Control Process on the Properties of High-Strength Low Alloy Steel*

Effect of the ThermoMechanica1-Control Process on the Properties of High-strength Low Alloy Steel*

By Hirosh i TAMEHIRO,** Naoomi YAMADA** and Hiroo MATSUDA***

Synopsis tory rolling tests to examine the effect of the process The effect of the Thermo-Mechanical Control Process (TMCP) on conditions. The slabs were cut to 210 mm thick, the properties of high-strength low alloy steel has been examined and the 300 mm wide and 400 mm long and were rolled into following results have been obtained. 25 mm thick plates under various process conditions. The addition of niobium or titanium, especially the combination of The CR conditions adopted for the laboratory tests niobium and boron is effectivefor TMCP. Low-temperature toughness were : heating temperature, 1 150 to 1 200 °C; total of TMCP plate is not significantly influenced by the cooling conditions, but is mainly determined by the controlled-rolling (CR) conditions. rolling reduction below 900 °C, 70 to 75 %; finish TMCP alters the microstructure from ferrite pearlite to fine-grained rolling temperature, in the vicinity of the Ar3 tempera- ferrite-bainite and consequentlyincreases the strength without a loss in ture; cooling rate, 18 to 28 °C/s; finish water-cooling low-temperaturetoughness, compared with CR process. temperature, 410 to 470 °C and plate thickness, 16 The advantagesof TMCP plate are a decreasein the carbon equivalent, to 25 mm. improvementof HIC resistance and an increase in the impact energy. The mechanical properties in the transverse direc- tion were tested on full-thickness tensile and full-size I. Introduction Charpy V-notch impact test specimens taken from With increasingly strict quality requirements for the mid-thickness of the plate and on a Battelle Drop steels used in welded structures, various steel plate Weight Tear Test (BDWTT) specimen. Micro- scopic observation, hardness tests and HIC tests to production techniques, such as low-temperature slab- heating and rolling in the inter-critical temperature estimate the HIC resistance were also performed. region (Ar3 to An), have recently been developed and The HIC tests were carried out according to put into practical use. Of these, the Thermo-Me- NACE standard.4} Specimens of (t-2) mm x 20 mm chanical Control Process (TMCP),i-3) a combined x 100 mm (t: thickness) in size were polished, de- process of controlled-rolling (CR) and controlled-cool- greased and immersed in NACE solution for 96 h. ing is attracting wide attention. In TMCP, steel plate The NACE solution consisted of a H2S-saturated after CR is water-cooled during transformation at solution containing 0.5 % acetic acid plus 5 % NaCI a suitable cooling rate and then air-cooled. The as- having a pH of 3.5 to 3.8. After testing, the HIC cooled plate can be used as a structural material was estimated on the basis of the HIC area ratio using without any heat treatment. Since the microstruc- ultrasonic testing. ture of steel, as well as the grain size, can be controlled by TMCP, steel properties can be improved, plates of III. Experimental Results greater thickness can be produced, and the possibility of creating new properties exists. 1. Effect of the Alloying Elements On the other hand, some differences in the effect 1. Carbon and Manganese of the alloying elements and the microstructure of The tensile strength (TS) of TMCP plates of steel processed by CR and TMCP can be anticipated. C-Mn steel is correlated to C+Mn/9, as shown in Therefore, many problems still remain to be studied Fig. 1. An increase in the carbon content, though from the metallurgical viewpoint. it raises the TS, is not desirable because it reduces In this paper, the effect of the alloying elements and the low-temperature toughness of the steel, especially the process conditions on the properties of High- the impact energy. Manganese lowers the Ar3 tem- Strength Low Alloy (HSLA) steel and the advantages perature and facilitates grain refinement by CR and of TMCP plate over CR plate are discussed. consequently enhances the toughness of TMCP plates. Manganese is also needed in suitable amounts in order II. Experimental Procedure to promote bainitic transformation by controlled- To examine the effect of the alloying elements, cooling and to increase strength. Since the carbon laboratory steels were made in 150 kg vacuum- and manganese contents are the basis for alloy de- induction-melted heats and cast as 125 mm thick signing, they should be properly determined in terms ingots. The ingots were then rolled under suitable of the required properties. process conditions. In addition, commercial con- 2. Niobium, Vanadium and Titanium tinuously cast slabs of 0.07%C-1.5%Mn-0.04%Nb- Niobium, vanadium and titanium suppress the 0.08%V-0.01 %Ti steel were also prepared for labora- recrystallization of austenite during rolling, and refine

* Manuscript received March 9, 1984; accepted in the final form on June 25, 1984. 1985 ISIJ ** Kimitsu R & D Laboratory, Nippon Steel Corporation, Kimitsu, Kimitsu 299-11. *** Kimitsu Works, Nippon Steel Corporation, Kimitsu, Kimitsu 299-11.

(54) Research Article Transactions IsI1, Vol. 25, ].985 (55) the microstructure and simultaneously strengthen the change in strength caused by controlled-cooling is steel by inducing precipitation-hardening. In the more pronounced. The increases in YS and TS microalloyed-Ti range of 0.01 to 0.02 %, titanium are approximately 6 kgf/mm2 and 4 to 7 kgf/mm2, reacts with nitrogen to produce fine titanium nitride respectively. It seems that improvement in the TS (TiN). Finely dispersed TiN inhibits the grain of TMCP plates significantly depends on the increased growth of austenite during slab-heating and recrystal- hardenability of the steel, which is in turn brought lized austenite during rolling, consequently yielding about by niobium dissolved in the austenite, in ad- an uniform and fine-grained microstructure.5~ dition to precipitation-hardening, while that of CR The effects of niobium and titanium contents on plates is attributable mainly to precipitation-hard- the mechanical properties of 20 mm thick TMCP ening. In fact, the addition of niobium increases the plates are shown in comparison with those of CR volume fraction of bainite in spite of the austenite- plate in Figs. 2 and 3. grains being refined. The TS gradually increases For Nb-free steel, controlled-cooling increases the with the increase in niobium until it reaches its TS of TMCP plates by approximately 4.5 kgf/mm2 maximum level determined by the solubility limit of and decreases the yield strength (YS) by some niobium during heating. 2 kgf/mm2. For Nb steel, on the other hand, the On the other hand, there can be seen no degrada- tion of low-temperature toughness due to controlled- cooling, and the addition of niobium raises the Charpy impact energy (VE). Although no difference in the Charpy impact transition temperature (VTrs) is apparent between TMCP plates and CR plates because the v TJrsare lower than -100 °C due to the refining effect of finely dispersed TiN, the fact that higher Nb steels yield finer grain suggests a beneficial effect of higher niobium on ~~l ls. The reason for the increase of VE-40 under the TMCP is attributed to the self tempered uniform and fine-grained microstructure formed by interrupted water-cooling. Titanium improves steel properties in virtually the same way as niobium. For Ti steel, controlled- cooling increases the TS and YS by approximately 6 kgf/mm2 and 0 to 4 kgf/mm2, respectively. In the case of Ti-free C-Mn steel, controlled-cooling decreases

Fig. 1. Relationship between tensile strength and the the YS by some 1 kgf/mm2 while the TS is increased carbon equivalent in C-Mn steel. by approximately 4 kgf/mm2. Here again, as in the

Fig. 2. Effect of niobium content on the mechanical properties of 0.07%C -1 .55%Mn-0.001 %S- 0.018%Ti- 0.004%N steel.

Fig. 3. Effect of titanium content on the mechanical properties of 0.10%C-1.58%Mn-0.001 %S-0.003%N steel. ( 56) Transactions ISIJ, Vol. 25, 1985

case with niobium, the increase in the strength of tively. And the v Trs remains almost unchanged from TMCP plates is attributed to the bainitic micro- that of the base steel because the microstructure of structure and to precipitation-hardening. Nb-B steel is sufficiently fine-grained. This increase The v Tis of CR plates is lower than that of TMCP in TS results from the increase in the volume fraction plates in the titanium range from 0 to 0.05 %. of bainite. The reason for this is thought to be because the How the combined addition of niobium and boron austenite-grains of Ti-free and 0.05 % Ti steels are increases the TS of CR and TMCP steels has yet to coarser than those of Ti-microalloyed steels (0.01 N be studied. However, niobium dissolved during slab- 0.02 % Ti). As shown in Fig. 3, v Tr.Sdegrades as heating is likely to play an important role because the strength increases through controlled-cooling. More effect of the combined addition of niobium and boron specifically, for 0.05 % Ti steel containing titanium is significantly influenced by the process conditions stoichiometrically in excess of nitrogen, the TiN employed, especially the slab-heating temperature. formed is comparatively larger in size and the aus- If a slab is reheated at a low temperature, the amount tenite-grains are coarser than in Ti-microalloyed steel of dissolved niobium decreases, lowering the effect where austenite-grains are refined to a great extent. of the combined addition. CR also reduces the in- Therefore, to increase strength without losing low- crease of TS, because it accelerates the precipitation temperature toughness, the austenite-grains must be of niobium carbide during rolling. kept as small as possible prior to water-cooling. The effects of carbon and manganese on the Vanadium contributes less to the improvement of mechanical properties of 16 mm thick TMCP plates mechanical properties by controlled-cooling than of Nb-B steel are indicated in Figs. 5 and 6, respec- niobium or titanium does. This is especially so for tively. As evident from Fig. 5, an increase in carbon Ti-microalloyed steel where nitrogen is fixed as TiN. content leads to no appreciable improvement in the A 0.05 % addition of vanadium increases TS by only strength-toughness balance of Nb-B steel. Even with about 2 kgffmm2 and hardly improves the low-tem- an increase of the carbon content from 0.01 to 0.06 %, perature toughness at all. the increase in TS is only 4 kgf/mm2 or so. The 3, Boron reason for this is thought to be due to the fact that in- Boron is usually utilized in quenched and tempered creasing the carbon content decreases the amount of (QT) steels to achieve a TS greater than 60 kgf/mm2. niobium dissolved during slab-heating and, as a result, Recently, however, it is also used in controlled-rolled reduces the volume fraction of bainite. For both CR steel plates to reduce both carbon and the carbon and TMCP plates, therefore, the significant effect of equivalent (Ceq) and to obtain high weldability.6~ the combined addition of niobium and boron is more Boron increases the strength of QT steel because pronounced at lower carbon contents. In fact, as boron atoms segregated at the austenite-grain bound- can be seen in Fig. 6, TMCP plates of low C-Nb-B aries retard the nucleation of ferrite, and consequently steel have excellent strength and toughness, even with increase the hardenability of steel. Although it has a relatively low manganese content. not yet been clarified why the addition of boron exerts It has been known that decreasing the manganese a significant influence on the strength of CR plates content elevates the Ar3 temperature, lowers the effect which transforms from non-recrystallized austenite, of CR on the grain-refinement of austenite and reduces it does undeniably suppress the nucleation of ferrite low-temperature toughness. So far as low C-Nb-B and accelerates bainitic transformation. To use steel is concerned, however, this tendency is not microalloyed boron effectively for the improvement apparent. of mechanical properties in CR plates, it is essential to lower the nitrogen content as low as possible, fix 2. Effect of Process Gonditions"3,s~ the nitrogen as TiN to prevent the formation of Figure 7 shows the effect of the slab-heating tem- boron nitride (BN) and simultaneously add niobium and boron in combination. A significant improvement in the strength-toughness balance of 20 mm thick plate as a result of the combined addition of niobium and boron is shown in Fig. 4.7) In general, if the cooling rate after CR is not as high as that of quenching, the addition of boron alone does not increase the strength of low Ceq, steel. The addition of boron to the base steel in Fig. 4 where the nitrogen is fixed with titanium leads to little increase in the strength of either CR or TMCP plates but tends rather to decrease low-temperature toughness. The addition of niobium results in improved strength and toughness as already discussed, but the increase in strength is only slight. In con- Fig. 4. Significant improvement in the strength and tough- trast, the combined addition of both niobium and ness balance by the combined addition of niobium boron increases the TS of both CR and TMCP plates and boron. Base composition is 0.05%C-1.5%Mn- by as much as 10 kgf/mm2 and 12 kgf/mm2, respec- 0.02%Ti. Transactions ISIJ, Vol. 25, 1985 (57)

Fig. 5. Effect of carbon content on the mechanical properties of 1.05° Mn-0.001 °0S -0 .04%Nb-0.02%Ti- 0.0015%,B steel. Fig. 7. Effect of the slab-heating temperature on the mc- chanical properties of steel

Fig. 6. Effect of manganese content on the mechanical properties of 0.03%C-0.001%S--0.04°0Nb-0.02%Ti- O.00l4%B steel. Fig. 8. Effect of the finish rolling temperature on the me- perature. A lower slab-heating temperature yields chanical properties of steel. finer austenite-grains during heating, refines the microstructure of the plates and as a consequence enhances low-temperature toughness. The TS, however, decreases because lower heating temperatures reduce the amount of dissolved niobium. There is no difference between TMCP and CR plates in the dependence of tensile strength and toughness on the slab-heating temperature. Figure 8 shows the effect of the finish rolling temperature. The Ar3 temperature of the test steel is around 760 °C, at which point the TS reaches its minimum value and then increases whether or not the finish rolling temperature is above or below the Ar3 temperature. The increase in TS occurring

above the Ar3 temperature, results from an increase Fig. 9. Effect of the cool ing rate on the mechanical prop- in the volume fraction of bainite, while that below the erties of steel. Ar3 temperature results from an increase in deformed ferrite and the volume fraction of bainite transformed 4E and BDWTT transition temperature. from enriched residual austenite. The effect of the cooling rate on the mechanical The best low-temperature toughness in terms of properties of TMCP plates is shown in Fig. 9. The ,1 and BDWTT transition temperature is obtained TS of TMCP plates, is about 3.5 kgl7m,m2 higher than at a finish rolling temperature around the Ar3 tem- that of CR plates at a cooling rate of 10 °C/s and perature. Rolling in the inter-critical region, if about 5.5 kgf/mm2 higher at 20 °C/s. The YS also practiced properly, causes separation and lowers the increases as the cooling rates increase. This increase BDWTT transition temperature. However, exces- of strength is ascribed to the rise in the volume frac- sive rolling in the inter-critical region followed by tion of bainite. At a cooling rate of 30 °C/s, marten- water-cooling has a detrimental effect on both the site develops on the plate surface. At cooling rates (58 ) Transactions ISI1, Vol. 25, 1985 higher than 10 °C/s, VE-40 increases by roughly 3 kgf-m, but the BDWTT transition temperature deteriorates by about 5 °C. The cooling rate has little effect on the low-temperature toughness. This implies that the toughness of TMCP plates is mainly controlled by the austenite-grain size before water- cooling, i.e., by the applied CR conditions. By the application of controlled-cooling, the microstructure changes from ferrite-pearlite to fine-grained ferrite-bainite and the banded structure diminishes. The effect of the finish water-cooling temperature on the mechanical properties of TMCP plates is indicated in Fig. 10. Although the TS increases as the finish water-cooling temperature decreases, it is only below about 520 °C that the increase of TS becomes pronounced, where the TS of TMCP plates becomes higher by roughly 4 kgf/mm2 than that of CR plates. In the temperature range of 520N400 °C, the TS Fig. 10. Effect of the finish water-cooling temperature on hardly changes. Below 400 °C, however, it rapidly the mechanical properties of steel. increases because the volume fraction of bainite increases in the microstructure and, at the same time, island martensite is formed. For this reason, below 400 °C the stress-strain curve shows continuous yielding and YS decreases sharply. As for low-temperature toughness, a slight increase can be seen in vE-40 between 520 °C and 400 °C but a loss below 400 °C, while the BDWTT is hardly affected by the finish water-cooling temperature. The formation of island martensite below 400 °C greatly influences ~E-40 but has hardly any affect on the BDWTT transition temperature. It is evident from the above findings that the BDWTT is little influenced by cooling conditions such as finish water-cooling temperature and cooling rate and is mainly determined by the CR conditions applied.

Iv. Steel Properties Improved by TMCP 1. Carbon Equivalent2) Fig. 11. Relationship between the carbon equivalent and Controlled-cooling after CR alters the microstruc- tensile strength. ture of plates from ferrite-pearlite to fine-grained ferrite-bainite, thereby increasing their strength. Considering the softening in the heat-affected zone (HAZ) of weld joints, the increase in TS over that of CR plates generally ranges from about 4 to 6 kgf/mm2. For TMCP plates, as shown in Fig. 11, this increase in TS makes it possible to lower the Ceq. by approximately 0.04 0/ Although the Ceq. of CR plates of Nb-B steel is lower than that of TMCP plates of conventional steel, it is possible to further reduce the Ceq. by TMCP. At 0.3 % Ceq,, a strength of TS 60 kgf/mm2 can be attained. A reduction in C°(11 leads to substantial improvement in the weldability Fig. 12. Effect of the finish rolling temperature on the and toughness of the HAZ. separation index of Charpy V-notch impact test specimens. 2. Charpy Impact Energy TMCP simultaneously increases the strength and and TMCP plates of 0.06%C-1.5 %Mn-0.003%S- even the impact energy of plate because of reduced 0.2%Ni-0.04%Nb-0.07%V steel. Even at the same separation on fracture surface. Figure 12 shows the finish rolling temperature, TMCP plates have a lower relationship between the finish rolling temperature SI. The reduction of separation is considered to be and the separation index (SI) for 20 mm thick CR a result of the disappearance of ferrite-pearlite banded Transactions ISIJ, Vol. 25, 19$5 (59) structure by controlled-cooling (Photo. 1).1) distribution of TMCP plate is fairly uniform in com However, even for ultra-low sulphur steels, with parison with that of CR plate, though the concen- impact energies high enough to ensure separation- trations of manganese, phosphorus and other alloying free characteristics (Figs. 2 and 3), controlled-cooling elements are equally distributed in both plates. The further increases impact energy. This increase is concentration of carbon in the segregated zone occurs attributed to the self-tempered uniform and fine because segregation of manganese and other elements ferrite-bainite structure formed by water-cooling. retards the austenite to ferrite transformation in the segregated zone, thus facilitating the diffusion of 3. HIS' Resistance4> carbon from the normal zone into the segregated zone When the ferrite-pearlite banded structure vanishes, during transformation. For TMCP plates cooled at fine-grained ferrite-bainite microstructure forms by higher rates, such a diffusion of carbon is effectively TMCP, as shown in Photo. 1. A similar change of suppressed and the volume fraction of hardened the microstructure also appears in the segregated structures is reduced and hard phases are finely dis- zone at the mid-thickness of plates rolled from con- tributed. As a result, HIC resistance increases con- tinuously cast slabs. The detrimental effect of segre- siderably. gation at the mid-thickness on the HIC resistance results mainly from the presence of hardened structures V. Conclusions which are caused by the segregation of carbon, The effect of TMCP conditions on the properties manganese and phosphorus, and so on. If the con- of HSLA steel was examined. As a result, the fol- trolled-cooling condition applied after CR is ap- lowing conclusions were obtained. propriate, such hardened structures can be reduced (1) The tensile strength of TMCP plates of C- effectively and the HIC resistance is enhanced. Figure 13 shows the effect of finish water-cooling temperature on HIC resistance and maximum hardness of the segregated zone at the mid-thickness of 16 mm thick Nb and Nb-B steel plates. In the case of grade X70, hardness is lowest and HIC is suppressed at a finish water-cooling temperature around 490 °C. If water-cooling is interrupted at an ap- propriate temperature, the percentage of hardened structures in the segregated zone is reduced and the hard phases such as high-carbon martensite are finely distributed. The microstructures at the mid-thickness of CR and TMCP plates containing niobium and vanadium are shown in Photo. 2. The microstructure of TMCP plates consists of fine-grained ferrite-bainite while that of CR plates consists of a banded structure of ferrite-bainite-martensite. The change in the hardened structure caused by TMCP is due to the inhibi- tion of carbon diffusion during the transformation from austenite to ferrite by water-cooling.

The distribution of carbon concentrations at the Fig. 13. Effect of the finish-water cooling temperature on mid-thickness of plates as measured by the Ion Micro the HIC resistance and maximum hardness of Analyzer (IMA) is indicated in Fig. 14. The carbon segregated zones at the mid-thickness of plates.

Photo, 1. Microstructures of CR and TMCP plates containing niobium and vanadium. (60) Transactions ISIT, Vol. 25, 1985

Photo. 2. Microstructures at the mid-thickness of CR and TMCP plates containing niobium and vandadium.

Fig. 14. Distribution of the carbon concentration of segregated zone at the mid-thickness of plates by IMA (0.08%C-1.33%Mn-Nb-V steel).

Mn steel is correlates to C + Mn/9. largely from the increase in the volume fraction of (2) Niobium and titanium serve simultaneously bainite in the microstructure. to refine the microstructure and improve the low- (6) TMCP increases TS by approximately 4 to temperature toughness and strength of TMCP plates. 6 kgff mm2 without a loss in low-temperature toughness The increase in TS of TMCP plate is attributable to and consequently lowers the Ceq. by about 0.04 %. the ferrite-bainite structure formed by niobium and (7) The microstructure of TMCP plates is free titanium dissolved in austenite, in addition to precipi- of a banded structure, less susceptible to separation, tation-hardening. and improves Charpy impact energy. (3) The combined addition of niobium and boron (8) With TMCP, the volume fraction of hardened alters the microstructure to a fine-grained ferrite- structures at the mid-thickness of plates is reduced bainite structure and significantly enhances the and hard phases such as martensite and bainite are strength-toughness balance. Since this effect is more finely distributed. As a result, the segregated zone pronounced for lower C steels, the combined addition becomes less hard, thereby considerably enhancing of niobium and boron is especially effective in reducing HIC resistance. the C0(,,. (4) The low-temperature toughness of TMCP REFERENCES plates is determined mainly by the CR conditions, 1) C. Ouchi, T. Okita and S. Yamamoto : Tetsu-to-Hagane, such as the slab-heating temperature and finish rolling 67 (1981), 129. temperature. 2) K. Tsukada, K. Matsumoto, Y. Yamazaki, K. Hirabe, K. (5) The increase of TS in TMCP plates results Arikata and K. Takeshige: Tetsu-to-Hagane, 70 (1984), Transactions ISI1, Vol. 25, 1985 (61)

89. 6) H. Nakasugi, H. Matsuda and H. Tamehiro : Alloys for 3) C. Shiga, K. Amano, T. Hatomura, Y. Saito, K. Hirose the Eighties, Climax Molybdenum Co. Conf., (1980), 213. and T. Choji : Proceedings of an International Confer- 7) H. Tamehiro, H. Matsuda, M. Ohashi, Y. Kawada, Y. ence on Steels for Line Pipe and Pipeline Fittings, The Onoe and K. Nakajima: Tetsu-to-Hagane, 68 (1982), 5336. Metal Soc., London, (1981), 127. 8) T. Takeda, M. Murata, H. Tamehiro, H. Matsuda, N. 4) NACE Standard TM-02-84. Yamada and Y. Onoe: Tetsu-to-Hagane, 69 (1983), 5655. 5) H. Gondo, H. Nakasugi, H. Matsuda, H. Tamehiro and 9) T. Takeda, H. Tamehiro, N. Yamada, S. Eiro and S. Matsu- H. Chino: .Nippon Steel Technical Report, Overseas, (1979), da : Tetsu-to-Hagane, 70 (1984), 5544. No. 14, 55.