Development of Cold Roll Manufacturing Technology2019年08月08日
Cold roll manufacturing process can be divided into three stages: the first stage is 1940-1960, when forged steel rolls were mostly used as cold rolls, smelted in electric arc furnace without vacuum degassing. At that time, the goal was to uniform the hardness of the roll body and avoid catastrophic fracture in the process of roll manufacture and use. The content of Cr in roll steel is only 1.5%~2.0%. The second stage is 1960~1970. During this period, high quality cold rolls were manufactured by electroslag remelting. The inclusion grade was greatly improved and the quality of rolls was obviously improved. During this period, the process of smelting steel for rolls by vacuum degassing appeared. The rolls were inspected by non-destructive testing method, which greatly reduced the accidents of roll breakage and spalling. The effective hardening depth of cold rolls containing 2.5%-3.0% Cr reached 15.8-19 mm. The third stage is from 1980 to 1991. With the improvement of rolling mill, the quality requirement of cold rolled sheet is improved, so the quality requirement of roll is improved. At this time, the level of roll management is also improved, and the roll spalling is greatly reduced.
In 1970s, induction heating and cold treatment technology appeared in roll manufacturing technology, which made the depth of hardened layer of roll increase from less than 10 mm to about 15 mm. After induction hardening, the surface hardened layer produces residual compressive stress and the core produces residual tensile stress. Because the induction heating is confined to a certain depth of the surface and the temperature inside the roll is still low, the residual stress after quenching is smaller than that after whole quenching, so the roll does not need to drill the central hole. Due to the advantages of induction hardening, it has rapidly become the mainstream technology in cold roll manufacturing.
In the process of cold rolling, the rollers have to bear a lot of rolling stress, and the welds, inclusions and edge cracks of the rolled parts are easy to cause instantaneous high temperature, which will cause the work rolls to be subjected to intense thermal shock, resulting in cracks, sticking rolls, spalling and even scrap. Therefore, the cold roll should have the ability to resist cracking and spalling caused by bending stress, torsional stress and shear stress, as well as high wear resistance, high contact fatigue strength, high fracture toughness and thermal impact strength. Therefore, how to improve the service life of rolls has always been a major problem faced by the roll manufacturing industry.
Commonly used materials for cold rolling work rolls at home and abroad are GCr15, 9Cr2, 9Cr, 9CrV, 9Cr2, 9Cr2Mo, 60CrMoV, 80CrNi3W, 8CrMoV, 86CrMoV7, Mo3A and Cr5 series. In order to improve the depth of hardened layer and contact fatigue life, reduce the brittleness and superheat sensitivity of hardened layer, and meet the further requirements of mechanical properties and service properties of cold rolled work rolls, chemical composition optimization of 5% Cr cold rolled steel has been carried out in foreign roll mills since the mid and late 1980s, mainly in 5% Cr steel. The content of Mo and V was increased or Ti and Ni were added. Ti precipitates in the matrix in the form of TiCN compound in 5% Cr steel roll with about 0.1% Ti. After friction loss, TiCN falls off and scratches are formed on the surface of the roll to regenerate the appropriate roughness. In the practical operation of tinplate mill, high speed rolling can be realized from the early stage of rolling by effectively utilizing the advantage of low roughness reduction.
At present, the material of cold roll is still developing. Some experts and scholars believe that cold roll should be developed in the direction of high chromium, even high-speed steel and Semi-high-speed steel. Because of the increase of chromium content, the depth of hardening layer increases, and the types of carbides in the material can all be converted into M23C7 type, whose representative countries are mainly Britain, the United States, Japan, etc. Some experts and scholars insist on low chromium roll and secondary quenching, the reason is that when micro-cracks appear on the roll surface, if not removed in time, follow suit. During rolling, micro-cracks propagate slowly along both radial and circumferential directions, and then spalling occurs. In severe cases, the depth of radial propagation generally reaches the depth of hardened layer. If the hardening layer is very deep, the possibility of cracks occurring in the interior during quenching will increase, so spalling will directly cause scrap and great loss.