RU2237101C1 - Steel for deep drawing and article made from the same (variants) - Google Patents

Abstract

FIELD: metallurgy, in particular steel composition for cold sheet-metal pressworking.

SUBSTANCE: invention relates to steel for production of shape irregular article by cold sheet-metal pressworking, preferably of automobile parts. Claimed steel contains (mass.%) carbon 0.002-0.010; silicium 0.002-0.30; manganese 0.07-0.30; phosphorus 0.04-0.12; sulfur 0.005-0.012; chromium 0.007-0.040; nickel 0.007-0.040; copper 0.2-0.6; aluminum 0.02-0.06; nitrogen 0.002-0.007; optionally boron 0.0007-0,0050; and balance iron and inevitable impurities: with the proviso that total content of manganese, phosphorus and copper is not less than 0.55 %. Three variants of steel composition are described. Claimed steel exhibits improved corrosion resistance and technological ductility when treatment is carried out with cold-working die by sheet-metal pressworking method. Said composition provides preservation of strength characteristic of the steel and end product.

EFFECT: steel with excellent corrosion resistance and technological ductility.

4 cl, 1 tbl, 5 ex

Description

The invention relates to ferrous metallurgy, and in particular to the chemical composition 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 and corrosion resistance, in particular, under atmospheric conditions. Recently, requirements to the level of strength characteristics have been increasing. This is dictated by the need to improve safety, as well as toughening environmental requirements to reduce harmful substances in the exhaust of a car, requiring a reduction in its mass.

Steel is known and the product made from it is a cold-rolled structural sheet for stamping parts of automobiles, tractors and agricultural machinery, containing carbon, manganese, copper, aluminum, boron, phosphorus, nitrogen and iron in the following ratio of components, wt.%:

Carbon 0.02-0.07

Manganese 0.15-0.30

Copper 0.02-0.30

Aluminum 0.03-0.07

Boron 0.0005-0.005

Phosphorus 0.04-0.10

Nitrogen 0.001-0.007

Iron Else

(AS USSR No. 1741459, IPC С 22 С 38/16, publ. 10.11.1995).

The cold-rolled sheet of such steel has high formability and corrosion resistance in some environments, in particular in an aqueous solution of sulfuric acid. However, in the atmosphere and in aqueous media containing chlorine ions, the corrosion resistance of a cold-rolled sheet of such steel is insufficient. In addition, the increased aluminum content can also have a negative effect on corrosion resistance due to the presence of a significant amount of non-metallic inclusions in the structure.

Known steel and products made from it, for example, in the form of hot-rolled strips 2-4 mm thick, cold-rolled strips 0.5-1.5 mm thick, designed for stamping parts of complex shape. Steel contains the following components, wt.%:

Carbon 0.01-0.1

Manganese 0.1-0.9

Silicon 0.01-0.50

Copper 0.01-0.10

Aluminum 0.02-0.07

Boron 0.0001-0.0050

Calcium 0.0005-0.0040

Titanium 0.001-0.030

Nitrogen 0.002-0.010

Phosphorus 0.005-0.12

Vanadium 0.001-0.08

Niobium 0.001-0.05

Chrome 0.01-0.10

Nickel 0.01-0.10

Iron and inevitable impurities

(RF patent 2190685, IPC C 22 C 38/54, publ. 10.10.2002). The specified invention is aimed at providing the possibility of manufacturing parts from sheet metal of complex shape with the necessary strength, including thin-walled and low weight. However, products made of such steel have a low corrosion resistance. In particular, in the presence of calcium in steel, the presence of non-metallic inclusions is possible, which adversely affects the corrosion resistance in a number of media, in particular in the atmosphere.

Steel is known and products made from it, for example, in the form of hot-rolled strips about 3 mm thick, cold-rolled strips 0.7-0.8 mm thick, intended for the manufacture of complex products by sheet metal stamping, mainly automobile parts, including those with protective coatings. Steel contains the following components, 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 inevitable impurities

(RF patent 2190684, IPC C 22 C 38/50, publ. 10.10.2002). The specified invention allows to increase the values of the coefficient of normal plastic anisotropy and the hardening index in the absence of aging of the steel after recrystallization annealing. However, both corrosion resistance and strength characteristics of such steel are low.

The closest analogue is high-strength steel for deep drawing and products made from it, including cold-rolled sheets with a thickness of 0.8-0.9 mm, designed for the manufacture of automobile parts with a high level of loading. Steel contains the following components, wt.%:

Carbon 0.002-0.007

Silicon 0.005-0.05

Manganese 0.5-1.7

Sulfur 0.005-0.015

Phosphorus 0.005-0.015 or 0.05-0.15

Aluminum 0.005-0.07

Nickel 0.005-0.3

Copper 0.005-0.3

Titanium 0.02-0.15

Nitrogen 0.001-0.007

Chrome 0.005-0.3

Iron Else

The steel may additionally contain 0.01-0.15 wt.% Niobium and / or 0.0005-0.005 wt.% Boron (RF Patent 2061782, IPC C 22 C 38/50, publ. 06/10/1996 - prototype of inventions ″ steel ″ And ″ product ″).

The specified invention (prototype) is aimed at stabilizing the strength characteristics with a texture favorable for stamping, increasing plastic and corrosion properties, increasing the yield, hardening when drying the paintwork in finished parts, increasing the uniformity of the distribution of mechanical properties along the length of the roll, reducing the temperature of the brittle transition and ensuring recycling waste. However, products from the specified steel have low corrosion resistance in the atmosphere and in aqueous chlorine-containing environments. In addition, when the carbon and titanium contents in the steel are closer to the upper limits indicated in the formula, a reduction in the formability characteristics is possible.

The problem to which the invention is directed is to optimize the chemical composition of steel while providing a technical result in the form of increased corrosion resistance and technological plasticity of steel during cold deformation in sheet metal stamping, characterized by high plastic deformation, while maintaining the strength characteristics of steel and the finished product.

The technical result is achieved by the fact that in the known steel for deep drawing containing carbon, silicon, manganese, sulfur, phosphorus, chromium, nickel, copper, aluminum, nitrogen, iron and inevitable impurities, according to the invention, the content of components in the ratio, wt.% Is assigned :

Carbon 0.002-0.010

Silicon 0.002-0.030

Manganese 0.07-0.30

Phosphorus 0.04-0.12

Sulfur 0.005-0.012

Chrome 0.007-0.040

Nickel 0.007-0.040

Copper 0.2-0.6

Aluminum 0.02-0.06

Nitrogen 0.002-0.007

Iron and inevitable impurities Else,

the total content of manganese, phosphorus and copper should be at least 0.55 wt.%, the steel additionally contains 0.0007-0.0050 wt.% boron, as well as the fact that the product is made of the specified steel. Alternatively, the technical result is also achieved by the fact that in the known steel for deep drawing containing carbon, silicon, manganese, sulfur, phosphorus, chromium, nickel, copper, aluminum, titanium, nitrogen, iron and inevitable impurities, according to the invention, the content of components in the ratio, wt.%:

Carbon 0.002-0.010

Silicon 0.002-0.030

Manganese 0.07-0.30

Phosphorus 0.04-0.12

Sulfur 0.005-0.012

Chrome 0.007-0.040

Nickel 0.007-0.040

Copper 0.2-0.6

Aluminum 0.02-0.06

Titanium 0.02-0.12

Nitrogen 0.002-0.007

Iron and inevitable impurities Else,

the total content of manganese, phosphorus and copper should be at least 0.55 wt.%, while the ratio of the contents of titanium, nitrogen, sulfur and carbon must satisfy the condition:

(Ti _total. -3.43N-1.5S) × (C) ≤0.00015, (1)

where Ti, N, S and C are the contents of titanium, nitrogen, sulfur and carbon, respectively, wt.%, steel additionally contains 0.0007-0.0050 wt.% boron, and also the fact that the product is made of the specified steel.

Alternatively, the technical result is also achieved by the fact that in the known steel for deep drawing containing carbon, silicon, manganese, sulfur, phosphorus, chromium, nickel, copper, aluminum, titanium, niobium, nitrogen, iron and inevitable impurities, according to the invention appoint the content of components in the ratio, wt.%:

Carbon 0.002-0.010

Snake 0.002-0.030

Manganese 0.07-0.30

Phosphorus 0.04-0.12

Sulfur 0.005-0.012

Chrome 0.007-0.040

Nickel 0.007-0.040

Copper 0.2-0.6

Aluminum 0.02-0.06

Titanium 0.02-0.12

Niobium 0.001-0.060

Nitrogen 0.002-0.007

Iron and inevitable impurities Else,

the total content of manganese, phosphorus and copper should be at least 0.55 wt.%, while the ratio of the content of titanium, nitrogen, sulfur and carbon must satisfy the condition

(Ti _total. -3.43N-1.5S) × (C) ≤0.00015, (1)

where Ti, N, S and C are the contents of titanium, nitrogen, sulfur and carbon, respectively, wt.%, steel additionally contains 0.0007-0.0050 wt.% boron, and also the fact that the product is made of the specified steel.

The essence of the invention lies in the fact that to ensure high corrosion resistance requires the joint alloying of steel with copper and phosphorus while limiting the content of silicon and manganese in it. At the same time, the required strength characteristics of steel are provided by solid-solution hardening due to the presence of substitution elements in the solution - manganese, copper and phosphorus. For this purpose, the minimum value of their total content is limited to at least 0.55%. At the same time, phosphorus favorably affects plastic anisotropy, thereby improving the formability of steel. To ensure high formability of steels with increased strength and corrosion resistance, it is advisable to microalloy steel with titanium or titanium together with niobium in such an amount as to ensure removal of interstitial impurities — carbon and nitrogen — from the solid solution, which is implemented in the second and third embodiments of the invention. The formulas currently used to calculate the required content of microalloying elements take into account the need to bind carbon, nitrogen, and sulfur to carbides, nitrides, and carbosulfides. At the same time, at a sufficiently high content of carbon and titanium in steel, the temperature of the precipitation of particles of titanium carbide increases (above 900 ° C) and their precipitation during hot rolling becomes possible, which negatively affects the characteristics of the grain structure and texture of the steel, impairing the formability. A related need is to limit the content of titanium remaining in the solid solution after the binding of nitrogen and sulfur, and the carbon content in accordance with expression (1). The negative point that arises when phosphorus is introduced into steel may be embrittlement of the metal due to the tendency of phosphorus atoms to segregate at grain boundaries. Additional microalloying with boron, whose atoms prevent the migration of phosphorus to the grain boundaries, ensures the preservation of high ductility of steel.

The limitation of the lower limit of carbon content is determined by the fact that the presence of carbon in the amount of at least 0.002 wt.% Is necessary for the implementation of the BH-effect (Bake Hardening Effect), which allows you to combine a low yield strength (which is important for stampability) with the ability of steel to a significant hardening during the drying process of the paintwork applied to the stamped product.

The limitation of the lower limit of nitrogen 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.

An 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 a deterioration in stampability (with a large amount of carbon and titanium), and also increases the cost of metal products.

The upper limits of the content of silicon and manganese are associated with a negative effect of these elements on the corrosion resistance of steel. 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 lower limits of the content of copper and phosphorus, respectively, 0.2 and 0.04% are standardized to guarantee increased corrosion resistance.

The limitation of the upper limit of the phosphorus content of 0.12% is due to the fact that at its increased content in steel embrittlement of grain boundaries can occur.

The limitation of the copper content to 0.6% is necessary to prevent the appearance of cracks during hot rolling, as well as to rationally limit the strength of steel before stamping.

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 due to its negative effect on corrosion resistance.

A decrease in the content of chromium and nickel below 0.007% leads to a decrease in the corrosion resistance of steel, and an increase in their content above 0.04% can adversely affect stampability.

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 leads to a rise in price of steel without a positive effect on the properties.

The minimum boron content is determined by the advisability of preventing embrittlement of steel in the presence of phosphorus. An increase in boron content above the upper limit can lead to a decrease in the formability of steel.

Examples of specific embodiments of the invention

Five melts of ultra-low-carbon steels were smelted in a 300-ton converter of Severstal OJSC and cast in a continuous casting unit into slabs with a section of 250 × 1290 mm. The slabs were rolled hot in a mill of ″ 2000 ″ into strips 3.0 mm thick. The temperature of the end of rolling was 900 ° C. Strips after chilling were wound into rolls at a temperature of 700 ° C. After etching and cold rolling into strips 0.9 mm thick, strips in metal rolls were heat treated in bell-type furnaces with a hydrogen protective atmosphere at a temperature of 680 ° C. After training with a compression ratio of 1.0%, complex mechanical tests of both cold rolled and galvanized metal were carried out, as well as corrosion tests of cold rolled metal.

Smelting 1 - steel containing 0.01% carbon, 0.015% silicon, 0.20% manganese, 0.05% phosphorus, 0.008% sulfur, 0.011% chromium, 0.020% nickel, 0.4% copper, 0.04% aluminum, 0.004% nitrogen, iron and inevitable impurities, while the total content of manganese, phosphorus and copper was 0.65%, the smelting corresponds to claims 1 and 3 of the claims.

Smelting 2 - steel containing 0.004% carbon, 0.012% silicon, 0.25% manganese, 0.040% phosphorus, 0.009% sulfur, 0.012% chromium, 0.012% nickel, 0.4% copper, 0.02% aluminum, 0, 05% titanium, 0.003% nitrogen, iron and inevitable impurities, while the total content of manganese, phosphorus and copper was 0.69%, and the product on the left side of expression (1) was 0.00010%, melting corresponds to items 4 and 6 claims

Smelting 3 - steel containing 0.005% carbon, 0.012% silicon, 0.18% manganese, 0.045% phosphorus, 0.007% sulfur, 0.015% chromium, 0.011% nickel, 0.55% copper, 0.06% aluminum, 0.025% titanium, 0.05% niobium, 0.004% nitrogen, iron and inevitable impurities, while the total content of manganese, phosphorus and copper was 0.775, and the product on the left side of expression (1) was 0.000039, melting corresponds to items 7 and 9 claims

Melting 4 - steel containing 0.06% carbon, 0.011% silicon, 0.25% manganese, 0.045% phosphorus, 0.007% sulfur, 0.010% chromium, 0.015% nickel, 0.3% copper, 0.04% aluminum, 0.004% nitrogen, 0.001% boron, iron and inevitable impurities, while the total content of manganese, phosphorus and copper was 0.595%, smelting corresponds to claims 2 and 3 of the claims;

Melting 5 - steel containing 0.005% carbon, 0.012% silicon, 0.56% manganese, 0.05% phosphorus, 0.008% sulfur, 0.03% chromium, 0.04% nickel, 0.09% copper, 0, 05% aluminum, 0.09% titanium, 0.004% nitrogen, iron and inevitable impurities, while the total content of manganese, phosphorus and copper was 0.7, and the product on the left side of expression (1) was 0.00032, melting does not correspond none of the variants of the invention due to the higher manganese content, low copper content and the failure of the conditions (1).

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, an analysis of the machine tensile diagram was used for control).

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 (base 26 mm); 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 tested in tension, determining the magnitude of the BH effect, as the difference between the yield strength σ _t (BH) and σ ₂ .

The results of mechanical and corrosion tests are given in the table. The main mechanical characteristics determined by the normative and technical documentation (NTD) for steels of this class were determined. These include yield strength σ _t , tensile strength σ _c , the ratio of yield strength to ultimate strength, elongation δ ₄ , normal plastic anisotropy coefficient r and strain hardening coefficient n. The criterion for ensuring the required strength was considered to be to obtain a yield strength of cold rolled steel of at least 220 N / mm ² or to obtain a yield strength of not less than 200 N / mm ² while providing a BH effect of at least 30 N / mm ² . The criterion of high technological ductility and stampability was considered to be ensuring the value of the coefficient of normal plastic anisotropy r not less than 1.8, the values of the coefficient of strain hardening n not less than 0.18, the ratio of yield strength to tensile strength not more than 0.65 and the relative elongation not less than 36% .

As a method of corrosion testing of cold-rolled steel samples, we used a method of alternating immersion of steel sheet samples in a solution of 3.5% NaCl with 10 minutes in it and subsequent air release (50 minutes), in accordance with ASTM G 44-75 (Method 1). Corrosion resistance was evaluated by weight gain (gain) per unit surface area of the sample for 30 test cycles. If the value of the mass gain was not more than 5 g / m ² , then the corrosion resistance was considered satisfactory. In addition, the following accelerated tests were used to assess corrosion resistance: immersion in a 5% solution of H ₂ SO ₄ (method 2), holding in a special chamber with air at a temperature of 40 ° C and humidity 98% (method 3), tests in a special laboratory setup, the working space of which is filled with fog, formed from the introduction of an aqueous solution of NaCl into it (method 4). When using methods 2 and 3, corrosion resistance was evaluated by weight loss. Using method 4, samples were tested after applying a paint coating and applying an incision on the coating. Corrosion resistance was evaluated by the width of the zone of corrosion propagation from the place of incision. Test results for the investigated methods are also presented in the table.

For melts 2, 3, 5, the yield strength values were obtained not lower than 220 N / mm ² . The yield strength of 200-210 N / mm ^{2 was} obtained for melts 1 and 4, however, due to the tendency of these melts to the HV effect at the level of 40-45 N / mm ² in the finished product from this steel, the yield strength will be 240-255 N / mm ² . That is, all studied melts have increased strength. For casting steels 1-4, high stampability indicators were obtained: low values of the ratio of yield strength to tensile strength, high values of r and n and elongation. A particularly high complex of mechanical characteristics was obtained for steel 2-4. For melting 5, unsatisfactory values of n, elongation, and the ratio of yield strength to tensile strength were obtained.

Cold rolled steel from smelting steel 5 has a low corrosion resistance due to its high manganese content and low copper content. For the rest of the heats, the corrosion resistance is satisfactory.

Thus, cold-rolled steel from steel casting 1-4, corresponding to claims 1 to 9 of the claims, has high strength and stampability with satisfactory corrosion resistance. Rolled steel 5 has unsatisfactory stampability and low corrosion resistance.

That is, the use of this proposal significantly increases the corrosion resistance and technological ductility of steel for deep drawing and products made from it, while maintaining the strength characteristics.

Claims (9)

1. Steel for deep drawing containing carbon, silicon, manganese, sulfur, phosphorus, chromium, nickel, copper, aluminum, nitrogen, iron and inevitable impurities, characterized in that it contains components in the ratio, wt.%: Carbon 0.002-0.010 Silicon 0.002-0.030 Manganese 0.07-0.30 Phosphorus 0.04-0.12 Sulfur 0.005-0.012 Chrome 0.007-0.040 Nickel 0.007-0.040 Copper 0.2-0.6 Aluminum 0.02-0.06 Nitrogen 0.002-0.007 Iron and inevitable impurities while the total content of manganese, phosphorus and copper should be at least 0.55 wt.%. 2. Steel according to claim 1, characterized in that it further comprises 0.0007-0.0050 wt.% Boron. 3. The product is made of steel for deep drawing, characterized in that it is made of steel according to claim 1 or 2. 4. Steel for deep drawing containing carbon, silicon, manganese, sulfur, phosphorus, chromium, nickel, copper, aluminum, titanium, nitrogen, iron and inevitable impurities, characterized in that it contains components in the ratio, wt.%: Carbon 0.002-0.010 Silicon 0.002-0.030 Manganese 0.07-0.30 Phosphorus 0.04-0.12 Sulfur 0.005-0.012 Chrome 0.007-0.040 Nickel 0.007-0.040 Copper 0.2-0.6 Aluminum 0.02-0.06 Titanium 0.02-0.12 Nitrogen 0.002-0.007 Iron and inevitable impurities the total content of manganese, phosphorus and copper should be at least 0.55 wt.%, and the ratio of the content of titanium, nitrogen, sulfur and carbon must satisfy the condition (Ti _total. -3.43N-1.5S) × C≤0.00015, where Ti, N, S and C are the contents of titanium, nitrogen, sulfur and carbon, respectively, wt.%. 5. Steel according to claim 4, characterized in that it further comprises 0.0007-0.0050 wt.% Boron. 6. The product is made of steel for deep drawing, characterized in that it is made of steel according to claim 4 or 5. 7. Steel for deep drawing containing carbon, silicon, manganese, sulfur, phosphorus, chromium, nickel, copper, aluminum, titanium, niobium, nitrogen, iron and inevitable impurities, characterized in that it contains components in the ratio, wt.% : Carbon 0.002-0.010 Silicon 0.002-0.030 Manganese 0.07-0.30 Phosphorus 0.04-0.12 Sulfur 0.005-0.012 Chrome 0.007-0.040 Nickel 0.007-0.040 Copper 0.2-0.6 Aluminum 0.02-0.06 Titanium 0.02-0.12 Niobium 0.001-0.060 Nitrogen 0.002-0.007 Iron and inevitable impurities the total content of manganese, phosphorus and copper should be at least 0.55 wt.%, and the ratio of the content of titanium, nitrogen, sulfur and carbon must satisfy the condition (Ti _total. -3.43N-1.5S) × C≤0.00015, where Ti, N, S and C are the contents of titanium, nitrogen, sulfur and carbon, respectively, wt.%. 8. Steel according to claim 7, characterized in that it additionally contains 0.0007-0.0050 wt.% Boron. 9. A product made of steel for deep drawing, characterized in that it is made of steel according to claim 7 or 8.

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