EP1865086B1 - Use of a steel flat product produced from a manganese boron steel and method of its production - Google Patents

Abstract

Manganese-boron-steel (I) comprises (in wt.%): carbon (0.1-0.2); silicon (0.05-0.3), manganese (0.8-1.8), nickel (0.5-.8); phosphorus (= 0.015), sulfur (= 0.003), boron (0.0002-0.008), optionally titanium (0.01-0.1), optionally aluminum (0.01-0.05), optionally nitrogen (0.002-0.005) and rest of iron and unavoidable impurities. Independent claims are included for: (1) a flat steel product comprising (I); and (2) a process for the preparation of the steel product comprising smelting a steel melt containing (I), pouring the melt to slabs or thin slabs, thermally-rolling the slabs or thin slabs at 850-900[deg]C to form a hot-rolled strip and hasping the hot-rolled strip at 560-600[deg]C.

Description

The invention relates to a use of a steel flat product produced from a manganese-boron steel, and a method for its production.

MnB steels of the type used according to the invention are used in particular in the field of automobile construction for the production of components which are said to have a high strength in addition to a low weight.

The high safety requirements placed on the deformation behavior of modern automobiles in the event of a crash today can be met by using higher-strength steels for producing the deformation behavior of the vehicle bodywork and the particularly critical components in the event of an accident. The problem is, however, that high-strength steels usually have insufficient formability and tend to spring back in their cold deformation to the respective component.

This problem is overcome in practice by the fact that sheets produced from higher-strength steels are deformed to the respective component in the warm state in so-called "hot pressing". This approach has been proven and allows the production of complex shaped body and chassis parts made of high-strength steels.

Manganese-boron steels of the type in question, which are standardized in EN 10083-3, have proven particularly suitable for the hot-pressing deformation. These steels have a good hardenability, which allows safe process control during hot pressing and by which it is economically possible to effect a martensite hardening in the mold without additional cooling during the hot deformation.

A MnB steel proven in practice for the purposes summarized above is known as "22MnB5" and has been given the material number 1.5528 in the steel key, 2004 edition. Typically, 22MnB5 steel on the market contains, in addition to iron and unavoidable impurities (in wt%), 0.220-0.250% C, 1.2-1.4% Mn, 0.2-2.3% Si, up to 0 , 02% P, up to 0.005% S, 0.020-0.050% Al, 0.020-0.050% Ti, 0.11-0.2% Cr, 0.002-0.0035% B and in each case up to 0.1% Mo, Cu and Ni (Material Data Sheet 11-112 of Salzgitter Flachstahl GmbH, November 2005 edition).

For appropriately composite, hot-rolled and provided with an Al-coated steel sheets is in the EP 0 971 044 B1 an alloying specification according to which a MnB steel, in addition to iron and unavoidable impurities (in% by weight), has a carbon content of more than 0.20%, but less than 0.5%, a manganese content of more than 0.5%, but less than 3%, a silicon content of more than 0.1%, but less than 0.5%, a chromium content of more than 0.01% but less than 1%, a titanium content of less than 0.2%, an aluminum content of less than 0.1%, a phosphorus content of less than 0.1%, should have a sulfur content of less than 0.05% and a boron content of more than 0.0005%, but less than 0.08%. An Indian EP 0 971 044 B1 As an exemplary embodiment, provided with an Al coating steel sheet has accordingly (in wt .-%) 0.21% C, 1.14% Mn, 0.020% P, 0.0038% S, 0.25% Si, 0, 04% Al, 0.009% Cu, 0.020% Ni, 0.18% Cr, 0.0040% N, 0.032% Ti, 0.003% B and 0.0050% Ca. After a heat treatment, the strength of this steel sheet should be more than 1500 MPa.

Furthermore, from the JP 2004-211197 a steel sheet for structural components of automobile bodies, which in addition to iron and unavoidable impurities (in wt .-%) 0, 20 - 0.35% C, 0, 01 - 1.0% Si, 0.3 - 2.0% Mn , 0.01-0.10% Al, more than 1-5% Ni, 0.005-0.1% Ti, 0.0005-0.005% B, 0.001-0.01% N, up to 0.02% P , up to 0.01% S, up to 0.01% O. In the case of this alloy, for the Ti and N contents, the condition "Ti / 47.88 - N / 14.01 ≤ 0.001", for the Al and N contents, the condition "0 ≤ Al / 26.98 - N / 14.01 ≤ 0, 0015 "and for the C, Si, Mn, Ni, Cr, Cu, S and P contents the condition" -28.8 x C ² + 38.0 x C + 1.7 x Si + 7.5 x Mn + 1.4 x Ni + 43.2 x Cr + 8.7 x Cu + 53.5 x S + 57.1 x P + 17.9 ≥ 35 ", the steel sheet in question should have a particularly good hardenability after hot deformation with at the same time particularly good behavior in the event of a sudden impact.

What is common to the above-mentioned conventional MnB steels is that they each have high strengths and good hardenability. Their high strength after hot pressing results from the carbon dissolved in the martensite grid, while their good hardenability is largely determined by the Mn and Cr contents and supplemented by B.

The residual fracture strain of the known steels, however, is typically only 5-6%. In order to meet the ever-increasing demands on the crash behavior of vehicle bodies, however, steels are needed, in addition to a high strength and an improved elongation behavior of the manufactured from such a steel component after hot pressing.

In addition to the above-discussed prior art are from the EP 1 160 346 A1 A high-strength hot-dip galvanized steel sheet excellent in plating adhesion and press-formability, and a method for producing such a steel sheet are described. The steel sheet consists of a steel, which in addition to iron and unavoidable impurities (in wt .-%) C: 0.05 to 0.2%, Si: 0.2 to 2.0%, Mn: 0.2 to 2 , 5%, Al: 0.01 to 1.5%, Ni: 0.73 to 5.0%, P: <0.03% and S: <0.02%, and optionally one or more of the following ingredients Cu: <0.2%, B: 0.0002 to 0.01%, Co: <0.3%, Sn: <0.3%, Mo: <0.5%, Cr: <1%, V : <0.3%, Ti: <0.06%, Nb: <0.06%, rare earth metals <0.05%, Ca: <0.05%, Zr: <0.05%, Mg: < 0.05%, Zn: <0.02%, W: <0.05%, As: <0.02%, N: <0.03%, and O: <0.05%. At the same time, the ratio of Si to Al should be set so that the Condition 0.4 (%) ≦ Si + 0.8 Al (%) ≦ 2.0% is satisfied. Likewise, the volume percentage of retained austenite in the steel sheet should be 2 to 20%. Then, on the steel sheet surface, the ratio of Ni, Cu, Co, Sn and Si, Al in 0.5 μm of the steel sheet surface layer is set so that the condition Ni + Cu + Co + Sn (%) ≥ 1/4 Si + 1/3 Al (%), a Zn-plated layer specifically alloyed should adhere particularly well to the flat steel product. For this purpose, the plating layer comprises Al: ≦ 1%, optionally Fe: 8 to 15% and optionally one or more constituents selected from Mn: <0.02%, Pb: <0.01%, Fe: <0.2% , When the optional 8 to 15% Fe is not contained in the Zn plating layer, the Zn layer may also be Sb: <0.01%, Ni: <3.0%, Cu: <1.5%, Sn: < 0.1%, Co: <0.1%, Cd: <0.01%, and Cr: <0.05%. The principle used according to this prior art thus presupposes certain minimum contents of Si and Al, whereas the combination of the Mn and B contents plays only a minor role. Instead, like the one in the EP 1 160 346 A1 specified embodiments and explanations show, the Cu content of particular importance.

Finally, out of the JP 2005-336541 a method is known for producing a higher strength plate which is at least 70 mm thick and which is composed of a steel containing (in% by weight) 0.04-0.20% C, 0, 0002-0.0-03 % B and 0, 0003 - 0, 0025% Ca, balance iron and unavoidable impurities, the carbon content being C% in relation to the boron content B% of Steel is set so that the condition 0.1 ≤ C% + 100 x B% ≤ 0.32 is met. At the same time certain conditions are to be met for the bainite content of the steel sheet. The addition of Ca is intended to improve the toughness in the heat-affected zone of a weld, which is produced when welding the heavy plate. It is obvious that heavy plates of this type are neither suitable for their range of characteristics nor their dimensions for hot pressing.

Based on the above-described prior art, the invention therefore an object of the invention to provide a use of a flat steel product, which is due to its composition and properties particularly suitable for the production of components that in terms of in practice from such flat steel products , like steel strips or steel sheets, manufactured components certainly meet requirements. In addition, a method for producing such a flat steel product should be specified.

With regard to use, this object has been achieved by using a manganese-boron steel according to the invention for the production of a hot-pressed component, which contains (in% by weight) 0.1-0.20% C, 0.05 - 0.30% Si, 0.8 - 1.8% Mn, 0.5 - 1.8% Ni, up to 0.015% P, up to 0.003% S,
0.0002 - 0.0080% B and the remainder contains iron and unavoidable impurities. In addition, the MnB steel of the present invention may optionally contain 0.01-0.1% Ti, 0.01-0.05% Al, 0.002-0.005% N each in combination or alone. A component which is hot-pressed under the use of such a flat steel product according to the invention has a tensile strength R _m of at least 1,000 MPa and has an elongation at break A ₈₀ of more than 10%.

In contrast to the MnB steels conventionally used for hot pressing, in which the mechanical properties are largely determined by the C, Mn and Cr contents, contents of Cr have been dispensed with in the alloying concept according to the invention.

Instead, MnB steel used according to the invention contains contents of Ni in order to significantly increase the elongation at break and toughness of components obtained from the steel used according to the invention after a hardened state obtained by hot press forming. At the same time, the presence of Ni improves the hardenability of MnB steels used according to the invention, so that the Mn contents could be reduced and the addition of critical Cr in terms of its influence on the residual ductility could be dispensed with.

The C content of steels used according to the invention has also been modified in such a way that the martensite hardness produced in the course of hot pressing is reduced or the components obtained after hot pressing have a bainitic structure in favor of improved extensibility, by virtue of the toughness and elongation at break of steel used according to the invention produced components is also positively influenced. If emphasis is placed on increased strength of the steel used according to the invention, in the Under the invention, the C content in the range of 0.15 to 0.20 wt .-% can be adjusted. In contrast, C contents of 0.10-0.15% by weight have proved to be useful for a less solid, but particularly good residual ductility and toughness, even after the hot-pressing molding and the steel having hardened therein.

By modifying the Mn content, it is also possible to influence the residual elongation and hardenability of the steel used according to the invention. Reduced Mn contents in the range of 0.8-1.6% by weight of improved residual extensibility are beneficial, while at Mn contents of 1.0-1.8% by weight improved hardenability of the inventively used Steel is present.

Likewise, by a suitable adjustment of the nickel content, the residual elongation and the hardenability of the steel used according to the invention can be adjusted. Thus, lower Ni contents lying in the range of 0.5-1.8% by weight, in particular in the range of 0.8-1.5% by weight, lead to higher hardenability and at the same time to improved toughness, whereby together an improved residual elongation is achieved.

A steel used according to the invention with high strength and improved residual elongation after hardening achieved during hot deformation accordingly has (in% by weight) 0.15-0.2% C, 0.05-0.30% Si, 0.8 - 1.6% Mn, 0.5 - 1.0% Ni, max. 0.015% P, max. 0.003% S, 0.0002 - 0.0080% B and the remainder iron and unavoidable impurities.

In contrast, a steel used according to the invention with lower strength but with further improved residual elongation after hardening achieved during hot deformation has (in% by weight) 0.10-0.15% C, 0.05-0.30% Si , 0 - 1.8% Mn, 0.8 - 1.8% Ni, max. 0.015% P, max. 0.003% S, 0.0002 - 0.0080% B and the remainder iron and unavoidable impurities.

Regardless of which of the various possible variants of the alloy used according to the invention are used, the hardening effect of boron in a steel used according to the invention occurs in particular when the B content is at least 0.0008% by weight.

The flat steel product used according to the invention may be a hot-rolled or cold-rolled steel strip or sheet. In this case, composite steel strips or steel sheets used according to the invention are outstandingly suitable for surface refinement by application of a metallic coating. This can be applied for example by Feueralumierung or hot dip galvanizing and by a combination of these methods. In addition, the corrosion protection respectively obtained by the metallic coating can additionally be improved by additionally providing the flat steel product used according to the invention with an organic or inorganic coating.

• Erschmelzen einer der entsprechend der Zusammensetzung des erfindungsgemäß verwendeten Stahlflachproduktes zusammengesetzten Stahlschmelze,
• Vergießen der Schmelze zu Brammen oder Dünnbrammen,
• Warmwalzen der Brammen oder Dünnbrammen bei einer Warmwalzendtemperatur von 850 - 900 °C zu einem Warmband,
• Haspeln des Warmbands bei Haspeltemperaturen von 560 - 600 °C.

With regard to the manufacturing process, the above object is finally achieved by a process for the preparation of a flat steel product used in accordance with the invention, in which the following working steps are carried out:

• Melting one of the steel melt composed according to the composition of the flat steel product used according to the invention,
• Casting the melt into slabs or thin slabs,
• Hot rolling of the slabs or thin slabs at a hot rolling end temperature of 850 - 900 ° C into a hot strip,
• Coiling of the hot strip at reel temperatures of 560 - 600 ° C.

The resulting hot strip can be descaled for further processing in a conventional manner, for example by pickling. If the resulting flat steel product is to be fed directly as a hot strip of the forming to a component, can now be applied as corrosion protection, a metallic coating in the manner already described above. Alternatively, the hot strip may be cold rolled after the pickling, if necessary, to cold strip. The degrees of cold rolling achieved during cold rolling, preferably without intervening intermediate annealing, should be at least 40% in order to ensure complete recrystallization.

The invention will be explained below with reference to exemplary embodiments.

In a conventional converter, two molten steels S1, S2 used according to the invention have been melted, the compositions of which are given in Table 1. In addition, in Table 1, the composition of a conventional 22MnB5 steel V is shown for comparison.

The steel melts S1, S2 have subsequently been cast into thin slabs in a likewise conventional cast roll mill.

Hot strips were then hot rolled from the resulting thin slabs. The hot rolling end temperature was 850 ° C. Subsequently, the obtained hot strips were coiled at a coiling temperature which was 560 ° C.

After coiling, the hot-rolled strips were pickled and cold-rolled to cold-rolled strips. With a view to minimizing production costs, cold rolling was carried out without intermediate annealing. The degree of cold deformation achieved in the course of cold rolling (cold rolling degree = reduction in thickness / thickness of the cold rolled hot strip achieved by the cold rolling) was 40%.

Subsequently, samples separated from the obtained cold tapes were coated with a metallic coating by fire aluminizing. In the case of a first sample, the metallic coating consisted of 89-92% by weight of Al, balance Si. A second sample was fire-aluminized with a coating containing 55% by weight of Al, 43.4% by weight of Zn and 1.6% by weight of Si.

Other samples separated from the obtained cold tapes were subjected to a hot-dip galvanizing process in which a first sample was coated with a zinc-zinc layer (Zn content at least 99% by weight) and a second sample with a layer of 95% by weight Zn, balance Al has been. A third sample was coated in a manner known per se by galvanic plating with a pure zinc coating.

The samples thus treated have been hot stamped into body parts. The components thus obtained had tensile strengths of more than 1,000 MPa and an elongation at break A ₈₀ of at least 10%. By applying a paint layer applied by cataphoresis dip coating with subsequent drying, the lower yield strength could be increased by at least 80 MPa, without the strength and elongation at break of the components changed.

Depending on the respective temperature control in the tool used for the hot-pressing deformation, a martensitic, a bainitic or a mixed martensitic-bainitic structure was present in the components obtained.

In Diag. 1, for the steels S1, S2, the hardness profiles calculated according to the Jominiy test are compared with the hardness curve of the conventionally assembled steel V. The respective hardness H is plotted over the respective distance A to the quenched surface. Table 1 S1 S2 V C 0.20 0.15 0.233 Si 0.24 0.24 0.24 Mn 1.00 1.00 1.23 P <0.015 <0.015 0,018 S <0.001 <0.001 0.001 al 0.03 0.03 0,035 N 0,003 0,003 0.0031 Ni 0.65 0.65 0.023 Ti 0.033 0.033 0.033 B 0.0025 0.0025 0.0022 Cr <0.05 <0.05 0.14

Claims (17)

1. Use of a steel flat product produced from a manganese boron steel which contains (in wt. %) C: 0.1 - 0.20 %, Si: 0.05 - 0.30 %, Mn: 0.8 - 1.8 %, Ni: 0.5 - 1.8 %, P: ≤ 0.015 %, S: ≤ 0,003 %, B: 0.0002 - 0.0080 % and optionally one or more of the following elements Ti, Al and N according to the following proportions: Ti: 0.01 - 0.1 %, Al: 0.01 - 0.05 %, N: 0.002 - 0.005 %, the remainder being iron and unavoidable impurities, for manufacturing a hot-pressed component which has a tensile strength R_m of at least 1,000 MPa and possesses an elongation at fracture A₈₀ of more than 10 %.
2. Use according to Claim 1, characterised in that the steel contains 0.15 - 0.20 wt. % of carbon.
3. Use according to Claim 1, characterised in that the steel contains 0.10 - 0.15 wt. % of carbon.
4. Use according to any one of the preceding claims, characterised in that the steel contains 0.8 - 1.6 wt. % of manganese.
5. Use according to any one of Claims 1 to 3, characterised in that the steel contains 1.0 - 1.8 wt. % of manganese.
6. Use according to any one of the preceding claims, characterised in that the steel contains 0.5 - 1.0 wt. % of nickel.
7. Use according to any one of Claims 1 to 5, characterised in that the steel contains 0.8 - 1.8 wt. % of nickel.
8. Use according to any one of the preceding claims, characterised in that the steel contains at least 0.0008 wt. % of boron.
9. Method for manufacturing a steel flat product for use according to any one of Claims 1 to 8 comprising the following production steps:

   - smelting a manganese boron steel melt which contains (in wt. %): C: 0.1 - 0.20 %, Si: 0.05 - 0.30 %, Mn: 0.8 - 1.8 %, Ni: 0.5 - 1.8 %, P: ≤ 0.015 %, S: ≤ 0,003 %, B: 0.0002 - 0.0080 % and optionally one or more of the following elements Ti, Al and N according to the following proportions: Ti: 0.01 - 0.1 %, Al: 0.01 - 0.05 %, N: 0.002 - 0.005 %, the remainder being iron and unavoidable impurities,

   - casting the melt into slabs or thin slabs,

   - hot-rolling the slabs or thin slabs at a hot-rolling final temperature of 850 - 900 °C into a hot-rolled strip,

   - coiling the hot-rolled strip at coiling temperatures of 560 - 600 °C.
10. Method according to Claim 9, characterised in that the hot-rolled strip is de-scaled after the hot-rolling.
11. Method according to Claim 9 or 10, characterised in that the hot-rolled strip is cold-rolled into a cold-rolled strip after the hot-rolling.
12. Method according to Claim 11, characterised in that the cold-rolling is carried out without intermediate annealing.
13. Method according to Claim 11 or 12, characterised in that the cold-rolling degree obtained when cold-rolling is at least 40 %.
14. Method according to any one of Claims 9 to 13, characterised in that the de-scaled, hot-rolled strip or the cold-rolled strip is surface-coated by a metallic coating.
15. Method according to Claim 14, characterised in that the de-scaled, hot-rolled strip or the cold-rolled strip is hot-dip aluminium-coated.
16. Method according to Claim 14, characterised in that the de-scaled, hot-rolled strip or the cold-rolled strip is hot-dip galvanized.
17. Method according to any one of Claims 14 to 16, characterised in that the de-scaled, hot-rolled strip or cold-rolled strip provided with the metallic coating is provided with an organic or inorganic coating.