Three-dimensional Numerical Simulation and Experimental Analysis of Austenite Grain Growth Behavior in Hot Forging Processes of 300M Steel Large Components

The microstructure models were integrated into finite element（FE）code,and a three-dimensional（3D）FE analysis on the entire hot forging processes of 300 M steel large components was performed to predict the distributions of effective strain,temperature field and austenite grain size.The simulated results show that the finest grains distribute in the maximum effective strain region because large strain induces the occurrence of dynamic recrystallization.However,coarse macro-grains appear in the minimum effective strain region.Then,300 M steel forging test was performed to validate the results of FE simulation,and microstructure observations and quantitative analysis were implemented.The average relative difference between the calculated and experimental austenite grain size is 7.56%,implying that the present microstructure models are reasonable and can be used to analyze the hot forging processes of 300 M steel.

Feasibility study of creep feed grinding of 300M steel with zirconium corundum wheel

The ultrahigh strength 300M steel has been commonly used in the manufacture of aircraft landing gear and rotor shaft parts due to its excellent mechanical properties.Creep feed grinding is one of the essential operations during the whole component manufacturing processes.In this work,the feasibility of creep feed grinding of 300M steel by using the hard zirconium corundum wheel was theoretically and experimentally evaluated.A variety of responses including grinding forces,temperature fields,specific grinding energy,surface integrity and chip modes were carefully recorded.Besides,the mechanism of ground surface profile generation and the spatial frequency spectrum of the surface profile were tentatively analyzed.It was found that the wheel speed has a relative influence on the grinding forces and temperatures of which the work hardening effect dominates the material removal with lower wheel speed while the thermal softening becomes crucial as the wheel speed exceeds the critical value for the studied 300M steel.Furthermore,a scattered spatial frequency spectrum for the generated surface profile was noticed with lower wheel speed while the spectrum gathers towards the lower frequency values with higher amplitude as the wheel speed increases.The shearing chip and flowing chip dominates the main chip type,indicating the excellent abrasive sharpness during the grinding process.In general,the used zirconium corundum wheel presents feasibility for the creep feed grinding of 300M steel because of the high material removal rate,absence of surface burn,low wheel wear and higher compressive residual stresses.

Hot Deformation Behavior of Modified CNS-Ⅱ F/M Steel

Modified CNS-Ⅱ F/M steel was designed for in-core components of supercritical water cooled reactor. Study on the hot deformation behavior of modified CNS- Ⅱ F/M steel is of great importance for processing parameter planning and microstructure controlling during hot deformation. The hot deformation behavior of modified CNS-Ⅱ F/M steel was investigated through isothermal hot compression test at temperature ranging from 1 223 to 1 373 K and strain rate 0.01 to 10 s^-1. The true stress true strain data gained from compression tests were used to built constitu- tive equation of modified CNS-Ⅱ F/M steel. The influence of strain on the accuracy of constitutive analysis was in corporated, assuming strain has a influence on material constants. A 5th order polynominal equation with very good accuracy was used to represent the influence of strain on material constant. The flow stresses calculated from the constitutive equation were compared with test values in the whole experiment range and the absolute average error for the constitutive equation in predicting fl.ow stress is 4. 728 %. At last, the recrystallization behavior of modified CNS-Ⅱ F/M steel was investigated. The relationship of critical strain and peak strain with Zener-Hollomon parameter were given by an experimental equation.

Fatigue life assessment of a high strength steel 300 M in the gigacycle regime

The fatigue behavior of a high strength steel 300 M in the gigacycle regime was investigated. Fully reversed tension - compression fatigue tests at ambient temperature were performed using an ultrasonic fatigue system operating at 20 kHz.The staircase test method was employed to obtain accurate values of the mean fatigue strength corresponding to fixed numbers of cycles up to 10~9.These results were compared to the curve which is estimated by the data tested in the mid-long life regime on conventional servo hydraulic test machine at 20 Hz.Results indicate that the fatigue strength determined from ultrasonic fatigue testing is lightly higher than conventional testing in the range of 10~6-10~7 cycles.It is obvious that nucleations of fractures tend to occur below the surface, if fractures happen after more than 10~7 cycles.All the fractured specimens fails from internal SiO_2 inclusions or smaller carbides and carbide clusters.

Weldability of 780 MPa Super-High Strength Heavy-Duty Truck Crossbeam Steel

CO2-shielded welding experiments of newly developed, 780 MPa super-high strength heavy-duty truck crossbeam steel were conducted, and the microstructure, microhardness, mechanical properties, and impact tough- hess of the welded joint were studied. The evolution of the microstructure of the welded joint occurred as follows.. welding seam （acicular ferrite＋proeutectoid ferrite）→fusion zone （granular bainite-long strip M/A island）→coarse grain zone （granular bainite-long strip or short bar M/A island）→fine grain zone （ferrite＋ pearlite＋ blocky M/A is- land）→mixed grained zone （ferrite＋granular bainite＋blocky M/A island）→base metal （proeutectoid ferrite＋gran- ular bainite-hlocky or granular M/A island）. Increasing the density of the grain boundaries can effectively improve the impact toughness, and the blocky M/A island hindered crack propagation more effectively than the long strip M/A island. The new hot-rolled 780 MPa super-high strength steel had excellent weldability. The welding technology was applied under the following conditions： welding voltage was 20 to 21 V, welding current was 200 to 210 A, and the gas flow rate was 25 L/rain.

Corrosion Study and Passive Film Characterization of 11% Cr FIM and 15% Cr ODS Steels

An ODS （oxide dispersion strengthened） steels are one of the most notable structural materials being developed for future high-temperature energy production technologies, and several studies have been devoted to the development of ODS materials for such applications. However, only little paper focuses on corrosion behavior of F/M （ferritic martensictic） and ODS steels. The corrosion behavior of 11% Cr F/M steel and 15% Cr ODS steel were evaluated using electrochemical methods in borate buffer and 1 kmol m＂3 HNO3 with or without NaCI and also in boiling 60% nitric acid. The corrosion resistance results clearly indicated the influences of steel alloys composition and chloride ions. The XPS （X-ray photo-electron spectroscopy） results of the pre-passivated surface revealed that the oxide formed were composed predominantly of Fe203 along with Cr203, and Y203 layers in ODS steel. The corrosion rate measured in boiling nitric acid for 48 h for both the steels shows high corrosion rate in boiling condition. The SEM （scanning electron microscopy） observation of the pit morphology after corrosion tests appears with shallow pit in both steel surfaces The corrosion degradation behavior in relation to the composition of the passive oxide film in different electrolytic solutions is discussed in this paper.

Influence of Cerium on Solidification Microstructure of M2 High Speed Steel

The influence of Ce on the solidification microstructures of M2 high speed steel was studied. The results show that Ce has the effect of alleviating the segregation of alloying elements such as W and Mo in high speed steel. With the addition of Ce, the amount of eutectic carbides is decreased and the flakes of the carbides are refined. Ce mainly segregates onto the interface between the eutectic carbide and austenite, and a Dart of Ce enters M2C carbide. Ce can also enhance the breaking and spheroidizing of the network eutectic carbides during high temperature heat treatment.

Microstructural evolution and mechanical properties of 300M steel produced by low and high power selective laser melting

300 M ultra-high strength steel has been widely used in critical structural components for aviation and aerospace vehicles,owing to its high strength,excellent transverse plasticity,fracture toughness and fatigue resistance.Herein,low and high power selective laser melting(SLM)of 300 M steel and their microstructural evolution and mechanical properties have been reported.The results show that the optimal energy density range with the highest relative density for SLMed 300 M steel is between 60 and160 J/mm^3.Furthermore,molten pools for deposition exhibit a conduction mode with semi-elliptical shape at a lower laser power of 300~600 W but a keyhole mode with"U"shape at a higher laser power of 800~1900 W.The heterogeneous microstructure of as-built samples is cha racterized by a skin-core structure which is that tempered troostite with the coarse non-equiaxed grains in the molten pool is wrapped by tempered sorbite with the fine equiaxed grains in the heat-affected zone.The skin-core structure of SLMed 300 M steel has the characteristics of hard inside and soft outside.The average microhardness of samples varies from 385 to 341 HV when laser power increases from 300 to 1900 W.Interestingly,ultimate tensile strength(1156-1193 MPa)and yield tensile strength(1085-1145 MPa)of dense samples fabricated at diffe rent laser powers vary marginally.But,the elongation(6.8-9.1%)of SLMed 300 M steel is greatly affected by the laser power.

Metallurgical analysis of a failed maraging steel shear screw used in the band separation system of a satellite launch vehicle

Maraging steels have excellent combination of strength and toughness and are extensively used for a variety of aerospace applications. In one such critical application, this steel was used to fabricate shear screws of a stage separation system in a satellite launch vehicle. During assembly preparations, one of the shear screws which connected the separation band and band end block has failed at the first thread. Microstructural analysis revealed that the crack originated from the root of the thread and propagated in an intergranular mode. The failure is attributed to combined effect of stress and corrosion leading to stress corrosion cracking.

Wet and Dry Tribological Behaviors of Circular Islandic Protrusion Patterns on M2 Steel Discs under Spinning Condition

Under spinning conditions, lubricant on islandic spot patterned M2 steel disc experiences centrifugal and tangential force components. Depending upon the relative position of the spots and the flow of lubricant, accumulation of lubricant in front of patterned islandic spots creates thrusting to mating part and subsequently reduces contact between the mating couple. Whilst wear debris is likely to be spun off the plateau of the spots to their neighbouring valleys so as to reduce wear. Hence, it gives favorable tribological characteristics. Aiming at verifying such mechanisms, studies were performed on M2 steel disc specimens slid with ASSAB 17 tool steel pin. The M2 steel disc specimens were respectively （i） machined with non-patterned （NP）, （ii） etched to produce in-lined （INE） islandic patterns, and （iii） etched to produce staggered （STE） islandic spot patterns. Results indicated that the INE patterned discs gave most favorable wear characteristics, the NP of the worse characteristics whilst the STE ranged in the middle. However, the actual contact mechanism leads to the descending sequence of favorable friction behaviors nominally as： NP, INE and STE.

Semi-solid Casting of High Speed Steel Ingots Using Inclined Slope Pre-crystallization Method

Semi-solid casting of M2 high speed steel ingots was investigated by inclined slope pre-crystallization method. Effects of casting temperature and slope length on the microstructure of M2 HSS ingots were investigated. M2 cast ingots of non-dendritic primary austenite and fine eutectic ledeburite network carbide structure were obtained, with the casting temperature, slope length and angle of 1480 ℃, 500 mm and 60° respectively. Meanwhile, the microstructure of cast samples was quantitatively assessed by Image tool software. Results show that optimum mean equivalent diameter of primary austenite crystal grain is 50.8 μm, shape factor is 0.83, and mean thickness of network carbide is 5.21 μm.

Study on M/A islands in pipeline steel X70

The fine structure of M/A islands in pipeline steel X70 has been studied by transmission electron microscope (TEM). It is shown that the M/A islands are about 1-2 μm in size and distribute at the grain boundary of irregular massive ferrite or acicular ferrite in the microstructure of the steel undergoing TMCP (thermo-mechanical controlled processing). The analysis of diffraction contrast shows that the M/A islands consist of retained austenite and some martensite lamellae different in size and orientation. The microt winning and midrib exist in the lamella of lenticular martensite, which exhibit the typical character of high-carbon martensite. The influences of TMCP parameters on M/A islands have been studied carefully. With the increase of the cooling speed, the amount of M/A islands decrease slightly and the morphology of M/A islands changes to thin dispersive short bars from thick irregular long strips.

Effect of deformation and cooling rate on the transformation behavior and microstructure of X70 steels

The effects of the deformation in the non-recrystallization region of austenite and the cooling rate on the transformation behavior and microstructure of low-carbon low-alloy steel for pipeline application were studied on the thermal-mechanical simulator Gleeble-1500. It was shown that an increase in deformation amount can greatly increase the nucleation site of ferrite when deformed in the non-recrystallization region of austenite, and an increase in nucleation ratio can greatly refine grains. When the cooling rate is accelerated, the driving force of nucleation is increased and the nucleation rate also improves. Ultra-refine grains can be obtained by controlled rolling. The high density of ferrite nucleus, which forms along the austenite grain boundary, twin interface, and deformation band are introduced in the matrix of austenite by the control of hot rolling, after which the microstructure can be refined. It was found that the acicular ferrite has a very fine sub-structure, high dislocation density, and a thin slab with ultra-fine grains. Small M/A islands and cementite are precipitated on the matrix of the slabs by the analysis technique of TEM and SEM.

韩国钢铁企业应对跨国并购的国际化经营战略--以Mittal Steel公司跨国并购为例

在经济全球化浪潮下,跨国企业并购日益加剧。从韩国Mittal Steel公司的跨国并购来看,尽管韩国钢铁企业拥有跨国并购的优势,但并没有处于主动的地位。为迅速适应跨国并购的国际化趋势,韩国钢铁企业应当积极通过跨国并购,进一步加强与钢铁生产相关企业以及同已经持有股份的国外企业之间的战略联盟。韩国政府也应当把跨国并购当做企业发展的重要战略,制定相关政策,积极支持企业跨国并购。

INFLUENCE OF AUSTENITIZING TEMPERATURE ON IMPURITY SEGREGATION ALONG GRAIN BOUNDARIES IN STEEL 4330M

The effect of austenitizing temperature on segregation of impurities along grain boundaries in steel 4330M has been examined by AES.The impurity segregation was computed quantitatively.Results showed that the quantity of impurity segregation changed with the austenitizing temperature.The limitation of the McLean′s expression for equilibrium segrega- tion was discussed.

ON THE SWERVING OF STRENGTH CURVE FOR STEEL 20Cr11MoVNbNB

Study was made on the swerving of the curves of strength and hardness vs L-M parameter P=T(20+lgt)for steel 20Crl 1MoVNbNB aged at 550.600 and 650℃ for 0—5000 h,T being aging temperature in K and taging time in h.The swerving seems to be due to:(1)the coarsening of the mean size of principal precipitate M_(23)C_6,(2)the precipitation of Laves phase MoFe_2;(3)the decrease of Mo content in matrix and(4)the recovery and recrystallization of the matrix phase.

Effect of TMCP Parameters on the Microstructure and Properties of Q460qNH Steel

The effects of TMCP parameters, consisting of finish cooling temperature and start rolling temperature in non-recrystallization region, on the final microstructure and mechanical properties of Q460 qNH steel were studied by tensile, Charpy impact tests and optical microscopy. The TMCP parameters for Q460 qNH steel were optimized by laboratory experiments. The results show that the yield strength and tensile strength increase with the finish cooling temperature, and the microstructure of Q460 qNH steel consists of ferrite and granular bainite.

Interaction of Mechanical and Electrochemical Factors duringCorrosion Fatigue of Fe-26Cr-1Mo Stainless Steel in 1M H_2SO_4 Solution

The cyclic plastic straining electrode technique has been used to investigate the transient electrochemical behaviour of Fe-26Cr1Mo stainless steel in 1M H2SO4 solution at a passive potential.The influence of plastic strain amplitude and plastic strain rate on the dissolution current response was analysed. The experimental results showed that the transient current was dependent on the competitive process of the surface film rupture and repassivation of the new surface. The high plastic strain amplitude and the high plastic strain rate caused a change of electrochemical activity of specimen surface. In the condition of low strain amplitude and strain rate, the characteristics of current response was mainly relative tp the process of new surface repassivation.The competition kinetics has been analysed through the comparison of plastic strain rate and repassivating rate

440MPa级HSLA钢焊缝低温韧性分析

采用自制的2种实心焊丝对440 MPa级HSLA钢进行了对接MAG焊(80%Ar+20%CO_(2)),通过对比分析2种焊缝的显微组织和夹杂物尺寸及成分,对2种焊缝低温韧性出现显著差异的原因进行了阐释。结果表明,J1焊丝的焊缝由针状铁素体、侧板条铁素体、先共析铁素体和M-A组元组成,而J2焊丝的焊缝由针状铁素体和M-A组元组成;2种焊丝的焊缝中夹杂物的粒径分布、尺寸、数量和成分差异小,不是导致2种焊缝低温韧性出现显著差异的主要原因;与J1焊丝的焊缝相比,J2焊丝的焊缝中针状铁素体含量升高、M-A组元含量降低、条状和块状M-A组元占比降低,是导致其低温韧性显著优于J1焊丝的主要原因。

Effect of Nb on the transformation kinetics of low carbon(manganese) steel during deformation of undercooled austenite

The hot compression tests using Gleeble 1500 were performed by varying the true strain up to 1.6 （80% reduction） in Nbfree and Nb-microalloyed steels. The effect of Nb addition on the transformation kinetics during deformation of undercooled austenite was investigated. It was found that as compared with Nb-free steel, the transformation incubation period of Nb-bearing steel was prolonged and the transformation kinetics curves parallelly moved to higher strain because of the solute Nb drag effect. Studies on kinetics also showed that the deformation-enhanced ferrite transformation （DEFT） of the two steels were composed of three stages, which can be expressed by the J-M-A equations individually. However, the parameter n related to the mode of nucleation and growth is somewhat different in the first and second stages of the two steels, and the same in the third stage for both the steels corresponding to the nucleation Of retained austenite.