Effect of carbon and microalloy additions on hot-stamped boron steel

Hot stamping was performed with seven novel boron steels, exhibiting various carbon contents and microalloy additions of Mo, V and Ni compared to the standard 22MnB5 boron steel. The lower carbon content of 15MnB5 resulted in total elongation in excess of 11.0%. The higher carbon content of 38MnB5 resulted in ultimate tensile strength in excess of 2000?MPa. The vanadium addition resulted in ultimate tensile strength in excess of 1600?MPa combined with total elongation in excess of 11.0%. The carbon and microalloy additions were concluded to give rise to improved mechanical properties in hot-stamped boron steel for automotive body engineering applications.     

Development of carbon–manganese–chromium steels for automotive hot stamping technologies

The hot stamping potential of three commercial carbon–manganese–chromium steels was demonstrated. The steels were selected with the intention of producing ultrahigh strength ferritic–martensitic dual phase microstructures from hot stamping heat treatment, exhibiting superior tensile properties for ‘impact energy absorptive’ crash performance compared to the conventional carbon–manganese–boron steel for automotive hot stamping technologies, 22MnB5. Compared to 22MnB5, each of the carbon–manganese–chromium steels provided at least twice the total elongation, yet with a relatively small loss to ultimate tensile strength. It was concluded that the carbon–manganese–chromium steels provide superior tensile properties for impact energy absorptive crash performance, while exhibiting ‘lean’ chemical compositions compared to previously investigated steels with similar aims and offer opportunities for use in the ‘tailor welded blank’ concept.     

Dual phase versus TRIP strip steels: comparison of dynamic properties for automotive crash performance

Abstract

The need to simultaneously reduce vehicle emissions and increase the safety of passengers is encouraging the automotive industry to incorporate new technologies and materials into today's vehicles. To remain competitive, the steel industry has developed steel grades with increased energy absorbing properties allowing down gauging of body in white components to address the competition from alternative materials such as aluminium alloys and composites. Two of the more important developments are the introduction of dual phase (DP) and transformation induced plasticity (TRIP) grades for the automotive industry. These grades offer superior strength/formability and work hardening properties compared to conventional high strength grades of similar tensile strength. Utilising thinner gauge components with increased energy absorbing properties would permit addressing the mass/safety issues by the automotive industry. This paper relates the crash performance of a range of both commercial and experimental DP and TRIP grades. Dynamic tensile testing was performed at low and very high strain rates within the range of 0·001–200 s^?1, to allow an extensive analysis of the effect of strain rate on the material properties. Crash testing was also performed on closed top hat sections at low, medium and high strain rates and the results compared to the dynamically tested tensile specimens. This study helped clarify the enhanced performance offered by high strength DP and TRIP strip steel grades during dynamic tensile testing and impact loading conditions. This advantageous behaviour is attributed to the favourable microconstituents present in these novel grades and their deformation characteristics. This paper concentrates only on the crash properties measured from dynamic tensile tests. The microstructural analysis is presented in a separate publication.     

Semi-hot Stamping as an Improved Process of Hot Stamping

Reducing the forming load,deletion of springback,increasing the formability of sheets as well as producing high strength parts are the main reasons to apply hot stamping process.Hot stamping process and 22MnB5 steels are the state of the art process and grades,respectively;however novel processes and steel grades are under considerations.In the current research,behavior of the steel grade MSW1200 blanks under semi and fully hot stamping processes was characterized.During semi-hot stamping process,the blank was firstly heated to a temperature of about 650℃ and then formed and quenched in the die assembly,simultaneously.Microstructure and mechanical properties of semi and fully hot stamped blanks were studied and the results were compared with those of normally water/air quenched blanks.The hot stamped blanks attained the strength values as high as water quenched blanks.The highest ductility and consequently,the best formability were achieved for the blank which had been semi-hot stamped.It was concluded that for the mentioned steel,semi-hot stamping process could be considered as an improved thermo-mechanical process which not only guaranteed a high formability,but also led to ultra high strength values.     

Novel cold-rolled martensitic ultra-high-strength steels for roll forming technologies

Roll forming is one of the fastest developing forming technologies for martensitic ultra-high-strength steels in the automotive industry. As the automotive industry continues to demand materials exhibiting higher strength in addition to improved formability, development of novel martensitic ultra-high-strength steels for roll forming technologies is a priority topic in the steel industry. Six such novel steels were developed, exhibiting ultimate tensile strength from 1500 to 2000?MPa, total elongation from 4% to 11%, bend ratio from 5t to 2t and hole-expansion capacity from 18% to 80%. It was concluded that the novel steels exhibit very attractive properties for roll forming technologies and provide low cost down-gauging opportunities with improved crashworthiness for the automotive industry.     

Dual phase versus CMn strip steels: comparison of quasi-static and dynamic connection properties for road safety barriers

Abstract

The use of high strength steel grades in automotive applications has been widely recorded. This is due largely to vehicle weight reduction programmes as well as increases in vehicle crash safety legislation. This represents the steel industry’s response to the challenge that vehicle components manufactured from steel could get replaced with alternative materials, such as aluminium and polymers. Consequently, new high strength steel grades have been developed to offer credible alternatives. Recently, the UK government has released a new specification, BS EN 1317-1-2-3-4: Road Restraint Systems, to which all new safety barrier designs have to comply. However, much of this development and subsequent usage has been targeted at automotive manufacturers. Road safety barrier technology has not evolved in the same way when compared to vehicle technology. Therefore, a study has been undertaken to assess the outcome of using some of these novel high strength steel grades for the manufacture of road safety barrier components. Quasi-static and dynamic tensile testing at different velocities was undertaken. Representative connection coupons were used to understand the energy absorbing properties of a dual phase steel grade when compared to the current CMn steel grade. The present study presents some initial results as to the increased performance that could be gained from utilising new high strength steel grades for the production of road safety barrier systems.     

400MPa超级钢板材冲压性能的实验

对400MPa超级钢板材进行了拉伸试验、冷弯试验和金相组织检验,获得了超级钢板的硬化指数、塑性应变比、屈服强度、极限强度等力学性能,并分析了400MPa超级钢板的冲压成形性能,了解这些性能对超级钢冲压件在汽车行业里的应用与生产具有一定的指导作用.     

A novel ultra-strong hot stamping steel treated by quenching and partitioning process

In the present study, we design a novel hot stamping steel containing high amounts of C and Si and micro-alloying element (i.e. Nb). The steel was subjected to quenching and partitioning (Q&P) process. The new Q&P treated hot stamping steel exhibits a significantly improved mechanical property in terms of strength, ductility and impact toughness compared with the traditional 22MnB5 hot stamping steel. The influence of partitioning time on the microstructure and mechanical properties was investigated. The retained austenite (RA) fraction and the carbon content of RA significantly increased with higher partitioning times. With increasing partitioning time, the uniform elongation, total elongation, strength-ductility balance and impact energy was also remarkably enhanced. The maximum strength-ductility balance achieves around 23?GPa %.     

Sensitivity of novel 460 MPa proof strength titanium–vanadium microalloyed high strength low alloy steels to annealing linespeed and soak temperature

High strength low alloy steels are characterised by predominantly ferritic microstructures, strengthened by grain boundary and precipitation strengthening. Both of these strengthening mechanisms traditionally arise from the niobium addition. Increasing the niobium addition would theoretically increase strength. However, increasing niobium content above ~ 0.04 wt.% is not recommended in industrial practice due to narrowing of the annealing process window. Two novel grades exhibiting different additions of titanium and vanadium in place of the traditional niobium addition were investigated. Sensitivity to annealing linespeed and soak temperature was investigated to conclude whether a practically achievable process window exists and moreover, to conclude whether proof strength in excess of 420 MPa could be achieved while satisfying the maximum ultimate tensile strength and minimum total elongation specifications of CEN Grade HC420LA under European Standard EN 10268:2006. One of the two novel grades, exhibiting higher manganese and vanadium contents, met the minimum proof strength target, while almost satisfying the maximum ultimate tensile strength and minimum total elongation specifications. However, the annealed microstructure was found to be partially recrystallised, which is not recommended in industrial practice. Moreover, sensitivity to annealing linespeed and soak temperature was considered too great to obtain a practically achievable process window.     

Forgeability test of extruded Mg–Sn–Al–Zn alloys under warm forming conditions

Magnesium (Mg) alloys have been thoroughly researched to replace steel or aluminum parts in automotives for reducing weight without sacrificing their strength. The widespread use of Mg alloys has been limited by its insufficient formability, which results from a lack of active slip systems at room temperature. It leads to a hot forming process for Mg alloys to enhance the formability and plastic workability. In addition, forged or formed parts of Mg alloys should have the reliable initial yield and ultimate tensile strength after hot working processes since its material properties should be compatible with other parts thereby guaranteeing structural safety against external load and crash. In this research, an optimal warm forming condition for applying extruded Mg–Sn–Al–Zn (TAZ) Mg alloys into automotive parts is proposed based on T-shape forging tests and the feasibility of forged parts is evaluated by measuring the initial yield strength and investigating the grain size in orientation imaging microscopy (OIM) maps.     

热成形淬火对QP1180/22MnB5激光拼焊板组织与性能的影响

目的 对QP1180和22MnB5激光拼焊板进行热成形试验,以解决超高强钢板材焊后的软化问题。方法 选择QP1180和22MnB5异种高强钢作为母材进行激光自熔焊,对焊后的激光拼焊板进行热成形试验,通过体式显微镜、扫描电子显微镜、液压拉伸试验机和维氏硬度计等手段,分析热成形前后激光拼焊板微观组织和力学性能的变化。结果 与焊态拉伸试样相比,热成形试样抗拉强度提高了135%,断后伸长率降低了55%,拉伸试样都在22MnB5母材处断裂,均为塑性断裂。在热成形后,对焊接接头进行组织分析,发现QP1180母材区马氏体含量增加,22MnB5母材区和临界热影响区组织由珠光体和铁素体转变为马氏体,焊接接头热影响区各亚区的组织均转变为大小不同的板条马氏体。硬度测试结果表明,焊态试样焊接接头的QP1180临界区存在软化现象,硬度值最低为335HV,22MnB5侧硬度值由母材处向焊缝升高,母材硬度最低为170HV;而在热成形后,QP1180临界区软化现象消失,硬度值趋于平缓,22MnB5母材处硬度比焊态试样硬度高了2倍。结论 与焊态试样相比,经热成形后激光拼焊板的焊后软化问题得到了解决。     

可淬火硼钢板热冲压成形实验研究

为研究淬火加热温度、保温时间及冷却水流速等热冲压工艺参数对热冲压零件力学性能及微观组织的影响规律,通过在不同工艺参数条件下进行弯曲件热冲压工艺试验,测量弯曲件的力学性能并观察其金相组织.结果表明,在所设计的模具上可实现高强硼钢热成形零件的有效淬火,热冲压弯曲件的抗拉强度可达到1500MPa以上,主要形成均匀细小的马氏体组织.确定了热冲压工艺参数的选择范围.     

A Comparative Study on the Production of a Hat Profile by Roll Forming and Stamping

Lightweight design using high-strength aluminum alloys has gained importance due to the continuing need for weight reduction and increasing crash safety requirements in the automotive industry. There are various manufacturing processes available for processing high-strength aluminum alloys. Herein, the production of high-strength aluminum parts by roll forming and stamping based on the example of an AA7075-T6 hat profile is compared. Roll forming represents a continuous manufacturing process, while stamping is a discontinuous process. Different process routes (T6, W-Temper and O) for roll forming as well as for stamping (T6, W-Temper, O and hot forming) are in focus of the investigation. Fundamental differences of the forming processes and the tempering condition are observed and criteria for the choice of the manufacturing process and process route are presented. The temperature-supported process routes improve the poor cold formability of AA7075 alloy and thus enhance the process window. Potential is offered for both manufacturing processes by applying tailored properties achieved through targeted quenching.     

Development of high strength hot rolled steel sheet for wheel disc application

Abstract

A hot rolled steel with unique combination of high strength and excellent ductility coupled with high hole expansion ratio has been developed for automotive applications using a new metallurgical approach. The steel chemistry and processing parameters were designed in such a way that austenite–ferrite phase transformation takes place at lower temperature with interphase precipitation of very fine carbides. A fine ferritic microstructure was produced with an average grain size of 3·4 μm resulting into attractive mechanical properties. Interphase precipitation was used as one of the major strengthening mechanisms apart from grain refinement. This led to an achievement of tensile strength as high as 800 MPa (min.) with an excellent stretch formability and low impact transition temperature. This is a potential grade with unique features which can have wider applications.     

Materials selection for hot stamped automotive body parts: An application of the Ashby approach based on the strain hardening exponent and stacking fault energy of materials

Complex stamping operations are becoming widespread in the automotive industry to produce vehicle body parts with adequate mechanical strength and reduced wall thickness. The need for weight reduction drives the development of new metallic materials capable of achieving a good balance between formability and mechanical properties. Advanced high strength steels play a major role in this scenario. The aim of this work was to develop a materials selection strategy for hot stamped automotive body parts using the Ashby approach. The selection process was based on the formability of metallic alloys derived from two fundamentals materials properties, the strain hardening exponent and the stacking fault energy.     

基于AUTOFORM模拟的顶盖前边梁加强板热冲压工艺

目的 研究顶盖前边梁加强板热冲压成形过程中不同参数对零件成形的影响规律,为生产提供技术指导。方法 通过AUTOFORM模拟零件热成形过程,并对模拟结果对比分析,得到了板料尺寸、成形温度、保压时间、成形压力、冷却时间等工艺参数对成形性能的影响。结果 优化了板料尺寸及板料定位,并通过调节模具间隙保证了零件的成形性,得到零件最大减薄率小于15%,平均抗拉强度达到1450 MPa,平均硬度达到HV475,从而确定了顶盖前边梁加强板的热冲压工艺。结论 结合此有限元分析方法试制出抗拉强度大于1450 MPa的合格零件,为此类零件批量化生产提供理论依据。     

Hot-rolled and continuously cooled bainitic steel with good strength–elongation combination

The current work demonstrates the microstructural evolution and mechanical property evaluation of a newly designed steel composition after hot rolling in laboratory-scale rolling mill, followed by continuous cooling. The steel thus developed has typically about 80% carbide-free bainite; about 20% retained austenite and can deliver ～1400?MPa ultimate tensile strength along with more than 20% total elongation. The presence of ultra-fine bainite plates (～100–130?nm thick) with high dislocation density was thought to be responsible for ultra-high strength. Excellent ductility at such strength level could be due to the presence of sufficient amount of retained austenite (～20%) thermally stable at room temperature but starts transforming to martensite during deformation exhibiting transformation-induced plasticity effect.     

Ductile ultra high strength hot stamped steel obtained by Q&P treatments

Current hot stamped steels can offer ultra high strength (1500?MPa) with limited elongation (～5%). Research is ongoing to develop hot stamped steels with similar strength and improved ductility by using various microstructural approaches. The current work describes in depth the characterisation of microstructural features of a steel that was processed via a quenching and partitioning method in a modified hot stamping thermal cycle to deliver a good combination of strength and ductility. It was found that the microstructure thus produced is very complex comprising many different phases. As it is also beyond the ability of one single technique to fully characterise it, several techniques were employed in order to properly understand the various features of the different constituents of this microstructure.     

超高强度马氏体时效钢的力学行为与微观组织演化的关系

用高分辨透射电镜(HRTEM)和原子探针层析技术(APT)等手段研究了2.4 GPa级超高强度马氏体时效钢在时效过程中析出相的演化规律及其与材料力学性能的关系。对组织观察的结果表明,马氏体时效钢在时效过程中析出相的演化规律分为三个阶段：时效初期富Ni和富Ti团簇的形成、峰时效期金属间化合物Ni₃Ti及其界面处富Mo相的形成、过时效阶段Ni₃Ti的粗化和富Mo相过渡为Ni₃Mo。力学性能的实验结果表明,随着时效时间的延长抗拉强度呈现先提高后降低的趋势,时效时间为4 h时抗拉强度达到最大值2560 MPa。断裂韧性呈现与抗拉强度相反的变化趋势,时效时间为4 h时的断裂韧性值最低,仅为20 MPa·m^1/2。根据不同时效阶段材料中析出相的演化规律,探讨了马氏体时效钢的力学行为与析出相的关系。