Microstructures and properties of 550 MPa grade high strength thin-walled H-beam steel

The microstructures and mechanical properties of 550 MPa grade lightweight high strength thin-walled H-beam steel were experimentally studied. The experimental results show that the microstructure of the air-cooled H-beam steel sample is consisted of ferrite, pearlite and a small amount of granular bainites as well as fine and dispersive V（C,N） precipitates. The microstructure of the water-cooled steel sample is consisted of ferrite and bainite as well as a small amount of fine pearlites. The microstructure of the water-cooled sample is finer than that of the air-cooled sample with the average intercept size of the surface grains reaching to 3.5 gna. The finish rolling temperature of the thin-walled high strength H-beam steel is in the range of 750 ~C-850 ~C. The lower the finish rolling temperature and the faster the cooling rate, the finer the ferrite grains, the volume fraction of bainite is increased through water cooling process. Grain refinement strengthening and precipitation strengthening are used as major strengthening means to develop 550 MPa grade lightweight high strength thin- walled H-beam steel. Vanadium partially soluted in the matrix and contributes to the solution strengthening. The 550 MPa grade high-strength thin-walled H-beam steel could be developed by direct air cooling after hot rolling to fully meet the requirements of the target properties.     

磨损中钢的硼化物层及过渡区亚结构变化

用透射电子显微镜对经渗硼的45钢作了观察。结果表明,磨损表层的硼化物中产生孪晶和层错,过渡区的铁素体中产生位错网,珠光体片层间距变小,部分渗碳体碎化。     

Microstructural Features During Strain Induced Ferrite Transformation in 08 and 20Mn Steels

The microstructure evolution during strain induced ferrite transformation was followed in thermal-simulation tests of clean 08 and 20Mn steels. The influences of carbon equivalence and initial austenite grain size on ferrite grain refinement and the volume frac- tion of ferrite during straining were inspected. The results revealed that the accelerating effect of ferrite transformation by strain was increased as the carbon equivalence decreased. However, finer ferrite grains were obtained at higher carbon content. At strain of -1 .5 ferrite grains less than 3 μm and 2 μm can be obtained in 08 and 20Mn steels respectively. Whereas the ferrite grain refinement in 08 steel was due to both effects of strain induced transformation and ferrite dynamic recrystallization, that in 20Mn was mainly due to st fain induced transformation. Heavy strain can produce fine ferrite grains in coarse austenite grained 08 steel, but it would lead to band microstructure in coarse austenite grained 20Mn.     

20g钢珠光体球化后拉伸断裂过程组织变化

采用现场金相显微镜对珠光体球化的20g钢拉伸过程进行了原位观察,用扫描电镜分析变形组织及裂纹。结果表明：珠光体发生球化的20g钢在拉伸过程中,在某些晶粒内部首先发生变形,产生大量的滑移带,并在晶界处产生塞积。随后其它部分晶粒转动,形成对变形有利的取向,各晶粒协调变形。拉伸过程中,裂纹首先产生于试样边缘的珠光体组织。试件内部,微裂纹空洞在铁素体和珠光体的晶界处萌生,由于应力集中,产生扩展连接,呈穿晶和沿晶断裂。     

Effect of deformation and cooling rate on the transformation behavior and microstructure of X70 steels

The effects of the deformation in the non-recrystallization region of austenite and the cooling rate on the transformation behavior and microstructure of low-carbon low-alloy steel for pipeline application were studied on the thermal-mechanical simulator Gleeble-1500. It was shown that an increase in deformation amount can greatly increase the nucleation site of ferrite when deformed in the non-recrystallization region of austenite, and an increase in nucleation ratio can greatly refine grains. When the cooling rate is accelerated, the driving force of nucleation is increased and the nucleation rate also improves. Ultra-refine grains can be obtained by controlled rolling. The high density of ferrite nucleus, which forms along the austenite grain boundary, twin interface, and deforma- tion band are introduced in the matrix of austenite by the control of hot rolling, after which the microstructure can be refined. It was found that the acicular ferrite has a very fine sub-structure, high dislocation density, and a thin slab with ultra-fine grains. Small M/A islands and cementite are precipitated on the matrix of the slabs by the analysis technique of TEM and SEM.     

Ultra-fine ferrite grains obtained in the TSDR process

By careful design of rolling schedule,ultra-fine （～2μm） ferrite grains in a low carbon high niobium （0.09wt%Nb） microalloying steel with average austenite grain sizes above 800 μm can be achieved in the simulated thin slab direct rolling process. The 5-pass deformation was divided into two stages： the refinement of austenite through complete recrystallization and the refinement of ferrite through dynamic strain-induced transformation. The effects of Nb in solution and strain-induced NbCN precipitates on the ferrite transformation were also extensively discussed.     

Mechanical properties of fine-grained dual phase low-carbon steels based on dynamic transformation

The fine grained dual phase (FG-DP) steel with ferrite grains of 2-4.5 μm and martensite islands smaller than 3 μm was obtained through the mechanism of deformation-enhanced ferrite transformation (DEFT). Mechanical properties of the steel were tested at room temperature. The results indicated that with a similar volume fraction of martensite (about 20vol%),FG-DP steel ex-hibited a superior combination of higher strength and more rapid strain hardening at low strains compared with the coarse-grained dual ph...     

Analysis on the deformation and fracture behavior of carbon steel by in situ tensile test

1. IntroductionIn materials science, the emphasis is on the under-lying relationship between the microstructure andproperties of materials. Materials science and engi-neering has since long times aimed to develop the accu-rate relationship between the pro…     

Microstructure evolution and precipitation behavior of low carbon steel hot strips produced by CSP

The microstructures of low carbon steel before, during and after rolling deformation of each stand were observed using optical microscope. The result showed that the microstructures were very fine after six passes rolling deformation. The effect of the first stand reduction on microstructure refinement was very distinct. During the rolling process, with the increase of the accumulated strain, the microstructures would further refine, and the density of dislocation would increase at the same time. In continuous casting thin slabs and each finishing stand, lots of observed precipitates were mainly A12O3 and MnS along the grain boundaries or in grains,which played an important role in the mechanical properties of the hot strips of low carbon steel produced by CSP (compact strip production) technology.     

20CrMnTi钢的位错密度及晶体结构

采用X射线衍射法及新的线性分析理论,准确地测量了不同应变量时20CrMnTi钢的位错密度,分析了应变量与位错密度的关系。结果表明:位错密度随变形量的增加而提高,退火状态时位错密度较低。透射电镜试验证明了20CrMnTi退火后的组织为铁素体和珠光体,晶内位错表态为"曲折"形的位错线,变形后形成胞状结构,同时有孪晶出现。胞状结构的出现大大提高了钢的强度。     

Relaxation of Deformed Austenite and Refinement of Bainite in a Nb-Containing Microalloyed Steel

By means of stress relaxation and strain-induced precipitation in deformed austenite, bainite with micron sheaves size was ob-tained in Nb-containing steel. Microstructures of deformed samples isothermally relaxed for various time followed by cooling in water were examined. Stress relaxation test and transmission electron microscopy (TEM) were employed to detect precipitation of microalloy elements, such as Nb, Ti, during isothermal holding after deformation. All the samples were constituted by lath-like bainite along with acicular ferrite, but the size of bainitic sheaves and the amount of acicular ferrite were changed with relaxation time. To achieve optimum refinement, relaxation should be confined in the stage when the precipitates have sufficiently grown and started to coarsen. The sample having not undergone relaxation does not exhibit obvious refinement despite of its higher dislocation density. These results indicate that relaxation promotes bainite to refine, which is because deformed austenitic grains are divided by dislocation walls formed during rela-xation and acicular ferrite formed before bainitic transformation.     

热加工对管线钢针状铁素体组织形成的影响

以X70管线钢为实验材料,研究不同变形量和冷却速率对管线钢显微组织的影响。结果表明,在奥氏体未再结晶区进行适量的变形,从而形成位错、形变带和胞状组织等缺陷,可以增加铁素体在奥氏体晶内的形核位置和形核率,增大相变驱动力,有利于在随后的冷却过程中得到晶粒细小的针状铁素体组织;其中变形量2ε=0.4、冷却速率为30~60℃/s（油冷）下冷却的试样,能够得到最佳的针状铁素体组织,可以满足工程上要求组织中针状铁素体占80%以上的要求。     

65Mn钢大塑性变形后的组织与力学性能

在650℃下对65Mn钢进行了C方式的等径弯曲通道变形(Equal Channel AngularPressing,简称ECAP)研究。重复挤压时试样沿轴线旋转180°再装入模具。通过光学及透射显微镜研究发现:ECAP变形后65Mn钢的累积等效真应变达到5左右,片层状的珠光体组织演变成了超细的渗碳体颗粒均匀分布于亚微晶铁素体基体组织中;变形5道次后铁素体基体为均匀的等轴晶,平均晶粒尺寸约为0.3μm。65Mn钢经ECAP变形后,硬度明显提高。     

Comparison of Ferrite Refinement Mechanisms by Different Processing Schedules in 08 and 20Mn Steels

The influence of deforming temperature on ferrite refinement was analyzed by comparing the microstructures obtained by de-formation at above Ar3, in two-phase region of(α+γ) and at below A1 in clean 08 and 20Mn steels. The results indicate that ferrite refinement through strain induced transformation by deformation at above Ar3 is more effective than that by deformation simply through ferrite dynamic recrystallization. The main problem of ferrite refinement by deformation at below Ar3 is the inhomogeneity of microstructure which is controlled by the orientations and sizes of ferrite grains and the distribution of second phases. Ferrite dynamic recrystallization after strain induced transformation can further effectively refine ferrite.     

Controlled rolling and cooling process for low carbon cold forging steel

Effect of controlled rolling and cooling process on the mechanical properties of low carbon cold forging steel was investigated for different processing parameters of a laboratory hot rolling mill. The results show that the specimens with fast cooling after hot rolling exhibit very good mechanical properties, and the improvement of the mechanical properties can be attributed mainly to the ferrite-grain refinement. The mechanical properties increase with decreasing final cooling temperature within the range from 670 °C to 570 °C due to the finer interlamellar spacing of pearlite colony. The specimen with fast cooling after low temperature rolling shows the highest values of the mechanical properties. The effect of the ferrite grain size on the mechanical properties was greater than that of pearlite morphology in the present study. The mechanical properties of specimens by controlled rolling and cooling process without thermal treatment were greatly superior to that of the same specimens by the conventional rolling, and their tensile strength reached 490 MPa grade even in the case of low temperature rolling without controlled rolling. It might be expected to realize the substitution medium-carbon by low-carbon for 490 MPa grade cold forging steel with controlled rolling and cooling process.     

600MPa级钛微合金化高强钢的组织与性能研究

采用光学显微镜、透射电镜以及能谱分析等对转炉CSP流程600 MPa级钛微合金化高强钢的组织及性能进行研究。结果表明,试验钢具有良好的综合力学性能,其典型组织为多边形铁素体加粒状贝氏体;位错和位错胞的强化作用成为钛微合金钢的主要强化机制之一;钢中M/A岛在增加试验钢强度的同时并未明显降低其韧性和塑性;试验钢中存在TiN、TiC和TiS等析出物,为钢的细晶强化和析出强化提供了保证;试验钢中存在大量纳米级铁碳析出物,其沉淀强化作用不容忽视。     

高扩孔型铁素体/贝氏体双相钢组织性能研究

对540MPa级铁素体/贝氏体双相钢的组织性能进行分析研究。结果表明,试验钢的显微组织为85%左右的铁素体加上15%左右的贝氏体;铁素体晶粒尺寸细小,基体中有较高的位错密度和大量细小弥散的析出物;贝氏体在铁素体基体上分布均匀,以板条状为主,板条间分布有较多碳化物颗粒。通过合适的成分设计和控轧控冷手段获得的铁素体/贝氏体双相钢组织形态,可较好地保证材料所需的强度-拉伸凸缘性能匹配。     

铸态锰钢中的碳化物

对铸态锰钢中的碳化物作了研究，铸态锰钢中有两类碳化物：一类是凝固时生成的弥散分布在基体上的团球状碳化物，另一类是在塑性变形时，在应变马氏体形成以前，优先 析出的呈弥散分布的细小颗粒，这两类碳化物一起构成了铸态锰钢组织中的第二相硬质点，这些硬质点和细化的奥氏体晶粒的界面都是位错运动的障碍，造成了位错密度的提高，进而提高了铸态锰钢的加工硬化能力和抗磨性能。     

薄板坯连铸连轧工艺生产的低碳钢冷轧基板力学特性

以采用薄板坯连铸连轧工艺生产的低碳钢作为冷轧基板,在实验室二辊可逆轧机上进行不同程度的冷变形,对不同变形后的冷轧板进行室温拉伸试验和显微硬度值测定,分析了其加工硬化情况,并对不同冷变形后试验钢的显微组织进行分析。结果表明:低碳钢经冷变形后,屈服强度和抗拉强度增大,加工硬化显著;试验钢的显微组织为铁素体和少量的珠光体,晶粒随着变形量的增大而拉长细化。     

高速车轮钢断裂韧性与组织结构的关系

以CLSOD车轮钢为研究对象,对断裂韧性水平不同的试样断口进行显微组织分析,研究组织中晶粒尺寸、铁素体含量、珠光体片层间距对材料断裂韧性的影响。结果表明,车轮钢断裂韧性随着平均晶粒尺寸、5％最大平均晶粒尺寸的增大而降低,晶粒越均匀,断裂韧性越高;断裂韧性起初随铁素体含量的增加而增大,达到一个峰值后随着铁素体含量的增加而减小;在珠光体片层间距为0．13-0．16μm的范围内,断裂韧性与珠光体片层间距的相关性不大。