Nb含量对低碳微合金钢热处理组织与性能的影响

设计了3种含铌量不同的微合金试验钢,采用热处理模拟方法研究了900 ℃奥氏体化550 ℃等温处理后试验钢的组织、性能及第二相粒子的析出行为.结果表明,低铌钢热处理后组织为多边形铁素体加少量板条贝氏体铁素体加极少量珠光体,随着铌含量增加,试验钢组织中板条贝氏体铁素体含量明显增加,且组织细化;同时随着铌含量的增加强度提高,中高铌试验钢的屈服强度可达400 MPa,且屈强比较小(＜0.8).3种试验钢中均有铌的碳氮化物粒子析出,但析出量较少.     

稀土对10SiMnNb钢显微组织的影响

冶炼低硫钢10SiMnNb 和10SiMnNbRE 钢。测定了10SiMnNbRE 钢中固溶0.035wt%RE。研究了稀土元素对先共析铁素体析出,珠光体转变,贝氏体转变和马氏体转变的影响。结果表明,固溶稀土增加了过冷奥氏体的稳定性,在相同的冷却速度下,添加稀土元素和无稀土元素比较,显微组织显然不同,先共析铁素体量减少,珠光体量增加,粒状贝氏体量增加,并细化了马氏体组织。     

钒-氮微合金化超低碳贝氏体钢的连续冷却转变特性

研究N含量不同的2种超低碳Fe-Mn-Mo贝氏体钢的连续冷却转变特性.采用热膨胀法测试试验钢不同冷速下的连续冷却转变曲线,用40MAT型激光显微镜观察相应的显微组织转变特征,并进行硬度及强度测试.试验结果表明,Fe-Mn-Mo系超低碳贝氏体钢中添加V和N对贝氏体相变具有显著的影响.贝氏体相区明显扩大.其中高氮钢的贝氏体转变区更宽,晶粒大小为相同冷速下低氮钢的1/3左右,维氏硬度值高出低氮钢40HV,强度高出100 MPa.V-N微合金化超低碳贝氏体钢中析出的V(C,N)粒子促进了贝氏体转变、扩大了贝氏体相区.     

合金元素对TRIP钢连续冷却固态相变和硬度的影响

为了研究Al和P合金元素在TRIP钢固态相变过程中的重要作用,利用热膨胀实验、金相观察、显微硬度测量等方法绘制了4种不同Al和P含量的C-Mn-Al-P TRIP钢的CCT图.结果表明,Al元素强烈缩小奥氏体相区,提高Ac3与Ms,促使CCT图左移和上移.P元素能够阻碍碳化物的生成,当钢中w(P)达到0.14%时,能显著将CCT曲线图中的珠光体区与贝氏体区右移.P元素对先共析铁素体相变和马氏体相变没有显著影响.随着冷却速率的增加,材料的显微硬度增加.对于每一种成分超过其临界冷却速率时将得到完全的马氏体组织.添加固溶强化元素可以强化铁素体基体,增加铁素体基体的硬度,P元素固溶强化能力最强,Mn元素稍弱,Al元素很弱.     

热变形参数对贝氏体非调质冷镦钢组织性能的影响

通过实验测试贝氏体非调质冷镦钢的相变点,采用Gleeble3500热模拟实验机研究变形程度和冷却速度对非调质贝氏体冷镦钢组织转变和力学性能的影响。结果表明,增加冷却速度,强度变高。增加变形程度,有利于低冷速条件下的铁素体转变,而在冷却速度一定的情况下,增加变形量,硬度与强度也有所降低。实验表明,在40%变形量,10℃/s冷却速度条件下,强度相对较高。     

核电SA508-3钢在不同冷速下的显微组织

针对由某公司生产首次应用到核电上的SA508-3钢,采用膨胀法在Gleeble1500D热模拟机上测定了其焊接连续冷却转变的膨胀曲线,结合显微组织和硬度,获得了每个冷速下对应的相变点温度.结果表明,在0.015～0.05℃/s冷速范围内.为高温转变的铁素体和珠光体区,0.1～7℃/s冷速范围内,为中温转变的贝氏体区,20～80℃/s的冷速范围内,为低温转变的马氏体区;随着冷却速度的增大,硬度值也越来越大,0.015℃/s对应的硬度值为199 HV,80℃/s对应的硬度值为546 HV,这为制定合理的焊接工艺提供了依据.     

无钼热轧双相钢相变和组织的研究

研究了Ｓｉ－Ｍｎ－Ｃｒ型热轧双相钢的相变动力学，发现在ＣＣＴ曲线上存在温度间隔约１００℃的奥氏体亚稳区。随着冷却速率的降低，连续冷却转变后的组织中多边表铁素体量增多，贝氏体量减少，当冷却速度减小到一定程度时组织中出现针状马氏体。     

冷却速度对热轧双相钢相变的影响及双相化的可行性

讨论了随着冷却速度的降低，Ｓｉ－Ｍｎ－Ｃｒ－Ｍｏ系热轧双相钢连续冷却转变的组织中多边形铁素体量增多，贝氏体量减少，且贝氏体由羽毛状向粒状转化，当冷速减小到１．２４－０．２℃／ｓ的范围时，组织中出现针状马氏体。此外，该钢的ＣＣＴ曲线上出现了温度间隔约１８０℃的奥氏体亚稳区，为实现双相化热轧工艺提供了理论依据。     

35K钢过冷奥氏体连续冷却转变曲线研究

利用膨胀法结合金相-硬度法，在Gleeble-1500热模拟机上测定了35K钢的临界点Ar1，Ar3以及Ms；测定了不同冷却速度下连续冷却转变的膨胀曲线，获得了该钢的连续冷却转变曲线(CCT曲线)；研究了35K钢的连续冷却过程中奥氏体转变过程及转变产物的组织和性能；此外，通过对CCT曲线的分析，确定了避免铁素体呈现魏氏组织形貌和产生贝氏体组织的临界冷却速度，为生产实践和新工艺的制定提供了参考依据。     

20MnVBH钢CCT曲线的测定

用膨胀分析法测定了20MnVBH钢在1050℃和860℃连续冷却下的CCT曲线。并对转变后的试样进行了组织分析和硬度测定。结果发现:与马氏体区紧密相邻的宽阔的中间转变区域内存在粒状贝氏体组织;提高奥氏体化温度和加快冷却速度都有利于粒状贝氏体的形成。     

Effect of zirconium addition on the austenite grain coarsening behavior and mechanical properties of 900 MPa low carbon bainite steel

The ultra-free bainitic microstructure of a 900 MPa low carbon bainitic Cu-Ni-Mo-B steel was obtained by a newly developed relaxation precipitation control (RPC) phase transformation processing.In a pan-cake like prior-anstenite grain,the microstructure consisted of lath bainite,a little of abnormal granular bainite,and acicular ferrite.The effect of zirconium carbonitrides on the austenite grain coarsening behavior was studied by transmission electron microscopy (TEM).The results show that,the lath is narrower with increasing cooling rate.The ratio of all kinds of bainitic microstructure is proper with the intermediate cooling rate;and Zr-containing precipitates distribute uniformly,which restrains austenite grain growing in heat-affected welding zone.     

Modeling of microstructure evolution and mechanical properties during hot-strip rolling of Nb steels

An integrated metallurgical model was developed for Nb steels to predict the microstructure evolution and mechanical properties during the hot-strip rolling and cooling process. On the basis of the industrial data, the transformation kinetics, strength, and elongation rate were evaluated for different chemical compositions and processing parameters. The yield strength and tensile strength increase with increasing Nb content or decreasing finishing temperature. The bainite distributed in finer ferrite matrix, which is produced at relatively low coiling temperatures, can greatly increase the strength of steel, especially tensile strength, thereby decreasing the yield ratio. A reasonable agreement was found between the predicted and measured results. It indicates that the present models can be used to simulate the actual production process.     

High performance low cost steels with ultrafine grained and multi-phased microstructure

Ultrafine grained ferrite was obtained through tempering cold rolled martensite with an average grain size of 200―400 nm in a low carbon and a microalloyed steel. Thermal and mechanical stability of the two steels was studied. Due to the pinning effect of microalloyed precipitates on the movement of dislocations and grain boundaries, the recrystallization and grain growth rate were retarded, and the thermal stability of ultrafine grained microstructure was improved. The ultrafine grained ferritic steel was ...     

空冷高硅中碳低合金钢的组织与性能

研究了高硅中碳低合金钢空冷态和空冷＋回火态的显微组织和力学性能.试验钢在860℃保温0.5 h奥氏体化后空冷处理,随后分别在250℃和400℃保温1 h回火.结果表明：试验钢空冷后组织为贝氏体/马氏体和残余奥氏体的混合组织,硬度约为41 HRC;而250℃回火后组织变化不大,硬度明显升高,约为49 HRC,韧性明显增加,由44 J/cm2增加到66 J/cm2,抗拉强度、屈服强度和延伸率明显下降.回火温度进一步增加对力学性能影响不大.     

Microstructure evolution during reheating of a Nb-bearing steel II. Dislocation-precipitate interaction

Cooled in water after the isothermal relaxation of deformed austenite for different times, a Nb-bearing microalloyed steel always exhibits the synthetic microstructure in which bainitic ferrite dominates. Strain-induced precipitates do not occur in an unrelaxed sample while they distribute outside dislocations in the sample relaxed for long time. Most of the strain induced precipitates distribute along dislocations in the sample relaxed for proper time. After bainitic transformation, the dislocations formed in the deformed austenite remain to be pinned by the precipitates so that the thermostability of the bainitic ferrite is improved. Coarsening of the precipitates accompanied by their distribution density change has caused the first hardness peak of bainite during reheating. The second hardness peak is attributed to the precipitates, which nucleate in bainite. Dislocations inside the laths getting rid of the pinning of precipitates and their polygonization play the precursor to the evolution of microstructures during reheating.     

Microstructure evolution during reheating of a Nb-bearing steelⅡ. Dislocation-precipitate interaction

Cooled in water after the isothermal relaxation of deformed austenite for different times, a Nb-bearing microalloyed steel always exhibits the synthetic microstructure in which bainitic ferrite dominates. Strain-induced precipitates do not occur in an unrelaxed sample while they distribute outside dislocations in the sample relaxed for long time. Most of the strain induced precipitates distribute along dislocations in the sample relaxed for proper time. After bainitic transformation, the dislocations formed in the deformed austenite remain to be pinned by the precipitates so that the thermostability of the bainitic ferrite is improved. Coarsening of the precipitates accompanied by their distribution density change has caused the first hardness peak of bainite during reheating. The second hardness peak is attributed to the precipitates, which nucleate in bainite. Dislocations inside the laths getting rid of the pinning of precipitates and their polygonization play the precursor to the evolution of microstructures during reheating.     

Microstructure evolution during reheating of a Nb-bearing steel Ⅱ. Dislocation-precipitate interaction

Cooled in water after the isothermal relaxation of deformed austenite for different times, a Nb-bearing microalloyed steel always exhibits the synthetic microstructure in which bainitic ferrite dominates. Strain-induced precipitates do not occur in an unre-laxed sample while they distribute outside dislocations in the sample relaxed for long time. Most of the strain induced precipitates distribute along dislocations in the sample relaxed for proper time. After bainitic transformation, the dislocations formed in the deformed austenite remain to be pinned by the precipitates so that the thermostability of the bainitic ferrite is improved. Coarsening of the precipitates accompanied by their distribution density change has caused the first hardness peak of bainite during reheating. The second hardness peak is attributed to the precipitates, which nucleate in bainite. Dislocations inside the laths getting rid of the pinning of precipitates and their polygonization play the precursor to the evolution of microstructures during reheating.     

热穿-热轧法生产的新型贝氏体中空渗碳钢的组织和性能

用热穿-热轧法制备了新型贝氏体中空钢．研究了热处理对新型贝氏体钢和渗碳处理对中空钢组织和性能的影响．结果表明：新型贝氏体钢正火＋低温回火热处理后的组织为贝氏体铁素体和奥氏体,淬火＋低温回火后的组织由马氏体、贝氏体和奥氏体组成;正火或淬火＋低温回火后,新型贝氏体中空钢具有良好的强韧性．正火＋低温回火后,中空钢的组织为贝氏体铁素体和残余奥氏体组织．新型贝氏体中空钢渗碳后空冷,渗层的组织为高碳马氏体和残余奥氏体组织,非渗层为贝氏体铁素体和残余奥氏体组织,实体中空钢具有较好的强韧性和渗碳效果．     

Effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region

The effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region was investigated using a Gleeble 1500 thermomechanical simulator and a continuous annealing thermomechanical simulator. The results showed that the microstructure consisted of ferrite and bainite as the main phases with a small amount of retained austenite and martensite islands at cooling rate of 5 and 50 ℃/s, respectively. Fast cooling after continuous annealing affected all constituents of the microstructure. The mechanical properties were improved considerably. Ultimate tensile strength(UTS) increased and total elongation(TEL) decreased with increasing cooling rate in all specimens. The specimen 1 at a cooling rate of 5 ℃/s exhibited the maximum TEL and UTS×TEL(20% and 27 200 MPa%, respectively) because of the competition between weakening by presence of the retained austenite plus the carbon indigence by carbide precipitation, and strengthening by martensitic islands and precipitation. The maximum UTS and YS(1 450 and 951 MPa, respectively) were obtained for specimen 2 at a cooling rate of 50 ℃/s. This is attributed to the effect of dispersion strengthening of fi ner martensite islands and the effect of precipitation strengthening of carbide precipitates.