Revealing Precipitate Development During Hot Rolling and Cooling of a Ti–Nb Micro-Alloyed High Strength Low-Alloy Steel through X-Ray Scattering

High-energy synchrotron X-ray small-angle scattering (SAXS) is used to study the precipitate development during hot rolling and cooling of a commercial Ti–Nb micro-alloyed, high-strength, low-alloy (HSLA) steel. To study precipitation during hot rolling conditions, Gleeble and dilatometer trials are made. Samples are then studied at room temperature using SAXS in conjunction with transmission electron microscopy (TEM). TEM is used to determine the morphology and composition of the precipitates, whilst both TEM and SAXS are used to study the particle sizes. One major advantage with high-energy SAXS is the ability to make measurements after a minimum of sample preparation and in transmission geometry, as opposed to just at prepared surfaces, plus the possibility to determine volume fractions of the precipitates. The measurements show that after deformation at high temperature, particle coarsening occurs and the volume fraction of precipitates increases after holding for 20 s at 900 °C which confirms strain-induced precipitation at finishing rolling conditions. The measurements show that holding at 600 or 650 °C for one hour gives a larger volume fraction of nanosized particles. Coiling simulations with slow cooling from 600 to 470 °C show coarsening of particles and an increase in the volume fraction of the smaller particles compared to holding at a constant temperature.     

两次淬火对HSLA钢组织和冲击韧性的影响

研究了两次淬火+回火和传统的一次淬火+回火热处理对HSAL钢的显微组织和力学性能的影响。结果表明,在不显著降低强度的条件下,两次淬火使实验钢的冲击功明显提高,还改善了低温韧性和稳定性。两次淬火回火热处理可细化钢的组织,使原始奥氏体晶粒的尺寸和有效晶粒尺寸减小、大角度界面的密度和解离裂纹的扩展偏折频率提高。组织的细化和大角度晶界的增多抑制了裂纹的扩展,使韧性大幅度提高。     

Effect of Cooling Rate and Deformation on Microstructures and Critical Phase-Transformation Temperature of Boron-Nickel Added HSLA H-Beams

 Microstructures and critical phase-transformation temperature of boron-nickel added Nb-treated high strength low alloy (HSLA) H-beams cooled at different cooling rate, with different deformation were investigated. Continuous cooling transformation (CCT) diagram of this new type of steel was obtained by using Gleeble 1500 thermomechanical simulator. Microstructures and hardness, especially micro-hardness of the experimental steel were investigated by optical microscopy (OM), scanning electron microscope (SEM), Rockwell and Vickers hardness tests. Phase analysis was also studied by X-ray diffraction (XRD). The results indicated that with increase of cooling rate, microstructures of continuous cooled specimens gradually transformed from polygonal ferrite and pearlite, grain boundary ferrite and bainite, bainite and martensite to single martensite. The CCT diagram revealed that slow cooling was needed to avoid austenite-bainite transformation to ensure toughness of this steel. By plastic deformation of 40%, austenite-ferrite transformation temperature increased by 46 ℃, due to deformation induced ferrite transformation during continuous cooling, but Rockwell hardness has little change.     

Effects of Nb on stress corrosion cracking of high-strength low-alloy steel in simulated seawater

In this study, simulated heat-affected zone (HAZ) of Nb-free and Nb-bearing steel were obtained, and SEM, TEM, and slow strain rate tensile (SSRT) tests were performed to investigate the effect of Nb on the stress corrosion cracking (SCC) behavior of high-strength low-alloy (HLSA) steel in simulated seawater with or without hydrogen charging. The addition of Nb significantly refined the grains and uniformed the microstructure of HLSA. Nb hardly affected the SCC susceptibility of BM and HAZ without hydrogen-charging. However, after charging with 10 mA cm⁻², the SCC resistance of Nb-bearing steel, especially the coarse grain HAZ (CGHAZ) improved drastically, and the process of crack initiation and propagation was inhibited owing to the hydrogen trap function of NbC precipitates.     

高Nb高强度低合金耐候钢中Nb、Ti析出相的特征

用光学显微镜、热场发射扫描电子显微镜以及透射电子显微镜对高Nb高强度低合金耐候钢中的Nb、Ti析出相特征进行了观察分析。结果表明,试验钢中的Nb、Ti析出相为面心立方结构,大部分为Nb、Ti相一起析出,且尺寸较大,主要分布于晶界。单相有两种,一种为立方TiN,尺寸较大,另一种为富Nb或者富Ti的Nb,Ti碳氮化物,大部分尺寸超过50 nm;复合析出为以富Ti析出(Ti, Nb)C为基形成的“核心”和以富Nb析出(Nb, Ti)C为边缘形成的“帽子”,平均尺寸为150 nm。     

Simultaneously Improving Mechanical Properties and Stress Corrosion Cracking Resistance of High-Strength Low-Alloy Steel via Finish Rolling within Non-recrystallization Temperature

The effect of hot rolling process on microstructure evolution,mechanical properties and stress corrosion cracking (SCC) resistance of high-strength low-alloy (HSLA) steels was investigated by varying the finish rolling temperature (FRT) and total rolling reduction.The results revealed granular bainite with large equiaxed grains was obtained by a total rolling reduction of 60％ with the FRT of 950 ℃ (within recrystallization temperature Tr).The larger grain size and much less grain boundaries should account for the relatively lower strength and SCC resistance.A larger rolling reduction of 80％ under the same FRT resulted in the formation of massive martensite-austenite (M/A) constituents and resultant low ductility and SCC resistance.In contrast,a good combination of strength,ductility and SCC resistance was obtained via 80％ rolling reduction with the FRT of 860 ℃ (within non-recrystallization temperature Tnr),probably because of the fine grain size and M/A constituents,as well as a high density of grain boundary network.     

Ca微合金化钢焊接接头组织性能的研究

针对Ca微合金化HSLA钢,采用斜Y型坡口焊接冷裂纹试验评价了焊接冷裂纹倾向,研究了其不同t_(8/5)条件下模拟CGHAZ和气体保护焊接头的组织结构和力学性能.结果表明:该钢的焊接冷裂纹敏感性较低:Ca形成的氧化物(CaO)有效地钉扎焊接CGHAZ奥氏体晶界,细化焊接CGHAZ晶粒,有利于IFA形成,改善其焊接CGHAZ韧性;Ca微合金化HSLA钢具有优良的焊接力学性能.     

硼对低合金高强钢大热输入焊缝韧性的影响

对不同硼(B)的质量分数的低合金高强钢大热输入焊缝进行组织性能研究,分析、讨论B对低合金高强度(High strength low alloy, HSLA)钢焊缝韧性的影响规律。研究表明,低合金高强钢焊缝中适量增加B元素质量分数可提高组织中针状铁素体质量分数,有效细化焊缝组织,而当B元素过量时则使焊缝组织中的针状铁素体质量分数下降,组织粗化。同时,适量B元素还可抑制晶界先共析铁素体的产生;随着焊缝中B质量分数的增大,贝氏体转变得到促进,M-A组元总量提高,从不含B时的2.4%提高到B质量分数为0.008 8%时的5.7%,且其尺寸也相应增大,由平均尺寸2.14 μm增加至2.83 μm。由于B质量分数增加先促进针状铁素体后促进贝氏体形成,因此其对焊缝韧性的影响呈现抛物线变化规律,即低温冲击吸收能量随着B质量分数的增加先上升后下降。在焊缝Ti质量分数约为0.03%的条件下,B质量分数为0.005 2%时,焊缝获得了最佳的低温冲击韧度。     

Zr对高强度钢热影响区第二相粒子及韧性影响

研究了0.012 4%锆对低合金高强度钢焊接热影响区粗晶区第二相粒子和冲击韧性的影响.结果表明,模拟20 kJ/cm焊接线能量下无锆钢焊接热影响区粗晶区中第二相粒子为Al-Ti复合氧化物和（Ti,Nb） N析出物.而含锆钢则是Zr-Al-Ti复合氧化物及（Al,Ti,Nb） N和（Ti,Nb） N析出物.同时,定量数据分析表明含锆钢中氧化物和氮化物粒子密度更高且尺寸更加细小.这些高密度的细小的第二相粒子在焊接过程中能有效钉扎晶界移动,抑制奥氏体晶粒粗化,在焊接热影响区粗晶区中得到尺寸相对细小均匀的原奥氏体晶粒,使得含锆钢焊接热影响区粗晶区呈现韧性断裂和极好的低温冲击韧性.     

低合金高强钢焊缝强韧性研究

通过焊丝向焊缝中加入不同合金元素，研究这些合金元素对焊缝强度、冲击韧性尤其是低温冲击韧性的影响。结果表明．尽管所加合金元素不同，焊缝金属冲击功与温度关系曲线上平台能均具有较高水平，但焊缝金属低温冲击韧性受合金化程度的影响较大。采用Ti、B及M等元素多元合全化的焊缝金相组织中针状铁素体比例高，具有较高的低温冲击韧性和强度。