What are the commonly used materials for roll manufacturing? How is it classified?
The Chinese national standard GB/T13314 <General Technical Conditions for Forged Steel Cold-rolled Work Rolls> lists 7 common cold-rolled roll materials. In recent years, Chinese roll manufacturing enterprises have formed a new material series after long-term development and accumulation. Cold roll materials are usually classified according to the Cr content of the material, and are divided into 2%Cr series, including 9Cr2, 9Cr2Mo, 9Cr2MoV, etc.; 3% series, including 9Cr3Mo and MC3, etc.; 5%Cr series such as MC5, MC5A, etc. In addition, there are some special materials such as high-speed steel, semi-high-speed steel and high-alloy die steel.
What are the characteristics of commonly used materials for roll manufacturing?
The characteristic requirements of the cold roll material are determined by its working conditions. Due to the high working load on the surface of the cold roll, the material of the roll must have high strength. At the same time, there is mutual friction between the work roll and the rolling material, and between the work roll and the backup roll, so the material of the cold roll must have high wear resistance. Cold rolls often encounter rolling accidents during use, and the material of the roll must have high accident resistance, as well as texturing properties, corrosion resistance, grinding properties, etc.
How to choose the roll material?
The selection of roll material is based on the design and working conditions of the rolling mill. Commonly used roll materials can generally meet the basic use conditions of rolling, so the material is first determined according to the maximum unilateral use depth of the rolling mill design. The effective hardened layer depth of commonly used 2%Cr material can be 15-20mm, 3%Cr material can reach 20-30mm, and 5%Cr material can reach 20-50mm hardened layer depth when matched with different quenching processes . Secondly, when selecting the roll material, the working conditions of rolling and the requirements of rolled products should also be considered. For example, in the initial stage of the rolling mill, it is recommended to use 2% Cr material with better accident resistance. After normal operation, the material of high chromium deep hardened layer can be considered, and the hardness uniformity of 3%Cr and 5%Cr rolls is better. Therefore, it can be given priority in rolling high-demand plates.
What are the smelting processes of cold roll materials?
The cold roll is generally made of medium-high alloy high-carbon chromium steel, and the use conditions of the roll are heavy load and high fatigue, so the requirements for steel smelting are relatively high. Usually, the smelting process of large forged steel roll materials includes electric arc furnace smelting, mainly to achieve various chemical components of steel and control the content of harmful elements. The molten steel in the electric arc furnace is sometimes refined outside the furnace to adjust the chemical composition of the steel and eliminate the excessively high gas content in the steel.
What are the heat treatment contents in the manufacturing process of forged steel cold rolls?
A cold roll undergoes multiple heat treatments during the manufacturing process. After the steel ingot is cast, in order to reduce the segregation in the steel, diffusion annealing must be carried out. The forged billet must be annealed by spheroidizing and hydrogen expansion to reduce the gas content in the steel and improve the distribution of carbides in the matrix. Quenching and tempering heat treatment should be carried out after rough machining, so that the core of the roll can obtain good comprehensive mechanical properties, and provide tissue preparation for subsequent heat treatment. Finally, the roll body is quenched and heat treated. Through the whole or surface heating, the surface layer or the whole of the roll body is heated to above the austenite transformation temperature of the steel, and the temperature is kept for a certain period of time. Then, it is rapidly cooled to form a high-hardness martensite layer on the surface of the roll, and then tempered at a low temperature to adjust the surface hardness and residual stress of the roll. In addition, some rolls also require local heat treatment of the bearing gear to achieve a certain hardness.
What is the cold treatment of the roll and what is its function?
Roll cold treatment is a process of putting the forged steel roll after quenching and before tempering into the -40 to -120 low temperature tank for cryogenic insulation.
How is the final heat treatment of the roll body carried out?
The final heat treatment of the roll usually refers to the quenching of the roll body, which is a key process in the manufacture of rolls. Through the final heat treatment, the roll body forms tempered martensite suitable for cold rolling conditions. According to the difference of quenching heating method, commonly used process methods include two kinds of overall heating method and surface heating method.
The overall heating is to preheat the roll and then heat it in a heating furnace with different heating media, so that the surface and inside of the entire roll reach the required austenitization temperature, and then cool down rapidly. The characteristics of this method are that the process method is relatively simple, and the temperature control is relatively easy, but there are quality problems such as large quenching deformation, large internal stress, and easy cracking.
Surface heating, especially induction surface heating, and roll quenching is a heat treatment heating method commonly used in the manufacturing process of large forged steel rolls. According to the difference in the frequency of the heating power supply, the commonly used ones are low-medium frequency, power frequency and dual-frequency heating. On the special quenching machine tool, when the preheated roll passes through the energized induction coil, due to electromagnetic induction, an induced current is formed on its surface, thereby heating the surface. The heating temperature is adjusted by adjusting the power through temperature measurement feedback, The characteristics of this process are rapid heating, complex process equipment, reasonable non-distribution of quenching layer, and small internal stress.
What are the typical technical requirements for cold rolled work rolls?
Answer: Different rolling mills have different requirements for rolls. The following are the technical requirements for the work rolls of general strip mills:
① The hardness unevenness of the roller body is 2-3HS;
②The depth of the hardened layer in the radial direction is ≥15mm;
③Roll body metallographic structure: martensite + fine dispersed carbide + retained austenite;
④The carbides on the roller body are evenly distributed, and the mesh shape is less than 2.5 grades;
⑤The austenite content in the hardened layer is ≤15%;
⑥The width of the soft belt is ≤60mm.
What are the manufacturing processes of forged steel cold rolled work rolls?
According to the different working conditions of the roll and the different process methods of each manufacturer, the roll manufacturing process is different. A complete roll manufacturing process includes:
Product Design – Smelting (EF, EF＋LFV – Pre-furnace Inspection – Protective Casting Electrode – Electrode Annealing – Electrode Surface Cleaning – ESR Ingot Casting – High Temperature Diffusion – Forging (Normalizing) Spheroidizing Hydrogen Expansion Annealing –
Roll surface roughing (cut test block – inspection (UT, low magnification, high magnification, size, appearance) – storage –
Picking – Rough machining – Ultrasonic flaw detection – Quenching and tempering treatment – Hardness test – Semi-finishing – Roll neck surface and thrust end face quenching and tempering – Roll neck hardness test – Semi-finishing – Ultrasonic flaw detection – Roll body surface quenching (+ Cold treatment + low temperature tempering) – roll body hardness inspection – finishing – inspection – storage – rust prevention, packaging and transportation.
What is the role of each alloy element in the cold roll material?
Answer: In the 86CrMoV7 work roll, C is an important alloying element in the steel, and its function is mainly to strengthen the steel through solid solution in the steel matrix. At the same time, some carbon and other carbide-forming elements in the steel form carbides, which are distributed in the steel matrix. Play the role of strengthening and increasing wear resistance. The role of Cr is to increase hardenability, improve the strength and corrosion resistance of steel. At the same time, he forms M3C or M7C3 carbides with carbon, which effectively increases the wear resistance of the roll.
The role of Mo is to increase the hardenability of the roll, significantly improve the strength of the steel, and eliminate the temper brittleness of the roll. The function of V is to refine the grain of the steel structure, increase the hardenability and strength of the roll, and increase the toughness and plasticity of the material by refining the grain.