Tomohiko Hojo

Effects of Fine Particle Peening on Fatigue Strength of Transformation-Induced Plasticity (TRIP)-Aided Martensitic Steel

The effects of fine particle peening on the torsional strength of a transformation-inducedplasticity (TRIP)-aided martensitic (TM) steel were investigated for applications to precision gears. Fine particle peening increased the fatigue limits and lowered the notch-sensitivities of steel, compared with quenched and subsequently tempered SNCM420 steel. In addition, fine particle peening lowered the fatigue crack propagation rate in TM steel with a high threshold value of the stress intensity range. These results were associated with (1) higher Vickers hardness, (2) higher compressive residual stress, and (3) a larger amount of untransformed retained austenite in the surface layer when compared with SNCM420 steel.

Microstructure and Mechanical Properties of Ausformed Ultra High-Strength TRIP-Aided Steels

Ultra high-strength low-alloyed TRIP-aided steels possess high impact toughness, fatigue strength and hydrogen embrittlement performance, as well as good formability. So, it is expected that the TRIP-aided steels are applied to some automotive parts such as not only center pillar but also driving parts, spring and bolts. In the present study, the effects of ausforming conditions (temperature and strain) after austenitizing on tensile properties and toughness of TRIP-aided steels with bainitic ferrite matrix were investigated to develop a new type of ultra high-strength forging steel. TRIP-aided steel with chemical composition of 0.2%C, 0.5%Si, 1.5%Mn, 1.0%A1, 0.02%Nb and 0.1 %Mo achieved large total elongation and high impact toughness by ausforming to 0-50% strain at 600-900°C, followed by austempering at 400°C for 500s. This was mainly caused by refined microstructure and stabilized retained austenite.

Critical assessment 29: TRIP-aided bainitic ferrite steels

ABSTRACT Transformation-induced plasticity (TRIP)-aided bainitic ferrite steels developed for automotive applications have attractive mechanical properties such as ductility, formability, toughness, fatigue strength and delayed fracture strength. These mechanical properties are principally associated with a ductile lath-structure matrix and the strain-induced transformation of the metastable-retained austenite films of 3–20 vol.%. In this paper, data on the microstructural and mechanical properties of the low-carbon TRIP-aided bainitic ferrite steels are critically assessed, as well as their deformation mechanism.

Impact properties of low-alloy transformation-induced plasticity-steels with different matrix

The effects of matrix types on Charpy impact properties were investigated in Fe–0.2%C–1.5%Si–1.5%Mn (mass%) transformation-induced plasticity steels. The steels with annealed martensite and bainitic ferrite matrix possessed higher upper shelf Charpy impact absorbed energy than the steel with polygonal ferrite and martensitic matrix. In addition, the low ductile–brittle fracture appearance transition temperatures were achieved in annealed martensite and martensite types in comparison with those of other steels.

Fatigue strength of a vacuum-carburised TRIP-aided martensitic steel

ABSTRACT Fatigue properties of a transformation-induced plasticity-aided martensitic steel subjected to vacuum carburisation under carbon potentials ranging from 0.70 to 0.80 mass% and subsequent fine-particle peening were investigated for the fabrication of precision transmission gears. The fatigue limits of smooth specimens increased by 34–41% with increasing carbon potential, compared with that of heat-treated and fine-particle peened steel. The notched fatigue limits increased to a lesser degree except in case of carbon potential of 0.70 mass%. The increased smooth fatigue limits were associated with the high Vickers hardness and compressive residual stress via severe plastic deformation and the strain-induced martensitic transformation during fine-particle peening, as well as a 50% reduction of retained austenite fraction during fatigue deformation.

Effect of Heat-Affected Zone on Spot Weldability in Automotive Ultra High Strength Steel Sheet

Effect of heat-affected zone (HAZ) on spot weldability in automotive hot stamping (HS) steel sheet was investigated for automotive applications. Tensile test was performed on a tensile testing machine at a crosshead speed of 3 mm/min, using spot welded test specimen (Parallel length: 60 mm, Width: 20 mm, Thickness: 1.4 mm, Tab: 20×20 mm). The spot welding test was carried out using spot welded test specimen with welding current (I) of 6.3 kA to 9.5 kA. Hardness was measured with the dynamic ultra micro Vickers hardness tester. In HS steel, has very high strength of 1 500 MPa, tensile strength (TS) and total elongation (TEl) of the spot welded test specimen of HS steel were lower than those of base metal test specimen. The spot welded test specimen broke in the weld. The Vickers hardnesses (HVs) of base metal and fusion zone of hot stamping steel were around HV500. In addition, the hardness of HAZ was under HV300. The difference of hardness between fusion zone and HAZ was around HV200. The hardness distribution acted as a notch. On the other hand, in dual phase (DP) steel, has low strength of 590 MPa, the TS of spot welded test specimen of DP steel was the same as the base metal test specimen because of the breaking of base metal. The TEl of the spot welded test specimen of DP steel was smaller than that of base metal test specimen. In the spot welded test specimen of DP steel, the hardness of base metal was around HV200 and the fusion zone was around HV500. The hardness distribution did not act as a notch. The difference in hardness between base metal and HAZ acted on a crack initiation at HAZ softening.

Evaluation for hydrogen embrittlement properties of ultra high-strength steel sheets by 4-Point Bending Technique

Hydrogen embrittlement properties of SCM435 and vanadium added steel sheets were investigated by using 4-Point Bending Technique (4PBT) to propose the standardization for the evaluation of hydrogen embrittlement properties. Maximum fracture strengths of steel sheets using 4PBT were decreased with increasing diffusible hydrogen concentration. vanadium added steels indicated high hydrogen embrittlement resistances compared with SCM435 steels. The tendencies between hydrogen concentration and maximum fracture strength obtained by 4PBT in SCM435 and vanadium added steels were similar to those using Conventional Strain Rate Technique (CSRT) although maximum fracture strengths using 4PBT were higher than those using CSRT in both steels.

Evaluation for Hydrogen Embrittlement Properties of Tempered Martensitic Steel Sheets Using Several Testing Technique

近年,自動車の衝突安全性向上と自動車車体軽量化によ る燃費向上のため,自動車用薄鋼板の超高強度化の要求が 高まっており,多くの自動車用衝突安全部材に980 MPa超 級の超高強度鋼板が適用され始めている。そのため,自動 車用超高強度鋼板も水素脆化(遅れ破壊)の発生が懸念さ れるようになり,耐水素脆化特性に優れた超高強度薄鋼板 の開発 ,および水素脆性発生のメカニズムに関する研 究 が積極的に行われるようになった。日本の水素脆化 に関する研究は,1960年代,橋梁に用いられていた,引張 強さが1300 MPa級のF13T高力ボルトが突然破壊したこと によって始まった 。そのため,これまでの水素脆化に関 する研究のほとんどは丸棒を用いた研究 であった。こ こで,一般的に静的荷重を負荷したあと,時間遅れをとも なって水素起因による破壊を発生する現象を遅れ破壊,材 料中に侵入した水素によってじん性が著しく低下する現象 を水素脆化と呼ぶが,遅れ破壊は水素脆化の一種であるた め,本研究では両者とも水素脆化と呼ぶこととする。 現在までに水素脆化特性を評価する方法として,鋼材が 破壊しない限界拡散性水素量(HC)と鋼材の使用中に環境 から侵入する拡散性水素量(侵入拡散性水素量(HE))の関 係から水素脆化が生ずるか否かを判定する方法 が提案 されている。このなかで,丸棒のHCを評価する実験手法と しては,一定荷重法(CLT:Constant Load Test),低ひず み速度引張試験法(SSRT:Slow Strain Rate Technique), および通常のひずみ速度引張試験法(CSRT:Conventional Strain Rate Technique)などが用いられている。超高 強度薄鋼板においては上記の試験法に加えて4点曲げ 法(4-Point Bending Technique)やU曲げボルト締め法 (U-Bending Technique)による水素脆化特性の評価も可 能である。 上記のように,これまでの水素脆化に関する研究は各 研究機関でそれぞれの試験法,評価法で議論されてきた。 2010年に発足した一般社団法人日本鉄鋼協会材料の組織 と特性部会「水素脆化研究の基盤構築」研究会では,それ ぞれの研究機関が共通基盤技術を構築し,各研究機関で評 価された水素脆化特性を共通的に議論できるようにするこ 種々の試験法による焼戻しマルテンサイト鋼板の 水素脆化特性評価