RU2256707C1 - Method of production of the steel with homogeneous properties - Google Patents

Abstract

FIELD: metallurgy; methods of production of steels with homogeneous properties.

SUBSTANCE: the invention is pertaining to the field of metallurgy, in particular, to production of deep-drawing steels used for production of items of the complex configuration, predominantly - he details of motor-cars. The technical result of the invention is increased homogeneity of mechanical properties of the steel and reduction of cost of its production. This technical result is achieved due to the fact, that they conduct a continuous casting of the steel containing of no more than 0.007 mass % of carbon and 0.006 mass % of nitrogen in slabs, heating and hot rolling in bands, refrigeration, a reeling in rollers, etching, cold rolling with compaction of no less than 70 %, annealing and temper rolling. The annealing of cold-rolled steel conduct in the pusher-type furnace at temperatures of 750 ÷ 900°C during 5 ÷ 18 minutes.

EFFECT: the invention ensures increased homogeneity of mechanical properties of the steel and reduction of cost of its production.

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Description

The present invention relates to metallurgy, specifically to the production of steel for deep drawing, used for the manufacture of products of complex configuration, mainly automobile parts.

Widely used technology for the production of steel for deep drawing includes smelting and continuous casting, hot rolling, pickling, cold rolling, annealing and tempering. The main characteristic of the quality of steel are mechanical properties. These properties depend on the chemical composition, structure parameters and texture of the finished steel. The structure and texture are formed during all technological operations. Training is carried out to give the strips a good flatness. For a long time, 08U steel was used for deep drawing. However, the improvement of the smelting technology made it possible to obtain cast steel slabs with a low content of such interstitial atoms as carbon and nitrogen. Steel was developed that differs from 08Yu steel in a better combination of strength and plastic properties and is called IF-steel. To vary the properties of this steel, in some cases it is purposefully microalloyed with titanium, niobium, phosphorus and some other chemical elements. For example, the following chemical composition of such steel is known, wt.%: 0.003 ÷ 0.015 carbon, 0.005 ÷ 0.020 silicon, 0.05 ÷ 0.20 manganese, 0.004 ÷ 0.012 sulfur, 0.005 ÷ 0.015 or 0.05 ÷ 0.15 phosphorus, 0.015 ÷ 0.060 aluminum, 0.004 ÷ 0.03 nickel, 0.006 ÷ 0.05 copper, 0.001 ÷ 0.006 nitrogen, 0.00005 ÷ 0.005 boron, the titanium content is determined from the ratio - 1.5 × Sulfur + 3.43 × Nitrogen + 6 × Carbon <Titanium <1.5 × Sulfur + 3.43 × Nitrogen + 10 × Carbon, the rest is iron [RF Patent 2034088, IPC C 22 C 38/50, 38/54, 1995].

There is also known a method of manufacturing sheet steel for cold stamping [RF Patent 2197542, IPC C 21 D 8/04, 9/48, 2003], including continuous casting of slabs, their heating and hot rolling into strips, water cooling, winding into rolls , etching, cold rolling, annealing and training. In this method, the technical result is to improve the exhaust properties and increase the yield of conditioned steel sheet. To achieve this result, steel containing, wt.%: 0.002 ÷ 0.007 carbon, 0.005 ÷ 0.050 silicon, 0.08 ÷ 0.16 manganese, 0.01 ÷ 0.05 aluminum, 0.05 ÷ 0.12 titanium, not more than 0.015 phosphorus, not more than 0.010 sulfur, not more than 0.04 chromium, nickel and copper, not more than 0.006 nitrogen, iron - the rest is subjected to continuous casting at a speed of 0.4 ÷ 1.6 m / min at a temperature of cast steel 1500 ÷ 1580 ° C in slabs. The slabs are heated to 1150 ÷ 1240 ° C, incubated for 2.5 ÷ 4 hours and rolled into strips with a temperature of rolling end not lower than 870 ° C. Hot rolled strips are cooled with water to 550 ÷ 730 ° C and wound into rolls. After trawling, the strips are subjected to cold rolling with a total compression of at least 70% and annealing at 700–750 ° С with holding at this temperature for 11–34 hours. Then the bands are trained with a compression of 0.4 ÷ 1.2% in rolls with a surface roughness of 2 ÷ 4 μm Ra and a peak density of 60 ÷ 120 1 / cm. An embodiment of the method is possible, in which the hot-rolled strips are annealed at 660 ÷ 680 ° C with holding for 10 ÷ 18 hours. This method is selected as a prototype.

It allows you to improve the mechanical properties and increase the yield of conditioned steel sheet. However, in the production of steel, an important characteristic of its quality is the uniformity of mechanical properties along the length and width of the strips. The prototype method does not solve this problem, although it is aimed at increasing the yield of conditioned steel.

The technical problem solved in the proposed invention is to increase the uniformity of the mechanical properties of steel and reduce the cost of its production.

The problem is achieved in that according to the proposed method, continuous casting of steel containing not more than 0.007 wt.% Carbon and 0.006 wt.% Nitrogen is carried out in slabs, heating and hot rolling into strips, cooling, winding into coils, pickling, cold rolling with reductions of at least 70%, annealing and training. Annealing of cold rolled steel is carried out in continuous furnaces at temperatures of 750-900 ° C for 5 ÷ 18 minutes.

The invention consists in the following. In the prototype method, annealing of coils of cold rolled steel weighing at least 10 tons is carried out in bell furnaces. However, with such annealing, the heating rate of the external and internal parts of the rolls varies significantly and is the reason for the increase in heterogeneity of properties. Usually bell-type furnaces are heated at a speed of about 50 ÷ 60 ° C / hour. Moreover, the heating rate of the inner parts of the rolls does not exceed 7 ° C / hour. The use of annealing in continuous furnaces allows for uniform and uniform heating along the width and length of the strips and to reduce the heterogeneity of the properties. In addition, the cost of steel annealing in a continuous furnace is lower than in a bell furnace. Production costs are reduced. Moreover, the modes of technological operations are selected so that the mechanical properties of the finished steel meet the requirements of consumers, are similar to the properties of the prototype method.

The mechanical properties of steel are influenced by the modes of all technological operations, starting from smelting. During smelting, the required chemical composition of the steel is achieved. The chemical elements that make up the steel affect its mechanical properties in two ways: one - the effect on the formation of structure and texture in the process of steel production; the other is the effect on the level of residual stresses, bond strength between atoms, the number of dispersed second phases in the finished steel. In subsequent technological operations after smelting, the necessary thickness, texture and structure are achieved, i.e. steel properties. A positive effect in this method is achieved due to the same heating conditions along the length and width of the strips of cold rolled steel. However, obtaining the necessary mechanical properties of steel is determined by the totality of the regulated chemical composition of steel and the modes of technological operations.

The requirement for the content in steel of not more than 0.007 wt.% Carbon and 0.006 wt.% Nitrogen is caused by the fact that at high contents of atoms of the introduction of these chemical elements in the finished steel, it becomes impossible to obtain the necessary combination of strength and plastic properties. The reason for this is the interaction of nitrogen and carbon atoms with defects in the crystal lattice [J. Aldazabal et al. Hall-Petch behavior induced by plastic strain gradients. Vaterials Science and Engineering, A 365. 2004. h.186-190].

Compression of at least 70% during cold rolling is necessary to obtain the maximum number of texture components ({111} <uvw> and others) that provide the necessary level and minimum anisotropy of the mechanical properties of the finished steel.

To obtain the optimal grain size and texture, certain temperature-time regimes of annealing of cold-rolled steel are required. As studies have shown, steel annealing can be carried out both in bell-type and continuous continuous furnaces. However, annealing in the passageways provides better flatness of the strips and a higher uniformity of mechanical properties along the width and length of the strips, lower cost of steel. Optimum temperature-time modes of annealing provide complete passage in the steel of primary recrystallization and a certain development of collective recrystallization. With insufficient or excessive temperature or annealing time, the degree of development of collective recrystallization does not correspond to the formation of optimal parameters of the structure and texture, i.e. mechanical properties of steel. The proposed annealing modes in continuous furnaces are optimal for cold rolling with reductions of at least 70%, because compression is realized not only in the form of texture, but also in the form of the level of energy accumulated by steel. A different energy level requires certain annealing modes to obtain the necessary properties of steel.

In the proposed method, only the totality of the claimed features allows to achieve the goal. The search for such a totality of features in Russian and foreign scientific and technical literature has not yielded results. We can assume that the invention meets the criterion of “Novelty”.

An example implementation of the method

Steel is smelted in a converter, and slabs are produced by continuous casting. The chemical composition of the steel is shown in table 1. Steel No. 3 and 4 is beyond the scope of the claimed chemical composition, prototype No. 5.

Before hot rolling, the slabs are placed in a methodical furnace with gas heating and walking beams. In the oven, the slabs are kept at a temperature of 1180 ° C for 2.5 hours. Hot rolling is carried out in a mill consisting of four roughing and seven finishing stands to a strip thickness of 3.20 mm. The temperature of the end of rolling is 870 ° C. The strips are choked on the discharge roller table and the winding temperature is 640 ° С. After cooling, a layer of oxides (scale) is removed from the surface of the strips by etching them in a solution of hot sulfuric acid. Then cold rolling is carried out on a four-stand mill to a strip thickness of 0.80 mm. The total reduction is 75%. Cold rolled strips are annealed in continuous furnaces at a temperature of 820 ° C for 9 minutes. Annealed strips are trained with a compression of 1%.

Table 2 shows the mechanical properties, their heterogeneity along the length and width of the strips, and production costs. Production costs are given as a percentage in comparison with the existing production method and prototype.

Example No. 1 fully complies with the claimed parameters. In example No. 2, the carbon and nitrogen content is on the border of the claimed values of these elements. In examples No. 3 and 4, the maximum permissible content of carbon and nitrogen was exceeded, and a yield tooth appeared on the tensile test curves, which immediately prevents the use of such steel for stamping parts of complex shape. Example No. 5 is taken from the prototype method for comparison with annealing in a bell furnace.

The inventive method provides an output of conditioned steel, identical to the prototype method. Moreover, the heterogeneity of the yield strength along the length of the strips is reduced by 4–5%, along the width of the strips - by 4%, and the cost of production compared with the existing and prototype method is reduced by 6%. The values of the coefficients r and n of the relative elongation δ and tensile strength σ _in the proposed method and the prototype method are high and much higher than those required by GOST for these steels, GOST 9045-93.

Table 1
The chemical composition of steel, wt.% No. FROM Si Mn S R Al Cr Ni Cu Ti N Fe 1 0.004 0.017 0.11 0.010 0.010 0.06 0,03 0.01 0,03 0.08 0.005 Ost. 2 0.007 0,021 0.11 0.010 0.011 0.06 0,03 0.01 0,03 0.09 0.006 Ost. 3 0.005 0,020 0.11 0.010 0.011 0.05 0,03 0.02 0.02 0.09 0.008 * Ost. 4 0.009 * 0.019 0.11 0.009 0.010 0.05 0,03 0.01 0,03 0.09 0.005 Ost. 5 0.0045 0,028 0.13 0.007 0.010 0,03 0.01 0.01 0.02 0.09 0.003 Ost. table 2
Mechanical properties, yield of conditioned steel and production costs No. σ _0.2 , N / mm ² σ _In , N / mm ² δ,% r n Inhomogeneity σ _0.2 ,% Production costs,% By lenght In width 1 170 310 54 2.7 0.31 4 4 94 2 165 305 52 2.7 0.30 5 4 94 3 σ _t 235 325 36 1.9 0.20 5 4 94 4 σ _t 230 325 36 1.8 0.20 5 4 94 5 162-170 300-305 55-56 2.9 0.36 nine 8 100 Note to table No. 2 r is the coefficient of normal plastic anisotropy. It is defined as the ratio of true strains in width and thickness when the sample is stretched by 17% (GOST 11701). The larger the coefficient r, the greater the normal anisotropy, the resistance to thinning and the ability of the metal to draw, n is the strain hardening index characterizing the ability of the metal to harden under uniform plastic deformation. It is determined by two points during deformation of the sample by 10 and 17%. The greater the exponent n, the greater the metal’s ability to draw.

Claims (1)

A method for the production of steel with uniform properties, including continuous casting of steel containing not more than 0.007 wt.% Carbon and 0.006 wt.% Nitrogen, into slabs, heating and hot rolling into strips, cooling, winding into coils, pickling, cold rolling with compression not less than 70%, annealing and training, characterized in that the annealing of cold rolled steel is carried out in continuous furnaces at temperatures of 750 ÷ 900 ° C for 5 ÷ 18 minutes