RU2699696C1 - Method of producing cold-resistant rolled sheet of increased strength - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly, to production of structural steels for use in shipbuilding, construction and other industries. Method of producing high-strength hot-rolled stock in thicknesses of 8–50 mm with high level of cold resistance involves steel making, containing, wt. %: carbon - 0.07–0.12, manganese - 0.30–0.80, silicon - 0.10–0.40, chromium - 0.20–0.50, nickel - 1.70–2.50, molybdenum - 0.20–0.50, copper - 0.35–0.60, vanadium - 0.02–0.05, aluminium - 0.01–0.06, nitrogen is not more than 0.008, sulphur is not more than 0.005, phosphorus not more than 0.015, iron - balance, steel pouring on slabs, slab heating to temperature of 1230–1260 °C, deformation to final thickness, wherein deformation is finished at temperature for thicknesses of 8.0–10.0 not more than 920 °C, and for thicknesses of 10.1–50.0 mm not more than 940 °C, further rolling is subjected to two-stage heat treatment.

EFFECT: increase strength, cold resistance and improve weldability of steel.

1 cl, 3 tbl

Description

The invention relates to metallurgy, and more particularly to the production of structural steels with increased strength and cold resistance, improved weldability for use in shipbuilding, construction and other industries.

For structures of northern design for various purposes (parts of ship hulls, offshore stationary drilling platforms, floating drilling rigs and infrastructure facilities), a cold-resistant high-strength steel with thickness up to 40 mm with high performance is required.

A number of low alloy steels of increased strength and cold resistance are known for the manufacture of this kind of metal products.

For example, the method of production of rolled products (RF Patent No. 2345149, IPC C21D 8/02, C22C 38/12, C21D 9/46 publ. 04/10/2008 [1]), of steel of the following chemical composition, wt. %:

Carbon - 0.04-0.10

Manganese - 1.00-1.40

Silicon - 0.15-0.35

Vanadium - 0.02-0.10

Niobium - 0.02-0.06

Aluminum - 0.02-0.06

Sulfur - 0.001-0.008

Phosphorus - 0.003-0.012

Nickel - 0.10-0.80

Molybdenum - 0.01-0.08

Iron is the rest.

A method for the production of cold-resistant rolled products includes smelting steel of the specified chemical composition in a converter, casting metal into continuously cast billets, heating slabs for rolling, preliminary deformation with a total reduction ratio of 58-65% with regulated minimum reductions in the first four passes: (12-15%) - (13-17%) - (14-18%) - (14-20%), at a temperature of 940-990 ° C, cooling the resulting workpiece by 70-100 ° C, final deformation at a temperature of 830-750 ° C s total reduction ratio of 35-42%, accelerated cooling to temperatures of 550-400 ° C, then h slow cooling in the caisson to a temperature not higher than 150 ° C.

The disadvantages of this method of production are that the rental has significantly lower performance in cold resistance and strength.

Also known is a method of producing steel of the following chemical composition (RF Patent 2432403 C1, C21D 8/02 C22C 38/08, publ. 10.27.2011), wt. %:

Carbon 0.06-0.12 Manganese 0.60-1.20 Silicon 0.15-0.35 Aluminum 0.02-0.05 Nickel 0.05-0.40 Niobium 0.02-0.06 Molybdenum 0.003-0.08 Titanium 0.002-0.02 Vanadium 0.02-0.05 Nitrogen 0.001-0.008 Sulfur 0.001-0.008 Phosphorus 0.003-0.012 Calcium 0.005-0.03 Copper 0.05-0.30 Iron rest

steel casting into billets, austenitization of the billet at a temperature of 1180-1210 ° C, preliminary deformation with regulated compressions of at least 12% at a temperature of 1000-1050 ° C, cooling of the obtained billet in air to the temperature of the beginning of the final deformation, final deformation at a temperature of 880-770 ° C, with each subsequent reduction of 1-4% more than the previous one. The temperature of the end of the rolling of the sheets is calculated by the formula: Tkp = Ar ₃ + (100-130) -37,7ln (t), where t is the thickness of the sheet, accelerated cooling is carried out in the temperature range 620-510 ° C, then the sheet metal is slowly cooled to stacked to ambient temperature. The disadvantages of this method are the lack of strength characteristics and cold resistance.

Closest to the described invention in technical essence and the achieved result is a prototype method of steel of the following chemical composition (RF Patent 2629420 C1, C21D 8/02 C22C 38/44 B21B 1/26, publ. 08.29.2017), wt. %:

Carbon 0.07-0.12 Manganese 0.20-0.70 Silicon 0.10-0.50 Chromium 1.00-1.40 Nickel 1,50-2,00 Molybdenum 0.10-0.30 Copper 0.20-0.50 Niobium 0.02-0.05 Aluminum 0.01-0.06 Nitrogen no more than 0,008 Sulfur no more than 0,005 Phosphorus no more than 0,010 Iron rest

in this case, slabs with a given chemical composition are heated to a temperature of 1240-1260 ° C in furnaces and rolled into sheets to a final thickness at a rolling end temperature of not more than 890 ° C, cooled in air, then heated to a temperature of 920-940 ° C with a total exposure of 2.0-3.0 min / mm followed by quenching in water, heating for tempering sheets to a temperature of 690-740 ° C with an exposure of 1.5-2.8 min / mm depending on the thickness and subsequent cooling on air.

The disadvantage of steel of known composition is the lack of strength of metal.

The technical problem solved by the invention is to significantly increase the strength of steel while maintaining the level of impact strength of more than 200 J / cm ² at -60 ° C, while providing a high level of relative narrowing in the direction of thickness. This task is complex, since an increase in strength characteristics entails a reduction in toughness and relative narrowing in the thickness direction.

The technical problem is achieved in that in a method for the production of cold-resistant sheet metal with a thickness of 8-50 mm, including smelting, casting onto billets, heating under deformation, rolling in a given temperature range, cooling in air and further heat treatment:

Carbon 0.07-0.12 Manganese 0.30-0.80 Silicon 0.10-0.40 Chromium 0.20-0.50 Nickel 1.70-2.50 Copper 0.35-0.60 Vanadium 0.02-0.05 Molybdenum 0.20-0.50 Aluminum 0.01-0.06 Nitrogen no more than 0,008 Sulfur no more than 0,005 Phosphorus no more than 0.015 Iron rest,

the slabs are heated to a temperature of 1230-1260 ° C in methodological furnaces and rolled into sheets to a final thickness at a starting rolling temperature of not more than 1020 ° C and a rolling end of not more than 940 ° C with cooling in air and subsequent two-stage heat treatment, including heating to a temperature of 910-930 ° C with a total exposure of 2.0-4.0 min / mm and quenching in water, re-heating to a temperature of 700-710 ° C with a total exposure time of 1.0-3.0 min / mm depending on the thickness of the rental followed by cooling in air to ambient temperature Reda.

The essence of the invention is as follows. Ensuring the specified mechanical properties of thick sheets is achieved both by optimizing the chemical composition of the steel and the modes of their subsequent deformation-temperature and heat treatment. After rolling, a finely dispersed microstructure is formed in the steel of the proposed composition, and subsequent heat treatment allows one to obtain desired and uniform properties in the thickness range of 8.0–40.0 mm.

Carbon reinforces steel. When the carbon content is less than 0.07%, the required hardenability and strength of the steel are not achieved, and when its content is more than 0.12%, the toughness of steel deteriorates.

Silicon deoxidizes steel, increases its strength characteristics. At a silicon concentration of less than 0.10%, the strength of the steel is lower than acceptable, and at a concentration of more than 0.40%, ductility decreases.

Manganese deoxidizes and strengthens steel, binds sulfur. When the manganese content is less than 0.30%, the strength of the steel is insufficient. Content over 0.80% leads to an overrun of alloying and an increase in cost.

Chromium provides an increase in strength at elevated temperatures, and also provides high hardenability of steel. At a concentration of less than 0.20%, the strength is below acceptable values. An increase in the chromium content of more than 0.50% leads to a loss of ductility.

Silicon is introduced into steel to increase strength characteristics. When the content of this element is less than 0.10%, the positive effect is practically not manifested, and when the content is more than 0.40%, the cold resistance decreases.

When the nickel content is less than 1.70%, the strength and toughness of steel decreases, the content of more than 2.50% leads to an overuse of alloying and increase cost.

Molybdenum increases the strength and toughness of steel, grinding grain microstructure. When the molybdenum content is less than 0.20%, the strength of the steel is lower than the required level, and an increase in its content of more than 0.50% worsens the ductility and leads to an excessive consumption of alloying elements.

Copper helps to increase strength properties. But if the content of this element for a given composition exceeds 0.60%, then there may be a decrease in the toughness of steel at freezing temperatures. When the content is less than 0.35%, the doping efficiency is insufficient.

Phosphorus, nitrogen and sulfur in steel are harmful impurities, their concentration should be as low as possible. However, when the concentration of phosphorus is not more than 0.015%, nitrogen is not more than 0.008% and sulfur is not more than 0.005%, their negative effect is negligible.

Vanadium forms fine particles of V (C, N), which, by choosing the appropriate mode, are used to limit the growth of austenite grain and to regulate the recrystallization process. With a content of less than 0.02%, the effect of niobium is practically absent, with a content of more than 0.05% there is an overspending of ferroalloys.

Aluminum is introduced into steel as a deoxidant, as well as for the purpose of grinding grain (this effect is achieved when the content is not less than 0.01%). When the aluminum content in the steel is more than 0.06%, the purity of the steel by non-metallic inclusions of the aluminum oxide system decreases, which adversely affects the mechanical properties of the base metal.

The main distinguishing features of the production method are:

- the optimum heating temperature for rolling is 1240-1260 ° C, providing complete dissolution of carbonitrides and eliminating excessive growth of austenite grain;

- ensuring the temperature of the beginning of the finish rolling in the range of not more than 1020 C and the temperature of the end of the rolling of sheet metal not more than 940 ° C to exclude hereditary coarse grains during subsequent heating for heat treatment;

- the optimum temperature for hardening is 910-930 ° C with a shutter speed of 2.0-4.0 min / mm, which allows to achieve the necessary cooling rate during hardening and eliminates the growth of austenite grain;

- regulation of tempering temperature and holding time in the range of 700-710 C with a total holding time of 1.0-3.0 min / mm allows you to remove internal stresses and provide the required complex of strength and cold-resistant characteristics.

Tests of sheet metal manufactured by this technology showed that the proposed modes for steel of the selected chemical composition provide stable impact strength at temperatures up to minus 60 ° C, provided that high strength characteristics and relative narrowing in the direction of the thickness of the steel are obtained.

Implementation example

Smelting was carried out in an oxygen converter, poured into slabs. Slabs with a given chemical composition were heated to a temperature of 1230-1260 ° C in methodological furnaces and rolled on a 2800 plate mill into sheets to a final thickness (8.0-40.0 mm) with a finish rolling start temperature of not more than 1020 C and a rolling end temperature for thicknesses of 8.0-10.0 mm not more than 920 ° C, for thicknesses of 10.1-50.0 mm not more than 940 ° C. After the deformation process was completed, the final rolling of sheet metal in air was carried out to ambient temperature. Then, heating was carried out in roller furnaces to a temperature of 910–930 ° C with a total exposure time of 2.0–4.0 min / mm and further quenching in water in a roller quenching machine. After quenching, the metal was subjected to tempering according to the regime:

8.0-40.0 mm temperature - 700-710 ° C, holding time - 1.0-3.0 min / mm

After tempering, the metal was cooled in air to ambient temperature.

From the table. 1-3 it follows that the proposed steel (compositions 5-6) has higher strength characteristics and comparable to the prototype level of impact strength at low temperatures (-60 ° C). In addition, steel is characterized by high relative narrowing in the thickness direction.

With transcendental concentrations of elements (compositions 1-4), the strength characteristics and toughness of steel deteriorate. Also lower properties in terms of strength and toughness have steel according to the prototype (composition 7).

Claims (3)

A method for the production of cold-resistant rolled sheets of increased strength, including steel smelting, casting into slabs, heating for rolling, deformation in a given temperature range, cooling to ambient temperature in air, heating under quenching, quenching in water, tempering and subsequent cooling in air to temperature environment, characterized in that the steel is smelted composition, wt. %: Carbon 0.07-0.12 Manganese 0.30-0.80 Silicon 0.10-0.40 Chromium 0.20-0.50 Nickel 1.70-2.50 Copper 0.35-0.60 Vanadium 0.02-0.05 Molybdenum 0.20-0.50 Aluminum 0.01-0.06 Nitrogen no more than 0,008 Sulfur no more than 0,005 Phosphorus no more than 0.015 Iron rest, the slabs are heated to a temperature of 1230-1260 ° C in methodical furnaces and rolled on a plate mill into sheets to a final thickness of 8-50 mm at a temperature of the start of finish rolling of not more than 1020 ° C and the end of rolling of not more than 940 ° C with air cooling and subsequent two-stage heat treatment, including heating to a temperature of 910-930 ° C with a total exposure of 2.0-4.0 min / mm and quenching in water, and re-heating to a temperature of 700-710 ° C with a total exposure time of 1.0-3, 0 min / mm depending on the thickness of the rolled product, followed by cooling in air to an ambient temperature kg Ambient.