RU2330887C1 - Method of producing cold-rolled steel for deep-drawing - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention concerns the ferrous metallurgy field. Particularly it concerns manufacturing of strip for producing odd-shaped items, mainly car parts including protective coating. While drying of lacquer coating on completed part so-called BH-effect (bake-hardening effect), it is implemented melting of steel contained mass%: carbon - 0.001-0.008, silicon - 0.002-0.025, manganese - 0.05-0.30, phosphorus - 0.005-0.060, sulphur - 0.003-0.012, aluminium - 0.010-0.070, nitrogen - 0.002-0.006, titanium - any more than 3.43N+0.005, where N - nitrogen content, niobium - any more than 7.736C - 0.0035, where C - carbon content, iron and inevitable admixture are the rest, teeming, hot rolling, winding of strips in rolls, cold rolling, recrystallisation annealing at temperature T_annealing no less 750°C, which value is selected at dependence: |T_annealing|≥875-62500(|C|-|Nb| /7.736), |C|, |Nb| - absolute value carbon and niobium content, conformably, refrigeration before 450-500°C at a rate of V °C/c, which absolute value is defined from ratio 0.04T_annealing.-23 ≤ |V| ≤ 20, where T_annealing. - annealing temperature, °C, 0.04 - empirical coefficient, 1/°C, galvanised coating and pinch rolling.

EFFECT: support of strip high blanking, heightened level of strength characteristics, which is achieved as a result of strengthening.

6 ex, 1 tbl

Description

The invention relates to ferrous metallurgy, in particular to the chemical composition and technological parameters of the production of steels intended for the manufacture of products of complex configuration, mainly automobile parts, including with protective coatings. The main requirements for such steels are high stampability, including that which is retained after application of the protective coating, as well as an increased level of strength characteristics achieved as a result of hardening during drying of coatings on the finished parts of the so-called BH-effect (bake-hardening effect) .

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

chrome 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, IPC description C22C 38/50, published on 10/10/2002).

The specified method allows to obtain high values of the coefficient of normal plastic anisotropy and the hardening index, which indicates high stampability. However, such steel may not be prone to the HH effect.

A known method for the production of cold rolled strips of ultra-low carbon steel, including steelmaking, containing, wt.%:

carbon 0.006-0.10,

Manganese 0.01-0.15,

phosphorus ≤0.07,

nitrogen ≤0.0025,

aluminum ≤0.04,

niobium 0.031-0.06,

sulfur ≤0.008,

iron and unavoidable impurities - the rest,

casting, heating slabs to 1150-1200 ° C, hot rolling with a temperature of rolling end at 910-920 ° C, winding at 740-750 ° C, cold rolling with a total compression of at least 70%, heating the strip at a speed of 10-20 ° C / s to the annealing temperature, determined depending on the Nb / C ratio by the formulas

at 3.1≤Nb / C≤4.65,

T _ref = 7.52 · (Nb / C) ² + 45.55 · Nb / C + 791 ° C

at 4.65 <Nb / C≤10,

T _anne = 1.75 · (Nb / C) ² + 33.81 · Nb / C + 730 ° C,

where Nb and C are the niobium and carbon content in steel, wt.%, holding at annealing temperature for 50-60 s and cooling at a speed of 15-25 ° C / s to 340-360 ° C.

If necessary, cold-rolled strips are coated.

The method is aimed at stabilizing the complex of mechanical properties while providing a category of very particularly complex hoods while simultaneously obtaining a strengthening effect (BH effect) of at least 40 MPa.

(RF patent No. 2212457, IPC C21D 8/04, publ. September 20, 2003, prototype description).

The disadvantage of this method is the unstable value of the HV effect depending on the continuous annealing mode.

A known method of producing high-strength cold-rolled sheet steel for deep drawing, with the ability to harden when drying coatings (VN effect). Steel intended for automotive bodies contains the following components, wt.%:

carbon 0.001-0.003,

silicon not more than 0.25,

manganese 0.1-1.0,

phosphorus 0.035-0.1,

sulfur 0.004-0.015,

aluminum 0.01-0.1,

nitrogen 0.001-0.003,

titanium not more than 0.005,

iron and inevitable impurities rest.

The billet of steel is subjected to hot rolling with a final temperature of ≥880 ° C, then cold rolling is carried out, followed by continuous annealing at a temperature higher than or equal to the recrystallization temperature but lower than Ar ₃ , and final rolling with elongation ≥1.6% and tension ≥ 7.0 kg / mm ² .

(Japanese application laid out No. 4-2729, IPC C21D 9/48, C21D 8/04, publ. 01/07/1992, declared 04/20/1990 No. 2 - 105806).

The disadvantage of this method is that with a low titanium content and the absence of niobium in steel, nitrogen is bound into finely divided particles of aluminum nitride, which leads to grain refinement and a decrease in formability.

Closest to the claimed is a method of manufacturing cold rolled steel for deep drawing, including steel smelting, casting, hot rolling, winding strips into coils, cold rolling, recrystallization annealing and tempering, if necessary, applying a zinc coating, in which steel containing carbon, silicon is smelted , manganese, phosphorus, sulfur, aluminum, nitrogen, titanium, niobium, iron and inevitable impurities, including oxygen, in the following ratio, wt.%:

carbon 0.001-0.006,

silicon 0.002-0.020,

manganese 0.07-0.30,

phosphorus 0.005-0.020,

sulfur 0.005-0.010,

aluminum 0.015-0.050,

nitrogen 0.002-0.006,

titanium 0.02-0.08,

niobium in an amount determined from the ratio

0.005% ≤ (Nb) ≤7.75 (C)

or in an amount determined from the ratio

7.75 (C) <(Nb) ≤0.06%,

where (C) and (Nb) are the carbon and niobium content, respectively, wt.%,

oxygen 0.001-0.005,

iron and inevitable impurities rest,

the total content of aluminum and titanium is 0.07-0.12%, and the ratio of aluminum to oxygen content is at least 5.0, while the minimum content of titanium is calculated from the ratio:

(Ti) _min = 3.43 (N) +2.4 (S),

where (Ti), (N) and (S) are the contents of titanium, nitrogen and sulfur, respectively, wt.% (RF Patent No. 2233905, description, IPC C22C 38/14, publ. 08/10/2004 - prototype of the invention).

The specified method (prototype) provides high stampability of cold rolled steel for deep drawing, in particular after zinc coating, however, the strength of the finished product and the magnitude of the HV effect are not stable when changing the chemical composition within the specified limits and technological parameters of production.

The problem to which the invention is directed, is to optimize the chemical composition and technological parameters of steel production while providing a technical result in the form of an increase in strength characteristics, a tendency to the HV effect, while maintaining high formability.

The technical result is achieved by the fact that in the known method of producing 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, zinc coating and training, according to the invention, steel is melted containing components in the following ratio, wt.%:

carbon 0.001-0.008,

silicon 0.002-0.025,

manganese 0.05-0.30,

phosphorus 0.005-0.060,

sulfur 0.003-0.012,

aluminum 0.010-0.070,

nitrogen 0.002-0.006,

titanium not more than 3.43N + 0.005, where N is the nitrogen content

niobium not more than 7.73 6C - 0.0035, where C is the carbon content,

iron and unavoidable impurities - the rest,

annealing is carried out at a temperature T _{annealing of} at least 750 ° C, the value of which is selected according to

| T _ar | ≥875-62500 (| C | - | Nb | / 7,736),

| C |, | Nb | - the absolute values of the carbon and niobium content, respectively, and cooling after annealing to a temperature of 450-500 ° C is carried out at a speed of V ° C / s, the absolute value of which is determined from the ratio

0.04T _OT -23≤ | V | ≤20,

where T _anne - annealing temperature, ° C,

0.04 is an empirical coefficient, 1 / ° C.

The invention consists in the following.

To ensure high formability and the required magnitude of the HV effect, the content of free carbon in a solid solution (ferrite) is not more than 0.002%. In this case, nitrogen must be bound into stable particles (nitrides), for example, titanium nitride and aluminum nitride. When the titanium content in steel is more than 3.43 N + 0.005% after nitrogen is bound to titanium nitride, a certain amount of titanium remains in the solid solution and can participate in the formation of titanium carbide particles, thereby reducing the magnitude of the BH effect. This is associated with the limitation of the maximum titanium content in steel in accordance with the claims: titanium - not more than 3.43 N + 0.005, where N is the nitrogen content. If the titanium content is insufficient for complete nitrogen binding, the nitrogen remaining in the solution will be bound to aluminum nitride. In any case, the presence of titanium in steel only in the form of nitrides makes it possible to control the carbon content in the solid solution by providing a certain ratio between the content of carbon and niobium in the steel. Moreover, the limitation of the upper value of the niobium content depending on the carbon content: niobium is not more than 7.736C-0.0035, where C is the carbon content, is a guarantee of ensuring the steel's tendency to the HH effect, since it is possible to ensure the carbon content in the solid a solution of at least 0.0006% - taking into account the possibility of partial dissolution of niobium carbide particles during annealing. With a high content of niobium in steel — closer to the upper limit of the claims, it is possible to increase the carbon content in the solid solution, and hence the magnitude of the HV effect, by partially dissolving the particles of niobium carbide during the annealing process, which is achieved by increasing the annealing temperature. _{Related to} this is the need to set the annealing temperature in accordance with the claims: | T _anne. | ≥875-62500 (| C | - | Nb | / 7,736).

The limitation of the lower value of the annealing temperature - 750 ° C is associated with the need for a sufficiently complete course of recrystallization processes to ensure the required stampability. In the process of subsequent cooling, the carbon that has passed into the solid solution during the annealing process can again be released as niobium carbide if the cooling rate to a temperature of 450-550 ° C is not large enough. Related to this is the need to limit the cooling rate to the indicated temperatures in accordance with the claims: 0.04 _T OT -23≤ | V | ≤20. It is especially important to increase the cooling rate with increasing annealing temperature and dissolution of niobium carbide particles, which is reflected in the left side of the ratio. This allows you to save the dissolved carbon in a solid solution and provide the HV effect.

The limitation of the upper value of the cooling rate is associated with the capabilities of existing units (an increase in the cooling rate is associated with an increase in the speed of the strip in the unit and, consequently, with a decrease in the annealing time, which can lead to a decrease in stampability due to insufficient recrystallization processes), as well as that a further increase in speed does not lead to an increase in the BH effect. At lower temperatures (below 450-550 ° C), the diffusion mobility of carbon atoms decreases and the rate of further cooling does not affect the magnitude of the HV effect.

The limitation of the lower limit of carbon, nitrogen and sulfur in steel is determined by the capabilities of steelmaking technologies existing today. 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 these elements above the upper limits of the claims leads to the need to increase the number of micro-laminating elements to maintain stampability, which can lead to lower surface quality, poor adhesion of the protective coating, and also increases the cost of metal products.

The upper limits of the content of silicon, manganese, and phosphorus are associated with the need to limit the solid solution hardening of steel to the limits that negatively affect 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 a factor that positively affects the formability of steel and the adhesion of protective coatings.

Examples of specific performance of the method.

Six ultralow-carbon steels were smelted in a 300-ton converter at Magnitogorsk Iron and Steel Works OJSC, cast on a continuous steel casting plant into slabs with a cross section of 250 × 1300 mm, of which strips of 2.8 mm thickness were obtained by hot rolling at the 2000 mill, providing the temperature of the end of rolling is 880-900 ° C. Strips after chilling were wound into rolls at a temperature of 710-730 ° C. After etching and cold rolling on strips with a thickness of 0.8 mm, the strips were processed in a hot dip galvanizing unit with continuous annealing at a temperature of 750-850 ° C, a cooling rate to a temperature of a zinc bath of 450 ° C 6-15 ° C / s and a zinc coating coverings. After training with a compression ratio of 0.8%, complex mechanical tests were performed.

The following steel options were tested:

option 1 - steel containing 0.004% carbon, 0.012% silicon, 0.11% manganese, 0.015% phosphorus, 0.008% sulfur, 0.04% aluminum, 0.013% titanium, 0.022% niobium, 0.004% nitrogen, iron and unavoidable impurities while the expression 3.43N + 0.005 = 0.0187, which exceeds the titanium content in the steel, that is, the titanium content corresponds to the claims, the expression 7.736C-0.0035 = 0.027, which exceeds the niobium content in the steel, that is, the niobium content also consistent with the claims, the annealing temperature for this option was 830 ° C, which corresponded to the ratio | T _anne. | ≥875-62500 (| C | - | Nb | / 7,736), according to which the minimum annealing temperature was 803 ° C; the cooling rate after annealing was 12 ° C / s, which corresponded to a ratio of 0.04 T _anh -23≤ | V | ≤20, according to which its value should be at least 10.2 ° C / s; thus, this option is fully consistent with the claims;

option 2 - steel containing 0.005% carbon, 0.014% silicon, 0.15% manganese, 0.009% phosphorus, 0.009% sulfur, 0.03% aluminum, 0.023% titanium, 0.025% niobium, 0.004% nitrogen, iron and inevitable impurities while the expression 3.43N + 0.005 = 0.0187, which is lower than the titanium content in the steel, that is, the titanium content does not correspond to the claims, the expression 7.736C-0.0035 = 0.035, which exceeds the niobium content in the steel, that is, the content niobium corresponds to the claims, the annealing temperature for this option was 800 ° C, which corresponded to the ratio | T _anne. | ≥875-62500 (| C | - | Nb | / 7.736), according to which the minimum annealing temperature was 764 ° C; the cooling rate after annealing was 10 ° C / s, which corresponded to a ratio of 0.04 T _anh -23≤ | V | ≤20, according to which its value should be at least 9 ° C / s; thus, this option did not meet the claims for titanium content;

option 3 - steel containing 0.004% carbon, 0.011% silicon, 0.12% manganese, 0.010% phosphorus, 0.008% sulfur, 0.04% aluminum, 0.011% titanium, 0.030% niobium, 0.004% nitrogen, iron and unavoidable impurities while the expression 3.43N + 0.005 = 0.0187, which exceeds the titanium content in the steel, that is, the titanium content corresponds to the claims, the expression 7.736C-0.0035 = 0.027, which is lower than the niobium content in the steel, that is, the niobium content does not correspond to the claims, the annealing temperature for this embodiment was 870 ° C, which corresponded to the ratio | T _anne. | ≥875-62500 (| C | - | Nb | / 7.736), according to which the minimum annealing temperature was 867 ° C; the cooling rate after annealing was 15 ° C / s, which corresponded to a ratio of 0.04T _anne. -23≤ | V | ≤20, according to which its value should be at least 11.8 ° C / s; thus, this option did not meet the claims for niobium content;

option 4 - steel containing 0.003% carbon, 0.007% silicon, 0.14% manganese, 0.040% phosphorus, 0.007% sulfur, 0.05% aluminum, 0.015% titanium, 0.019% niobium, 0.005% nitrogen, iron and unavoidable impurities while the expression 3.43N + 0.005 = 0.0222, which exceeds the titanium content in the steel, that is, the titanium content corresponds to the claims, the expression 7.736C-0.0035 = 0.0197, which exceeds the niobium content in the steel, that is the niobium content also corresponds to the claims, the annealing temperature for this option was 830 ° C, which did not correspond to the ratio | T _anne | ≥875-62500 (| C | - | Nb | / 7,736), s according to which the minimum annealing temperature was to be 841 ° C; the cooling rate after annealing was 12 ° C / s, which corresponded to a ratio of 0.04 T _anh -23≤ | V | ≤20, according to which its value should be at least 10.2 ° C / s; Thus, this option did not correspond to the claims according to the annealing temperature:

option 5 - steel containing 0.004% carbon, 0.022% silicon, 0.40% manganese, 0.070% phosphorus, 0.008% sulfur, 0.05% aluminum, 0.011% titanium, 0.018% niobium, 0.006% nitrogen, iron and unavoidable impurities the expression 3.43N + 0.005 = 0.0256, which exceeds the titanium content in the steel, that is, the titanium content corresponds to the claims, the expression 7.736C-0.0035 = 0.027, which exceeds the niobium content in the steel, that is, the niobium content also corresponds to the claims, the annealing temperature for this option was 800 ° C, which corresponded to the ratio | T _anne | ≥875-62500 (| C | - | Nb | / 7,736), according to As to which the minimum annealing temperature should have been 770 ° C; the cooling rate after annealing was 10 ° C / s, which corresponded to a ratio of 0.04 T _anh -23≤ | V | ≤20, according to which its value should be at least 9 ° C / s, so this option did not correspond to the formula inventions for the content of manganese and phosphorus;

option 6 - steel containing 0.004% carbon, 0.015% silicon, 0.10% manganese, 0.010% phosphorus, 0.009% sulfur, 0.04% aluminum, 0.014% titanium, 0.022% niobium, 0.005% nitrogen, iron and unavoidable impurities while the expression 3.43N + 0.005 = 0.0222, which exceeds the titanium content in the steel, that is, the titanium content corresponds to the claims, the expression 7.736C-0.0035 = 0.027, which exceeds the niobium content in the steel, that is, the niobium content also corresponds to the claims, the annealing temperature for this option was 830 ° C, which corresponded to the ratio | T _anne | ≥875-62500 (| C | - | Nb | / 7,736), as Asno, the minimum annealing temperature should have been 803 ° C; the cooling rate after annealing was 6 ° C / s, which did not correspond to a ratio of 0.04 T _anh -23≤ | V | ≤20, according to which its value should be at least 10.2 ° C / s, so this option did not meet the claims for cooling rate.

Mechanical testing of galvanized steel samples 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 (which is characteristic, in particular, for IF steels), the yield strength was determined by the readings of the tensometer taking into account the linear portion of the tensile diagram (in addition, for analysis, we used the analysis of the machine tensile diagram).

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, the elongation δ _{4 was} determined on the basis of 80 mm (A ₈₀ ).

Tests to determine the hardening of steel during drying of the paintwork (VN effect) were carried out in the following sequence:

1) the samples were stretched to a strain of 2%, which was determined by an extensometer (26 mm base); in this case, σ _{2 was} determined — stress at strain 2%;

2) the samples were placed in an oven heated to a temperature of 170 ± 10 ° C and kept for 20 minutes;

3) the samples were tensile, determining the magnitude of the HV effect as the difference between the yield strength σ _t (HV) and σ ₂ .

The results of mechanical tests of galvanized steel samples of all options are given in the table.

The stamping of steel was evaluated by the basic mechanical characteristics provided by the normative and technical documentation (NTD) for steels of this class. These include yield strength σ _t , tensile strength σ _B , elongation δ ₄ , normal plastic anisotropy coefficient r and strain hardening coefficient n. The criterion for ensuring high punchability was considered to be the yield strength of cold-rolled steel not more than 210 N / mm ² , the relative elongation not less than 40%, the normal plastic anisotropy coefficient r not less than 2.0, the value of the strain hardening coefficient n not less than 0.20. The criterion for ensuring the required propensity for the BH effect was considered to be its value of at least 40 N / mm ² .

It can be seen that for options 2, 3, 4 and 6, due to inconsistency with the claims in terms of titanium, niobium content, annealing temperature and cooling rate, respectively, the required HH effect was not provided. For option 5, due to the high content of manganese and phosphorus, high values of strength characteristics, but insufficient stampability, were obtained. Thus, only the steel obtained according to embodiment 1, corresponding to the claims, has mechanical properties at the level of the VOSV drawing category at the required value of the HV effect.

That is, the use of this proposal significantly increases the magnitude of the VN effect of hot-dip galvanized steel and the strength of the finished product from it while maintaining high characteristics of stampability.

Table
The results of mechanical tests of steels obtained by the used options. Option No. σ _t , N / mm ² σ _in , N / mm ² δ ₄ ,% r n BH effect, N / mm ² one 190 315 42 2.1 0.20 45 2 180 305 44 2.2 0.20 twenty 3 185 305 45 2.2 0.20 10 four 205 330 40 2.1 0.22 5 5 230 335 38 1.8 0.21 fifty 6 185 310 43 2.2 0.20 thirty

Claims (1)

A method of manufacturing a cold rolled strip 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, strip winding into coils, cold rolling, recrystallization annealing, zinc coating and training, characterized in that smelted steel containing components in the following ratio, wt.%: carbon 0.001-0.008, silicon 0.002-0.025, manganese 0.05-0.30, phosphorus 0.005-0.060, sulfur 0.003-0.012, aluminum 0.010-0.070, nitrogen 0.002-0.006, titanium no more than 3.43N + 0.005, where N is the nitrogen content niobium no more than 7,736С-0,0035, where С - carbon content iron and inevitable impurities rest, annealing is carried out at a temperature T _{annealing of} at least 750 ° C, the value of which is selected according to: | T _wr | ≥875-62500 (| C | - | Nb | / 7,736) where | C |, | Nb | - the absolute values of the carbon and niobium content, respectively, and cooling after annealing to a temperature of 450-500 ° C is carried out at a speed of V ° C / s, the absolute value of which is determined from the ratio: 0.04 _Tcr -23≤ | V | ≤20, where T _anne - annealing temperature, ° С, 0.04 - empirical coefficient, 1 / ° С.