Hot Ductility Evolution Mechanism of Titanium-Bearing Microalloyed Steels

Titanium-bearing (Ti-bearing) microalloyed steels have high strength and toughness by grain refinement effect of carbonitride precipitates. However, they can induce surface cracks of continuous casting slab when the Ti alloyed content is high. A microalloyed steel with Ti content (0.10–0.15 wt%) is carried out by thermalmechanical simulator over 600–1350 °C to analyze hot ductility evolution mechanism. Fracture surface morphology, phase transition, and behavior of precipitates of the tensile samples are investigated by experimental detection and thermodynamic calculation. The ductility–temperature curves show that the third brittle temperature range is 600–890 °C, which is mainly attributed to the thin proeutectoid ferrite film and precipitated titanium carbonitride particles, widening the embrittlement temperature ranges through of steel. In addition, the tensile samples at 890–1350 °C have good hot ductility, indicating the dynamic recrystallization of deformed austenite can trigger grain boundaries migration away from cracks and avoid the side effect of the Ti (C,N) particles on hot ductility. The first brittle temperature range of 1350 °C-melting point is mainly ascribed to the partial melting of the grain boundaries with element segregation of sulfur and phosphorus, and microporosity loose among dendrites.     

Influence of Strain Rate on Hot Ductility of a V‐Microalloyed Steel Slab

The hot ductility and malleability of a vanadium‐microalloyed steel is investigated by means of tensile and compression tests at temperatures ranging from 700 to 850°C and strain rates of 3 × 10^?4 to 0.3 s^?1. The deformation tests are performed after austenitization and cooling to test temperature. The so‐called second ductility minimum is located around 750°C for all strain rates except for the highest one, where no ductility trough is observed. Ductility steadily increases with strain rate at a given temperature, and the fracture mode progressively changes from intergranular to transgranular. In the region of minimum ductility, intergranular cracking occurs at low strain rates by void nucleation, growth and coalescence within thin layers of deformation induced ferrite covering the austenite grain boundaries. Cracking is favoured by V(C,N) precipitation associated with the γ/α phase transformation. Ductility remains low above the temperature of minimum ductility, where no apparent ferrite formation is observed (790 °C). Void formation takes place as a result of grain boundary sliding in combination with matrix and grain boundary precipitation. These voids are able to grow and link up forming intergranular cracks. Ductility increases with strain rate mainly due to the short time available for precipitation as well as for intergranular void growth and coalescence.     

Q550d钢板坯热送热装制度的探讨

通过对均匀奥氏体化的该钢在不同装炉温度下进行淬火操作并观察金相组织,研究了该钢在不同装炉温度下组织的演变过程,对其加热制度进行了探讨。实验结果表明,在奥氏体单相区热装的淬火组织为灰色马氏体;在共析线温度下热装的淬火组织为大体积的铁素体晶粒团和极少量的黑色珠光体;冷装时的淬火组织为铁素体和珠光体。共析线温度下低温区热装是Q550d钢热装的理想区间,在工业生产中,应该努力提高低温区的热装温度,热装温度应在Ar1以下并尽可能地靠近Ar1。     

不同实验条件对冷轧双相钢高温热塑性的影响及影响机制

 采用Gleeble 2000高温力学性能模拟实验机对不同冷却速率及不同拉伸速率下600 MPa级Al Mo系冷轧双相钢高温热塑性进行了研究。结果表明,随拉伸应变速率增大,双相钢的高温热塑性明显提高;降低冷却速率,能显著提高双相钢高温区(t＞1 100 ℃)的塑性性能。为了避免铸坯在连铸过程中产生表面裂纹,矫直温度应保证在1 050~1 150 ℃范围内,同时二次冷却应采用弱冷水制度,以降低冷却速率。金相观察发现,沿奥氏体晶界呈网状分布的铁素体薄膜是造成两相区塑性低谷的主要原因,而AlN、FeO等析出相致使奥氏体单相区脆化。     

含铌微合金高强度钢Q345C连铸坯的热塑性

 通过Gleeble-2000 试验机研究了Q345C钢连铸坯的高温热塑性。利用扫描电镜、金相显微镜、透射电镜观察了第Ⅰ、Ⅲ脆性温度区内拉伸试样断口部位的显微组织及形貌,分析了动态再结晶、相变、析出物等对微合金化钢高温延塑性的影响。结果表明：在1×10-3/s应变速率下, Q345C钢存在两个脆性温度区,即第Ⅰ脆性区（1200～1300℃）和第Ⅲ脆性区（600～875℃）,无第Ⅱ脆性区出现;最高塑性出现在1050℃左右,断面收缩率（Z）达到85.8%;在第Ⅲ脆性区,沿奥氏体晶界析出膜状铁素体抗拉能力较低,晶界处存在夹杂物以及微合金元素的析出物,是钢的热塑性降低的主要原因。     

含钒低合金钢铸坯高温延塑性研究

采用Gleeble-1500热模拟试验机测试了含钒低合金钢铸坯的高温延塑性,利用扫描电镜、金相显微镜对断口形貌及金相组织进行分析。低合金钢的第Ⅰ脆性温度区在Ts~1 370℃之间,第Ⅲ脆性温度区在915~710℃之间。第Ⅲ脆性区间由奥氏体低温域晶界滑移楔形裂纹造成的沿晶脆性断裂和奥氏体晶界先共析铁素体薄膜造成的沿晶韧性断裂两部分组成。钢中的V对钢的第Ⅲ脆性凹槽的影响比较大,脆化向低温区域延伸。     

送装工艺对板坯再加热过程奥氏体晶粒细化的影响

连铸坯下线至加热炉的温度制度及其表层组织演变与热送或粗轧裂纹密切相关.基于热模拟实验分析了送装工艺对奥氏体转变特征和再加热晶粒尺寸的影响.高温共聚焦激光扫描显微镜原位观察表明，含Nb J55钢在双相区700℃热装时，组织为晶界膜状先共析铁素体、魏氏体和大量残留奥氏体，再加热至1200℃，奥氏体晶粒大小、位置都不变；单相区600℃温装时，组织为大量铁素体+珠光体，再加热至1200℃时，奥氏体晶粒明显细化.马弗炉模拟SS400钢双相区不同热装温度发现，铁素体转变量至少达70%时才可细化再加热后的奥氏体晶粒.在临界转变量以上，基体中铁素体转变量越多晶粒细化程度越明显.      

Effect of Boron Precipitation Behavior on the Hot Ductility of Boron Containing Steel

The effect of boron (B) precipitation behavior on the hot ductility of B containing steel was investigated. Hot ductility of B containing steel was sensitive to the cooling rate (CR) in the range of 1 to 20 K/s (1 to 20 °C/s), whereas that of B-free steel showed little change with CR. Increased CR causes deepening and widening of the ductility trough in B containing steel. Particle tracking autoradiography (PTA) analysis and transmission electron microscope (TEM) image of the samples show that boron nitride (BN) particles form along prior austenite grain boundaries, and that as CR increases, these particles become smaller and more numerous. This increase in the number of small BN precipitates may promote intergranular fracture, leading to a decrease in hot ductility in the lower austenite temperature region (1173 to 1273 K (900 to 1000 °C)). Furthermore, the formation of filmlike ferrite at ~1123 K (850 °C) causes a decrease in the hot ductility of this steel regardless of B addition and CR.     

High-Temperature Mechanical Properties of Shipbuilding Steel BC in Continuous Casting

The mechanical property of shipbuilding steel BC has been studied by means of tensile test at various temperatures from 700 ℃ to 1000℃ with theGleeble-1500D thermo-mechanical simulator.The results indicate that the yield strength and tensile strength of steel have an analogous change pattern as temperature decreasing,and the transition in variation rate of strength occurs at 800℃ in both of them;the hot ductility trough of steel BC is a temperature range from 725℃ to 800℃,while the best hot ductility ranges from 875℃ to 1000℃ with the ductility value over 80%.For the sake of understanding the fracture mechanism of the steel,fracture surface and microstructure of the specimens have been examined by scanning electron microscope and metalloscope correspondingly.The results show that both the second phase particles and the pro-eutectoid ferrite surrounding the austenite boundaries play a significant role to the variation of hot ductility of steel BC.Deservedly,the research is important to the improvement and the further studies on the quality of steel during slab continuous casting process.     

Influence of Isothermal Treatment on MnS and Hot Ductility in Low Carbon,Low Mn Steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 × 10^?4 s^?1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The higher-carbon plain C-Mn steel only displayed a narrow trough less than the Ae₃ temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility.     

微合金钢红送裂纹形成的试验研究

 在对析出相行为进行试验研究的基础上,结合铸坯热应力变化与组织演变分析讨论了微合金钢红送裂纹的形成机制。结果表明：铸坯输送方式的不同以及装炉温度的变化对装炉时铸坯中析出相的行为具有显著影响,但不同输送方式以及不同装炉温度条件下再加热结束后的铸坯中析出相的存在状态较为相似;微合金钢红送裂纹是在析出相行为、组织演变和热应力变化三者的共同作用下形成的,其中起主导作用的是铸坯组织的演变,即两相区中奥氏体晶界先共析铁素体网膜的生成。     

Corner crack mechanism of boron-containing LCAK steel slabs

At Baoshan Iron Steel Co., Ltd., corner cracks of boron containing LCAK steel slabs had caused remarkable quality loss and mass flow disorder. With the help of fractography and thermodynamics analysis,the embrittlement mechanism of this steel grade was studied and the results are as follows: ① The transformation from γ to α starts at the austenite grain boundaries and a layer of thin ferrite film gradually forms around the austenite grains. Strain concentration will preferentially start inside the ferrite phase when the stress accumulates to a certain level.② The coarse BN particles acceleratedly precipitated at the γ/α interfaces further deteriorate the ductility of the ferrite film,and brittleness results in strain concentration and microvoid coalescence inside the ferrite film.Therefore the austenite grain boundaries are prone to intergranular failure.③ The stoichiometry among Al,N and B is a very important factor influencing the hot ductility of this steel grade.By controling the B-to-N atomic ratio to above 1,all N can be fixed by B instead of Al.Thus coarse-grained steel is available and fewer grain boundaries and higher ductility can reduce the risk of corner cracks.④ By adjusting the B-to-N atomic ratio,Baoshan Iron Steel Co. ,Ltd.successfully reduced the number of cracks to nearly one tenth of that in the past and the hot tensile tests confirmed remarkable improvement in the hot ductility of this steel.     

热装热轧微合金钢板表面裂纹分析

 利用金相观察、扫描电镜及能谱分析和透射电镜等手段,对热装热轧微合金钢板出现的表面裂纹进行分析研究,并与使用同批次连铸坯冷装热轧无裂纹的钢板进行比较,分析产生表面裂纹的原因。实验结果表明热装热轧微合金钢板产生表面裂纹的原因是铸坯冷却或加热过程中Cu、As低熔点元素在奥氏体晶界的偏聚。与热装热轧板相比,冷装热轧板晶粒尺寸小直径在10μm左右,而热装热轧板晶粒尺寸大且不均匀。热轧板析出物尺寸在15～25nm之间,裂纹源处较基体多,大量细小的Nb(C,N)化合物在奥氏体晶界析出,降低了晶界强度。     

Effect of heat treatment on microstructure and mechanical properties of duplex stainless steel

Present study concerns the effect of deformation and heat treatment on the microstructure and mechanical properties of a duplex stainless steel. While hot rolling causes the coarse distribution of the constituent phases (ferrite and austenite), 50% cold rolling results into the elongated and splintered two — phase structure. Supersaturated ferrite structure established by water quenching from 1300°C results into the strengthening due to the formation of fine dispersed austenite precipitates within ferrite grain after isothermal heat treatment (1000°C, 0.5 hour). Duplex structure consisting of ferrite and austenite in a fine-grained form is obtained after isothermal heat treatment of cold rolled sample. Cold deformed and heat treated steel exhibits best combination of strength and ductility among all the investigated steel samples.     

The Influence of Hot Charging Conditions on Microstructure and Precipitation of High Strength Nb–Ti Complex Microalloyed Steels

A comprehensive microstructure analyses were conducted for hot charging processed Nb, Ti microalloyed steel, especially focusing on the evolution of microstructure and precipitation behavior. Hot charging process was simulated in the laboratory in the current research. Precipitation and microstructure in Ti–Nb microalloyed HSLA steels were investigated in different hot charging temperature conditions using transmission electron microscope (TEM), optical microscopy, analytical transmission electron microscopy, and energy dispersive spectroscopy (EDS). The results showed that the microstructure consists of fine and homogeneous lath bainite at 1000°C, lath bainite and granular bainite at 900°C. As the hot charging temperature decreased, there appeared a small fraction of proeutectoid ferrite along grain boundary at 800°C. The grain sizes in hot charging are coarse compared with that of 700°C due to the phase transformation didn't occur. The chemical composition of complex carbonitrides changes from Ti rich to Ti–Nb uniform with decreasing temperature.     

耐火耐侯钢热装温度对高温性能与组织的影响

对耐火耐侯钢的高温性能进行热模拟研究,发现该钢在500℃装炉时钢的高温强度最低,比冷装炉钢的高温强度低20MPa,600℃装炉时钢的高温强度最高,比冷装炉钢的高温强度高40MPa。总体来说,热装不会影响该钢的高温性能,热装温度高于600℃对该钢的高温性能更为有利。600℃以下装炉,该钢加热奥氏体晶粒度与冷装的差不多。600℃以上装炉,加热奥氏体晶粒度比冷装炉的要小l级左右。     

Q345B中厚钢板表面裂纹原因分析

通过板坯表面酸洗、钢板表面抛丸、氮氧分析、扫描电镜能谱仪和金相显微镜等手段,对唐钢所生产Q345B中厚钢板的表面裂纹处进行观察、检测,研究了热装板坯在轧制过程中产生表面裂纹的原因和机理.同时还进行了板坯热装、温装、冷装对比试验.结果表明,含铝低合金钢板由于板坯热装温度处于第三低温脆性区域,冷却过程中奥氏体向铁素体的转变不完全,AIN在奥氏体晶界析出,削弱晶界能,体积膨胀加剧了晶界强度的减弱,在轧制时扩展形成表面裂纹.     

铸态超级双相不锈钢S32750热加工性能

 为研究热变形参数对铸态超级双相不锈钢S32750热变形行为和显微组织的影响,运用Gleeble-3800热模拟试验机对S32750进行不同温度和应变速率下的高温拉伸和压缩试验。结果表明,S32750在1000~1200℃范围内具有较好的热塑性。在变形温度较低、应变速率较低时,流变曲线表现出不同于单相不锈钢的“类屈服平台”特征;当应变速率较高或变形温度较高、应变速率较低时,流变曲线为典型的动态再结晶特征。微观组织演变显示,铁素体和奥氏体两相都发生动态再结晶,且铁素体的再结晶先于奥氏体。降低应变速率,提高变形温度,可促进动态再结晶发生。基于热变形动力学模型建立了本构方程,表观应力指数为3.99,热变形激活能为393.75kJ/mol。S32750的高温软化机制与Zener-Hollomon(Z)参数有关,随Z参数增加,热变形峰值应力增加。     

Effect of Hot Torsion Parameters on Development of Ultrafine Ferrite Grains in Microalloyed Steel

 Hot torsion testing was performed on a low carbon Nb-Ti microalloyed steel to study the effects of hot torsion parameters, strain and strain rate, on ultrafine ferrite grains production through dynamic strain-induced transformation, at a deformation temperature just above Ar3. The initiation and evolution of ultrafine ferrite grains were studied. The results show that the amount of strain and strain rate has conversely effect on the volume fraction and grain size of ultrafine ferrite grains. With increasing strain, the interior of austenite grains become activated as nucleation sites for fine ferrite grains. As a result, ferrite grains continuously nucleate not only at the former austenite grain boundaries but also inside the austenite grains which leads to a rapid increase in volume fraction of ultrafine grains. Increasing of strain rate reduces the tendency of ferrite grains coarsening so that ultrafine ferrite grains are achieved, while the volume fraction of ultrafine grains decreases at the same strain level.     

Hot Ductility of DSS and Its Microstructure Observation During Hot Deformation

By selecting several typical duplex stainless steels (DSS), i. e., 00Cr22Ni5Mo3N, 00Cr21Ni2Mn5N and 00Cr25Ni7Mo4N, as research materials, hot ductility characteristic of DSS was studied by thermal simulation method and microstructure evolution during hot compression was observed through TEM. The results show that the optimum hot ductility temperature range of DSS is 1050–1200°C. 00Cr25Ni7Mo4N exhibits the worst hot ductility and 00Cr21Ni2Mn5N has similar hot ductility to 00Cr22Ni5Mo3N. During hot compression, the dynamic recovery of austenite occurs in DSS while the dynamic recovery and reerystallization of ferrite take place in 00Cr22NioMo3N and 00Cr21Ni2Mn5N, but only the dynamic recovery of ferrite can be observed in 00Cr25Ni7Mo4N.