Effects of overaging temperature on the microstructure and properties of 600 MPa 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 (BH2) values. The effects of overaging temperature on the microstructure, mechanical properties, and BH2 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 BH2 value initially increases and then decreases. The maximum BH2 value of 83 MPa was observed for the specimen overaged at 350°C.     

Effect of temper rolling on the bake-hardening behavior of low carbon steel

In a typical process, low carbon steel was annealed at two different temperatures (660℃ and 750℃), and then was temper rolled to improve the mechanical properties. Pre-straining and baking treatments were subsequently carried out to measure the bake-hardening (BH) values. The influences of annealing temperature and temper rolling on the BH behavior of the steel were investigated. The results indicated that the microstructure evolution during temper rolling was related to carbon atoms and dislocations. After an apparent increase, the BH value of the steel significantly decreased when the temper rolling reduction was increased from 0% to 5%. This was attributed to the increase in solute carbon concentration and dislocation density. The maximum BH values of the steel annealed at 660℃ and 750℃ were 80 MPa and 89 MPa at the reductions of 3% and 4%, respectively. Moreover, increasing the annealing temperature from 660 to 750℃ resulted in an obvious increase in the BH value due to carbide dissolution.     

Effects of microrolling parameters on the microstructure and deformation behavior of pure copper

Microrolling experiments and uniaxial tensile tests of pure copper under different annealing conditions were carried out in this paper. The effects of grain size and reduction on non-uniform deformation, edge cracking, and microstructure were studied. The experimental results showed that the side deformation became more non-uniform, resulting in substantial edge bulge, and the uneven spread increased with increasing grain size and reduction level. When the reduction level reached 80% and the grain size was 65 μm, slight edge cracks occurred. When the grain size was 200 μm, the edge cracks became wider and deeper. No edge cracks occurred when the grain size was 200 μm and the reduction level was less than 60%; edge cracks occurred when the reduction level was increased to 80%. As the reduction level increased, the grains were gradually elongated and appeared as a sheet-like structure along the rolling direction; a fine lamellar structure was obtained when the grain size was 20 μm and the reduction level was less than 60%.     

C-Si-Mn系热轧双相钢显微组织与力学性能之间的关系

通过对热轧试样的组织性能测试,研究了热轧双相钢显微组织与力学性能之间的关系.研究发现,铁素体和马氏体强度是影响力学性能的主要因素.建立了C-Si-Mn系热轧双相钢力学性能的数学模型,可以作为热轧双相钢基本力学性能参数预测的理论依据.     

C—Si—Mn系热轧双相钢显微组织与力学性能之间的关系

通过对热轧试样的组织性能测试,研究了热轧双相钢显微组织与力学性能之间的关系。研究发现,铁素体和马氏体强度是影响力学性能的主要因素。建立了C-Si-Mn系热轧双相钢力学性能的数学模型,可以作为热轧双相钢基本力学性能参数预测的理论依据。     

Influence of rapid heating process on the microstructure and tensile properties of high-strength ferrite-martensite dual-phase steel

Three low-carbon dual-phase (DP) steels with almost constant martensite contents of 20vol%were produced by intercritical an-nealing at different heating rates and soaking temperatures. Microstructures prepared at low temperature (1043 K, FH1) with fast-heating (300 K/s) show banded ferrite/martensite structure, whereas those soaked at high temperature (1103 K, FH2) with fast heating reveal blocky martensite uniformly distributed in the fine-grained ferrite matrix. Their mechanical properties were tested under tensile conditions and compared to a slow-heated (5 K/s) reference material (SH0). The tensile tests indicate that for a given martensite volume fraction, the yield strength and total elongation values are noticeably affected by the refinement of ferrite grains and the martensite morphology. Metallographic observations reveal the formation of microvoids at the ferrite/martensite interface in the SH0 and FH2 samples, whereas microvoids nucleate via the fracture of banded martensite particles in the FH1 specimen. In addition, analyses of the work-hardening behaviors of the DP micro-structures using the differential Crussard–Jaoul technique demonstrate two stages of work hardening for all samples.     

Corrosion behavior of tempered dual-phase steel embedded in concrete

Dual-phase (DP) steels with different martensite contents were obtained by appropriate heat treatment of an SAE1010 structural carbon steel, which was cheap and widely used in the construction industry. The corrosion behavior of DP steels in concrete was investigated under various tempering conditions. Intercritical annealing heat treatment was applied to the reinforcing steel to obtain DP steels with different contents of martensite. These DP steels were tempered at 200, 300, and 400℃ for 45 min and then cooled to room temperature. Corrosion experiments were conducted in two stages. In the first stage, the corrosion potential of DP steels embedded in concrete was measured every day for a period of 30 d based on the ASTM C 876 standard. In the second stage, the anodic and cathodic polarization values of these steels were obtained and subsequently the corrosion currents were determined with the aid of cathodic polarization curves. It was observed that the amount of second phase had a definite effect on the corrosion behavior of the DP steel embedded in concrete. As a result of this study, it is found that the corrosion rate of the DP steel increases with an increase in the amount of martensite.     

预应变对不同晶粒度合金钢缺口试样断裂行为的影响

对两种不同晶粒度的低合金高强钢(WCF-62)的试样在常温下获得均匀预应变后(0～20%),在-125 ℃下对预应变和未预应变的试样进行了四点弯曲试验,测量了宏观和微观力学参数,以及断口参数.并结合有限元计算所得的不同预应变条件下的缺口前端的应力应变场分布,分析了预应变对不同晶粒度低合金高强钢缺口试样断裂行为的影响.结果表明:粗晶和细晶材料在室温预应变3%后,缺口韧性有明显的降低,但是随预应变进一步增加,缺口韧性基本保持不变.原因是预应变3%后两种材料解理断裂都转变为起裂控制.     

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℃ for 90 s and then either rapidly cooled to 450℃ and held for 30 s (process A) or rapidly cooled to 350℃ and then reheated to 450℃ (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, A₈₀=24.3%, n=0.17) was obtained via process A.     

工艺参数对双相不锈钢瞬间液相连接组织结构和等温凝固的影响

采用3种非晶态镍合金作中间层,对双相不锈钢进行瞬间液相连接试验研究。试验结果表明:采用MBF-30作中间层在1 060℃和300 s进行连接时,等温凝固完成,连接中间区以γ-Ni相为主,BN出现在基体与中间层的界面处。但是,MBF-35作中间层在1 060℃和300 s进行连接时,等温凝固未能完成,连接中心部含有Ni3Si;采用MBF-50作中间层在1175℃和300 s进行连接时,BN和(Cr,Ni)3Si相出现在连接中间区,等温凝固仍未能完成(等温凝固完成时间大于14400 s),而采用溶质分布模型计算结果仅为71 s。微观结构及等温凝固的差异归因于在较高温度下存在Si,N和Cr等合金元素并在连接中心处富集。     

Effects of chromium on the microstructure and hot ductility of Nb-microalloyed steel

It is well-known that the surface quality of the niobium microalloy profiled billet directly affects the comprehensive mechanical properties of the H-beam. The effects of chromium on the γ/α phase transformation and high-temperature mechanical properties of Nb-microalloyed steel were studied by Gleeble tensile and high-temperature in-situ observation experiments. Results indicated that the starting temperature of the γ→α phase transformation decreases with increasing Cr content. The hot ductility of Nb-microalloyed steel is improved by adding 0.12 wt% Cr. Chromium atoms inhibit the diffusion of carbon atoms, which reduces the thickness of grain boundary ferrite. The number fractions of high-angle grain boundaries increase with increasing chromium content. In particular, the proportion is up to 48.7% when the Cr content is 0.12 wt%. The high-angle grain boundaries hinder the crack propagation and improve the ductility of Nb-microalloyed steel.     

铬对铌微合金钢显微组织和热塑性的影响分析

It is well-known that the surface quality of the niobium microalloy profiled billet directly affects the comprehensive mechanical properties of the H-beam. The effects of chromium on the γ/α phase transformation and high-temperature mechanical properties of Nb-microalloyed steel were studied by Gleeble tensile and high-temperature in-situ observation experiments. Results indicated that the starting temperature of the γ→α phase transformation decreases with increasing Cr content. The hot ductility of Nb-microalloyed steel is improved by adding 0.12wt% Cr. Chromium atoms inhibit the diffusion of carbon atoms, which reduces the thickness of grain boundary ferrite. The number fractions of high-angle grain boundaries increase with increasing chromium content. In particular, the proportion is up to 48.7% when the Cr content is 0.12wt%. The high-angle grain boundaries hinder the crack propagation and improve the ductility of Nb-microalloyed steel.     

Effect of cooling temperature on the microstructure and corrosion behavior of X80 pipeline steel

Dual-phase accelerated cooling(DPAC) was applied to X80 pipeline steel to obtain its microstructure with different amounts of bainite and ferrite. The microstructure, hardness, and polarization behaviors of the steel, cooled to different temperatures, were investigated. Results showed that, with decreasing cooling temperature, the amount of polygon ferrite(PF) increased while that of acicular ferrite(AF) decreased. The amount of bainite correspondingly decreased, except when cooled to 760°C. Moreover, the grain size of ferrite increased. The corrosion behaviors of different phases were distinct. Martensite/austenite(M/A) islands presented at the grain boundary of the PF phase caused small pits. Numerous micro-corrosion cells were formed in the AF and bainite phases, where micropores were prone to form. X80 pipeline steel cooled to 700°C had the best corrosion resistance in the simulated seawater. The decreased amount of the PF phase reduced the area of cathode, resulting in slight corrosion. About 40 vol% of the bainite phase provided strength while the PF phase provided adequate ductility to the X80 steel. It was concluded that the appropriate cooling temperature was 700°C for ideal corrosion resistance and mechanical properties.     

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-1.8Mn) were investigated by different heat treatments for automobile applications. The results show that F-TRIP steel, a traditional TRIP steel containing as-cold-rolled ferrite 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 microstructure, 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, resulting in a lower transformation rate and consequently contributing to a higher elongation compared to F-TRIP steel. Work hardening behavior is also discussed for both types of steel.     

Numerical simulation on the microstress and microstrain of low Si-Mn-Nb dual-phase steel

According to the stress-strain curves of single-phase martensite and single-phase ferrite steels, whose compositions are similar to those of martensite and ferrite in low Si-Mn-Nb dual-phase steel, the stress-strain curve of the low Si-Mn-Nb dual-phase steel was simulated using the finite element method (FEM). The simulated result was compared with the measured one and they fit closely with each other, which proves that the FE model is correct. Based on the FE model, the microstress and microstrain of the dual-phase steel were analyzed. Meanwhile, the effective factors such as the volume fraction of martensite and the yield stress ratio between martensite and ferrite phases on the stress-strain curves of the dual-phase steel were simulated, too. The simulated results indicate that for the low Si-Mn-Nb dual-phase steel, the maximum stress occurs in the martensite region, while the maximum strain occurs in the ferrite one. The effect of the volume fraction of martensite (f_M) and the yield stress ratio on the stress-strain curve of the dual-phase steel is small in the elastic part, while it is obvious in the plastic part. In the plastic part of this curve, the strain decreases with the increase of f_M, while it decreases with the decrease of the yield stress ratio.     

工艺参数对45钢激光表面强化组织与性能的影响

通过改变激光功率和扫描速度等参数,研究其对45钢激光表面强化组织与性能的影响。实验结果表明,单道扫描时,当保持扫描速度v为15mm/s时,增加激光功率P,可增加硬化层的深度,最大深度可达1.5mm以上。另外,P/v比值越大,硬化层深度越大;而当P/v比值保持不变时,硬化层深度随着激光功率的增加而增加,其中激光功率从1.2kW到1.8kW时,硬化层深度值增加较快;当激光功率大于1.8kW后,深度值的增长随功率增加变缓;而且硬化层的硬度都达到700HV以上,远高于基体的硬度。在激光多道搭接扫描时,激光能量的再次输入会导致靠近搭接区的前一道硬化层产生回火软化,其硬度接近基体的硬度。     

回火工艺参数对DP600热轧双相钢组织和性能的影响

采用低温回火实验模拟热轧双相钢实际生产过程中卷取后的冷却过程, 分析回火温度与保温时间对 0.09C-0.1Si-1.3Mn-0.5Cr-0.05P热轧双相钢组织及力学性能的影响.结果表明,回火过程中主要是碳原子通过扩散影响铁素体与马氏体中的微结构,从而对其组织和力学性能产生影响;200 ℃回火后双相钢基本保持原有的组织与性能,230℃以上随回火温度升高和保温时间延长,马氏体分解逐渐明显,铁素体中碳原子钉扎可动位错造成屈服现象的发生;在230~250℃温度范围回火时,保温时间的延长较温升对回火程度的影响更加明显.     

双相纳米结构CuZr合金力学行为的模拟研究

采用分子动力学方法研究了在拉伸载荷作用下,晶粒尺寸对纳米多晶Cu和双相纳米结构CuZr复合材料塑性变形机制的影响.研究结果表明,小晶粒尺寸的纳米多晶Cu的变形机制以晶粒旋转以及晶界迁移为主,并伴随着少量位错的成核与滑移.晶粒尺寸较大的纳米多晶Cu的塑性变形机制则以裂纹的成核与生长为主.对于双相纳米结构Cu/CuZr复合材料,非晶相的塑性变形在复合材料的塑性变形过程中起主导作用,且这种现象与晶粒尺寸无关.此外,当晶粒尺寸增加到一定尺寸时,复合材料的晶界处也出现了晶界裂纹,但非晶相明显延缓和阻碍了裂纹的成核与扩展.研究表明非晶相的引入能有效提升纳米多晶Cu的塑性.     

纤维双相钢的组织形成及对性能的影响

研究了纤维双相15Mn2Nb钢组织的形成及对性能的影响.结果表明,纤维双相钢具有优异的强塑性匹配,是一种天然的复合材料.其纤维组织的获取主要取决于合金化和热处理工艺规程,而合金元素Nb阻碍铁素体再结晶是形成纤维组织的关键.