铌微合金化1800 MPa级热成形钢的高温热塑性

Hot ductility of a Nb-microalloyed 1800MPa grade hot-stamping steel

在碳中和背景下,以热成形钢为代表的超高强钢在汽车白车身中的应用逐渐增加。微合金化技术有助于提高热成形钢的力学性能,但微合金元素对热成形钢高温热塑性的影响仍待探索。采用Gleeble-3500热模拟试验机评估了铌质量分数为0.04%的1 800 MPa级铌微合金化热成形钢(0.04Nb钢)在不同高温条件下的热塑性,并分析了不同温度区间内0.04Nb钢的断裂失效机制。结果表明,0.04Nb钢具有2个脆性温度区间和1个塑性温度区间。试验数据分析表明,第I脆性温度区间为1 250~1 300℃,试样呈沿晶脆性断裂,原因是高温使晶界结合力减弱,同时可能存在低熔点的硫、磷等夹杂物,进一步弱化晶界结合力;塑性温度区间为850~1 250℃,在这个区间里钢的热塑性较高且断面收缩率均大于60%,虽然在此温度范围内开始有Nb(C,N)析出相的析出,但高温促进晶界和位错的运动以及动态再结晶的发生,保证了0.04Nb钢高的热塑性;第III脆性温度区间为650~800℃,在这个区间里钢的热塑性随温度降低呈下降的趋势,沿晶界呈网状分布的先共析铁素体和在晶界析出的细小Nb(C,N)析出相导致拉伸时产生的应力集中是本阶段诱发脆性断裂的主要原因。0.04Nb热成形钢在实际的连铸过程中,应避开650~800℃的脆性温度区间,而在900℃以上的塑性温度区间进行矫直操作。

Under the background of carbon neutrality,the use of hot-stamping steels representing one of the ultrahigh strength steels in automotive body-in-white is gradually increasing.Microalloying technology is helpful to improve the mechanical properties of hot-stamping steels.However,the effect of microalloying elements on the hot ductility of hot-stamping steels remains unknown.The hot ductility of a Nb-microalloyed 1800 MPa grade hotstamping steel(0.04%Nb steel,designated as 0.04Nb steel)was evaluated using Gleeble-3500thermal simulation testing machine,and the fracture mechanisms at various test temperatures were analyzed.The results indicated there were two hot brittle ranges and a temperature range with high ductility for the 0.04Nb steel.The first hot brittle range,spanning from 1250℃to 1300℃,showed intergranular brittle fracture due to the reduction in grain boundary adhesion caused by high temperatures.Meanwhile,the inclusions formed by S and P with a low melting point may exist,which can further weaken the grain boundary adhesion.The temperature range spanning from 850℃to 1250℃is featured by high hot ductility with the reduction of area greater than 60%.The high ductility was attributed to the occurrence of dynamic recrystallization and the intensive movement of grain boundaries and dislocations,despite that Nb(C,N)starts to precipitate at the temperature range.The third hot brittle range spans from 650℃to 800℃,where the hot ductility of the 0.04Nb steel generally decreases with decreasing temperatures.The poor ductility was caused by intergranular proeutectoid ferrite and fine Nb(C,N),which leads to stress concentration at grain boundaries during straining and results in brittle fracture.During the actual continuous casting process of 0.04Nb steel,the hot brittle range of 650-800℃should be avoided,while the straightening operation should be carried out in the temperature range above 900℃.