SE204211C1 - - Google Patents

Description

Inventors: R S Knoth and N A Matthews The present invention relates to legally alloy steels for use as pre-rolls in casting mills, plate name rolling mills, coarse rolling mills, pre-rolling mills and the like. These rollers, which are used to roll goods into blanks or platinums, are subject to heavy shock loads and excessively high pressures and must be made of a steel which has different physical properties than the steels used for the rollers of medium or fine rolling mills. The rollers of the medium or fine rolling mills must be so hard that they are too brittle for rolling with a large reduction. Industrial rolls, which often weigh 45,000 kg or more and have diameters of 1.2 m or more, must have such a combination of half-strength and toughness that the spread of heat cracks, which result from heat transfer from a hot cast to the roll, is prevented or reduced. as small as possible (the Overford heat results in different length extensions of the surface, whereby small surface cracks are formed). If the spread of these hot crackers can be prevented, the hot cracks can actually be to their advantage, as they provide some gripping force and facilitate the movement of a cast through the rolling mill. In addition, suitable hardness is required for good wear resistance, but the hardness Mr should not be so great that the surface of the roll is spread.

Got roll mills' rolls are usually made of a steel containing about 0.71.2% carbon, up to about 1% chromium, 0.5% molybdenum, up to 1.75% nickel, 0.41% manganese, up to 0, 5% silicon and sometimes small amounts of other elements, although some steels for casting mills contain as little as 0.4% carbon or as much as 1.8% and although others are still nickel free. Rollers, which are made of these different steels, have a tendency to crack after a short time, and a tendency to duplicate. at b: 39 / to crack or break Or for present okande. It is assumed that the increased number of cracked rollers can be attributed to, among other things, the increasingly responsive operating conditions, e.g. higher roll pressures, which Yid ambient temperature lead to the spread of the cracks. The normally different steels are characterized by the fact that their transition between sprout and ductile state yid heating is yid significantly higher temperatures On 150 ° C, e.g. 175 ° C, the result being that the transition from a spray to a ductile state under normal operating conditions takes place at too high a temperature for the propagation of spray cows to be effectively prevented.

The steel for irrigation as casting rollers according to the invention contains 0.55-0.7% carbon, 0.2-0.6% silicon, 0.3-0.7% manganese, 3.4-3.8% nickel, 0, 8-1.2% chromium, 0.350.45% molybdenum and 0.08-0.25% vanadium, the remainder, apart from impurities and in alloy slat normally present, unintentional elements, Or jam. This steel is distinguished from the fact that, when treated with suitable salt, it has a bainitic microstructure, which is considerably more advantageous than the perlite microstructures which distinguish the alloy steels commonly used for rollers.

The various elements will now be treated. If the carbon content Or lagre On 0.55%, the result is insufficient half-strength and nit resistance, while carbon in an amount Over 0.7% leads to pre-explosion. Due to this, it is appropriate that the carbon content does not exceed 0.65%. The silicon content must not exceed 0.6% and should not exceed 0.3%, as silicon lowers the impact strength. Manganese in a content of more than 0.7% also has a detrimental effect on the impact strength, but at least 0.3% manganese must be present to bind any sulfur present and to contribute to the hardenability. Nickel in amounts of Over 3.8% is disadvantageous, as segregation and formation of residual austenite in that case occurred. In addition, the critical temperatures (conversion temperatures) are collected, if too large amounts of nickel are not present. Preferably the nickel content does not exceed 3.7%. At least 3.4% nickel is required to ensure hardness, toughness and desired conversion properties, ie. to give a bainitic structure, so that the beneficial effect of nickel on the toughness of the annealed structure is maintained.

Both chromium and molybdenum have a tendency to form hard and brittle carbides at the grain boundaries, and these are unsuitable. The chromium content preferably does not exceed 1%, and the sum of the chromium and molybdenum contents preferably does not exceed 1.4%. On the other hand, both chromium and molybdenum must be present, since perlite has a tendency to form, if the chromium content is lower than 0.8%, and because the conversion properties are adversely affected, ie. since perlite can be formed, about molybdenum content for storage at 0,%. Vanadium contributes in an important way to the toughness and to the attainment of a low temperature of conversion between sprayed and ductile state upon heating. In addition, vanadium is preferably used as a deoxidizing agent instead of aluminum, since vanadium does not form film-like or chain-like oxide or sulfide inclusions.

Of the common pollutants, sulfur and phosphorus are kept as low as possible, e.g. up to 0.035% (preferably up to 0.03%) sulfur and up to 0.015% (preferably up to 0.01%) phosphorus.

Cast steel rolls according to the invention can be removed from the molds while having a temperature in the range of 425-650 ° C. They can then be heat treated, the first step comprising heating for 515 hours at 485-595 ° C to eliminate cotton and equalize the temperature Mom the entire roller. The rollers are then homogenized in the temperature range of about 955-1065 ° C for 2-12 hours, preferably 2 hours at 1065 ° C or about 12 hours at 955 ° C, and then the rollers are cooled to and kept in the temperature range of 620-315 ° C to achieve conversion for grain refinement. The rollers are then heated anyo to an austenitizing temperature of 760-8 ° C, after which they areo cooled, preferably in air, to a temperature of 370-220 ° C but above the Ms temperature of the steel. The rollers are walled approximately isothermally within this temperature range for a sufficiently long time to form bainite, e.g. under 10 hrs. It is important that the cooling be carried out immediately after the austenitization treatment at a rate of at least 22 ° C / h in order to form a bainitic and not a perlite structure. The cooling rate should not be so fast that it causes the formation of martensite. The rollers are then tempered to achieve the desired hardness. It is preferred to perform one. double annealing at 540-595 ° C. The annealing temperature should not exceed 650 ° C, as flake conversion to austenite in sh cases may arise in segregated areas, which upon cooling will then be converted back to non-annealed, relatively scattered structures.

An example of the steel according to the invention is given below under the designation 5 "steel A", in which case two steel (B and C) are also given, which are typical examples of steel which has hitherto been used for casting rolls.

Table I.

StalC% Si% Mn% Ni% Cr% Mo% V% S% P% A0,630,520,43,40,80,40,120,0280,009 0,580,330,472,0,40,220,0040,009 0,770,0,481,40,390,230,0130, 016 Stal A gays the following heat treatment: 1) tilting at 540 ° C for 5 hours; 2) heating to 955 ° C and homogenization for 5 hours; 3) cooling to 315 ° C per hour; 4) heating to an austenitizing temperature of 815 ° C and holding at this temperature for about 2 hours; 5) cooling to 315 ° C for 18 hours and then double annealing at 640 ° C for a total of 12 hours (6 hours for normal annealing); and 6) oven cooling.

States B and C were heat treated in the same manner as follows: 1) Mining at 540 ° C for 5 hours; 2) heating to 955 ° C and Mining at denim temperature for 3 hours; 3) cooling to 315 ° C in 18 hours; 4) heating to 870 ° C and tilting at this temperature for 3 hours; 5) cooling to 315 ° C for 15 hours and then annealing at 565 ° C for 10 hours; and 6) cooling oven to 205 ° C for 15 hours and then air cooling.

Sample rods of the heat-treated steels were tested at room temperature in terms of tensile strength, tensile strength (0.2-strength) requirement of 50.8 mm, contraction, hardness, impact strength and wrap temperature, "Fibrous Appearance Transition Temperature" (FATT), ie. . the temperature at which 50% fiber breakage occurs (in the case of a change from a sprout to a tough state).

Table IL Traction Fracture Border (k13 / m 10,280 1.27 104 6,269 0.69 149 260.39 183 Extension Contract Hardness tion% (Brinell) These data show that steel A is characterized by high half-strength and good toughness along with a considerably lower transition temperature between crack and ductile condition than steel B and C. The combination of properties obtained is not obtained at the expense of any particular property, for example, the high toughness is not obtained at the expense of true hardness or hall strength.

Claims (5)

Patent claim: Alloy steel, characterized in that it contains 0,55-0,7% carbon, 0,2-0,6% silicon, 0,3-0,7% manganese, 3,4-3,8% nickel , 0.8-1.2 chromium, 0.85-0.45% molybdenum and 0.08 -0.25% vanadium, the remainder, apart from Iran impurities and in alloy steel normally present unintentional elements, are jam. 2. Still according to claim 1, characterized in that the carbon content is 0.55-0.65%, the silicon content 0.2-0.55%, the nickel content from 3.4 to 3.7%, the chromium content is 0.8- 1.0% and the vanadium content is 0.1-0.2%. Stat. according to patent claim 2, characterized in that the silicon content is at most 0.3%. Steel according to claim 2 or 3, characterized in that the sum of the chromium and molybdenum contents is at most 1.4%. 5. Roll for cast or coarse rolling mills, made from a steel according to any one of the preceding patent claims. Request publications: