Effect of HER property in bainite-martensite complex steels according to Si and Al compositions with isothermal heat treatment temperatures

Abstract

Various elements are used to make high strength and high elongation alloy steels. Among the lots of alloy elements, Si is the representative element to improve strength of steels because it has solid solution strengthening effects to the ferrite matrix. On the other hand, Si makes wettability problems in the hot-dip galvanizing process, so Al is being considered one of the elements discussed to replace it.

To make bainite and martensite mixed complex steels by performing isothermal heat treatments before/after Ms temperature through Q&P process, two different types of chemical composition alloy steels are used. One is high Si contents alloy steels (Si7Cr2 alloys), and the other one is Al contents steels (Al2Cr2 alloys) in which Si contents are replaced partially by Al.

As a result of confirming the stretch-flangeability through the HER (Hole Expansion Ratio) test, there is no significant differences in HER because chemical compositions between two alloy steels. But, in common, they show that the HER is decreased as the isothermal heat treatment temperature is increased.

By EBSD (Electron BackScatter Diffraction) analysis of HER specimens, it is found that the smaller EGs (Effective Grain size), the higher HER. And the larger aspect ratio (the microstructures are longer and lath shape), the higher HER. Through this, it is found that the chemical compositions of alloy elements by replacing parts of the Si contents with Al contents have a little effect on the HER characteristics. And also, it is confirmed that the most important factors affecting to the HER characteristics are the size and shape of the microstructures according to the isothermal heat treatment temperatures.

고강도/고연신 합금강을 만들기 위하여 다양한 합금원소가 사용되고 있다. 그중에서 Si은 대표적인 합금원소로 ferrite matrix에 고용강화 효과를 부여하여 강도 향상에 기여하지만, 용융도금공정에서 낮은 젖음성 문제를 일으켜 이를 대체하기 위하여 Al이 거론되고 있다.

높은 함량의 Si를 가진 합금강(Si7Cr2 alloys)과 Si의 일부를 소량 Al로 대체한 합금강(Al2Cr2 alloys)을 Q&P 프로세스를 통해 Ms 온도 전/후로 등온 열처리를 진행하여 bainite와 martensite가 혼합된 복합조직강을 만들었다.

HER(Hole Expansion Ratio) 시험을 통해 Stretch-flangeability 특성을 확인한 결과, 두 강종 간에 합금원소 함량 차이에 의한 HER 차이는 크게 없었으며, 공통적으로 등온 열처리 온도가 증가함에 따라서 HER이 감소하는 경향을 보였다.

HER 시편에 대하여 EBSD(Electron BackScatter Diffraction) 분석 결과, EGs(Effective Grain size)가 작을수록 HER이 증가하고, Aspect Ratio가 클수록(microstructure가 길쭉한 lath 형태일수록) HER이 증가한다는 것을 알 수 있었다. 이를 통해 HER 특성에 대하여 Si 함량 일부를 Al로 대체함에 따른 합금원소 영향은 거의 없으며, 등온 열처리 조건에 따른 microstructure의 크기와 형상이 HER에 영향을 미치는 주요 인자라는 것을 알 수 있었다.