RU2255989C1 - Method of production of cold-rolled steel for deep-drawing - Google Patents

Abstract

FIELD: ferrous metallurgy; motor-car industry; production of steels intended for manufacture of items of a complex configuration with the help of cold sheet stamping.

SUBSTANCE: the invention is pertaining to the field of ferrous metallurgy and motor-car industry, in particular, to methods of production of steels intended for manufacture by cold sheet stamping of items of a complex configuration, predominantly details for motor cars. The technical problem is to boost steel stamping, to improve the quality of a surface of a steel strip and hence to improve adhesion of a protective cover. The method includes a steel smelting, casting, hot rolling, strips reeling in rolls, a cold rolling, a recrystallization annealing and a temper rolling. The steel contains components in the following ratio (in mass %): Carbon - 0.002 - 0.008, silicon - 0.005-0.025, manganese - 0.05-0,20, phosphorus - 0.005-0.025, sulfur - 0.003-0.012, aluminum - 0.02-0.07, titanium - 0.02-0.05, niobium - 0.001 0.080, iron and imminent impurities - the rest. The hot rolling is completed at the temperature determined from the ratio: T_f.r≥ 7300 / (3.0-Ig [Nb] [C]) - 253, where T_f.r - temperature of the end of the rolling, °C; [Nb] and [C] - the shares of niobium and carbon in the steel accordingly in mass %, and the recrystallization annealing is carried out in a pusher-type furnace at the temperature assigned depending on the contents of niobium in steel according to the equation: T_an= (750 + 1850 [Nb]) ± 20, where T_an - a temperature of the thermal treatment, °C; [Nb] - the contents of niobium in the steel, in mass %.

EFFECT: the invention allows to boost the steel stamping, to improve the quality of the steel strip surface and adhesion of a protective cover.

4 ex, 1 tbl

Description

The invention relates to ferrous metallurgy, and in particular to a method for the production of steels intended for the manufacture of products of complex configuration using cold sheet stamping, mainly automobile parts.

The main requirements for such steel are high formability, including that which is preserved after applying a protective coating, as well as after heat treatment in continuous annealing units, ensuring good adhesion of the protective coating to the steel strip. In this case, the stampability of steel is determined by the combination of a low yield stress σ _t and high values of the coefficient of normal plastic anisotropy r, the coefficient of strain hardening n and the relative elongation δ ₄ .

A known method for the production of hot rolled steel strips with a thickness of about 3 mm and cold rolled steel strips with a thickness of 0.7-0.8 mm, intended for the manufacture of complex shapes by sheet metal stamping, mainly automobile parts, including with protective coatings. The method includes smelting steel of the following composition, wt.%:

carbon - 0.002-0.015,

silicon - 0.005-0.020,

Manganese - 0.05-0.2

sulfur - 0.005-0.015,

phosphorus - 0.005-0.015,

aluminum - 0.015-0.06,

chromium - 0.005-0.04,

nickel - 0.004-0.03,

copper - 0.006-0.05,

nitrogen - 0.001-0.006,

titanium - 0.02-0.15

calcium - 0.0003-0.0010,

iron and unavoidable impurities - the rest,

casting, hot rolling, strip winding into coils, cold rolling, recrystallization annealing and training (RF Patent 2190684, C 22 C 38/50).

The disadvantage of this method is that after the heat treatment in the continuous annealing of steel, such aggregates can have high yield stress values b _m and low elongation values b _4. In addition, when a given chemical composition of calcium is introduced into steel, a special type of nonmetallic inclusions may be formed that impair the adhesion of the protective coating.

The closest analogue is a method for the production of cold rolled steel for deep drawing, including steel smelting, casting, hot rolling, winding strips into coils, cold rolling, recrystallization annealing and tempering, and steel of the following chemical composition is melted in the ratio of components, wt.%:

carbon - 0.003-0.015,

silicon - 0.005-0.02,

Manganese - 0.05-0.20,

sulfur - 0.004-0.012,

phosphorus - 0.005-0.015

or 0.05-0.1,

aluminum - 0.015-0.06,

chromium - 0.005-0.04,

nickel - 0.004-0.03,

copper - 0.006-0.05,

nitrogen - 0.001-0.006,

niobium - 0.01-0.15

1.5 sulfur + 3.43 nitrogen + 6 carbon <titanium <1.5 sulfur + 3.43 nitrogen + 10 carbon, iron - the rest.

Steel may additionally contain 0.0005-0.005 wt.% Boron. (RF patent 2034088, C 22 C 38/50, 38/54).

The disadvantage of this method is that when the content of carbon and niobium in the steel is closer to the upper limits indicated in the formula, there is a decrease in the characteristics of stampability, especially during recrystallization annealing in the passage units and applying a protective coating. In addition, the high titanium content in steel, closer to the upper limit indicated in the formula, leads to a decrease in the surface quality of the steel strip and, accordingly, the adhesion of the zinc coating.

The technical problem solved by the invention is to increase the stampability of steel, regardless of the mode of heat treatment and applying a protective coating, as well as improving the surface quality of the steel strip, and therefore, the adhesion of the protective coating.

The technical result is achieved by the fact that in the method of manufacturing cold rolled steel for deep drawing, including the smelting of steel containing carbon, silicon, manganese, phosphorus, sulfur, aluminum, nitrogen, titanium, niobium, iron and inevitable impurities, casting, hot rolling, winding strips in rolls, cold rolling, recrystallization annealing and training, smelted steel containing components in the following ratio, wt.%:

carbon - 0.002-0.008,

silicon - 0.005-0.025,

Manganese - 0.05-0.20,

phosphorus - 0.005-0.025,

sulfur - 0.003-0.012,

aluminum - 0.02-0.07,

nitrogen - 0.002-0.007,

titanium - 0.02-0.05,

niobium - 0.001-0.080.

iron and unavoidable impurities - the rest,

hot rolling is completed at a temperature determined from the ratio:

where T _c.p. - temperature of the end of rolling, ° C,

[Nb] and [C] - the content of niobium and carbon in steel, respectively, wt.%,

and recrystallization annealing is carried out in a continuous furnace at a temperature assigned depending on the niobium content in the steel in accordance with the equation:

where T _otzh. - heat treatment temperature, ° C,

[Nb] - the content of niobium in steel, wt. %

The essence of the invention lies in the fact that to ensure high punchability, the absence of interstitial impurities — carbon and nitrogen — is required in the ferrite, as well as a relatively large ferrite grain, which is determined by the initial ferrite grain in the hot-rolled steel, as well as the completeness of recrystallization processes during annealing.

The proposed chemical composition of the steel provides a fairly complete removal from the solid solution of interstitial impurities - carbon and nitrogen; in this case, nitrogen binds predominantly to titanium nitride, and carbon - to niobium carbide.

For the formation of a relatively large grain of ferrite after hot rolling, it is necessary that the rolling be completed at temperatures higher than the temperature at which the release of niobium carbide begins. With a sufficiently high content of carbon and niobium in the steel, the temperature of the release of niobium carbide particles rises (above 900 ° C), and it becomes possible to separate them during hot rolling, which negatively affects the characteristics of the grain structure and texture of the steel, impairing the formability. This necessitates the appointment of the temperature of the end of rolling depending on the niobium content and carbon content in accordance with expression (1). When this condition is met, dynamic recrystallization processes are completed in the absence of niobium carbide particles, while grain grinding does not occur.

The presence of niobium in the steel shifts the temperature range of recrystallization during annealing towards higher temperatures, which is especially noticeable during continuous annealing. Therefore, the annealing temperature in the continuous furnace should be set depending on the niobium content in the steel in accordance with equation (2). This ensures that recrystallization processes take place during continuous annealing and obtain the required stampability characteristics: high values of normal plastic anisotropy coefficient r, strain hardening coefficient n, elongation δ ₄ , low yield stress σ _t .

To ensure high quality of the surface of the steel strip and, therefore, the adhesion of the protective coating, it is necessary to limit the maximum value of the titanium content to not more than 0.05%. This is due to the fact that with an increase in the titanium content in steel, the number of nonmetallic inclusions containing titanium and having a negative effect on the surface quality increases, in particular, leading to the appearance of films during rolling. This, in turn, impairs the adhesion of the protective coating to the steel strip.

The limitation of the lower limit of the content of carbon, nitrogen, phosphorus and sulfur in steel is determined by the capabilities of existing steelmaking technologies. A further decrease in the content of these elements does not cause a significant improvement in consumer properties, but leads to a significant increase in the cost of metal products.

The increase in the content of carbon, nitrogen and sulfur above the upper limits of the claims leads to the need to increase the number of microalloying elements, which, as shown above, can lead to poor stampability (with a large amount of carbon and niobium), and also increases the cost of metal products.

The upper limit of the silicon content is associated with the negative effect of the increased silicon content on the adhesion of the coating. The upper limit of the manganese content is associated with its negative effect on the formability. The restriction of the lower limits of the content of these elements is mainly dictated by economic considerations, since a further decrease in their content does not lead to an increase in the quality of steel.

The minimum aluminum content in steel is determined by the need for sufficient deoxidation of the steel. The limitation of the upper limit of the aluminum content is associated with its negative effect on the adhesion of the protective coating due to an increase in the amount of aluminum nitrides and, consequently, structural heterogeneity.

The minimum content of titanium and niobium is determined by the requirement of sufficient removal of interstitial impurities from the solid solution. An increase in the content of titanium and niobium above the upper limit, in addition to the negative effect on the adhesion of the protective coating and on stampability, leads to an increase in the cost of steel.

Examples of specific performance of the method

Four melts of ultra-low-carbon steels were smelted in a 300-ton converter of OJSC Magnitogorsk Iron and Steel Works and cast in a continuous casting plant into slabs with a cross section of 250 × 1290 mm. Hot rolling of slabs into strips with a thickness of 3.0 mm was carried out at the 2000 mill. The temperature of the end of rolling was 880-920 ° C. Strips after chilling were wound into rolls at a temperature of 700 ° C. After etching and cold rolling into 0.8 mm thick strips, the strips were subjected to recrystallization annealing with zinc coating in a continuous hot dip galvanizing unit. The annealing temperature was 750–880 ° С. After training with a compression ratio of 0.8%, complex mechanical tests of galvanized metal were performed, and the quality of the coating was evaluated.

Option 1 - steel containing 0.008% carbon, 0.014% silicon, 0.18% manganese, 0.018% phosphorus, 0.008% sulfur, 0.04% aluminum, 0.005% nitrogen, 0.02% titanium, 0.08% niobium, iron and inevitable impurities, the expression 7300 / (3,0-Ig [Nb] [C]) - 253 was 906 ° C, and the temperature of the end of the rolling was 880 ° C, that is, the ratio (1) did not correspond to the claims, the expression (750 + 1850 [Nb]) ± 20 was 878-918 ° C, and the annealing temperature was 880 ° C, that is, it corresponded to the claims.

Option 2 - steel containing 0.006% carbon, 0.013% silicon, 0.20% manganese, 0.015% phosphorus, 0.009% sulfur, 0.02% aluminum, 0.003% nitrogen, 0.03% titanium, 0.07% niobium, iron and inevitable impurities, while the expression 7300 / (3.0-Ig [Nb] [C]) - 253 was 871 ° C, and the temperature of the end of the rolling was 900 ° C, that is, the ratio (1) was consistent with the claims, expression (750 + 1850 [Nb]) ± 20 was 860-900 ° C, and the annealing temperature was 850 ° C, that is, did not comply with the claims.

Option 3 - steel containing 0.004% carbon, 0.011% silicon, 0.17% manganese, 0.016% phosphorus, 0.006% sulfur, 0.06% aluminum, 0.025% titanium, 0.05% niobium, 0.004% nitrogen, iron and unavoidable impurities, while the expression 7300 / (3,0-Ig [Nb] [C]) - 253 was 817 ° C, and the temperature of the end of the rolling was 910 ° C, that is, the ratio (1) was consistent with the claims, expression (750 +1850 [Nb]) ± 20 was 823-863 ° С, and the annealing temperature was 850 ° С, that is, it corresponded to the claims. This option is fully consistent with the claims.

Option 4 - steel containing 0.005% carbon, 0.014% silicon, 0.15% manganese, 0.016% phosphorus, 0.008% sulfur, 0.05% aluminum, 0.07% titanium, 0.05% niobium, 0.004% nitrogen, iron and inevitable impurities, the expression 7300 / (3,0-Ig [Nb] [C]) - 253 was 833 ° C, and the temperature of the end of the rolling was 920 ° C, that is, the ratio (1) was consistent with the claims, expression (750 + 1850 [Nb]) ± 20 was 823-863 ° C, and the annealing temperature was 850 C, that is, consistent with the claims. This option did not meet the claims for titanium content - 0.07 instead of 0.02-0.05%.

Mechanical testing of samples of cold-rolled steel from the steel of the above-mentioned heats was carried out on an INSTRON-1185 electromechanical testing machine. The dimensions of the sample were 20 × 120 mm.

The tests were carried out in a semi-automatic mode with a longitudinal strain tensometer (strain gauge base 12.5 mm). The tensile rate was 10 mm / min.

In the case of tensile curves without a physical yield strength, the yield strength was determined from the readings of the tensometer taking into account the linear portion of the tensile diagram (in addition, for analysis, an analysis of the machine tensile diagram was used).

The hardening index was determined in the deformation range from 10 to 17%.

The coefficient of normal plastic anisotropy r was determined when the tests were stopped (when reaching 17%) by manually measuring the width of the sample (in three sections).

For samples with a width of 20 mm, an elongation of 64 was determined on the basis of 80 mm (A ₈₀ ).

The results of the mechanical tests are shown in the table. The main mechanical characteristics were determined: yield strength σ _t elongation b ₄ , normal plastic anisotropy coefficient r and strain hardening coefficient n. The criterion for ensuring the required stampability was considered to be the yield strength of cold rolled steel σ _t not higher than 175 N / mm ² , the relative elongation b ₄ not less than 40%, the normal plastic anisotropy coefficient r not less than 2.0, and the strain hardening coefficient n not less than 0 ,20.

To assess the quality of the connection of the zinc coating with the base metal, a bending test method at an angle of 180 ° was used without sending in accordance with GOST 14918-80. The test results for the studied swimming trunks are also presented in the table.

For steel according to option 1, the yield strength σ _{t was} obtained above the required value (190 N / mm ² instead of 175 N / mm ² ), which is associated with the grinding of ferrite grains after hot rolling due to the failure of relation (1). For option 2, due to the incomplete occurrence of recrystallization processes (non-fulfillment of relation 2), low values of ductility δ ₄ (38% instead of 40%) and stampability r (1.8 instead of 2.0) were obtained. For options 3 and 4, the set of mechanical characteristics is very high. However, for option 4, due to the increased titanium content, an unsatisfactory quality of the coating to the base connection was obtained.

Thus, only cold-rolled steel obtained according to option 3, corresponding to the claims, has high strength and stampability with satisfactory quality of the connection of the coating with the base.

That is, the use of the present invention significantly improves the formability of steel even after recrystallization annealing in a through-flow unit and applying a zinc coating, as well as the quality of the connection of the coating with the base.

Table The mechanical properties of galvanized steel and the quality of the connection of the coating with the base No. of swimming trunks σ _t N / mm ² δ _4% r n The quality of the connection of the coating with the base 1 190 40 2.0 0.22 beats 2 175 38 1.8 0.20 beats 3 170 43 2.2 0.22 beats 4 165 45 2.2 0.21 unsuccessful.

Claims (1)

A method of manufacturing cold rolled steel for deep drawing, including the smelting of steel containing carbon, silicon, manganese, phosphorus, sulfur, aluminum, nitrogen, titanium, niobium, iron and inevitable impurities, casting, hot rolling, winding strips into coils, cold rolling, recrystallization annealing and training, characterized in that smelted steel containing components in the following ratio, wt.%: Carbon 0.002-0.008 Silicon 0.005-0.025 Manganese 0.05-0.20 Phosphorus 0.005-0.025 Sulfur 0.003-0.012 Aluminum 0.02-0.07 Nitrogen 0.002-0.007 Titanium 0.02-0.05 Niobium 0.001-0.080 Iron and inevitable impurities hot rolling is completed at a temperature determined from the ratio T _c.p. ≥ 7300 / (3,0-Ig [Nb] [C]) - 253, where T _c.p. - temperature of the end of rolling, ° C; [Nb] and [C] - the content of niobium and carbon in steel, respectively, wt.%, And recrystallization annealing is carried out in a continuous furnace at a temperature assigned depending on the content of niobium in steel in accordance with the equation T _otzh. = (750 + 1850 [Nb]) ± 20, where T _otzh. - heat treatment temperature, ° С; [Nb] is the niobium content in steel, wt.%.