RU2758716C1 - Method for production of hot-rolled steel products from tool steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of ferrous metallurgy, namely to the production of high-strength sheet metal for high-precision machine-building equipment. Steel is melted with the following chemical composition, wt. %: carbon 0.4-0.8, silicon 0.4-1.2, manganese 0.1-0.7, sulfur no more than 0.03, phosphorus no more than 0.03, chromium 0.7-1.5, nickel 0.001-0.5, copper 0.001-0.04, nitrogen no more than 0.012, vanadium 0.001-0.2, titanium 0.001-0.15, molybdenum 0.001-0.3, tungsten no more than 0.2 with its subsequent casting. Hot rolling with a total compression of at least 88% at the temperature of its beginning 1110-1200°C is performed and finished at a temperature of 830-930°C. After that, the sheets are placed in a furnace heated to a temperature of 790-840°C, exposure is carried out for 20-50 minutes, then the temperature is lowered to 740-800°C and the exposure is carried out for another 20-50 minutes, and then the sheets are cooled in air.

EFFECT: reduction in the tendency of steel to crack during quenching is achieved, an increase in the strength and viscosity of steel after final heat treatment, as well as ensuring uniform hardness across the section of rolled metal.

4 cl, 2 tbl, 1 ex

Description

Method for the production of hot rolled steel from tool steel

The invention relates to ferrous metallurgy, in particular to the production of tool high-strength sheet metal for high-precision engineering equipment.

Known tool steel with a given chemical composition, as the final heat treatment of which is used normalization from a temperature of 870 ºС with tempering at a temperature of 600 - 610 ºС [USSR author's certificate No. 601322, IPC С22С38 / 22, 1978].

The disadvantage of this method is the increased content of manganese and molybdenum, selenium, as well as the inability to obtain granular perlite in the structure when carrying out classical normalization with tempering on steels with the specified chemical composition, which in turn impairs the machinability of steel by cutting and mechanical processing.

Closest to the proposed invention in technical essence is a method comprising smelting steel containing, by weight. %: 0.20-0.38 C, 0.20-1.10 Si, 0.50-1.00 Mn, 0.50-1.45 Cr, 0.70-1.30 Ni, 0.20 -0.80 Mo, 0.02-0.16 V, 0.02-0.08 Al, 0.001-0.010 N, no more than 0.25 Cu, 0.001-0.030 Nb, 0.001-0.020 Ti, no more than 0.008 S , not more than 0.013 P, the rest Fe, obtaining a continuously cast slab, its hot deformation, water quenching at a temperature of 930-980 ° C, tempering at a temperature of 575 ± 25 ° C [Patent RU No. 2631063, IPC C21D8 / 02, C22C38 / 54, C21D9 / 42, 2017].

The disadvantage of this invention is the lack of the possibility of obtaining the optimal microstructure required for subsequent quenching and tempering.

The technical result of the proposed method for the production of hot rolled steel from tool steel is to reduce the tendency of steel to cracking during quenching, increase the strength and toughness of steel after the final heat treatment, as well as ensuring uniform hardness over the section of the rolled metal.

This technical result is achieved by the fact that in the method for the production of hot rolled products from tool steel, including steel smelting, casting, hot rolling and heat treatment, according to the invention, steel of the following chemical composition, wt. %:

Carbon 0.4 - 0.8 Silicon 0.4 - 1.2 Manganese 0.1 - 0.7 Sulfur no more than 0.03 Phosphorus no more than 0.03 Chromium 0.7 - 1.5 Nickel 0.001 - 0.5 Copper 0.001 - 0.04 Nitrogen no more than 0.012 Vanadium 0.001 - 0.2 Titanium 0.001 - 0.15 Molybdenum 0.001 - 0.3 Tungsten no more than 0.2

At the same time, hot rolling begins at a temperature of 1110 - 1200 ºС, and ends at a temperature of 830 - 930 ºС.

After that, the sheets are planted in a furnace preheated to a temperature of 790 - 840 ºС, hold for 20 - 50 minutes, then the temperature is reduced to 740 - 800 ºС and hold for another 20 - 50 minutes, and then the sheets are cooled on air.

Hot rolling is carried out with a total reduction of at least 88%.

After planting the sheets in the oven, their total exposure is determined depending on the thickness of the sheets:

for thicknesses of 15 - 25 mm, the total exposure is 40 - 55 minutes;

for thicknesses of 26 - 35 mm, the total exposure is 45 - 75 minutes;

for thicknesses 36 - 50 mm the total exposure time is 65 - 100 min.

The steel has a pearlite or ferrite-pearlite structure with a granular pearlite content of at least 70%, while the content of oxide, sulfide and silicate non-metallic inclusions in the steel does not exceed 3 points each.

The essence of the proposed method is as follows.

This chemical composition and the structure of granular perlite obtained after the claimed (preliminary) heat treatment makes it possible to ensure, in the future, a uniform structure and high mechanical properties of rolled metal after final heat treatment (at the consumer).

When the carbon content is less than 0.4%, the required strength is not achieved after the final heat treatment. With a carbon content of more than 0.8%, the brittleness of the metal increases during mechanical cutting, and after heat treatment, cracks may form.

Silicon is a deoxidizing agent, and also contributes to an increase in the strength and elasticity of steel after final heat treatment. A silicon content of more than 1.2%, combined with a high carbon content, results in high brittleness of the steel.

Manganese acts as a deoxidizer, an element that binds sulfur. Also, manganese increases the strength of the steel after the final heat treatment. A manganese content of more than 0.7% with a high carbon and silicon content can lead to a decrease in the ductility of the steel. In general, manganese adversely affects the formation of granular pearlite, but joint alloying of steel with manganese and chromium in an amount of about 1.0% each facilitates spheroidization.

Sulfur and phosphorus are harmful impurities that degrade the quality of steel; therefore, the content of these chemical elements should be limited to less than 0.03% each.

Chromium provides not only high mechanical properties after final heat treatment, but also expands the permissible temperature range of preliminary heat treatment.

Tungsten, molybdenum, vanadium, titanium, copper contribute to the independent growth of cementite surrounded by ferrite, that is, abnormal eutectoid decomposition, while manganese, chromium, nickel prevent it.

The metal that has undergone preliminary heat treatment can be subjected to further cold or warm plastic processing (for example, stamping). In this case, a high homogeneity of the microstructure of rolled products is required, there should be no macrostructure defects, the number and size of non-metallic inclusions should be minimal, there should be no mesh along the grain boundaries, and the amount of harmful impurities should be minimized.

With the initial structure of granular pearlite, the tendency to growth of austenite grains is less, the permissible range of quenching temperatures is wider, the tendency to cracking during quenching is less, and the strength and toughness of steel after final heat treatment are higher (small globules are evenly distributed in martensite).

A narrow temperature range is characteristic for obtaining the structure of granular pearlite. Its lower limit should be slightly higher than point A ₁ to form a large number of carbide isolation centers during the subsequent cooling. The upper limit should not be too high, since lamellar pearlite is formed on cooling due to the dissolution of carbide precipitation centers in austenite. The more carbon in steel, the narrower the temperature range of heat treatment.

The spheroidization rate depends on the initial microstructure and is maximum for a finely dispersed structure; therefore, after rolling, it is important that the structure does not contain coarse lamellar pearlite and bainite. Rolling should be carried out without cooling and accelerated cooling.

With one-stage heat treatment, complete spheroidization of cementite does not occur, therefore the proposed method implies a two-stage heat treatment with alternating heating and cooling about A _c1 (the first stage at temperatures of 790 - 840 ° C, the second stage at temperatures of 740 - 800 ° C). In this case, the cementite plate partially dissolves in the austenite with each heating. Dissolution proceeds mainly from the tops and edges of the plates. At each cooling, cementite is released from the austenite on the undissolved remains of cementite plates, the isolation occurs mainly far from the tops of the ribs.

The final structure depends on the cooling rate during heat treatment. The lower the cooling rate, the larger the size of the globules during the decomposition of austenite and the higher the hardness of granular pearlite.

The microstructure of steel after proper annealing is fine-grained pearlite, and after subsequent heat treatment at the consumer's site it is needleless martensite with a certain amount of undissolved excess carbide grains.

Hot rolling is carried out with a total reduction of at least 88% in order to work out the cross-sectional structure and ensure the required grain size.

The content of oxide, sulfide and silicate nonmetallic inclusions in steel should not exceed 3 points each, since, in the future, rolled products are subjected to cold sheet or die forging, and in this case, a high homogeneity of the microstructure of rolled products and a minimum amount of nonmetallic inclusions are required.

With an increase in the thickness of the rolled products, in order to obtain the same temperature of the sheets over the section, the holding time increases.

An example of implementation.

Table 1 shows the chemical compositions of steel with different content of elements. Table 2 shows the controlled characteristics of the steel.

According to the data presented in tables 1 and 2, subject to the specified heat treatment modes, rolled metal from the claimed tool steel has the required characteristics: microstructure, hardness, dimensions of non-metallic inclusions before the final heat treatment, and therefore, lends itself well to mechanical processing, cutting and can be used for the manufacture of parts subjected to subsequent heat treatment.

Table 1

Chemical compositions of steel

Fuse C Si Mn P S Cr Ni Cu N Mo V Ti W 1 0.63 0.81 0.275 0.016 0.0035 1.12 0.068 0.118 0.0081 0.007 0.0043 0.0032 0.0035 2 0.66 0.67 0.285 0.015 0.0054 1.09 0.079 0.114 0.008 0.016 0.005 0.0023 0.003 3 0.63 0.71 0.21 0.011 0.0017 1.1 0.121 0.107 0.0077 0.028 0.0042 0.0021 0

table 2

Controlled steel properties

Fuse Rolled steel thickness, mm Brinell hardness Decarbonized layer (pure ferrite) Microstructure Non-metallic inclusions No. mm HB mm % ratio Score 1 15 207-217 0.06-0.08 80% granular perlite, 20% lamellar pearlite Non-deformable silicates 1.5-3.0
Brittle silicates 1.5-3.0
String nitrides and carbonitrides up to 0.5
Sulfides 1.0-2.0
Point oxides 1.5
Line oxides 0.5-3.0
Lamellar silicates 0.5-1.0 17 207-217 0.05-0.08 80% granular perlite, 20% lamellar pearlite Non-deformable silicates 1.5-3.0
Brittle silicates 1.5-3.0
String nitrides and carbonitrides up to 0.5
Sulfides 1-2
Point oxides 0.5
Line oxides 0.5
Lamellar silicates 0.5-1.0 22 207-217 0.05-0.10 85% granular perlite, 15% lamellar pearlite Non-deformable silicates 1.5-2.0
Brittle silicates 0.5-2.0
String nitrides and carbonitrides up to 0.5
Sulfides 1-2
Point oxides 0.5
String oxides no
Lamellar silicates 1 2 17 225-226 0.08-0.12 80% granular perlite, 20% lamellar pearlite Non-deformed silicates 1.0-1.5
Brittle silicates 1.0-3.0
String nitrides and carbonitrides no
Sulfides 2.0-2.5
Point oxides 0.5
String oxides no
Lamellar silicates 1 eighteen 217-223 0.12-0.15 80% granular perlite, 20% lamellar pearlite Non-deformed silicates 1.0-1.5
Brittle silicates 1-3
String nitrides and carbonitrides no
Sulfides 2.5
Point oxides 0.5
String oxides no
Lamellar silicates 1-2 twenty 219-228 0.12-0.18 85% granular perlite, 15% lamellar pearlite Non-deformable silicates 1-2
Brittle silicates 0.5-1.5
String nitrides and carbonitrides no
Sulfides 1.5-2.5
Point oxides 0.5
String oxides no
Lamellar silicates 0.5-1.0 3 twenty 223-229 0.12-0.14 70% granular perlite, 30% lamellar pearlite Non-deformable silicates 2.5
Brittle silicates 1.0-1.5
String nitrides and carbonitrides no
Sulfides 2.5
Point oxides 0.5-1
Line oxides 1.0-1.5
Lamellar silicates 1-2 thirty 230-233 0.09-0.15 70% granular perlite, 30% lamellar pearlite Non-deformable silicates 1.5-2.0
Brittle silicates 1
String nitrides and carbonitrides no
Sulfides 1.0-1.5
Point oxides 0.5
String oxides no
Lamellar silicates 0.5-1.0 40 210-217 0.16-0.24 70% granular perlite, 30% lamellar pearlite Non-deformable silicates 1.0-2.0
Brittle silicates 0.5
String nitrides and carbonitrides no
Sulfides 1.0-1.5
Point oxides 0.5
String oxides no
Lamellar silicates 1-2 50 210-217 0.15-0.20 70% granular perlite, 30% lamellar pearlite Non-deformable silicates 1.0-2.0
Brittle silicates 0.5
String nitrides and carbonitrides no
Sulfides 1.0-1.5
Point oxides 0.5
String oxides no

Claims (9)

1. A method for the production of hot rolled steel from tool steel, including steel smelting, casting, hot rolling and heat treatment, characterized in that the steel is smelted of the following chemical composition, wt. %: Carbon 0.4 - 0.8 Silicon 0.4 - 1.2 Manganese 0.1 - 0.7 Sulfur no more than 0.03 Phosphorus no more than 0.03 Chromium 0.7 - 1.5 Nickel 0.001 - 0.5 Copper 0.001 - 0.04 Nitrogen no more than 0.012 Vanadium 0.001 - 0.2 Titanium 0.001 - 0.15 Molybdenum 0.001 - 0.3 Tungsten no more than 0.2 in this case, hot rolling begins at a temperature of 1110 - 1200 ° C, ends at a temperature of 830 - 930 ° C, after which the sheets are planted in a furnace heated to a temperature of 790 - 840 ° C, hold for 20 - 50 minutes, then the temperature is reduced up to 740 - 800 ° C and hold the exposure for another 20 - 50 minutes, and then the sheets are cooled in air. 2. The method according to claim 1, characterized in that the hot rolling is carried out with a total reduction of at least 88%. 3. The method according to claim 1, characterized in that after the sheets have been planted in the oven, their total exposure is determined depending on the thickness of the sheets: for thicknesses of 15 - 25 mm, the total exposure is 40 - 55 minutes; for thicknesses of 26 - 35 mm, the total exposure is 45 - 75 minutes; for thicknesses 36 - 50 mm the total exposure time is 65 - 100 min. 4. The method according to claim 1, characterized in that the steel has a pearlite or ferrite-pearlite structure with a granular pearlite content of at least 70%, while the content of oxide, sulfide and silicate non-metallic inclusions in the steel does not exceed 3 points each.