【摘要】 采用恒载荷延迟断裂、氢热分析(TDS)等实验方法,研究了一种低碳Mn-B系超高强度热成形钢板在热成形态和常规淬火回火态的氢致延迟断裂行为.结果表明,经热成形后,试样缺口临界应力和断裂寿命均高于常规淬火态,并达到常规淬火态+100℃回火试样的水平.热成形试样经200℃回火处理后,具有更高的缺口临界应力和断裂寿命.SEM观察发现,在外加应力和腐蚀液的作用下,热成形试样裂纹源区的断裂机制由常规淬火试样的沿晶+穿晶准解理的混合断裂变为穿晶准解理+少量沿晶的混合断裂.TDS实验结果表明,在临界应力作用下,在腐蚀液中加载100 h后,试样中的H含量均明显提高.由于试样在热成形过程中发生较高程度的自回火,导致经应力腐蚀后,试样在断裂过程中能承受的H量高于淬火态试样和常规淬火态+100℃回火试样,使试样具有更高的延迟断裂抗力.因此,经热成形处理后,试样具有良好的耐延迟断裂性能,且回火处理能使试样的耐延迟断裂性能得到进一步提高.更多还原

【Abstract】 In consideration of the light weight and the impact safety of cars,in recent years,hot stamping has been increasingly applied to the manufacture of car parts,which has driven forward the increasing use of ultra-high strength steel sheet in the automobile industry.However,when the tensile strength of steel is more than about 1200 MPa,the steel sheet is very susceptible to hydrogen induced delayed fracture.Up to now,some research has been carried out on delayed fracture behavior of both high strength and ultra-high strength steel sheet.However,the delayed fracture behavior of a low-carbon Mn-B type steel sheet widely used for hot stamping has not been paid enough attention and fully studied.There are also a scarcity of reports on hydrogen induced delayed fracture behavior of this steel sheet after hot stamping.With the development of the hot stamping,the evaluation of delayed fracture behavior of ultra-high strength steel sheet for hot stamping,especially the delayed fracture behavior of steel sheet after hot stamping has become very urgent.For these reasons,hydrogen induced delayed fracture behavior of a low-carbon Mn-B type ultra-high strength hot stamping steel sheet at both the hot stamped status and the common quenched and tempered status was studied by means of constant load delayed fracture test and hydrogen thermal desorption spectrometry(TDS)analysis.The results show that both the critical delayed fracture stress σ_c and delayed fracture Ufe ofthe steel sheet after hot stamping process are higher than as-quenched sample,and it even matches the level of the quenched +100℃ tempered sample.Moreover,the σ_c of the steel sheet could be further improved by tempering at 200℃.The delayed fracture surface observed by SEM shows that,under the effect of applied stress and corrosive liquid,the fracture characteristic of the hot stamped sample at crack initiation area changes from brittle intergranular failure in the as-quenched condition to ductile transgranular failure in the hot stamped condition.Furthermore,the TDS result shows that the hydrogen content of steel sheet significantly increases when the steel sheet was applied at critical stress for 100 h in the corrosive liquid.In addition,the hot stamping steel sheet can withstand more hydrogen content than as-quenched sample and quenched +100 ℃ tempered sample due to the sufficient self tempering.As a result,the hot stamping sample gets a higher critical delayed fracture stress.Therefore,the steel has good delayed fracture resistance after hot stamping,and the delayed fracture resistance of the steel sheet could be further improved by tempering.更多还原