The research on the material of cold-rolled work rolls has always been highly valued by the roll industry. From the initial bearing steel, it has gradually developed to 2% Cr, 3% Cr, and 5% Cr section steel. Its development is usually based on the continuous improvement of chromium content. In recent years, semi high-speed steel and high-speed steel roll materials have been developed and there have been some reports on their microstructure and carbide research. The technicians have studied the quenching process of a newly developed forged high-speed cold-rolled work roll material and analyzed the quenching temperature on the structure, grain size, residual austenite and hardness of the high-speed steel rolls, which provides the theoretical and experimental basis for the development of high-end cold-rolled work rolls.
The new type forged high-speed steel roll material used in the test was smelted in a 50kg vacuum induction furnace and then forged at a high temperature to produce a 100mm × 100mm × 250mm billet, which was annealed at 700°C for 4h to obtain the test raw material. The metallographic sample is a 15mm × 15mm × 15mm block, which is heated at different temperatures by a box-type resistance furnace and quenched by air cooling. The hardness was measured using a 660RLD / T Rockwell hardness tester and the residual austenite was detected using an X’perPro X-ray diffractometer, the structure and grain size were measured using an Axiovert200MAT optical microscope, and the carbides were observed using a QUANTA400 scanning electron microscope.
The microstructure of the new forged high-speed steel rolls is lower bainite + bainite + a small amount of martensite + bar carbide + granular carbide. Among them, large bar carbide is precipitated during the solidification of molten steel. Primary carbides, granular carbides are secondary carbides precipitated from austenite during cooling, and the carbide content is 3.6%.
The quenching temperature of the forged high-speed steel roll
(1) The quenching temperature affects the dissolution of carbides. As the quenching temperature increases, the carbide content in the quenched structure of high-speed steel rolls gradually decreases. When the temperature increased to 1200 °C, the granular carbides had been basically dissolved, leaving only a small amount of massive carbides.
(2) With the increase of quenching temperature, the grains of high-speed steel continues to grow. When the quenching temperature exceeds 1040 °C, the grain growth tendency is obvious. The quenching temperature exceeds 1160 °C and the grain size reaches 4.5, then the grain coarsening will become very serious.
(3) As the quenching temperature increases, the residual austenite content increases continuously. When the quenching temperature is below 1080 °C, the residual austenite content increases relatively slowly and then its content increases sharply. When the quenching temperature reaches 1160 °C, the residual austenite content increases to 38%. So the quenching temperature should be lower than 1080°C, considering the control of residual austenite content.
(4) When the quenching temperature reaches to 1040 °C, the hardness reaches its peak value of 64.1HRC. At this time, the matching of the structure, grain size and residual austenite content could be the best for the high-speed steel roll.
(5) The quenching temperature of the new-type forged high-speed steel roll materials should be selected between 1020 ~ 1080 °C.