Microstructure,properties, and age hardening behavior of a thermomechanically processed ultralow-carbon cu-bearing high-strength steel

An ultralow-carbon steel alloyed with Ni, Mn, Mo, and Cu and microalloyed with Nb and Ti was subjected to a three-stage controlled rolling operation followed by water quenching. The effect of thermomechanical processing on the microstructure, mechanical properties, and age-hardening behavior of the steel was evaluated. The precipitation behavior of Cu at different aging temperatures was studied by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The high strength values obtained in the present steel are due to the fine-lath martensite structure along with tiny precipitates of microalloying carbide and carbonitride of Ti and Nb at all finish rolling temperatures (FRTs). The increased strength value at the lower FRT is due to the finer lath width and packet size of martensite. The large TiN particles and the coarse martensite-austenite (MA) constituents impaired the impact-toughness value of the steel at subambient temperature. On aging at different temperatures, a wide variation in structure and properties has been obtained. At low aging temperatures, coherent Cu particles form and a peak strength is obtained due to the formation of fine ε-Cu precipitates. On increasing aging temperatures, the Cu particle size increases, with a simultaneous decrease in dislocation density in the matrix resulting in a continuous decrease in strength.     

Microstructure,properties, and age hardening behavior of a thermomechanically processed ultralow-carbon Cu-bearing high-strength steel

An ultralow-carbon steel alloyed with Ni, Mn, Mo, and Cu and microalloyed with Nb and Ti was subjected to a three-stage controlled rolling operation followed by water quenching. The effect of thermomechanical processing on the microstructure, mechanical properties, and age-hardening behavior of the steel was evaluated. The precipitation behavior of Cu at different aging temperatures was studied by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The high strength values obtained in the present steel are due to the fine-lath martensite structure along with tiny precipitates of microalloying carbide and carbonitride of Ti and Nb at all finish rolling temperatures (FRTs). The increased strength value at the lower FRT is due to the finer lath width and packet size of martensite. The large TiN particles and the coarse martensite-austenite (MA) constituents impaired the impact-toughness value of the steel at subambient temperature. On aging at different temperatures, a wide variation in structure and properties has been obtained. At low aging temperatures, coherent Cu particles form and a peak strength is obtained due to the formation of fine ε-Cu precipitates. On increasing aging temperatures, the Cu particle size increases, with a simultaneous decrease in dislocation density in the matrix resulting in a continuous decrease in strength.     

Evolution of Phases and Mechanical Properties of Thermomechanically Processed Ultra High Strength Steels

The present study concerns the development of two low carbon microalloyed ultra high strength steels on a pilot scale. This recent endeavour has been made towards the reduction of weight by achieving high strength to weight ratio together with improved weldability for the various prospective high performance defence applications such as explosive ammunition, gun barrel, missile skins, light-weight military bridges etc. These steels were thermomechanically processed and finished at different finish rolling temperatures followed by water quenching. Variation in microstructure and mechanically properties at different finished rolling temperatures was studied. The experimentally determined continuous cooling transformation diagrams have revealed that adequate hardenability is achievable in these steels usually at a cooling rate >5 °C/s. Lath martensite along with the microalloy (Ti, Nb) CN precipitate particles are the characteristic microstructural feature of the investigated steels. The high strength value obtained in the present steels is due to the accumulated contribution of fine grained pan-caked austenite, highly dislocated lath martensite along with the presence of tiny precipitates of microalloy carbide/carbonitride and Cu rich precipitates. The good combination of strength (1,364–1,538 MPa) and ductility (11–16 %) has been achieved for the selected range of finish rolling temperature. The Charpy impact toughness values (30–80 J) reveal approximately consistent fall with the lowering of testing temperature.     

Phase Transformation and Mechanical Properties of an Ultrahigh Strength Steel

The present study deals with the development of a low carbon high strength steel by thermomechanical controlled processing on a pilot scale.The continuous cooling transformation has yielded a flat top "C" curve with the phase field occupied by a mixture of bainite and martensite.The microstructure of water quenched steel essentially consists of highly dislocated lath martensite along with fine (Ti,Nb)CN precipitates and twins.High strength steel with 1217-1298 MPa yield strength and 1372-1513 MPa ultimate tensile strength along with 16-12% total elongation has been obtained in the range of 850 to 750℃ finish rolling temperature.The impact toughness value in the range of 45-72J was also achieved in the present steel.     

终轧温度对低合金铌钛贝氏体钢组织和性能的影响

 采用光学显微镜、透射电子显微镜（TEM）、EDS能谱分析仪和拉伸冲击试验机,研究了终轧温度对TMCP(thermo-mechanical controlled processing)低合金铌钛贝氏体钢组织和性能的影响。结果表明：随着终轧温度的降低,强度和韧性先升高后降低。终轧温度为815 ℃时,由于冷却前温度已降低到奥氏体-铁素体两相区,在晶界形成大量先共析铁素体,造成了强度和韧性的下降。终轧温度为870 ℃时,得到细小的板条贝氏体＋少量的马氏体组织,在贝氏体板条上有30～50 nm的Nb、Ti析出相弥散分布,获得了最优异的性能,其屈服强度为805 MPa,抗拉强度为1 005 MPa,－20 ℃冲击功的平均值为197 J。     

An Ultra-low Carbon, Thermomechanically Controlled Processed Microalloyed Steel: Microstructure and Mechanical Properties

In the current study, a novel ultra-low carbon, high-molybdenum-bearing microalloyed steel has been thermomechanically processed. Transformation of this steel during continuous cooling has been assessed. Variation in the microstructure and mechanical properties at different finish rolling temperatures has been studied. The average grain size, misorientation of grain boundary, and distribution of ferrite grains have been analyzed by using electron backscatter diffraction. The lower yield strength (251 to 377?MPa) with moderate tensile strength (406 to 506?MPa) along with high ductility (30 to 47?pct) has been achieved in the selected range of finish rolling temperatures. Superior impact toughness value in the range of 153 to 162?J is obtained in the subsize specimen even at subzero temperatures (233?K [?40?°C]), which is attributed to fine average ferrite grain size. The acicular ferrite dominated microstructure obtained at the 1023?K (750?°C) finish rolling temperature is the most attractive microstructure for pipeline applications due to its excellent combination of strength and toughness.     

Correlation of rolling condition,microstructure, and low-temperature toughness of X70 pipeline steels

Correlation of rolling conditions, microstructure, and low-temperature toughness of high-toughness X70 pipeline steels was investigated in this study. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot rolling, and their microstructures were varied by rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature fracture properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below −100 °C when the finish cooling temperature range was 350 °C to 500 °C, showing excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85 pct, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture occurred at some finish cooling temperatures.     

Relationship Among Microstructure and Properties and Heat Treatment Process of Ultra-High Strength X120 Pipeline Steel

 The variation of heat treatments including directed quenching and tempering off-line after controlled rolling (DQT) and quenching off-line and tempering off-line after controlled rolling (RQT) with microstructure and mechanical properties of a low-carbon microalloyed steel was compared and analyzed. For DQT, the quenched steel was obviously banded microstructure, with increasing tempering temperature, lath martensite coarsened, the cusp carbide precipitated at grain boundaries, the yield strength fluctuated slightly, and the fracture-separation was obvious. The impact toughness was better in the steel tempered at 500 ℃ for 1 h. In RQT, with increasing tempering temperature, lath martensite degenerated, intragranular and intergranular finer precipitations with smaller than 30 nm precipitated and grew up and were distributed dispersedly, the stripe-like carbides were distributed at grain boundaries, and the yield strength and tensile strengthen decreased obviously. The impact toughness of RQT process was much better than that of DQT process, and the comprehensive mechanical properties were better for the steel tempered at 500 ℃ for 1 h of RQT process.     

Structure and Properties of a Low-Carbon, Microalloyed, Ultra-High-Strength Steel

The current study concerns the development of a low-carbon, microalloyed ultra-high-strength steel on a pilot scale. The continuous cooling transformation has been evaluated, and a flat top “C” curve with a mixed microstructure of bainite and martensite has been obtained at a lower transformation temperature. The steel has been processed thermomechanically, followed by air cooling and water quenching. In addition, a variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. Although a mixture of granular bainite and bainitic ferrite with interlath and intralath precipitation of NbC/NbC(N)/TiC(N) particles are the characteristic microstructural feature of air-cooled steel, the lath martensitic structure along the fine NbC/NbC(N)/TiC(N) precipitate is obtained in case of a water-quenched steel. The high-strength value obtained in the current steel is caused by the accumulated contribution of fine-grained, pancaked austenite, highly dislocated fine lath martensitic structure along with the presence of tiny precipitates of microalloy carbide/carbonitride.     

Evolution of Microstructure and Mechanical Properties of Thermomechanically Processed Ultrahigh-Strength Steel

In the present study, low carbon microalloyed ultrahigh-strength steel was manufactured on a pilot scale. Transformation of the aforesaid steel during continuous cooling was assessed. The steel sample was thermomechanically processed followed by air cooling and water quenching. Variation in microstructure and mechanical properties at different finish rolling temperatures (FRTs) was studied. A mixture of granular bainite and bainitic ferrite along with interlath and intralath precipitation of (Ti, Nb)CN particles is the characteristic microstructural feature of air-cooled steel. On the other hand, lath martensitic structure along with a similar type of microalloying precipitates of air-cooled steels is obtained in the case of water-quenched steel also. The best combination of strength (1440 to 1538 MPa) and ductility (11 to 16 pct) was achieved for the selected range of FRTs of water-quenched steel.     

终轧温度对高氮奥氏体钢组织和力学性能的影响

通过加压冶炼、控制轧制方式获得氮质量分数为0.59%的Mn18Cr18N钢板,研究了终轧温度对高氮奥氏体钢组织和力学性能的影响。结果表明,在再结晶区轧制并且终轧温度为970 ℃的钢板,组织为奥氏体等轴晶和部分孪晶,强度较低,塑性、冲击韧性较好;终轧温度为910 ℃的钢板,大部分组织为变形奥氏体晶粒,有少量再结晶晶粒,随着终轧温度降低钢板强度升高,塑性和冲击韧性降低;在未再结晶区轧制并且终轧温度为780 ℃的钢板,组织为变形严重的奥氏体晶粒,强度最高,塑性、韧性最低。所有试验钢有晶界析出的Cr₂N相,降低终轧温度和减缓轧后冷却速度,会增加Cr₂N相的析出。     

热轧条件下Nb-Mo及Nb微合金化X100管线钢的组织特征及力学行为

采用实验室热轧、显微分析及力学性能检测手段,对Nb-Mo及Nb微合金化X100管线钢在不同工艺条件下的组织特征及力学行为的变化规律进行了研究.分析结果表明:工艺参数对Nb-Mo复合成分试验钢影响较大,控轧控冷工艺条件下Nb-Mo及Nb微合金化X100管线钢力学性能均能达到API 5L中X100管线钢要求,但Nb-Mo复合成分力学性能富余量较大,性能较优.随冷却速度的增加及终冷温度的降低,试验钢强度增加,韧性及塑性恶化.板条马氏体与贝氏体复相组织较板条马氏体可大大提高试验钢的塑性及低温冲击韧性.     

回火温度对Cu-Cr-Ti马氏体耐磨钢组织及强韧性的影响

摘要：矿山机械用耐磨钢构件服役环境恶劣而常常出现磨损失效，研究适用于复杂工况下的高耐磨钢成分、工艺与组织性能的关系，有利于提高耐磨构件的服役寿命并降低经济损失。利用SEM、TEM、洛氏硬度计、万能拉伸试验机及冲击试验机等，研究了160～400℃不同回火温度下Cu-Cr-Ti马氏体耐磨钢的组织形貌、强度硬度及-20℃冲击韧性的变化。结果表明，试验钢淬火态组织主要为板条马氏体，当回火温度为160℃时，马氏体板条依然清晰，但随回火温度升高到400℃，马氏体板条界渐渐消失，基体中出现大量片状或粒状渗碳体。EDS分析发现样品钢基体中含有纳米级Ti、Nb的碳氮化物。随回火温度升高，基体组织演变导致强化机制发生变化，回火温度为300℃，综合力学性能最佳，其抗拉强度为1500MPa，屈服强度1100MPa，伸长率为15.5%。随回火温度升高，-20℃冲击韧性由60J/cm2逐渐降低到36.3J/cm2。     

Some aspects of possible bearing steel microstructure modifications

Microstructural changes and mechanical properties with Mn-Cr steel (0.65 % C) under effect of cold rolling, rapid austenitizing, immediate oil quenching and low temperature tempering were investigated. Compared to properties of conventionally treated standard Cr-C bearing steel, higher ultimate strength, increased fracture toughness and enhanced rolling contact fatigue life were observed at comparable yield stress levels. The increased stress relaxation ability of the lath martensite matrix, and the absence of transformation microcracks, both due to the reduced carbon content in steel and also in γ-solid solution, are the main reasons of the properties' improvement. The double transfer of substructural defects from cold worked ferrite to austenite and back into martensite, and the austenite heterogeneity control, both enabled by rapid austenitizing followed by quenching, was the key for obtaining the effects mentioned.     

退火温度对非调质960 MPa高强钢组织和性能的影响

为了稳定热轧高强钢的组织和性能,需进行退火处理。了解不同的温度对组织的影响规律至关重要,为此对相同终轧温度条件下退火温度对非调质960 MPa高强钢组织和性能的影响进行研究。结果表明,试验钢在不同退火温度下处理后,均得到回火马氏体组织,并出现明显的纤维组织,使得马氏体板带具有明显的方向性,且随着退火温度的升高马氏体组织粗化。经不同退火温度处理后,试验钢的强度在RD方向(轧制方向)的下降趋势低于TD方向(轧件宽度方向),说明TD方向的强度对温度变化较敏感;同时随着退火温度的升高,伸长率和冲击韧性在RD方向为先升后降,在TD方向伸长率持续升高,冲击韧性几乎不变。综合考虑,确定退火温度为500 ℃时为最佳退火工艺方案。     

无碳化物贝氏体耐磨钢板组织与性能的研究

研究了无碳化物贝氏体耐磨钢板组织、力学性能及焊接性能。结果表明,在低碳贝氏体钢基础上,通过加入一定量的硅元素,利用其在贝氏体组织转变过程中抑制碳化物析出作用,得到由非等轴铁素体加马氏体和残余奥氏体(M-A)岛或由板条状铁素体及其板条间残余奥氏体(Ar)膜组成的无碳化物贝氏体组织,以此得到既具有高强度、高硬度,又具有较高的低温冲击韧性,同时具有较好的焊接性能。     

Structure and mechanical properties of Fe-Cr-Mo-C alloys with and without boron

A study of the structure and mechanical properties of Fe-Cr-Mo-C martensitic steels with and without boron addition has been carried out. Nonconventional heat treatments have subsequently been designed to improve the mechanical properties of these steels. Boron has been known to be a very potent element in increasing the hardenability of steel, but its effect on structure and mechanical properties of quenched and tempered martensitic steels has not been clear. The present results show that the as-quenched structures of both steels consist mainly of dislocated martensite. In the boron-free steel, there are more lath boundary retained austenite films. The boron-treated steel shows higher strengths at all tempering temperatures but with lower Charpy V-notch impact energies. Both steels show tempered martensite embrittlement when tempered at 350 °C for 1 h. The properties above 500 °C tempering are significantly different in the two steels. While the boron-free steel shows a continuous increase in toughness when tempered above 500 °C, the boron-treated steel suffers a second drop in toughness at 600 °C tempering. Transmission electron microscopy studies show that in the 600 °C tempered boron-treated steel large, more or less continuous cementite films are present at the lath boundaries, which are probably responsible for the embrittlement. The differences in mechanical properties at tempering temperatures above 500 °C are rationalized in terms of the effect of boron-vacancy interactions on the recovery and recrystallization behavior of these steels. Although boron seems to impair room temperature impact toughness at low strength levels, it does not affect this property at high strength levels. By simple nonconventinal heat treatments of the present alloys, martensitic steels may be produced with quite good strength-toughness properties which are much superior to those of existing commercial ultra-high strength steels. It is also shown that very good combinations of strength and toughness can be obtained with as-quenched martensitic steels.     

Microstructure and Abrasive Wear Behavior of Medium Carbon Low Alloy Martensitic Abrasion Resistant Steel

The effect of processing parameters such as hot rolling and heat treatment on microstructure and mechanical properties was investigated for a new 0.27mass% C and Ni,Mo-free low alloy martensitic abrasion resistant steel.The three-body impact abrasive wear behavior was also analyzed.The results showed that two-step controlled rolling besides quenching at 880℃and tempering at 170℃could result in optimal mechanical property:the Brinell hardness,tensile strength,elongation and-40 ℃impact toughness were 531,1 530 MPa,11.8% and 58J,respectively.The microstructure was of fine lath martensite with little retained austenite.Three-body impact abrasive wear results showed that wear mechanism was mainly of plastic deformation fatigue when the impact energy was 2J, and the relative wear resistance was 1.04times higher than that of the same grade compared steel under the same working condition.The optimal hardness and toughness match was the main reason of higher wear resistance.     

屈服强度785 MPa低碳含铜船板钢的组织和性能

 研究了一种屈服强度大于785 MPa的船板钢,测试了其动态连续冷却相变曲线（CCT）,研究了试验钢经控制轧制+直接淬火+回火（DQ- T）工艺处理后的组织性能。结果表明,直接淬火（DQ）钢板组织为板条马氏体（LM）,回火后铜、铌元素呈弥散析出。经500 ℃回火钢板的强度最高,冲击韧性（KV2）最低。钢板经710 ℃回火,其组织为二次马氏体（SLM）+铁素体,屈服强度（Re）为810 MPa,抗拉强度（Rm）为 1 066 MPa,伸长率（A）为17%,在-80 ℃下KV2为97 J,达到最佳强韧性匹配。     

热处理对2 000 MPa超高强度不锈钢组织和性能的影响

 运用Thermal-calc热力学软件、光学显微镜（OM）、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等手段研究了热处理工艺对一种新型的超高强度不锈钢微观组织及力学性能的影响。结果表明,经过固溶处理后钢的基体为高密度位错的板条马氏体组织;强度随着时效温度的升高而逐渐升高,在520～540 ℃时可达到2 000 MPa,且冲击吸收功在540 ℃时达到最大值37 J。此时在板条马氏体上析出大量、细小、弥散以μ相为主的第二相,同时在板条与板条界面上有块状的逆转变奥氏体生成,这是该钢具有超高强度与高韧性的主要原因。