New cold rolling work rolls for rolling high strength steel

In order to meet the needs of rolling high-strength steel and ultra-high—strength steel，a new type of cold work roll was developed by the alloy composition design and manufacturing process optimization．

Wear resistance of the working layer and strength—toughness of the roll body have been significantly improved. The application results at the work site show that the average rolling tonnage in a single—cycle increased by about 40％ ，and the grinding depth after serving was reduced by about 7％．Its performance is better than that of the imported cold work roll of the same type．The new work roll can meet the demand of rolling high strength automobile steel，and it also has a broad application prospect in cold rolling of silicon steel，stainless steel and other steel sheets which are difficult to be rolled．

Keywords：Rolling mill rolls ， work rolls

Tempering work rolls is a key factor to ensure stable production of high-strength steel. According to the production of high-strength steel, ordinary 5% Cr series cold-rolled work rolls have weak shape control ability and poor wear resistance. And the problem of poor roughness maintenance leads to difficulties in controlling the strip shape and surface quality, and shortens the roll changing cycle, which affects product quality, unit capacity and production costs, and becomes a bottleneck restricting the production of cold-rolled high-strength steel. With the continuous improvement of the strength grade and quality requirements of high-quality cold-rolled plates, in order to meet the needs of high-strength steel special-purpose units for mass production of high-strength steel, it is urgent to develop new cold-rolled work rolls that can meet the rolling high-strength steel requirements, and to explore the use of such rolls Use technology.

1. Development of new cold-rolled work rolls for rolling high-strength steel

1.1 Design of new cold-rolled work roll material

The cold-rolled work roll material is essentially an aggregate of two components, iron-based solid solution (martensite) and alloy carbide, and its performance depends on the respective properties and distribution of these two components. The research results of cold-rolled work roll materials show that strengthening the matrix is of great significance to improve the material properties of cold-rolled work rolls. Non-carbide forming elements such as Si and Mn can exist in the martensitic matrix in the form of solid solution, resulting in significant hardening and strengthening, and its effect is better than that of Cr, Mo, V and other alloying elements. Carbide is the main second phase of cold-rolled work roll materials, and its type, volume fraction, shape, size and distribution also have an important impact on wear resistance.

Therefore, on the basis of ordinary 5% Cr cold-rolled work roll material, the design of new cold-rolled work roll material composition for rolling high-strength steel was carried out, the content of solid solution elements such as Si and Mn was increased, and the content of Cr, Mo, V , and other carbide-forming elements to strengthen the matrix and optimize the carbide phase to improve the strength and wear resistance of cold-rolled work roll materials.

1.2 Development of new cold-rolled work roll manufacturing process

The new cold-rolled work roll still adopts the manufacturing process of smelting→forging→post-forging heat treatment→rough machining→preparatory heat treatment→semi-finishing→final heat treatment→finishing.

The smelting of the roll blank adopts the technological process of electric furnace smelting→outside refining→vacuum treatment→electrode rod casting→electroslag remelting→electroslag ingot. After vacuum treatment, argon gas weak stirring technology is adopted, electrode rod casting adopts argon gas protection casting technology, and electroslag remelting adopts melting speed control technology to ensure the casting quality of electroslag ingots.

Before forging, the electroslag ingot is subjected to high-temperature homogenization treatment to improve the liquid precipitation carbide. The forging deformation adopts the compound deformation process of upsetting + elongation to obtain a dense roll blank structure. After forging, the billet is subjected to normalizing and annealing heat treatment of spheroidization and hydrogen expansion.

Ultrasonic non-destructive testing and quality inspection are performed on the roll blank after rough turning. The chemical composition, gas content and harmful elements meet the design requirements, the inspection results of high and low magnification structures and non-metallic inclusions, and the results of ultrasonic non-destructive testing meet the technical requirements, so far the roll blank can be deemed qualified.

Preliminary heat treatment, that is, quenching and tempering heat treatment process, aims to improve the morphology and distribution characteristics of carbides, make necessary organizational preparations for final heat treatment, and at the same time ensure that the roll body core and roll neck obtain good mechanical properties. Preliminary heat treatment adopts short-term austenitizing heat treatment technology to control the temperature of the core of the roll and greatly reduce the heat capacity of the core, which is conducive to increasing the cooling rate during quenching and increasing the depth of the quenched and tempered layer.

The final heat treatment uses a dual-frequency induction hardening machine tool to heat and harden the effective working layer of the roll body, and uses an atmosphere circulation box furnace for low-temperature tempering to ensure the hardness and uniformity of the roll surface.

1.3 Radial hardness and microstructure of new cold-rolled work rolls

The test results of the radial hardness distribution of the new cold-rolled work rolls are shown in Figure 1, and the microstructure of the effective working layer is shown in Figure 2.

Table 1  Quantitative analysis of the carbides

The cross-sectional microstructure of the new cold-rolled work roll is shown in Figure 3. There is a small amount of lower bainite about 40mm away from the roll surface, which is a transition zone structure, and the hardness (HRC) is lower than 60. After entering the transition zone, the hardness decreases until it reaches the lowest value about 70mm away from the roller surface. Here, the temperature under the influence of induction heating reaches below the critical point of austenite, which is equivalent to a high-temperature tempering. The structure is spherical pearlite, and the hardness (HRC) is about 25. Below 75 mm from the roll surface is a mixed structure of tempered sorbite and spherical pearlite, with a hardness (HRC) of 25-29.

1.4 Performance characteristics of new cold-rolled work rolls

The mechanical properties of the new cold-rolled work rolls and ordinary 5% Cr cold-rolled work rolls (at the 1/2 radius of the roll neck) are shown in Table 2.

The yield strength of the new cold-rolled work roll reaches 772MPa, which is 16% higher than that of ordinary 5% Cr cold-rolled work rolls, and its tensile strength is 1 060 MPa, which is about 11% higher than that of ordinary 5% Cr cold-rolled work rolls. In addition, the impact energy of the new cold-rolled work roll reaches 12.7 J, while that of the ordinary 5% Cr cold-rolled work roll is about 9.0 J. The strength and toughness of the new cold-rolled work rolls are significantly improved compared with ordinary 5% Cr cold-rolled work rolls, and the anti-deformation and shape control capabilities are improved.

The wear resistance of the working layers of the new cold-rolled work rolls and common 5% Cr cold-rolled work rolls was evaluated by the ring block wear test. Test conditions: oil lubrication, load 1000 N, friction ring speed 200 r/min, each test runs for 2 hours.

The wear volumes of the two roll samples are shown in Table 3.

The wear volume of the working layer of the new cold-rolled work roll is about 50% smaller than that of the ordinary 5% Cr cold-rolled work roll, and its wear resistance is significantly improved. The hardness of the working layer of the new cold-rolled work roll has no obvious change after wear, and its tempering stability has been improved to a certain extent.

Application of new cold-rolled work rolls for rolling high-strength steel

Four new cold-rolled work rolls were successfully developed using newly developed materials and manufacturing processes, and the use assessment and verification were carried out for more than 6 months in the tempering machine of Baosteel’s high-strength steel special unit. It is mainly used in the production of high strength and high surface quality automotive sheets. The new cold-rolled work roll adopts the standard operation and maintenance specification to carry out on-board management, grinding, flaw detection, surface roughening and chrome plating.

During the use assessment period, the new cold-rolled work rolls were compared with 6 imported cold-rolled work rolls for rolling high-strength steel under the same working conditions. The single-cycle minimum rolling tonnage, maximum rolling tonnage and average rolling tonnage of the new cold-rolled work rolls are all better than imported cold-rolled work rolls. Among them, the average rolling tonnage of a single cycle has increased by about 40%, and the average grinding amount of a single off-machine has decreased by about 7%, which proves that the performance of the new cold-rolled work roll is better than that of the imported cold-rolled work roll of the same type.

On the basis of ordinary 5% Cr cold-rolled work roll materials, through alloy composition design and manufacturing process development, a new type of cold-rolled work roll for rolling high-strength steel has been developed, and its microstructure is different from ordinary 5% Cr cold-rolled work rolls. Significant changes, while the matrix is strengthened, the type, size and distribution of carbides are more reasonable, and the strength and toughness of the roll body and the wear resistance of the working layer are significantly improved. The performance of the new cold-rolled work rolls is better than that of imported cold-rolled work rolls of the same type. The new cold-rolled work rolls can extend the roll changing cycle and reduce the amount of grinding rolls, and are suitable for the rolling production of high-strength steel for automobiles.