钢中的碳含量和显微组织对屈强比的影响_英文_于庆波

第 16 卷 第 6 期 2009 年 12 月 塑性工程学报 JOURNAL OF PLASTICITY ENGINEERING Vol 16 No 6 Dec 2009 doi 10 3969 j issn 1007 2012 2009 06 024 钢中的碳含量和显微组织对屈强比的影响 徐州工程学院 机电工程学院 徐州 221008 于庆波 孙 莹 摘 要 通过对低碳合金钢的屈强比实验 研究了碳含量和显微组织对屈强比的影响 理论分析和实验结果表明 铁素体晶粒大小会影响钢的屈强比 既铁素体晶粒越细小 则钢的屈强比越高 另外 不同的显微组织也会影响钢 的屈强比 由铁素体 贝氏体组织所组成钢 其屈强比低于由铁素体 珠光体组织所组成钢 而且钢中弥散分布的 M A 岛颗粒会提高钢的抗拉强度进而降低钢的屈强比 关键词 钢结构建筑 屈强比 M A 岛 弥散强化 中图分类号 TB30 TG14 文献标识码 A 文章编号 1007 2012 2009 06 0119 08 Effect of carbon content and microstructure on the yield strength ratio of steel YU Qing boSUN Ying Mechanical and Electronic Department Xuzhou Institute of Technology Xuzhou 221008 China Abstract The experiment on the yield strength ratio of low carbon alloy steel was performed and the effect of carbon content and microstructures on the yield strength ratio of steel was discussed Theoretical analysis and experimental results indicate that the ferrite grain size can affect the yield strength ratio of steel i e the smaller ferrite grain size the higher the yield strength ra tio In the meantime different microstructures can also affect the yield strength ratio of steel The yield strength ratio of steel composed of polygonal ferrite and bainite is lower than that of ferrite and pearlite Furthermore grains of M A island in large quantities distributed in diffusion can increase the tensile strength of steel remarkably and so decrease the yield strength ratio Key words steel structure construction yield strength ratio M A island dispersion strengthening 江 苏 省 高 校 自 然 科 学 研 究 计 划 资 助 项 目 08KJD460012 江苏省高校 青蓝工程 资助项目 于庆波 E mail yuqb1970 sina com 作者简介 于庆波 男 1970 年生 博士 副教授 研 究方向为钢铁材料组织性能 收稿日期 2009 05 21 修订日期 2009 07 07 Introduction To substitute the reinforced concrete structure by the steel structure has become a development trend in high rise constructions for the many virtues of the steelstructureconstruction 1 7 InJapan the steel structure constructions have achievedmore than 40 of total construction area and in Ameri ca and Western Europe the industrial and civil ar chitectures newly built also use steel frame At present among the highest one hundred buildings in the world the steel structure constructions have exceeded one half The rapid development of the steel structure constructions results from their vir tues such as light structure big space industriali zation production saving energy and so on 8 9 And especially after the large earthquake taken place in Kobe Japan these steel structure constructions show better earthquake resistant capability than re inforced concrete construction The earthquake re sistant capability of the steel structure constructions Funded by the Natural Science Research Project for colle ges and universities in Jiangsu No 08KJD460012 Spon sored by Qing Lan Project in Jiangsu YU Qing bo E mail yuqb1970 sina com First author YU Qing bo male born in 1970 doctor associate professor Received date 2009 05 21 Approved date 2009 07 07 results from the work hardening of the steel and the yield strength ratio is an important parameter to judge the earthquake resistant capability of the steel forhigh risebuilding InEurope theyield strength ratio should be lower than 0 90 but in Ja pan it should be lower than 0 80 10 Over the past several decades some researches on the yield strength ratio of steel have been per formed 11 13 but the micro mechanism on the low yield strength ratio of steel has not been reported In this paper the law s of chemical composition ferrite grain size and the volume fraction of pearli te and bainite on the yield strength ratio are ob tained by thermo mechanical controlledprocess Furthermore the regression formulas on yield and tensile strength have been derived 1 Experimental procedures 1 1 Experimental materials To further investigate the yield strength ratio of steel four kinds of steel were designed in which steel A does not contain alloying element Nb steel B contains Nb the carbon content of steel C is rather high and steel D contains more alloying ele ments such as manganese chromium and niobium Table 1 The ingots employed for the experiment were prepared by melting in vacuum induction fur nace and then forged into the specimens of 150mm 100mm 45mm Tab 1 Chemical composition of experimental steels in wt 表 1 实验钢的化学成分 wt SteelCSiMnCrMoNbPS A0 040 0 240 530 320 40 0 008 0 0102 B0 041 0 210 500 350 42 0 015 0 007 0 0085 C0 080 0 220 490 350 42 0 016 0 0090 009 D0 040 0 281 430 400 20 0 055 0 0120 003 1 2 Experimental process Process A The hot rolling experiments of four kinds of steel were carried out at 375mm 375mm two high re versing mill The billets were heated at 1200 for 1h Two stages rolling were performed The de formation process was 45mm 33 30mm 50 15mm 45 8mm 25 6mm After hot rolling the steel plates were immediately cooled down to specified fi nal temperature by spray cooling at 12 s and then were aircooled to roomtemperature as shown in Fig 1 Fig 1 Schematic illustration of controlled rolling and controlled cooling process Process A 图 1 控轧控冷工艺示意图 工艺 A Process B According to the process illustrated in Fig 2 the thermo mechanical controlled experimental process of steels A and B were performed It can be seen that the heating up temperature the deformation a mount of each pass and the deformation temperature of each pass are the same as that of experimental process A The difference from the experimental process A is that the steel plates in process B were not immediately cooled and spray cooling did not start until the steel plates were air cooled to 700 as shown in Fig 2 Fig 2 Schematic illustration of controlled rolling and controlled cooling process Process B 图 2 控轧控冷工艺示意图 工艺 B 2 Results 2 1 Mechanical properties After the thermo mechanical controlled experi 120塑性工程学报第 16 卷 mental process of steels A D were accomplished the mechanical properties of the steel plates was tested and the test results are given in Table 2 Tab 2 Hot rolling technological parameters and the mechanical properties Process A 表 2 热轧工艺参数和力学性能 工艺 A No Finish rolling temp Final temp Yield strength MPa Tensile strength MPa Yield streng thratio Ferrite grain size m A18906203334230 7879 13 A28905503374430 7619 08 A38606203304150 7959 20 A48605503354350 7708 98 A58306203154050 7779 12 A68305503304300 7679 22 B18906203774550 8297 28 B28905503674700 7817 15 B38606203674470 8217 13 B48605503704700 7877 14 B58306203744420 8467 06 B68305503804650 8177 01 C18906203704810 7707 31 C28905503734880 7607 36 D8305504906500 750 Tab 3 Hot rolling technological parameters and the mechanical properties Process B 表 3 热轧工艺参数和力学性能 工艺 B No Finish rolling temp Start cooling temp Final temp Yield strength MPa Tensile strength MPa Yield streng thratio Ferrite grain size m A78907006203004070 735 15 11 A88907005502953970 730 15 28 B78907006203534500 783 13 46 B88907005503554570 780 13 35 2 2 Mechanical properties Process A When the finish cooling temperature is 620 the microstructure of steels A1 A3 and A5 are composed of proeutectoid ferrite and pearlite but steels B1 B3 and B5 are composed of proeutectoid ferrite pearlite and small amount of bainite When the finishcoolingtemperature isdecreasedto 550 the microstructure of steels A and B is com posed of proeutectoid pearlite and small amount of bainite In addition it can be observed by compari son in Fig 3 that steel B is finer in proeutectoid fer rite grains than steel A under the same condition 121 第 6 期于庆波 等 钢中的碳含量和显微组织对屈强比的影响 Fig 3 Microstructures of steels A and B obtained in process A 图 3 工艺 A 中的 A 钢和 B 钢的显微组织 122塑性工程学报第 16 卷 It can be observed in Fig 4 that the microstruc ture of the steel C1 is composed of proeutectoid fer rite pearlite and small amount of bainite and that of the steel C2 is composed of proeutectoid ferrite bainite and small amount of pearlite It can be seen by comparison that the steel C is lower in the vol ume fraction of ferrite than the steels A and B but the steel C is higher in the volume fraction of pearli te and bainite than the steels A and B Process B Because these steel plates were not immediately cooled after rolling overcooling austenite had been cooled to ferrite austenite dual phase region before starting spray cooling and so proeutectoid ferrite grains had nucleated and grown up Therefore under the same finish rolling temperature and finish cooling temperature condition in comparison with the steels obtained in process A the ferrite grains of the steels A7 and B7 were coarser than those of the steels A1 and B1 and those of the steels A8 and B8 were coarser than those of the steels A2 and B2 123 第 6 期于庆波 等 钢中的碳含量和显微组织对屈强比的影响 3 Discussion 3 1 Effect of chemical composition and microstruc ture on the yield strength ratio The results in Table 2 show that under the same thermal mechanical controlledprocessparameter condition the steels B and C differ in yield strength by 8MPa but the tensile strength of the steel C is higherthanthatofsteel Bapproximatelyby 22MPa Accordingly the yield strength ratios of the steels C1 and C2 are 0 77 and 0 76 and those of the steels B1 and B2 are 0 829 and 0 781 respec tively The bigger difference of the yield strength ratio between the steels B and C result from their different microstructures The carbon content of the steel C is 0 08 and the steel B is 0 041 It is generally believed that the volume fraction of pro eutectoid ferrite decreases with the increase of car bon content The test results show that the volume fraction of the ferrite and pearlite of the steel B1 are 91 57 and 8 72 and the volume fraction of ferrite andpearlite bainite ofthe steel C1 are 77 87 and 22 13 respectively The relation be tween their strength and pearlite bainite content is shown in Fig 6a The volume fraction of ferrite and pearlite of the steel B2 is 91 57 and 8 43 and the volume fraction of the ferrite and pearlite bain ite of the steel C2 is 77 28 and 22 72 respec tively The relation between theirstrength and pearlite bainite volume fraction is shown in Fig 6b Fig 6 shows that the yield strength does not var ied remarkably as the volume fraction of pearlite bainite increases but the tensile strength increases remarkably After all the test data were regressed the equations on the yield and tensile strength were obtained as follow s s 38 99d 1 2 78 03 R 0 95 1 b 25 4d 1 2 4 2 P B 120 1 R 0 94 2 Where d is the ferrite grain diameter P Bis the volume fraction of pearlite bainite and R is multi ple correlation coefficient It can be known by Eq 1 that the yield strength is mainly affected by fer Fig 6 Effect of the volume fraction of pearlite bainite on yield strength and tensile strength 图 6 珠光体 贝氏体体积分数对屈服强度 和抗拉强度的影响 rite grain size and has nothing to do with the volume fraction of the pearlite bainite In addition it can be known by Eq 2 that the influence of ferrite grain size on the yield strength is more remarkable than that on the tensile strength and the tensile strength of steel also increases as the volume frac tion of pearlite and bainite increases Accordingly the regression equation can represent the nature of the steel in strength The tensile test of the steel composed of large amount of ferrite small amount of pearlite and bainite includes three stages as shown in Fig 7 Fig 7 Stress strain curve of low carbon steel 图 7 低碳钢的应力应变曲线 In the first stage region the deformation of soft and hard microstructure is in elastic deforma 124塑性工程学报第 16 卷 tion state In the second stage region the plastic deformation of the soft microstructure has begun but the hard microstructures are still in e lastic deformation state In the third stage re gion both soft microstructure and hard micro structure have been in plastic deformation state When a specimen composed of ferrite pearlite and bainite is pulled the plastic deformation of ferrite grains first occurs because they are soft microstruc ture However they can not bear larger stress Therefore in the process of consequent deforma tion the raise of tensile strength is due to the ex istence of harder microstructure such as pearlite and bainite Although above experiment results indicate that the yield strength ratio decreases with the in crease of carbon content but the yield strength ra tio of steel D is also considerably low 0 75 when carbon content is only 0 04 It is because more alloying elements such as Mn Cr and Nb were added into the steel D These allo ying elements remarkably increase the harden ability of the steel D and avoid the formation of pearlite Hence the steel D is composed of large amount of granular bainite and small amount of quasi polygo nal ferrite as shown in Fig 8 Under the same chemical composition the strength of steel plate composed of large amount of granular bainite and small amount of quasi polygonal ferrite is higher than that composed of polygonal ferrite and pearli Fig 8 Microstructure of steel plate D 图 8 钢板 D 的显微组织 te especially in tensile strength The steel plate D is also composed of hard and soft microstructure with the soft microstructure being quasi polygonal ferrite and bainitic ferrite and the hard microstruc ture being M A islands Martenite retained Aus tenite These small islands are not only in large a mount but also distribute diffusely in ferrite matrix as shown in Fig 9 And thereby based on Eq 2 it can be concluded that the yield strength ratio of the steel D is low Fig 9 M A islands in the granular bainite 图 9 粒状贝氏体中的 M A 岛 3 2 Effect of ferrite grain size on the yield strength ratio It can be known by the well known Hall Patch formula that the grain size affects the strength of steel remarkably It can be found by the comparison between the steels A1 and A7 Fig 10a that al though their chemical composition finishrolling temperature and finish cooling temperature are iden tical but the ferrite grain of the steel A1 9 13 m is finer than that of the steel A7 15 11 m and so the yield strength ratio of the steel A1 is 0 787 and that of steel A7 is 0 735 Similarly the ferrite grain of the steel A2 9 08 m is finer than that of the steel A8 15 28 m so the yield strength ratio of the steel A1 is 0 787 and that of steel A7 is 0 735 In addition it is found by the comparison be tween the ferrite grain size and the yield strength ratio of the steels B1 and B7 the steels B2 and B8 that the finer the ferrite grain the higher the yield strength ratio as shown in Fig 10b It is because the refinement of ferrite grain increases the strength of the ferrite and so to reduce the difference of strength between ferrite soft microstructure and 125 第 6 期于庆波 等 钢中的碳含量和显微组织对屈强比的影响 Fig 10 Effect of ferrite grain size on yield strength ratio 图 10 铁素体晶粒大小对屈强比的影响 pearlite bainite hard microstructure According ly when the ferrite grains yield the pearlite bain ite also begins to yield and pearlite bainite no longer serves for hard microstructure to receive much stress coming from the ferrite grains Consequent ly the yieldstrength ofsteel plate increases which results in the raise of the yield strength ratio In addition it can be known by the regression e quations 1 and 2 that with the refinement of ferrite grain the increase of yield strength is higher than that of tensile strength and so the increase of the yield strength ratio would happen 4 Conclusions 1 When the strength difference is relatively large between the hard microstructure and soft mi crostructure for the steel composed of multi micro structure the yield strength ratio of steel is relative ly low In turn when the strength difference is rela tively small the yield strength ratio is relatively high 2 The research results indicate that the grain size canremarkably affect the yield strength of steel that is the finer the ferrite grain the high er the yield strength ratio of steel 3 Grains of M A island in large amount distribu ted in diffusion can increase the tensile strength of steel remarkablyand so to decrease the yield strength ratio 参考文献 1 SUN Bangming YANG Caifu ZHANG Yongquan De velopment of the Steel for High Rise Building J Width Thick Plate 2001 7 3 1 2 Gong Shihong Sheng Guanmin Effect of V Ti microal loying on anti seismic properties of buildin