Aspects of microstructure in low carbon steels produced by the CSP process

The solidification structure, microstructure evolution during rolling andprecipitates with nanometers in dimension of the low carbon steels produced by CSP process with thinslabs have been studied in recent years. Important differences in microstructure and mechanicalproperties between the CSP products and the conventional one were observed. These differences mayarise from the much rapider solidification rate and cooling rate after casting of the thin slabs.Some aspects of the microstructure for the low carbon steels of the CSP thin slabs are summarizedand compared with the conventional one.

Grain refinement of low carbon steel produced by CSP process

In comparison with conventional production for hot strips, compact strip production (CSP) brings about some new micro-structural phenomena. Investigations were carried out to clarify the grain refinement mechanism of low carbon steel strips produced by the EAF-CSP process. Samples, obtained from the same rolling stock during continuous rolling, were examined through SEM, TEM and XEDS. Thin slabs have a dominant columnar structure and the spacing of the secondary dendrite arms ranges from 90 to -125 μm. The average grain sizes for the central area of the samples from the 1st to 6th pass are 41.6, 25.2, 21.4, 20.2, 13.1, 6.7 μm, respectively. Large number of nanometer oxide and sulfide have been found in the low carbon steel produced by the CSP process. The grain refinement mechanism can be summarized as follows: finer solidification structure of the thin slab; austenite recrystalliza-tion at higher temperature and stain accumulation at lower temperature caused by the great reduction of single rolling pass during continuous rolling; nano-scaled precipitates of sulfide and oxide which drag grain boundaries of austenite or ferrite to prevent the grain coarsening.

Oxide and Sulfide Dispersive Precipitation and Effects on Microstructure and Properties of Low Carbon Steels

Electron microscopic investigation on low carbon steel strips produced by the CSP process has been carried out. Large number of oxide dispersive precipitates have been observed in the ferrite matrix of the steel strips. Dimension of them is about 10~20 nm. Electron diffraction study showed that the structure of these precipitates consists with cubic system spinel structure. Their lattice parameter is about 0.83 nm. The results implied that they should be complex oxides of Fe, Al et al. Small sulfide particles with 100-300 nm in size have also been observed. Remarkable strengthening and grain refinement effects can be obtained by the precipitations. The oxygen and sulfur in steels could play beneficial role under certain conditions.

Solidification Structure of Low Carbon Steel Strips with Different Phosphorus Contents Produced by Strip Casting

In the present paper, low carbon steel strips with different phosphorus contents were produced using a twin roll strip casting process. The solidification structure was studied and its features were analyzed in detail. It was found that the strips possessed a fine microstructure compared with the mould cast steels. With increasing phosphorus content more ferrite has been formed with finer grains.

Influence of Equal Channel Angular Pressing on Tribological Properties of Low Carbon Steel (Fe-0,09C-0,64Si-,26Mn)

This paper presents the results of equal channel angular pressing (ECAP) and subsequent heat treatment (HT) as a method to improve the wear resistance of metallic materials in friction sliding. The effect of ECAP and HT on the microstructure and mechanical properties of low carbon steel is investigated in this work. The mechanisms of wear resistance of steel with ultrafine and nanostructures produced by equal-channel angular pressing is analyzed. The results show that ECAP at room temperature and annealing at 350&degC and 450&degC can be used as a technology of reducing wear in friction sliding.

Layer Structure Analysis of Low-Carbon Steel Containing Rare Earth by High-Temperature Carburizing of Liquid Cast-Iron

The layer structure of low-carbon steel containing RE by high-temperature (T>1200 ℃) carburizing of liquid cast-iron was studied and the diffusion activation energy of carbon was calculated by metallographic microscpe, chemical analysis etc. The result shows that the technology of carburizing in liquid cast-iron can expedite caburization distinctly and changes the carburizing layer structure. The carburizing rate is 60～80 times of that of the traditional technology, and there is about 43% decrease in the activation energy compared with gas-carburization. In outer structure layer, cementite is formed simultaneously both on the crystal boundary reticularly and inside the crystal grains stripedly. In inner carburizing layer, there is undissolved blocky ferrite in reticular cementite. Besides, rare earth element can expedite carburization process.

Flat friction stir spot welding of low carbon steel by double side adjustable tools

2 mm low carbon steel plates were successfully welded by the flat friction stir spot welding(FSSW) using double side adjustable tools, by which the keyhole formed in the conventional FSSW was eliminated and a flat surface on both the top and bottom sides of the welded joints was obtained. In addition, the hook shape usually generated in the conventional FSSW was eliminated by this technique, and the unbonded interface was parallel to the surface of the sheets. Owing to the enlarged bonded interface width by eliminating the keyhole and the intermixed interface by the adjustable probe, the plug fracture occurred under all the welding conditions in the present study. Due to the suppression of the thickness thinning and elimination of the hook shape, the joint performance was improved in the plug fracture mode. The shear tensile performance was considered to strongly depend on the microstructure in the tip area of the unbonded interface and the maximum shear fracture load of 23.0 kN was achieved in this study.

Banded structure control of low carbon microalloyed steel based on oxide metallurgy

Banded structure is a common harmful microstructure for low carbon microalloyed steel,which seriously shortens the service life of processed parts.In order to study the effect of oxide metallurgy on improving banded structure,the Ti-Zr deoxidized low carbon microalloyed steel that can play the oxide metallurgical role of inclusion was chosen as the research object,and the inclusion characteristics,microstructure and transverse and longitudinal mechanical properties after hot rolling were analyzed.The results showed the inclusion number density increased in all experimental steels after hot rolling,and a large number of long strip inclusions with aspect ratio greater than 3 appeared along the rolling direction.In addition,after hot rolling,there were element segregation bands in the experimental steels,and granular bainite bands were formed in the element enrichment zone.However,the intragranular ferrite generated in the cooling process destroyed the continuity of granular bainite bands,so that the microstructure anisotropy indexes of experimental steels were small.The mechanical properties analysis showed that the anisotropy of performance was mainly reflected in plasticity and toughness in the experimental steels.Among them,the difference ratio of elongation,section shrinkage and impact energy of No.2 steel was 1.69%,3.87% and 1.69%,respectively,which were less than those of No.1 steel and No.3 steel.The anisotropy of microstructure and mechanical properties of No.2 steel that full played the role of oxide metallurgy were improved,and the banded structure control of low carbon microalloyed steel can be realized by oxide metallurgy technology.

CSP低碳钢的晶粒细化与强韧化

生产统计表明:成分与Ｑ１９５接近的ＣＳＰ工艺低碳钢ＺＪ３３０其屈服强度成倍提高,可达到３１０－４１０ＭＰａ,同时延伸率达到３０%－４５%．本文对ＣＳＰ工艺生产的低碳钢板以及同一块钢坯经不同阶段轧制冷却后的试样进行了研究,应用ＴＥＭ＋ＸＥＤＳ技术在薄晶体和萃取复型试样中观察到沿晶界和亚晶界析出的许多细小氧化物和硫化物粒子．研究表明:纳米级氧化物与硫化物沉淀粒子可阻碍晶粒长大,而微量杂质元素在晶界偏聚可以通过阻碍晶界迁移和降低γ－α转变温度而起到细化晶粒的作用,由沉淀和偏聚导致的有效晶粒尺寸减小是这类钢板强度与塑性同时大幅提高的重要原因之一．

CSP生产低碳钢的组织演变和析出物研究

为了阐明EAF-LF-CSP工艺生产的低碳钢组织细化机理,在薄板坯和不同道次变形后的同一轧件上取样,利用金相、SEM、TEM、XEDS等技术研究了连轧过程中显微组织演变和钢中第二相析出物.结果表明:与普通连铸板坯相比薄板坯的凝固组织更加细小;随轧制道次增加,薄板坯表面和心部的组织差异逐渐减小,轧后室温组织细化;CSP生产的低碳钢中存在大量纳米尺寸的氧化物和硫化物,起到细化晶粒的作用.CSP生产中采用快速冷却和凝固工艺、单道次大压下连轧工艺和层流冷却工艺,是成品组织细化的主要原因.

CSP工艺热轧低碳带钢位错形貌及密度分析

采用H-800透射电子显微镜及正电子湮没技术,观察了CSP工艺热轧低碳带钢轧制过程中位错形貌,并计算了终轧后轧件的位错密度。结果表明:随着轧制过程的进行,累积变形量的增加,位错密度逐渐提高。CSP工艺比传统工艺生产的同规格产品位错密度高约一个数量级。

Q345含Nb低碳钢CSP轧制时动态再结晶的研究

用Gleeble -1 50 0热模拟试验机模拟研究了包钢生产的含NbQ3 4 5C低碳钢( % :0 0 56C ,1 2 0Mn ,0 . 0 52Nb ,0 . 0 3 3V ,0 .0 1 2Ti,0 0 .0 52N) 52mm连铸坯在CSP(紧凑式带材生产)轧制时的高温变形过程中真应力应变曲线,回归得出动态再结晶数学模型和动态再结晶的临界条件:Z =9.70×1 0 11exp( 2 5εc)和Z =1 .77×1 0 12 exp( 1 5.8εs) ,式中Z- 变形因子,εc -开始发生动态再结晶的临界变形量,εs -发生完全再结晶的临界变形量。利用动态再结晶判定图,避开轧制道次间的部分再结晶区进行轧制,可有效地降低钢板中的混晶组织。

CSP制无取向硅钢热轧、常化、冷轧织构的演变

运用EBSD和X射线衍射技术,研究了模拟CSP流程生产无取向硅钢在热轧-常化-冷轧过程中织构的演变。热轧板沿板厚方向应力场和温度场的差异导致由表至中织构锋锐度增高,织构类型存在明显变化,表层主织构为B类纤维织构,次表层为A类纤维织构,中心层为旋转立方织构{001}<110>。常化过程削弱了这种差异性,但中心层仍保留了一定强度的立方织构{001}<100>。冷轧板表层及中心层的主织构均为{112}<110>、{111}<112>,表层织构锋锐度较中心层的强。分析表明热轧、常化、冷轧织构的演变与基体初始织构、组织密切相关。表层、次表层热轧板织构经常化演化成散漫、分布较均匀的织构,中心层主要织构由{001}<-1-10>绕RD逆时针旋转45°演化至{001}<0-10>。冷轧后织构演变为以{223}<110>为主的B类织构和以{111}<112>为主的A类织构。

基于CSP工艺下稀土元素对低碳钢冷轧板再结晶的影响

在实验室进行了冷轧板再结晶退火实验,结合再结晶实验结果及再结晶形核和长大动力学模型,研究了稀土元素对低碳钢冷轧薄板再结晶及晶粒长大的影响。结果表明,低碳钢冷轧薄板中加入稀土元素提高了再结晶温度,阻碍了再结晶形核及再结晶晶粒长大。低碳钢冷轧薄板中加入稀土元素增加了晶粒长大激活能。稀土元素提高了低碳钢冷轧薄板的再结晶形核激活能约3.87 kJ/mol,提高再结晶晶粒长大激活能约11.7%。

CSP冷轧低碳钢板再结晶晶粒长大阶段微区织构的研究

利用电子背散射衍射(EBSD)技术研究了CSP冷轧低碳钢板再结晶晶粒长大阶段组织和微区织构的变化。结果表明具有{111}取向晶粒的数量和尺寸在再结晶刚完成时都具有很大的优势,并一直保持到退火完成。这是再结晶织构中γ取向线密度很高的一个重要原因。但是{111}晶粒的生长在晶粒长大后期停滞,造成了γ取向线密度的下降。晶粒长大动力学的差异对应着取向差的变化,{111}晶粒生长的停滞可以用{111}晶粒同周围晶粒取向差的减小来解释。

CSP-CDCM-BA流程钢板组织性能演变规律

通过低碳铝镇静钢CSP-CDCM-BA流程的工业试验,借助光学显微镜、X射线衍射仪和专用拉伸试验机等设备,测定了热轧、冷轧、退火和平整后钢板的组织、织构、力学性能和基本成形性能,并对其特征、演变规律及影响因素进行了分析,验证了退火平整板r值的变化与{111}/{100}的线性关系。结果表明,增加{111}和减少{100}织构组分对提高薄钢板成形性能同样重要。

CSP-低碳钢DC03冷轧板的组织和织构

研究了CSP工艺生产的0.04%C低碳钢DC032.9 mm热轧基板、0.9 mm的冷轧板和退火平整板的组织与织构。结果表明,热轧基板组织为铁素体+少量沿晶界分布的珠光体,铁素体基体中有少量40～100 nm MnS粒子,没有观察到AlN;冷轧板组织为含大量位错的带状铁索体-珠光体,{111}织构的择优取向较明显;退火平整板组织为铁素体和游离渗碳体,并观察到10～30 nm的析出相,{111}织构轴密度达5.15,{100}轴密度减为0.61。

CSP冷轧低碳钢板的再结晶织构

采用基于薄板坯连铸连轧(CSP)工艺条件下的低碳钢板作为冷轧基料,在实验室模拟现场工艺进行了冷轧和罩式退火,利用X射线衍射和电子背散射衍射(EBSD)分析了退火过程中的织构和微区取向的变化,并对CSP条件冷轧板再结晶织构的形成机制进行了讨论。结果表明:γ取向线在再结晶发生后增加比较明显,但在晶粒长大阶段却略有降低。形变亚晶在再结晶过程中发生合并长大,这些具有大角度晶界的亚晶将是再结晶形核的基础。以较小的晶内平均取向差和较大的晶粒间取向差为判据,利用EBSD技术选取了最有可能成为再结晶晶核的亚晶,这些亚晶存在着以{111}<112>取向为主的择优取向。再结晶晶粒的生长速度在随后的整个退火过程中存在较大差异,{111}再结晶新晶粒的生长速度在晶粒长大阶段受到抑制,可能是其最终成品γ取向线取向分布密度下降的原因。再结晶初期晶核的择优取向与其生长速度的差异共同作用决定了再结晶的最终织构。

CSP热轧低碳钢板轧制过程中析出行为分析

采用H-800透射电镜研究CSP热轧低碳钢轧制过程中析出物形貌、尺寸、及分布等,结果表明:累积变形量较小,变形温度较高时,第二相粒子主要在晶界或相界上形成,数量较少且比较粗大,尺寸多在150nm以上;随着轧制过程的进行,累积变形量的增大和轧制温度的降低,第二相粒子主要在晶内析出,细小、弥散且数量较多,尺寸大多数在20～100nm之间,析出物主要为Al2O3、MnS、Cu7S4和Fe3C。