RU2478729C2 - Method of making steel strip (versions) - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: proposed method comprises steel smelting, continuous casting of slabs to be heated, hot rolled, and water cooled; strip reeling, etching, cold rolling, recrystallisation annealing, application of zinc coating and/or pinch rolling. Smelt steel contains following substances, in wt %: carbon 0.001 - 0.015, silicon 0.001 - 0.50, manganese 0.05 -1.0, phosphorus not over 0.12, sulfur not over 0.025, chromium not over 0.30, nickel not over 0.30, copper not over 0.19, aluminium not over 0.02 - 0.15, nitrogen not over 0.010, iron and unavoidable impurities making the rest, including titanium and niobium making the rest. Note here that [Mn]/[S] ratio equals ≥ 4.8 while total content of titanium and niobium does not exceed 0.012 wt %. Slabs are heated at 1060°C to 1200°C. Hot rolling is terminated at 800-930°C. Strip reeling is performed at 550 to 730°C while cold rolling is performed to total reduction of 40-95%. Recrystallisation annealing is carried out in bell-type furnace at 600 - 750°C with holding for 6-35 hours or, in applying zinc coating in pusher furnace, at 750 - 900°C.

EFFECT: higher strength, better forming properties, lower costs.

4 cl, 4 tbl, 1 ex

Description

The invention relates to the field of metallurgy, and specifically to a method for the production of cold-rolled and hot-dip galvanized steel strips with additional hardening when drying a paint coating on a stamped product (VN effect). Steel strips are designed for the manufacture of automobile products by stamping.

The BH effect is the hardening that occurs during the drying process of a paint coating on a stamped part. In steels with the HV effect, high plastic properties in the delivery state and high strength characteristics after stamping and drying are combined. High-strength steels with a BH effect are resistant to dents, which makes it possible to reduce the thickness of sheet steel, and therefore leads to fuel economy. The BH effect at the box office is due to artificial aging associated with the presence of free carbon atoms in a solid solution of ferrite, which form Cotrell clouds - the thinnest carbide precipitates. These carbides fix mobile dislocations in ferrite at elevated temperatures, which leads to hardening of stamped parts during drying.

One of the defining qualities of steel strips for stamping is the ability to stretch, a combination of strength and plastic characteristics, the strips must correspond to a certain set of mechanical properties, for example, according to the requirements of the standards SEW 094, RENAULT 11-04-804 (table 1).

Table 1 Type of product Standard Mark Yield strength σ _0.2 (R _el ), N / mm ² Tensile Strength (R _m ), N / mm ² Elongation A ₈₀ % r90 n90 The increase in yield strength due to heat treatment of BH ₂ , N / mm ² No less He less He less No less Cold
tany SEW 094 ZStE 180 BH 180-240 300-380 32 - - 40 ZStE 220 BH 220-280 320-400 thirty - - 40 ZStE 260 BH 260-320 360-440 28 - - 40 ZStE 300 BH 300-360 400-480 26 - - 40 Cold
Tany and galvanized
ny RENAULT 11-04-804 E 220 BH 220-260 340-420 32 1.7 0.19 40 E 260 BH 260-310 370-440 32 1,5 0.17 40

A known method for the production of sheet steel, including casting slabs of steel containing, wt.%:

Carbon 0.002-0.007 Silicon 0.005-0.050 Manganese 0.08-0.16 Aluminum 0,0-0,05 Titanium 0.05-0.12 Phosphorus no more than 0.015 Sulfur 0.003-0.010 Chromium no more than 0,04 Nickel no more than 0,04 Copper no more than 0,04 Nitrogen no more than 0,006 Iron rest,

heating slabs to 1150-1240 ° C, hot rolling with a temperature of rolling end not lower than 870 ° C, water cooling to 550-730 ° C, winding into coils, cold rolling with a total compression of at least 70%, annealing at 700-750 ° With exposure at this temperature for 11-34 hours, training with compression 0.4-1.2% [RF Patent No. 2197542 IPC C21D 8/04, C21D 9/48, 06/28/2001].

The disadvantage of this method of production is low strength, high cost of steel due to the presence of expensive titanium, low strength, the absence of VN effect, as well as increased sorting of metal defects "captivity" and "non-metallic inclusions."

A known method for the production of hot-dip galvanized metal of the highest drawing categories, including hot rolling with a winding temperature of 500 ± 30 ° C, cold rolling with a total compression of not more than 70%, annealing in a bell furnace in a protective atmosphere with single-stage heating at a temperature of 680-710 ° C and thermal metal processing in the line of the continuous hot dip galvanizing unit at temperatures of 490-510 ° C with a heating rate of 10.8-11.4 ° C / s in the first stage, at temperatures of 520-560 ° C with a heating rate of 0.4-0.8 ° C / s in the second stage and exposure at these temperatures 85 rounds with cooling, overaging and applying very thin zinc coating [RF patent №2128719 IPC C21D 9/48, C21D 8/04, 2/40 S23S, of 04.10.1999].

The disadvantage of this method of production is low strength, the lack of VN effect.

Closest to the technical nature of the present invention is a method of producing a steel sheet with high hardenability during drying, characterized by the content in steel of the following components, wt.%:

Carbon 0.0001-0.2 Silicon ≤2.0 Manganese ≤3.0 Phosphorus ≤0.15 Sulfur ≤0.015 Chromium ≤2.5 Nickel + Copper 0.001-1.0 Aluminum ≤0.1 Nitrogen 0.001-0.1 Titanium 0.0001-0.1 Niobium 0.001-0.03 Iron and inevitable impurities rest,

including continuous casting of slabs, heating of slabs at a temperature of 1200 ° C or higher, hot rolling, which is completed at a temperature not lower than 600 ° C, cooling with water, winding into coils at a temperature of 550 ° C or lower, cold rolling with a total compression of 95% or below, recrystallization annealing, and if it is carried out in continuous furnaces, at a temperature of 600-1100 ° C and then applying a zinc coating, training the strips with compression of 3% or less. [EP No. 1306456 A1, C21D 9/46, 05/02/2003].

A disadvantage of the known steel and method of production is low strength.

The technical result of the invention is to optimize the composition of steel and the production technology of steel strips, cold-rolled and hot-dip galvanized, with increased strength characteristics, with good formability, with hardening when drying the paint coating in finished parts (with VN effect), as well as reducing production costs for by eliminating the use of expensive alloying elements such as titanium, niobium.

The technical result is achieved in that the method for the production of steel strip includes the smelting of steel containing components in the following ratio, wt.%:

Carbon 0.001-0.015 Silicon 0.001-0.50 Manganese 0.05-1.0 Phosphorus no more than 0.12 Sulfur no more than 0,025 Chromium no more than 0.30 Nickel no more than 0.30 Copper no more than 0,19 Aluminum from more than 0.02 to 0.15 Nitrogen no more than 0,010 Iron and inevitable impurities, including titanium and niobium, rest,

the ratio [Mn] / [S] ≥ 4.8 is satisfied and the sum of titanium and niobium contents is not more than 0.012 wt.%, while the steel may additionally contain 0.0005-0.005% boron and / or 0.0005-0.005 % calcium, continuous casting of slabs. The slabs are heated at a temperature of 1060 to less than 1200 ° C, the hot rolling is completed at a temperature of 800-930 ° C, then the strips are cooled with water and the strips are coiled into coils at a temperature of more than 550 to 730 ° C. After hydrochloric etching, cold rolling is carried out with a total compression of 40-95%. Further, according to the first embodiment, recrystallization annealing is carried out in bell furnaces at a temperature of 600-750 ° C with holding at this temperature for 6-35 hours, then the bands are trained.

According to a second embodiment of the zinc coating method, recrystallization annealing is carried out in continuous furnaces at a temperature of 750-900 ° C and then the bands are trained.

The essence of the invention lies in the fact that the mechanical properties of steel strips depend on the chemical composition of the steel and the modes of deformation-heat treatment at rolling stages.

Carbon is one of the reinforcing elements. When the carbon content is less than 0.001%, the strength of the steel is below acceptable, there is no VN effect. An increase in carbon content of more than 0.015% leads to a decrease in the ductility of steel. Due to the presence of free carbon in the ferrite matrix as a result of aging, hardening is achieved during the drying process of the paintwork (BH effect).

Silicon in steel is used as a deoxidizer and an alloying element. When the silicon content is less than 0.001%, a sufficient deoxidizing effect is not achieved. An increase in silicon content of more than 0.50% leads to an increase in the number of silicate non-metallic inclusions, reduces ductility and increases the embrittlement of steel.

Manganese and phosphorus create solid solution hardening, which ensures the necessary complex of mechanical properties. When the manganese content is less than 0.05%, the strength of the steel is below acceptable. An increase in the manganese content of more than 1.0% and phosphorus of more than 0.12% excessively strengthens the steel, worsens its ductility and formability.

Sulfur is an impurity element and strengthens the ferrite matrix due to the formation of manganese sulfides. An increase in sulfur content of more than 0.025% leads to a decrease in ductility and formability.

Sulfur intensely binds to manganese with the formation of relatively refractory manganese sulfides MnS. MnS inclusions can serve as centers of nucleation of other phases (nitride, carbide). With an increased sulfur content and a low manganese content, sulfur is present in steel in the form of iron sulfide FeS, which forms a low-melting eutectic with iron located at the grain boundaries, which leads to the formation of cracks during rolling. To avoid this, the content of manganese and sulfur is related by the dependence: [Mn] / [S] ≥ 4.8.

Chromium, nickel, and copper strengthen the ferrite matrix. The content of chromium, nickel more than 0.30% and copper more than 0.19% excessively strengthens the steel, worsens its ductility.

Aluminum is introduced into steel as a deoxidizing agent, stabilizes steel, prevents its aging. When the aluminum content is less than 0.02%, the ductility of the steel decreases, the steel becomes prone to aging. An increase in aluminum content of more than 0.15% leads to a deterioration in the complex of mechanical properties.

Nitrogen is a nitride forming element that strengthens steel. An increase in nitrogen content of more than 0.010% leads to a decrease in ductility and contributes to the aging of steel.

Boron prevents the excessive growth of ferritic grains, homogenizes the microstructure. The presence of boron prevents the formation of phosphorus segregation along grain boundaries. When the boron content is less than 0.0005%, the strength properties of rolled products are below the permissible level. An increase in boron content of more than 0.005% leads to a decrease in ductility of rolled products.

Calcium in the range from 0.0005 to 0.005% is used to increase the efficiency of the deoxidizing action of aluminum and to improve the shape of aluminum oxide inclusions and their globularization.

Heating slabs to a temperature above 1200 ° C leads to the dissolution of large particles (carbonitrides, carbides, nitrides) in solid solution, which are subsequently released in the form of fine particles, which leads to excessive hardening of steel and deterioration of stampability. A decrease in the heating temperature below 1060 ° C leads to a deterioration in plastic properties. Heating of slabs in the temperature range of 1060-1200 ° C ensures minimal oxidation of steel during heating and rolling.

When the temperature of the end of rolling is less than 800 ° C and the temperature of the winding is less than 550 ° C, the steel acquires a fine-grained microstructure and a texture unfavorable for cold stamping, and the ductility of rolled products deteriorates. At a temperature of the end of rolling above 930 ° C and a winding temperature above 730 ° C, an uneven microstructure with large grains on the surface of the strip is formed in the steel, which negatively affects the formability.

Cold rolling with a total compression of 40-95% provides a stamping-friendly texture. With exorbitant values, the ductility and flatness of the strip deteriorate.

Recrystallization annealing at 600-750 ° С with holding at this temperature for 6-35 hours - in bell-type furnaces or 750-900 ° С - in continuous continuous furnaces is optimal for primary recrystallization in steel and to some extent collective recrystallization. If the annealing temperature is insufficient or excessive, an unrecrystallized or inhomogeneous microstructure with heterogeneous grains is formed, which negatively affects the mechanical properties and formability of the rolled product. Exposure to bell furnaces for less than 6 hours leads to the formation of an unrecrystallized microstructure, and exposure to more than 35 hours leads to unjustified energy consumption.

Tests to determine the hardening during drying of the BH ₂ paint coating (BH effect) were determined according to SEW 094 in the following sequence:

1. Samples were stretched to a strain of 2% and R _{p2.0 was} determined — stress at a strain of 2%.

2. Samples were annealed at a temperature of 170 ° C for 20 minutes.

3. Samples were tensile to break and R _{eL was} determined.

4. Expected BH ₂ = R _eL -R _p2,0

Method implementation examples

6 oxygen steel melts were smelted in an oxygen converter, the chemical composition of which is given in table 1.

The smelted steel was cast in slabs on a continuous casting machine. The slabs were heated in a walking beam heating furnace and rolled on a 2000 continuous broadband mill into strips 2.0–5.5 mm thick. Hot rolled strips on the discharge roller table were cooled with water and wound into rolls. Chilled rolls were subjected to hydrochloric acid etching in a continuous etching unit. Then, the etched strips were rolled on a 5-stand mill to a thickness of 0.6-2.0 mm. The strips were annealed in bell-type furnaces with a hydrogen protective atmosphere or continuous continuous furnaces with coating. Annealed strip trained. The deformation-thermal regimes at the rolling stages are shown in table 3.

The mechanical properties of the experimental swimming trunks are shown in table 4.

From tables 2-4 it is seen that in the case of the implementation of the proposed method (compositions No. 2-5), mechanical properties are achieved with an increased level of strength, high ductility and HH effect of more than 40 N / mm ² (yield strength corresponds to 210-330 N / mm ² , relative elongation - 29-40%, hardening during drying of BH ₂ - 40-47 N / mm ² ). With transcendent values of the declared parameters, properties are achieved either with low strength (yield strength 150 N / mm ² - composition No. 1) or with low ductility and low hardening when drying the paint coating (elongation 25%, VN ₂ = 34 N / mm ² - composition No. 6). When using the prototype method (composition No. 7), the properties of rolled products with low strength (yield strength 160 N / mm ² ) are achieved.

Steel strips produced by the proposed technology have good stampability and additional hardening during drying of the paintwork of at least 40 N / mm ² .

table 2 The chemical composition of the experimental swimming trunks (wt.%) No.
composition FROM Si Mn P S Cr Ni Cu Al N AT Sa Mn / s Ti + Nb Iron and the inevitable
impurities one 0,0005 0,0005 0.04 0.01 0.015 0.01 0.02 0.02 0.04 0.001 - 0.006 2.7 0.013 Ost. 2 0.015 0.001 0.15 0.02 0.010 0,03 0.06 0.09 0.05 0.002 0,0005 0.005 5,0 0.006 Ost. 3 0.010 0.15 0.10 0.05 0,021 0.08 0.10 0.10 0.04 0.004 0.005 - 4.8 0.007 Ost. four 0.001 0,03 0.50 0.09 0,025 0.15 0.18 0.18 0.10 0.005 - 0,0005 twenty 0.012 Ost. 5 0.005 0.50 1.00 0.12 0,030 0.30 0.30 0.19 0.15 0.010 - - 33 0.012 Ost. 6 0.016 0.55 1.05 0.13 0,031 0.35 0.35 0.20 0.16 0.011 0.006 - 34 0.014 Ost. 7 0.0034 0.01 0.10 0.009 0.006 0.39 0.01 0.02 0,044 0.0022 - - Not Reg. Not Reg. Ost. Prototype

Table 3 Deformation-thermal regimes at rolling stages Composition number Strip type Slab heating temperature, ° С The temperature of the end of rolling, T _CP , ° C Winding temperature, T _cm , ° C The total compression during cold rolling,% Recrystallization Annealing Temperature and Exposure one cold-smoked 1050 790 495 39 595 ° C - 5 hours 2 cold-smoked 1060 800 500 40 600 ° C - 6 hours 3 cold-smoked 1100 850 600 65 750 ° C - 35 hours four ots 1150 900 680 80 750 ° C 5 ots 1190 930 730 95 900 ° C 6 ots 1300 935 735 96 905 ° C cold-smoked 755 ° C - 36 hours 7 cold-smoked 1000-1160 922 730 95 and less 650-800 ° C Prototype ots 700-900 ° C

Table 4 Mechanical properties of experimental swimming trunks Composition number Yield strength σ _0.2 (R _el ), H / mm ² Tensile Strength (R _m ), N / mm ² Elongation A ₈₀ % Anisotropy coefficient
r ₉₀ Strain hardening coefficient
n ₉₀ The increase in yield strength due to heat treatment, BH ₂ , N / mm ² one 150 (low value) 300 45 2.7 0.25 0 2 210 340 40 2.1 0.21 47 3 250 370 35 2.0 0.20 45 four 270 375 34 1.8 0.19 40 5 330 420 29th 1.7 0.18 45 6 340 430 25 (low value) 1.4 0.15 34 (low value) 7 160 (low value) 292 52 there is no data there is no data 42 Prototype

Claims (4)

1. A method of manufacturing a steel strip, including steelmaking, continuous casting of slabs, heating them, hot rolling, water cooling, winding strips into rolls, pickling, cold rolling, recrystallization annealing in bell furnaces and tempering, characterized in that the steel is smelted containing the following components, wt.%:
carbon 0.001-0.015 silicon 0.001-0.50 manganese 0.05-1.0 phosphorus no more than 0.12 sulfur no more than 0,025 chromium no more than 0.30 nickel no more than 0.30 copper no more than 0,19 aluminum from more than 0.02 to 0.15 nitrogen no more than 0,010 iron and inevitable impurities, including titanium and niobium rest,
the ratio [Mn] / [S] ≥ 4.8 is satisfied and the sum of the contents of titanium and niobium is not more than 0.012 wt.%, the slabs are heated at a temperature of 1060 to less than 1200 ° C, hot rolling is completed at a temperature of 800-930 ° С, strip winding is carried out at a temperature from more than 550 to 730 ° С, cold rolling is carried out with a total compression of 40-95%, and recrystallization annealing is carried out at a temperature of 600-750 ° С with holding at this temperature for 6-35 hours. 2. The method according to claim 1, characterized in that the steel further comprises 0.0005-0.005% boron and / or 0.0005-0.005% calcium. 3. A method of manufacturing a steel strip, including steelmaking, continuous casting of slabs, heating them, hot rolling, water cooling, winding strips in rolls, pickling, cold rolling, recrystallization annealing in continuous furnaces, applying a zinc coating and training, characterized in that steel is melted containing the following components, wt.%:
carbon 0.001-0.015 silicon 0.001-0.50 manganese 0.05-1.0 phosphorus no more than 0.12 sulfur no more than 0,025 chromium no more than 0.30 nickel no more than 0.30 copper no more than 0,19 aluminum from more than 0.02 to 0.15 nitrogen no more than 0,010 iron and inevitable impurities, including titanium and niobium rest,
the ratio [Mn] / [S] ≥ 4.8 is satisfied and the sum of the contents of titanium and niobium is not more than 0.012 wt.%, the slabs are heated at a temperature of 1060 to less than 1200 ° C, hot rolling is completed at a temperature of 800-930 ° C, the strip winding is carried out at temperatures from more than 550 to 730 ° C, cold rolling is carried out with a total compression of 40-95%, and recrystallization annealing is carried out at a temperature of 750-900 ° C. 4. The method according to claim 3, characterized in that the steel further comprises 0.0005-0.005% boron and / or 0.0005-0.005% calcium.