KR100536645B1 - Highly stable, steel and steel strips or steel sheets cold-formed, method for the production of steel strips and uses of said steel - Google Patents

Abstract

A steel strip or sheet steel having good cold formability and high-strength is described, comprising a light steel having (in weight-percent) C: <=1.00%, Mn: 7.00-30.00%, Al: 1.00-10.00%, Si: >2.50-8.00%, Al+Si: >3.50-12.00%, B: >0.00-<0.01%, as well as alternately Ni: <8.00%, Cu: <3.00%, N: <0.60%, Nb: <0.30%, Ti: <0.30%, V: <0.30%, P: <0.01%, with the remainder iron and unavoidable impurities.

Description

High Stability Cold Formed Steel and Steel Strips or Steel Sheets, Steel Strips Manufacturing Methods and Applications of the Steels

The present invention relates to Fe-Mn-Al-Si light steel and steel strips or steel sheets with excellent cold formability and high strength. The invention also relates to a method for producing a strip from the steel and to a suitable use of the steel.

Hard steel is known from German Patent No. 197 27 759 C2 for the manufacture of body parts and for low temperature applications. The hard steel contains, in addition to Fe, 10% to 30% Mn, 1% to 8% Al and 1% to 6% Si, and the sum of Al and Si contents does not exceed 12%. Although carbon components are contained in such known steels, the range of their contents is within the range of impurities.

In contrast, hard structural steel known from German Patent Application Publication No. 199 00 199 Al contained carbon as an optional alloy component. Known steels include more than 7% to 27% Mn, more than 1% to 10% Al, more than 0.7% to 4% Si, less than 0.5% C, less than 10% Cr, less than 10% Ni and It contains less than 0.3% Cu. In addition, the steel includes N, V, Nb, Ti and P in a range where the total content does not exceed 2%.

Hard steels known from European Patent Application No. 1 067 203 Al contain carbon in the range of 0.001 to 1.6%. The steel also contains, in addition to Fe, from 6 to 30% Mn, up to 6% Al, up to 2.5% Si, up to 10% Cr, up to 10% Ni and up to 5% Cu. The steel also contains V, Ti, Nb, B, Zr and rare earth components in a range where the total content does not exceed 3%. In addition, the known steels include P, Sn, Sb and As in a range where the total content does not exceed 0.2%.

Steels having a composition of the kind described above have been shown to be difficult in hot and cold rolling despite the inclusion of carbon. Thus, instability or cracks frequently appear at the strip edge, making it difficult to manufacture large-scale strips or sheets substantially with such steel. In addition, the steels have a very strong isotropic strain behavior which shows high Δr values. Steel sheets produced according to the above known methods are more difficult to perform further processing because they lack formability.

In order to produce components provided in toothed or similar shaped elements, good formability steel with high strength is required. The component is generally a gear part provided with a serration inside or outside. These can be efficiently produced by flow forming with high dimensional accuracy and low cost.

A method for producing gear parts by flow molding is known from German Patent No. 197 24 661 C2. According to the known method, blanks are made from microalloyed high strength structural steels having a sheet and a lower yield point of at least 500 N / mm ² . This blank is then cold formed into gears by flow forming. In the process of forming into a saw tooth, the sheet material is reshaped to the limit of its forming ability. The surface of the workpiece provided as serrated is then cured without thermal deformation while maintaining the original temperature.

It is an object of the present invention, starting from the related art as described above, to provide hard steels and / or steel strips or steel sheets made therefrom having good formability and good strength which can be easily manufactured on an industrial scale. Also disclosed are methods of making steel strips or steel sheets and preferred uses of such steels.

It is an object of the present invention to formulate the following composition (in weight percent):

C: 1.00% or less,

Mn: 7.00% to 30.00%,

Al: 1.00% to 10.00%,

Si: greater than 2.50% to 8.00%,

Al + Si: greater than 3.50% to 12.00%, and

B: greater than 0.00% to less than 0.01%;

Optionally,

Ni: less than 8.00%,

Cu: less than 3.00%,

N: less than 0.60%,

Nb: less than 0.30%,

Ti: less than 0.30%,

V: less than 0.30%, and

P: achieved by hard steel containing less than 0.01% of components.

The balance consists of Fe and unavoidable impurities. In this case the impurities include sulfur and oxygen.

Surprisingly, the addition of trace amounts of boron has been shown to significantly improve the properties and productivity of the steel according to the invention. As a result, the content of boron contained in the steel according to the present invention lowered the yield point of the steel and thus significantly improved the formability. When the carbon content is from 0.10 to 1.00 weight percent, ie if carbon can be detected at least 0.10 weight percent in the steel according to the invention, the desired effect of the alloy on the mechanical-technical properties of the steel according to the invention is further enhanced.

In this case, the incorporation of these components can yield very good results in both mechanical and technical properties. Thus, the steel and / or steel strips or steel sheets produced therefrom according to the invention have significantly lower Δr values than known steel sheets through the relevant art of the kind discussed herein.

In addition, the cold rolled steel strips and steel sheets containing the components according to the invention have a relatively low yield point, improved elongation at elevated hardening index (n value), elevated deep drawing quality (r value), and low plane It is characterized by an increase in the product of yield point and elongation as well as anisotropy (Δr value). Thus, the tensile strength of the steel strip and the steel sheet according to the invention is at least 680 MPa. The product of tensile strength and elongation is at least 41,000 MPa ·%. The yield point of the steel strip and the steel sheet according to the invention does not exceed 520 MPa. At the same time, the steels and / or sheets and strips produced therefrom according to the invention have extremely high uniform elongations from 20% to 45% or more, with n values up to 0.7 being obtained.

As a result, particularly good cold form hard steel strips or steel sheets through this method are well suited for the production of components for vehicle bodies due to their relatively high strength and low density. Also for the manufacture of wheels for vehicles, in particular motor vehicles, for the manufacture of components which are hydroforming internally or externally, for the production of high strength engine parts such as camshafts or piston rods, and pulsed stresses such as shocks. A significant proportion of the strength and weight of the steel sheet according to the invention is suitable for the manufacture of components, such as armor plates, which are to be protected against), and especially for the protective elements which allow people to be protected against impact. Calculated. Especially when applied to the latter, the relatively low weight of the steel sheet according to the invention and at the same time its high strength acts as a sure effect.

In addition, the steel sheets according to the invention are well suited for the production of nonmagnetic components if they have a fully austenitic microstructure.

In addition, the steels according to the invention have been shown to maintain their strength, especially at low temperatures. Thus, the steels are well suited for the manufacture of components used in cryogenic engineering, such as containers or pipes for cryogenic engineering.

In particular, the significant effect of boron contained in the steel used according to the invention can be reliably achieved if the boron is contained in 0.002wt% to 0.01wt%, in particular 0.003 to 0.008wt%.

C contained in the range of 0.1% to 1.0% improved the productivity of the steel sheet and steel strip according to the invention. In the steel according to the invention, the intermetallic phase formation is prevented due to carbon. Thus, cracks and instability in the strip edge region caused in steel strips made of known steels were significantly reduced, in particular the instability was reduced with an increase in the C content. The addition of boron further improved the strip edge quality. As a result, strip edge instability can be almost completely eliminated by the mixed addition of C and B.

Boron replaces the action of the alloying element Mn in its effect on mechanical-technical properties. As a result, steels containing 20% Mn and 0.003% boron have a similar property profile as steels containing 25% Mn but no B. Therefore, the hard structural steel according to the present invention may have a relatively high strength despite having a relatively small Mn content. This reduced the cost for the alloying components and the hard steels used in accordance with the present invention were made through an easier metallurgical melting process.

In addition, the contents of C and B provided according to the invention represent a wide range of hot rolling parameters. As a result, the characteristics of the steel according to the invention obtained when a high hot rolling finish temperature and a coiling temperature were selected were shown to be essentially the same as those of a steel obtained at a low hot rolling finish temperature and a coiling temperature. Insensitivity in the production of hot strips also facilitated the steel sheet productivity according to the present invention.

Since the Si content is limited to at least 2.50 wt%, preferably at least 2.70 wt%, the steel strips and steel sheets according to the present invention have improved cold formability compared to hard steel strips or sheets having a low Si content. A large amount of Si showed more uniform yield point and tensile strength values, high fracture elongation and uniform elongation. In addition, the Si contained in the steel according to the present invention made the r and n values higher and led to an isotropic implementation of mechanical properties. The upper limit of the sum of the Al and Si contents was set to 12% because the sum of the Al and the Si contents exceeded the upper limit, causing the risk of brittleness.

The steel strips and sheets according to the invention are produced by casting an input stock such as slabs, thin slabs, or strips made of the steel according to the invention with the components described above, and the cast inputs. The material is heated to 1100 ° C. or higher or used directly at this temperature, the preheated input is hot rolled into a hot strip at a hot rolling finish temperature of at least 800 ° C., and the finished rolled hot strip is 450 ° C. to It was preferably prepared by a method consisting of winding up at a winding temperature of 700 ° C.

It is necessary to make heating of an input material into 1100 degreeC or more in order to perform hot rolling by making hot rolling finish temperature 800 degreeC or more. Since the hot strips are hot rolled at a hot rolling finish temperature of at least 800 ° C. and wound up at lower temperatures in accordance with the invention, the positive effects of carbon, in particular of the disclosed boron, have been fully exploited. As a result, boron and carbon provided high tensile strength and yield values of the hot rolled strips in an even more acceptable range of fracture elongation. As the hot rolling finish temperature increases, the tensile strength and yield point decrease, while the draw value increases. By varying the hot rolling finish temperature within the range provided by the present invention, the desired properties of the steel strip can be easily changed as desired.

By restricting the winding temperature to a value of 700 ° C. or less, material brittleness can be easily avoided. A brittle phase is formed, for example, at high winding temperatures that can cause material flaking, which has been shown to make the further process more difficult or even impossible. In addition, in order to develop a steel structure which ensures an optimized ratio of yield strength and ductility, the lower limit of the winding temperature must be maintained at 450 ° C.

Hot strips made according to the invention are also distinguished by their excellent application properties. If thinner sheets or strips have to be produced, the hot strips can be cold rolled into cold strips after winding, wherein the cold rolls are in the range of 30% to 75% for the production of cold rolled steel strips with optimized ratios of toughness and ductility. It is preferable to carry out at a reduction ratio. Inevitably work hardening occurs by cold rolling, and for the balance of work hardening, it is preferable to perform annealing at an annealing temperature of 600 ° C to 1100 ° C after cold rolling. In this case the annealing is carried out in the hood in the temperature range of 600 ° C. to 750 ° C. or continuously in the annealing furnace at a temperature of 750 ° C. to 1100 ° C. During hood annealing, the cold strip is held in the hood furnace for a long time in the form of a coil to sufficiently heat the entire coil. Therefore, the temperature of the hood annealing is set to a temperature at which the target result (balance of work hardening) can be obtained. During continuous annealing, as the strip continuously passes through the annealing furnace at high speed, the strip is exposed to heat in the annealing furnace for only a short time. Thus, in order for the center of the strip to reach the temperature at which work hardening is achieved, the temperature of the continuous annealing must be higher than the hood annealing temperature. Finally, in view of cold formability and surface forming, it is preferable to dress the cold strip in the finishing step. The dressing is a "pinch pass", "skin pass" as a pass of a small strain applied to an annealed cold rolled strip to prevent the formation of kink or stretcher strain markings during subsequent cold work. Or "mild rolling".

Furthermore, a preferred use of the steel and / or steel strips and sheets produced therefrom according to the invention is in the product of cold formed components by flow molding. For this purpose, the blanks are made from steel finished by flow forming. Because of their particular property profile, for this purpose, in particular the steel and / or sheet blanks produced therefrom according to the invention are suitable. During flow molding, the prepared "pre-form" is stretched under high pressure. In particular, flow forming is a method used for producing a rotationally symmetric hollow member. For this purpose, the preformed blank is fixed between the mandrel and the tail stock. The feature of this process is that the metal is formed by local compression due to the radial pressure of the rollers. The metal will flow and be shaped along the contour of the inner mandrel in a single roller pass.

Microstructures that are fully austenitic or comprise a mixed structure of ferrite and austenite with martensite can be obtained by the action of the component in the steel according to the invention. The steels according to the invention thus form fairly well. They are compressed more strongly than the known high-strength microalloyed or multiphase steels used for making through flow forming in the cold forming process. As a result, the strength of the component can be achieved in the range of 1400 N / mm ² to 2200 N / mm ² depending on cold forming. Thus, additional hardening of the components produced after cold forming is unnecessary. Furthermore, in connection with the intended purpose, in particular with respect to the intended purpose, the steel according to the invention used for producing the toothed gear component is reduced in density due to the high content of low specific gravity components such as Si and Al. It has a desirable effect for the product.

If the steel made according to the invention is used, subsequent heat treatment or surface hardening of the flow molded component becomes unnecessary. Thus, in use, if the steel according to the invention is used to produce toothed elements that are locally stressed, there is no longer any risk of warping and scaling caused by the related art by further processing steps. . As a result, the steel according to the invention makes it possible to economically produce light, very robust and dimensionally stable components through cold forming, in particular flow forming.

Hereinafter, the present invention will be described in more detail with reference to the following typical examples and comparative examples.

In Table 1 the components of five steels A, B, C, D and E are indicated, of which steels A, B and C correspond to the alloys provided according to the invention, while steels D and E are comparative examples. .

River C Mn Al Si B Fe, impurity A 0.5 15 3 3 0.003 Balance B 0.5 20 3 3 0.003 Balance C - 20 3 3 0.003 Balance D - 14 3 3 - Balance E - 19 3 3 - Balance

Steels A to E, of associated composition, were melted and cast into slabs. Subsequently, the slab was preheated to a temperature of 1150 ° C. The preheated slabs were then hot rolled and then wound up.

The respective hot rolling final temperature (ET) and winding temperature (HT), the respective tensile strength (R _m ), yield point (R _e ), elongation (A ₅₀ ), uniform elongation (A _g1 ) and n of the obtained hot strip The properties of the values are shown in Table 2.

River ET [℃] HT [℃] R _e [N / mm ² ] R _m [N / mm ² ] A ₅₀ [%] A _gl [%] n A 960 500 486 792 42 38 0.31 B 930 500 509 825 46 42 0.32 C 920 500 496 818 31 27 - D 820 500 610 920 26 - - E 840 500 430 700 30 - -

Except for the strip made from steel D, which is not cold rollable, but not the inventive steel, the hot strip obtained was then cold rolled to approximately 65% strain and subsequently annealed at 950 ° C. The mechanical properties of the cold rolled steel sheet obtained through this method are listed in Table 3.

River R _e [N / mm ² ] R _m [N / mm ² ] A ₅₀ [%] A _gl [%] n r Δr A 408 775 64 64 0.33 1.02 -0.1 B 411 785 61.1 61.1 0.33 1.0 -0.06 C 284 714 58 56.8 0.39 1.05 -0.17 D Cold rolling not possible E 382 744 52.5 50.3 0.32 0.82 -0.25

Steel strips made with steels A to C according to the invention show excellent cold formability. In this case, at high strength and high elongation at break, each of the steels has a definite isotropic strain factor r-1, Δr-0. Even in steel strips made from steel C according to the present invention without carbon and containing boron, they have low yield points, elevated fracture and uniform elongation and isotropic molding factors.

Thus, all variants of the steel sheets according to the invention are particularly intended for the manufacture of bodywork components, in particular for automobile bodies, wheels for vehicles, in particular motor vehicles, nonmagnetic components, containers used in cryogenic engineering, internal or external Hydraulically molded components, tubes used for the manufacture of high-strength engine parts such as camshafts or piston rods, components or armor plates used to protect against pulsed stresses such as impacts, or for human or animal bodies Suitable for exterior panels of protective elements such as body armor. Also for this purpose very sturdy gear parts can be produced from the steel according to the invention which are distinguished by light weight and good application properties without further heat treatment.

Claims (50)

In weight percent, C: 1.00% or less, Mn: 7.00% to 30.00%, Al: 1.00% to 10.00%, Si: greater than 2.50% to 8.00%, Al + Si: greater than 3.50% to 12.00%, and B: Excellent cold formability and high strength steel, which contains a component of more than 0.00% to less than 0.01% and comprises a balance of Fe and unavoidable impurities. 2. The excellent cold formability and high strength steel according to claim 1, wherein the carbon content is 0.10 to 1.00 weight percent. 3. The excellent cold formability and high strength steel according to claim 1 or 2, wherein the Si content is greater than 2.70 weight percent. 3. The excellent cold formability and high strength steel according to claim 1 or 2, wherein the boron content is 0.002 to 0.01 weight percent. In weight percent, C: 1.00% or less, Mn: 7.00% to 30.00%, Al: 1.00% to 10.00%, Si: greater than 2.50% to 8.00%, Al + Si: greater than 3.50% to 12.00%, and B: A high strength steel strip or steel sheet having excellent cold formability, comprising a component of more than 0.00% and less than 0.01% and comprising a balance of Fe and unavoidable impurities. The high strength steel strip or steel sheet having excellent cold formability according to claim 5, wherein the tensile strength is 680 MPa or more. The high strength steel strip or steel sheet according to claim 5 or 6, wherein the product of tensile strength and elongation is 41,000 MPa ·% or more. The high strength steel strip or steel sheet having excellent cold formability according to claim 5 or 6, wherein the yield point is 520 MPa or less. In weight percent, C: 1.00% or less, Mn: 7.00% to 30.00%, Al: 1.00% to 10.00%, Si: greater than 2.50% to 8.00%, Al + Si: greater than 3.50% to 12.00%, and B: casting an input material such as slab, thin slab, or strip from a steel having a composition containing more than 0.00% and less than 0.01% and containing a balance of Fe and unavoidable impurities; Heating the cast input material to 1100 ° C. or more or immediately using the same at a temperature; Hot rolling the preheated input raw material into a hot strip at a hot rolling finishing temperature of 800 ° C. or higher, Winding the finished rolled hot strip at a winding temperature of 450 ° C to 700 ° C. 10. The method of claim 9, wherein the hot strip is cold rolled into a cold strip after winding. The method of claim 10, wherein the cold strip is annealed at an annealing temperature of 600 ° C. to 1100 ° C. 12. 12. The method of claim 11, wherein the annealing is performed with hood annealing at an annealing temperature of 600 ° C to 750 ° C. 12. The method of claim 11 wherein the annealing is carried out in a continuous annealing at an annealing temperature of 750 ° C to 1100 ° C. 14. A method according to any one of claims 10 to 13, wherein the cold strip is dressed. The method according to any one of claims 10 to 13, wherein the cold rolling is carried out at a cold reduction ratio of 30% to 75%. 14. A cold formable high strength steel strip or steel according to any one of claims 9 to 13, characterized in that a blank is produced from each hot or cold strip, and the blank is subsequently cold formed into the final component. Sheet manufacturing method. 18. The method of claim 16, wherein the cold forming is performed by flow forming. A method of using the excellent cold formability and high strength hard steel according to claim 1 to produce a support body component. A method of using the excellent cold formability and high strength hard steel according to claim 1 or 2 for producing exterior body parts. A method of using the excellent cold formability and high strength hard steel according to claim 1 for producing wheels for vehicles such as automobiles. A method of using the excellent cold formability and high strength steel according to claim 1 or 2 to produce a non-magnetic component having a fully austenitic microstructure. A method of using the excellent cold formability and high strength hard steel according to claim 1 or 2 for producing components such as containers or pipes used in cryogenic engineering. A method of using the good cold formability and high strength hard steel according to claim 1 or 2 to produce internally or externally hydroformed components. A process using the excellent cold formability and high strength steel according to claim 1 or 2 for the manufacture of tubes used for producing high strength engine parts such as camshafts or piston rods. A method of using good cold formability and high strength steel according to claim 1 or 2 to produce components such as armor plates used to protect against pulse stresses such as impacts. A method of using the excellent cold formability and high strength steel according to claim 1 or 2 to produce protective components such as helmets and body armor used to protect people against pulsed stresses such as impacts. A method of using the good cold formability and high strength hard steel according to claim 1 or 2 for producing a component by flow forming. A method of using the good cold formability and high strength hard steel according to claim 1 for producing a gear part. 29. The method of claim 28 wherein a tooth is formed in the gear part. 30. The method of claim 29, wherein the gear part is manufactured by flow forming. 3. The method of claim 1, wherein Ni: less than 8.00%, Cu: less than 3.00%, N: less than 0.60%, Nb: less than 0.30%, Ti: less than 0.30%, V: less than 0.30%, and P: 0.01 Excellent cold formability and high strength steel, characterized in that it also contains one or two or more components selected from the group consisting of less than%. 5. The excellent cold formability and high strength steel according to claim 4, wherein the boron content is 0.003 to 0.008 weight percent. The high strength steel strip or steel sheet having excellent cold formability according to claim 5, wherein the carbon content is 0.10 to 1.00 weight percent. 34. The method of claim 5 or 33, wherein Ni: less than 8.00%, Cu: less than 3.00%, N: less than 0.60%, Nb: less than 0.30%, Ti: less than 0.30%, V: less than 0.30%, and P: 0.01. A high strength steel strip or steel sheet with excellent cold formability, characterized in that it further contains one or two or more components selected from the group consisting of less than%. 34. The high strength steel strip or steel sheet according to claim 5 or 33, wherein the Si content is greater than 2.70 weight percent. 34. The high strength steel strip or steel sheet according to claim 5 or 33, wherein the boron content is 0.002 to 0.01 weight percent. 37. The high strength steel strip or steel sheet according to claim 36, wherein the boron content is 0.003% to 0.008% by weight. 10. The method of claim 9, wherein the carbon content is 0.10 to 1.00 weight percent. 39. The method of claim 9 or 38 wherein Ni: less than 8.00%, Cu: less than 3.00%, N: less than 0.60%, Nb: less than 0.30%, Ti: less than 0.30%, V: less than 0.30%, and P: 0.01. A method for producing a cold formable high strength steel strip or steel sheet, characterized in that it also contains one or two or more components selected from the group consisting of less than%. 39. The method of claim 9 or 38, wherein said Si content is greater than 2.70 weight percent. 39. The method of claim 9 or 38, wherein said boron content is from 0.002 weight percent to 0.01 weight percent. 42. The method of claim 41 wherein the boron content is from 0.003 weight percent to 0.008 weight percent. 39. The method of claim 38, wherein the hot strip is cold rolled into a cold strip after winding. 45. The method of claim 43, wherein the cold strip is annealed at an annealing temperature of 600 ° C to 1100 ° C. 45. The method of claim 44, wherein the annealing is performed with hood annealing at an annealing temperature of 600 ° C to 750 ° C. 45. The method of claim 44, wherein the annealing is carried out in a continuous annealing at an annealing temperature of 750 ° C to 1100 ° C. 47. The method of any of claims 43-46, wherein the cold strip is dressed. 47. The method according to any one of claims 43 to 46, wherein the cold rolling is performed at a cold reduction rate of 30% to 75%. 47. The cold formable high strength of any one of claims 38 or 43-46, wherein a blank is made from each hot or cold strip, and the blank is subsequently cold formed into the final component. Method of manufacturing steel strips or steel sheets. 50. The method of claim 49, wherein the cold forming is performed by flow forming.