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Description: This article introduces their respective working conditions about cold and hot rolls, and how to choose mill roll materials correctly

Keywords: Cold rolling mill rolls, hot rolled rolls, mill roll materials

Cold rolled rolls and hot rolled rolls

The roll is a tool for plastically deforming (rolled material) metal, and is an important consumable part that determines the efficiency of the rolling mill and the quality of the rolled material. The roll is an important part of the rolling mill in the rolling mill. It uses the pressure generated by a pair or a group of rolls to roll and roll the steel. It is mainly subjected to dynamic and static loads, wear and temperature changes during rolling.

There are two types of rolls that we usually use, cold rolled rolls and hot rolled rolls.

There are many types of materials made of cold rolling rolls, such as 9Cr, 9Cr2, 9Crv, 8CrMoV, etc. This type of roll has two requirements that are

1: The surface of the roll must be quenched

2: The hardness of the surface must be HS45~105.

The materials made of hot rolling rolls generally include 60CrMnMo, 55Mn2, etc. This type of roll is used in a wide range of fields, and can be used in some processing such as section steel, bar, rebar, high-end wire, seamless steel pipe, billet, etc. It has withstood strong rolling force, severe wear and tear. Influenced by thermal fatigue, and hot rolls work at high temperatures, and allow diameter wear within a unit workload, so surface hardness is not required, only high strength, toughness and heat resistance are required. The hot roll only adopts overall normalizing or quenching, and the surface hardness requires HB190~270 hardness.

Working conditions of rolls

1. Working conditions of cold rolls

During the working process, the cold roll needs to bear a lot of rolling cycle stress, strong friction, etc. The working conditions of the cold roll mainly include:

1. The roller surface must have high and uniform surface hardness, usually greater than 62HRC

2. A deep hardened layer is required, and the hardness of the edge of the roll must be relatively low;

3. The roller neck has low hardness

4. Cold rolls must have sufficient resistance to cracking and peeling caused by torsional stress, bending stress, and shear stress;

5. It should have high wear resistance, contact fatigue strength, fracture toughness and thermal shock strength, etc.

2. Working conditions of hot rolls

Hot rolls often work in a high temperature environment of 700°C to 800°C, and they can withstand strong rolling forces. Its working conditions are usually as follows:

1. It is required that the hot roll material must have high hardenability

2. Must have a low coefficient of thermal expansion

3. Must have high thermal conductivity and high temperature yield strength;

4. Must have high oxidation resistance and high temperature creep strength;

5. It should have excellent tempering resistance and uniform surface hardness, etc.

What aspects should be considered when selecting roll materials

1. The roll must have a good biting property, so it is required to select a thick slab.

2. The material selected for the roll must have good thermal crack resistance and thermal fatigue resistance;

3. The diameter of the roll made of material is large, the length of the roll body is long, and the rolling force it bears is high

4. The material is required to have high fracture resistance and high strength.

5. Since the rough rolling and finishing rolling are completed in the same stand, it must be considered that the thickness of the rolled piece is large and the width is small during rough rolling.

Material selection of commonly used rolls

Selection of work roll material

Work rolls in the front section of rough rolling (R1): The work rolls in the front section of rough rolling must have strength, toughness, wear resistance and thermal crack resistance. The hardness range is about HS40-55. Generally, rolls made of 60CrNiMo cast steel and other materials are used.

Work rolls in the back section of rough rolling (R2): The work rolls in the back section of rough rolling are required to use materials with good thermal cracking resistance. Generally, semi-steel, high-chromium steel, high-speed steel and other materials are used.

Work rolls in the front section of finish rolling (F1-4): The temperature of the work rolls in the front section of finish rolling is high and the load is heavy. Generally, materials such as cast semi-steel and high-chromium centrifugal composite cast iron are used. High chromium centrifugal composite cast iron has high roll surface wear resistance and thermal crack resistance, and can suppress roll surface spotting defects.

Work roll after finish rolling (F5-7): The work roll after finish rolling is used in the final stage of rolling, which has a very important impact on product quality and surface condition. The performance requirements of the roll are mainly high hardness, wear resistance, indentation resistance, spalling resistance and thermal crack resistance. Generally, infinite chilled cast iron (common type, improved type) and other materials are used.

Material selection of backup rolls and vertical rolls

Whether it is for rough rolling or finish rolling, the back-up rolls are required to have good thermal crack resistance, wear resistance, fatigue resistance and high strength. In terms of materials, composite cast steel, alloy forged steel (Cr3, Cr5) and other materials are generally used.