CMnAlSi TRIP Steels: Part One

요약:

Continued material development in the automotive has given rise to a new generation of HSLA grades which are characterized by excellent strength and high ductility levels.
Studies of microstructure changes in high strength CMnAlSi steel after austenitization show that it is not possible to obtain a fully austenitic region with the addition of Al or Si higher than about 1.5% and actually these two elements strongly stabilize ferrite.

The CMn steels with the addition of Al or/and Si is a new generation of HSLA grade. These kinds of steels are used in automotive application, offering great potential for bodies-in-white. These steels are characterized with ferritic-bainitic microstructure with retained austenite, which undergoes martensitic transformation during technological forming of products, additionally contributing the strengthening of a finished part.

Steels strengthened through the TRIP effect usually consist of around 0.2% C, 1 2% Mn and around 1.5% Si. Participation of retained austenite, usually being equal from 10 to 15% has a decisive contribution for obtaining high strength and ductility of these steels. It generally depends on carbon concentration in austenite increasing in individual stages of production of sheets made of this group of steels.

Thermodynamic stability of retained austenite is defined by the temperature of martensitic transformation start Ms. Depending on the required sheet thickness and specific application, TRIP-type steels are manufactured through intercritical annealing of cold rolled sheet with successive isothermal holding in range of bainitic transformation temperature or through energy-saving method of thermo-mechanical processing. The majority of research concerning steels with TRIP effect refers to the elaboration of optimum heat treatment conditions after cold rolling, allowing achievement of desired participation of retained austenite. Moreover, their behavior during cold plastic deformation with variable strain rate in room temperature, lowered and also increased temperature is also investigated.

Obtaining a fine-grained microstructure of the steel before the beginning of direct cooling from the temperature of last roll pass requires knowledge of chemical composition influence, and in particular, impact of microadditions introduced into the steel in strengthening processes and recrystallization occurring in hotworking conditions. So far, problems regarding TRIP steels haven’t received much attention, whereas they are essential for correct design of hot rolling.

A similar approach should be applied in the case of microalloyed steels as well as IF and DP-type steels, which are the subject of physical simulations and mathematic modeling. Apart from strict monitoring of temperature–time conditions during hot rolling, precise control of the course of cooling after last roll pass, particularly in a range of γ→α transformation and during isothermal holding in bainitic range is also required.

Many studies of TRIP-steels have been concerned with the mechanical properties and the heat-treatment parameters of CMnSi TRIP-steels. There are few studies focused on the microstructure and mechanical properties of CMnAlSi TRIP-steels. Also, the effects of mechanical stability of retained austenite and the heat-treatment conditions on the resulting mechanical properties have not been investigated sufficiently.

In the paper of Gajda B. and Klis A.K. is presented the results of investigations of microstructure changes in the modern high-strength CMnAlSi steel after austenitization at (α+γ) temperature 900°C/60s in order to determine the influence of the cooling rate on the phase transformations and obtaining multiphase TRIP-aided microstructure. Also the effect of alloying elements on the Ac1 and Ac3 temperatures and the volume fractions of austenite in various (α+γ) austenitization temperatures for the investigated steel were presented.

It is shown in Figures 1 and 2 that the additions of Al and/or Si haven’t got the strong influence on Ac1 temperature, it varies from 688°C to 733°C, but with the increase of Al and/or Si content in the steel the Ac3 temperature grows strongly. It is not possible to obtain the fully austenitic region for the steel with the addition of Al or Si higher than about 1.5%. Al and Si are the elements strongly stabilizing ferrite.



Figure 1: The influence of the Al content on Ac1 and Ac3 temperatures for CMnSi steel (0.15%C, 1.55%Mn, 1.01%Si)



Figure 2: The influence of the Si content on Ac1 and Ac3 temperatures for CMnAl steel (0.15%C, 1.55%Mn, 1.09%Al)

In the investigated steel after austenitization at 900°C/60s about 50% of austenite is obtained. Cementite is completely dissolved. The new austenite saturated with carbon and manganese transforms during cooling to various phases. Dilatometric curves of the samples heat treated at 900°C/60s were used for creation of the CCT diagram of the investigated CMnAlSi steel. The diagram is shown in Figure 3.



Figure 3: CCT diagram of investigated CMnAlSi steel after austenitization at 900°C/60s



Figure 4: Typical TRIP microstructure of CMnAlSi austenitized at 900°C/60s cooled with the rate 20°C/s, SEM

Steel CMnAlSi is well suited for production of TRIP grades in a large range of temperatures from 800°C to 900°C at the cooling rates of about 10°C/s to 40°C/s. The amount of 50% austenite at temperature 900°C allows for production of a TRIP microstructure with stable retained austenite.

기술 자료 검색

검색할 어구를 입력하십시오:

검색 범위

본문
키워드

머릿글
요약

Total Materia는 다양한 나라와 규격에 따른 금속학 이미지에 대한 정보를 포함하고 있습니다.

메뉴 표시줄에 특별히 디자인된 금속학 탭을 이용하여, 금속학 데이터가 포함된 관심 재질을 리스트에서 선택하실 수 있습니다.

또한 금속학 데이터는 표준 빠른 검색을 통해 찾을 수 있으며 규격 내 소그룹 페이지를 통해 이용 가능한 관련 자료들이 표시됩니다.

재질명을 '재질'창에 입력하신 후 규격을 알고 계신다면 규격을 선택하고 '검색' 버튼을 클릭합니다.


미세 구조에 대한 일반적인 정보가 관련 관심 재질의 화학 조성과 함께 출력됩니다.


구조의 세부 범위를 보여주는 여러 배율에서의 이미지가 가능하다면 제공됩니다.

다양한 조건을 선택할 수 있으며, '조건 선택' 메뉴를 사용하여 다양한 공정 및 열처리에 따른 금속학 이미지를 보여줍니다.



Total Materia 데이터베이스를 사용해 보실 수 있는 기회가 있습니다. 저희는 Total Materia 무료 체험을 통해 150,000명 이상의 사용자가 이용하고 있는 커뮤니티로 귀하를 초대합니다.