Modern large-scale cold strip rolling mills have achieved endless and semi-endless rolling. The requirements for the quality of strips, such as the surface flatness and thickness difference of cold-rolled strips, are getting higher and higher, and the quality requirements for work rolls are also getting higher and higher. The higher it is, the higher the requirements for surface hardness and the depth of the hardened layer are. The research on the material of cold-rolled work rolls has always been highly valued by the roll industry. It has gradually developed from the initial bearing steel to 2%Cr, 3%Cr, and 5%Cr section steel, and its development is usually characterized by the continuous increase of chromium content. In recent years, semi-high-speed steel and high-speed steel roll materials have appeared, and there have been some reports on the research on their microstructure and carbides. The technicians of Anhui Huanbowan High-speed Steel Roll Co., Ltd. studied the quenching process of a newly developed forged high-speed steel cold-rolled work roll material, and analyzed the effect of quenching temperature on the structure, grain size, retained austenite of high-speed steel rolls The influence of hardness and hardness provides a theoretical and experimental basis for the development of high-end cold-rolled work rolls.
The new-type forged high-speed steel roll material used for the test was smelted in a 50kg vacuum induction furnace, and then forged into a 100mm×100mm×250mm billet at high temperature, and the original material for the test was obtained after annealing at 700°C for 4h. The metallographic sample is a 15mm×15mm×15mm square, which is heated at different temperatures in a box-type resistance furnace and quenched by air cooling. 660RLD/T Rockwell hardness tester was used to test the hardness, X’perPro X-ray diffractometer was used to test the retained austenite, Axiovert200MAT optical microscope was used to test the structure and grain size, and QUANTA400 scanning electron microscope was used to observe the carbide.
The as-cast microstructure of the new forged high-speed steel roll is lower bainite + ledeburite + a small amount of martensite + block carbide + granular carbide, of which the large block carbide is precipitated during the solidification of molten steel. Primary carbides, granular carbides are secondary carbides precipitated from austenite during cooling, with a carbide content of 3.6%. research shows:
(1) The quenching temperature affects the dissolution of carbides. With the increase of the quenching temperature, the carbide content in the quenched structure of the high-speed steel roll gradually decreases. When the temperature was raised to 1200°C, the granular carbides were basically dissolved, and only a small amount of massive carbides remained.
(2) With the increase of quenching temperature, the grains of high-speed steel continue to grow. When the quenching temperature exceeds 1040 °C, the grain growth trend is obvious. When the quenching temperature exceeds 1160 °C, the grain size reaches 4.5, and the grain coarsening is very serious.
(3) With the increase of quenching temperature, the content of retained austenite increases continuously. When the quenching temperature is below 1080℃, the content of retained austenite increases gradually, and then increases sharply. When the quenching temperature reaches 1160℃, the retained austenite content increases to 38%. Therefore, considering the control of retained austenite content, the quenching temperature should be lower than 1080 °C.
(4) When quenched at 1040°C, the hardness reaches a peak value of up to 64.1HRC. At this time, the high-speed steel roll structure, grain size, and retained austenite content match the best.
(5) The quenching temperature of the new forged high-speed steel roll material should be selected between 1020 and 1080 °C.