Experimental study on the forming characteristics of 1.5 GPa ultrahigh-strength dual-phase steel

The DP1500 steel series successfully produced by Baosteel is a marked improvement over the cold-rolled hot-dip galvanized dual-phase steel series.Sufficient parameter data related to forming characteristics are needed for the successful application of dual-phase steel series in engineering structures.Therefore,differences in the mech-anical properties,forming limit,hole expansion ratio,and stretch bend limit of the 1.5 GPa ultrahigh-strength steel,including DP1500,QP1500,and MS1500,have been systematically studied.Results show that the DP1500 exhibits good plastic deformation performance and approximately 5% uniform elongation,and its true major strain minimum on the forming limit curve(FLC_(0)) value is approximately 0.083,which is higher and lower than the FLC_(0) values of MS1500 and QP1500 of the same strength grade,respectively.DP1500 also exhibits good flanging and pore expansion capabilities and superior performance to QP1500 and MS1500.The minimum radius-to-thickness(R/T) ratio(1.4) of DP1500 in the 90° bend tests transverse to the rolling direction is between the R/T ratios of MS1500 and the QP1500.Overall,the formability performance of DP1500 is between that of MS1500 and QP1500.Its excellent crash energy absorption and formability performance render it a suitable structural component,and it has been successfully tested and verified on a typical complex ultrahigh-strength steel skeleton structure.

Electroslag Refining of CrNiMoWMnV Ultrahigh-Strength Steel

Increasing demands for ultrahigh-strength steels in commercial as well as military applications have raised interest in finding alternatives to the high-cost high-alloyed steel and super-alloys currently used, e.g. the use of economic low-alloy compositions processed via low-cost air induction melting and electroslag refining (ESR). In this work the yield of alloying elements and the removal of the impurities nitrogen, sulphur and phosphorus as a result of electroslag refining (ESR) in a newly developed CrNiMoWMnV ultrahigh-strength steel (UHSS) have been studied in relation to their activities in the molten metal pool. Six experimental heats of CrNiMoWMnV UHSS with different chemical compositions were designed, melted in an induction furnace (IF) and refined using ESR. This was followed by hot forging of the ingots at 1100°C to 950°C. ESR using a CaF2-CaO-Al2O3 slag system led to a high yield in Cr, Ni, Mo, W, Mn and V, while the yield of Si is low. The desulphurization of all six UHSS grades was pronounced with most of the sulphur removed either to the slag or by gas reactions. The degree of dephosphorization was only 5% irrespective of the steel composition. On the other hand, denitrification (removal of nitrogen) was achieved. It ranged from 8% to 63% depending on the steel composition. The yield of the alloying elements and removal of impurities from the steel during ESR depends on the chemical and physical properties of the ESR slag and the activity of the elements in the molten state, taking into account elemental interactions.

Relation between Modulus of Elasticity and Compressive Strength of Ultrahigh-Strength Mortar with Mixed Silicon Carbide as Fine Aggregate

Ultrahigh-strength mortar mixed surface-oxidized silicon carbide as a fine aggregate was prepared by means of press-casting followed by curing in an autoclave. The relation between modulus of elssticity up to 111 GPa and compressive strength up to 360 MPa of mortar mixed silicon carbide was discussed and it was revealed that the contributions of the aggregate hardness and of the interfacial strength between the aggregate and the cement paste on the elasticity of mortar were imporant.

Numerical investigation of welding residual stress in Q960 ultrahigh-strength steel multipass joints

This study investigates Q960 ultrahigh-strength steel as the research object.Based on software,a thermo-metallurgical-mechanical finite element model(FEM)is established to simulate the welding temperature field and residual stress distribution.At the same time,the hole-drilling(HD)method is used to measure the residual-welding stress distribution on the surface of the single-pass.Numerical simulation and experimental results show that the predicted value of numerical simulation agrees well with the experimentally measured value,which verifies the accuracy of the FEM.Based on the verification model,the surface and internal stress distribution characteristics of Q960 ultrahigh-strength steel during the multipass remelting of Q960 ultrahigh-strength steel considering solid-state phase transformation(SSPT)are analyzed.The results show that when SSPT is considered,after single-pass welding of Q960 ultrahigh-strength steel,the welded joint is dominated by tensile residual stress,and the peak stress is located in the heat-affected zone(HAZ).At the same time,the effect of SSPT can significantly reduce the size of the residual stress in the weld and affect the distribution of the lateral residual stress.Additionally,as the number of weld passes increased,the transverse residual stress at the center of the weld showed a“stepped”trend,and a local compressive stress peak appeared at the location of the HAZ.

THE VALENCE ELECTRON STRUCTURE OF AUSTENITE IN LOW ALLOY ULTRAHIGH-STRENGTH STEELS AND ITS INFLUENCES ON KINETICS OF PHASE TRANSFORMATION

Based on S. H. Yu's empirical electron theory of solids and molecules, the valence electron structure of austenite in low alloy ultrahigh-strength steels, 30CrMnSiNi_2A and 40 CrMnSiMoVA is established. The behavior of various main-added elements in the kinetics of phase transformation is discussed on the basis of C-Me segregation caused by the valence electron structure, The influence of alloying elements on the structure and morphology of transformed products is discussed from the viewpoint of interaction between the driving force of phase transformation and segregating force. Then on the basis of the valence electron structure of the alloy, the composition design of ultrahlgh-strength steels is discussed.

Carbide dissolution and austenite grain growth behavior of a new ultrahigh-strength stainless steel

The isothermal grain growth behavior for a new ultrahigh-strength stainless steel (UHSSS) is investigated in temperature range from 900 to 1150 ℃ and holding time range from 0 to 20 min. In the temperature range from 1000 to 1050 ℃, a bimodal grain size distribution was induced by different austenite grain growth rates which resulted from the weakened pin-ning effect by the partial dissolution of M6C particles along austenite grain boundaries. Further raising heating temperatures, M6C particles almost dissolved and the bimodal grain size distribution phenomenon became weakened, indicating that the austenite grain coarsening temperature of the new UHSSS was close to 1050 ℃. According to the present experimental results, a pragmatic mathematical model based on the Arrhenius equations was developed to predict the austenite grain growth process, which elaborated the influence of heating temperature, holding time and initial grain size on the austenite grain growth. Predictions for the new UHSSS presented a good agreement with experimental results.

Evolution mechanisms of microstructure and mechanical properties in a friction stir welded ultrahigh-strength quenching and partitioning steel

Ultrahigh-strength quenching and partitioning(Q&P) steels have attracted strong interests in the auto manufactory,while the comprehensive understanding in the microstructure and mechanical behavior of their welded joints is highly needed to enrich their applications.In the present work,it is designed to make an insight into these imperative conundrums.Equal strength Q&P 1180 steel joints to parent metal were successfully fabricated via friction stir welding(FSW) technique under different parameters. Apparent hardening and softening were observed in stir zone(SZ) and heat-affected zone(HAZ) respectively,whose microstructures strongly depended on the peak temperature and cooling rate during welding.The formation of fresh martensite was the main mechanism for the SZ hardening,while the decomposition of metastable phases played key roles in the microhardness drop of the HAZ.A heat source zone-isothermal phase transition layer model was proposed to clarify the impregnability of the joint strength under parameter variation.The dual-phase structure,nano-carbide particles,tempered initial martensite,and ultrafine-grained ferrite synergistically improved the strain hardening ability of the HAZ,which eventually resulted in the equal strength FSW joints.

Effect of cooling rate and composition on microstructure and mechanical properties of ultrahigh-strength steels

The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels(UHSSs)with different levels of C,Cr and Ni has been evaluated for the as-cooled and untempered condition.One UHSS had higher contents of C and Cr,while the other one had a higher Ni content.On the basis of dilatation curves,microstructures,macrohardness and microhardness,continuous cooling transformation diagrams were constructed as a guide to heat treatment possibilities.Cooling rates(CRs)of 60,1 and 0.01°C/s were selected for more detailed investigations.Microstructural characterization was made by laser scanning confocal microscopy,field emission scanning electron microscopy combined with electron backscatter diffraction,electron probe microanalysis and X-ray diffraction.Mechanical properties were characterized using macrohardness,tensile and Charpy V-notch impact tests.UHSS with the higher C and Cr contents showed lower transformation temperatures and slower bainite formation kinetics than that with the higher Ni content.Higher cooling rates led to lower volume fractions and carbon contents of retained austenite together with finer prior austenite grain size,as well as effective final grain size and lath size.These changes were accompanied by higher yield and tensile strengths.The best combinations of strength and toughness were obtained with martensitic microstructures and by avoiding the formation of granular bainite accompanied by proeutectoid carbides at low CR.For the cooling rates studied,UHSS with the higher C and Cr contents showed the higher hardness and strength but at the cost of toughness.

Microstructure and Mechanical Properties of an Ultrahigh-strength Titanium alloy Ti-4.5Al-5Mo-5V-6Cr-1Nb Prepared Using Laser Directed Energy Deposition and Forging:A Comparative Study

The application of titanium alloys in aerospace put forward the requirement for higher strength.Additive manu-facturing is a promising method for the efficient and economical processing of titanium alloys.However,research on the additive manufacturing of ultrahigh-strength titanium alloys is still limited.The mechanisms of microseg-regation for high alloying elements and poor plasticity are still not clear.In this study,an ultrahigh-strength titanium alloy Ti-4.5Al-5Mo-5V-6Cr-1Nb(TB18)was prepared using two methods:laser direct energy deposi-tion(LDED)and forging.The LDEDed alloy contains three zones with similar grain morphologies but different microstructure.The microsegregation of the alloy is limited due to the rapid solidification and almost eliminated after the thermal cycle and solution treatment.With stress relief treatment,the LDEDed alloy exhibits anisotropic mechanical properties.After solution and aging treatments,its ultimate strength is enhanced;however,its plas-ticity is relatively lower than that of the wrought alloy with equally high strength.The excellent balance of the strength and plasticity of the wrought alloy can be ascribed to the formation of𝛼WGB and multiscale𝛼laths,which provides enlightenment for optimizing the properties of the LDEDed alloy.

Effect of low-temperature ausforming on bainitic transformation and mechanical properties in ultrahigh-strength bainitic steel

Low-temperature ausforming(LT-AF)prior to bainitic transformation leads to a noticeable acceleration of bainitic transformation kinetics;however,the effect of LT-AF on the retained austenite(RA)features and the resulting mechanical properties is still unclear.LT-AF was applied to ultrahigh-strength bainitic steel before austempering.The deformation behavior and the resulting dislocation substructures were investigated by thermomechanical simulator and transmission electron microscopy(TEM).The planar dislocation structures produced during deformation at 350℃ accelerate the bainitic transformation kinetics during isothermal holding.The effect of LT-AF on the bainitic transformation kinetics and the features of RA was elucidated via dilatometer measurement,TEM,scanning electron microscopy,and X-ray diffraction.It is observed that LT-AF not only retains more RA content but also facilitates improved RA stability.This trend is mainly due to the large amounts of planar dislocations in RA and bainitic laths inherited from undercooled austenite caused by LT-AF,the decrease in bainitic sheaves size,and the increase in filmy RA content compared to the sample without ausforming.A large fraction of filmy RA with high stability and the refinement of bainitic sheaves obtained by LT-AF remarkably enhance the strain hardening capacity and achieve significantly better ductility compared to the directly austempered sample.

掺氢氧化钙对超高强混凝土力学性能影响的机理

鉴于含矿物掺合料较多的超高性能混凝土(Ultra-high performance concrete,UHPC)中火山灰反应所需氢氧化钙含量不足的状况,在配制超高强混凝土(UHPC基体)时掺加氢氧化钙,研究其对超高强混凝土力学性能的影响机理。结果表明,超高强混凝土力学性能的改善源于掺入的氢氧化钙与矿物掺合料中的SiO_(2)发生火山灰反应生成C-S-H及C-A-S-H凝胶,且在组合养护(90℃热水养护2 d+250℃干热养护3 d)下,部分C-(A)-S-H凝胶向托勃莫来石与硬硅钙石晶体转变,改善了超高强混凝土的微观结构。

钢管超高强混凝土受弯力学性能试验研究

以官盛渠江大桥为依托,通过3组共6个受弯试件的模型试验,探讨钢管超高强混凝土(抗压强度f_(cu)=80.3~115.2MPa)的受弯力学性能,研究混凝土强度对钢管超高强混凝土的抗弯承载力、变形特征与失效模式的影响。试验结果表明:钢管超高强混凝土的受弯破坏模式与普通钢管混凝土相同,主要为挠度过大而失效,呈整体弯曲破坏,受压区有局部鼓屈;试件进入屈服阶段后,承载力降低较少,具有很好的弯曲延性性能;管内混凝土主要对钢管提供横向约束,避免钢管过早受弯压陷屈曲;在截面含钢率一定时,管内混凝土强度增长对钢管超高强混凝土受弯构件的受弯破坏模式、承载力与延性性能的影响较小。

挤压成形和回火温度对DT300钢力学性能的影响分析

本文研究了挤压成形和不同回火温度对DT300钢力学性能的影响,并分析了各工艺条件下的影响规律,以获得最佳材料力学性能,为指导企业生产、促进DT300钢的推广应用提供了有效的试验数据。

热处理对喷射成形超高强Al-Zn-Mg-Cu系铝合金的影响

研究了2种不同热处理方式对喷射成形超高强度Al Zn Mg Cu系铝合金的显微组织和力学性能的影响。观察了沉积态、挤压态、固溶及时效处理后样品的显微组织,对经时效处理的样品进行了力学性能测试。结果表明:沉积态合金晶粒均匀细小;挤压态合金存在大量的第二相颗粒,为富铜相;固溶处理后,合金出现了再结晶现象。在T6条件下,采用常规470℃单级固溶和时效处理,其抗拉强度仅为710MPa,延伸率为6.5%;采用双级固溶和时效处理,其抗拉强度超过800MPa,延伸率达到9.3%。

Zn含量对喷射成形7×××系高强铝合金组织与性能的影响

采用喷射成形技术制备了不同Zn含量的7×××系超高强铝合金,研究了Zn含量对材料的显微组织及室温力学性能的影响.结果表明:喷射成形工艺可显著细化晶粒,有效抑制合金内的偏析,获得细小、均匀的等轴晶组织,采用相同工艺制备的不同Zn含量的材料的晶粒尺寸为10～20μm.喷射成形制备的7×××系超高强铝合金中的主要组成相为:α(Al)、六方晶格的MgZn2、四方晶格的Al2Cu和面心斜方晶格的Al2CuMg.Zn含量在9.5%～11.5%时,经过适当的热处理,材料的强度可以达到800 MPa以上.综合考虑材料的组织和性能,确定喷射成形7×××系铝合金中的Zn含量应控制在9.5%～11.5%.

超高强度钢板热冲压成形研究与进展

超高强度钢板的热冲压成形技术是减轻车身质量、提高汽车抗冲击和防撞性能的重要途径之一。在分析热冲压技术对钢板及模具材料、设计要求的基础上,总结了国内外对于热冲压超高强度钢板开发及研究概况,分析了超高强度钢板热冲压成形技术的研究现状及主要研究方向,讨论了超高强度钢板热冲压成形领域要解决的关键问题,对于超高强度钢板热冲压成形技术在汽车工业中的应用有一定的指导意义。

7055超高强、超高韧铝合金力学性能分析

用拉伸试验研究了经不同固溶和时效工艺处理后7055铝合金的力学性能,并结合显微组织进行了分析。结果表明,复合固溶（双级）＋特殊时效处理新工艺比传统热处理工艺更合理,可使铝合金获得超高强、超高韧的有效结合。试验确定了最佳工艺,即强化固溶：455℃×（10-30）min＋470℃×（20-40）min＋（480-500）℃×（10-30）min;特殊时效：135℃×16 h＋（190-210）℃/（10-20）min,铝合金新状态T78的σb=720 MPa、σ0.2=690 MPa、δ5=12%。

应力比和腐蚀环境对超高强度钢AerMet100疲劳裂纹扩展的影响

采用载荷控制与应力强度因子控制两种方法研究超高强度钢AerMet100在不同裂纹扩展速率区域受到应力比和腐蚀环境影响的疲劳裂纹扩展行为。利用扫描电子显微镜(SEM)对断口进行观察。结果表明:疲劳裂纹扩展速率da/dN≤1×10-5mm/cycle时,当应力比R≤0.5,疲劳裂纹扩展门槛值ΔK th随着应力比R增加而减小,当应力比R≥0.5,疲劳裂纹扩展门槛值变化趋于平缓;在da/dN≥1×10-5mm/cycle时,疲劳裂纹扩展速率出现不受应力比影响的收敛现象。在da/dN≥1×10-5mm/cycle时,腐蚀环境会加速疲劳裂纹扩展;在裂纹扩展速率da/dN≤1×10-5mm/cycle时,由于氧化致裂纹尖端闭合效应,腐蚀环境导致裂纹扩展变慢,并使裂纹扩展门槛值增大。

航空用高强韧Al-Zn-Mg-Cu系铝合金的研究进展

简述了传统铸造超高强Al-Zn-Mg-Cu系铝合金的发展历程、成分设计、制备与加工工艺、热处理制度、微量元素的作用,研究了其微观组织和性能,并提出了超高强铝合金以后的发展方向。

回火工艺对Aermet100超高强度钢组织与韧性的影响

采用SEM、TEM等方法研究了不同回火温度对Aermet100超高强度钢的显微组织和韧性的影响。结果表明,Aermet100钢经885℃淬火后,在472～492℃温度范围内回火,显微组织为高位错密度的板条马氏体(板条M)和少量逆转变奥氏体(AR)。随着回火温度的升高,板条M宽度略有增加,韧化相AR的含量有所提高。高密度位错马氏体的存在使得Aermet100钢保持较高的韧性水平,而逆转奥氏体含量的增加则导致Aermet100钢的韧性显著提高。