenched martensite of AISI 1010 steel is shown in Figure 3, revealing the typical lath martensite structure in the original austenite grains of the low-carbon steel. The parallel direction martensite structure in the original austenite grains of the low-carbon steel. The parallel direction of of the laths shows various angles with respect to the axes of the sample. The lath bundles are the laths shows various angles with respect to the axes of the sample. The lath bundles are randomly randomly distributed in multiple orientations and there is no specific angle with the subsequent distributed in multiple orientations and there is no specific angle with the subsequent deformation deformation direction. direction. The structure of lath martensite has been widely studied and reported in the literature. There is a three-level hierarchy in this morphology: (I) lath, a single crystal of martensite including high density lattice defects; (II) block, aggregation of laths with the same crystallographic orientation (variant); and (III) packet, aggregation of the blocks with the same habit plane [32–35]. The SEM images of the microstructure of the 10 and 30% cold rolled specimens are shown in Figure4a,b, respectively. According to Ueji et al. [32], the structure of cold rolled martensite is divided into three kinds of microstructures, designated by the alphabetical characters (A, B, and C) and defined as follows: A Very fine lamellar structure mainly elongated parallel to the RD. B Irregularly bent lamellar structure. Materials 2020, 13, x FOR PEER REVIEW 4 of 15

Materials 2020, 13, 459 4 of 14

C FigureLump 3. of Optical martensite microscopy laths with (OM) shear image bands (a) and that scanning is parallel electron to the microscopy ND. (SEM) image (b) Materialsshowing 2020, 13the, x microstructures FOR PEER REVIEW of the as-quenched martensite in the experimental steel. 4 of 15

The structure of lath martensite has been widely studied and reported in the literature. There is a three-level hierarchy in this morphology: (I) lath, a single crystal of martensite including high density lattice defects; (II) block, aggregation of laths with the same crystallographic orientation (variant); and (III) packet, aggregation of the blocks with the same habit plane [32–35]. The SEM images of the microstructure of the 10 and 30% cold rolled specimens are shown in Figure 4a,b, respectively. According to Ueji et al. [32], the structure of cold rolled martensite is divided into three kinds of microstructures, designated by the alphabetical characters (A, B, and C) and defined as follows:

A Very fine lamellar structure mainly elongated parallel to the RD.

B Irregularly bent lamellar structure. FigureFigure 3. 3. OpticalOptical microscopymicroscopy (OM)(OM) imageimage ((aa)) andand scanningscanning electronelectron microscopymicroscopy (SEM)(SEM) imageimage (b(b)) showing the microstructures of the as-quenched martensite in the experimental steel. C Lumpshowing of themartensite microstructures laths with of the shear as-quenche bands dthat martensite is parallel in the to experimentalthe ND. steel.

The(a) structure of lath martensite has been widely(b) studied and reported in the literature. There is a three-level hierarchy in this morphology: (I) lath, a single crystal of martensite including high density lattice defects; (II) block, aggregation of laths with the same crystallographic orientation (variant); and (III) packet, aggregation of the blocks with the same habit plane [32–35]. The SEM images of the microstructure of the 10 and 30% cold rolled specimens are shown in Figure 4a,b, respectively. According to Ueji et al. [32], the structure of cold rolled martensite is divided into three kinds of microstructures, designated by the alphabetical characters (A, B, and C) and defined as follows: ND ND

RD 10μm 10μm A Very fine lamellar structure mainly elongated parallelRD to the RD.

B Figure Irregularly 4. SEMSEM bent images images lamellar showing showing structure. the the microstructures microstructures of the the experimental experimental steel cold-rolled to a reduction reduction of 10% ( a) and and 30% 30% ( (b).). The The initial initial microstructure microstructure was was martensi martensite.te. Alphabetic Alphabetic characters characters denote denote which C type Lump of microstructureof martensite is laths exhibited. with shear A, A, lamellar lamellar bands di dislocation thatslocation is parallel cell cell (LDC); (LDC); to the B, B, ND. irregularly irregularly bent bent lath lath (IBL); C, kinked lath (KL); D, lath martensite (M)). (a) (b) The symbolssymbols ofof A, A, B, B, C, C, and and D D denote denote the the lamellar lamellar dislocation dislocation cell (LDC),cell (LDC), irregularly irregularly bent lathbent (IBL), lath (IBL),kinked kinked lath (KL) lath structures (KL) structures and lath and martensite lath martensite structures structures (M), in (M), Figure in4 Figure, respectively. 4, respectively. The broken The brokenlines in lines Figure in4 Figure indicate 4 indicate the boundaries the boundaries between between the di ff theerent different structures structures with the with A, B,the C, A, and B, C, D andmicrostructures. D microstructures. In the SEM In the image SEM in image Figure in4a Figure reveals 4a that reveals the microstructures that the microstructures in the 10% cold-rolledin the 10% cold-rolledspecimen of specimen low carbon of martensiticlow carbon steel martensiti is mainlyc steel M structure. is mainly The M deformed structure. microstructure The deformed is microstructuredisordered and is randomly disordered distributed and randomly in diff erentdistributed orientations in different and is orientations at no specific and angle is at with no specific respect angleto the with RDN D direction.respect to the The RD IBL direction. and LDC The structures IBL and NLDC wereD structures easily categorized were easily by categorized their characteristic by their characteristicmorphology. morphology.RD However, the However, LDC and the martensite LDC10μm and structures martensite both structures show similarboth show lamellar similar10μm structures, lamellar RD structures,so they are so sometimes they are sometimes difficult to distinguish.difficult to distin Whenguish. the lamellar When the structure lamellar is structure elongated is inelongated a direction in withina directionFigure 20◦ from within 4. SEM the 20°images RD, from it isshowing consideredthe RD, the it microstructures is as considered an LDC structure, asof thean experimentalLDC otherwise structure, thesteel otherwise structure cold-rolled was the to a categorizedstructure reduction was as a martensiteof 10% ( structurea) and 30% (M) (b).[ The33]. initial According microstructure to the images was martensi shownte. in Alphabetic Figure4, all characters three kinds denote of deformed which structurestype of (LDC, microstructure IBL, and KL)is exhibited. occur in A, the lamellar 30% colddislocation rolled cell specimens, (LDC); B, andirregularly a fairly bent large lath amount (IBL); of martensiteC, kinked structure lath (KL); still D, remains. lath martensite The area (M)). occupied by the LDC, IBL, and KL structures increases at the same time, and the LDC structure occupies the largest area. The martensite lath also begins to be The symbols of A, B, C, and D denote the lamellar dislocation cell (LDC), irregularly bent lath elongated along the RD. (IBL), kinked lath (KL) structures and lath martensite structures (M), in Figure 4, respectively. The The as-quenched martensitic steel was cold rolled to a reduction of 10%, and 30%, and the fractions broken lines in Figure 4 indicate the boundaries between the different structures with the A, B, C, of M, LDC, IBL, and KL of low-carbon martensitic steel under different rolling reductions are as shown and D microstructures. In the SEM image in Figure 4a reveals that the microstructures in the 10% in Figure5. The fractions of the M, LDC, IBL, and KL structures in the 10% cold rolled sheet were 49%, cold-rolled specimen of low carbon martensitic steel is mainly M structure. The deformed 24%, 14% and 13%, respectively. The area fraction of the LDC, IBL, and KL increased with increasing microstructure is disordered and randomly distributed in different orientations and is at no specific the reduction, however, the area fraction of the M decreased. When the cold rolled reduction reaches angle with respect to the RD direction. The IBL and LDC structures were easily categorized by their characteristic morphology. However, the LDC and martensite structures both show similar lamellar structures, so they are sometimes difficult to distinguish. When the lamellar structure is elongated in a direction within 20° from the RD, it is considered as an LDC structure, otherwise the structure was

Materials 2020, 13, x FOR PEER REVIEW 5 of 15 categorized as a martensite structure (M) [33]. According to the images shown in Figure 4, all three kinds of deformed structures (LDC, IBL, and KL) occur in the 30% cold rolled specimens, and a fairly large amount of martensite structure still remains. The area occupied by the LDC, IBL, and KL structures increases at the same time, and the LDC structure occupies the largest area. The martensite lath also begins to be elongated along the RD. The as-quenched martensitic steel was cold rolled to a reduction of 10%, and 30%, and the fractions of M, LDC, IBL, and KL of low-carbon martensitic steel under different rolling reductions are as shown in Figure 5. The fractions of the M, LDC, IBL, and KL structures in the 10% cold rolled sheetMaterials were2020 49,, 1324,, 45914, and 13%, respectively. The area fraction of the LDC, IBL, and KL increased with5 of 14 increasing the reduction, however, the area fraction of the M decreased. When the cold rolled reduction reaches to 30%, the area fraction of M decreases to 12%, but the area fraction of LDC, IBL, andto KL 30%, increases the area to fraction 40, 31, of and M decreases17%, respectively. to 12%, but In theother area words, fraction the of marten LDC, IBL,site andlath KLrotates increases with to increasing40%, 31% cold and rolled 17%, reduction. respectively. The In lath other is words,continuously the martensite elongated lath along rotates the withRD and increasing gradually cold tends rolled to reduction.become parallel The lath to isthe continuously RD. The martensitic elongated micros alongtructure the RD and is effective gradually totends obtain to ultrafine become parallelgrains, to andthe the RD. blocks The martensiticand packets microstructure of martensite is have effective angular to obtain and rugged ultrafine shape grains, [36]. and Such the blockshigh density and packets of high-angleof martensite boundaries have angular and complicated and rugged shape shape of [36blocks]. Such and high packets density would of high-angle lead to inhomogeneous boundaries and deformationcomplicated (grain shape subdivision) of blocks and packetsduring wouldplastic lead deformation, to inhomogeneous which deformationcan result in (grain an subdivision)ultra-fine deformationduring plastic microstructure deformation, with which large can local result misori in anentations. ultra-fine After deformation cold rolling microstructure deformation with at low large strainslocal (Figure misorientations. 4a,b), the martensitic After cold structure rolling deformation mostly remains at low the strainssame. (Figure4a,b), the martensitic structure mostly remains the same.

100 M KL IBL LDC

Area Fraction(%)

0 0 102030 Reduction/%

Figure 5. Fraction of the areas showing lamellar dislocation cells (LDCs), irregularly bent lath (IBL), Figure 5. Fraction of the areas showing lamellar dislocation cells (LDCs), irregularly bent lath (IBL), kinked lath (KL), and lath martensite (M) in the sheets cold rolled to a reduction of 10% and 30%. kinked lath (KL), and lath martensite (M) in the sheets cold rolled to a reduction of 10 and 30%. The The initial microstructure was martensite. initial microstructure was martensite. 3.2. Mechanical Properties after Cold Rolling 3.2. Mechanical Properties After Cold Rolling The stress-strain curves of the as-quenched specimen and cold rolled specimens (10% and 30% reduction)The stress-strain are shown curves in Figure of the6. Theas-quenched stress-strain specimen curve of and the normalizedcold rolled specimenspecimens is (10 also and shown 30% in reduction)order to compareare shown with in Figure the as-quenched 6. The stress-strai specimen.n curve The yield of the strength, normalized tensile specimen strength, is and also elongation shown in order to compare with the as-quenched specimen. The yield strength, tensile strength, and of the experimental steel after normalizing (85 ◦C held for 10 min, air cooled) are 233 MPa, 320 MPa, elongationand 32.2%, of the respectively. experimental The steel results after presented normalizing in Figure (85 °C6 revealheld for that 10 min, cold air deformation cooled) are signiﬁcantly 233 MPa, 320enhances MPa, and the 32.2%, strength respectively. and decreases The the results plasticity presented compared in withFigure the 6 quenchedreveal that specimen. cold deformation The strength significantlyof the martensitic enhances steel the gradually strength increases, and decrease whiles thethe plasticity plasticity gradually compared decreases with the with quenched increasing specimen.reduction.Materials 2020 The When, 13 strength, x FOR the PEER deformationof theREVIEW martensitic reaches steel to 30%, gradually the tensile increases, strength while of thethe low-carbon plasticity gradually martensitic6 of 15 decreasessteel reaches with toincreasing about 1600 reduction. MPa. When the deformation reaches to 30%, the tensile strength of the low-carbon martensitic steel reaches to about 1600 MPa.

30% cold rolling 1500 10% cold rolling

1000 0%, as-quenched

Stress/MPa 500 Normalizing

0 0 5 10 15 20 25 30 35 40 Engineering strain/% Figure 6. TensileTensile curves of the experimental steels at different diﬀerent states (quenched and cold rolled states).

The yieldyield strength, strength, tensile tensile strength, strength, hardness, hardness, elongation, elongation, and impact and toughness impact oftoughness the experimental of the steelexperimental after quenching steel after and coldquenching rolling areand shown cold rollin in Figureg are7. Aftershown quenching, in Figure the 7. strengthAfter quenching, and hardness the strength and hardness markedly increased, while the elongation decreased compared with the normalized specimen. The hardness and tensile strength of the initial martensite were 440 HV and 1302 MPa, respectively.

-2 1600 90 1400 80

1200 Yield stress(MPa) 70 Tensile strength(MPa) 60 Strength/MPa 1000 -2 Toughness/J·cm Toughness (J·cm ) 50

Hardness(HV) 14 Elongation(%) 500 12 10 Hardness/HV Elongation/% 400 8 0 5 10 15 20 25 30 Reduction/%

Figure 7. Mechanical properties of high-strength steel.

After a cold rolled reduction of 30%, the hardness and tensile strength increase to 466 HV and 1585 MPa, respectively. The elongation and impact toughness of the quenched martensite were 15% and 95 J/cm2, respectively. With the increase of the cold rolled reduction to 30%, the elongation and impact toughness were reduced to 10.2% and 67 J/cm2, respectively. During the deformation process, the laths are also sheared by the micro-shear band while migrating toward the RD, thereby destroying the initial lath boundaries and causing a large misorientation within the martensite structure [30,32]. Then, the microstructure is further refined and the crystal defect density is gradually increased. As a result, the tensile strength and hardness of the experimental steel increase with the increasing cold rolled reduction, and the elongation tends to decrease. The tensile specimens were also selected in the direction of 0°, 45°, and 90° with respect to the RD of the experimental steel. The differences in the elongation and tensile strength of the experimental steel when the selected specimens were in different directions to the RD are shown in Figure 8. The strength along the 0° RD is little higher than that along the 45° and 90° RD, whereas the change in elongation is the opposite.

Materials 2020, 13, x FOR PEER REVIEW 6 of 15

30% cold rolling 1500 10% cold rolling

1000 0%, as-quenched

Stress/MPa 500 Normalizing

0 0 5 10 15 20 25 30 35 40 Engineering strain/% Figure 6. Tensile curves of the experimental steels at different states (quenched and cold rolled states).

The yield strength, tensile strength, hardness, elongation, and impact toughness of the experimental steel after quenching and cold rolling are shown in Figure 7. After quenching, the Materials 2020, 13, 459 6 of 14 strength and hardness markedly increased, while the elongation decreased compared with the normalized specimen. The hardness and tensile strength of the initial martensite were 440 HV and 1302markedly MPa, respectively. increased, while the elongation decreased compared with the normalized specimen. The hardness and tensile strength of the initial martensite were 440 HV and 1302 MPa, respectively.

-2 1600 90 1400 80

1200 Yield stress(MPa) 70 Tensile strength(MPa) 60 Strength/MPa 1000 -2 Toughness/J·cm Toughness (J·cm ) 50

Hardness(HV) 14 Elongation(%) 500 12 10 Hardness/HV Elongation/% 400 8 0 5 10 15 20 25 30 Reduction/%

Figure 7. Mechanical properties of high-strength steel. Figure 7. Mechanical properties of high-strength steel. After a cold rolled reduction of 30%, the hardness and tensile strength increase to 466 HV and 1585After MPa, respectively.a cold rolled Thereduction elongation of 30%, and the impact hardne toughnessss and oftensile the quenched strength martensiteincrease to were 466 HV 15% and and 158595 J/cm MPa,2, respectively. respectively. With The the elonga increasetion and of the impact cold rolled toughness reduction of the to quenched 30%, the elongation martensite and were impact 15% andtoughness 95 J/cm were2, respectively. reduced to With 10.2% the and increase 67 J/cm 2of, respectively. the cold rolled During reduction the deformation to 30%, the process, elongation the lathsand impactare also toughness sheared bywere the reduced micro-shear to 10.2% band and while 67 J/cm migrating2, respectively. toward During the RD, the thereby deformation destroying process, the theinitial laths lath are boundaries also sheared and causing by the a largemicro-shear misorientation band withinwhile migrating the martensite toward structure the [RD,30,32 thereby]. Then, destroyingthe microstructure the initial is further lath boundaries refined and and the crystalcausing defect a large density misorientation is gradually within increased. the martensite As a result, structurethe tensile [30,32]. strength Then, and the hardness microstructure of the experimental is further refined steel and increase the crystal with defect the increasing density is cold gradually rolled increased.reduction, As and a theresult, elongation the tensile tends strength to decrease. and hardness of the experimental steel increase with the increasingThe tensile cold rolled specimens reduction, were alsoand selectedthe elongation in the direction tends to ofdecrease. 0◦, 45◦, and 90◦ with respect to the RD of theThe experimental tensile specimens steel. The were diff erencesalso selected in the elongationin the direction and tensile of 0°, strength45°, and of90° the with experimental respect to steel the RDwhen of the the selected experimental specimens steel. were The in di differencesfferent directions in the to theelongation RD are shown and tensile in Figure strength8. The strength of the Materials 2020, 13, x FOR PEER REVIEW 7 of 15 experimentalalong the 0◦ RD steel is littlewhen higher the selected than that specimens along the were 45◦ and in different 90◦ RD, whereasdirections the to change the RD in are elongation shown in is Figurethe opposite. 8. The strength along the 0° RD is little higher than that along the 45° and 90° RD, whereas the change in elongation is the opposite.

(a) (b) 0° 0° 16 ° 1.9 %

45 0.8 % 0.39 % 0.39 45° 0.65 % 1600 90° 90° 2.1 %

14 1.0 %

1.9 % 1.9 0.75 % 1400 12 0.38 % Elongation/% 0.54 % 0.98 % Tensile strength(MPa) 0.49 % 0.49 10 1200 01030 01030 Reduction/% Reduction/%

Figure 8. Elongation (a) and tensile strength (b) of low-carbon martensitic steel in the directions at

anglesFigure 0◦ ,8. 45 Elongation◦, and 90◦ with (a) and respect tensile tothe strength RD. (b) of low-carbon martensitic steel in the directions at angles 0°, 45°, and 90° with respect to the RD. The mechanical properties (elongation and strength) of the undeformed martensite steel at the differentThe angles mechanical are almost properties the same, (elongation and the and diff strengerencesth) between of the undeformed them are very martensite small (less steel than at the 0.7%).different There angles is no obviousare almost orientation the same, concentration and the diff inerences the initial between martensitic them steelare very (Figure small3), and(less the than martensite0.7%). There lath is randomlyno obvious distributed orientation in concentration all directions. in The the deviations initial martensitic for the elongation steel (Figure and 3), strength and the betweenmartensite 0◦ and lath 45 is◦ (orrandomly 0◦ and 90distributed◦) slightly in increase all directions. after cold The rolling. deviations The martensite for the elongation lath bundle and underwentstrength between extension, 0° bending, and 45° or(or torsion 0° and after 90°) coldslightly rolling increase (Figure after4), which cold rolling. resulted The in anisotropymartensite of lath thebundle structure underwent in different extension, orientations. bending, or torsion after cold rolling (Figure 4), which resulted in anisotropy of the structure in different orientations. When the cold rolled reduction reaches to 10%, the deviation of the elongation between 0° and 45° (or 0° and 90°) is 0.75% (or 1.9%), and the deviation of the strength between 0° and 45° (or 0° and 90°) is 1.0% (or 2.1%). When the cold rolled reduction reaches to 30%, the deviation of the elongation between 0° and 45° (or 0° and 90°) is 0.49% (or 0.98%), and the deviation of the strength between 0° and 45° (or 0° and 90°) is 0.8% (or 1.9%). These results reveal that the anisotropy of material properties is not obvious for the martensitic steel and the cold rolled martensitic steel at low strain (below 30% reduction). The fracture morphology of the tensile specimens for the experimental steel is shown in Figure 9. Specifically, the fracture morphology of the as-quenched martensitic steel is shown in Figure 9a, which reveals that the fracture mode is mainly based on a ductile fracture, with fibers that are parallel to the initial fracture surface and arranged in rows. In the microscopic image of the fracture (Figure 9b), there are obvious equiaxed dimples, a large number of which are aggregated. The matching dimples are elongated in the same direction. Images of the fracture morphology of a 10% cold rolled specimen are shown in Figure 9c,d. Tearing edges appear at the heart of the fracture and the shear lip is mainly distributed around the fracture with a large number of voids. The dimple size and depth are gradually reduced compared with the as-quenched martensitic steel. When the reduction reaches 30%, there is a distinct tear-like structure in the core of the fracture displayed in Figure 9e. Compared with the 0% and 10% cold rolled specimens, the ductile fracture zone becomes very small and the material fracture mode is a mixed fracture mode. In Figure 9f, the dimple size and depth are significantly reduced.

Materials 2020, 13, 459 7 of 14

When the cold rolled reduction reaches to 10%, the deviation of the elongation between 0◦ and 45◦ (or 0◦ and 90◦) is 0.75% (or 1.9%), and the deviation of the strength between 0◦ and 45◦ (or 0◦ and 90◦) is 1.0% (or 2.1%). When the cold rolled reduction reaches to 30%, the deviation of the elongation between 0◦ and 45◦ (or 0◦ and 90◦) is 0.49% (or 0.98%), and the deviation of the strength between 0◦ and 45◦ (or 0◦ and 90◦) is 0.8% (or 1.9%). These results reveal that the anisotropy of material properties is not obvious for the martensitic steel and the cold rolled martensitic steel at low strain (below 30% reduction). The fracture morphology of the tensile specimens for the experimental steel is shown in Figure9. Specifically, the fracture morphology of the as-quenched martensitic steel is shown in Figure9a, which reveals that the fracture mode is mainly based on a ductile fracture, with fibers that are parallel to the initial fracture surface and arranged in rows. In the microscopic image of the fracture (Figure9b), there are obvious equiaxed dimples, a large number of which are aggregated. The matching dimples are elongated in the same direction. Images of the fracture morphology of a 10% cold rolled specimen are shown in Figure9c,d. Tearing edges appear at the heart of the fracture and the shear lip is mainly distributed around the fracture with a large number of voids. The dimple size and depth are gradually reduced compared with the as-quenched martensitic steel. When the reduction reaches 30%, there is a distinct tear-like structure in the core of the fracture displayed in Figure9e. Compared with the 0% and 10% cold rolled specimens, the ductile fracture zone becomes very small and the material fracture mode isMaterials a mixed 2020 fracture, 13, x FOR PEER mode. REVIEW In Figure9f, the dimple size and depth are significantly reduced.8 of 15

Figure 9.FigureTensile 9. Tensile fracture fracture morphology morphology of of thethe experimental steel steel quenched quenched (a,b) and (a,b cold) and rolled cold to rolleda to a reductionreduction of 10% of 10% (c,d ()c, ord) or 30% 30% (e (,ef,).f).

3.3. Effects of Aging The tensile strength, elongation and hardness of the specimens cold rolled and aging at 120 °C are shown in Figure 10a,b, respectively. The data in Figure 10a reveal that the tensile strength of the initial martensite and deformed martensite show an increasing trend at first, and then a decreasing trend with increasing aging time, and the tensile strength reaches a maximum value after aging for 5 h. The tensile strength of the as-quenched martensitic steel and the 30% cold rolled martensitic steel increases to 1339 and 1625 MPa, respectively, after aging treatment for 5 h. When the aging time reaches to 36 h, the tensile strength of the as-quenched martensitic steel and 30% cold rolled martensitic steel is 1320 and 1604 MPa, respectively. The elongation of the initial martensite and deformed martensite also increases with increasing aging time. When the aging treatment time reaches to 36 h, the elongation of the as-quenched martensitic steel increases from 15% to 17%, and the elongation of the 30% cold rolled specimen increases from 10.2% to 11%. The results in Figure 10b reveal that there are differences in the changes of the hardness with the aging time between the as- quenched and deformed martensite steel specimens. The hardness of the as-quenched specimen decreases gradually, decreasing from 440 to 411 HV with the increase of the aging time to 36 h.

Materials 2020, 13, 459 8 of 14

3.3. Eﬀects of Aging

The tensile strength, elongation and hardness of the specimens cold rolled and aging at 120 ◦C are shown in Figure 10a,b, respectively. The data in Figure 10a reveal that the tensile strength of the initial martensite and deformed martensite show an increasing trend at first, and then a decreasing trend with increasing aging time, and the tensile strength reaches a maximum value after aging for 5 h. The tensile strength of the as-quenched martensitic steel and the 30% cold rolled martensitic steel increases to 1339 and 1625 MPa, respectively, after aging treatment for 5 h. When the aging time reaches to 36 h, the tensile strength of the as-quenched martensitic steel and 30% cold rolled martensitic steel is 1320 and 1604 MPa, respectively. The elongation of the initial martensite and deformed martensite also increases with increasing aging time. When the aging treatment time reaches to 36 h, the elongation of the as-quenched martensitic steel increases from 15% to 17%, and the elongation of the 30% cold rolled specimen increases from 10.2% to 11%. The results in Figure 10b reveal that there are differences in Materials 2020, 13, x FOR PEER REVIEW 9 of 15 the changes of the hardness with the aging time between the as-quenched and deformed martensite steel specimens. The hardness of the as-quenched specimen decreases gradually, decreasing from However, the hardness of the cold rolled deformed martensitic steel increases at first, but then 440 to 411 HV with the increase of the aging time to 36 h. However, the hardness of the cold rolled decreases with increasing aging time. In particular, the hardness of the 30% cold rolled specimens deformed martensitic steel increases at first, but then decreases with increasing aging time. In particular, increased from 466 to 479 HV after aging time for 5 h, but decreased gradually with increasing aging the hardness of the 30% cold rolled specimens increased from 466 to 479 HV after aging time for 5 h, time. When the aging time reaches to 36 h, the hardness becomes 470 HV. but decreased gradually with increasing aging time. When the aging time reaches to 36 h, the hardness becomes 470 HV.

(a) (b) 1650 17 480 1600 16 1550 460 15 1500 Tensile strength Elogation 14 440 120℃- 0%

1450 0% 13 120℃-10% 10% 120℃-30% 1350 Elogation/% 420 30% 12 Hardness/HV Tensile strength/MPa Tensile 11 1300 400 10 0 5 30 35 0 5 10 15 20 25 30 35 40 Aging time/h Aging time/h

Figure 10. Tensile strength, elongation (a), and hardness (b) of the experimental steel after aging at

120Figure◦C for 10. various Tensile time strength, periods. elongation (a), and hardness (b) of the experimental steel after aging at 120 °C for various time periods. The SEM images showing the microstructure of the as-quenched specimens and the 30% cold rolled specimens,The SEM images and then showing aging the at 120microstructure◦C for various of the time as-quenched periods are specimens displayed inand Figure the 30% 11a–c. cold Specifically,rolled specimens, Figure 11 anda,b showthen aging that after at 120 the °C aging for various treatment, time the periods microstructures are displayed are not in soFigure different 11a–c. fromSpecifically, the as-quenched Figure 11a,b one. show The that microstructure after the aging still treatment, exhibits the the typical microstructures tempered are lath not martensite so different structuresfrom the composed as-quenched of recovered one. The lathmicrostructure martensite. Afterstill exhibits aging at the 120 typical◦C for 36tempered h, a few lath fine carbidesmartensite werestructures found. Thecomposed segregation of recovered and redistribution lath martensite. of carbon After atoms aging take at 120 place °C intofor 36 lattice h, a few defects fine such carbides as dislocations,were found. lath The boundaries, segregation and and prior redistribution grain boundaries. of carbon In addition, atoms take the transitionalplace into lattice epsilon-carbides defects such alsoas formeddislocations, in this lath aging boundaries, stage [37– 39and]. Theprior locations grain boundaries. of the precipitation In addition, of fine the carbides transitional are various epsilon- throughoutcarbides also the formed deformed in this lath aging martensite stage and[37–39]. are worthThe locations studying of furtherthe precipitation in the future. of fine As carbides shown in are Figurevarious 11c, throughout when the reductions the deformed reached lath tomartensite 30%, the martensiteand are worth phase studying interface further becomes in the blurred future. and As theshown substructure in Figure becomes 11c, when coarser the thatreductions the aging reache timed wasto 30%, 36 h. the Figure martensite 11d showed phase ainterface TEM image becomes of a 30%blurred cold and rolled the specimen substructure after becomes aging at 120coarser◦C for that 36 th h,e which aging cantime be was clearly 36 h. seen Figure that 11d the showed high-density a TEM dislocationsimage of a still 30% exist cold between rolled specimen the martensite after aging laths andat 120 the °C lath for martensite 36 h, which boundary can be clearly becomes seen blurred. that the Carbideshigh-density precipitated dislocations which still showing exist abetween diffuse distribution.the martensite The laths carbide and particlesthe lath martensite are small with boundary size ofbecomes 10–20 nm. blurred. Carbides precipitated which showing a diffuse distribution. The carbide particles are small with size of 10–20 nm.

Materials 2020, 13, 459 9 of 14 Materials 2020, 13, x FOR PEER REVIEW 10 of 15

Figure 11. The SEM microstructures (a–c) and TEM microstructure (d) of the cold rolled sheets tempered Figure 11. The SEM microstructures (a–c) and TEM microstructure (d) of the cold rolled sheets at 120 C for various time periods, (a) quenched specimens with subsequent aging for 5 h, (b) quenched tempered◦ at 120 °C for various time periods, (a) quenched specimens with subsequent aging for 5 h, specimens with subsequent aging for 36 h, (c) and (d) 30% cold rolled specimens with subsequent (b) quenched specimens with subsequent aging for 36 h, (c) and (d) 30% cold rolled specimens with aging for 36 h (Arrows point to the precipitated nano-carbides). subsequent aging for 36 h (Arrows point to the precipitated nano-carbides). 4. Discussion 4. Discussion The X-ray diffraction measurement results were used to analyze the samples under different conditions,The X-ray and diffraction the spectra measurement of the (110), (200),results (211), wereand used (220) to analyze crystal planethe samples with relatively under different strong diconditions,ffraction intensityand the spectra were extracted. of the (110), Jade (200), software (211), solves and the(220) full crystal width plane at half with maximum relatively (FWHM) strong valuesdiffraction of each intensity peak adds were the extracted. FWHM of Jade the foursoftware crystal solves faces the of the full sample, width andat half introduces maximum the empirical(FWHM) Formulavalues of (1) each [40 ]peak to calculate adds the the FWHM dislocation of the density four curvecrystal of faces different of the samples. sample, and introduces the empirical Formula (1) [40] to calculate the dislocation density curve of different samples. β2 ρ = (1) 2 lnβ 22πb2 ρ = (1) π 2 where ρ is the dislocation density, β is the full width2ln2 at halfb maximum of each peak, b is the Berk vector, where bρ =is0.247 the dislocation nm [41]. density, β is the full width at half maximum of each peak, b is the Berk vector,The where variation b = 0.247 of the nm dislocation [41]. density for the as-quenched and deformed low-carbon martensitic steelThe as a functionvariation ofof the the tempering dislocation time density at 120 ◦foCr isthe shown as-quenched in Figure 12and. Thedeformed dislocation low-carbon density ofmartensitic the as-quenched steel as a and function deformed of the low tempering carbon martensitictime at 120 °C steels is shown showed in Figure a tendency 12. The to dislocation increase at firstdensity and of then the began as-quenched to decrease and after deformed low temperature low carbon tempering martensitic for 1steels h. Without showed aging a tendency treatment, to 15 2 15 2 theincrease dislocation at first density and then of low-carbon began to decrease martensitic afte steelr low increased temperature from tempering 1.18 10 form− 1 toh. 1.45Without10 agingm− × × withtreatment, increasing the dislocation cold rolled reduction.density of However,low-carbon the martensitic dislocation steel density increased of the undeformed from 1.18 × low-carbon1015 m−2 to martensitic1.45 × 1015 steelm−2 with was higherincreasing than cold that ofrolled the coldreduction. rolled specimenHowever, after the agingdislocation treatment. density When of the agingundeformed time reached low-carbon to 36 martensitic h, the dislocation steel was density higher than of the that undeformed of the cold androlled deformed specimen low-carbon after aging 15 2 15 2 martensitictreatment. When steel were the 1.64aging 10timem reached− and 1.57to 36 10h, them −dislocation, respectively. density The dislocationof the undeformed density of and the × × colddeformed rolled low-carbon specimens continued martensitic to increasesteel were with 1.64 the × increase 1015 m− of2 and the amount1.57 × 10 of15 cold m−2, rolled respectively. deformation The afterdislocation aging treatment.density of Forthe cold the specimens rolled specimens with cold continued rolling of to 10% increase and 30%, with thethe dislocationincrease of densitythe amount was 15 2 15 2 1.55of cold10 rolledm− deformationand 1.62 after10 maging− after treatment. aging treatment For the specimens for 1 h, respectively. with cold rolling When of the 10% aging and 30%, time × × 15 2 15 2 increasedthe dislocation to 36 density h, the dislocation was 1.55 × density1015 m−2 wasand 1.62 1.53 × 101015 mm−2− afterand aging 1.57 treatment10 m− for, respectively. 1 h, respectively. × × When the aging time increased to 36 h, the dislocation density was 1.53 × 1015 m−2 and 1.57 × 1015 m−2, respectively.

MaterialsMaterials2020 2020, 13, 13, 459, x FOR PEER REVIEW 1011 of of 14 15

1.6 ) -2 m • 15

1.4 120 °C - 0%

120 °C - 10% 120 °C - 30%

location density(10 location 1.2 Dis

0246 35 40 Aging time/h

Figure 12. Dislocation density of the as-quenched and deformed martensitic steel with subsequent Figure 12. Dislocation density of the as-quenched and deformed martensitic steel with subsequent aging at 120 C for various time periods. aging at 120◦ °C for various time periods. Since the carbon content of the experimental steel is low, when low-carbon martensite is tempered Since the carbon content of the experimental steel is low, when low-carbon martensite is at 120 ◦C for 1 h, carbides will not precipitate after a short period of aging. Only carbon atoms will tempered at 120 °C for 1 h, carbides will not precipitate after a short period of aging. Only carbon be segregated near the dislocations [42] and the Cottrell atmosphere will be formed. Speich [43] atoms will be segregated near the dislocations [42] and the Cottrell atmosphere will be formed. Speich used calculations to find that in the case of steels containing less than 0.2 wt% C, almost 90% of the [43] used calculations to find that in the case of steels containing less than 0.2 wt % C, almost 90% of carbon segregates to dislocations and lath boundaries during quenching. When tempered at lower the carbon segregates to dislocations and lath boundaries during quenching. When tempered at temperatures for a shorter period of time, these martensites cause additional segregation but no carbide lower temperatures for a shorter period of time, these martensites cause additional segregation but precipitation. Due to the segregation of carbon atoms, result in an increase in dislocation density and no carbide precipitation. Due to the segregation of carbon atoms, result in an increase in dislocation tensile strength of martensitic steel after tempering for 1 h. When the tempering time exceeds 1 h, density and tensile strength of martensitic steel after tempering for 1 h. When the tempering time the recovery of the partial lath martensite occurred, and a small amount of supersaturated carbon exceeds 1 h, the recovery of the partial lath martensite occurred, and a small amount of supersaturated precipitated from the martensite laths that can form ε-carbide and pinning movable dislocations [44–46]. carbon precipitated from the martensite laths that can form ε-carbide and pinning movable At this time, the dislocation density began to decrease, but the tensile strength continued to increase. dislocations [44–46]. At this time, the dislocation density began to decrease, but the tensile strength In other words, the phenomenon of “secondary hardening” occurred. After the aging time exceeded 5 h, continued to increase. In other words, the phenomenon of “secondary hardening” occurred. After as the aging time continued to increase, the martensite microstructure phase interface became blurred the aging time exceeded 5 h, as the aging time continued to increase, the martensite microstructure and the martensite lath bundles began to decompose. Additionally, the carbides precipitated between phase interface became blurred and the martensite lath bundles began to decompose. Additionally, the martensite lath bundles began to undergo rearrangement and annihilation, which caused the the carbides precipitated between the martensite lath bundles began to undergo rearrangement and martensite laths to coarsen. As a result, the dislocation density continued to decrease, and the tensile annihilation, which caused the martensite laths to coarsen. As a result, the dislocation density strength began to decrease. Caron and Krauss [47] pointed out that when fine carbides in the martensite continued to decrease, and the tensile strength began to decrease. Caron and Krauss [47] pointed out laths were precipitated, the recovery was suppressed, causing the lath bundles to be effectively pinned that when fine carbides in the martensite laths were precipitated, the recovery was suppressed, and moved, thereby decreasing the dislocation density and resulted in the occurrence of the coarsen causing the lath bundles to be effectively pinned and moved, thereby decreasing the dislocation phenomenon. After aging for 1–36 h, the carbides gradually precipitated and the martensite laths density and resulted in the occurrence of the coarsen phenomenon. After aging for 1–36 h, the gradually coarsened, which is the same as the conclusion obtained in reference [47]. After the aging carbides gradually precipitated and the martensite laths gradually coarsened, which is the same as treatment, the deformed martensite lath was coarsened (from Figure 11) which suppressed the recovery, the conclusion obtained in reference [47]. After the aging treatment, the deformed martensite lath so that the dislocation density of the undeformed martensitic steel was higher than the dislocation was coarsened (from Figure 11) which suppressed the recovery, so that the dislocation density of the density of the cold rolled deformed martensitic steel. undeformed martensitic steel was higher than the dislocation density of the cold rolled deformed The hardness and tensile strength for the deformed low-carbon martensite steel followed by martensitic steel. aging were increased during the early aging time period, and when the aging time more than 5 h, the hardnessThe hardness (tensile strength)and tensile is reducedstrength from for the 479 deform HV (1625ed MPa)low-carbon to 470 HVmartensite (1604 MPa), steel respectively, followed by ataging 30% deformation.were increased The during variation the early of the aging tensile time strength period, forand the when as-quenched the aging time martensite more than specimen 5 h, the ishardness the same (tensile as that ofstrength) the deformed is reduced one, from but the479 hardness HV (1625 always MPa) to decreased 470 HV from(1604 440MPa), HV respectively, to 411 HV (Figureat 30% 10 deformation.). The variation The of variation the tensile of the strength tensile and strength the hardness for the foras-quenched the quenched martensite martensite specimen steel is is dithefferent, same indicating as that of that the a deformed clear correspondence one, but the does hardness not exist always between decreased the hardness from and440 strengthHV to 411 when HV the(Figure quenched 10). The steel variation was aging of at the low tensile temperature. strength Whenand the the hardness aging time for reaches the quenched 36 h, the martensite nano-carbide steel underwentis different, a slightindicating precipitation that a clear (Figure correspondence 11d). A large does number not exist of dislocations between the have hardness been and introduced strength duringwhen the martensiticquenched steel transformation, was aging at andlow thetemperatur structuree.of When the martensite the aging time has beenreaches changed 36 h, the by coldnano- deformation,carbide underwent whichleads a slight to anprecipitation increase in the(Figure deformation 11d). A large dislocations. numberThe of dislocations diffusion channels have been of Cintroduced atoms increase, during and the the martensitic diffusion activation transformation energy, and of C the atoms structure decreases, of the which martensite may lead has to been the precipitationchanged by ofcold nano-carbides deformation, in martensiticwhich leads steel to an during increase low-temperature in the deformation aging treatment.dislocations. After The diffusion channels of C atoms increase, and the diffusion activation energy of C atoms decreases,

Materials 2020, 13, 459 11 of 14 martensitic transformation and cold deformation treatment followed by the low temperature aging treatment, the precipitation behavior of nano-carbides is needed to further research and discussion. At the same time, the dislocations migrated to form a dislocation interface in the recovery process that resulted in interface enhancement. The arrangement of the dislocation interface in the tensile process increased the strength of the lath martensitic steel, but the effect on the hardness was relatively small. Saeglitz and Krauss [48,49] have examined low temperature aging by varying time and temperature, and pointed out that for the low-carbon martensitic steels subjected to low-temperature aging treatment, the high hardness and strength are very much dependent on the density of the transition carbides and dislocations built into the low-temperature aging substructure of the tempered martensite crystals, and these carbon-dependent densities control the strain hardening that leads to high hardness and ultimate tensile strengths. Cahn et al. [50] pointed out that in brass, copper, or nickel with low stacking fault energy, there is no drop-in hardness during recovery, the recovery being accomplished by dislocation rearrangement, in contrast to aluminum and iron, where stacking fault energy is high and substantial decreases in hardness during recovery are accomplished apparently by reductions in dislocation density. The variation of the hardness and strength after aging treatment remains to be further studied. Huang et al. and Tsuji et al. [51,52] comparatively studied the cold rolled deformation of nanostructured pure aluminum and IF steel. After aging treatment, the stress of pure aluminum and IF steel increases while the dislocation density decreases, and this indicates that the presence of a certain amount of interior dislocations in the nanostructures produces softening rather than hardening as is observed in conventional coarse grained materials. This correlation between dislocation source density and intensity is usually observed in nanoscale metals through experiments [53] and through atomic models [54]. The availability of dislocation sources or the lack of such sources may therefore significantly affect the yield stress. The results of this study indicate that dislocations can cause softening and will have important applications in the study of new nanomaterials. The typical age hardening and over-aging softening behavior were observed through experiments. Due to the small content of experimental steel alloy elements used in this study, the changes of the mechanical properties are mainly related to the changes of dislocation motion and dislocation density. The softening phenomenon in the experiments of this paper is basically consistent with the experimental conclusions obtained in the literature.

5. Conclusions The AISI 1010 low-carbon steel was quenched with ice water to obtain the lath martensite structure, followed by cold rolling at low strains (<30%) and aging at low temperature (120 ◦C). The mechanical behavior of the deformed martensitic structure was studied. The microstructural evolution of the martensite was examined. The major results are summarized as follows: (1) The deformed structures (LDC, IBL, and KL) were present in the 10% and 30% cold rolled specimens and fairly large amount of the martensite structure remained the same. The area occupied by the LDC, IBL and KL structure increased from 24, 14, and 13% to 40, 31, and 17% with increasing rolling reductions. The martensite lath began to be elongated along the RD and the martensite structure was gradually refined. (2) As the reduction increased, the hardness and the tensile strength gradually increased, and the elongation decreased. When the reduction reached to 30%, the hardness and tensile strength increased to 466 HV and 1585 MPa, respectively. However, the elongation decreased to 10.2%. When the tensile specimens were selected at 0◦, 45◦, and 90◦ in the RD, the specimens exhibited different strength and elongation. The deviation was calculated to be within 3%. The anisotropy of the material properties was not obvious for the martensitic steel and the cold rolled martensitic steel at low strain (below 30% reduction). (3) The strength of the initial martensite and deformed martensite showed an increasing trend at first, and then a decreasing trend with increasing aging time, the tensile strength reached to a maximum value after aging for 5 h. The tensile strength of the as-quenched martensitic steel Materials 2020, 13, 459 12 of 14

and 30% cold rolled martensitic steel increased to 1339 and 1625 MPa after aging treatment for 5 h, respectively. The elongation of the initial martensite and deformed martensite increased with increasing aging time. When the aging treatment time reached to 36 h, the elongation of the as-quenched martensitic steel and 30% cold rolled specimen increased to 17 and 11%, respectively.

Author Contributions: Data curation, Z.L. and L.Q.; Formal analysis, L.Q.; Funding acquisition, Z.L. and S.Q.; Investigation, Z.L. and L.Q.; Methodology, Z.L.; Project administration, Z.L. and L.Q.; Resources: Z.L., L.Q., S.L. and L.Z.; Supervision, Z.L.; Validation, L.Q.; Visualization, L.Q.; Writing-original draft, L.Q.; Writing-review & editing: Z.L. and L.Q. All authors have read and agreed to the published version of the manuscript. Funding: The projects are supported by the Natural Science Foundation of Hebei Province for Distinguished Young Scholar (E2017203036) and the Natural Science Foundation of China (No.51675466). Acknowledgments: The authors would like to express their thanks to M. Li of Yanshan University for his help in experiments of this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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