Influence of silicon on the microstructures, mechanical properties and stretch-flangeability of dual phase steels

Uniaxial tension tests and hole-expansion tests were carried out to determine the influence of silicon on the microstructures, mechanical properties, and stretch-flangeability of conventional dual-phase steels. Compared to 0.03wt% silicon, the addition of 1.08wt% silicon induced the formation of finer ferrite grains （6.8μm ） and a higher carbon content of martensite （Cm≈ 0.32wt%）. AS the silicon level increased, the initial strain-hardening rate （n value） and the uniform elongation increased, whereas the yield strength, yield ratio, and stretch-flangeability decreased. The microstructures were observed after hole-expansion tests. The results showed that low carbon content martensite （Cm ≈ 0.19wt%） can easily deform in coordination with ferrite. The relationship between the mechanical properties and stretch-flangeability indicated that the steel with large post-uniform elongation has good stretch-flangeability due to a closer plastic incom- patibility of the ferrite and martensite phases, which can effectively delay the production and decohesion of microvoids.

Microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels

The microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels were studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and tensile test. The results show that Si can promote the transformation of austenite （γ） to ferrite （α）, enlarge the （α＋γ） region, and increase the aging stability of martensite by inhibiting carbide precipitation. Adding Cr leads to the formation of retained austenite and martensite/austenite （M/A） constituents, as well as the decomposi- tion of martensite during the overaging stage. Both of the steels show higher initial strain-hardening rates and two-stage strain-hardening characteristics. The C-Mn-Si-Nb steel shows the higher strain-hardening rate than the C-Mn-Cr-Nb steel in the first stage; however, there is no significant difference in the second stage. Although the tensile strength and elongation of the two steels both exceed 1000 MPa and 15%, respectively, the comprehensive mechanical properties of the C-Mn-Si-Nb steel are superior.

Effect of hot-dip galvanizing processes on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel

A C–Mn dual-phase steel was soaked at 800°C for 90 s and then either rapidly cooled to 450°C and held for 30 s(process A) or rapidly cooled to 350°C and then reheated to 450°C(process B) to simulate the hot-dip galvanizing process. The influence of the hot-dip galvanizing process on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel(DP600) was investigated using optical microscopy, scanning electron microscopy(SEM), transmission electron microscopy(TEM), and tensile tests. The results showed that, in the case of process A, the microstructure of DP600 was composed of ferrite, martensite, and a small amount of bainite. The granular bainite was formed in the hot-dip galvanizing stage, and martensite islands were formed in the final cooling stage after hot-dip galvanizing. By contrast, in the case of process B, the microstructure of the DP600 was composed of ferrite, martensite, bainite, and cementite. In addition, compared with the yield strength(YS) of the DP600 annealed by process A, that for the DP600 annealed by process B increased by approximately 50 MPa because of the tempering of the martensite formed during rapid cooling. The work-hardening coefficient(n value) of the DP600 steel annealed by process B clearly decreased because the increase of the YS affected the computation result for the n value. However, the ultimate tensile strength(UTS) and elongation(A80) of the DP600 annealed by process B exhibited less variation compared with those of the DP600 annealed by process A. Therefore, DP600 with excellent comprehensive mechanical properties(YS = 362 MPa, UTS = 638 MPa, A_(80) = 24.3%, n = 0.17) was obtained via process A.

Microstructure evolution and mechanical properties of 1000 MPa cold rolled dual-phase steel

The microstructure evolution of 1 000 MPa cold rolled dual-phase (DP) steel at the initial heating stages of the continuous annealing process was analyzed. The effects of different overaging temperatures on the microstructures and mechanical properties of 1 000 MPa cold rolled DP steel were investigated using a Gleeble-3500 thermal/mechanical simulator. The experimental results show that ferrite recovery and recrystallization, pearlite dissolution and austenite nucleation and growth take place in the annealing process of ultra-high strength cold rolled DP steel. When being annealed at 800 ℃ for 80 s, the tensile strength and total elongation of DP steel can reach 1 150 MPa and 13%, respectively. The microstructure of DP steel mainly consists of a mixture of ferrite and martensite. The steel exhibits low yield strength and continuous yielding which is commonly attributed to mobile dislocations introduced during cooling process from the intercritical annealing temperature.

Effects of overaging temperature on the microstructure and properties of 600MPa cold-rolled dual-phase steel

C–Mn steels prepared by annealing at 800°C for 120 s and overaging at 250–400°C were subjected to pre-straining（2%） and baking treatments（170°C for 20 min） to measure their bake-hardening（BH_2） values. The effects of overaging temperature on the microstructure, mechanical properties, and BH_2 behavior of 600 MPa cold-rolled dual-phase（DP） steel were investigated by optical microscopy, scanning electron microscopy, and tensile tests. The results indicated that the martensite morphology exhibited less variation when the DP steel was overaged at 250–350°C. However, when the DP steel was overaged at 400°C, numerous non-martensite and carbide particles formed and yield-point elongation was observed in the tensile curve. When the overaging temperature was increased from 250 to 400°C, the yield strength increased from 272 to 317 MPa, the tensile strength decreased from 643 to 574 MPa, and the elongation increased from 27.8% to 30.6%. Furthermore, with an increase in overaging temperature from 250 to 400°C, the BH_2 value initially increases and then decreases. The maximum BH_2 value of 83 MPa was observed for the specimen overaged at 350°C.

STUDY ON STRUCTURE AND PROPERTIES OF 18Si MnCu DUAL PHASE STEEL

After intercritical treatment,the structure and properties of 18SiMnCu cold rolled sheet steel is studied. When the sheet steel held at 780℃ and followed by 25℃/ s colling rate, a composite structure of (F+M+A) and optimum mechanical properties were obtained:σ_s=300MPa,σ_b = 750MPa,δ=27%, n= 0.27. The transformation process of retained austenite was examined by x-ray and TEM.

Effects of TMCP Parameters on Microstructure and Mechanical Properties of Hot Rolled Economical Dual Phase Steel in CSP

Effects of chemical compositions, finish rolling temperature, isothermal temperature on runout table and coiling temperature on microstructure and mechanical properties of economical dual phase steel produced on CSP line were investigated. Experimental results showed that martensite volume fraction could be enhanced and banding mi- crostructure could be reduced by controlling Mn, Si contents and applying proper finish rolling temperature. Opti- mized processing-parameters were obtained for DP580 production on CSP line of Wuhan Iron and Steel （group） Co （WISCO） in China. Optimal mierostructure and mechanical properties could be achieved when the strip was finished rolling at the range of 790 to 830 ~C, isothermally holding at 680 to 740 ~C and coiling below 250 ~C.

On significance of initial microstructure in governing mechanical behavior and fracture of dual-phase steels

Different initial microstructures were obtained through combination of intercritical annealing and cold-rolling. Subsequently, steels with different microstructures of ferrite-pearlite （FP）, ferrite-martensite （FM） and complete martensite （M） were intercritically annealed at 780 ℃ for 5 min and water quenched to obtain ferrite-martensite microstructure. The significance of initial microstructures on ultimate microstructure, mechanical properties, strain-hardening ability and fracture behavior in dual-phase steels has been elucidated. Initial microstructures of FP, FM and M yielded different martensite morphologies, notably chain-like network structure, fine and fibrous martensite structure, respectively. Furthermore, with increasing martensite content in the initial microstructure, the average grain size of ferrite was significantly refined from about 12.3 to 2.1 μm, which results in that the ultimate tensile strength （UTS） and yield strength were increased, total elongation remained unaffected, and uniform elongation （UE） and strain-hardening ability were increased. A comparison of mechanical properties for different initial microstructures suggested that when the initial microstructure was complete martensite, the steel had excellent mechanical properties, with UTS × UE of 122.5 J cm^-3, which was 24% greater than the conventional continuously annealed steels with ferrite-pearlite initial microstructure （98.8 J cm^-3）. The variation in tensile properties, strain-hardening ability and fracture mechanism of steels with different initial microstructures were discussed in relation to the ultimate microstructures.

微合金化低碳马氏体钢的显微组织及力学性能

制备了钛、铌和钼等元素掺杂的微合金化低碳马氏体钢,并对其进行热轧+880℃淬火+不同温度(170,190,210,230,250℃)回火处理,研究了不同热处理态试验钢的显微组织、拉伸性能和冲击性能,以190℃回火态钢为例分析了强韧化机制。结果表明:热轧态试验钢由铁素体和贝氏体组成,淬火态试验钢由铁素体和马氏体组成,不同温度回火态试验钢均由回火马氏体和少量铁素体组成,回火态试验钢的铁素体面积分数在15%~20%,低于热轧态和淬火态;淬火态试验钢的抗拉强度和布氏硬度最高;随着回火温度升高,回火态试验钢的抗拉强度和布氏硬度下降,断后伸长率、屈服强度和冲击吸收功均先增大后减小;190℃回火态试验钢的断后伸长率和冲击吸收功最高,拉伸断口和冲击断口处纤维区中韧窝占比最高,抗拉强度、布氏硬度和屈服强度适中,综合力学性能最好;190℃回火态试验钢中析出的纳米级小尺寸(Ti,Nb,Mo,Cr)C相起到沉淀强化作用,纳米级大尺寸(Ti,Nb,Mo,Cr)C相起细晶强化作用,大角度晶界占比较高,提高了裂纹扩展功,因此试验钢强度和韧性提高。

1500双相钢的微观组织及力学性能研究

对Fe-C-Mn-Si系双相(dual-phase,DP)钢通过Nb的微合金化以及设计后续连续退火热处理工艺,制备出抗拉强度达1500 MPa级Nb微合金化的Fe-C-Mn-Si-Nb系DP钢。利用扫描电子显微镜、透射电子显微镜、万能拉伸试验机等设备系统研究了不同退火温度以及Nb微合金化对Fe-C-Mn-Si系DP钢微观组织及力学性能的影响。实验结果表明,Nb的微合金化使奥氏体、铁素体和马氏体晶粒细化。在800~850℃退火温度范围内,随着退火温度的升高,试样的屈服强度逐渐增加,抗拉强度先增加后减少,含Nb钢850℃退火,240℃过时效后的抗拉强度和伸长率分别为1537.6 MPa和15.27%。经850℃退火后,含Nb钢的抗拉强度和伸长率相较无Nb钢的分别提升41.1 MPa和2.4%。

不同预处理工艺对P20钢两相区淬火组织和性能的影响

采用扫描电镜、光学显微镜、洛氏硬度计、材料拉伸试验机等研究了不同预处理工艺对P20钢两相区淬火组织和性能的影响。结果表明:由于预处理工艺不同,P20钢两相区淬火的显微组织存在明显差异。A组试样(860℃×1h空冷+860℃×30 min炉冷退火)预处理在两相区淬火的铁素体为块状,碳化物颗粒较多;B组试样(860℃×1 h空冷+860℃×30 min油冷淬火)预处理在两相区淬火的铁素体为长条状,碳化物颗粒较少;B组试样两相区淬火的力学性能优于A组试样,其原因是B组试样两相区淬火时,碳化物得以充分固溶扩散,淬火组织中碳化物颗粒少而小。

高温暴晒和火灾服役条件下高强钢DP800的力学性能变化规律

为了揭示极端高温暴晒服役条件和火灾服役条件下DP800钢的力学性能,采用“加热-保温-空冷”与“加热-保温-水冷”方法分别模拟两种高温服役条件,通过显微组织观测和静载拉伸试验研究高温服役后DP800钢的微观组织形貌特征和力学性能变化规律。结果表明:不同高温空冷条件下(室温~200℃),DP800钢的组织均为铁素体和分布在其晶界上的岛状马氏体,随着服役温度的升高和时间的增加,其力学性能得到明显改善;高温水冷对DP800钢的微观组织和力学性能影响显著,随着服役温度的不断升高,两种保温时间下DP800钢的微观组织均经历了“马氏体铁素体化(300~700℃)→铁素体+马氏体双相组织(700~900℃)”的变化规律,其抗拉强度和屈服强度均呈现出“不断下降(300~700℃)→持续增大(700~850℃)→再次下降(850~900℃)”的变化规律。采用非线性回归法,构建了服役时间不超过60 min时高温水冷条件下DP800钢的抗拉强度、屈服强度随服役温度变化的经验预测模型,最大预测误差仅为7.88%,该模型可为高温服役后DP800钢力学性能变化评价提供理论参考。

模拟过火后汽车结构钢DP780的组织演变及力学性能变化

采用热处理模拟手段,设定加热温度、保温时间和冷却方式,研究汽车结构钢DP780钢经火灾受热后的组织演变和力学性能变化。结果表明:在200~700℃加热条件下,DP780钢的显微组织均为铁素体和马氏体,但温度升高后,铁素体晶粒长大,马氏体发生再结晶;无论空冷还是水冷,随加热温度的升高,DP780钢的屈服强度和抗拉强度先略有提高后快速下降,伸长率略低于原始母材。在800~1000℃加热条件下,空冷和水冷对DP780钢的组织和性能产生明显影响。800℃加热时,空冷和水冷组织均由原始铁素体、再生铁素体和马氏体构成,但后者马氏体体积分数显著增加,强度远高于前者;900~1000℃加热时,奥氏体晶粒长大,形成粗大的贝氏体和马氏体组织,强度高但塑性低。DP780钢的高温(200~700℃)力学性能与室温力学性能相比,当拉伸温度低于400℃时,强度提高,伸长率降低,高于400℃时强度急剧下降,伸长率升高。结果证实,只要DP780钢在200~1000℃受热,其综合力学性能均会发生下降,这将对采用DP780钢制结构件的事故车辆的继续服役产生一定的安全隐患。

双相钢热镀锌前热处理工艺研究

对DP600钢进行热镀锌前的热处理工艺研究,探讨不同再加热工艺条件下DP600钢的组织和力学性能。通过扫描电子显微镜和透射电子显微镜观察,分析快速冷却温度、再加热过程保温时间对组织演变的影响。拉伸试验表明,不同的微观结构组成表现出不同的力学性能和加工硬化行为,随着快速冷却温度从250℃降低到150℃,粒状残余奥氏体晶粒的体积分数增加,抗拉强度从658 MPa降低到643 MPa;快速冷却温度为200℃时,随着保温时间的增加,屈服强度(YS)、拉伸强度(TS)和屈服强度与拉伸强度的比值(YS/TS)均呈下降趋势。此外,在两阶段的加工硬化行为中,保温时间为10 s的样品与30 s和50 s样品相比,在阶段Ⅰ和阶段Ⅱ皆略高,这归因于未转化奥氏体在较短保温时间内稳定性较低,在随后的冷却过程中已转化为马氏体。

Heat Treatment of Cold-Rolled Low-Carbon Si-Mn Dual Phase Steels

The effects of overageing （OA） and vanadium addition on microstructure and properties of cold-rolled lowcarbon Si-Mn dual phase （DP） steel sheets are studied. The results show that the microstructure and mechanical properties of DP steels are greatly affected by OA temperature. When OA is done at low temperatures （473 -- 573 K）, the elongation rate increases sharply while both the yield strength and tensile strength decrease significantly, When OA is done at high temperatures （〉573 K）, the elongation does not further increase, but the yield strength increases and tensile strength decreases, which deteriorates the mechanical properties of the steel. The results also show that both the strength and the tempering stability can be significantly improved by vanadium microalloying. Finally, the optimum OA temperature for the traditional steel appears to be around 523 K, while it is around 573 K for the V-containing steel, resulting in the best overall mechanical properties.

Microstructure and Properties of Mo Microalloyed Cold Rolled DP1000 Steels

Two kinds of ultra-high strength cold rolled dual phase steels have been developed by designing C-Si-Mn-Cr and C-Si-Mn-Cr-Mo alloy systems. Tensile strength and elongation of both steels exceed 1100 MPa and 10%,respectively. The microstructures of both steels consist of massive martensite and ferrite. And the massive martensite of Mo-free steel disperses in the ferrite with volume fraction of 64%. However,the massive martensite of Mo-containing steel is connected or closed by small martensite islands each other,and martensite volume fraction is 69%. As to Mo-free steel,the yield strength,yield ratio,and work hardening exponent n are 548 MPa,0.49,and 0.26,respectively. As for Mo-containing one,the yield strength,yield ratio,and n value are 746 MPa,0.66,and 0.33,respectively. Besides,the ferrite of Mo-free steel is deformed at the initial stage of plastic deformation. However,for Mo-containing one,Mo solution strengthened ferrite and small overaged martensite islands are deformed preferentially at small strain,which causes the yield strength to reach approximately 200 MPa higher than that of Mo-free steel.

Microstructure and properties in dissimilar/similar weld joints between DP780 and DP980 steels processed by fiber laser welding

The microstructure and mechanical properties（strength, fatigue and formability） of dissimilar/similar weld joints between DP780 and DP980 steels were studied. The microstructure in fusion zone（FZ） was lath martensite（LM）, and alloying elements in the FZ were uniformly distributed. The hardness in the FZ of dissimilar weld joint was similar to the average value（375 HV） of the two similar weld joints. The microstructural evolution in heat affected zone（HAZ） of dissimilar/similar weld joints was as follows：LM（coarse-grained HAZ） →finer LM（fine-grained HAZ） →M-A constituent and ferrite（intercritically HAZ） →tempered martensite（TM） and ferrite（sub-critical HAZ）. Lower hardness in intercritically HAZ and sub-critical HAZ（softening zones） was observed compared to base metal（BM） in dissimilar/similar weld joints. The size of softening zone was 0.2-0.3 mm and reduction in hardness was ~7.6%-12.7% of BM in all the weld joints, which did not influence the tensile properties of weld joints such that fracture location was in BM. Formability of dissimilar weld joints was inferior compared to similar weld joints because of the softening zone, non-uniform microstructure and hardness on the two sides of FZ. The effect of microstructure on fatigue life was not influenced due to the presence of welding concavity.

00Cr12Ni双相组织不锈钢焊接接头区域特征与性能

采用熔化极活性气体保护焊(metal active gas arc welding,MAG焊)、等离子弧焊(plasma arc welding,PAW)和高频感应焊接方法获得铁素体+马氏体双相组织不锈钢00Cr12Ni的焊接接头,对其组织区域特征和力学性能进行了研究.典型的焊接接头热影响区(heat affected zone,HAZ)可分为晶粒粗大,铁素体为优势相的高温热影响区(high temperature heat affected zone,HTHAZ)和晶粒细小,马氏体为优势相的低温热影响区(low temperature heat affected zone,LTHAZ).通过测量实际焊接热循环曲线的方法确定了HTHAZ及LTHAZ的温度范围,并采用热模拟研究HAZ不同区域的力学性能.结果表明,HTHAZ的热循环峰值范围为1200℃至熔点,晶粒粗大呈现为脆性;LTHAZ热循环峰值范围为800~1200℃,室温组织为非平衡低碳板条马氏体,韧性较好,但低于0℃时呈脆性.MAG焊接头由于奥氏体焊缝为钟罩形,HAZ冲击试验时断面包括奥氏体焊缝,因此冲击性能较好;PAW接头在1000℃奥氏体快冷可获得细晶粒马氏体韧性提高;高频感应焊接接头中无HTHAZ,但焊接过程中的加工硬化导致室温呈脆性,通过去应力退火后韧性恢复.

焊后热处理对激光焊接DP980钢组织和性能的影响

采用SEM观察、硬度测试、拉伸试验等手段,研究了焊后热处理对DP980钢激光焊接接头组织与性能的影响。结果表明,DP980钢激光焊接接头中各区域马氏体相的比例存在明显差异:相对于母材,熔合区的马氏体比例更高,而热影响区马氏体比例明显下降,形成了明显的软区,降低了接头的力学性能。焊后热处理可以改善接头各区域的组织均匀性,有效消除软区,从而提高接头的硬度和强度,但接头的塑韧性降低。

Effect of Microstructure on Hydrogen Induced Cracking Behavior of a High Deformability Pipeline Steel

The hydrogen induced cracking （HIC） behavior of a high deformability pipeline steel was investigated with three different dual-phase microstructures, ferrite and bainite （F＋B）, ferrite and martensite/austenite islands （F＋M/A） and ferrite and martensite （F＋M）, respectively. The HIC test was conducted in hydrogen sulfide （H2S）-saturated solution. The results showed that the steels with F＋B and F＋M/A dual-phase microstructures had both higher deformability and better HIC resistance, whereas the harder martensite phase in F＋M microstructure was responsible for the worst HIC resistance. The band-like hard phase in dual-phase mi- crostructure was believed to lead to increasing susceptibility to HIC.