RU2442831C1 - Method for production of high-strength steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: the invention relates to metallurgy, in particular, to construction weldable steels used in production of weldable constructions and heavy vehicle platforms to work in northern regions. In order to improve strength and production yield the slabs are produced from the steel with the following chemical composition, %: 0,13-0,19 C; 0,5-1,1 Mn; 0,3-0,7 Si; 1,1-1,7 Cr; 0,03-0,08 Nb; 0,02-0,06 Al; 0,002-0,030 Ca; 0,003-0,008 N; less than 0,010 S; less than 0,015 P; rest - iron. The slabs are heated to at least 1150°C, the hot rolling is carried out with the total reduction of the cross-sectioned area of at least 80% and finished with the temperature ranging from 750 to 950°C, the sheets are hardened with the temperature beginning at 890-930°C, and the high-temperature relief is carried out at 600-680°C.

EFFECT: improved strength and production yield.

1 ex, 3 tbl

Description

For the manufacture of welded structures and platforms of BelAZ heavy vehicles operating at low temperatures, heat-improved hot-rolled sheet metal 10–40 mm thick from welded cold-resistant low alloy steel is used. Hot rolled sheets after thermal improvement should combine high strength and toughness at low temperatures. The required mechanical properties of the hot rolled sheets in the delivery state are given in Table 1.

Table 1 Mechanical properties of high strength sheet steel σ _in , N / mm ² σ _t , N / mm ² δ ₅ ,% KCU ⁺²⁰ , J / cm ² KCU ^-40 , J / cm ² KCV ^-40 , J / cm ² Hall. 180 ° bend d = 3a no less 850-930 700-835 twenty 56 39 29th having stood. Note: d is the diameter of the mandrel; a - sheet thickness

A known method for the production of sheet steel, including smelting and continuous casting into slabs of low alloy steel of the following chemical composition, wt.%:

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 Rest.

The cast slabs are heated to a temperature of 1250 ° C and rolled with a total compression of at least 75%. Laminated sheets are subjected to quenching from the austenitic region and high-temperature tempering [1].

The disadvantages of this method are that plate steel has low strength and toughness at low temperatures. This makes it impossible to use it for the manufacture of platforms for heavy vehicles.

There is also known a method of manufacturing high strength sheets of steel grade 17GS of the following chemical composition, wt.%:

Carbon 0.14-0.20 Manganese 1.0-1.4 Silicon 0.4-0.6 Chromium no more than 0.30 Nickel no more than 0.30 Copper no 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 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) [2].

The disadvantage of this method is that it does not provide high strength 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 high-strength sheet 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 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 [3].

The disadvantages of this method are that the sheet steel after quenching and high temperature tempering has low strength properties, which, in turn, leads to a decrease in yield.

The technical problem solved by the invention is to increase the strength properties and yield.

For this, in the known method for the production of high-strength sheet steel, including the manufacture of steel slabs, their heating, hot rolling into sheets and hardening of sheets with subsequent high-temperature tempering, according to the proposal, the slabs are made of steel of the following chemical composition, wt.%:

Carbon 0.13-0.19 Manganese 0.5-1.1 Silicon 0.3-0.7 Chromium 1.1-1.7 Niobium 0.03-0.08 Aluminum 0.02-0.06 Calcium 0.002-0.030 Nitrogen 0.003-0.008 Sulfur no more than 0,010 Phosphorus no more than 0.015 Iron Rest,

moreover, the heating of the slabs is carried out to a temperature not lower than 1150 ° C, hot rolling of the sheets is carried out with a total relative compression of at least 80% and is completed in the temperature range of 750-950 ° C, the hardening of the sheets is carried out from a temperature of 890-930 ° C, and high-temperature tempering at a temperature of 600-680 ° C.

The invention consists in the following. The heating of slabs of low alloy steel of the proposed chemical composition to a temperature of not lower than 1150 ° C ensures its austenitization, complete dissolution of sulfides, phosphides, nitrides, alloying and impurity compounds, carbonitride reinforcing particles in the austenitic matrix. Due to this, the technological plasticity and deformability of steel during rolling are increased. In addition, since during the rolling process with a total relative compression of at least 80%, a continuous decrease in the temperature of the metal occurs, at the indicated heating temperature, at the time the sheets are finished rolling, their temperature decreases to a predetermined value T _cn = 750-950 ° C (temperature of the end of rolling), which contributes to the intensification of the allocation of reinforcing carbonitride particles and grinding of the microstructure of steel. After thermal improvement, at the same time as hardening, the steel acquires a cellular structure that increases viscosity at low temperatures.

Heating sheets to a temperature of 890-930 ° C, water quenching and tempering at a temperature of 600-680 ° C provide an increase in the level and stability of the strength, viscosity and plastic properties of hot-rolled sheets. Due to thermal improvement, fluctuations in the contents of chemical elements in steel, as well as temperature instability of the process, are inevitably existing in the practice of industrial production, are not leveled, which favorably affects the stability of the mechanical properties of the sheets and increases the yield.

It was experimentally established that a decrease in the temperature of heating slabs below 1150 ° C leads to incomplete dissolution of austenite carbonitride hardening particles, reducing the plastic and viscosity properties of the sheets.

When the total relative compression during rolling is less than 80% or the temperature of the end of rolling above 950 ° C, the required degree of dispersion and strain-hardening of the proposed steel sheet is not achieved. As a result, the strength properties of sheets after thermal improvement (hardening and high-temperature tempering) are below acceptable values. Lowering the rolling temperature below 750 ° C leads to the formation of a longitudinal texture, which reduces the impact strength of thermally enhanced sheets.

Carbon reinforces steel. With a carbon content of less than 0.13%, the required strength of the steel is not achieved, and with its content of more than 0.19%, the toughness and weldability of the steel deteriorate.

Manganese deoxidizes and strengthens steel, binds sulfur. When the manganese content is less than 0.5%, the strength of the steel is insufficient. An increase in manganese content of more than 1.1% leads to a decrease in viscosity at low temperatures and a deterioration in ductility.

Silicon deoxidizes steel, increases its strength. At a silicon concentration of less than 0.30%, the strength of the steel is lower than permissible, and at a concentration of more than 0.70%, ductility decreases, the steel does not stand the test for cold bending.

Chrome increases the strength and toughness of steel. At a concentration of less than 1.10%, the strength and viscosity are below acceptable values. An increase in the chromium content of more than 1.7% leads to a loss of ductility due to the growth of carbides, and a decrease in the yield of thermally improved rolled sheets.

Niobium contributes to the grinding of the microstructure of low alloy steel by sheet thickness, increasing cold resistance. However, if the niobium content is more than 0.08%, the weldability of steel will deteriorate, which is unacceptable. With a decrease in the niobium content of less than 0.03%, a high impact strength at low temperatures is not achieved.

Aluminum deoxidizes steel and grinds grain. When the aluminum content is less than 0.02%, its effect is small, the viscosity properties of steel deteriorate. An increase in the content of this element of more than 0.06%, it binds nitrogen, which leads to a decrease in strength characteristics.

Calcium is a modifying element. In addition, it binds sulfur to globular sulfides, increasing the viscosity properties of steel. At a calcium concentration of less than 0.002%, its effect is weak. An increase in calcium concentration of more than 0.030% increases the number and size of non-metallic inclusions, toughness at low temperatures and yield of sheet metal deteriorate.

Nitrogen provides hardening of steel due to the separation of fine nitride and carbonitride particles from a solid solution. When the nitrogen content is less than 0.003%, the strength of plate steel is insufficient. An increase in the nitrogen content of more than 0.008% leads to a decrease in the viscosity properties of high-strength sheet steel at low temperatures.

Sulfur and phosphorus in this steel are harmful impurities, their concentration should be as low as possible. However, at a sulfur concentration of not more than 0.010% and phosphorus not more than 0.015%, their negative effect on the properties of steel is negligible. At the same time, deeper desulfurization and dephosphorization of steel will significantly increase the cost of its production, which is impractical.

Heating of hot-rolled sheets for hardening to temperatures above 930 ° C leads to a decrease in the toughness of sheet steel. Lowering this temperature to less than 890 ° C does not provide a stable obtaining of the specified strength properties, which reduces the yield.

The tempering of tempered sheets at temperatures above 680 ° C reduces their strength properties below the permissible level. A decrease in tempering temperature below 600 ° C leads to a loss of plastic and viscous properties of high-strength sheets, which reduces their yield.

Method implementation examples

Steel of various chemical composition was smelted in an electric arc furnace. In the ladle, steel was deoxidized by ferrosilicon, ferromanganese, alloyed with ferrochrome, metallic aluminum and niobium were introduced. Using synthetic slag, excess sulfur and phosphorus were removed. Calcium was introduced into the melt in the form of silicocalcium. The chemical composition of smelted steels is given in table.2.

Table 2. The chemical composition of high strength steels Composition number The content of chemical elements, wt.% C Mn Si Cr Nb Al Ca N S P Fe one. 0.12 0.4 0.2 1,0 0.02 0.01 0.002 0.002 0.003 0.010 Ost. 2. 0.13 0.5 0.3 1,1 0,03 0.02 0.003 0.003 0.004 0.012 -: - 3. 0.16 0.8 0.5 1.4 0.05 0.04 0.006 0.006 0.008 0.013 -: - four. 0.19 1,1 0.7 1.7 0.08 0.06 0.008 0.008 0.010 0.015 -: - 5. 0.20 1,2 0.8 1.8 0.09 0,07 0.009 0.009 0.011 0.016 -: - 6. 0.10 1,5 0.4 0.2 0.04 0.04 - 0.007 0.004 0.014 -: -

Note: composition 6 steel additionally contains 0.10% V and 0.02% Ti.

Steel with composition No. 3 was poured into slabs with a thickness of H = 200 mm. Then the slabs were heated to austenitization temperature T _a = 1200 ° C and rolled on a plate reversing mill 2800 into sheets with a thickness of h = 12 mm with a total relative compression ε equal to:

The temperature of the end of the rolling sheets was: T _KP = 800 ° C.

The hot-rolled sheets were quenched in a roller quenching machine by heating to a temperature T _s = 920 ° C and cooling with water. The hardened sheets were subjected to high tempering at a temperature T _from = 640 ° C.

After thermal improvement, samples were taken from the sheets and mechanical properties tested.

Implementation options for the production method of high-strength sheet steel and indicators of their effectiveness are given in table 3.

From tables 2 and 3 it follows that the proposed modes of production of high-strength sheet steel (options No. 2-4) provide an increase in strength properties and yield. Finished sheets have the highest strength and toughness, withstand the cold bend test.

In cases of transcendental values of the concentrations of chemical elements in steel, modes of hot rolling, hardening and high tempering (options No. 1 and No. 5), as well as the use of the prototype method (option No. 6), there is a decrease in the strength properties of the finished sheets and yield. In these cases, sheet steel is used for less critical purposes.

The technical and economic advantages of the proposed method consist in the fact that the simultaneous optimization of the chemical composition of steels, temperature and deformation modes of hot rolling, as well as subsequent thermal improvement, can increase the strength properties while ensuring specified and stable viscosity and plastic properties

Table 3. The modes of production of sheet steel and indicators of its quality No. p / p Composition number T _a , ° C ε,% T _CP , ° C T _s , ° C T _from , ° C σ _in , N / mm ² σ _t , N / mm ² δ ₅ ,% KCU ⁺²⁰ , J / cm ² KCU ^-40 , J / cm ² KCV ^-40 , J / cm ² Hall. bend Yield,% one. one 1100 95 760 880 590 840 690 17 54 35 27 I can’t stand it. 78,4 2. four 1230 90 950 930 680 900 820 22 57 40 thirty Aging 99.7 3. 3 1200 94 800 920 640 930 835 24 58 42 32 Aging 99.9 four. 2 1150 80 750 890 600 920 700 22 56 41 31 Aging 99.8 5. 5 1140 75 960 880 590 850 690 16 52 37 26 I can’t stand it. 79.5 6. 6 1170 72 810 910 650 640 530 thirty 54 38 28 Aging 97.0

high-strength sheet steel for welded structures and platforms of heavy vehicles. It also contributes to increased yield.

The prototype method was used as a basic object in determining the technical and economic efficiency. The application of the proposed method provides an increase in the profitability of the production of high-strength sheet steel by 12-15%.

Literary sources

1. Japanese application No. 61-163210, IPC C21D 8/00, 1986

2. Matrosov Yu.I. and others. Steel for gas pipelines. M .: Metallurgy, 1989, p. 242-244, 268.

3. Patent of the Russian Federation No. 225123, IPC C21D 8/02, C22C 38/58, 2005 - prototype.

Claims (1)

A method of manufacturing high-strength sheet steel, including the manufacture of steel slabs, their heating, hot rolling into sheets, hardening of sheets followed by high-temperature tempering, characterized in that the slabs are made of steel of the following chemical composition, wt.%:
carbon 0.13-0.19 manganese 0.5-1.1 silicon 0.3-0.7 chromium 1.1-1.7 niobium 0.03-0.08 aluminum 0.02-0.06 calcium 0.002-0.030 nitrogen 0.003-0.008 sulfur no more than 0,010 phosphorus no more than 0.015 iron rest,
moreover, the heating of the slabs is carried out to a temperature not lower than 1150 ° C, hot rolling of the sheets is carried out with a total relative compression of at least 80% and is completed in the temperature range of 750-950 ° C, the hardening of the sheets is carried out from a temperature of 890-930 ° C, and high-temperature tempering at a temperature of 600-680 ° C.