Improving the Fatigue Performance of the Welded Joints of Ultra-Fine Grain Steel by Ultrasonic Peening

Contrast tests were carried out to study the fatigue performance of the butt joints treated by ultrasonic peening, aiming at the improvement of ultrasonic peening treatment(UPT) on welded joints of a new material. The material is a new generation of fine grain and high purity SS400 steel that has the same ingredients as the traditional low carbon steel. The specimens are in two different states:welded and ultrasonic peening conditions. The corresponding fatigue testing data were analyzed according to the regulation of the statistical method for fatigue life of the welded joints established by International Institute of Welding(IIW). Welding residual stress was considered in two different ways: the constant stress ratio R=0.5 and the Ohta method. The nominal stress-number (σ-N)curves were corrected because of the different plate thickness compared to the standard and because there was no mismatch or angular deformation. The results indicated that: 1) Compared with the welded specimens, when the stress range was 200 MPa, the fatigue life of the SS400 steel specimens treated by ultrasonic peening is prolonged by over 58 times, and the fatigue strength FAT corresponding to 106 cycles is increased by about 66%; 2) As for the SS400 butt joint (single side welding double sides molding), after being treated by UPT, the nominal S-N curve (m=10) of FAT 100 MPa(R=0.5) should be used for fatigue design. The standard S-N curves of FAT 100 MPa(R=0.5, m=10) could be used for fatigue design of the SS400 steel butt joints treated by ultrasonic peening.

Austenite grain growth behavior of Q1030 high strength welded steel

The austenite grain growth behavior of Q1030 steel was studied under different heating conditions. The austenite grain size increases with the heating temperature and holding time increasing. Austenite grains grow in an exponential manner with rising heating tem- perature and in a parabolic manner with prolonging holding time. A mathematical model for describing the austenite grain growth behavior of Q 1030 steel was obtained on the basis of experimental results using regression analysis. When the heating temperatures lie between 1000 and 1100℃ at a certain holding time, abnormal grain growth appears, which causes mixed grains in Q1030 steel.

Microstructure and mechanical properties of high-strength low alloy steel by wire and arc additive manufacturing

A high-building multi-directional pipe joint(HBMDPJ)was fabricated by wire and arc additive manufacturing using high-strength low-alloy(HSLA)steel.The microstructure characteristics and transformation were observed and analyzed.The results show that the forming part includes four regions.The solidification zone solidifies as typical columnar crystals from a molten pool.The complete austenitizing zone forms from the solidification zone heated to a temperature greater than 1100℃,and the typical columnar crystals in this zone are difficult to observe.The partial austenitizing zone forms from the completely austenite zone heated between Ac1(austenite transition temperature)and1100℃,which is mainly equiaxed grains.After several thermal cycles,the partial austenitizing zone transforms to the tempering zone,which consistes of fully equiaxed grains.From the solidification zone to the tempering zone,the average grain size decreases from 75 to20μm.The mechanical properties of HBMDPJ satisfies the requirement for the intended application.

Influence of rapid heating process on the microstructure and tensile properties of high-strength ferrite–martensite dual-phase steel

Three low-carbon dual-phase （DP） steels with almost constant martensite contents of 20vo1% were produced by intercritical annealing at different heating rates and soaking temperatures. Microstructures prepared at low temperature （1043 K, FH1） with fast-heating （300 K/s） show banded ferrite/martensite structure, whereas those soaked at high temperature （1103 K, FH2） with fast heating reveal blocky martensite uniformly distributed in the fine-grained ferrite matrix. Their mechanical properties were tested under tensile conditions and compared to a slow-heated （5 K/s） reference material （SH0）. The tensile tests indicate that for a given martensite volume fraction, the yield strength and total elongation values are noticeably affected by the refinement of ferrite grains and the martensite morphology. Metallographic observations reveal the formation of microvoids at the ferrite/martensite interface in the SH0 and FH2 samples, whereas microvoids nucleate via the fracture of banded martensite particles in the FH1 specimen. In addition, analyses of the work-hardening behaviors of the DP microstructures using the differential Crussard-Jaoul technique demonstrate two stages of work hardening for all samples.

IMPROVEMENT ON SUSCEPTIBILITY OF ULTAR-HIGH STRENGTH STEEL TO STRESS CORROSION CRACKING BY HIGH TEMPERATURE QUENCHING

The effect of quenching temperature on the stress corrosion cracking of 30Cr3SiNiMoV ultra-high strength steel in 3.5% NaCl aqueous solution has been studied.The threshold K_(ISCC) may continuously increase with the quenching temperature raised from 870 to 1200℃ . All of the fractures are intergranular.The analyses of the segregation along prior austenitic grain boundaries,grain size and other microstructural factors reveal that the inerease of K_(ISCC) is mainly due to the coarsening of prior austenitic grains.

Effects of Microstructural Modification Using Friction Stir Processing on Fatigue Strength of Butt-Welded Joints for High-Strength Steels

Friction stir processing (FSP) is an effective surface-microstructure modification technique using a rotational tool to refine and homogenize microstructure of metallic materials. In this study, FSP was conducted on the surface of the heat-affected zone (HAZ), which is a region exhibiting degraded mechanical properties and shown to have microstructural changes, of butt-welded joints for two high-strength steels with tensile strength grades of 490 MPa and 780 MPa (hereafter HT490 and HT780, respectively). Inhomogeneous mixing of materials derived from weld metals and base metals (BMs) in a stir zone (SZ) produced inhomogeneous distribution of elements and microstructure depending on the set of the advancing side and retreating side in the SZs. The welded joints with FSP for HT490 exhibited higher hardness than that of the BM through whole of the SZ surface (fine polygonal ferrite grains and bainite structure with laths at the Mn-rich and Mn-poor regions, respectively). On the other hand, those for HT780 exhibited the minimum hardness value similar to that of the BM at the SZ surface (a few polygonal ferrite grains in the matrix of martensite laths). Fatigue strength increased by about 35 MPa and 15 MPa in stress amplitude at 107 cycles as fatigue limit due to FSP. Fatigue failure occurred at the BM and the SZ, respectively, in the welded joints modified by FSP for HT490 and HT780, in comparison with the HAZs in the as-welded joints for both grade steels. The difference in fatigue strength increase due to FSP and failure location between the welded joints for HT490 and HT780 can be attributed to the topmost SZ microstructures and their distribution.

Development of grain boundary allotriomorphic ferrite/granular bainite duplex steel

A new hot-rolled low alloy high strength steel with grain boundaryallotriomorphic ferrite/granular bainite duplex microstructure has been developed through novelmicrostructure and alloying designs without any noble metal elements such as nickel and molybdenum.Its as-rolled microstructure and mechanical properties, fatigue crack propagation behavior comparedwith single granular bainitic steel as well as continuous cooling transformation, were investigatedin detail. The measured result of CCT (continuous cooling transformation) curve shows that suchduplex microstructure can be easily obtained within a wide air-cooling rate range. More importantly,this duplex microstructure has much better combination of toughness and strength than the singlegranular bainite microstructure. It is found that the grain boundary allotriomorphic ferrite in thisduplex microstructure can blunt the microcrack tip, cause fatigue crack propagation route branchingand curving, and thus it increases the resistance to fatigue crack propagation, improves steeltoughness. The mechanical properties of the above commercial duplex steel plates have achieved orexceeded 870 MPa ultimate tensile strength, 570 MPa yield strength, 18 percent elongation and 34 JCharpy V-notch impact energy at -40 deg C, showing good development potential.

Effects of Phosphorus Grain Boundary Segregation and Hardness on the Ductile-to-Brittle Transition for a 2.25CrMo Steel

The combined effect of phosphorus grain boundary segregation and hardness on the ductile-to-brittle transition was examined for a P-doped 2.25CrlMo steel by using Auger electron spectroscopy in conjunction with hardness measurements, Charpy impact tests and scanning electron microscopy. With prolonging time at 540 ~C after water quenching from 980℃, the segregation of phosphorus increases and the hardness decreases. The DBTT （FATT） increases with increasing phosphorus segregation and decreases with decreasing hardness. The effect of phosphorus segregation is dominant until 100 h aging and after that the hardness effect becomes dominant. This effect makes the DBTT （FATT） decrease with further prolonging ageing time although the segregation of phosphorus still increases strongly.

A study on HAZ’s microstructure and grain size of 10CrNi3MoV steel

This paper deals with microstructure and grain size of HAZ of 590 MPa high strength ship-structure steel with welding simulator. While t 8/5 time is short, 10CrNi3MoV steel’s microstructure of coarse grain heat-affected zone (CGHAZ) is lath martensite structure and a small amount of twined martensite structure; while t 8/5 time is long, it is mainly granular bainite structure. After second thermal cycle with peak temperature between critical temperature Ac 1′ and Ac 3′, CGHAZ has the inheritance of coarse grain and coarse microstructure. Welding energy input (determined by t 8/5 time) has an effect on the inheritance of coarse microstructure, but no influence on the inheritance of coarse grain. In detail, with a shorter t 8/5 time, there is an inheritance of coarse microstructure and grains. With a longer t 8/5 time, there is only the inheritance of coarse grains. While t 8/5 increases, the scope of temperature of causing it diminishes. Therefore, the inheritance of coarse microstructure connects with the high speed of heating and cooling.

Partial Inverse Grain Size Dependence of Strength in High Mn Steels Microalloyed With Nb

Two steels without and with Nb addition were chosen to investigate the effects of Nb on the microstructures and the mechanical properties of Fe-Mn-Al-Si steels.The results revealed that Nb refined the grains markedly and both TRIP and TWIP effects occurred during deformation process.The Nb containing steel possesses higher yield strength and much lower tensile strength , the latter being explained by the suppression of TRIP effect due to the increase of stacking fault energy.This indicates grain refining is secondary for strengthening of steels when TRIP or TWIP effect exists during the deformation of low carbon and high Mn steels.

High performance low cost steels with ultrafine grained and multi-phased microstructure

Ultrafine grained ferrite was obtained through tempering cold rolled martensite with an average grain size of 200―400 nm in a low carbon and a microalloyed steel. Thermal and mechanical stability of the two steels was studied. Due to the pinning effect of microalloyed precipitates on the movement of dislocations and grain boundaries, the recrystallization and grain growth rate were retarded, and the thermal stability of ultrafine grained microstructure was improved. The ultrafine grained ferritic steel was strengthened, but its strain hardening rate was reduced. It seems that the tiny carbide precipitates have no significant effect on work hardening rate. The ultrafine grained ferrite+martensite dual phase microstructure was obtained in the microalloyed steel through intercritically annealing cold rolled martensite. The resulting multiphase microstructure has a tensile strength higher than 1.0 GPa with a yield ratio lower than 0.7. Another type of multiphase microstructure with nanoscaled lath bainite+ retained austenite was obtained through an isothermal heat treatment in low temperature bainite transformation region in high carbon steel. The tensile strength was as high as 1.64 GPa with a yield ratio of 0.84.

Achievements of New Generation Steel Project in China

Major achievements of the national project, Fundamental Research on New Generation of Iron and Steel Materials in China (NG Steel), are reviewed in the paper. Ultrafine grained steel technology, based on deformation induced ferrite transformation (DIFT)and successive microstructure changes, is illustrated for grain refinement in both plain low carbon steel and microalloyed steel. Delayed fracture resistance of alloy structure steel can be improved through prior austenite grain refinement. It is shown by results that nano scale precipitates play an important role to grain refinement in thin slab casting and rolling (TSCR)process. Progresses on super cleanliness, high homogeneity, welding and metallurgical process simulation are also briefly introduced.

新型汽车覆盖件钢板性能研究

UF340作为一种新型超细晶高强钢,在汽车外覆盖件的应用上具有替代HC180BD的潜力,研究了UF340力学性能和成形性能。结果表明,在塑性、加工硬化指数和各向异性方面,UF340与HC180BD具有相似的力学性能,与HC180BD相比UF340没有明显的烘烤硬化性,2种材料的成形性能基本一致。通过Keeler模型计算获得UF340的成形极限曲线与成形试验结果存在一定差异,采用线性分段拟合方式获得的成形极限图与试验结果吻合度更高,采用该方法获得的成形极限曲线可为新材料的成形应用提供参考。

Development of Nb-V-Ti Hot-Rolled High Strength Steel With Fine Ferrite and Precipitation Strengthening

A hot-rolled steel with high yield strength of 700 MPa, good elongation of about 20% and low ductile-brittle transition temperature （DBTT） lower than -70℃ has been developed in laboratory. The results show that adopting finishing rolling temperature of around 800℃ is rational, and coiling temperature is between 400 and 500℃ The strength of developed 700 MPa hot-rolled high strength steel is derived from the cumulative contribution of fine grain size, dislocation hardening and precipitation hardening. The fine grain strengthening and precipitation hardening are the dominant factors responsible for such high strength, and good elongation and toughness are predominantly due to fine grain ferrite.

Investigation on Precipitation Hardening High Strength IF Steel Sheet for Automotive Application

High strength IF steel sheets with sufficient formability had been extensively used in automotive industry.In this paper,a new type of high strength cold-rolled IF steel with higher carbon and niobium contents was studied.Thermal plastic and continuous annealing were performed on thermo-mechanical simulator.The transformation points were tested by thermal expansion apparatus.Optical microscopy and transmission election microscope (TEM) were used to analyze the microstructure and the secondary precipitates of the steel.The results showed,the ductibility temperature range was from 950℃ to 1250℃ and the transformation points were 887℃ and 913℃ respectively.The grain size of this steel was smaller than that of conventional high strength IF steel.At the mean time,there were many fine Nb(C,N) precipitates distributed in the intra-granular regions and the PFZ (precipitate free zone) were formed in the neighborhood of grain boundaries.Due to the unique micro-structural feature,the yield strength and the yield ratio of the steel were decreased while the tensile strength was increased.With the increasing of the annealing temperature,the strength decreased,the total elongation A50,r-value at 15% strain and n-value were all increased.In order to obtain the favorable mechanical properties,the skin-pass rolling rate should be chosen at 0.6-0.8%.

Fatigue Property of Low Cost and High Strength Wheel Steel for Commercial Vehicle

The fatigue properties of the newly developed wheel steel used for commercial vehicles were studied using push-pull axial loading fatigue tests with stress ratio R=-1. Q235B steel, which is conventionally applied to commercial vehicle wheels, was also analyzed for comparison. Although the chemical composition and microstructure （ferrite and pearlite） of newly developed wheel steel were similar to those of Q235B, the 107 cycles fatigue limit of the new wheel steel was 260 MPa, which is 24% higher than that of Q235B （210 MPa）. The improvement of the fatigue strength of the new wheel steel can be attributed to grain refinement. In order to investigate the effect of the decrease in thickness of the wheel steel on the fatigue property of the wheel, dynamic cornering fatigue tests were conducted on full scale wheels with the model of 8.25 × 22.5. The results indicated that the newly developed wheel steel had outstanding fatigue life even if the mass loss was 10% in comparison with Q235B.

高强钢多道次热压缩晶粒尺寸模型

为准确描述300M高强钢在多道次热压缩过程中的晶粒尺寸演变行为,解决晶粒尺寸在不同再结晶过程之间不连续的问题,提出了包含15个待定参数的晶粒尺寸演化模型。在温度950~1150℃、应变速率0.01~10 s^(-1)、应变0~0.9、道次间保温时间0~600 s的范围内开展了双道次热压缩实验,结果表明,随着温度的升高和应变速率的降低,平均晶粒尺寸呈增加趋势;亚动态再结晶和静态再结晶在变形后保温大约3 min就发生地比较完全,在后续的保温过程中,晶粒长大非常缓慢。随着应变的增加,再结晶体积分数增加。基于实验结果,通过参数识别获得模型参数,平均相对误差为6.90%,平均相对偏差为3.3μm。将模型集成在DEFORM中,模拟了Ф90 mm×200 mm的圆柱形试样的平均晶粒尺寸变化,并在模锻压力机上开展了验证实验,结果表明所建立的双道次压缩晶粒尺寸演化模型能很好地预测300M高强钢平均晶粒尺寸的演化。

Effect of Boron on CGHAZ Microstructure and Toughness of High Strength Low Alloy Steels

Effect of boron on the microstructure and impact toughness in the coarse-grained heat-affected zone（CGHAZ）of two high strength low alloy steels,boron-free and boron-containing,was investigated by means of weld thermal simulation test.The result shows that,for the boron-free steel,a microstructure consisting of grain boundary ferrite degenerates pearlite and granular bainite for longer t8/5（the cooling time from 800 to 500 ℃）,while lath bainite for shorter t8/5.For the boron-containing steel,granular bainite is dominant for a wide range of t8/5.Continuous cooling transformation（CCT）study on the CGHAZ indicates that the transformation start temperature decreases by about 50-100℃under different t8/5,for the boron-containing steel compared with the boron-free steel.The presence of boron suppresses the nucleation of ferrite at prior austenite grain boundaries and hence enlarges the range of t8/5for granular bainite transformation.However,the addition of boron deteriorates the impact toughness of CGHAZ,which may be due to a markedly increased fraction of martensite-austenite（M-A）constituents and decreased fraction of high angle grain boundaries.

Microstructure and Properties of 980MPa UltrafineGrained Cold Rolled Dual-Phase Steels

Two kinds of 980MPa grade cold rolled dual phase steels have been developed by designing C-Si-Mn and C-Si-Mn-Nb alloy systems.The microstructure of martensite in Nb-free steel is consisted of lath martensite and twined martensite with the volume fraction of 67%.However,the main hard phase in Nb-containing one is twined martensit with the volume percent of 59%.The size of martensite islands in Nb-containing steel is from 1μm to 3μm,and the size of NbC precipitates is from 15nm to 40nm.As to Nb-containing steel,the yield strength,tensile strength,yield ratio and elongation are 501MPa,1035MPa,0.48 and 17.5% respectively.Futhermore,Nb-containing steel has higher working hardening rate value above the critical strain 6.5%.And it decreases slowly with increasing the strain.This is mainly because of ultrafine grain size and nano-precipitates in ferrite,which improves the compatibility of phases and reduces the stress concentration at the phase interface.

汽车用双相钢的研究与生产现状

双相钢以其优良的力学性能和成型性能在汽车行业得到了大力发展,成为理想的汽车用钢板。目前国内与国外相比,无论是在双相钢板基础理论研究上还是实际生产产品上都还存在着相当大的差距;介绍了国内外双相钢的研究现状及重点研究方向,指出国内应注重在双相钢的高强化、镀锌化、细晶化等方面加强研究,以适应我国对汽车用双相钢板迅猛增长的需求。