RU2362814C2 - Low-alloy steel and product implemented from it - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to ferrous metallurgy, particularly to manufacturing of steel composition and products, implemented from them, and can be used at manufacturing of automobile's components, agricultural machinery, chemical equipment by means of cold deformation. Steel contains carbon, silicon, manganese, sulphur, aluminium, nitrogen, phosphorus, copper, nickel, chrome, molybdenum and iron, at following ratio of components, wt %: carbon 0.05 - 0.09, silicon 0.01 - 0.10, manganese 0.15 - 0.40, copper 0.01 - 0.50, nickel 0.01 - 0.50, chrome 0.01 - 0.30, sulphur 0.005 - 0.025, aluminium 0.03 - 0.8, nitrogen 0.003 - 0.009, phosphorus 0.04 - 0.10, molybdenum 0.002 - 0.20, iron and unavoidable admixtures are the rest. Phosphorus content connected to content of carbon, nitrogen and manganese, and content of molybdenum is connected to content of phosphorus, chrome, aluminium, copper and nickel by following correlations: 0.2(C + N + Mn) / P ≤ 1.2 and [0.75 - (20P - 2.4Cr - 0.4Al - 0.9(Cu + Ni))] / 2Mo ≤ 2.0.

EFFECT: there are increased strength and plastic performances and corrosion resistance of steel and products implemented from it.

2 cl, 2 tbl

Description

The invention relates to ferrous metallurgy, and in particular to compositions of steels and to products made from them, and can be used in the manufacture of automobile parts, agricultural machinery, chemical equipment manufactured by cold deformation. Currently, the leading engineering industries make high demands on the physicomechanical and service properties of structural hot-rolled and cold-rolled strips and a sheet of low-carbon low-alloy steels, in particular, strength, deformability, corrosion resistance to ensure manufacturability, reliability, and cost-effectiveness of critical products.

Known steel for the production of hot-rolled and cold-rolled sheet, containing, wt.%:

carbon 0.002-0.015; manganese 0.10-0.35; chromium 0.05-0.40; titanium 0.02-0.15; aluminum 0.01-0.06; nitrogen <0.006; sulfur <0.02; oxygen <0.01;

iron and inevitable impurities - the rest, when the ratio Ti / (C + N) <0.8. Steel may additionally contain silicon up to 0.1%, boron up to 0.0035%, rare-earth metals up to 0.05% (US Patent No. 4331488, NKI 148/36, publ. 06/25/1982).

The disadvantages of this steel:

a small ratio of aluminum content at the lower limit (0.01 wt.%) and nitrogen (<0.006 wt.%), which for a given carbon content (0.002-0.015 (wt.%) is not enough for refining and degassing of steel melt. at the upper limit of titanium (0.15 wt.%) and chromium (0.4 wt.%), which can cause steel contamination with inclusions of titanium and chromium oxides, increase the fine-grained, hardness and brittleness of the ferrite base of the product, cause the formation of external and internal defects .

Known steel for the production of cold-rolled sheet, containing, wt.%

carbon <0.005; manganese <0.5; phosphorus <0.1; nitrogen <0.005; oxygen 0.016-0.035;

iron and unavoidable impurities are the rest.

Steel may additionally contain boron <0.005%, niobium <0.08%, vanadium <0.1%.

A hot-rolled sheet up to 3 mm thick is made of steel, then a cold-rolled sheet up to 0.8 mm thick (European application No. 0119088, IPC S22C 38/12, publ. 04/19/1984).

The disadvantages of this steel:

the absence of complex deoxidizers with a low carbon content <0.005 wt.% does not provide purification from oxygen and nitrogen. Therefore, steel may experience severe strain aging. The high content of microalloying elements at the upper limit (niobium - 0.08 wt.%, Vanadium - 0.1 wt.%) Can cause their strong enrichment of the boundaries of primary crystallites - dendrites, the formation of brittle layers of carbide eutectic, as well as the precipitation of carbide inclusions in solid state of austenite or ferrite, grain refinement, reduction of high-temperature crack resistance. This affects the deformability and welding, creates the instability of mechanical properties.

Known hot rolled strips with a thickness of 2.0 mm or more, cold rolled - with a thickness of 0.5 mm or more, made of steel containing, wt.%:

carbon 0.02-0.07; manganese 0.15-0.30; copper 0.02-0.30; aluminum 0.03-0.07; boron 0.0005-0.005; phosphorus 0.04-0.10; nitrogen 0.001-0.07;

iron is the rest.

(RF patent No. 1741459, IPC С22С 38/16, publ. 10.11.1995)

The disadvantages of this steel and products made from it:

the composition of the steel does not provide dispersion hardening during tempering-drying of the coating after stamping the part. This reduces the margin of service strength and reliability. Boron in an amount of 0.005 wt.% (Especially in combination with manganese, wt.% = 0.30) can reduce the coefficient of plastic anisotropy and reduce the ability to complex drawing. In addition, the combined presence of boron, aluminum and nitrogen, forming complex non-metallic inclusions during crystallization and in the solid state, increases the possibility of pitting corrosion, and grain-boundary segregation of copper and boron impairs weldability.

The closest analogue of the present invention are products of low alloy steel containing, wt.%:

carbon 0.004-0.010; silicon ≤0.05; manganese 0.1-1.20; phosphorus 0.01-0.05; sulfur ≤0.02; aluminum 0.01-0.10; nitrogen ≤0.004; oxygen ≤0.003 niobium 0.01-0.14;

iron and unavoidable impurities are the rest.

Steel may additionally contain up to 0.05% titanium and up to 0.002% boron.

Products are hot-rolled sheets with a thickness of 1.3 to 6.0 mm, cold-rolled sheets with a thickness of 0.8 mm, designed for the manufacture of external parts of the car, such as the hood, panel, wings. Properties of steel in the cold-rolled state: (σ _B = 340-440 MPa, σ _T = 200-297 MPa, δ ₄ = 26-40% (European patent No. 10522302, IPC C22C 38/12, C21D 9/46, publ. 14.02 .2007 - description) (prototype).

The disadvantages of steel and products made from it:

the composition at the selected content of alloying elements does not provide stability properties. At wt.% Silicon and phosphorus at 0.05, manganese ≥1.0, the ferrite grain is crushed, the hardness increases. At wt.% Aluminum 0.10, niobium 0.14, carbon up to 0.010, a large number of oxide inclusions are formed, and the different grain size is enhanced. The combination of wt.% Carbon up to 0,010, manganese up to 1,0, silicon up to 0.05 causes enhanced natural aging and instability of properties. A content of wt.% Niobium up to 0.14 causes a concentration-structural heterogeneity — the formation of brittle layers of carbide eutectic at the grain boundaries and precipitates of carbonitride inclusions. This affects the deformability, crack resistance, corrosion resistance.

The objective of the present invention is the production of low alloy steel for the production of thin hot-rolled and cold-rolled sheets, intended for parts with high mechanical and service characteristics (manufacturability, strength, corrosion resistance).

EFFECT: obtaining stably high mechanical properties of products made of low alloy steel, reducing the corrosion rate.

The technical result is achieved in that the product of low alloy steel according to the invention is made in the form of a cold-rolled strip or sheet with a thickness of 0.15-3.0 mm from steel containing carbon, silicon, manganese, sulfur, aluminum, nitrogen, phosphorus, copper, nickel, chromium, molybdenum and iron, in the following ratio of components, wt.%:

carbon 0.05-0.09;

silicon 0.01-0.10;

manganese 0.15-0.40;

copper 0.01-0.50;

nickel 0.01-0.50;

chromium 0.01-0.30;

sulfur 0.005-0.025;

aluminum 0.03-0.08;

nitrogen 0.003-0.009;

phosphorus 0.04-0.10;

molybdenum 0.002-0.20;

iron and unavoidable impurities - the rest, while the phosphorus content is related to the content of carbon, nitrogen and manganese, and the molybdenum content is related to the content of phosphorus, chromium, aluminum, copper and nickel with the following dependencies:

0.2 (C + N + Mn) / P≤1.2;

[0.75- (20P-2.4Cr-0.4Al-0.9 (Cu + Ni))] / 2Mo≤2.0.

The technical result is also achieved by the fact that the low-alloy steel product according to the invention is made in the form of a hot rolled strip or sheet with a thickness of 2.0 to 10.0 mm from steel containing carbon, silicon, manganese, sulfur, aluminum, nitrogen, phosphorus, copper, nickel, chromium, molybdenum and iron, in the following ratio of components, wt.%:

carbon 0.05-0.09;

silicon 0.01-0.10;

manganese 0.15-0.40;

copper 0.01-0.50;

nickel 0.01-0.50;

chromium 0.01-0.30;

sulfur 0.005-0.025;

aluminum 0.03-0.08;

nitrogen 0.003-0.009;

phosphorus 0.04-0.10;

molybdenum 0.002-0.20;

iron and unavoidable impurities - the rest, while the phosphorus content is related to the content of carbon, nitrogen and manganese, and the molybdenum content is related to the content of phosphorus, chromium, aluminum, copper and nickel with the following dependencies:

0.2 (C + N + Mn) / P≤1.2;

[0.75- (20P-2.4Cr-0.4Al-0.9 (Cu + Ni))] / 2Mo≤2.0.

The resulting sheets are used for the manufacture of load-bearing or facing parts of automobiles.

The necessary properties of hot-rolled and cold-rolled low-alloy steel products are provided by the influence of the selected chemical composition and component ratio on the structural state of the products.

The most effective according to the results, technological and reliable way of regulating the strength and plastic characteristics of low-carbon steels with a ferritic structural base is alloying with elements forming solid solutions of substitution or incorporation. Among elements of the type of substitution, phosphorus is the strongest hardener. It increases hardness, grinds ferrite grains during recrystallization, increases strain hardening and uniform tensile strain, and causes dispersion hardening after deformation and tempering. Phosphorus also affects the intragranular texture of ferrite after rolling or annealing: it inhibits the development of simple cubic orientations parallel to the rolling plane and enhances the development of octahedral orientations. This increases plastic anisotropy. However, when choosing the optimal phosphorus content, it is necessary to take into account the combined effect of other elements that affect the structure and properties. In low-carbon steels, this is primarily carbon, nitrogen, and manganese. Increasing the thermal stability of austenite, increasing the amount of perlite (or eutectoid carbide and other phases), they can cause an increase in strength, but at the same time weaken the positive effect of phosphorus on deformability.

In addition, with a low content of these three elements, especially in combination with increased phosphorus, the process of eutectoid transformation of austenite can be replaced: instead of lamellar perlite colonies, ferrite and coarse ("crab") carbide inclusions are separately formed. This degrades ductility and contributes to pitting corrosion. It was experimentally established that to obtain high ductility, strength and corrosion resistance, the phosphorus content is related to the content of carbon, nitrogen and manganese by dependence (1), and the limits of their content are as follows: the minimum content of P wt.% = 0.04 is taken to increase the plastic anisotropy coefficient R, the maximum content of P wt.% = 0.10 - to prevent embrittlement. The minimum content of C wt.% = 0.05, Mn wt.% = 0.15, N wt.% = 0.003 - to obtain a uniform grain structure of ferrite and prevent coarse carbide precipitations.

With the formation of a solid solution of phosphorus in ferrite (up to 0.15 wt.% P), passivation (increase in electrolytic potential) of steel in aggressive media - acidic, saline - is possible. The practical use of this important effect requires preventing the formation of concentration inhomogeneity during segregation of phosphorus at the boundaries of ferrite grains after rolling and heat treatment. Without this, embrittlement and the development of intergranular corrosion are possible. Among elements of the type of substitution in iron alloys, the diffusion of phosphorus molybdenum, which is also a surface-active element, is especially slow. Moreover, as a positive strong passivator, molybdenum, replacing phosphorus at the grain boundaries, increases the intergranular corrosion resistance. According to experimental data in aggressive environments (acid, salt solutions), the corrosion rate of steel can be reduced by controlling the content of Mo, wt.%:

[0.75- (20P-2.4Cr-0.4Al-0.9 (Cu + Ni))] / 2Mo≤2.0.

The minimum content of Mo molybdenum, wt.% = 0.002 was chosen to prevent grain boundary segregation of phosphorus and improve corrosion resistance, the maximum content of its wt.% = 0.20 was chosen to exclude the formation of molybdenum carbide inclusions at the boundaries of ferrite grains. In combination with the maximum carbon content, wt.% = 0.09, nitrogen = 0.009, manganese = 0.40, it provides the required strength without reducing the ductility necessary for the stampability of complex products. The lower limit of aluminum content, wt.% = 0.03, is taken to bind nitrogen and oxygen during deoxidation of the melt and in the solid state and to limit the growth of grain size uniformity, its upper limit wt.% = 0.08 prevents the formation of nitride or carbonitride inclusions aluminum, preservation of ductility, protection against grain-boundary and intragranular corrosion. The lower limit of copper content, wt.% = 0.01, nickel, wt.% = 0.01, and chromium, wt.% = 0.01, is taken to prevent ferrite heterogeneity, worsening plastic anisotropy, the upper limit is copper content, wt. .% = 0.50, nickel, wt.% = 0.50 and chromium, wt.% = 0.30 - to improve corrosion resistance without excessive hardening.

Examples of specific performance.

Oxygen-converter method produced 2 steel melts of the proposed composition (see table 1). After refining the melt in the ladle and introducing the necessary additives, continuous casting was performed on flat billets, with their cutting into slabs. The slabs were hot rolled into strips (3.5 mm thick), etched, cold rolled (1.0 mm thick), annealed, and trained. The mechanical properties and corrosion rate of hot-rolled and cold-rolled sheets in aggressive solutions (accelerated test method) were determined (see table 2).

Tests for corrosion resistance of steel was carried out according to the method

determination of corrosion rate in the following environments:

- concentrates of an aqueous solution (NaCl = 25%),

- seawater (NaCl up to 5%; MgCl up to 2%; MgSO4 up to 6%).

The analysis method is weighing. Measurements were made after - 1, 3, 6 months.

Bearing fasteners of the car body were made from hot-rolled sheet by cutting down, body parts were stamped from cold-rolled sheet - wind window frame racks, roof reinforcement, wheel arch, floor protective screen, etc. All details passed the full-scale tests. The results of the tests confirm the receipt of the claimed technical result.

Claims (2)

1. The product of low alloy steel, made in the form of a cold-rolled strip or sheet, characterized in that it is made of a thickness of 0.15-3.0 mm from steel containing carbon, silicon, manganese, sulfur, aluminum, nitrogen, phosphorus, copper, nickel, chromium, molybdenum and iron, in the following ratio of components, wt.%:
carbon 0.05-0.09 silicon 0.01-0.10 manganese 0.15-0.40 copper 0.01-0.50 nickel 0.01-0.50 chromium 0.01-0.30 sulfur 0.005-0.025 aluminum 0.03-0.08 nitrogen 0.003-0.009 phosphorus 0.04-0.10 molybdenum 0.002-0.20 iron and inevitable impurities rest,
the phosphorus content is associated with the content of carbon, nitrogen and manganese, and the molybdenum content is associated with the content of phosphorus, chromium, aluminum, copper and nickel according to the following relationships:
0.2 (C + N + Mn) / P≤1.2,
[0.75- (20P-2.4Cr-0.4Al-0.9 (Cu + Ni))] / 2Mo≤2.0. 2. A low-alloy steel product made in the form of a hot-rolled strip or sheet, characterized in that it is made from a thickness of more than 2.0 to 10.0 mm from steel containing carbon, silicon, manganese, sulfur, aluminum, nitrogen, phosphorus, copper, nickel, chromium, molybdenum and iron, in the following ratio of components, wt.%:
carbon 0.05-0.09 silicon 0.01-0.10 manganese 0.15-0.40 copper 0.01-0.50 nickel 0.01-0.50 chromium 0.01-0.30 sulfur 0.005-0.025 aluminum 0.03-0.08 nitrogen 0.003-0.009 phosphorus 0.04-0.10 molybdenum 0.002-0.20 iron and inevitable impurities rest,
the phosphorus content is associated with the content of carbon, nitrogen and manganese, and the molybdenum content is associated with the content of phosphorus, chromium, aluminum, copper and nickel according to the following relationships:
0.2 (C + N + Mn) / P≤1.2,
[0.75- (20P-2.4Cr-0.4Al-0.9 (Cu + Ni))] / 2Mo≤2.0.