Tempering characteristics of a vanadium containing dual phase steel

Dual phase steels are characterized by a microstructure consisting of ferrite, martensite, retained austenite, and/or lower bainite. This microstructure can be altered by tempering with accompanying changes in mechanical properties. This paper examines such changes produced in a vanadium bearing dual phase steel upon tempering below 500 °C. The steel mechanical properties were minimally affected on tempering below 200 °C; however, a simultaneous reduction in uniform elongation and tensile strength occurred upon tempering above 400 °C. The large amount of retained austenite (≅10 vol pct) observed in the as-received steel was found to be essentially stable to tempering below 300 °C. On tempering above 400 °C, most of the retained austenite decomposed to either upper bainite (at 400 °C) or a mixture of upper bainite and ferrite-carbide aggregate formed by an interphase precipitation mechanism (at 500 °C). In addition, tempering at 400 °C led to fine precipitation in the retained ferrite. The observed mechanical properties were correlated with these microstructural changes. It was concluded that the observed decrease in uniform elongation upon tempering above 400 °C is primarily the consequence of the decomposition of retained austenite and the resulting loss of transformation induced plasticity (TRIP) as a contributing mechanism to the strain hardening of the steel. B. V. N. RAO, formerly Senior Research Engineer, Analytical Chemistry Department, General Motors Research Laboratories     

The formability of a dual phase steel

The mechanical properties and the forming behavior of a dual phase sheet steel have been assessed. The effect of downgaging on forming properties was especially considered because of its importance in lighter weight, energy saving materials. A lower bound method has been used to determine the forming limit diagrams of the steel. Results have shown that the effect of sheet thickness on the stretch formability of the dual phase steel is comparable to some results published previously but not to others. The discrepancy could be due to variations in composition, processing and method used to determine the forming limit diagram of the steel. New formability curves and design guidelines for the steel have been obtained using a recently developed technique for the assessment of formability under simultaneous stretching and bending conditions.     

The deformation behavior of a vanadium-strengthened dual phase steel

A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when a V containing HSLA steel is cooled in a controlled manner from either the austenite or austenite plus ferrite phase fields. Such a heat treatment results in the pearlite regions and carbide particles of the standard V steel being replaced by martensite; this leads to a decrease in the yield stress and an increase in ductility while the tensile strength is essentially unchanged. The fatigue of dual phase steels is slightly superior in the high strain life (ductility controlled) region and slightly inferior in the low strain life (yield dominated) region when compared to standard V steel. The replacement of the pearlite and cementite particles which can nucleate cracks, by more ductile martensite islands results in improved Charpy impact properties. The strength and the ductility of the dual phase materials is shown to be in agreement with a theory of composites with two ductile phases. This theory then allows one to understand the relative importance of various microstructural features in controlling strength and ductility. In this way it is found that the key to the superior elongation (at a constant tensile strength) is largely due to the high strength (fine grained), highly ductile ferrite matrix.     

Tempering of steel

Tempering of martensitic steels involves the segregation of carbon, the precipitation of carbides, the decomposition of retained austenite, and the recovery and recrystallization of the martensitic structure. Because these several reactions overlap and occur on such a fine scale, it is only recently that our knowledge of the resulting structures has become reasonably complete. Our present understanding of the processes involved in the tempering of iron-carbon martensites and how they are affected by alloying elements is reviewed.     

Cr-Mo系列钢种质量研究

为满足汽车工业对高档齿轮用钢的需求 ,北满特钢系统研究了Cr-Mo系列钢种的冶金特性 ,成功开发了此系列钢 ,并使其各项技术指标均达到了日本同类产品的水平 ,且取得了良好经济效益。     

Tempering behaviour of a dual-phase low-alloy steel

The tempering behaviour of a dual phase steel of 0.08% C, 1.21% Mn, 1.00% Si, 0.42% Cr, and 0.41% Mo composition with two different martensite contents of 30 and 52%. (obtained by intercritical treatments at 820 and 860°C, respectively) has been studied. The ultimate tensile strength decreased and percentage elongation increased continuously with increasing tempering temperature up to 600°C for both intercritical treatments. The yield strength has, however, increased up to 300°C, beyond which it decreased for the steel with 30% martensite. In contrast it remained almost constant for 52% martensite up to 300°C, beyond which it decreased. The martensite of dual-phase steel for both the intercritical treatments has undergone microstructural changes on tempering that are akin to those of fully martensitic low carbon steels. The SEM fractographs from the as-quenched specimens indicate that the tensile specimens failed by microvoid coalescence with the martensite areas appearing facetted and featureless while those for 600°C tempered condition by the formation of equiaxed dimples.     

Plastic behavior of dual phase steel following plane-strain deformation

Dual-phase, high-strength steel sheet has been prestrained in plane-strain tension. Residual hardening and ductility properties were evaluated by performing subsequent uniaxial tensile tests in either co-axial or noncoaxial principal strain axis orientations. In contrast to similar work on aluminum-killed 1008/1010 steel sheet, only minor changes were found in the subsequent flow behavior of dual-phase steel, and no significant difference was found between the two orientations. The small effect of an abrupt strain path change observed in this study is consistent with the low work hardening rate of this alloy.     

Temper embrittlement in a 3-pct Cr-Mo turbine disc steel

The kinetics of isothermal temper embrittlement have been studied in a 3 pet Cr-Mo turbine disc steel at tempering temperatures from 450 to 580‡C. In the cast examined embrittlement results principally from the segregation of phosphorus to prior austenite grain boundaries and it is shown that the present results are consistent with an equilibrium segregation process. These data have been used to predict the degree of embrittlement resulting from continuous cooling from the tempering temperature and the results have been compared with laboratory tests for a wide range of cooling rates. The probable effect of post-tempering cooling rate on toughness and the feasibility of using retempering treatments as a means of rehabilitating temper embrittled components are also considered.     

热镀锌双相钢的发展和应用

简要介绍了热镀锌和双相钢的发展历程,说明了发展热镀锌双相钢的必要性。介绍了国内外的热镀锌双相钢的生产和应用情况。     

Tempering of steel during laser treatment

This study concentrates on the annealing effects of a laser-treated area by a successive laser pass in a plain carbon, a chromium, and a tungsten steel. Transmission electron microscopy (TEM), optical microscopy (OM), and wear and hardness measurements revealed substantial effects on the properties and microstructure of the plain carbon and tungsten steel which have a partly martensitic cellular structure after the first laser treatment. Close to the melt zone, martensite was again formed, whereas further away, nucleation of carbides was observed. The hardness increased in the former area and decreased in the latter. An analytical model sustained that the softening is due to carbon diffusion and martensite tempering. The softer zone in the tungsten steel exhibited a decrease of wear rate relative to the not-annealed zone in contrast to the plain carbon steel. Besides some diffusion, no annealing effects were found in the chromium steel which possessed an austenitic cellular structure after the first laser treatment. Formerly Graduate Students, Department of Applied Physics, Materials Science Centre, University of Groningen     

Effect of post rolling stress on phase transformation behavior of microalloyed dual phase steel

The slow phase transformation of microalloyed dual phase steel makes the nonuniform stress and temperature fields during the post rolling cooling process have a significant impact on the phase transformation process.Given the relatively slow phase transformation of DP780 steel within the microalloyed dual phase steel series,the influence of stress on the phase transformation behavior of DP780 steel was investigated.To quantify the nonuniform thermal and stress conditions in the steel coil,a thermo-mechanical coupled finite element model of the hot-rolled strip cooling process was established.Based on the simulation data,DP780 steel was chosen as the research material,and Gleeble 3500 thermal simulation equipment was used for experimental validation.The thermal expansion curves were analyzed through regression to establish the dynamic model of DP780 steel phase transformation under stress.Subsequently,metallographic analysis was conducted to determine phase transformation type and grain size of DP780 steel.The results confirmed that the stress promotes the occurrence of semi-diffusion-type bainite transformation.Furthermore,an appropriate level of stress facilitates the growth of bainitic grains,while the increased stress inhibits the growth of ferritic grains.     

V微合金化热轧双相钢的组织与性能

研究了V微合金化热轧试验钢的组织与性能,并对V(C,N)粒子的析出及其对强度的影响进行了分析.结果表明,试验钢的组织由多边形铁素体、贝氏体和少量马氏体组成,多边形铁素体的平均晶粒尺寸约7μm,试验钢的抗拉强度高于570 MPa,综合力学性能良好.通过透射电镜观察发现,试验钢中有V(C,N)粒子的析出,析出方式主要是在铁素体内随机析出和沿位错线析出.由于第2相粒子的析出,试验钢的强度提高了约45 MPa.     

Microstructure failure in ferrite-martensite dual phase steel under in-situ tensile test

To investigate microstructure failure in ferrite-martensite dual phase steel,in-situ observations were performed on multiple plate DP800 specimens during uniaxial tensile tests.Microstructure evolution of the observed region was investigated in details.The experimental data showed that micro-cracks in various regions differed in the initiation time,and micro-failures mainly occurred from the locations with typical characteristics of stress concentration (i.e.ferrite interiors,the interfaces of ferrite-martensite grains and the martensite-martensite interfaces).Growth of micro-crack generally experienced the following stages:cracking from martensite boundaries,tiny particles in ferrite interiors,or martensite interiors,propagating in ferrite,bypassing martensite boundaries,or passing through martensite-martensite interfaces,finally ending on martensite boundaries.Martensite was one important source of micro-failure and changed the propagation of micro-cracks significantly.Microstructure deformation was inhomogeneous in the stage of plastic deformation.     

Dry sliding friction and wear in plain carbon dual phase steel

In view of the potential of plain carbon dual phase (DP) steel as wear resistant material, the wear and friction characteristics of this steel, which consists of hard martensite islands embedded in a ductile ferrite matrix, have been investigated and compared with those observed in plain carbon normalized (N) steel that has the same composition of 0.14 wt pct carbon. Dry sliding wear tests have been carried out using a pin-on-disk wear testing machine at normal loads of 14.7, 24.5, and 34.3 N and at a constant sliding velocity of 1.15 m/s. Weight loss in the samples has been measured over time on the same specimen, and the variation of cumulative wear loss with sliding distance has been described by two linear segments, for both the DP and the N steel. At these loads, the mechanism of wear is primarily oxidative, although subsurface cracking and delamination wear could also be observed in a few places. The second linear segment could result from a dynamic steady state wear of the transfer layer of compacted oxide wear debris on the sliding surfaces. The wear rate calculated on the basis of the first linear segment varies linearly with normal load, which is indicative of Archard’s law, and it is significantly lower for the DP steel than for the N steel. The wear rate calculated on the basis of the second linear segment, however, varies with load linearly for the DP steel but nonlinearly in the N steel. In the first linear segment, the wear coefficient is about 0.39 × 10⁻⁴ for the DP steel and is 0.40 × 10⁻⁴ for the N steel. Higher hardness and, consequently, a lower real area of contact in the DP steel at all the loads have compensated for the lower wear rates, and have resulted in a wear coefficient similar to that in the N steel. The steady state wear coefficient from the second linear segment is 0.29 × 10⁻⁴ for the DP steel at all loads; for the N steel, these are 0.21 × 10⁻⁴ and 0.64 × 10⁻⁴, respectively, for lower and higher loads.     

High cycle fatigue behavior of gas-carburized medium carbon Cr-Mo steel

High cycle fatigue properties of gas-carburized 4140 steel were assessed to compare with those of 8620 steel which is widely used as a carburizing steel. Fatigue limit was evaluated associated with microstructure, case depth, and distribution of retained austenite and compressive residual stress near the surface. Test results indicated that the reheat quenching method of 4140 and 8620 steels produced a reduction in grain size, retained austenite level, and compressive residual stress at the surface and an increase in fatigue limit. The fatigue limit of direct-quenched 4140 steel shows substantially lower value than that of direct-quenched 8620 steel due to larger grain size of direct-quenched 4140 steel. However, the fatigue limit of reheat-quenched 4140 steel is greatly improved and is comparable to the reheat-quenched 8620 steel. This is attributed to the larger reduction ratio in grain size and deeper case depth of reheat-quenched 4140 steel as compared to direct-quenched and reheat-quenched 8620 steels.     

合金元素对Cr-Mo系锅炉钢性能的影响

叙述了不同工作温度下使用的锅炉钢种类,分析了Ｃｒ、Ｍｏ、Ｖ、Ｎｂ、Ｎ等合金元素对提高锅炉钢蠕变强度和焊接性能的影响。     

冷轧双相超高强钢氢致延迟断裂门槛应力研究

以抗拉强度为1 000 MPa级的汽车用冷轧双相钢为对象,采用电化学充氢和恒载荷拉伸试验方法,研究了材料在不同应力和充氢条件下的延迟断裂行为以及材料延迟断裂门槛应力与氢浓度之间的关系.结果表明,在充氢条件下材料延迟断裂时间随着应力的增大而缩短,延迟断裂的门槛应力随着充氢电流密度增大而下降,延迟断裂门槛应力与临界可扩散氢浓度的对数呈线性关系.     

Effect of thermomechanical treatment on structure and mechanical properties of Mo-bearing dual phase steel

The effects of hot rolling of a dual phase steel in the (α + γ) range on microstructure and mechanical properties was investigated by using two thermomechanical (TMT) routes. The first consisted of heating A_c3, soaking, cooling to deformation temperature in the (α + γ) range. The second comprises heating to deformation temperature in the (α + γ) range, followed by rolling and quenching. Parameters varied were temperature (with the first route) and extent of deformation (with the second). The microstructures were characterised by optical and transmission electron microscopy. The results indicate a distinct difference in the final structure and properties due to the two different TMT routes. The first TMT route resulted in a greater amount of ferrite, finer lath width of martensite, finer ferrite grain size and increased density of dislocations. The strength properties decreased, the YS/UTS ratio decreased and ductility increased with the increase in the extent and temperature of deformation. However, TMT route 2 resulted in an increase in the amount of martensite, finer ferrite grain size, decrease in the martensite lath width and increased dislocation density. The strength properties increased, YS/UTS ratio increased and ductility decreased with increase in the extent and temperature of deformation.     

Fatigue of a dual- phase steel

A study has been made of the fatigue of a V containing dual-phase steel, whose tensile strength is equivalent to that of SAE 980X high strength, low-alloy (HSLA) steels, as a function of prestrain. It is found that the cyclic stress-strain curve, strain-life response and notch sensitivity are little affected by pre-strains of up to 8 pct: This is in contrast to monotonie flow strength which increases substantially with prestrain. The fatigue performance of the dual-phase steel, while different in detail from that of other HSLA steels, is intermediate between that for SAE 950X and 980X steels. However, the notch fatigue behavior is equivalent to that of 980X steels. The fatigue response of dual-phase steel can be understood in terms of its high rate of work hardening which is a consequence of its ferrite plus martensite microstructure.