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.

Interaction of Hydrogen and Retained Austenite in Bainite/Martensite Dual-Phase High Strength Steel

The hydrogen trapping phenomena in two bainite/martensite dual-phase high strength steels(U20Si and U20DSi)were investigated by electrochemical permeation technique.The hydrogen diffusivity was calculated from data of permeation delay time,and the diffusion coefficient in U20 Si is far less than that in U20 DSi.Moreover,the hydrogen diffusivity decreases as the volume percent of retained austenite increases.The experiment results show that there are different hydrogen trappings and different volume percents of retained austenite in U20 Si and U20 DSi.The retained austenite is precipitated as films.The trap binding energy for the retained austenite and hydrogen is calculated to be 40.4kJ·mol-1.

Improving fatigue strength of bainite/martensite dual-phase steels in very high cycle fatigue regime by refining microstructures

Very high cycle fatigue behaviors of two bainite/martensite dual-phase steels were investigated.One of the steels was cyclic rapid heat treated and its microstructures were refined. Fatigue strength of the steel is 225 MPa higher than that without refining.Observation of fracture surfaces show that the fatigue cracks initiate at bainites for non-refined steel and at non-metallic inclusions for the refined steel.The size of inclusions is much smaller than that of bainites which results in the improvement of fatigue strength.

Microstructure design and feasibility of hot stamping for new generation high strength multi-phase steels

Optimization of microstructure for new generation advanced high strength steels(AHSS ) for automobiles was briefly reviewed.Two different heat treatments(quenching partitioning austempering/QPA and quenching partitioning tempering/QPT) have been investigated to obtain optimal microstructures,which are made up of martensite(hard phase),retained austenite(soft phase),and carbide or nano-bainite.Combination of hot stamping and newly developed heat treatments is discussed.

Influence of original microstructure on the transformation behavior and mechanical properties of ultra-high-strength TRIP-aided steel

The transformation behavior and tensile properties of an ultra-high-strength transformation-induced plasticity （TRIP） steel （0.2C-2.0Si-I.SMn） were investigated by different heat treatments for automobile applications. The results show that F-TRIP steel, a tradi- tional TRIP steel containing as-cold-rolled ferfite and pearlite as the original microstructure, consists of equiaxed grains of intercritical ferrite surrounded by discrete particles of M/RA and B. In contrast, M-TRIP steel, a modified TRiP-aided steel with martensite as the original mi- crostlucture, containing full martensite as the original microstructure is comprised of lath-shaped grains of ferrite separated by lath-shaped martensite/retained austenite and bainite. Most of the austenite in F-TRIP steel is granular, while the austenite in M-TRIP steel is lath-shaped. The volume fraction of the retained austenite as well as its carbon content is lower in F-TRIP steel than in M-TRIP steel, and austenite grains in M-TRIP steel are much finer than those in F-TRIP steel. Therefore, M-TRIP steel was concluded to have a higher austenite stability, re- sulting in a lower transformation rate and consequently contributing to a higher elongation compared to F-TRIP steel. Work hardening be- havior is also discussed for both types of steel.

Research and development of hot-rolled ultra-high strength steel at Baosteel

The effects of the composition and cooling process on the microstructures and properties of hot-rolled ultra-high strength low alloy （HSLA） steel, complex phase steel and martensite steel were studied in the laboratory. And S700MC and MP1200 ultra-high strength steels were trial produced at the 1 880 mm hot-rolling line of Baosteel. Compared with conventional hot-rolled high strength products,the idea that water is alloy was applied in the newly developed hot-rolled ultra-high strength steel. By the use of the economical composition design and controlled cooling after hot-rolling effectively,ultra-high strength steel of different steel grades can be obtained.

In-situ Observation of Dark Phase Precipitation during Heating and Soaking Process of a High Nickel Steel

In-situ observation of microstructural evolution during heating and soaking process was carded out for a high nickel steel using HTCLSM. Dark phases were observed when soaking at 900℃. Results showed that the number of the dark phases culminated in about 50 s during soaking at 900℃. With the increase of soaking time the area proportion of the dark phases increased and reached the maximum value in about 3 min, When temperature rose from 900 ℃, the dark phases remained steady initially, but started to dissolve into the matrix at about 1 060 ℃ and completely disappeared at 1 132℃. When the specimen soaked at 900 ℃ was cooled down to room temperature （RT）, the dark phases kept stable. Energy spectrum analysis results showed that the dark phases contained much more Cr and Mn elements than the matrix and,were also rich in V. Tensile test results showed that the dark phase strengthened the steel with the maximum tensile strength obtained after soaking at 900 ℃ for 3 minutes.

Research of DP600 high strength steel wheels

Steel has been used as a traditional wheel materials for a long time.Before 1980' s,over 90% wheels were made of steels.Then,the yield strength of wheel steels increased gradually from 240 -350 MPa to 600 MPa or higher.The advanced high strength steels become the main wheel materials.Since 1990's,DP600 has been broadly applied in steel wheels oversea.However,there was little research of application of high strength steel wheels(especially wheel disc) in domestic wheel industrial.In presence, DP600 steel wheel is only developed in FAWSH by using imported CKD.Other domestic steel wheel manufacturers did little work on high strength DP600 wheel discs.Baosteel wheels Co.Ltd.is the first domestic company producing DP600 steel discs and has become the leading company in production of high strength wheel.Because of the high strength of DP600,its formability,springback,and wrinkle have become the keys of the successful application of DP600 steel.In this paper,the metallurgical and mechanical properties are firstly introduced and formation problems of DP600 are described compared to traditional steels.Secondly,FLD analysis has been done.Wrinkle and more springback are recognized.To solve these problems,an optimized solution is designed and the good result is obtained.The test has shown that the fatigue property of steel wheel discs made by DP600 have been greatly improved.

Effect of Microstructure on Corrosion Fatigue Behavior of 1500 MPa Level Carbide-Free Bainite/Martensite Dual-Phase High Strength Steel

Influence of microstructure of the experimental steels on the corrosion fatigue behavior in 3.5% of NaCl aqueous solution was studied.Experimental results show that compared with the full martensite（FM）steel,the carbide-free bainite/martensite（CFB/M）steel has higher corrosion fatigue strength and corrosion fatigue crack threshold（ΔKthcf）,and lower corrosion crack propagation rate [（da/dN）cf].

基于应变的应变硬化指数演化行为研究

为准确描述变形过程中双相高强钢板的应力应变关系,针对3种双相高强钢板应变硬化指数的演化行为进行了试验研究与理论模型预测。借助单向拉伸试验机和自主研发的DIC应变测量系统,获得了3种材料的真实应力-真实应变曲线,基于经典的Hollomon流动应力模型,得到了其应变硬化指数的演化历程,发现3种材料应变硬化指数与真实应变呈非线性变化关系。为了更好地预测应变硬化指数的演化行为,提出了可以描述其值变化的非线性数学模型,并结合经典的Hollomon流动应力模型,提出了基于n值变化的修正的Hollomon-n流动应力模型,最后通过3种材料的真实应力-真实应变曲线验证了所提出模型的有效性。

一种高强耐热轴承钢的微观组织及力学性能

利用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)、X射线衍射仪(XRD)、拉伸试验和洛氏硬度计等研究了一种新型成分优化的超高强度轴承钢经不同工艺热处理后的微观组织及力学性能。结果表明,在1040~1080℃范围内,随着固溶温度的升高,试验钢的抗拉强度逐渐升高,屈服强度逐渐降低。试验钢经1060℃固溶1 h后油淬及两次(-73℃冷处理2 h+540℃回火2 h)处理后,抗拉强度达到1997 MPa,屈服强度达到1600 MPa,断后伸长率达到12%,断面收缩率为55%,具有优异的强韧性能;随固溶温度的升高,试验钢的原始奥氏体晶粒尺寸逐渐增大,从19μm增加到50.9μm,未溶碳化物含量明显减少,残留奥氏体含量逐渐增多,其中,残留奥氏体含量增多是导致屈服强度逐渐降低的主要原因;试验钢中大量存在M_(6)C和Laves两种不同种类的析出相,其中M_(6)C碳化物在固溶阶段起到细化晶粒作用,Laves相在回火过程弥散析出,是试验钢获得超高强度的主要强化机制。

先进高强度双相钢板料成形极限研究

随着先进高强度双相钢的工业需求量日益增加,亟需实现双相钢板料成形极限曲线的精确预测,由于根据国标采用的胀形试验较难准确获取FLC_(0)点使FLC不准确。本文基于DP590、DP780、DP980、DP1180在常温下的胀形试验结果,结合MMC断裂模型的数值模拟,建立了4种板料的胀形断裂预测模型,通过模拟准确求出FLC_(0),结合试验数据,建立了基于MMC断裂准则的成形极限曲线(FLC),并进行参数化研究。结果表明,基于MMC准则的成形极限曲线拥有较好的准确性。

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。

Formability Investigations of High-Strength Dual-Phase Steels

Car manufacturing is always regarded as the key industry behind sheet metal forming, and thus, the requirements of and developments in car manufacturing play a decisive role in the development of sheet metal forming. The automotive industry is faced with contradictory demands and requirements： better performance with lower consumption and less harmful emissions, more safety and comfort; these are extremely difficult to supply simultaneously with conventional materials and conventional manufacturing processes. The fulfillment of these often contradictory requirements is one of the main driving forces in the automotive industry and thus in the material and process developments in sheet metal forming, as well. In recent years, significant developments can be observed in the application of high-strength steels. In this respect, the application of various dual-phase steels is one of the best examples. However, the application of these highstrength steels often leads to formability and manufacturing problems. One formability problem is the springback occurring after sheet metal forming. In the current research, we have dealt mainly with advanced high-strength steels, primarily with dual-phase steels. When applying them, the springback phenomenon is one of the most critical issues. To reduce the tremendous amount of experimental work needed, we also applied numerical simulation using isotropic–kinematic hardening rules. The isotropic–kinematic hardening behavior of a given material in the applied Auto Form numerical package may be characterized with three independent material parameters c, v and K（a detailed explanation of their meaning will be given in the main part of this paper）. However, we found that the material data included in simulation packages for these new high-strength steels are not fully adequate. For the determination of more reliable material parameters and to achieve better simulation results, a new testing device was developed. Numerical simulations were performed using the material parameters determined by the new device to show the sensitivity of springback behavior to these material parameters.

感应回火对含钒900 MPa级高强钢组织与性能的影响

对含钒Q960E高强钢进行热轧和900℃淬火,并对其分别在450、550和600℃下进行感应回火处理。通过扫描电子显微镜(SEM)与投射电子显微镜(TEM)研究了Q960E高强钢不同感应回火温度下的组织特征。结果表明:淬火后Q960E钢板组织由马氏体+贝氏体复相组织组成,并通过提高终轧和卷曲温度保证了感应回火前的板形质量。高温回火后组织为回火索氏体。通过V的添加,在感应回火过程中均匀弥散析出尺寸小于100 nm的析出相,保证了感应高温回火后的板材强度,并随回火温度的升高,部分回火马氏体消失,形成回火索氏体组织。在900℃/15 min淬火,(550~600℃)/15 min高频感应回火可以获得Q960E的最佳力学性能,通卷横向强度波动小于10 MPa,通卷纵向强度波动范围小于30 MPa,通卷横、纵向伸长率稳定在12%以上。

690 MPa低合金高强钢焊接技术及接头组织研究进展

屈服强度为690 MPa的低合金调质高强钢具有广阔的工程应用前景。该类调质高强钢焊接生产时易出现冷裂纹、热影响区软化或脆化,以及焊缝金属局部脆化问题。厘清高强钢组织演变原理,探索开发高效、高质焊接技术,是保证690 MPa调质高强钢在大型工程结构中安全应用的重要环节。本文调研了近年来围绕该类高强钢开展的焊接工艺优化和焊接新方法探索,总结了690 MPa调质高强钢焊接热影响区和焊缝金属组织演变的最新研究进展。

氮含量与终轧温度对钛微合金化高强钢CGLC700低温冲击韧性的影响

针对钛微合金化高强钢CGLC700低温冲击韧性差的问题,通过热力学计算与高温原位观察,采用电子背散射衍射、透射电镜、扫描电镜和光学显微镜对含Ti高强钢的夹杂物、第二相粒子、断口形貌和低温冲击韧性等进行了研究。结果表明,含Ti高强钢低温冲击韧性差的原因与钢中大尺寸脆性夹杂物和Ti(C,N)、TiN析出相有关。将钢中w[N]从0.0049%降低至≤0.0035%时,可以有效降低钢中脆性夹杂物的数量和尺寸,从而提高钢材冲击韧性;终轧温度从885~895℃降低至875~885℃,可以促使纳米级TiC第二相粒子析出和大角度晶界的生成,并降低有效晶粒尺寸,从而明显改善钢材的低温冲击韧性;同时降低氮含量至≤0.0035%与终轧温度在875~885℃时,钛微合金化高强钢中平均晶粒尺寸从3.1μm降至2.7μm,小尺寸有效晶粒占比高,大尺寸夹杂物及数密度降低,大角度晶界中占比增长了16.6%,钢材低温冲击功可以从14.75 J提高到37.35 J。

超级奥氏体不锈钢热轧工艺研究

利用热力学模拟软件计算了904L、254SMO、654SMO超级奥氏体不锈钢和316L常规奥氏体不锈钢相图以及析出相的析出行为,并通过数学回归方法研究了其高温强度。研究表明:超级奥氏体不锈钢凝固模式为全奥氏体凝固,固相线温度普遍偏低,较316L低80~110℃,σ相析出温度较316L高,高温强度较316L高,热加工难度大。

不同成分780 MPa级高强双相钢成形性能对比分析

针对不同成分高强双相钢HC420/780 DPD+Z开展了性能对比分析。首先,在常规C-Si-Mn化学成分体系的基础上,通过增加微合金元素或改变原有化学成分含量形成了3种同强度级别的材料,对3种材料的金相组织和力学性能进行了对比分析;基于液压成形试验机对3种材料的成形极限曲线进行了对比,并与3种理论预测模型进行了对比;对3种材料的扩孔性能和折弯性能进行了对比;基于AutoForm搭建了某车型纵梁零件成形过程仿真模型,分析了3种材料的应变分布;通过折弯性能验证材料是否满足纵梁的成形需求。结果表明,对于增加微合金元素的材料,其组织得到明显细化,马氏体呈现出弥散细小的状态,材料的拉延能力增强;对于高碳含量的材料,其C和Cr元素的含量提高,Si元素含量降低,有明显的带状马氏体组织,晶粒尺寸增大,材料的塑性应变比r值增大;根据成形极限曲线可知,微合金成分材料的平面主应变最大,约为0.175,扩孔率最高,达到26.2%,90°折弯极限圆角半径优于其他两种材料,应用纵梁零件的开裂风险最小;高碳含量材料的平面主应变最小,约为0.133;常规成分材料的扩孔率最低,仅为13.0%,折弯性能也最差;微合金元素材料具有良好的拉延和弯曲综合成形性能,而高碳含量可以明显提升材料的折弯和扩孔性能。

先进高强度双相钢弯扭复合回弹补偿控制研究

针对先进高强度双相钢弯扭复合回弹的预测与补偿问题,在弯扭复合回弹试验的基础上建立了有限元模型并进行了仿真模拟。首先,利用DP600材料在弯扭复合回弹试验中回弹量较大的特点,进行了单向拉伸试验和加载-卸载-反向加载的拉压试验以测定DP600的力学特性。其次,分别采用Ls-dyna提供的4种弹塑性材料模型建立了单向拉伸和拉压试验有限元模型,通过Ls-opt迭代求解筛选出合适的材料模型及参数,结果表明,采用MAT125材料模型预测的数值更接近弯扭复合回弹试验数据。最后,通过反向截面补偿法推导出底部支撑距离移动公式,通过反向位移补偿法和偏置曲面得到模具面,对弯扭复合回弹结果分别进行补偿。发现利用反向位移补偿法,并通过偏置曲面的方式,能够对在上压杆偏转角度增加时诱发的弯扭复合回弹进行有效补偿控制。