Patent Application
20100273024
The invention relates to a dual-phase steel, the structure of which substantially consists of martensite and ferrite and respectively bainite, it being possible for portions of retained austenite to be present and the dual-phase steel having a tensile strength of at least 950 MPa. The invention also relates to a flat product produced from a dual-phase steel of this type as well as to processes for the production of this flat product.
The generic term “flat product” as used herein typically includes steel strips and sheets of the type according to the invention.
In the field of vehicle body construction, there is a demand for steels which on the one hand have a high strength with a low weight, but on the other hand also have a good deformability. Numerous attempts are known at producing steels which combine these contradictory characteristics.
Thus, for example, EP 1 431 107 A1 discloses a steel which is not only to have an effective deep-drawing property but also a high tensile strength, and a flat product produced therefrom and a process for the production thereof. The known steel contains, in addition to iron and unavoidable impurities (in % by weight) 0.08-0.25% C, 0.001-1.5% Si, 0.01-2.0% Mn, 0.001-0.06% P, up to 0.05% S, 0.001-0.007% N and 0.008-0.2% Al. At the same time, it should have an average r-value of at least 1.2, an r-value in the rolling direction of at least 1.3, an r-value in a direction of 45° based on the rolling direction of at least 0.9 and an r-value transversely to the rolling direction of at least 1.2. In the known steel, a strength-increasing effect is attributed to silicon, the upper limit of 1.5% by weight having been chosen in respect of an effective coatability of the steel. The positive influence of Mn on the strength is also emphasised. In this respect, the upper limit of the Mn content of 1.5% was set in respect of the decrease in the r-values which accompany any exceeding of this limit, and to optimise the r-values of the known steel sheet, Mn contents ranging from 0.04 to 0.8% by weight, in particular from 0.04 to 0.12% by weight were considered advantageous.
To further increase the strength of the known steel, it can optionally also contain, in addition to other selectively added alloying elements, contents of B of 0.0001-0.01% by weight, of Ti, Nb and/or V in a total quantity of 0.001-0.2% by weight as well as contents of Sn, Cr, Cu, Ni, Co, W and/or Mo in a total quantity of 0.001-2.5% by weight. The total content of these elements is restricted to the respectively stated upper limit for reasons of cost.
If the steels described in EP 1 431 407 A1 have strengths of more than 850 MPa, they no longer have a dual-phase structure, but their structure either consists only of martensite or only of ferrite and respectively bainite. Furthermore, EP 1 431 407 A1 does not provide an example by which, for example the effects of Cr, Mo, Ti or B could be reproduced at the same time with small amounts of Si or relatively high contents of Mn. Instead, the examples given in EP 1 431 407 A1 prove that according to this prior art, the strength has been substantially adjusted by an appropriate coordination of the Mn and Si contents with the respective steel alloy.
A further possibility of producing flat products which consist of relatively high-strength dual-phase steels and which still have good mechanical-technological characteristics even after undergoing an annealing process with the inclusion of an overaging treatment is disclosed in EP 1 200 635 A1. In the process known from this document, a steel strip or sheet is produced which has a predominantly ferritic-martensitic structure in which the martensitic proportion is from 4 to 20%, the steel strip or sheet containing, in addition to Fe and melt-induced impurities (in % by weight) 0.05-0.2% C, up to 1.0% Si, up to 2.0% Mn, up to 0.1% P, up to 0.015% S, 0.02-0.4% Al, up to 0.005% N, 0.25-1.0% Cr, 0.002-0.01% B. The martensitic proportion of the respective steel preferably amounts to approximately 5 to 20% of the predominantly martensitic-ferritic structure. A flat product produced in this manner has strengths of at least 500 N/mm2 with a simultaneously good forming ability without requiring, for this purpose, particularly high contents of specific alloying elements.
To increase the strength, the transformation-influencing effect of the element boron is drawn on in the case of the steel described in EP 1 200 635 A1. In the known steel, the strength-increasing effect of boron is ensured in that at least one alternative nitride former, preferably Al and additionally Ti is added to the steel material. The effect of adding titanium and aluminium is to bind the nitrogen contained in the steel, such that boron is available to form hardness-increasing carbides. Supported by the necessarily present Cr content, a higher strength level is achieved in this manner compared to comparable steels. However, the maximum strength of the steels stated by way of example in EP 1 200 635 is less than 900 MPa in each case.
Finally, EP 1 559 797 A1 discloses a relatively high-strength dual-phase steel which has a structure comprising more than 60% ferrite and from 5-30% martensite and which contains, in addition to iron and unavoidable impurities (in % by weight) 0.05-0.15% C, up to 0.5% Si, 1-2% Mn, 0.01-0.1% Al, up to 0.009% P, up to 0.01% S and up to 0.005% N. In order to further increase the strength of this known steel, it is possible to add thereto 0.01-0.3% Mo, 0.001-0.05% Nb, 0.001-0.1% Ti, 0.0003-0.002% B, and 0.05-0.49% Cr. The known steel alloyed and obtained in this manner achieves tensile strengths of up to 700 MPa with a good deformability and surface finish. The objective of the development described in EP 1 559 797 A1 was an improvement in the mechanical characteristics of a steel of this type while avoiding an alloying of relatively large amounts of alloying elements, such as Si, P and Al which are critical in respect of surface finish, weldability and deformability.
Against the background of the prior art described above, the object of the invention was to develop a steel and a flat product produced therefrom which has a strength of at least 950 MPa and a good deformability. Furthermore, the steel should have a surface finish which, when using a simple production process, enables a flat product produced from this steel to be deformed in an uncoated state or in a state provided with an anti-corrosion coating, into a complexly formed component, such as a part of a car bodywork. In addition, a process is also to be provided which makes it easily possible to produce flat products obtained in the manner described above.
With respect to the material, this object is achieved according to the invention by the dual-phase steel stated in claim 1. Advantageous embodiments of this steel are set out in the claims referring to claim 1.
A flat product which achieves the aforementioned object is characterised according to the invention in accordance with claim 20 in that it consists of a steel which is composed and obtained according to the invention.
Finally, with respect to the production process, the aforementioned object is achieved according to the invention by the production methods stated in claims 26 and 27, the process stated in claim 26 relating to the production according to the invention of a hot strip and the procedural method stated in claim 27 relating to the production according to the invention of a cold strip. The claims referring to claims 26 and 27 each contain advantageous variants of the processes according to the invention. In addition, particularly advantageous embodiments are described below for the practical application of the processes according to the invention and of the variants thereof stated in the claims.
A steel according to the invention is characterised by high strengths of at least 950 MPa, in particular more than 980 MPa, while strengths of 1000 MPa and above are also routinely achieved. This steel simultaneously has a yield strength of at least 580 MPa, in particular at least 600 MPa, and has an elongation A80 of at least 10%.
Due to the combination of high strength and good deformability, the steel according to the invention is particularly suitable for the production of complexly formed components which are heavily stressed in practical use, as required for example in the field of car body construction.
Due to its dual-phase structure, the steel according to the invention has a high strength with a simultaneously good elongation. Thus, the alloy of a steel according to the invention is composed such that it has a martensitic proportion of at least 20%, preferably more than 30%, up to a maximum of 70%. At the same time, retained austenite portions of up to 8% can be advantageous, while smaller retained austenite proportions of at most 7% or less are generally preferred. The remainder of the structure of a dual-phase steel according to the invention consists respectively of ferrite and/or bainite (bainitic ferrite+carbides).
The high strengths and good elongation characteristics are achieved by the adjustment according to the invention of the dual-phase structure. This is enabled by a narrow choice of the contents of the individual alloying elements which are present in a steel according to the invention in addition to iron and unavoidable impurities.
Thus, the invention provides a C content of from 0.10-0.20% by weight. The minimum content of carbon of 0.10% by weight is selected in order to obtain the formation of the martensitic structure with sufficient hardness and to adjust the desired combination of characteristics of the steel according to the invention. However, where there are contents of more than 0.20% by weight, carbon hinders the formation of the desired ferritic/bainitic structural portion. Higher contents of carbon also have a negative effect on the welding suitability, which is particularly significant for the application of the material according to the invention in the field of automotive engineering, for example. The advantageous effect of carbon in a steel according to the invention can be used in a particularly reliable manner when the carbon content of a steel according to the invention is from 0.12 to 0.18% by weight, in particular from 0.15 to 0.16% by weight.
Si also serves in a steel according to the invention to increase the strength by hardening the ferrite or bainite. In order to be able to use this effect, a minimum Si content of 0.10% by weight is provided, the effect of Si emerging in a particularly reliable manner when the Si content of a steel according to the invention is at least 0.2% by weight, in particular at least 0.25% by weight. In respect of the fact that a flat product produced from a steel according to the invention is to have a surface finish which is optimum for further processing and, if necessary, for applied coatings, the upper limit of the Si content is simultaneously set at 0.6% by weight. The risk of grain boundary oxidation is also minimised when this upper limit is observed. An unfavourable influence of Si on the characteristics of the steel according to the invention can be avoided with even greater reliability by restricting the Si content of the steel according to the invention to 0.4% by weight, in particular to 0.35% by weight.
The Mn content of a steel according to the invention is within a range of from 1.5 to 2.50% by weight, in particular from 1.5 to 2.35% by weight in order to use the strength-increasing effect of this element. Thus, the presence of Mn promotes the formation of martensite. If a cold strip is produced from the steel according to the invention and said cold strip is annealed at the end of processing, the contents of Mn provided according to the invention prevent the formation of pearlite during cooling after annealing. These positive effects due to the presence of Mn in a steel according to the invention can be used in a particularly reliable manner when the Mn content is at least 1.7% by weight, in particular at least 1.80% by weight. However, in order to avoid a negative influence of Mn on the deformability, welding suitability and coatability, the upper limit for the contents of Mn is set at 2.5% by weight in the steel according to the invention. The possibly negative influences of Mn on a steel according to the invention can be ruled out with greater reliability by restricting the Mn content to 2.20% by weight, in particular 2.00% by weight.
Cr also has a strength-increasing effect in a dual-phase steel according to the invention in contents of from 0.2 to 0.8% by weight. This effect appears in particular when the Cr content is at least 0.3% by weight, in particular at least 0.5% by weight. However, the Cr content of a steel according to the invention is restricted at the same time to 0.8% by weight to reduce the risk of grain boundary oxidation and to ensure good elongation characteristics of the steel according to the invention. Furthermore, when this upper limit is observed, a surface is achieved which can be effectively provided with a metallic coating. Negative influences of the Cr contents are avoided in particular when the upper limit of the chromium content of a steel according to the invention is set at a maximum of 0.7% by weight, in particular at 0.6% by weight.
The presence of titanium in contents of at least 0.02% by weight also contributes to the increase in the strength of a steel according to the invention in that it forms fine deposits of TiC or Ti (C,N) and contributes to the grain refining. A further positive effect of Ti is the binding of nitrogen which may be present, thereby preventing the formation of boron nitrides in the steel according to the invention. These would have a strong negative influence on the elongation characteristics and also on the deformability of a flat product according to the invention. Thus, when boron is added to increase the strength, the presence of Ti also ensures that the boron can fully develop its effect. For this purpose, it can be favourable for Ti to be added in a quantity which is more than 5.1 times the respective N content (i.e. Ti content>1.5 (3.4×N content)). Excessively high Ti contents result, however, in unfavourably high recrystallisation temperatures, which has a particularly negative effect when cold-rolled flat products are produced from steel according to the invention which are annealed in the final processing stage. For this reason, the upper limit of the Ti content is restricted to 0.08% by weight, in particular to 0.06% by weight. The positive effect of Ti can be used in a particularly reliable manner on the characteristics of a steel according to the invention when its Ti content is from 0.03 to 0.055% by weight, in particular from 0.040 to 0.050% by weight.
The strength of the steel according to the invention is also increased by the contents of B of up to 0.002% by weight, which are optionally provided according to the invention and, as by the respective addition of Mn, Cr and Mo, when cold strip is produced from steel according to the invention, the critical cooling rate is reduced after annealing. For this reason, according to a particularly practice-oriented embodiment of the invention, the B content is at least 0.0005% by weight. However, at the same time excessively high contents of B can reduce the deformability of the steel according to the invention and adversely affect the development of the dual-phase structure which is desired according to the invention. Therefore, optimised effects of boron are provided in a steel according to the invention with contents of 0.0007-0.0016% by weight, in particular 0.0008-0.0013% by weight.
Like Boron or Cr in the aforementioned content ranges, the contents of molybdenum which are optionally present according to the invention also contribute to increasing the strength of a steel according to the invention. In this respect, according to experience, the presence of Mo does not have a negative effect on the coatability of the flat product with a metallic coating or on its extensibility. Practical tests have shown that the positive influences of Mo can be used particularly effectively up to contents of 0.25% by weight, in particular 0.22% by weight, also from a financial point of view. Thus, even contents of 0.05% by weight of Mo have a positive effect on the characteristics of the steel according to the invention. Where there are sufficient quantities of other strength-increasing elements, the desired effect of molybdenum in a steel according to the invention emerges in particular when its Mo content is from 0.065 to 0.18% by weight, in particular from 0.08 to 0.13% by weight. However, if the steel according to the invention contains less than 1.7% by weight of Mo and/or less than 0.4% by weight of Cr, it is advantageous to add from 0.05 to 0.22% by weight of Mo to ensure the required strength of the steel according to the invention.
When a steel according to the invention is melted, aluminium is used for deoxidisation and for binding nitrogen which may be contained in the steel. For this purpose, Al can be added if necessary in contents of less than 0.1% by weight to the steel according to the invention, the desired effect of Al ensuing in a particularly reliable manner when the contents thereof are within a range of from 0.01 to 0.06% by weight, in particular from 0.020 to 0.050% by weight.
Nitrogen is permitted in the steel according to the invention only in contents of up to 0.012% by weight particularly to avoid the formation of boron nitrides when B is simultaneously present. To reliably prevent the respectively present titanium from bonding completely with N and no longer being effective as a micro-alloying element, the N content is preferably restricted to 0.007% by weight.
Low contents of P which are below the upper limit provided according to the invention contribute to the good welding ability of the steel according to the invention. Therefore, according to the invention, the P content is preferably restricted to <0.1% by weight, in particular to <0.02% by weight, particularly good results being obtained with P contents of <0.010% by weight.
Where there are contents of sulphur below the upper limit provided according to the invention, the formation of MnS or (Mn,Fe) S is suppressed, thereby ensuring a good extensibility of the steel according to the invention and of the flat products produced therefrom. This is particularly the case when the S content is below 0.003% by weight.
In a manner according to the invention, flat products consisting of a dual-phase steel according to the invention can be delivered directly, i.e. without a subsequently performed cold rolling process, for further processing as a hot strip obtained after hot rolling. Thus, highly stress-resistant components in an uncoated state can be formed from the hot strip obtained according to the invention. If these components are to be protected in particular against corrosion, the hot strips can be provided with a protective metallic coating before or after they are formed into the respective component.
If, on the other hand, flat products of a relatively low thickness are required, the hot strips produced from the steel according to the invention can firstly undergo cold rolling and subsequently annealing in order to then be further processed as a cold strip, optionally after the application of a metallic anti-corrosion coating.
If the flat product according to the invention is provided with a protective metallic coating, this can be performed, for example by hot-dip galvanising, by a galvannealing treatment or by electrolytic coating. If required, a pre-oxidation process can be carried out before coating, in order to ensure a reliable bonding of the metallic coating on the substrate to be respectively coated.
To produce according to the invention a flat product which is present as a hot strip and has a tensile strength of at least 950 MPa and a dual-phase structure consisting to 20 to 70% of martensite, up to 8% of retained austenite and for the remainder of ferrite and/or bainite, a dual-phase steel, composed according to the invention, is firstly melted, the melt is cast into a pre-product, such as a slab or thin slab, the pre-product is reheated to or kept at a hot rolling starting temperature of from 1100 to 1300° C., the pre-product is hot rolled into the hot strip at a hot rolling final temperature of from 800 to 950° C. and the resulting hot strip is reeled at a reeling temperature of up to 570° C.
By suitably adjusting the reeling temperature within a range of from room temperature to 570° C., the dual-phase structure of the hot strip which is not then rolled any further as such can be adjusted in order to obtain the respectively desired combination of characteristics.
If the hot strip, obtained in the manner according to the invention, is to remain uncoated or is to be electrolytically coated as a hot strip with a metallic coating, the flat product does not have to be annealed. If, on the other hand, the hot strip is to be coated with a metallic coating by hot-dip galvanisation, it is firstly annealed at a maximum annealing temperature of 600° C. and then cooled to the temperature of the coating bath, which can be, for example, a zinc bath. After passing through the zinc bath, the coated hot strip can be cooled to room temperature in a conventional manner.
If a flat product according to the invention is to be provided in the form of a cold strip, then for this purpose a dual-phase steel composed according to the invention is melted, the corresponding steel melt is cast into a pre-product, such as a slab or thin slab, the pre-product is reheated to or kept at a hot rolling starting temperature of from 1100 to 1300° C., the pre-product is hot rolled into a hot strip at a hot rolling final temperature of from 800 to 950° C., the hot strip is reeled at a reeling temperature of from 500 to 650° C., the hot strip is then cold rolled, the resulting cold strip is annealed at an annealing temperature of from 700 to 900° C. and thereafter the cold strip is cooled in a controlled manner.
Reeling temperatures ranging up to 580° C. have proved to be particularly advantageous in connection with the production of a cold strip, because if the reeling temperature of 580° C. is exceeded, the risk of grain boundary oxidation increases. With low reeling temperatures, the strength and yield strength of the hot strip increase such that it becomes increasingly difficult to cold roll the hot strip. Accordingly, the hot strip which is to be cold rolled into a cold strip is preferably reeled at a temperature of at least 530° C., in particular at least 550° C.
If the cold strip produced according to the invention is to remain uncoated or is to be coated electrolytically, an annealing treatment is carried out in a continuous annealing furnace as a separate working step. The maximum annealing temperatures which are achieved are within a range of from 700 to 900° C. at heating rates of from 1 to 50 K/s. Subsequently, for the intentional adjustment of the combination of characteristics desired according to the invention, the annealed cold strip is preferably cooled such that cooling rates of at least 10 K/s are achieved within a temperature range of from 550 to 650° C. in order to suppress the formation of pearlite. After reaching the temperature in this critical range, the strip can be kept for a period of 10 to 300 s or can be cooled directly to room temperature at a cooling rate of from 0.5 to 30 K/s.
However, if the cold strip is to be coated by hot-dip galvanisation, the annealing and coating steps can be combined. In this case, the cold strip passes in a continuous sequence through various furnace sections of a hot-dip coating line, different temperatures prevailing in the individual furnace sections and reaching a maximum of from 700 to 900° C., in which case heating rates ranging from 2 to 100 K/s should be selected. After the respective annealing temperature has been attained, the strip is then kept at this temperature for 10 to 200 s. The strip is then cooled to the temperature, usually below 500° C., of the respective coating bath which is typically a zinc bath, and in this case as well the cooling rate should be more than 10 K/s within a temperature range of from 550 to 650° C. After reaching this temperature stage, the cold strip can optionally be kept at the respective temperature for 10 to 300 s. The annealed cold strip then passes through the respective coating bath which is preferably a zinc bath. Subsequently, the cold strip is either cooled to room temperature in order to obtain a conventional hot-dip galvanised cold strip, or is rapidly heated, then cooled to room temperature to produce a galvannealed cold strip.
If the hot strip is cold rolled into a cold strip, it has proved to be favourable to adjust cold rolling degrees of from 40 to 70%, in particular from 50 to 60% to achieve sufficiently high strengths of the rolled strip with an optimum utilisation of the respectively available installation engineering. A cold strip according to the invention which is cold-rolled in this manner typically has thicknesses of from 0.8 to 2.5 mm.
If necessary, the cold strip can undergo a skin pass rolling in a coated or uncoated state, with the adjustment of skin pass rolling degrees ranging up to 2%.
The invention will be described in detail in the following with reference to practical examples.
Sixteen steel melts 1 to 16, the compositions of which are stated in Table 1 were melted in conventional manner and cast into slabs. The slabs were then reheated in a furnace to 1200° C. and hot rolled in conventional manner starting from this temperature. The final rolling temperature was 900° C.
For a first series of tests, the hot strips obtained thus were reeled at a reeling temperature of 550° C. which was adjusted with an accuracy of +/−30° C., before they were cold rolled with a cold rolling degree of 50%, 65% and 70% into a cold strip having a thickness of from 0.8 mm to 2 mm.
The cold strips which were obtained then underwent annealing and controlled cooling procedures in the manner described above in a general form for a cold strip which is to be delivered uncoated.
Table 2 states the structural state, the mechanical characteristics and the respectively adjusted degrees of cold rolling and the strip thicknesses for the cold strips produced in the first series of tests from melts 1 to 16.
In three further series of tests, the hot strips produced from melts 1 to 16 in the manner described above were reeled at a reeling temperature below 100° C., at a temperature of 500° C. and at a temperature of 650° C. The characteristics determined for these hot strips are stated in Table 3 (reeling temperature 20° C.), Table 4 (reeling temperature=500° C.) and Table 5 (reeling temperature=570° C.). The hot strips obtained thus were not intended for cold rolling, but were forwarded for further processing into components, optionally after being provided with a protective metallic coating.
Amounts in % by weight, remainder iron and unavoidable impurities
| Number | Date | Country | Kind |
|---|---|---|---|
| 07114398.6 | Aug 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/060381 | 8/7/2008 | WO | 00 | 7/7/2010 |
