Method of production of cold-rolled steel for deep-drawing

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: Tf.r≥ 7300 / (3.0-Ig [Nb] [C]) - 253, where Tf.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: Tan= (750+ 1850 [Nb]) ± 20, where Tan - 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

 

The invention relates to ferrous metallurgy, in particular to a method of production of steel, for the manufacture of articles of complex shape using cold sheet metal forming, mainly parts of the car.

The main requirements of this steel are high stampability, including stored after application of the protective coating, as well as after thermal processing in units of continuous annealing ensuring good adhesion of the protective coating to the steel strip. This stampability steel is determined by the combination of low values of yield strength σtand high values of the coefficient of normal plastic anisotropy r, coefficient of strain hardening n and relative elongation δ4.

A method of producing hot rolled steel strips with a thickness of about 3 mm, cold-rolled steel strips with a thickness of 0.7-0.8 mm, for the manufacture of complex-shaped products by the method of sheet metal forming, mostly parts cars, including protective coatings. The method includes 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 is from 0.005 to 0.04,

Nickel - 0,004-0,03,

copper - 0,006-0,05,

titanium - 0,02-0,15

calcium - 0,0003-0,0010,

iron and inevitable impurities - the rest,

casting, hot rolling, winding the strips into rolls, cold rolling, recrystallization annealing and tempering (RF Patent 2190684, With 22 38/50).

The disadvantage of this method is that after the heat treatment in units of continuous annealing of such steel may have high values of yield strength btand low values of relative elongation b4. In addition, with the introduction of the steel of the chemical composition of calcium, the formation of a special type of non-metallic inclusions, which cause poor adhesion of the protective coating.

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

carbon - 0.003 to 0.015 g,

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 is from 0.005 to 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<Titan<1,5 sulfur+3,43 nitrogen+10 carbon, iron - rest.

Steel can complement the flax contain of 0.0005-0.005 wt.% Bora. (RF patent 2034088, With 22 38/50, 38/54).

The disadvantage of this method is that when the content of carbon and niobium in the steel closer to the upper limits specified in the formula, there is a reduction characteristics of stanoevska, especially when recristallization annealing in the walk-through units and the protective coating. In addition, the high content of titanium in the steel closer to the upper limit specified in the formula, leads to a decrease of the surface quality of the steel strip and, accordingly, the adhesion of the zinc coating.

The technical problem solved by the invention is to improve stanoevska steel regardless of the mode of heat treatment and application of protective coatings, as well as to improve the surface quality of the steel strip, and therefore, adhesion of the protective coating.

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

carbon - 0,002-0,008,

silicon is from 0.005 to 0.025,

manganese - 0,05-0,20,

phosphorus is from 0.005 to 0.025,

sulfur - 0,003-0,012,

aluminum - 0,02-0,07,

nitrogen - 0,002-0,007,

titanium - 0,02-0,05,

niobium is 0.001-0,080.

iron and inevitable impurities - the rest,

hot finish rolling at a temperature determined from the relationship:

where TC.P.- temperature end of the rolling, °C

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

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

where TOTG.- temperature heat treatment, °C

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

The invention consists in the fact that for high stanoevska requires the absence of the ferrite introduction of impurities of carbon and nitrogen, and relatively large ferrite grains, which is defined by the initial grain ferrite in hot-rolled and also the complete course of recrystallization processes during annealing.

The proposed chemical composition of steel provides a relatively complete removal of solid solution impurity introducing carbon and nitrogen; the nitrogen binds preferentially to titanium nitride, and carbon in the carbide of niobium.

For the formation of relatively large C the RNA ferrite after hot rolling is necessary, so rolling was finished at temperatures higher than the temperature of the allocation of niobium carbide. At a sufficiently high content in carbon steel and niobium increases the temperature of the allocation of particles of niobium carbide (above 900° (C), and it could be the selection in the hot rolling process, which negatively affects the characteristics of the grain structure and texture of steel, reducing stampability. Related to this is the need of the temperature of the end of the rolling, depending on the content of niobium and carbon content in accordance with the expression (1). In this case the process of dynamic recrystallization is completed in the absence of particles of niobium carbide, grinding grain does not occur.

The presence of niobium in steel shifts the temperature interval recrystallization annealing to higher temperatures, which is especially noticeable during continuous annealing. Therefore, the temperature of annealing in the passing of the furnace should be administered, depending on the content of niobium in steel in accordance with equation (2). It provides a complete course of recrystallization processes during continuous annealing and obtain the required characteristics of stanoevska: high values of the coefficient of normal plastic anisotropy r, coefficient of deformation uprock is of n, relative elongation δ4low values of yield strength σt.

To ensure the high quality of the surface of the steel strip and, consequently, the adhesion of the protective coating is necessary to limit the maximum value of the titanium content is not more than 0.05%. This is because with increasing titanium content of the steel increases the number of nonmetallic inclusions containing titanium and having a negative impact on the surface quality, in particular leading to the emergence of a prisoner during rolling. This, in turn, impairs the adhesion of the protective coating to the steel strip.

The restriction of the lower limit of the content of carbon, nitrogen, phosphorus and sulfur in steel is determined by the capabilities of the existing steelmaking technologies. Further reduction of the content of these elements does not cause a significant improvement of consumer properties, but leads to a significant rise in prices of metal products.

The increase in the content of carbon, nitrogen and sulfur above the upper limits of the claims leads to the necessity of increasing the number microeconomic elements, which, as shown above, can lead to deterioration of stanoevska (when a large amount of carbon and niobium), and also increases the cost of products.

The upper limit of the content of silicon is connected with the negative influence of high silicon concentration on the adhesion of the coating. The upper limit of the manganese content is connected with its negative impact on stampability. Restriction lower limits of the contents of these elements are mainly driven by economic considerations, so as to further decrease their content does not improve the quality of steel.

The minimum aluminum content in the steel is determined by the need for sufficient deoxidation of steel. Limitation upper limit of the content of aluminum is connected with its negative impact on the adhesion of the protective coating due to the increase in the number of nitrides of aluminum and, therefore, structural heterogeneity.

The minimum content of titanium and niobium is determined by the requirement for sufficient removal of the solid solution impurity introduction. The increase in the content of titanium and niobium is above the upper limit in addition to the negative impact on the adhesion of the protective coating and stampability leads to higher prices of steel.

Examples of specific implementation method

Four melting sverkhnizkochastotnye steels were melted in a 300-ton Converter OJSC “Magnitogorsk iron and steel works and bottled on the continuous casting into slabs section 250×1290 mm Hot rolling slabs into strips with a thickness of 3.0 mm was carried out on the mill “2000”. The temperature of the end of the rolling was 880-920°C. Strip after the souls of the Finance wound into rolls at a temperature of 700° C. After pickling and cold rolling the strip with a thickness of 0.8 mm strips were subjected to recrystallization annealing with zinc coating in continuous hot dip galvanizing. The temperature of annealing was 750-880°C. After tempering with the degree of compression of 0.8% conducted a comprehensive mechanical testing of galvanized metal, and also assessed the quality of the coating.

Option 1 - steel containing 0,008% of carbon, of 0.014% silicon, 0.18% manganese, 0,018% phosphorus, 0,008% sulfur, 0.04% of aluminum, 0.005% nitrogen, 0,02% titanium, 0.08% niobium, iron and inevitable impurities, and the expression 7300/(3,0-Ig[Nb] [C])-253 amounted to 906°and the temperature of the end of the rolling was 880°S, i.e. the ratio of (1) is not consistent with the claims, the expression (750+1850[Nb])±20 was 878-918°and the annealing temperature was 880°i.e. they correspond to the claims.

Option 2 - steel containing 0,006% of carbon, of 0.013% of silicon, 0.20% manganese, 0.015% phosphorus, 0,009% sulfur, 0.02% aluminum, 0,003% nitrogen, 0.03% of titanium, 0,07% niobium, iron and inevitable impurities, and the expression 7300/(3,0-Ig[Nb][C])-253 totaled 871°and the temperature of the end of the rolling 900°S, i.e. the ratio of (1) correspond to the claims, the expression (750+1850[Nb])±20 was 860-900°and the annealing temperature was 850°Since, that is not consistent with the formula izobreteny is.

Option 3 - steel containing 0,004% of carbon, to 0.011% silicon, 0,17% manganese, 0,016% phosphorus, 0,006% sulfur, to 0.06% aluminum, 0.025% titanium, 0.05% niobium, 0,004% nitrogen, iron and inevitable impurities, and the expression 7300/(3,0-Ig[Nb][C])-253 817°and the temperature of the end of the rolling was 910°S, i.e. the ratio of (1) correspond to the claims, the expression (750+1850[Nb])±20 was 823-863°and the annealing temperature was 850°i.e. they correspond to the claims. This option is fully consistent with the claims.

Option 4 - steel containing 0.005% carbon, of 0.014% silicon, 0.15% of manganese, 0,016% phosphorus, 0,008% sulfur, 0.05% aluminum, 0,07% titanium, 0.05% niobium, 0,004% nitrogen, iron and inevitable impurities, and the expression 7300/(3,0-Ig[Nb][C])-253 amounted to 833°and the temperature of the end of the rolling was 920°S, i.e. the ratio of (1) correspond to the claims, the expression (750+1850[Nb])±20 was 823-863°and the annealing temperature was 850 With, that is consistent with the claims. This option did not meet the claims on the content of titanium instead of 0.07 to 0.02-0.05%.

Mechanical testing of samples of cold rolled steel these melts were performed on an Electromechanical testing machine INSTRON-1185. The size of the sample was 20×120 mm

The tests were carried out in semi regimes the strain gauge longitudinal strain (base strain gauge 12.5 mm). Stretch speed was 10 mm/min

In the case of curves without physical strain yield stress value of the yield strength was determined by the strain gauge readings taking into account the linear section of the diagram expansion (in addition to control, used the analysis of machine diagram expansion).

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

The ratio of the normal plastic anisotropy r was defined by the stop trials (17%) by measuring manually the width of the sample (in three sections).

For samples with a width of 20 mm elongation 64 was determined on the basis of 80 mm (A80).

The results of the mechanical tests are shown in table. Defined the basic mechanical properties: yield strength σtelongation b4coefficient of normal plastic anisotropy r and the coefficient of strain hardening n. Criteria provide the required stanoevska considered getting the value of the yield strength cold-rolled σtnot above 175 N/mm2relative elongation b4not less than 40%, the coefficient of normal plastic anisotropy r is not less than 2.0 and the value of the coefficient of strain hardening n is not less than 0,20.

To evaluate the communication quality of the zinc coating with the main m the organism used method of bending tests at an angle of 180° without sending in accordance with GOST 14918-80. The test results for the studied melts also presented in the table.

For steel for option 1, the value of the yield strength σtreceived above the required (190 N/mm2instead of 175 N/mm2), which is associated with grinding grains of ferrite after hot rolling due to failure ratio (1). For option 2, due to incomplete course of recrystallization processes (default 2) obtained low values of the ductility characteristics δ4(38% instead of 40%) and stanoevska r (a 1.8 instead of 2.0). For options 3 and 4 complex mechanical characteristics very high. However, for option 4 because of the higher titanium content received unsatisfactory connection of the coating to the base.

Thus, only cold-rolled steel produced by variant 3, corresponding to the formula of the invention has high strength and stanoevska when a satisfactory connection of the cover with the base.

That is, the use of the present invention significantly increases the stampability steel even after recrystallization annealing in pass-through Assembly and application of the zinc coating, and the connection of the cover with the base.

/tr>
Table
Mechanical properties of galvanized steel and the connection of the coating to the base
No. smeltingσtN/mm2δ4%rnThe connection of the coating to the base
1190402,00,22ID
2175381,80,20ID
3170432,20,22ID
4165452,20,21neud

Method of manufacturing cold rolled steel for deep drawing, including steel containing carbon, silicon, manganese, phosphorus, sulfur, aluminum, nitrogen, titanium, niobium, iron and inevitable impurities, casting, hot rolling, winding the strips into rolls, cold rolling, recrystallization annealing and tempering, characterized in that the smelted steel containing components in the following ratio, wt.%:

Carbon 0,002-0,008

Silicon from 0.005 to 0.025

Manganese 0,05-0,20

Phosphorus from 0.005 to 0.025

Sulfur 0,003-0,012

Aluminum 0,02-0,07

Nitrogen 0,002-0,007

Titanium 0,02-,05

Niobium 0,001-0,080

Iron and inevitable impurities Else

hot finish rolling at a temperature determined from the relationship

TC.P.7300/(3,0-Ig[Nb][C]) - 253,

where TC.P.- temperature end of the rolling, ° C;

[Nb] and [S] is the content of niobium and carbon steel respectively, wt.%, and recrystallization annealing is carried out in the entrance of the furnace at a temperature that is assigned depending on the content of niobium in steel in accordance with the equation

TOTG.=(750+1850 [Nb])± 20,

where TOTG.- temperature heat treatment, ° C;

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



 

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5 cl, 1 ex, 1 dwg

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3 cl, 5 ex, 3 tbl

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EFFECT: the invention ensures improvement of the quality of a cold rolled sheet steel.

3 cl, 5 ex, 3 tbl

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