RU2709560C2 - High-strength manganese steel containing aluminium, method of producing sheet steel product from said steel and sheet steel product obtained according to said method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to high-strength aluminum-containing manganese steel used in automotive industry, shipbuilding, aerospace industry, and so forth. Steel has the following chemical composition, wt. %: C: 0.01 to 0.3, Mn: 4 to 10, Al: 1 to 4, Si: 0.01 to 1, Cr: 0.1 to 4, Mo: 0.02 to 1, P: less than 0.1, S: less than 0.1, N: less than 0.3, optionally, at least one of: V: from 0.01 to 1, Nb: from 0.01 to 1, Ti: from 0.01 to 1, Sn: from 0 to 0.5, Cu: from 0.005 to 3, W: from 0.03 to 3, Co: from 0.05 to 3, Zr: from 0.03 to 0.5, Ca: from 0.0005 to 0.1, iron and unavoidable impurities - the rest. Steel has tensile strength Rm from 800 MPa to 1700 MPa, elongation at breaking A50 from 6 % to 45 %, preferably from 8 % to 45 %, and microstructure containing from 5 to 65 % residual austenite.

EFFECT: steel has high deformation properties, high resistance to delayed formation of cracks and hydrogen embrittlement.

14 cl

Description

The invention relates to high-strength manganese steel with aluminum content, a method for producing a flat steel product from this steel, and to a flat steel product to be produced in accordance with this method.

European patent application EP 2383353 A2 discloses high-strength manganese-containing steel, a flat steel product made from this steel, and a method for manufacturing this flat steel product. Steel consists of elements (with a content in weight percent relative to the steel melt): C: up to 0.5; Mn: 4 to 12.0; Si: up to 1.0; Al: up to 3.0; Cr: from 0.1 to 4.0; Cu: up to 4.0; Ni: up to 2.0; N: up to 0.05; P: up to 0.05; S: up to 0.01; the remainder is iron and inevitable impurities. As an option, one or more elements of a group of V, Nb, Ti are presented, while the total content of these elements is a maximum of 0.5. It is argued that this steel is different in that it can be produced at lower cost than steel with a high manganese content, and at the same time it has great values in tensile fracture and, in connection with this, significantly better deformation ability. The method for producing a flat steel product from high strength steel with a manganese content, as described above, comprises the following working steps:

- smelting of the above steel melt;

- the production of the starting material for subsequent hot rolling, wherein the steel melt is poured into a stream from which at least one slab or thin slab is separated as a starting product for hot rolling, or into a cast strip that is supplied to the hot rolling process as a starting product

- heat treatment of the starting product to bring the starting product to a hot rolling temperature from 1150 ° to 1000 ° C;

- hot rolling of the starting product to obtain a hot strip with a maximum thickness of 2.5 mm, while hot rolling stops at a final temperature of hot rolling from 1050 to 800 ° C;

- winding a hot strip with the formation of a coil at a winding temperature of ≤700 ° C.

Based on this, the purpose of the present invention is to provide high-strength manganese steel with aluminum content with good deformation properties and with increased resistance to delayed cracking and hydrogen embrittlement, a method for producing a flat steel product from this steel, and a flat steel product to obtain in accordance with in this way, with a good combination of strength and deformation properties with respect to this steel.

This goal is achieved by high-strength manganese steel with aluminum content with the characteristics according to paragraph 1 of the claims, a method of manufacturing a flat steel product, in particular using the above steel, with the characteristics of paragraph 12 of the claims, and a flat steel product with obtaining in accordance with this the method according to paragraph 14 of the claims. Preferred embodiments of the present invention are described in the dependent claims.

In accordance with the present invention, high-strength manganese steel with aluminum content, having the following chemical composition, wt. %: C: from 0.01 to 0.3; Mn: 4 to 10; Al: 1 to 4; Si: from 0.01 to 1; Cr: from 0.1 to 4; Mo: from 0.02 to 1; P: less than 0.1; S: less than 0.1; N: less than 0.3; in this case, the remainder is iron with the inevitable elements associated with steel, when alloyed as an option with one or more of the following elements, wt. %: V: from 0.01 to 1; Nb: from 0.01 to 1; Ti: from 0.01 to 1; Sn: from 0 to 0.5; Cu: 0.005 to 3; W: from 0.03 to 3; Co: from 0.05 to 3; Zr: from 0.03 to 0.5; Ca: from 0.0005 to 0.1, offers a good combination of strength, tension and deformation characteristics. Moreover, the production of manganese steel in accordance with the present invention, with an average manganese content (steel with an average manganese content) based on alloying elements C, Mn, Cr, Al, Si, and Mo, is relatively inexpensive. Due to the increased Al content, steel has a lower specific gravity in comparison with other manganese steels with alloying with a small amount of Al and with an average manganese content. Manganese steel, in accordance with the present invention, is also characterized by increased resistance to delayed cracking (delayed fracture) and hydrogen embrittlement. This is achieved by the deposition of molybdenum carbide, acting as a hydrogen trap.

The steel in accordance with the present invention has a multiphase microstructure consisting of ferrite and / or martensite and / or bainite and residual austenite, with the effects of TRIP and / or TWIP. The residual austenite content is from 5% to 65%. Residual austenite is partially or completely converted to martensite through the TRIP effect upon application of high mechanical stresses. Thanks to the TRIP effect, elongation at break, in particular, uniform elongation, and tensile strength, are significantly improved.

The use of the term “do” in determining the content range, for example, from 0.01 to 1%, means that the limit values - 0.01 and 1 in this example - are also taken into account.

The steel in accordance with the present invention is particularly suitable for the production of high strength thick plates, hot and cold strips, which may have a metallic or non-metallic coating. Their application is possible, inter alia, in the automotive industry, shipbuilding, the development of technological equipment, infrastructure, in the aerospace industry and in household appliances.

Preferably, the steel has a tensile strength Rm from 800 MPa to 1700 MPa, and elongation at break A50 from 6% to 45%, preferably from 8% to 45%. Test piece A50 was used for elongation at break tests in accordance with DIN 50 125.

Alloying elements are usually added to steel to influence specific properties as needed. An alloying element can thus affect various properties in different steels. Influence and interaction usually depend heavily on the amount, presence of additional alloying elements and the state of the solution in the material. Correlations are volatile and complex.

The effect of the alloying elements in the alloy in accordance with the present invention will be described in more detail below.

The beneficial effects of alloying elements when used in accordance with the present invention will be described below:

Carbon C: necessary for the formation of carbides, stabilizes austenite and increases strength. A high carbon content impairs weldability, thereby impairing tension and toughness, for this reason the maximum content is set to less than 0.3%. To achieve sufficient material strength, a minimum addition of 0.01% is required.

Manganese Mn: stabilizes austenite and increases strength and toughness, allowing the formation of martensite induced by deformation and / or twinning in alloys in accordance with the present invention. A content of less than 4% is insufficient to stabilize austenite and this worsens the tensile properties, while at a content of 10% or more, the austenite stabilizes too much and this leads to tensile properties, in particular, yield strength, deteriorating. In accordance with the present invention, for manganese steel with an average manganese content, a range of 4% to 10% is preferred.

Aluminum Al: an aluminum content of more than 1% increases the strength and tension properties, lowering specific gravity and affects the behavior of the transformation of the alloy in accordance with the present invention. When Al content is more than 4%, the tensile properties deteriorate. A higher Al content also significantly degrades the casting behavior of the continuous casting process. This increases casting costs. At a content of less than 4%, Al delays the precipitation of carbides. Therefore, a maximum content of 4% is set with a minimum content of 1%.

Silicon Si: inhibits carbon diffusion and lowers specific gravity, and increases strength and tensile and toughness properties. In addition, with the addition of Si, an improvement in cold rolling ability is observed. With a content of more than 1%, the material becomes brittle and has a negative effect on the possibility of hot and cold rolling, and coating, for example, by galvanizing. Therefore, a maximum content of 1% is set with a minimum content of 0.01%. Preferably, the maximum content is less than 1%.

Chromium Cr: increases strength and reduces the rate of corrosion, delays the formation of ferrite and perlite and forms carbides. The maximum content is set to less than 4%, since with a higher content, the tensile properties deteriorate. The minimum chromium content is set to 0.1%.

Molybdenum Mo: acts as a carbide forming agent and increases strength, and increases resistance to delayed cracking and hydrogen embrittlement. A Mo content of more than 1% impairs the tensile properties, and therefore a maximum content of 1% and a minimum content of 0.02% are set.

Phosphorus P: P is a trace element of iron ore, with dissolution in the iron lattice, as a substitute atom. Phosphorus increases stiffness and improves the hardenability through mixed crystalline solidification. However, everything possible is done to reduce the phosphorus content to as small a value as possible, because, among other things, its low diffusion rate means that it has a strong tendency to segregate and greatly reduces the level of toughness. The addition of phosphorus to the grain boundaries usually causes cracks at the grain boundaries during hot rolling. In addition, phosphorus increases the temperature of the transition from hard behavior to brittle behavior to 300 ° C. For these reasons, the phosphorus content is limited to less than 0.1%.

Sulfur S: like phosphorus, attached like trace element in iron ore. Sulfur is undesirable in steel because it exhibits a strong tendency toward segregation and greatly increases brittleness, while tensile and toughness deteriorate. Therefore, everything possible is done to achieve the lowest possible sulfur content in the melt (for example, by vacuum treatment). For the above reasons, the sulfur content is limited to less than 0.1%.

Nitrogen N: N is also an element accompanying steel in production. When dissolved, it increases strength and toughness in steels with a manganese content greater than or equal to 4%. Steel with a lower manganese content <4 weight. % in the presence of free hydrogen have a pronounced aging effect. Nitrogen diffuses even at low temperatures in the dislocation and blocks them. Thus, it increases strength, coupled with a rapid loss of toughness. You can bind hydrogen in the form of nitrides, for example, by doping with aluminum, vanadium, niobium or titanium. For these reasons, the nitrogen content is limited to less than 0.3%.

Typically, microalloying elements are added in very small amounts (less than 0.1% per element). Unlike alloying elements, they mainly act due to the formation of precipitation, but can also affect the properties in a dissolved state. Despite the small amounts added, the microalloying elements strongly influence the production conditions and processing properties, and the final properties.

Typically, microalloying elements are vanadium, niobium and titanium. These elements can dissolve in the iron lattice with the formation of carbides, nitrides and carbonitrides with carbon and nitrogen.

Vanadium V and niobium Nb: they contribute to grain refinement, in particular through the formation of carbides, while improving strength, toughness and tensile properties. With a content of more than 1%, there are no additional benefits. For vanadium and niobium, the minimum content is set to greater than or equal to 0.02%, and the maximum content is less than or equal to 1%, as preferred as an option.

Titanium Ti: contributes to the grinding of grains as a carbide-forming agent, while at the same time, strength, toughness and tensile properties are increased while reducing intergranular corrosion. A Ti content of more than 1% impairs the tensile properties, and therefore a maximum content of 1% is set. Preferably, a minimum content of 0.02%.

Tin Sn: Tin increases strength, but, like copper, it accumulates under a layer of scale and at grain boundaries at high temperatures. This leads to the fact that, due to penetration into the grain boundaries, low-melting phases are formed, and, therefore, cracks in the microstructure form, and the solder is brittle, and therefore the maximum tin content is set to less than or equal to 0.5%, at minimum content of 0.005%.

Copper Cu: lowers corrosion rate and increases strength. With a content of more than 3%, production is complicated due to the formation of low-melting phases during casting and hot rolling, and therefore a maximum content of 3% with a minimum content of 0.005% is set. Preferably, a minimum content of 0.5%.

Tungsten W: acts as a carbide forming agent and increases strength and heat resistance. A W content of more than 3% impairs the tensile properties, and therefore a maximum content of 3% is set with a minimum content of 0.03%. Preferably, a minimum content of 0.05%.

Cobalt Co: increases the strength of steel, stabilizes austenite and improves heat resistance. When the content is more than 3%, the tensile properties deteriorate, and therefore, a maximum content of less than or equal to 3% with a minimum content of 0.05% is set. Preferably, a minimum content of 0.08%.

Zirconium Zr: acts as a carbide forming agent and increases strength. A Zr content of more than 0.5% impairs the tensile properties, and therefore a maximum content of 0.5% is set with a minimum content of 0.03%. Preferably, a minimum content of 0.05%.

Calcium Ca: calcium is used to modify non-metallic oxide inclusions, which otherwise could lead to unnecessary destruction of the alloy due to inclusions in the microstructure, which would act as stress concentration points, weakening the metal composite. In addition, calcium improves the uniformity of the alloy in accordance with the present invention. To achieve the corresponding effect, preferably a minimum content of 0.0005%. With a content of more than 0.1% calcium, there will be no additional benefits when modifying inclusions, but productivity will deteriorate, and this must be avoided due to the high vapor pressure of calcium in steel melts. Therefore, the maximum calcium content of 0.1% is set.

In accordance with the present invention, a method of manufacturing a flat steel product, in particular from steel described above, comprises the following steps:

- smelting steel containing, by weight. %: C: from 0.01 to 0.3; Mn: 4 to 10; Al: 1 to 4; Si: from 0.01 to 1; Cr: from 0.1 to 4; Mo: from 0.02 to 1; P: less than 0.1; S: less than 0.1; N: less than 0.3; in this case, the remainder is iron, including the inevitable accompanying steel elements, with alloying as an option with one or more of the following elements, wt. %: V: from 0.01 to 1; Nb: from 0.01 to 1; Ti: from 0.01 to 1; Sn: from 0 to 0.5; Cu: 0.005 to 3; W: from 0.03 to 3; Co: from 0.05 to 3; Zr: from 0.03 to 0.5; Ca: 0.0005 to 0.1;

- casting a steel melt to obtain a strip billet through a horizontal or vertical casting process approaching the final dimensions, or casting a steel melt to produce a slab or thin slab through a horizontal or vertical casting process of a slab or thin slab;

- re-heating the slab or thin slab to 1050 ° C - 1250 ° C and then hot rolling the slab or thin slab to obtain a hot strip or thick plate, or re-heating the resulting strip billet with approximation to the final dimensions, in particular, with a thickness of more than 3 mm, up to 1000 ° С - 1200 ° С and then hot rolling of the pre-strip to obtain a hot strip or thick plate, or hot rolling of a strip billet without reheating after casting, to obtain a hot strip or thick plate with intermediate heating as tion between the individual rolling passes in the hot rolling;

- winding a hot strip and, as an option, a thick plate at a winding temperature between 780 ° C and room temperature;

- as an option, annealing a hot strip or a thick plate with the following parameters: annealing temperature: 610-780 ° C, annealing duration: 1 minute - 48 hours;

- as an option, the cold rolling of a hot strip or the resulting strip billet with approximation to the final dimensions, with a thickness of less than or equal to 3 mm to obtain a cold strip;

- as an option, annealing a cold strip with the following parameters:

annealing temperature: 610-780 ° С, annealing duration: 1 minute - 48 hours,

which gives a flat steel product with a good combination of strength, tension and deformation properties, with increased resistance to delayed cracking and hydrogen embrittlement, with TRIP and / or TWIP effect under mechanical stress due to the residual austenite content in the microstructure.

As for other advantages, we turn to the above statements regarding steel in accordance with the present invention. The method gives a steel product in the form of a thick plate, hot strip or cold strip. There is a condition that the hot strip is wound at a maximum temperature of 780 ° C. The lower limit is room temperature, since the temperature of the winding has little effect on the properties during subsequent processing. In the context of the present invention, strips with a thickness of more than 3 mm are considered a thick plate, while such strips can definitely be wound, for example, with a thickness of 5 mm. A thick strip with a greater thickness, for example, 50 mm, turns into a sheet after hot rolling to obtain sheet material, because otherwise it cannot be wound. A hot strip or a cold strip can also be turned into a sheet if necessary.

Typically, the final hot rolling temperature is between 950 ° C and A _with 1 + 50 K.

Typically, the thickness ranges of the strip blank are from 1 mm to 35 mm for slabs, and for thin slabs from 35 mm to 450 mm. It is preferable that the slab or thin slab undergo hot rolling to obtain a hot strip or thick plate with a thickness of 70 mm to 1.5 mm, or a cast strip billet with approximation to the final dimensions passes hot rolling to obtain a hot strip with a thickness of 8 mm up to 1 mm. The cold strip in accordance with the present invention has a thickness of, for example, more than 0.15 mm.

In the context of the above method in accordance with the present invention, a strip preform produced in a two-roll casting process, approaching final dimensions with a thickness of less than or equal to 3 mm, preferably from 1 mm to 3 mm, is already understood as hot band. A strip blank made as a hot strip does not have a 100% cast structure due to deformation added by two rolls when moving in opposite directions. Therefore, hot rolling takes place already on the production line during casting with two rolls, and this means that there is no need for a separate hot rolling.

At temperatures in the range from 720 ° C to 1200 ° C, reheating takes place for hot rolling of the strip billet after casting to obtain a hot strip, with the possibility of intermediate heating between the individual phases of rolling during hot rolling. If only a few rolling phases are required, the reheat temperature can be selected from the lower limit of this range.

The hot strip, like a thick plate, can optionally undergo heat treatment at a temperature in the range between 610 ° C and 780 ° C from 1 minute to 48 hours, while higher temperatures are associated with a shorter processing time and vice versa. Annealing can also be carried out by means of a batch annealing process (longer annealing time), and, for example, by means of a continuous annealing process (shorter annealing time). Heat treatment can also be omitted if the hot strip or thick plate already has the desired final properties.

After the annealing process, the annealed hot strip can optionally be cold rolled to set the thickness to or greater than 0.15 mm, as required for its final use. After this, an additional annealing process can be carried out, if necessary, together with the coating process, and a training process by which the necessary surface structure is obtained.

Preferably, the flat steel product is hot dip galvanized or electrolytically galvanized, or a metal, inorganic or organic coating is applied.

The flat sheet product obtained by the method in accordance with the present invention in the form of a thick plate, hot strip or cold strip has a tensile strength Rm from 800 MPa to 1700 MPa, and elongation at break A50 from 6% to 45%, preferably from 8% up to 45%. In this case, high strength is linked to less tensile during fracture and vice versa.

Claims (94)

1. High-strength aluminum-containing manganese steel having the following chemical composition, wt.%: C: 0.01 to 0.3 Mn: 4 to 10 Al: 1 to 4 Si: 0.01 to 1 Cr: 0.1 to 4 Mo: from 0.02 to 1 P: less than 0.1 S: less than 0.1 N: less than 0.3 if necessary, at least one of: V: 0.01 to 1 Nb: 0.01 to 1 Ti: 0.01 to 1 Sn: 0 to 0.5 Cu: 0.005 to 3 W: 0.03 to 3 Co: 0.05 to 3 Zr: 0.03 to 0.5 Ca: 0.0005 to 0.1 iron and unavoidable impurities - the rest, however, it has a tensile strength Rm from 800 MPa to 1700 MPa, elongation at break A50 from 6% to 45%, preferably from 8% to 45%, and a microstructure containing from 5 to 65% residual austenite. 2. Steel under item 1, characterized in that it contains, wt.%: V: 0.02 to 1. 3. Steel under item 1 or 2, characterized in that it contains, wt.%: Nb: 0.02 to 1. 4. Steel according to any one of paragraphs. 1-3, characterized in that it contains, wt.%: Ti: 0.02 to 1. 5. Steel according to any one of paragraphs. 1-4, characterized in that it contains, wt.%: Sn: 0.005 to 0.5. 6. Steel according to any one of paragraphs. 1-5, characterized in that it contains, wt.%: Cu: 0.5 to 3. 7. Steel according to any one of paragraphs. 1-6, characterized in that it contains, wt.%: W: 0.05 to 3. 8. Steel according to any one of paragraphs. 1-7, characterized in that it contains, wt.%: Co: 0.08 to 3. 9. Steel according to any one of paragraphs. 1-8, characterized in that it contains, wt.%: Zr: 0.05 to 0.5. 10. A method of manufacturing a flat steel product from steel according to any one of paragraphs. 1-9, containing the following steps: - smelting of steel having the following chemical composition, wt.%: C: 0.01 to 0.3 Mn: 4 to 10 Al: 1 to 4 Si: 0.01 to 1 Cr: 0.1 to 4 Mo: from 0.02 to 1 P: less than 0.1 S: less than 0.1 N: less than 0.3 if necessary, at least one of: V: 0.01 to 1 Nb: 0.01 to 1 Ti: 0.01 to 1 Sn: 0 to 0.5 Cu: 0.005 to 3 W: 0.03 to 3 Co: 0.05 to 3 Zr: 0.03 to 0.5 Ca: 0.0005 to 0.1 iron and unavoidable impurities - the rest, - casting a steel melt to obtain a strip billet through a horizontal or vertical casting process approaching the final dimensions or casting a steel melt to produce a slab or thin slab through a horizontal or vertical casting process of a slab or thin slab; - re-heating the slab or thin slab to 1050 ° C-1250 ° C and then hot rolling the slab or thin slab to obtain a hot strip or thick plate, or re-heating the resulting strip blank with an approximation to the final dimensions, in particular with a thickness of more than 3 mm up to 1000 ° С-1200 ° С and then hot rolling of the strip billet with obtaining a hot strip or a thick plate or hot rolling of a strip billet without reheating after casting to obtain a hot strip or a thick plate with intermediate heating as ve options between the individual rolling passes in the hot rolling; - winding a hot strip and as an option a thick plate at a winding temperature between 780 ° C and room temperature; - if necessary, annealing a hot strip or a thick plate with the following parameters: annealing temperature: 610-780 ° C, annealing duration: 1 minute - 48 hours; - if necessary, cold rolling a hot strip or the resulting strip billet with approximation to the final dimensions to form a cold strip, - if necessary, annealing the cold strip with the following parameters: annealing temperature: 610-780 ° C, annealing duration: 1 minute - 48 hours. 11. The method according to p. 10, characterized in that the hot rolling of the slab is carried out to obtain a hot-rolled strip with a thickness of 70 mm to 1.5 mm or the hot-rolled strip is carried out to obtain a hot-rolled strip with a thickness of 8 mm to 1 mm. 12. A method of manufacturing a flat steel product from steel according to any one of paragraphs. 1-9, containing the following steps: - smelting of steel having the following chemical composition, wt.%: C: 0.01 to 0.3 Mn: 4 to 10 Al: 1 to 4 Si: 0.01 to 1 Cr: 0.1 to 4 Mo: from 0.02 to 1 P: less than 0.1 S: less than 0.1 N: less than 0.3 if necessary, at least one of: V: 0.01 to 1 Nb: 0.01 to 1 Ti: 0.01 to 1 Sn: 0 to 0.5 Cu: 0.005 to 3 W: 0.03 to 3 Co: 0.05 to 3 Zr: 0.03 to 0.5 Ca: 0.0005 to 0.1 iron and unavoidable impurities - the rest, - casting a steel melt to obtain a strip billet through a horizontal or vertical casting process, approaching the final dimensions, with a thickness of less than or equal to 3 mm; - if necessary, cold rolling the obtained strip billet to form a cold strip; - if necessary, annealing the cold strip with the following parameters: annealing temperature: 610-780 ° C, annealing duration: 1 minute - 48 hours. 13. Flat steel product obtained by the method according to any one of paragraphs. 10-12, characterized in that it has a tensile strength Rm from 800 MPa to 1700 MPa and elongation at break A50 flat steel product from 6% to 45%, preferably from 8% to 45%. 14. The flat steel product according to claim 13, characterized in that it has a metal, inorganic or organic coating, or a zinc coating obtained by hot dip or electrolytic method.