Study on Hot Ductility of Slab of 1 180 MPa Grade F/M High Strength Dual Phase Steel for Automobile
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摘要: 采用Gleeble3500热模拟试验机在温度区间650~1300℃对汽车用1 180 MPa级F/M双相高强钢进行高温热塑性研究,绘制热塑性曲线并对高温拉伸试样断口和显微组织进行观察。试验结果可知:该钢种在试验温度范围内存在1个脆性区,即910~675℃区间,800℃时断面收缩率达到最小值28.76%,在熔点~910℃温度区间内呈现良好塑性,断面收缩率均在60%以上;高温塑性区较窄,第Ⅲ脆性区“布袋”曲线明显且范围较大,该钢种裂纹敏感性高。断口观察可知,950℃和650℃断口均具有典型韧窝特征,属于韧性断裂;800℃断口为沿晶和解理混合型断口,属于典型脆性断裂。650℃断裂主要由先共析铁素体沿原奥氏体晶界析出引起,800℃脆性断裂主要由晶界弱化导致,1 050℃以上高温热强度低,拉伸超过材料所承受的最大强度而发生缩颈断裂。为避免板坯在矫直段产生裂纹,铸坯矫直温度应控制在950℃以上,避开第Ⅲ脆性区(910~675℃)。Abstract: The hot ductility of 1180 MPa F/M high strength dual phase steel for automobile had been investigated in the temperature range of 650℃ to 1300℃ using a Gleeble 3500 thermal simulator. The thermoplastic curve was drawn,and the fracture and microstructure of high temperature tensile specimen were observed. The test results show there is a brittle zone in the test temperature and the brittle temperature zone is in range of 675℃ to 910℃. The minimum shrinkage rate reaches 28.76% at 800℃,and it shows good plasticity in the temperature range of melting point to 910℃ with shrinkage rate above 60%. the high temperature plastic zone is narrow and the Ⅲ brittle zone is a typical "U" curve and wide range,indicating high crack sensitivity. Fracture observationI shows that the fractures at both 650℃ and 950℃ have typical dimple which belong to ductile fractures. Fracture at 800℃ is typical brittle mixing along grain boundary and cleavage fracture. The fracture at 650℃ is mainly caused by precipitation of pro-eutectoid ferrite along original austenite grain boundary. The brittle fracture at 800℃ is mainly caused by grain boundary weakening.The high-temperature strength above 1050℃ is low,because the tensile stress exceeds the maximum strength of the steel. In order to avoid slab cracking during straightening section,the slab straightening temperature should be controlled above 950℃ which fall out of the Ⅲ brittle zone(from 675℃ to 910℃).
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Key words:
- F/M high strength dual phase steel /
- slab /
- thermoplasticity /
- fracture analysis /
- microstructure
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