Patent Application
20150203946
The invention relates to a hot-rolled flat steel product with the mathematical product of tensile strength Rm and elongation A80 being at least 18 000 MPa*%. Flat steel products of this type are distinguished by a very high strength in combination with good elongation properties, and are suitable as such in particular for the production of components for motor vehicle bodies.
The invention similarly relates to a method for producing a flat steel product according to the invention.
The term “flat steel product” is to be understood here as meaning steel sheets or steel strips produced by a rolling process and also sheet bars and the like separated therefrom.
Where alloy contents are stated here merely in “%”, this always means “% by weight”, unless expressly stated otherwise.
The product of tensile strength Rm and elongation A80 is technically also referred to as “quality”.
EP 1 466 024 B1 (DE 603 15 129 T2) discloses a method for producing a flat steel product which is intended to have tensile strengths of considerably more than 1000 MPa. In order to achieve this, a steel melt comprising (in % by weight) 0.0005-1% C, 0.5-10% Cu, up to 2% Mn, up to 5% Si, up to 0.5% Ti, up to 0.5% Nb, up to 5% Ni, up to 2% Al and as remainder iron and impurities which are unavoidable for production-related reasons is produced. The melt is cast to form a strip, the thickness of which is at most 10 mm and which is cooled rapidly to a temperature of at most 1000° C. by sprinkling with water or a water-air mixture. Then, the cast strip is hot-rolled with a reduction rate of at least 10%. The hot-rolling is ended at an end temperature at which all of the copper is still in a solid solution in the ferrite and/or austenite matrix. Then, the strip is subjected to a step of rapid cooling, in order to keep the copper in a supersaturated solid solution in the ferrite and/or austenite solution. The strip thus cooled is finally wound to form a coil. The copper precipitations bring about precipitation hardening, by virtue of which the desired strength level of the steel is to be achieved. At the same time, the copper content is intended to increase the corrosion and embrittlement resistance of the steel through the formation of a protective oxide layer.
A hot strip having a tensile strength of above 1200 MPa and an elongation of up to 10% and a method for the production thereof are known from US 2009/0107588 A1. This known hot strip consists of a steel which, in addition to iron and unavoidable impurities, comprises (in % by weight) 0.10-0.25% C, 1-3% Mn, more than 0.015% Al, up to 1.985% Si, up to 0.30% Mo, up to 1.5% Co and up to 0.005% B, where the following should apply: 1%≦% Si+% Al≦2% (% Al=respective Al content, % Si=respective Si content) and % Cr+(3×% Mo)≧0.3% (% Cr=respective Cr content, % Mo=respective Mo content). At the same time, the steel is to have a microstructure which consists to an extent of at least 75% of bainite, to an extent of at least 5% of residual austenite and to an extent of at least 2% of martensite. To produce the hot strip, a melt of corresponding composition is cast to form a primary or preliminary product, which is then heated to more than 1150° C. and then hot-rolled at a hot-rolling end temperature at which the steel is still entirely austenitic. The hot strip obtained is then cooled in three steps. In the first step, the cooling is effected proceeding from a temperature lying above the Ar3 temperature of the steel at a cooling rate of at least 70° C./s to a first intermediate temperature of above 650° C. Proceeding from this first intermediate temperature, cooling is then effected to a second intermediate temperature, which lies between the bainite start temperature, i.e. the temperature at which bainite begins to form in the steel, and a lower limit temperature, which is 50° C. higher than the martensite start temperature, i.e. the temperature from which martensite forms in the steel. The cooling rate in this second cooling step is 20-90° C./s. This is followed by a third cooling step, in which the hot strip is cooled to room temperature. The temperature from which this third cooling step proceeds is determined here depending on the respective cooling rate.
Another method for producing a high-strength and readily deformable hot strip which is likewise based on the strength-increasing action of Cu precipitations is described in U.S. Pat. No. 6,190,469 B1. In this method, a steel comprising (in % by weight) 0.15-0.3% C, 1.5-2.5% Si, 0.6-1.8% Mn, 0.02-0.10% Al, 0.6-2.0% Cu, 0.6-2.0% Ni and as remainder iron and unavoidable impurities is cast to form slabs. The slabs are rolled to form hot strip, the hot-rolling end temperature being 750-880° C. The hot strip obtained is then cooled by means of water, proceeding from a start temperature of 680-740° C., to a coiling temperature which is at least the same as the temperature calculated on the basis of the formula 240×(% Mn+% Ni)−140 (where % Mn=respective Mn content, % Ni=respective Ni content) and not higher than 540° C. Then, the hot strip cooled to the coiling temperature is wound to form a coil. The hot strip obtained has a microstructure which, in addition to ferrite, comprises 5-20% residual austenite and 20-50% bainite, the microstructure containing copper precipitations which, through precipitation hardening, contribute to the strength of the hot strip obtained. The hot strip produced and provided in this way has an elongation of up to 23% combined with strengths lying in the region of 1000 MPa, and therefore as a whole high quality values of more than 20 000 MPa*% are achieved.
Against the background of the prior art explained above, it was an object of the invention to provide a hot-rolled flat steel product which can be produced in a simple and operationally reliable manner and has an optimized combination of a particularly high strength and good deformability. In addition, the intention was to provide a method for producing such a flat steel product.
In relation to the hot strip, this object has been achieved according to the invention by the hot-rolled flat steel product indicated in claim 1.
In relation to the method, the object mentioned above is achieved according to the invention in that at least the working steps indicated in claim 8 are performed to produce a hot-rolled flat steel product according to the invention.
Advantageous configurations of the invention are indicated in the dependent claims and will be explained in detail hereinbelow as the general concept of the invention.
The hot-rolled flat steel product according to the invention is distinguished by the fact that it contains, in addition to iron and unavoidable impurities (in % by weight):
A flat steel product according to the invention has a microstructure dominated by two phases, one dominating constituent of the microstructure being bainite and the second dominating constituent of the microstructure being residual austenite. In addition to these two main components, small proportions of martensite and ferrite may be present, but the contents thereof are too small to have an influence on the properties of the hot-rolled flat steel product. Accordingly, the microstructure of the flat steel product according to the invention consists of bainite to an extent of at least 50% by volume, in particular at least 60% by volume, and of residual austenite as the remainder in addition to optionally present proportions of up to 5% by volume ferrite and up to 10% by volume martensite, wherein at least part of the residual austenite is present in block form and at least 98% of the blocks of the residual austenite present in block form have a mean diameter of less than 5 μm.
The method according to the invention for producing a flat steel product provided according to the invention includes the following work steps:
The invention is based on the knowledge that it is beneficial to the required properties of the hot-rolled flat steel product if the residual austenite is present in block form, as long as the diameter of the residual austenite blocks does not exceed 5 μm. It has been assumed to date in the prior art that residual austenite present in block form is to be avoided in principle, since residual austenite in block form has been interpreted to be a cause of instabilities of the microstructure and an associated tendency toward the formation of undesirable martensite. Accordingly, to date the highest possible proportions of film-like residual austenite have always been sought in the prior art in the microstructure of a steel of the type in question here (see H. K. D. H. Bhadeshia and D. V. Edmonds “Bainite in silicon steels: new composition-property approach” published in Metal Science Vol. 17, September 1983, pages 411-419 (“Part 1”) and pages 420-425 (“Part 2”)).
Reference is made to “block-like” residual austenite in this context when the ratio of length/width, i.e. longest extent/thickness, of the microstructure constituents of residual austenite present in the microstructure is 1 to 5. By contrast, residual austenite is referred to as “film-like” when the ratio of length/width of the residual austenite accumulations present in the microstructure is greater than 5 and the width of the respective microstructure constituents of residual austenite is smaller than 1 μm. Film-like residual austenite is accordingly typically present as finely distributed lamellae.
The outlay which is still required according to the prior art to avoid residual austenite present in block form can therefore be avoided in the production of a flat steel product according to the invention by keeping the residual austenite blocks present in the microstructure of the obtained flat steel product according to the invention small, i.e. the extent thereof as expressed by their mean diameter is limited to less than 5 μm. It has surprisingly been found in this respect that residual austenite present in block form and having a diameter of smaller than 5 μm has a positive effect on the elongation properties of a steel of the type provided according to the invention. The residual austenite blocks present in this size prove to be more stable than block-like residual austenite present in coarser form. At the same time, they are not as stable as residual austenite present in film-like form and therefore enable the TRIP effect. The positive influence of residual austenite present in block form can be utilized particularly reliably when the extent of the block residual austenite measures at most 4 μm, in particular at most 3 μm. In this respect, it has been found in practice that, in flat steel products with a composition according to the invention and produced according to the invention, the maximum extent of the residual austenite present in block form regularly lies in the range of 1-3 μm, the maximum extent of the residual austenite blocks typically being limited on average to 2 μm. Complex, multi-step temperature control during the production of the flat steel product is surprisingly not required for this purpose.
Accordingly, a hot-rolled flat steel product according to the invention can be produced without any special expenditure while at the same time observing the parameters predefined according to the invention for the production method. In particular, complex cooling strategies or cooling strategies which require a high cooling power, as have still been deemed unavoidable in the prior art, are no longer required.
The positive influence of the residual austenite contents in the microstructure of a flat steel product according to the invention arises particularly reliably when the residual austenite content is at least 10% by volume, with beneficial effects to be expected with particular reliability given residual austenite contents of at least 15% by volume.
Hot-rolled flat steel products which are produced according to the invention regularly achieve tensile strengths Rm of more than 1000 MPa, in particular at least 1200 MPa, with elongations A80 which similarly regularly lie above 17%, in particular above 19%. Accordingly, the quality Rm*A80 of hot strips according to the invention is regularly in the range of 18 000-30 000 MPa*%. In particular, it is regularly at least 20 000 MPa*%. A flat steel product according to the invention as such has an optimum combination of extreme strength and good deformability.
The strength-increasing action of copper can also be utilized in a hot-rolled flat steel product according to the invention. In this respect, a minimum Cu content of 0.15% by weight can be present in the hot-rolled flat steel product according to the invention.
In the steel according to the invention, carbon delays the transformation to ferrite/pearlite, reduces the martensite start temperature MS and contributes to an increase in the hardness. In order to utilize these positive effects, the C content of the flat steel product according to the invention can be set at at least 0.3% by weight.
In the steel processed according to the invention, Mn in contents of up to 2.5% by weight, in particular up to 2.0% by weight, promotes the bainite formation, the Cu, Cr and Ni contents which are optionally additionally present likewise contributing to the formation of bainite. Depending on the respective other constituents of the steel processed according to the invention, it can be expedient here to limit the Mn content to at most 1.6% by weight.
In addition, the optional addition of Cr can also lower the martensite start temperature and suppress the tendency of the bainite to transform into pearlite or cementite. Moreover, in contents up to the upper limit of at most 2% by weight as predefined according to the invention, Cr promotes the ferritic transformation, with optional effects of the presence of Cr in a flat steel product according to the invention arising when the Cr content is limited to 1.5% by weight.
The optional addition of Ti, V or Nb can suppress the formation of a finer-grained microstructure and promote the ferritic transformation. In addition, these microalloying elements contribute to an increase in the hardness through the formation of precipitations. The positive effects of Ti, V and Nb can be utilized in a particularly effective manner in the flat steel product according to the invention when the content of each of these elements lies in the range of 0.002-0.15% by weight, in particular does not exceed 0.14% by weight.
Through the presence of Si and Al, the carbide formation in the bainite can be suppressed and, associated therewith, the residual austenite can be stabilized by dissolved carbon. In addition, primarily Si contributes to the solid solution solidification. In the steel processed according to the invention, Al can partly replace the Si content. For this purpose, a minimum Al content of 0.4% by weight can be provided. This applies in particular when the addition of Al is intended to set the hardness or tensile strength of the steel to a relatively low value in favour of improved deformability.
The positive influences of the simultaneous presence of Al and Si can be utilized particularly effectively when the Si and Al contents within the limits predefined according to the invention satisfy the condition % Si+0.8% Al>1.2% by weight or even the condition % Si+0.8% Al>1.5% by weight (where % Si: respective Si content in % by weight, % Al: respective Al content in % by weight).
The formation of the microstructure according to the invention can be ensured in particular by virtue of the fact that the Mn, Cr, Ni, Cu and C contents of the steel processed according to the invention and accordingly the Mn, Cr, Ni, Cu and C contents of the flat steel product according to the invention satisfy the following condition
1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2
where % Mn denotes the respective Mn content in % by weight, % Cr denotes the respective Cr content in % by weight, % Ni denotes the respective Ni content in % by weight, % Cu denotes the respective Cu content in % by weight and % C denotes the respective C content in % by weight.
To produce a flat steel product according to the invention, the preliminary product cast from a steel having a composition according to the invention is firstly brought to a temperature or held at a temperature which is sufficient to end the hot-rolling carried out proceeding from this temperature at a hot-rolling end temperature at which the hot strip obtained has a completely recrystallized, austenitic microstructure affording optimum preconditions for the bainite formation. This is the case when the hot strip obtained has a hot-rolling end temperature of at least 880° C. when it leaves the last rolling pass, it being possible for the method according to the invention to be executed with a particularly high level of operational reliability if the hot-rolling end temperature is set to at least 900° C. and does not exceed 1100° C., in particular 1050° C. To this end, it is typically the case that the preliminary product is heated to a temperature lying in the range of 1100-1300° C. before the hot-rolling. If the hot-rolling end temperature falls below 900° C., widespread softening of the austenite can be achieved by virtue of the fact that the main deformation of the hot strip takes place in the last hot-rolling passes. The hot strip thus obtained likewise has a microstructure with residual austenite proportions which satisfy the specifications according to the invention.
Subsequent to the hot-rolling, the hot strip is subjected to accelerated cooling with a cooling rate of at least 5° C./s to a coiling temperature lying in the range of 350-600° C. The cooling is optimally started here when 50-60% of the austenite has softened. In practice, a pause of for instance up to 2 s is provided for this purpose between the end of the hot-rolling and the start of the cooling. The minimum pause duration tp can be calculated by means of the following empirical formula:
tp=5·10+36·T−12.5,
where tp is the pause duration after the last deformation in seconds and T is the temperature in ° C. The formula gives the minimum time after which 50-60% of softened austenite is present. Pause durations calculated therefrom are:
Cooling to the coiling temperature is effected here in such a manner that no transformation of the austenite occurs up until the coiling. This has the effect that the bainite formation takes place over a sufficiently long time exclusively in the coil. Once the hot strip cooled in the manner described above has been wound to form a coil, for this purpose this coil is cooled in a temperature range having an upper limit which is the same as the temperature from which bainite forms from the austenite and having a lower limit which lies above the temperature from which martensite forms in the microstructure of the hot strip. The period of time for which the coil is held in this temperature range is in this respect chosen in such a way that the bainite proportion of at least 60% by volume as is desired according to the invention is achieved. In practice, a period of time of at least 0.5 h is regularly sufficient for this purpose, with higher bainite contents being set given a longer period of time.
Practical investigations have shown that a microstructural transformation between the end of the hot-rolling and the coiling can be avoided particularly reliably when the cooling rate is at least 10° C./s, with practical cooling rates lying in the range of up to 150° C./s, in particular being 10-50° C./s.
The formation of undesired martensite can be avoided particularly reliably by virtue of the fact that the lower limit of the coiling temperature is higher than the martensite start temperature by at least 10° C., in particular at least 20° C.
At the same time, the desired profile of the bainite formation can be ensured in practice by virtue of the fact that the upper limit of the coiling temperature is set at 550° C.
An optimum profile of the bainite formation taking place according to the invention in the coil arises when the coiling temperature at least corresponds to the temperature HTopt determined by the following formula:
HTMin=MS+(BS−MS)/3
Here, it is self-evident that the observance of this temperature is always subject to a certain tolerance under the operating conditions, i.e. this temperature is generally not satisfied exactly but instead is observed with a tolerance of typically +/−20° C.
The invention will be explained in more detail hereinbelow on the basis of exemplary embodiments.
Seven steels S1-S7 were melted, the composition thereof being shown in Table 1.
The steel melts of corresponding composition were cast in a conventional manner to form slabs and then heated similarly in a conventional manner to a reheating temperature OT.
The heated slabs were hot-rolled in a similarly conventional group of hot-rolling stands to form hot strips W1-W10 having a thickness of 2.0 mm.
The hot strips W1-W10 emerging from the group of hot-rolling stands were each at a hot-rolling end temperature ET, proceeding from which they were subjected to accelerated cooling at a cooling rate KR to a coiling temperature HT. The hot strips W1-W10 were wound to form coils at this coiling temperature HT.
The coils were then each cooled in a temperature range having an upper limit determined by the respective coiling temperature HT and a lower limit determined by the martensite start temperature MS determined for the respective steel S1-S7. The martensite start temperature MS here was calculated by the procedure explained in the article “Thermodynamic Extrapolation and Martensite-Start-Temperature of Substitutionally Alloyed Steels” by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180.
The period of time for which the coil was cooled in the temperature range defined in the manner described above was of such a magnitude that the hot strips thus obtained each had a microstructure consisting of bainite and residual austenite, in which the proportions of other microstructure constituents were present at most in ineffective quantities of virtually “0”.
The respective operating parameters of reheating temperature OT, hot-rolling end temperature ET, cooling rate KR, coiling temperature HT and martensite start temperature MS are indicated in Table 2.
Table 3 additionally shows the mechanical properties ascertained for the individual hot strips of tensile strength Rm, yield strength Rp, elongation A80, quality Rm*A80 and also the respective residual austenite content RA.
It was found that the tensile strength of at least 1200 MPa as desired here was not achieved in the case of the hot strip W3, which was produced from the steel S3 and had a comparatively low Si content.
In the case of the hot strip W5, which consisted of the steel S4 and was not produced according to the invention owing to the excessively low hot-rolling end temperature ET, up to 12% by volume of block-like, coarse residual austenite and also coarse martensite were present in the microstructure, which led to a considerably impaired elongation A80.
By contrast, the hot strip W4, which was likewise produced from the steel S4 but in a manner observing the specifications according to the invention, merely comprised up to 1% by volume of coarse block residual austenite with a mean extent of more than 5 μm. The remaining residual austenite was present in film-like and in finer-block form, with the result that a high elongation A80 was achieved.
In the case of the hot strip W7 produced from the steel S5 and in the case of the hot strip W10 produced from the steel S7, the minimum tensile strength of 1200 MPa as desired here was likewise not achieved. The reason in these cases lay in the respectively excessively high coiling temperature HT.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12178330.2 | Jul 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/065836 | 7/26/2013 | WO | 00 |
