Enhanced mechanical stability of ultrafine grained steel through intercritical annealing cold rolled martensite

The ultrafine grained ferritic steels possess high strength but low ductility due to the shortage of work hardening.Fine grained ferrite-martensite dual phase microstructure was obtained in a microalloyed steel and low carbon steels through intercritical annealing of the cold rolled martensite.The dual phase microstructure was uniform and the ferrite grain size was smaller in the microalloyed steel resulted from the pinning effect of microalloyed precipitates.But ferrite grown apparently and the volume fraction of the martensite was much higher without the addition of microalloying elements.By introducing martensite into the fine grained ferrite,the work hardening was effectively improved,leading to better mechanical stability.As a result of the fine ferrite grain size as well as uniform distribution of the martensite,the work hardening was enhanced,showing better strength-ductility balance in the microalloyed dual phase steel.     

Precipitation Behavior and Microstructural Evolution of Ferritic Ti–V–Mo Complex Microalloyed Steel

Precipitation behavior of(Ti,V,Mo)C and microstructural evolution of the ferritic Ti–V–Mo complex microalloyed steel were investigated through changing coiling temperature(CT).It is demonstrated that the strength of the Ti–V–Mo microalloyed steel can be ascribed to the combination of grain refinement hardening and precipitation hardening.The variation of hardness(from 318 to 415 HV,then to 327 HV) with CT(from 500 to 600–625 °C,then to 700 °C) was attributed to the changes of volume fraction and particle size of(Ti,V,Mo)C precipitates.The optimum CT was considered as 600–625 °C,at which the maximum hardness value(415 HV) can be obtained.It was found that the atomic ratios of Ti,V and Mo in(Ti,V,Mo)C carbides were changed as the CT increased.The precipitates with the size of \ 10 nm were the V-rich particles at higher CT of 600 and 650 °C,while the Ti-rich particles were observed at lower CT of 500 and 550 °C.Theoretical calculations indicated that the maximum nucleation rate of(Ti,V,Mo)C in ferrite matrix occurred around 630 °C,which was consistent with the 625 °C obtained from experiment results.     

Elevated-temperature properties of one long-life high-strength gun steel

The hardness, tensile strength and impact toughness of one quenched and tempered steel with nominal composition of Fe-0.25C-3.0Cr-3.0Mo-0.6Ni-0.1Nb (mass fraction) both at room temperature and at elevated temperatures were investigated in order to develop high-strength steel for long-life gun barrel use. It is found that the steel has lower decrease rate of tensile strength at elevated temperature in comparison with the commonly used G4335V high-strength gun steel, which contains higher Ni and lower Cr and Mo contents. The high elevated-temperature strength of the steel is attributed to the strong secondary hardening effect and high tempering softening resistance caused by the tempering precipitation of fine Mo-rich M2C carbides in the aaaaaaaaaaaaaaaa-Fe matrix. The experimental steel is not susceptible to secondary hardening embrittlement, meanwhile, its room-temperature impact energy is much higher than the normal requirement of impact toughness for high strength gun steels. Therefore, the steel is s     

Achieving higher strength in Cu–Ag–Zr alloy by warm/hot rolling

High-strength Cu–3Ag–0.5Zr alloy plates were produced by multi-pass rolling in the temperature range of500–800 °C. An increase in strength was observed by rolling in the aforementioned range without significant loss in ductility. All the rolled samples show higher strength than solution-treated and aged samples. The maximum strength was observed for plates rolled at 500 °C with a yield strength and ultimate tensile strength of 311 and385 MPa, respectively, and retaining a ductility of 23 %.Transmission electron microscopy(TEM) studies showed uniform distribution of fine silver precipitates and high dislocation density in the rolled samples. Nevertheless, the size of precipitates and dislocation density varied with the rolling temperature. The superior strength achieved in the rolled samples is attributed to grain refinement, dislocation strengthening, and precipitation hardening. This method can be employed to produce high-strength plates of precipitation hardenable copper alloys.     

Effect of Rare Earth and Cooling Process on Microstructure and Mechanical Properties of an Ultra-Cleaned X80 Pipeline Steel

In order to explore the eff ect of a small amount of rare earth addition in ultra-cleaned pipeline steel and the influence of the cooling process on the tensile and impact properties, three API X80 pipeline steels were fabricated by varying RE addition and the cooling process at the same time. Three microstructures with different features for a low C high Nb microalloyed high-strength pipeline steel and the corresponding mechanical properties were investigated. The results showed that even in the ultra-cleaned steel with O and S contents less than 10 ppm, the addition of RE would still cause an increase in the volume fraction of inclusions consisting of complicated RE oxysulfide and RE sulfide. More inclusions formed in the 112 ppm RE steel were harmful to the low temperature toughness, while few inclusions formed in the 47 ppm RE steel had almost no influence on the low temperature toughness. The two RE additions had no effect on strength of the steels. As the finishing cooling temperature was increased and the cooling rate was decreased within a certain range, the volume fractions of polygonal ferrite and quasi-polygonal ferrite as well as the number density and size of martensite–austenite islands were increased. Under such combined effect, the strength of the steels had almost no change. As the finishing cooling temperature was increased from 481 to 584 ℃ and the cooling rate was reduced from 20 to 13 ℃/s, for the steel with 112 ppm addition of RE, there was an obvious decrease in the low temperature toughness. The reduced value(about 33 J) of the USE of steel consisted of two parts including the influence(about 18 J) of more inclusions formed due to 112 ppm addition of RE and the eff ect(about 15 J) of the lower high-angle grain boundaries.     

Obtaining Ultra-High Strength and Ductility in a Mg–Gd–Er–Zn–Zr Alloy via Extrusion,Pre-deformation and Two-Stage Aging

The Mg–12Gd–1Er–1Zn–0.9 Zr(wt%) alloy with ultra-high strength and ductility was developed via hot extrusion combined with pre-deformation and two-stage aging treatment.The age-hardening behavior and microstructure evolution were investigated.Pre-deformation introduced a large number of dislocations,resulting in strain hardening and higher precipitation strengthening in the subsequent two-stage aging.As a result,the alloy showed a superior strength–ductility balance with a yield strength of 506 MPa,an ultimate tensile strength of 549 MPa and an elongation of 8.2% at room temperature.The finer and denser β' precipitates significantly enhanced the strength,and the bimodal structure,small β-Mg_5RE phase as well as dense γ' precipitates ensured the good ductility of the alloy.It is suggested that the combination of pre-deformation and two-stage aging treatment is an eff ective method to further improve the mechanical properties of wrought Mg alloys.     

Preparation and properties of W–Cu–Zn alloy with low W–W contiguity

The W–Cu–Zn alloy with a-brass matrix and low W–W contiguity was prepared by method of electroless copper plating combined with spark plasma sintering(SPS) method.The effects of process and parameters on the microstructure and mechanical properties of the alloy were investigated.The W–Cu–Zn alloy with a relative density of 96 % and a W–W contiguity of about 10 % was prepared by original fine tungsten particles combined with wet mixing method and SPS solid-state sintering method at 800 °C for 10 min.The microstructure analysis shows that Cu–Zn matrix consists of nano-sized a-brass grains,and the main composition is Cu_3Zn electride.The nano-sized Cu was coated on the surface of tungsten particles by electroless copper plating method,and the fairly low consolidation temperature and short solid-state sintering time result in the nano-sized matrix phase.The dynamic compressive strength of the W–Cu–Zn alloy achieves to1000 MPa,but the alloy shows poor ductility due to the formation of the hard and brittle Cu_3Zn electrides.The fine-grain strengthening and the solution strengthening of the Cu–Zn matrix phase are responsible for the high Vickers microhardness of about 300 MPa for W–Cu–Zn alloy.     

Effect of V–Ti Addition on Microstructure Evolution and Mechanical Properties of Hot-Rolled Transformation-Induced Plasticity Steel

In order to reveal the effect of V–Ti addition on the microstructure evolution and the mechanical properties of hot-rolled transformation-induced plasticity(TRIP) steel, two steels with 0.072 V–0.051 Ti steel(Bear-V–Ti steel) and 0.001 V–0.001 Ti steel(Free-V–Ti steel) were designed, respectively, and the comparison analyses were carried out by performing thermodynamic calculation and an experiment. With the thermodynamic calculation, the critical annealing temperature of a large fraction of retained austenite(～51%) obtained via solute enrichment was determined, and an optimized quenching at 650 °C and tempering at 200 °C adopted on the as-hot-rolled steel. The results show that the V–Ti TRIP steel displays more optimum mechanical stability during the tensile deformation, since the fraction and the mechanical stability of retained austenite are improved and the microstructure is also ultrarefined by V–Ti alloy precipitation. The yield strength of Bear-V–Ti steel increases from 650 to 800 MPa, and the ductility reaches 37%, showing that the comprehensive mechanical properties are greatly improved.     

Microstructure and Mechanical Properties of 1,000 MPa Ultra-High Strength Hot-Rolled Plate Steel for Coal Mine Refuge Chamber

The 1,000 MPa ultra-high strength hot-rolled plate steel with low-carbon bainitic microstructure was developed in the laboratory for coal mine refuge chamber. The static recrystallization behavior, microstructure evolution, and mechanical properties of this hot-rolled plate steel were investigated by the hot compression, continuous cooling transformation, and tensile deformation test. The results show that the developed steel has excellent mechanical properties at both room and elevated temperature, and its microstructure mainly consists of lath bainite, granular bainite, and ferrite after thermal–mechanical control process(TMCP). The ultra-high strength plate steel is obtained by the TMCP process in hot rolling, strengthened by bainitic transformation, microstructure refinement, and precipitation of alloying elements such as Nb, Ti, Mo, and Cu. The experimental steel has relatively low welding crack sensitivity index and high atmospheric corrosion resistance index. Therefore, the developed steel has a good balance of strength and ductility both at room and elevated temperature, weldability and corrosion resistance, and it can suffice for the basic demands for materials in the manufacture of coal mine refuge chamber.     

Effect of Isothermal Temperature on Microstructure and Mechanical Properties of High Al–Low Si TRIP Steel

In this study, the effect of isothermal temperature on microstructure and mechanical properties of a high Al–low Si TRIP steel was investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron back scattered diffraction, and tensile test. The results show that typical microstructure containing ferrite, bainite, and retained austenite can be obtained when two-stage heat treatment process was utilized. When annealing temperature is 840 °C and austempering temperature is 400 °C, the tensile strength is 542 MPa and the product of strength and elongation is 17,685 MPa%. The morphologies and stability of the retained austenite in low silicon/high aluminum TRIP steel were finally discussed.     

CT80级非调质连续油管用钢的组织和性能控制

通过两种成分非调质CT80连续油管用钢现场生产板卷工艺组织性能对比,分析了冷却速度、卷取温度、Mo和Nb元素含量等工艺参数对实验钢组织性能的影响。结果表明:当冷却速度由52℃/s提高到69℃/s后,铁素体形态为针状铁素体,实验钢屈服强度提高25 MPa;抗拉强度提高30 MPa。实验钢在530℃卷取时,组织中出现了3%的珠光体组织,抗拉强度低于性能指标10 MPa。而在400℃卷取时,组织中出现了3%的块状马氏体组织,使得屈服强度低于性能指标20 MPa;抗拉强度提高到690MPa。Mo元素含量提高,促进针状铁素体转变,实验钢淬透性提高,有利于获得M/A岛组织,保证获得高强度低屈强比性能。Nb元素含量提高,细晶强化和析出强化作用更明显。     

Effects of drawing strain and post-annealing conditions on microstructural evolution and tensile properties of medium- and high-carbon steels

Variation in the microstructure and tensile properties with the warm drawing strain and subsequent annealing of 0.45 wt% C (45C) medium-carbon steel and 0.82 wt% C (82C) high-carbon steel was investigated. The morphology and size of ferrite and pearlite changed considerably with applied strain during drawing and with annealing temperature and time, which made the tensile properties of the drawn steels vary considerably. With increasing drawing strain to ~2.5, the yield strength increased significantly from 393 to 1332 MPa for the 45C steel and from 673 to 1876 MPa for the 82C steel; this was attributed mainly to the strain hardening caused by severely deformed ferrite and/or enhanced particle hardening caused by cementite segmentation. During annealing of heavily drawn steels, ferrite grain growth and coarsening of broken cementite particles occurred simultaneously, which caused effective recovery of the ductility of severely drawn steels. These results demonstrate that the tensile strength and elongation of medium- and high-carbon steel wires can be tuned considerably by controlling the extents of drawing and annealing, thereby widening their applicability and facilitating their manufacture to match their mechanical properties to each application.     

回火温度对Ti-V-Mo微合金化马氏体钢析出相和力学性能的影响

利用微观分析和物理化学相分析法,对不同回火温度(550,600,650 ℃)保温1 h后的Ti-V-Mo微合金化马氏体钢的组织和析出相表征,并进行了强化分量的计算。结果表明,在600 ℃回火时具有最佳的综合力学性能:抗拉强度为1298 MPa,屈服强度为1286 MPa,伸长率为14%。强化分量计算结果表明:析出强化和细晶强化是主要的强化方式,约占总强度的40%和30%,其中析出强化分量σ_p为517 MPa,由5 nm以下的(Ti,V,Mo)C粒子(质量分数22%)提供。回火温度由550 ℃升高到600 ℃,抗拉强度和屈服强度均有增加,同时伸长率变化不大,其主要原因是σ_p对屈服强度的贡献量提高,在提高强度的同时改善了塑性。     

非调质CT80连续油管用钢的控轧控冷工艺

利用Gleeble-1500热模拟试验机进行了控轧控冷热模拟试验,分析了非调质CT80连续油管用钢的精轧变形温度、冷却速度和卷取温度对试验钢组织与性能的影响规律。基于控轧控冷热模拟试验结果,设定了试验钢实验室轧制工艺,在终轧温度830℃、冷却速度46℃/s和卷取温度450℃轧制工艺条件下,获得了具有针状铁素体+贝氏体+少量M/A岛组织构成的成品钢板,其屈服强度620 MPa,抗拉强度754 MPa,伸长率29.2%,屈强比0.82,各项性能均满足CT80连续油管用钢力学性能要求。     

Effect of alloying elements and coiling temperature on the recrystallization behavior and the bainitic transformation in TRIP steels

The effects of alloying elements and coiling temperature on recrystallization behavior and bainitic transformation were investigated based on 0.07C-Mn-Cr-Nb steel with a low carbon equivalent. Based on the ferrite recrystallization behavior, the proper intercritical annealing temperature of all studied steels was suggested to produce TRIP steel with good strength and elongation balance. All steels coiled at 550 °C showed much faster ferrite recrystallization behavior than steels coiled at 700 °C. In addition to the coiling temperature, the effect of increasing carbon content on the ferrite recrystallization was minor at a coiling temperature of 550 °C, but much more prominent at a coiling temperature of 700 °C. The highest Mo added steel showed the best strength and elongation balance, and the highest carbon and Mo added steel showed the highest tensile strength at a coiling temperature of 550 °C. The steel containing a higher amount of elemental Al (0.7 wt.% Al) exhibited much better elongation than the lower Al added steel (0.04 wt.% Al) in TS 780 MPa grade, about 24 % and 19 %, respectively.     

冷却方式对Nb-Ti微合金钢组织和性能及沉淀行为的影响

两阶段控制轧制后,采用不同的冷却路径进行冷却,研究冷却路径对Nb-Ti微合金钢组织和性能及沉淀行为的影响.结果表明,超快冷+空冷冷却路径可获得细晶组织,晶粒平均尺寸约为7.76μm,屈服强度高达425 MPa,抗拉强度高达500 MPa.超快冷+炉冷试样中存在细小的沉淀粒子,沉淀粒子尺寸主要集中在2—7 nm,而超快冷+空冷试样中只存在少量球形沉淀粒子,轧后直接空冷可获得相间沉淀粒子.不同冷却路径获得的热轧板在700℃下退火300 s后,沉淀粒子发生明显的粗化;退火处理后,超快冷+炉冷试样的晶粒平均尺寸减小为6.47μm,相对于退火前,其屈服强度和抗拉强度分别增加50和30 MPa、强度的增加主要源于细晶强化.对于含0.03%Nb(质量分数)的Nb-Ti微合金钢,由于沉淀粒子的体积分数有限,因此细晶强化效果远高于沉淀强化效果,强度的变化与晶粒尺寸的变化具有很好的对应性.另外,加工硬化指数与晶粒尺寸密切相关.随着晶粒平均尺寸的增加使加工硬化指数增加.     

铁素体—珠光体型非调质钢及其控锻控冷技术

综述了国内外铁素体－珠光体型非调质钢发展现状，探讨了锻造温度、锻造变形量、变形速率及锻后冷却速度对非调质钢强韧化的影响，指出了旨在控制先共析铁素体组织参数和沉淀硬化效应的控锻控冷新技术。为稳定锻造用铁素体－珠光体型非调质钢性能，推动其规模化生产应用，开拓了前景。     

冷却方式对Nb-Ti微合金钢组织和性能及沉淀行为的影响

两阶段控制轧制后,采用不同的冷却路径进行冷却,研究冷却路径对Nb-Ti微合金钢组织和性能及沉淀行为的影响.结果表明,超快冷+空冷冷却路径可获得细晶组织,晶粒平均尺寸约为7.76μm,屈服强度高达425 MPa,抗拉强度高达500 MPa.超快冷+炉冷试样中存在细小的沉淀粒子,沉淀粒子尺寸主要集中在2-7 nm,而超快冷+空冷试样中只存在少量球形沉淀粒子,轧后直接空冷可获得相间沉淀粒子.不同冷却路径获得的热轧板在700℃下退火300 s后,沉淀粒子发生明显的粗化;退火处理后,超快冷+炉冷试样的晶粒平均尺寸减小为6.47μm,相对于退火前,其屈服强度和抗拉强度分别增加50和30 MPa,强度的增加主要源于细晶强化.对于含0.03%Nb(质量分数)的Nb-Ti微合金钢,由于沉淀粒子的体积分数有限,因此细晶强化效果远高于沉淀强化效果,强度的变化与晶粒尺寸的变化具有很好的对应性.另外,加工硬化指数与晶粒尺寸密切相关,随着晶粒平均尺寸的增加使加工硬化指数增加.