退火温度对冷轧Fe20Mn0.3C钢组织及其拉伸变形行为的影响

Effect of annealing temperature on microstructure and tensile deformation behaviors of cold-rolled Fe20Mn0.3C steel

采用扫描电镜、准原位电子背散射衍射(EBSD)、X射线衍射和室温拉伸实验,研究了冷轧Fe20Mn0.3C钢在700~1000℃范围内退火1 h后的微观组织及其拉伸变形行为。结果表明,随着退火温度升高材料的屈服强度逐渐降低,而抗拉强度及伸长率则先升高后降低,当退火温度为800℃时,抗拉强度和伸长率达到峰值。800℃退火试样形成了均匀细小且非常稳定的奥氏体晶粒组织,其拉伸变形机制主要为孪生诱导塑性(TWIP效应);当退火温度进一步升高,奥氏体晶粒长大,其稳定性降低,空冷及拉伸过程中均发生马氏体相变,形变机理由TWIP效应转为相变诱导塑性(TRIP效应)。准原位拉伸EBSD研究表明:在拉伸变形过程中,退火试样中的淬火ε马氏体一方面通过γ→ε形式的TRIP效应增厚,另一方面通过ε→α'形式的TRIP效应转变成α'马氏体,而裂纹容易在α'马氏体界面形核扩展,因此,淬火ε马氏体越多,材料的伸长率越低。

Microstructure and tensile deformation behaviors of cold-rolled Fe20Mn0.3C TRIP/TWIP steels annealed at 700-1000 ℃ for 1 h were investigated by means of scanning electron microscopy, pseudo in-situ electron backseatter diffraction （EBSD） technique, X-ray diffraction and room temperature tension test. The results show that the yield strength of the steels decreases with the increasing of the annealing temperature, and the tensile strength and elongation first increases and then decreases. The tensile strength and elongation reach the peak values when the annealing temperature is 800 ~C. The specimens annealed at 800 ~C consist of small equiaxed austenitic grains with good stability and the deformation mechanisms is twinning induced plasticity （TWIP） in the process of room-temperature tensile test. With the increasing of annealing temperature, the austenitic grain size increases and the stability decreases. The austenite with large grain size can easily transform into martensite in the air-cooling and tensile process, and the deformation mechanism changes from TWIP into transformation inducing plasticity （TRIP）. The pseudo in-situ EBSD analyses show that in the process of deformation, the quenched ε- martensite of annealed specimens can be thickened through the TRIP effect of γ→ε, and the ε-martensite transforms into α′-martensite through the TRIP effect of ε--→α′ The cracks easily nucleate and grow at the interface of the α′-martensite. Therefore, the larger the quenched e-martensite content is, the lower the elongation of the steels is.