| Faced with environmental and energy pressures,the automotive industry is gradually developing towards light weight.Automobile chassis parts are made of low-alloy high-strength cast steel with higher performance,reducing the structural size of the cast steel parts,which can effectively achieve automobile weight loss.Based on the above considerations,this subject will design cast steel composition and heat treatment process,aiming to develop a low alloy high strength cast steel.The chemical composition of low-alloy high-strength cast steel in this subject is: C0.19%～0.26%,Si0.4%～0.5%,Mn0.4% ～1.1%,Cr0.8% ～0.9%,Ni1.0%～1.1 %,Mo0.2%～0.4%,Nb0.02%～0.07%,V0.7%～0.8%,B0.01%～0.03%.The subject designs three heat treatment processes: normalizing + tempering,quenching and tempering,and Q&P,and studies the effects of Nb,B and heat treatment processes on the structure and properties of low-alloy high-strength cast steel.And through OM,SEM,XRD and other analytical techniques to study the mechanism of Nb,B and residual austenite in Q&P steel.The main findings are as follows:(1)Nb can effectively refine the grain structure.When the Nb content reaches 0.042%,the structure of the low-alloy high-strength cast steel is even and fine,the Nb content further increases,and the structure is no longer significantly refined.(2)With the increase of Nb content,the strength of low-alloy high-strength cast steel increases,while the toughness decreases.When the Nb content is 0.042%,it exhibits excellent strength and toughness,and the tensile strength,yield strength,elongation after break,shrinkage at break and impact energy are 872 MPa,765MPa,15.5%,41% and 13.2J,respectively in the quenched and tempered state are 920 MPa,843MPa,14.7%,40% and 56.5J.(3)B can refine the structure of low-alloy high-strength cast steel.The B content does not exceed 0.0028%.With the increase of B content,the strength is slightly increased,and the impact toughness is significantly improved.Normalized+tempered low-alloy high-strength cast steel with B content of 0.0028% tensile strength,yield strength,elongation after fracture and shrinkage at break are 890 MPa,770MPa,14.5% and 39%,respectively,in the tempered state 930 MPa,795MPa,13.5% and 32% respectively.When the B content increases from 0.0012% to 0.0028%,the normalized and tempered impact energy increases from 10.7J to 15.6J,and the tempered impact energy increases from 28.5J to 35.7J.(4)The low-alloy high-strength cast steel with a B content of 0.0028% is processed by Q&P process.The results show that the structure of the low-alloy high-strength cast steel after the Q&P process at different quenching temperatures is made of lath martensite and tempered.Martensite and retained austenite.As the quenching temperature increases,the martensite content decreases and the residual austenite content increases.(5)As the quenching temperature rises,the strength of Q & P steel decreases and the toughness increases.It exhibits excellent strength and toughness when quenched at 310 ° C.At this time,the tensile strength,yield strength,elongation after break,shrinkage rate of section,impact energy and the residual austenite contents were 1148 MPa,798MPa,12.6%,15.1%,9.8J and 6.7%.(6)Based on the combined effects of Nb and B on the structure and properties of low-alloy high-strength cast steel,it is recommended that the composition of low-alloy high-strength cast steel for automobile chassis parts be: C0.19%～0.21%,Si0.4%～0.5% Mn0.4%～0.5%,Cr0.8%～0.9%,Ni1.0%～1.1%,Mo0.3%～0.5%,Nb0.04% ～ 0.05%,V0.7% ～ 0.8%,B0.02% ～ 0.03%,different heat treatment processes can be used according to performance requirements. |
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