RU2599654C1 - Method for production of high-strength steel sheet - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to ferrous metallurgy, particularly, to production of rolled plates from high-strength alloyed steel intended for crane production and light transport equipment. Obtained is a continuously cast slab from steel of the following chemical composition, wt%: 0.07-0.12 C, 0.05-0.30 Si, 1.1-1.7 Mn, 0.30-0.70 Cr, 0.90-1.20 Ni, 0.20-0.40 Mo, 0.03-0.07 V, 0.02-0.05 Al, from more than 0.006 to 0.010 N, 0.05-0.25 Cu, 0.02-0.09 Nb, from 0.003 to below 0.005 of Ti, not more than 0.005 of S, not more than 0.015 of P, Fe - the rest. To upgrade hardenability the composition of steel is additionally introduced with boron 0.001-0.005 wt%. Slab is heated, hot rolled to produce sheets, they are hardened and tempered. Hardening is carried out at the temperature of 930-980 °C, and tempering at the temperature of 500-600 °C.

EFFECT: provided are high strength properties of the sheets at maintaining sufficient ductility and impact strength.

1 cl, 4 tbl

Description

The invention relates to ferrous metallurgy, in particular to the production of plate products from high strength low alloy steel for crane production and light transport equipment.

Sheet metal for the manufacture of metal structures for heavy-duty cranes must combine high strength, toughness and weldability, undergo bending with minimal radii (both along and across the direction of rolling). The required set of properties of hot rolled sheets in the delivery state is given in table 1.

A known method of producing steel sheets, including smelting and continuous casting into slabs of low alloy steel, containing by weight,%: carbon 0.04-0.10, silicon 0.01-0.50, manganese 0.4-1.5, chromium 0.05-1.0, molybdenum 0.05-1.0, vanadium 0.01-0.1, boron 0.0005-0.005, aluminum 0.001-0.1, iron and impurities - the rest.

The cast slabs are heated to a temperature of 1250 ° C and rolled with a total compression of at least 75%. The rolled sheets are subjected to hardening from the austenitic region and high temperature tempering (JP, application No. 61-163210, IPC C21D 8/00, 1986).

The disadvantages of this method are that sheet steel has low plastic and viscous properties at low temperatures. Additional thermal improvement (hardening + tempering) after rolling does not provide an increase in the set of mechanical properties of the sheets to the required level.

There is also known a method of manufacturing high-strength sheets of steel grade 17GS (GOST 19281-89) of the following chemical composition, wt. %: carbon 0.14-0.20, manganese 1.0-1.4, silicon 0.4-0.6, chromium not more than 0.30, nickel not more than 0.30, copper not more than 0.30, phosphorus no more than 0.035, sulfur no more than 0.040, arsenic no more than 0.08, nitrogen no more than 0.008, iron - the rest.

The slabs are heated in a methodical furnace to a temperature of 1220-1280 ° C, subjected to rough rolling in the temperature range of 1050-1180 ° C to an intermediate thickness of 30-40 mm and finishing rolling in a regulated temperature range of 900-1050 ° C. To improve the mechanical properties, hot-rolled sheets are subjected to thermal improvement (hardening and high tempering) (Matrosov Yu.I. et al. Steel for gas mains. M: Metallurgy, 1989, pp. 242-244, 268).

The disadvantage of this method is the low strength properties for a given set of other mechanical properties of sheet steel.

The closest analogue to the present invention is a method for the production of strips of low alloy steel, including the manufacture of slabs of steel of the following chemical composition, wt. %: carbon 0.07-0.12, manganese 1.4-1.7, silicon 0.15-0.50, vanadium 0.06-0.12, niobium 0.03-0.05, titanium 0.010- 0.030, aluminum 0.02-0.05, chromium no more than 0.3, nickel no more than 0.3, copper no more than 0.3, sulfur no more than 0.005, phosphorus no more than 0.015, nitrogen no more than 0.010, iron - the rest .

The slabs are heated to a temperature of 1160-1190 ° C, subjected to rough rolling, finishing rolling with a total relative compression of at least 70% and a temperature of the end of rolling not higher than 820 ° C, after which the sheets are quenched with water from a temperature of 900-950 ° C and subjected to high temperature tempering at 600-730 ° C (RF patent No. 22525123, IPC C21D 8/02, C22C 38/58).

The sheets made from this steel has a tensile strength σ _s = 590-690 N / mm ^2, yield stress σ _s = 480-580 N / mm ^2, toughness at -20 ° C (KCV) at least 49 J and not less than 69 J at a temperature of -60 ° C (KCU).

The disadvantage of the prototype is that sheet steel after hardening and high temperature tempering has insufficiently high strength and toughness properties.

The technical result of the invention is to increase the strength and toughness properties of economically alloyed steel plate.

The specified technical result is achieved by the fact that in the known method for the production of high-strength sheet steel, which includes producing a continuously cast slab, heating it, hot rolling, hardening and tempering of sheets, in contrast to the closest analogue of a continuously cast slab, is obtained from steel of the following chemical composition, wt. %:

carbon 0.07-0.12 silicon 0.05-0.30 manganese 1.10-1.70 chromium 0.30-0.70 nickel 0.90-1.20 molybdenum 0.20-0.40 vanadium 0.03-0.07 aluminum 0.02-0.05 nitrogen from more than 0.006 to 0.010 copper 0.05-0.25 niobium 0.02-0.09 titanium from 0.003 to less than 0.005 sulfur no more than 0,005 phosphorus no more than 0.015 iron rest

while hardening is carried out at a temperature of 930-980 ° C, tempering is carried out at a temperature of 500-600 ° C. The steel composition additionally contains boron in the range of 0.001-0.005, wt. %

The essence of the invention lies in the fact that the final mechanical and functional properties of sheet steel are determined both by its chemical composition and the temperature conditions of quenching and tempering. In the process of conducting experimental studies, all significant factors were varied, achieving stable production of high strength characteristics of plate steel while maintaining sufficiently high ductility and toughness.

Carbon in low alloy steel of the claimed composition determines its strength. A carbon content of less than 0.07% leads to a decrease in strength properties below an acceptable level. An increase in carbon content of more than 0.12% affects the plastic and viscosity properties of steel.

Manganese deoxidizes and strengthens steel, binds sulfur. When the manganese content is less than 1.1%, the strength of the steel is insufficient. An increase in the manganese content of more than 1.7% leads to a decrease in the toughness of hardened steel.

When the silicon content is less than 0.05%, the deoxidation of steel deteriorates, and the strength properties of steel decrease. An increase in the silicon content of more than 0.30% leads to an increase in the number of silicate inclusions, reduces the toughness and weldability of steel.

A vanadium content of more than 0.07% leads to a deterioration in the weldability of steel and is not economically feasible in view of the increase in alloying costs. When the content of vanadium is less than 0.03%, the strength properties of steel do not reach the required level.

The niobium additives within the specified limits serve the purpose of dispersion hardening, and also inhibit the growth of austenitic grain and contribute to the appearance of a subgrain structure during cooling, which is fixed and stabilized by dispersed carbide particles. When the niobium content is less than 0.02%, sufficient hardening is not provided. An increase in the niobium content of more than 0.09% leads to a deterioration in the weldability of steel and is not economically feasible in view of the increase in alloying costs.

Titanium is a strong carbide forming element that strengthens steel. When the titanium content is less than 0.003%, the strength of the hot rolled sheets decreases. The titanium content of 0.005% and above does not provide further improvement in the properties of sheet steel, therefore, it is impractical.

Aluminum deoxidizes and modifies steel. At a concentration of less than 0.02%, its effect is weak, which affects the mechanical properties of steel. An increase in its content of more than 0.05% graphitizes carbon, which also affects the quality.

Chrome increases the strength of steel. When its concentration is less than 0.30%, the strength properties do not reach optimal values. An increase in chromium content of more than 0.70% leads to a loss of ductility.

Nickel helps to increase the plastic and viscous properties of sheet steel at low operating temperatures. When the nickel content is less than 0.90%, ductility and toughness are reduced, yield decreases. An increase in the nickel content of more than 1.20% leads to an increase in cost price, all other things being equal.

The addition of molybdenum in the specified range helps to obtain the required strength characteristics of steel, and also improves its hardenability. When the molybdenum content is less than 0.20%, the strength properties of steel do not reach the required level, and an increase in its content of more than 0.40% affects the weldability and ductility of hardened steel.

The addition of copper in the range of 0.05-0.25% increases the strength and corrosion resistance of steel. A higher copper content is not economically feasible.

Alloying with boron increases the strength properties after hardening and low tempering, without changing or slightly reducing viscosity and ductility. Boron added in the range of 0.001-0.005% significantly increases the hardenability of steel. Boron in an amount of more than 0.005% can contribute to the formation of embrittling particles Fe ₂₃ (C, B) ₆ (a form of iron borocarbide). To obtain the maximum effect on hardenability, a boron concentration of at least 0.001% is desirable.

The steel of the proposed composition may contain in the form of impurities not more than 0.005% sulfur, not more than 0.015% phosphorus. At the indicated maximum concentrations, these elements in the steel of the proposed composition do not have a noticeable negative effect on the viscosity properties of steel.

The upper limit of the nitrogen content of 0.010% is due to the need to obtain a given level of ductility and toughness of steel. A nitrogen content of more than 0.006% is necessary for the formation of carbonitrides of microalloying elements reinforcing steel.

Heating of hot-rolled sheets for hardening to temperatures above 980 ° C leads to an unacceptable decrease in the toughness of sheet steel. Lowering this temperature to less than 930 ° C does not provide a stable obtaining of the specified strength properties, which reduces the yield.

The tempering of tempered sheets at temperatures above 600 ° C reduces their strength properties below the permissible level. A decrease in tempering temperature below 500 ° C leads to a loss of plastic and viscous properties of high-strength sheets.

Thus, the full use of the resource of properties corresponding to low alloy steel of a given chemical composition is ensured by the heat treatment modes of plate products.

An example of the method

Using the induction melting furnace IST 0.03 / 0.05 I1, steel of various chemical composition was smelted (Table 2).

The obtained ingots were heated in a chamber furnace PKM 3.6.2 / 12.5 to a temperature of 1200 ° C. Then, ingots were crimped using a P6334 hydraulic press (rough rolling simulation) and 500 “DUO” single-strand reversible hot rolling mill (finishing rolling). The compression end temperature was from 850 ° C to 950 ° C. The ingots were rolled to a thickness of 8 mm and 10 mm. The resulting peals were cooled in air.

The heat treatment of the rolled samples consisted of quenching at a temperature of 900-1000 ° С and subsequent tempering at a temperature of 450-650 ° С (Table 3), after which the resulting peals were cut for tensile, hardness, impact bending, bending to parallelism parties.

Mechanical properties were determined on transverse samples in accordance with generally accepted conditions:

- tensile tests were carried out on flat samples according to GOST 1497;

- impact bending tests in accordance with GOST 9454 on samples with a V-shaped notch at a temperature of -40 ° C, with a U-shaped notch at a temperature of -50 ° C;

- bending test in accordance with GOST 14019.

The test results showed that in the sheet steel obtained by the proposed method (options No. 2-5, table. 4), a combination of the highest strength, plastic and viscosity properties is achieved.

In cases of transcendental values of the declared parameters (options No. 1 and No. 6), as well as when using the prototype method, the specified set of mechanical properties is not provided.

Thus, the application of the claimed method ensures the achievement of the desired result - obtaining high-strength sheet steel with a complex of difficult to combine properties: strength - conditional yield strength σ _0.2 not less than 700 N / mm ² , temporary tensile strength σ _in = 750-950 N / mm ² ; plastic - elongation δ ₅ not less than 12%; viscous - impact strength KCV ^-40 not less than 50 J / cm ² (KCU- ⁵⁰ not less than 39 J / cm ² ).

Claims (2)

1. A method of manufacturing high-strength sheet steel, including obtaining a continuously cast slab, heating, hot rolling, hardening and tempering of sheets, characterized in that the continuously cast slab is obtained from steel of the following chemical composition, wt. %:
carbon 0.07-0.12 silicon 0.05-0.30 manganese 1.10-1.70 chromium 0.30-0.70 nickel 0.90-1.20 molybdenum 0.20-0.40 vanadium 0.03-0.07 aluminum 0.02-0.05 nitrogen from more than 0.006 to 0.010 copper 0.05-0.25 niobium 0.02-0.09 titanium from 0.003 to less than 0.005 sulfur no more than 0,005 phosphorus no more than 0.015 iron rest
while hardening is carried out at a temperature of 930-980 ° C, and tempering is carried out at a temperature of 500-600 ° C. 2. The method according to p. 1, characterized in that the steel composition further comprises boron in the range of 0.001-0.005 wt. %