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 ap- parent 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.

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