热变形后的冷却速度对高碳铬铸钢组织和性能的影响

研究了经40％热变形后不同冷却速度下高碳铬铸钢的组织和性能。结果表明：热变形后的冷却速度与组织及性能有良好的对应关系。高碳铬铸钢变形后风冷（冷却速度约7℃／s）时,其综合力学性能最好。     

一种非调质连杆用高碳微合金钢的热变形行为

用Gleeble-3500热力模拟试验机在温度为1 223～1 323 K,应变速率为0.2～10 s-1的条件下对一种非调质连杆用高碳微合金钢进行了热压缩变形试验,测得了其流变曲线,并观察了变形后的组织.试验结果表明,流变应力和峰值应变随变形温度的降低和应变速率的提高而增大.试验用钢在真应变为0.8,温度为1 223～1 323 K,应变速率为0.2～10 s-1的条件下,发生完全动态再结晶.测得试验用钢的热变形激活能为289.9 kJ/mol,并得出了其热变形方程,以及动态再结晶晶粒尺寸与Zener-Hollomon参数之间的关系和动态再结晶状态图.     

Hot Toughness of Continuously Cast Steel Slabs during Reheating

Laboratory hot tensile tests on specimens taken from continuously cast steel slabs are executed by means of a Gleeble apparatus to investigate the ductility and hot toughness of steels during reheating of as cast conditions. The reduction in area at fracture and the fracture energy for 10 different heats with the carbon content in the range from 0.003 to 0.521 mass% and some variations in Nb, Cr and N are presented in the temperature range from room temperature up to 1025°C. Ductility minima regarding toughness are identified around 300°C and 650°C. Relationships of the reduction in area at fracture and of the specific fracture energy with the relevant elements of the steel composition are established. Equivalent carbon concentrations are defined which take the N concentration into account for the 300°C embrittlement and additionally the micro‐alloying element Nb for the 650°C ductility minimum. Limits for the reduction in area values and for the specific fracture energy are proposed to validate the crack sensitivity of as cast carbon steel slabs.     

Microstructure and Property of High Carbonic-Chromium Cast Steel with Different Hot Deformation Ratio

The microstructure and properties of high carbonic-chromium cast steel subjected to different hot deformation ratios were studied. The experimental results show that the microstructure and properties of high carbonic-chromium cast steel are obviously improved after hot deformation, and the best mechanical properties of the cast steel can be obtained under hot deformation ratio of 40 %-50 %, which leads to the morphology change of eutectic carbide and the precipitation of granular carbides.     

Strain‐Rate Sensitivity in Hot Forming of Steels – Influence of Microstructure,Temperature and Chemical Composition

A methodology to determine the strain‐rate sensitivity index was developed, based on rolling of a set of samples with the same draught but different speed at defined temperatures. It was shown that initial grain size has nearly negligible influence on the investigated variable, in contrast to phase composition whose influence is very considerable. Combined influence of strain rate and temperature on deformation resistance of various types of steel was studied. For a selected group of steels a universal equation was set up, which described, with a good accuracy, impact of reciprocal temperature and chemical composition (expressed simply by nickel equivalent) on strain‐rate sensitivity in hot state.     

The Microstructure and Mechanical Properties of Ultrafine Grained Plain C‐Mn Steels

An ultrafine microstructure was produced in plain C‐Mn steels with different carbon contents (0.15 ‐ 0.3 mass% C) by heavy warm deformation. The rolling was simulated by the plane strain compression test with a simulated post rolling coiling. The final microstructure consists of an ultrafine grained ferrite matrix with the average grain size of 1.1 ‐ 1.4 μm and spheroidized cementite particles of two different size groups. The fraction of high‐angle grain boundaries maintained in the range of 60% to 65%. With the increase of C content from 0.15 mass% to 0.3 mass% the strength increases by about 100 MPa, while the total elongation of 23% hardly changes. The (specific) upper shelf energy decreases from 320 J/cm² to 236 J/cm² but a rather low ductile‐to‐brittle transition temperature (DBTT) of about 206 K does not rise with increasing C content. The ultrafine steel with higher C content (0.3 mass%) exhibits a superior strength‐toughness combination.     

Bake Hardening of Hot Rolled Multiphase Steels under Biaxial Pre‐strained Conditions

Multiphase steels show a strong bake hardening effect being of importance for shaping of car body structural parts. The raised yield strength is exploited for improved crash resistance. Especially the automotive industry has a growing interest in using this effect. Normally the bake hardening effect is examined in tensile tests whereas under industrial conditions shaping of structural parts shows a wide spread of stress strain behaviour, from uniaxial conditions over plain strain to biaxial ones. So it is obvious that the bake hardening behaviour of a material cannot be described with results of the uniaxial tension test only. To give a first answer to this question, the dependence of the bake hardening effect on different biaxial prestrains was investigated for several hot rolled multiphase steels using various baking temperatures and holding times whereas the bake hardening effect under uniaxial prestrain had already been examined in [5]. Considering the choices to generate biaxial strain, a Marciniak forming tool with a diameter of 250 mm mounted on a 2500 kN hydraulic press was chosen. For control of plastic deformation and adjustment the non‐contact measuring system ARGUS, was used. To reduce the quantity of experiments “Design of Experiments” and statistical methods were applied for a martensitic steel, a dual phase steel, a complex phase steel, a ferrite‐bainite steel, and a retained austenite steel known as TRIP, all in hot rolled condition. As a result, a formula for yield stress, tensile strength and residual deformability was developed. Furthermore, a method was found to predict easily whether a steel under investigation is qualified for additional experiments in regard to bake hardening or more exactly its response to different baking temperatures and holding times.     

Development of Dual Phase and Multiphase High Strength Steels by Using a New Cooling Technology: The Howaq‐Twice Process

In order to develop low alloy dual and multiphase high strength steels, CRM and Arcelor implemented the Howaq‐twice process in a continuous annealing line. This new equipment combines a slow primary cooling with a fast secondary cooling. Various cooling strategies were investigated in regard to microstructure and mechanical properties of low alloyed steels. Depending on the cooling rate and the quenching temperature, the microstructure varied between fully martensitic, ferritic‐martensitic, and ferritic‐martensitic‐bainitic. The associated tensile strengths reached from 720 to 1100 MPa.     

Study of Bake Hardening Effect on Laser Welded Hot Rolled Ferrite‐Bainite Dual Phase Steels

The influence of bake hardening (BH) effect on laser welded hot rolled ferrite‐bainite dual phase steel (FBS) with respect to the process conditions was investigated. The samples were firstly laser welded and then pre‐strained. Pre‐strained the samples with defined degrees of deformation and a subsequent aging treatment leads to enhanced hardness and strengthening for both conditions. The microstructure of the fusion zone (FZ) and heat affected zone (HAZ) was studied. A high volume fraction of martensite could be observed in the FZ as well as in the HAZ. Both steels exhibited a clear BH effect in both, the as‐received and the laser welded conditions. The BH effect is more pronounced in the pre‐strained laser welded condition.     

A coupled Thermal‐mechanical FE Model of Flow Localization during the Hot Torsion Test

The hot torsion test (HTT) has been extensively used to analyse and physically model the flow behaviour and microstructure evolution of materials and alloys during hot deformation processes. The geometry of the specimen is a key factor for obtaining reliable results. In the present work, a thermo‐rigid viscoplastic FE code, THORAX.FOR, was developed to describe the interaction of thermal‐mechanical conditions and geometries of the HTT specimens. This was used to recommend the conditions for avoiding flow localization during HTT of API‐X70 microalloyed steel. The simulation results show how an inappropriate choice of both test specimen geometry and twist rate of deformation could lead to a significant temperature raise in the middle of the gauge section and temperature gradient in the radial and longitudinal direction of the specimen. This consequently causes flow localization during the test. Therefore, assumptions of isothermal forming conditions or uniform strain softening may not be valid in many test scenarios. These assumptions could introduce significant errors in the post results of the test such as flow curve and interpretation of microstructure evolution. Recommendations on proper specimen geometry for a specified strain rate will be given to avoid flow localization during the hot torsion test.     

In‐Situ Observation of the Formation of MnS during Solidification of High Sulphur Steels

A Confocal Scanning Laser Microscope equipped with a gold image furnace was used to directly observe the precipitation of MnS during solidification of high sulphur steels under isothermal conditions in the temperature region 1440 to 1480°C on the free surface of the steel melt. For the case of Al‐killed steels, below 1480°C MnS particles were found to precipitate with Fe forming simultaneously around them. This MnS containing structure continued to grow rapidly (264 μm/s) as a surface film. The film gradually changed, as the level of S in the melt decreased, into a eutectic structure (with lamella spacing of 2 μm) as predicted by thermodynamics. In Si‐ killed steels there was significantly lower tendency to form MnS both in terms of time until precipitation occurred and growth rate.     

Correlation between Microstructure and Mechanical Properties of High C‐Cr Cast Steel

The effect of carbide morphology and matrix structure on abrasion resistance of cast alloyed steel with 2.57% C, 16.2% Cr and 0.78% Mo was studied in the as‐cast and heat treated conditions. Samples were austenitized at three different temperatures of 980, 1050 and 1250 °C for 15 minutes and followed by tempering at 540 °C for 3 hours. The austenitizing temperature of 980 °C revealed fully martensitic structure with little amount of retained austenite, while at 1050 °C the matrix was austenitic with massive amount of coarse secondary carbides. The austenitic matrix with very fine secondary carbides was developed at 1250 °C. The maximum abrasion resistance was obtained at 1050 °C due to the highest structure hardness and existence of both eutectic and secondary carbides in larger size than the formed groove by the abrasive particles during the wear test. On the other hand, the as‐cast pearlitic structure showed high wear rate by an applied load of up to 0.2 bar, followed by very rapid increase in wear rate with higher applied loads. It could be considered that the austenitizing temperature of 1050 °C showed better combination of abrasion resistance and toughness in comparison with other heat treatment cycles.     

Modelling the Stress Relaxation Kinetics of Intercritically Deformed Austenite and Ferrite in C‐Mn Steel

In this study the softening kinetics of intercritically deformed C‐Mn steel have been characterised using the stress relaxation technique. In addition, the progress of softening has been monitored via optical microscopy of quenched samples. Physically based models for the softening kinetics of the separate phases were combined using the simple rule of mixtures, to predict the stress relaxation kinetics following intercritical deformation. Moreover, the strain and stress distribution developed during deformation has been taken into account using an analytical approach from the literature. Comparison of the model with experiments showed significant deviations. These were thought to be due to two effects concerning the role of phase interactions. Firstly, there was a region in the austenite phase having a low strain, leading to a fraction of non‐recrystallizing austenite. Secondly, the number of recrystallization nucleation sites was reduced. These two effects were tested by modifying the original model. The best agreement with experimental data was obtained when assuming the presence of a significant fraction of austenite that did not recrystallize.     

Effect of Hot Rolling Temperature on the Structure and Magnetic Properties of High Permeability Non‐Oriented Silicon Steel

1.3%Si steel hot bands were produced by hot rolling in a Steckel mill with two entrance temperatures: 1000°C (γ rolling) and 910°C (α+γ rolling). Hot band samples were processed with and without hot band annealing (900°C for 30s), cold rolled to final thickness of 0.5mm and annealed in H₂‐25%N₂ at 900°C for 40s. The combination of high hot rolling temperature and hot band annealing resulted in lower core loss and higher permeability. Although no significant differences were observed on the hot band microstructures, after hot band annealing the γ rolled sample showed a larger grain size. The final results were attributed to the effect of the initial grain size prior to cold rolling on microstructure and texture after annealing.     

The Structure And Properties of a Thermo‐mechanically Processed Low Carbon High Strength Steel

Research efforts were given towards development of low carbon high strength steels since recent past. The present study deals with the development of a low carbon high strength steel alloyed with Mn, Ni, Mo, Cu and microalloyed with Ti and Nb. The steel was subjected to three stage controlled rolling operation followed by accelerated cooling. The structure and properties of the steel at various processing conditions were evaluated. Microstructural observation reveals predominantly lath martensite along with twinned martensite structure at all processing conditions. High strength values at higher finish rolling temperatures have been obtained due to fine martensitic structure along with tiny precipitates of microalloying carbide and carbonitride. The strength value increases marginally at lower finishing temperature due to comparatively finer lath size of martensite and increased precipitation density of carbides, carbonitrides along with Cu particles. The variation in impact toughness properties at different finish rolling temperatures is found to be negligible at ambient and subambient temperatures. The formation of stable and large TiN/TiCN particles during casting have impaired the impact toughness values at ambient and at ‐40°C temperatures.