FLAT STEEL PRODUCT AND METHOD FOR THE PRODUCTION THEREOF

A flat steel product may have a tensile strength Rm≥950 MPa, a yield point ≥800 MPa, and an elongation at break A50≥8%. The flat steel product may comprise steel consisting of (in percent weight) 0.05%-0.20% C, 0.2%-1.5% Si, 0.01%-1.5% Al, 1.0%-3.0% Mn, ≤0.02% P, ≤0.005% S, ≤0.008% N, and in each case optionally 0.05%-1.0%, 0.05%-0.2% Mo, 0.005%-0.2% Ti, 0.001%-0.05% Nb, 0.0001%-0.005% B, the remainder being Fe and unavoidable impurities. A ratio ψ may conform to 1.5≤ψ≤3 with ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si), and % C, % Mn, % Cr, % Al, % Si being the respective C, Mn, Cr, Al, and Si contents of the steel. The flat steel product may have a microstructure consisting of (in percent area)≤5% bainite, ≤5% polygonal ferrite, ≥90% martensite, and ≤2% by volume of residual austenite, where at least half the martensite is annealed martensite.

Skip to: Description  ·  Claims  · Patent History  ·  Patent History
Description
CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Entry of International Patent Application Serial Number PCT/EP2016/059960, filed May 4, 2016, which claims priority to International Patent Application Serial Number PCT/EP2015/059968 filed May 6, 2015, the entire contents of both of which are incorporated herein by reference.

FIELD

The present disclosure generally relates to flat steel products with optimized strength and elongation characteristics, including methods for producing such flat steel products.

BACKGROUND

Where reference is made here to flat steel products, this means steel strips, sheets or sheet metal blanks obtained therefrom, such as sheet bars.

Unless explicitly stated otherwise, in the present text and the claims, the contents of particular alloy elements are each reported in % by weight and the proportions of particular microstructure constituents in area.

CA 2 734 976 A1 (WO 2010/029983 A1) discloses a steel having good ductility and formability, which is to have a tensile strength of at least 980 MPa. For this purpose, the steel comprises, as well as iron and unavoidable impurities (in % by weight), 0.17%-0.73% C, up to 3.0% Si, 0.5%-3.0% Mn, up to 0.1% P, up to 0.07% S, up to 3.0% Al and up to 0.010% N. The sum total of the Al and Si contents is to be at least 0.7%. At the same time, in each case in relation to the totality of all microstructure constituents, the martensite content in the steel microstructure is to be 10%-90%, the proportion of residual austenite within the range of 5%-50%, and the proportion of ferritic bainite originating from “upper bainite” at least 5%. “Upper bainite” refers here to a bainite in which fine carbide grains are distributed homogeneously, whereas these are not to be found in “lower bainite”. Higher contents of upper bainite of 17% or more are regarded as advantageous in order to generate the desired high residual austenite contents in the microstructure.

EP 2 524 970 A1 additionally discloses a flat steel product having a tensile strength Rm of at least 1200 MPa and consisting of a steel which, as well as Fe and unavoidable impurities, contains (in % by weight) C: 0.10%-0.50%, Si: 0.1%-2.5%, Mn: 1.0%-3.5%, Al: up to 2.5%, P: up to 0.020%, S: up to 0.003%, N: up to 0.02%, and optionally one or more of the elements “Cr, Mo, V, Ti, Nb, B and Ca” in the following contents: Cr: 0.1%-0.5%, Mo: 0.1%-0.3%, V: 0.01%-0.1%, Ti: 0.001%-0.15%, Nb: 0.02%-0.05%. The sum total Σ(V,Ti,Nb) of the contents V, Ti and Nb here is subject to the following criterion: Σ(V,Ti,Nb)≤0.2%, B: 0.0005%-0.005%, Ca: up to 0.01%. At the same time, the flat steel product has a microstructure having (in area %) less than 5% ferrite, less than 10% bainite, 5%-70% unannealed martensite, 5%-30% residual austenite and 25%-80% annealed martensite, with at least 99% of the iron carbides present in the annealed martensite having a size of less than 500 nm. Owing to its minimized proportion of overannealed martensite, a flat steel product having such characteristics has optimized formability.

EP 2 524 970 A1 likewise discloses a process for producing a flat steel product of the type elucidated above. In this process, first of all, a flat steel product having the aforementioned composition is heated at a heating rate θH1, θH2 of at least 3° C./s to an austenitization temperature THZ above the A3 temperature of the steel of the flat steel product and of not more than 960° C. The flat steel product is kept at that temperature for an austenitization period tHZ of 20-180 s, in order then to be cooled to a cooling finish temperature. The latter is greater than the martensite finish temperature and less than the martensite start temperature, the cooling being effected at a cooling rate at least equal to a minimum cooling rate determined as a function of the alloy contents of the steel. Then the flat steel product is kept at the cooling finish temperature for 10-60 s, in order then to be heated at a heating rate of 2-80° C./s to a partitioning temperature of 400-500° C. This may be followed by an isothermal hold of the flat steel product at the partitioning temperature over up to 500 s. Subsequently, the flat steel product is cooled down at a cooling rate of 3-25° C./s.

In the known process elucidated above, the heating and the optional additional holding at the partitioning temperature result in enrichment of the residual austenite in the microstructure of the flat steel product with carbon from the oversaturated martensite. This operation also is referred to in the art as “partitioning of the carbon” or “partitioning”. The partitioning can be conducted as early as during the heating, as what is called “ramped partitioning”, by means of holding at the partitioning temperature after the heating (called “isothermal partitioning”), or by means of a combination of isothermal and ramped partitioning. The slower heating rate which is the aim in ramped partitioning as compared with isothermal partitioning permits particularly exact actuation of the partitioning temperature specified in each case with a reduced energy input.

The steels having the characteristics and having been processed as elucidated above are among what are called the “AHSS steels” (advanced high strength steels).

Modern variants of these steels and flat steel products produced therefrom have very high strength with simultaneously high elongation, and are therefore particularly suitable for the production of safety-relevant components of automobile bodywork which are to absorb deformation energy in the event of a crash. However, it is found in practice that high residual austenite contents in the microstructure of such steels can improve the uniaxial elongation thereof by virtue of the known TRIP effect, but that they are not reliably successful in achieving equally good formability in all directions, as indicated, for example, by good hole expanding characteristics.

DETAILED DESCRIPTION

Although certain example methods and apparatus have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents. Moreover, those having ordinary skill in the art will understand that reciting ‘a’ element or ‘an’ element in the appended claims does not restrict those claims to articles, apparatuses, systems, methods, or the like having only one of that element, even where other elements in the same claim or different claims are preceded by “at least one” or similar language. Similarly, it should be understood that the steps of any method claims need not necessarily be performed in the order in which they are recited, unless so required by the context of the claims. In addition, all references to one skilled in the art shall be understood to refer to one having ordinary skill in the art.

One example object of the present disclosure is to provide a flat steel product that has not just an optimized combination of high strength and elongation, but also, coupled with improved use properties such as good suitability for welding, surface characteristics and suitability for coating with a metallic protective coating, has a microstructure that assures optimized formability irrespective of the direction of forming.

A process for producing such a flat steel product was likewise to be specified.

Advantageous configurations of the invention are specified in the dependent claims and are elucidated in detail hereinafter, as is the general concept of the invention.

A flat steel product of the invention accordingly features a tensile strength Rm of at least 950 MPa, a yield point of at least 800 MPa and an elongation at break A50 determined according to DIN EN ISO 6892, sample shape 1, of at least 8%. A flat steel product of the invention consists here of a steel consisting of, as well as iron and unavoidable impurities, (in % by weight)

    • C: 0.05%-0.20%,
    • Si: 0.2%-1.5%,
    • Al: 0.01%-1.5%,
    • Mn: 1.0%-3.0%,
    • P: up to 0.02%,
    • S: up to 0.005%,
    • N: up to 0.008%,
    • and optionally one or more of the elements from the group of “Cr, Mo, Ti, Nb, B” in the following contents:
      • Cr: 0.05%-1.0%,
      • Mo: 0.05%-0.2%,
      • Ti: 0.005%-0.2%,
      • Nb: 0.001%-0.05%,
      • B: 0.0001%-0.005%,
    • where the ratio


ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si)

    • with % C: respective C content of the steel
      • % Mn: respective Mn content of the steel
      • % Cr: respective Cr content of the steel
      • % Al: respective Al content of the steel
      • % Si: respective Si content of the steel
    • conforms to the following criterion:


1.5≤ψ≤3

    • and wherein the flat steel product has a microstructure consisting of
      • not more than 5 area % of bainite,
      • not more than 5 area % of polygonal ferrite,
      • not more than 2% by volume of residual austenite,
    • and
      • not less than 90 area % of martensite, wherein at least half the martensite is annealed martensite.

The invention is based on the finding that, through the choice of a suitable alloy, it is possible to obtain a flat steel product in which a microstructure comprising minimum residual austenite contents at most and characterized by a high content of annealed martensite and by ultrafinely distributed unannealed martensite results in a high strength coupled with very good deformability.

Typical tensile strengths Rm of flat steel products of the invention are 950-1300 MPa, coupled with a yield point which is at least 800 MPa and can reach as far as the respective tensile strength. The elongation A50 of flat steel products of the invention is typically 8%-20%. At the same time, a flat steel product of the invention, in the hole expanding test according to ISO 16630, regularly achieves hole expansion ratios of at least 30%.

These combinations of properties are accomplished in accordance with the invention through the exactly judged addition of inexpensive alloy constituents. These are matched to one another such that the desired mechanical properties are reliably achieved and the flat steel product obtained simultaneously exhibits good weld- and coatability.

Of essential significance here is the establishment of a suitable ratio between the elements that affect austenite formation and the hardenability of the steel, and the elements that suppress carbide formation. This ratio in the case of an alloy according to the invention is adjusted via the factor ψ, which is affected by the respective C, Mn, Cr, Al and Si contents of the steel. The factor ψ is not to be less than 1.5. Excessively high contents of silicon or aluminum would have an adverse effect on the coatability (silicon) or the castability (aluminum) of the steel. In the case of inadequate contents of carbon, manganese or chromium, the required strength would not be achieved. Relatively high values for the factor ψ of at least 1.6 have been found to be advantageous for the establishment of a stable production process, and values for the factor ψ of at least 1.8 have been found to be particularly advantageous for production stability. Excessive carbon and manganese can lead to an elevated residual austenite content, which would in turn result in lower formability. This is avoided in that the upper limit for the range in which the ψ factor of a steel of the invention lies has been set to the value of 3.0.

Carbon has several important functions in the steel of the invention. Firstly, the C content plays a major role in the formation of austenite and adjustment of the A3 temperature. An adequate C content enables full austenitization even at temperatures of less than 930° C. In the subsequent quenching, the residual austenite is stabilized by carbon. This stabilization can be assisted by an additional heat treatment step as envisaged by the invention in the process of the invention. The strength of the martensite is also greatly affected by the C content of the steel. On the other hand, the martensite start temperature is shifted to ever lower temperatures with rising C content, which leads to challenges in the production. For these reasons, the invention envisages, in the steel of a flat steel product of the invention, a C content of 0.05%-0.2% by weight, especially at least 0.065% by weight of C, and in practice the positive effect of C in the steel of the invention can be exploited in a particularly reliable manner when the C content is 0.07%-0.19% by weight.

For the specific judgement of the particular C content in each case, within the limits envisaged in accordance with the invention, it is also possible to cite what is called the carbon equivalent “CE”, the value of which is influenced to a crucial degree by the C content. For calculation of the carbon equivalent CE, the American Welding Society has proposed the following formula:


CE=% C+(% Si+% Mn)/5+(% Cr+% Mo)/6

  • with % C: respective C content of the steel
    • % Si: respective Si content of the steel
    • % Mn: respective Mn content of the steel
    • % Cr: respective Cr content of the steel
    • % Mo: respective Mo content of the steel

According to the invention, the carbon equivalent CE should be not more than 1.1% by weight, in order to assure good weldability. Particularly good suitability for welding can be assured in that the CE value is limited to not more than 1.0% by weight. However, the CE value should not be less than 0.254% by weight and especially not less than 0.29% by weight, in order to obtain the effect of the alloy elements that affect the calculation of the carbon equivalent CE and are envisaged in accordance with the invention.

The presence of silicon in the steel of a flat steel product of the invention suppresses the formation of cementite, which would bind carbon that would then no longer be available for the stabilization of the residual austenite, and which would worsen the elongation. The same effect can also be achieved by including Al in the alloy. However, a minimum of 0.2% by weight of Si should be present in the steel envisaged in accordance with the invention. However, Si contents of more than 1.5% by weight would have an adverse effect on the surface quality of a flat steel product of the invention. Therefore, in a flat steel product of the invention, the Si content is 0.2%-1.5% by weight, and in practice Si contents of at least 0.25% by weight or at most 0.95% by weight have been found to be particularly favorable and those of at most 0.63% by weight to be very particularly favorable.

Aluminum is added to the steel of a flat steel product of the invention in steel production for deoxidation and for binding of any nitrogen present. Al can additionally also be used for the suppression of cementite. However, in the presence of higher contents of Al, there is also a rise in the austenitization temperature. Therefore, the Al content of a steel envisaged for a flat steel product of the invention is limited to 0.01%-1.5% by weight. If low austenitization temperatures are to be assured, it may be appropriate to limit the Al content to a maximum of 0.44% by weight, especially to 0.1% by weight. Moreover, higher Al contents have an adverse effect on castability in steel production. Al contents of not more than 1.0% by weight, especially not more than 0.44% by weight, have been found to be favorable for assuring particularly good castability. In addition, aluminum can be bound by nitrogen to give aluminum nitride. Aluminum nitride precipitates present in the flat steel product can have an unfavorable effect on the formability of the flat steel product. Thus, with regard to optimization of formability, it may be appropriate to limit the Al content to not more than 1.0% by weight, especially to not more than 0.44% by weight.

In order to rule out any adverse effect of Si and Al in the flat steel product of the invention, the sum total of the contents of Al and Si in the steel of a flat steel product of the invention can be limited to not more than 1.7% by weight, and particularly favorable upper limits here have been found to be not more than 1.5% by weight, especially not more than 1.0% by weight, particularly with regard to optimization of suitability for welding. With regard to optimization of formability, advantageous upper limits for the sum total of the contents of Al and Si have likewise been found to be not more than 1.0% by weight, especially not more than 0.4% by weight.

Manganese is important for the hardenability of the steel of a flat steel product of the invention and additionally prevents the formation of unwanted pearlite during the cooling. The presence of Mn thus enables the formation of a starting microstructure (martensite and residual austenite) suitable for the formation of the microstructure stipulated in accordance with the invention. However, too high a Mn concentration would have an adverse effect on the elongation and weldability of the steel. Therefore, the range envisaged for the Mn content in accordance with the invention is 1.0%-3.0% by weight, especially at least 1.5% by weight or at most 2.4% by weight.

Phosphorus has an adverse effect on the weldability of a flat steel product of the invention. The P content should be as low as possible, but at least should not exceed 0.02% by weight, and should especially be less than 0.02% by weight or less than 0.018% by weight.

The presence of effective contents of sulfur in the steel of a flat steel product of the invention would lead to formation of sulfides, especially MnS or (Mn,Fe)S, which would have an adverse effect on the elongation. In order to avoid this, the S content of the steel should be kept as low as possible, but at least should not be higher than 0.005% by weight, especially less than 0.005% by weight or less than 0.003% by weight.

In order to avoid the formation of nitrides which could be detrimental to formability, the N content of the steel of a flat steel product of the invention is limited to not more than 0.008% by weight. Advantageously, the N content, for avoidance of any adverse effect, should be below 0.008% by weight, especially less than 0.006% by weight.

Chromium in contents of up to 1.0% by weight can optionally be utilized in the steel envisaged in accordance with the invention as an effective inhibitor of pearlite, and additionally contributes to strength. In the case of contents of more than 1.0% by weight of Cr, there is the risk of marked grain boundary oxidation. In order to be able to utilize the positive effect of Cr, at least 0.05% by weight is required. The presence of Cr has a particularly favorable effect in the steel of a flat steel product of the invention when at least 0.15% by weight of Cr is present, and an optimal effect is achieved at contents of up to 0.8% by weight.

Optionally, the steel of a flat steel product of the invention may additionally also contain molybdenum in contents of 0.05%-0.2% by weight. Mo in these contents likewise particularly effectively suppresses the formation of unwanted pearlite.

The steel of a flat steel product of the invention may additionally optionally contain contents of one or more micro alloy elements, in order to promote strength through the formation of very finely divided carbides. It has been found that contents of Ti and Nb are particularly suitable for this purpose.

Ti contents of at least 0.005% by weight and Nb contents of at least 0.001% by weight each lead, alone or in combination with one another, to freezing of the particle and phase boundaries during the heat treatment that a flat steel product of the invention undergoes in the course of production thereof in accordance with the invention. Ti can additionally be utilized for binding of the nitrogen present in the steel, in order to enable an effect of other alloy elements, especially boron. It has been found that particularly advantageous Ti contents are those of at least 0.02% by weight. However, too high a concentration of micro alloy elements would lead to carbides of excessive dimensions, which could initiate cracks at high degrees of deformation. Therefore, the Ti content of the steel of a flat steel product of the invention is limited to not more than 0.2% by weight and the Nb content thereof to not more than 0.05% by weight, and it is found to be advantageous for avoidance of adverse effects of the presence of micro alloy elements when the sum of the contents of Nb and Ti does not exceed 0.2% by weight.

The boron likewise optionally present in the steel of a flat steel product of the invention segregates to the phase boundaries and attenuates their movement. This leads to a fine-grain microstructure, which has an advantageous effect on the mechanical properties. In order that the effect of B can be utilized, Ti can be included in the steel alloy, as mentioned above. In order to be able to utilize the positive effect of B, the steel envisaged in accordance with the invention must contain at least 0.0001% by weight of B. In the case of contents of more than 0.005% by weight, no further increase in the positive effect of B can be identified.

In order to protect it from corrosive attacks, the flat steel product of the invention may have been provided with a metallic protective coating. This may especially have been applied by melt dip coating. Suitable coatings here for a flat steel product of the invention are especially Zn-based coatings.

The process of the invention for producing a high-strength flat steel product comprises the following operating steps:

  • a) providing an uncoated flat steel product consisting of a steel consisting of, as well as iron and unavoidable impurities, (in % by weight)
    • C: 0.05%-0.20%,
    • Si: 0.2%-1.5%,
    • Al: 0.01%-1.5%,
    • Mn: 1.0%-3.0%,
    • P: up to 0.02%,
    • S: up to 0.005%,
    • N: up to 0.008%,
    • and optionally one or more of the elements from the group of “Cr, Mo, Ti, Nb, B” in the following contents:
      • Cr: 0.05%-1.0%,
      • Mo: 0.05%-0.2%,
      • Ti: 0.005%-0.2%,
      • Nb: 0.001%-0.05%,
      • B: 0.0001%-0.005%,
    • where the ratio


ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si)

    • with % C: respective C content of the steel
      • % Mn: respective Mn content of the steel
      • % Cr: respective Cr content of the steel
      • % Al: respective Al content of the steel
      • % Si: respective Si content of the steel
    • conforms to the following criterion:


1.5≤ψ≤3;

  • b) heating the flat steel product to an austenitization temperature THZ which is above the Ac3 temperature of the steel of the flat steel product and is not more than 950° C., wherein the heating is effected up to an inflection temperature TW of 200-400° C. at a heating rate θH1 of 5-25 K/s and then up to the austenitization temperature THZ at a heating rate θH2 of at least 2-10° K/s;
  • c) holding the flat steel product at the austenitization temperature THZ over an austenitization period tHZ of 5-15 s;
  • d) first cooling the flat steel product over a cooling period tk of 50-300 s to an intermediate temperature TK of not less than 680° C.;
  • e) quenching the flat steel product, proceeding from the intermediate temperature TK, with a cooling rate of more than 30 K/s to a cooling finish temperature TQ conforming to the following criterion:


(TMS−175° C.)<TQ<TMS

    • with TMS=martensite start temperature of the steel of which the flat steel product consists;
  • f) keeping the flat steel product at the cooling finish temperature TQ for a holding period tQ of 10-60 s;
  • g) treating the flat steel product quenched to the cooling finish temperature TQ,
    • g.1) wherein the flat steel product, over a total treatment period tB of 10-1000 s, is kept at a treatment temperature TB at least equal to the cooling finish temperature TQ and not higher than 550° C., especially not higher than 500° C.,
    • or
    • g.2) wherein the flat steel product, proceeding from the cooling finish temperature TQ, is heated to a treatment temperature TB of 450-500° C., wherein the flat steel product is then optionally kept under isothermal conditions at this treatment temperature TB over a holding period tBI, wherein the heating to the treatment temperature TB is effected at a heating rate θBs of less than 80 K/s and the total treatment period tBT, formed as the sum total of the heating period tBR required for the heating and the holding time tBI, is 10-1000 s, and wherein the flat steel product, after the treatment, is passed through a melt bath in order to overcoat it with a metallic protective coating based on Zn;
  • h) cooling, proceeding from the treatment temperature TB, at a cooling rate θB2 of more than 5 K/s.

The principle of the procedure of the invention is illustrated in the diagram appended as FIG. 1.

In operating step a), a flat steel product consisting of a steel having the above-elucidated composition is provided. The flat steel product provided may especially be a cold-rolled flat steel product. However, it is also conceivable to process a hot-rolled flat steel product in the inventive manner.

For the heating of the flat steel product to the austenitization temperature THZ (operating step b)), two steps with one following on from the other without interruption are possible in principle, in which case the flat steel product in the first step is heated at a heating rate ΘH1 of 5-25 K/s up to an inflection temperature TW of 200-400° C. Favorable values of ΘH1 for the productivity of the process have been found to be at least 5 K/s, while a heating rate ΘH1 of more than 25 K/s has been found to be very energy-intensive and costly. Subsequently, the heating in the second step is continued at a heating rate ΘH2 of 2-10 K/s until the austenitization temperature THZ has been attained. In the second heating step, the alloy elements present in the flat steel product can diffuse within the flat steel product during heating operation. As the heating rate increases, there is a decrease in the time available for the diffusion process and hence for the homogenization of the alloy element distribution of the flat steel product. Inhomogeneously distributed alloy elements can lead to locally different microstructure transformations. For establishment of a homogeneous microstructure, it has been found to be advantageous to limit the heating rate ΘH2 to a maximum of 10 K/s. At the same time, values for the heating rate ΘH2 of less than 2 K/s have been found to be unfavorable for the economic viability of the process. Since there is an overlap in the ranges mentioned for the heating rates ΘH1, ΘH2, the heating to the austenitization temperature can also be effected in one run with a constant heating rate of 5-10 K/s. In that case, the heating rates θH1 and θH2 in operating step b) are the same.

The austenitization temperature THZ must be above the A3 temperature. The A3 temperature is dependent on the analysis and can be estimated by the following empirical equation (alloy contents used in % by weight):


A3[° C.]=910−203√{square root over (% C)}−15.2% Ni+44.7% Si+31.5% Mo−21.1% Mn

  • with % C: C content of the steel,
    • % Ni: Ni content of the steel,
    • % Si: Si content of the steel,
    • % Mo: Mo content of the steel,
    • % Mn: Mn content of the steel.

The alloying of the steel selected in accordance with the invention permits restriction of the austenitization temperature THZ to a maximum of 950° C. and hence allows the operating costs incurred for the performance of the process of the invention to be limited.

In order to prevent large austenite grains from forming, which would have an adverse effect on formability, the austenitization period tHZ over which the flat steel product is kept at the austenitization temperature THZ in operating step c) is limited to 5-15 seconds, where the austenitization period tHZ may be less than 15 s in order to avoid any unwanted grain growth.

In operating step d), there follows controlled and gradual cooling of the flat steel product proceeding from the austenitization period tHZ. This cooling can extend over 50-300 seconds and has to end at an intermediate temperature TK no lower than 680° C., in order to avoid the unwanted formation of ferrite. The upper limit in the intermediate temperature TK is preferably at temperatures of not more than A3, and is typically restricted to 775° C., since, in the case of higher intermediate temperatures TK, the cooling output required for the subsequent cooling is disproportionately high and thus puts the economic viability of the process into question.

After the gradual cooling in operating step d), the flat steel product, in operating step e), is quenched to an analysis-dependent cooling finish temperature TQ at a high cooling rate θQ. The high cooling rate θQ can be achieved, for example, with modern gas jet cooling.

The minimum cooling rate θQ necessary to avoid ferritic and bainitic transformation is more than 30 K/s. There is typically an upper limit to the cooling rate θQ arising from the plant, which is typically not more than 200 K/s. The range within which the cooling finish temperature TQ lies is limited at the upper end by the martensite start temperature TMS, and at the lower end by a temperature which is 175° C. below the martensite start temperature TMS ((TMS−175° C.)<TQ<TMS).

The martensite start temperature can be estimated by means of the following equation (alloy contents used in % by weight):


TMS(° C.)=539° C.+(−423% C−30.4% Mn−7.5% Si+30% Al)° C./% by wt.

  • with % C: C content of the steel,
    • % Mn: Mn content of the steel,
    • % Si: Si content of the steel,
    • % Al: Al content of the steel.

In operating step f), the flat steel product is kept at the cooling finish temperature TQ for a holding period tQ of 10-60 seconds, in order to establish the microstructure. In the course of this step, a martensitic microstructure is obtained with up to 30% residual austenite. The amount of martensite produced in this step depends essentially on the degree to which the cooling finish temperature is below the martensite start temperature TMS. The holding period tQ is at least 10 seconds, in order to assure homogenization of the temperature in the flat steel product and hence a homogeneous microstructure. In the case of longer holding periods of more than 60 seconds, the homogenization of the temperature is complete. The holding period tQ is not more than 60 seconds, in order to increase the productivity of the process.

By contrast with the prior art described at the outset, it is not an aim of the invention to stabilize residual austenite down to room temperature. Instead, the heat treatment of the flat steel product conducted in operating step g) has the aim of controlled redistribution of the carbon such that the microstructure of the flat steel product obtained on conclusion of the process consists essentially of two different kinds of martensite, namely an annealed martensite and an unannealed martensite.

According to the invention, operating step g) comprises two process variants g.1) and g.2), of which the first variant g.1) leads to an uncoated flat steel product of the invention and the second variant g.2) to a flat steel product of the invention provided with a Zn coating.

The temperature regime in each of the variants g.1), g.2) of the operating step g) is chosen such that the existing residual austenite present in the microstructure is enriched with carbon from the oversaturated martensite. The formation of carbides and the breakdown of residual austenite is deliberately suppressed via the inventive limitation of the total treatment period tBT. This period is 10-1000 seconds in order to enable sufficient redistribution of the carbon.

In respect of the first process variant g.1), the treatment of the flat steel product in operating step g) comprises keeping the flat steel product over the entire treatment period tBT at a treatment temperature TB at least equal to the cooling finish temperature TQ and not higher than 550° C., and a cooling finish temperature TQ of not more than 500° C. has been found to be particularly favorable. In the case of variant g.1), the treatment temperature TB may also be higher than the cooling finish temperature TQ. In this case, the flat steel product, proceeding from the cooling finish temperature TQ, is heated to the respective treatment temperature TB, where the heating should be effected at a heating rate ΘB1 of less than 80 K/s.

In the second alternative of operating step g), by contrast, the flat steel product is brought to a treatment temperature TB of 400-500° C. at a heating rate ΘB1 of less than 80 K/s, in order to enrich the residual austenite with carbon from the oversaturated martensite. The formation of carbides and the breakdown of residual austenite are deliberately suppressed by the inventive limitation of the total treatment period tBT, which in this variant g.2) of operating step g) is composed of the heating time tBR required for the heating and the holding period tBI over which the flat steel product is kept under isothermal conditions at the temperature TB. Given a sufficiently gradual heating rate ΘB1, the isothermal hold can also be dispensed with, and so the holding period tBI can be “0”.

In the second variant g.2) of operating step g), the flat steel product, after the heating and the optional hold at the treatment temperature TB, undergoes a melt dip-coating operation in which it is coated with a Zn coating. For this purpose, the treatment temperature TB can be chosen such that it corresponds to the inlet temperature at which the flat steel product is to enter the respective melt bath. Typically, for this purpose, the treatment temperatures TB are in the range of 450-500° C. This melt bath typically comprises, as well as zinc and unavoidable impurities, a total of up to 3.0% by weight of one or more elements from the group consisting of Al, Mg, Si, Pb, Ti, Ni, Cu, B and Mn.

Irrespective of which variant has been chosen, the flat steel product, on conclusion of operating step g), for new production of martensite, is cooled in a controlled manner at a cooling rate θB2 of more than 5 K/s, the cooling rates typically being not more than 50 K/s. θB2 is more than 5 K/s, in order to avoid the formation of pearlite and ferrite.

The process of the invention can be conducted in a continuous run in conventional calcining systems or belt coating systems that are typically provided for the purpose.

The flat steel product of the invention has a microstructure consisting

    • to an extent of at least, especially more than, 90 area % of martensite, of which at least, especially more than, 50 area % is annealed martensite from the first cooling step (operating step f)),
    • to an extent of at most, especially less than, 5 area % of bainite,
    • to an extent of at most, especially less than, 2% by volume of residual austenite and
    • to an extent of at most, especially less than, 5 area % of polygonal ferrite.

The microstructure of a flat steel product of the invention, with a mean grain size of less than 2 μm, is very fine and can barely be assessed by means of standard light-optical microscopy. Therefore, an assessment by means of scanning electron microscopy (SEM) with a minimum of 5000-fold magnification is recommended.

The maximum permissible residual austenite content, even in the case of high magnification, can be determined only with difficulty by light microscopy or scanning electron microscopy. Therefore, a quantitative determination of the residual austenite by means of x-ray diffraction (XRD) is recommended (according to ASTM E975), by which the residual austenite content is reported in % by volume.

Another measure that can be employed for the quality of the mechanical properties of a flat steel product of the invention is the distortion of the crystal lattice. This lattice distortion is very important for the initial resistance to plastic deformation. A suitable method for the measurement and quantification of lattice distortion is electron backscatter diffraction (EBSD). By the EBSD method, a sample is scanned point by point by SEM, with recording of a diffraction pattern at every measurement point, from which it is possible to determine the crystallographic orientation. Details of the measurement and of the various evaluation methods can be read in the handbooks. A useful EBSD evaluation method is what is called the kernel average misorientation (KAM—further description in the handbook “OIM Analysis v5.31” from EDAX Inc., 91 McKee Drive, Mahwah, N.J. 07430, USA), wherein the orientation of a measurement point is compared with the neighboring points. Beneath a threshold value, typically 50, adjacent points form part of the same (deformed) grain. Above the threshold value, the adjacent points form part of different (sub-)grains. Because the microstructure is so fine, a maximum step width of 100 nm is recommended in EBSD. For the assessment of the microstructure of the flat steel products of the invention, the KAM of the third adjacent points is evaluated. A flat steel product of the invention must have a mean KAM value from a measurement region of at least 75 μm×75 μm of more than 1.200, preferably more than 1.250

The invention is elucidated in detail hereinafter by working examples.

To test the invention, samples of steel sheets produced in a conventional manner have been provided, which consisted of steels A-I with the compositions specified in table 1.

Table 1 additional states, for each of the steels A-I, the factor ψ and the carbon equivalent CE which have been calculated by the already above-elucidated formulae


ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si)


and


CE=% C+(% Si+% Mn)/5+(% Cr+% Mo)/6

where % C is the respective C content, % Si the respective Si content, % Mn the respective Mn content, % Cr the respective Cr content, % Mo the respective Mo content and % Al the respective Al content of the steels A-I.

Steels E, F and G accordingly did not meet the demands on the tuning of the alloy elements essential to austenite formation and hardenability that are stipulated in accordance with the invention by the factor ψ.

Samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60 manufactured from steels A-I have undergone the process sequence shown in FIG. 1. These have firstly been heated at a heating rate θH1 to an inflection temperature TW and then at a heating rate θH2 to an austenitization temperature THZ, each of which was above the A3 temperature of the respective steel but lower than 950° C. The samples thus heated have subsequently been kept at the austenitization temperature THZ over an austenitization period tHZ and then cooled to an intermediate temperature TK over a cooling period tK. On attainment of the intermediate temperature TK, accelerated cooling at a cooling rate θQ has set in, in which the samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60 have been cooled to a cooling finish temperature TQ which, for each of samples 1-7, 11, 12, 16, 17, 19-23, 28-31, 33-35, 39, 40 and 43-60, was up to 175° C. lower and, for sample 18, higher than the martensite start temperature TMS of the respective steel A-I of samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60. Samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60 have been kept at the cooling finish temperature TQ for a holding period tQ of 10-60 s. Samples 1-7, 11, 12, 16, 17, 19-23, 28-31, 33-35, 39, 40 and 43-48 were subsequently heated at a heating rate θB1 over a heating time tBR to a treatment temperature TB at which they have been kept over an additional holding period tBI in some experiments. In an analogous manner, sample 18 was cooled to the treatment temperature TB. This was followed by cooling to room temperature at a cooling rate θB2. Samples 49-60, after being cooled down to the cooling finish temperature TQ and held at TQ for the holding period tQ in an isothermal manner without heating, were kept at the treatment temperature TB over a holding period tBI. For samples 49-60 too, this was followed by cooling to room temperature at a cooling rate θB2.

The aforementioned parameters employed in the experiments are specified in table 2. Of the samples 1-7, 11, 12, 16-23, 28-31 and 44-55 consisting of the inventive steels A-D, H and I, accordingly, samples 3 (θQ<30 K/s), 11 (THZ<A3), 18 (TQ>500° C.), 19 (θQ<30 K/s), 28 (THZ<A3), 29 (tHZ>15 s) and 48 (θB2<5 K/s) have not been treated in accordance with the invention.

In the context of the last cooling, samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40 and 43-60, in the cases where the treatment temperature TB was at a level of about 450° C. sufficient for entry into a Zn melt bath, could have passed through a melt bath. In the context of the experiments, however, this has been dispensed with, and so it did not affect the results of the study.

The mechanical properties of yield point Rp0.2, tensile strength Rm, the Rp0.2/Rm ratio, elongation at break A50 (according to DIN EN ISO 6892, sample form 1), the product Rm*A50, and the hole expansion ratios λ1, λ2 (according to ISO 16630) have been determined on the samples obtained after the heat treatment. Likewise ascertained have been the microstructure proportions of ferrite “F”, annealed martensite “AM”, residual austenite “RA”, unannealed martensite “M” and bainite “B”, and also the value “KAM” determined in accordance with the kernel average misorientation. The property values in question are reported in table 3 for each of the samples.

The mechanical properties attained in the calcined material with a quantification of the microstructure can be found in table 3. In the case of the samples that fulfill both the specifications of the invention in relation to the alloy of the respective steel and the conditions of the invention for the heat treatment, it is regularly the case that yield points Rp0.2 of more than 800 MPa, tensile strengths Rm of more than 950 MPa, and elongation at break values A50 of more than 8% are achieved, combined with hole expansion ratios λ1, λ2 of regularly more than 30%.

Comparative examples B11 and D28, by contrast, illustrate the effect of an insufficient austenitization temperature THZ. In these examples, the microstructure has not been fully austenitized, and so too much ferrite forms in the microstructure. This leads to extremely localized damage and early failure during forming.

Comparative example D29 shows how austenitization for too long a period at high temperatures can adversely affect formability.

Comparative examples A3 and C19 show that, in the case of excessively low cooling rates θQ, the desired yield point is not attained, which is attributable to the fact that ferrite formation could not be adequately prevented.

Comparative example C18, which was produced with too high a cooling finish temperature TQ, shows a yield point below that desired and low hole expansion ratios. These are attributable to an elevated level of ferrite and bainite in the microstructure.

Comparative examples E33-E35 and E56-E58 show a yield point and strength below those desired, which is attributable to the composition not in accordance with the invention and too high a ferrite content in the microstructure obtained. The high ferrite content is caused by inadequate prevention of carbide formation as a result of too low a silicon content and too low a content of aluminum and silicon in relation to carbon, manganese and chromium, and hence too high a ψ factor.

Finally, comparative examples F39, F40, F59 and F60 show the effects of too low a ψ factor, which also leads to departures from the microstructure desired. The minimum strength was attained in some cases, but the yield point and the hole expansion here are not within the target range.

Comparative example G43 makes it clear that too high a factor leads to excessively high residual austenite contents and reduced formability, which is manifested in poor hole expansion values λ1, λ2.

Comparative example 148 illustrates that too low a cooling rate θB2 leads to increased ferrite formation and hence to low yield points.

TABLE 1 C Si Mn Al P S N Cr Mo Ti Nb B CE Ψ Inventive? A 0.066 0.29 2.54 0.037 0.009 0.003 0.005 0.666 0.000 0.071 0.001 0.0013 0.74 2.09 YES B 0.085 0.30 2.75 0.030 0.000 0.003 0.005 0.750 0.100 0.070 0.000 0.0000 0.84 2.3  YES C 0.159 0.29 1.82 0.041 0.015 0.003 0.004 0.422 0.101 0.047 0.001 0.0010 0.67 1.79 YES D 0.180 0.30 1.95 0.030 0.010 0.003 0.003 0.300 0.000 0.050 0.000 0.0000 0.68 1.88 YES E 0.075 0.10 1.52 0.035 0.010 0.001 0.004 0.530 0.050 0.025 0.000 0.0030 0.50 3.46 NO F 0.164 0.72 1.90 0.041 0.012 0.001 0.005 0.370 0.010 0.114 0.001 0.0000 0.75 0.8  NO G 0.190 0.23 2.97 0.030 0.008 0.004 0.006 0.801 0.050 0.060 0.001 0.0009 0.97 3.53 NO H 0.186 0.40 2.20 0.029 0.009 0.001 0.005 0.350 0.080 0.000 0.000 0.0000 0.78 1.6  YES I 0.190 0.25 2.85 0.210 0.008 0.003 0.005 0.000 0.110 0.000 0.000 0.0000 0.83 1.65 YES Figures in % by weight, remainder Fe and unavoidable impurities Underlined and emboldened values denote values outside the specifications of the invention Ψ = (% C + % Mn/5 + % Cr/6)/(% Si + % Al) % C = C content, % Mn = Mn content, % Cr = Cr content, % Si = Si content, % Al = Al content

TABLE 2 Ser. ΘH1 TW ΘH2 A3 THZ tHZ TK tK ΘQ TQ TMS tQ ΘB1 tBR tBI TB ΘB2 Steel No. [K/s] [° C.] [K/s] [° C.] [° C.] [s] [° C.] [s] [K/s] [° C.] [° C.] [s] [K/s] [s] [s] [° C.] [K/s] A 1 10 300 5 817 860 10 760 105 −31 310 433 50 3 46.7 0 450 −11 A 2 11 270 4 817 860 12 760 100 −47 310 433 50 3 46.7 0 450 −11 A 3 11 270 4 817 860 12 760 100 −16 370 433 40 2 40.0 0 450 −11 A 4 5 270 5 817 860 10 775 100 −42 350 433 50 3 33.3 0 450 −10 A 5 5 270 5 817 860 10 775 100 −39 370 433 50 1.75 45.7 0 450 −9 A 6 5 270 5 817 860 12 775 120 −36 370 433 50 1.75 45.7 15 450 −20 A 7 5 270 5 817 860 12 775 120 −36 370 433 50 1 55.0 20 425 −20 B 11 5 300 2 809 780  8 760 135 −21 350 418 15 3 33.3 15 450 −10 B 12 5 300 2 809 840 10 760 110 −35 290 418 12 2 80.0 15 450 −12 B 16 8 300 2 809 860 10 740 120 −32 300 418 12 25 7.6 15 490 −15 B 17 5 300 2 809 840 12 740 120 −45 325 418 10 4 31.3 15 450 −15 C 18 9 255 3 807 860 10 740 105 −32 510 415 10 −1 60.0 16 450 −20 C 19 9 255 3 807 860 12 740 105 −15 350 415 10 3 33.3 0 450 −20 C 20 20 295 3 807 860 10 740 105 −49 290 415 50 3 53.3 22 450 −20 C 21 5 270 5 807 860 14 760 95 −42 350 415 50 3 33.3 0 450 −20 C 22 14 310 5 807 860 14 715 125 −39 350 415 50 3 33.3 0 450 −10 C 23 10 270 3 807 860 12 700 125 −39 350 415 50 1.5 50.0 0 425 −10 D 28 5 270 5 796 775 10 750 120 −32 290 402 11 3 45.0 0 425 −10 D 29 5 270 5 796 840 25 750 120 −44 290 402 10 3 53.3 25 450 −10 D 30 5 270 5 796 840 10 750 135 −38 250 402 12 3.5 57.1 0 450 −10 D 31 5 270 5 796 840 12 700 70 −50 350 402 15 3.5 28.6 0 450 −10 E 33 5 270 5 828 860 10 700 120 −54 300 461 50 3 50.0 5 450 −12 E 34 11 270 3 828 860 12 685 140 −49 300 461 50 3 50.0 5 450 −12 E 35 11 270 3 828 860 12 700 165 −42 350 461 50 3 33.3 5 450 −20 F 39 5 270 4 820 860 12 700 120 −31 310 408 50 3 46.7 5 450 −16 F 40 5 270 5 820 860 10 685 125 −33 310 408 20 3 46.7 0 450 −16 G 43 5 340 4 771 850 10 720 100 −21 325 368 25 8 40.0 25 465 −11 H 44 21 375 7 796 835 12 695 135 −37 230 391 14 3.5 57.1 0 430 −15 I 45 11 350 3 776 860  9 720 75 −41 295 376 11 2.8 53.6 0 445 −12 I 46 11 270 4 776 840 12 800 75 −35 290 376 10 0.012 833.3 0 300 −15 I 47 13 325 3.5 776 860 12 745 65 −45 240 376 13 6 36.7 0 460 −12 I 48 10 340 4 776 860 10 730 70 −31 350 376 15 3 46.7 0 450 −2 A 49 10 270 4 817 840 12 740 120 −32 300 433 10 0 0 420 300 −20 A 50 11 300 5 817 840 12 740 120 −32 325 433 10 0 0 420 325 −20 A 51 5 270 5 817 860 12 740 120 −31 325 433 10 0 0 420 325 −20 C 52 10 270 3 807 840 10 760 100 −32 300 415 12 0 0 420 300 −10 C 53 15 290 5 807 840 10 780 80 −32 325 415 12 0 0 470 325 −10 C 54 5 270 5 807 860 12 750 140 −31 325 415 12 0 0 470 325 −16 C 55 20 300 3 807 860 12 775 135 −32 350 415 12 0 0 380 350 −16 E 56 5 270 5 828 840 14 700 135 −22 300 461 15 0 0 410 300 −10 E 57 5 270 5 828 840 14 700 135 −20 325 461 15 0 0 460 325 −10 E 58 5 270 5 828 860  8 735 135 −21 325 461 15 0 0 460 325 −10 F 59 10 300 3 820 840 10 720 140 −22 350 408 13 0 0 770 350 −9 F 60 8 270 4 820 840 12 720 80 −22 300 408 13 0 0 420 300 −9 Underlined values denote values outside the specifications of the invention

TABLE 3 RA Ser. RP02 Rm RP02/ A50 Rm*A50 λ1 λ2 F AM [% by M B KAM Steel No. [MPa] Rm [%] [MPa*%] [%] [area %] vol.] [area %] [°] Inventive? A 1 1050  1063 0.99 9.3 9885.9 80 62 — 80 1 19 — 1.43 YES A 2 1090  1093 1.00 8.0 8744 64 80 — 90 0 10 — 1.45 YES A 3 661  952 0.69 11.2 10662 35 28 10 45 1 43 tr. 1.19 NO A 4 989 1072 0.92 9.9 10613 61 41 — 75 1 24 — 1.37 YES A 5 890 1063 0.84 10.2 10843 60 62 — 70   0.5 29 tr. 1.35 YES A 6 873 1056 0.83 10.7 11299 47 40 — 70   1.5 28 tr. 1.36 YES A 7 866 1071 0.81 8.8 9425 44 32 — 70 0 29 tr. 1.34 YES B 11 565 1197 0.47 11.2 13406 26 32 10 50   3.5 30   6.5 1.03 NO B 12 1030  1255 0.82 10.8 13554 54 49 — 75   0.5 24 tr. 1.29 YES B 16 980 1183 0.83 8.3 9819 38 41 — 60 1 38 tr. 1.32 YES B 17 1077  1292 0.83 10.5 13566 31 32 — 70   0.5 27 tr. 1.3  YES C 18 630 1056 0.60 12.7 13411 15 18 15  0 0 55 30  1.01 NO C 19 695  992 0.70 13 12896 35 29 20 65 1 14 — 1.09 NO C 20 1120  1123 1.00 8.3 9321 55 51 — 85 0 15 — 1.42 YES C 21 1026  1119 0.92 8.4 9400 48 47 — 75   0.5 23 tr. 1.4  YES C 22 927 1074 0.86 9.9 10633 46 43 — 75 1 23 tr. 1.34 YES C 23 908 1074 0.85 9.5 10203 45 40 tr. 65   0.5 33 tr. 1.31 YES D 28 701 1231 0.57 13.4 16495 24 17 20 30 2 40 8 1.03 NO D 29 979 1290 0.76 9.1 11739 31 29 tr. 50 4 45 tr. 1.38 NO D 30 1138  1366 0.83 8.9 12157 45 39 — 75   0.5 24 tr. 1.47 YES D 31 1091  1204 0.91 11.1 13364 31 34 tr. 65 1 33 — 1.45 YES E 33 416  616 0.68 10.5 6468 71 71 30 15 1 45 9 1.22 NO E 34 277  538 0.51 19.8 10652 76 78 45 10   0.5 40   4.5 1.03 NO E 35 283  540 0.52 23.4 12636 77 61 40 10 1 45 4 0.98 NO F 39 428  931 0.46 17.4 16199 20 23 35 30 1 30 4 1.02 NO F 40 442  977 0.45 17.4 17000 17 16 35 30   0.5 34 tr. 1.05 NO G 43 873 1253 0.70 14.3 17918 26 23 10 40 5 40 5 1.11 NO H 44 812 1079 0.75 16.7 18019 56 62 tr. 75   1.5 20 3 1.32 YES I 45 823 1156 0.65 15.9 18380 35 42 — 65 1 30 4 1.42 YES I 46 917 1109 0.83 16.2 17966 62 57 — 75   0.5 20   4.5 1.42 YES I 47 890 1047 0.85 13.1 13716 73 68 — 85   1.5 12 tr. 1.38 YES I 48 690  978 0.71 18.3 17897 14 12 10 60   2.0 20 8 1.21 NO A 49 861 1049 0.82 9 9441 82 75 — 60   0.5 39 tr. 1.34 YES A 50 816 1019 0.80 10.7 10903 62 53 — 50   1.5 48 tr. 1.29 YES A 51 875 1052 0.83 9.1 9573 72 76 — 60 1 38 tr. 1.31 YES C 52 893 1139 0.78 9.1 10365 35 35 tr. 65 0 34 — 1.29 YES C 53 862 1091 0.79 9 9819 47 36 tr. 70   0.5 27 tr. 1.27 YES C 54 959 1153 0.83 8.1 9339 55 38 — 50 0 50 — 1.41 YES C 55 1038  1144 0.91 8.4 9610 43 36 — 55 1 44 — 1.46 YES E 56 429  661 0.65 13.6 8990 78 76 25 25   1.5 45   3.5 1.17 NO E 57 398  628 0.63 15.9 9985 65 81 30 15 0 55 — 1.13 NO E 58 521  695 0.75 8.5 5908 71 72 15 35 0 50 — 1.26 NO F 59 491  841 0.58 19.7 16568 30 26 tr. 64 6 28 tr. 1.26 NO F 60 405  961 0.42 16.6 15953 21 20 15 35 0 40 10  1.18 NO “tr.” = proportion < 2 area %; Underlined values denote values outside the specifications of the invention

Claims

1.-9. (canceled)

10. A flat steel product having a tensile strength Rm of at least 950 MPa, a yield point of at least 800 MPa, and an elongation at break A50 of at least 8%, wherein the flat steel product comprises a steel consisting of iron, unavoidable impurities, and in percent by weight: wherein a microstructure of the flat steel product consists of

0.05%-0.20% C;
0.2%-1.5% Si;
0.01%-1.5% Al;
1.0%-3.0% Mn;
up to 0.02% P;
up to 0.005% S; and
up to 0.008% N,
up to 5% by area bainite;
up to 5% by area polygonal ferrite;
up to 2% by volume residual austenite; and
at least 90% by area martensite, wherein at least half of the martensite is annealed martensite.

11. The flat steel product of claim 10 wherein the steel further includes in percent by weight one or more of

0.05%-1.0% Cr;
0.05%-0.2% Mo;
0.005%-0.2% Ti;
0.001%-0.05% Nb; or
0.0001-0.005% B,
wherein a ratio ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si) conforms to 1.5≤ψ≤3, with % Mn being a respective Mn content of the steel, % Cr being a respective Cr content of the steel, % Al being a respective Al content of the steel, and % Si being a respective Si content of the steel.

12. The flat steel product of claim 11 wherein a sum total of Ti and Nb is at most 0.2% by weight.

13. The flat steel product of claim 11 comprising a carbon equivalent CE=% C+(% Si+% Mn)/5+(% Cr+% Mo)/6 that conforms to 0.254%≤CE≤1.1% by weight, with % C being a respective C content of the steel, % Si being a respective Si content of the steel, % Mn being a respective Mn content of the steel, % Cr being a respective Cr content of the steel, and % Mo being a respective Mo content of the steel.

14. The flat steel product of claim 13 wherein the carbon equivalent CE is at most 1.0% by weight.

15. The flat steel product of claim 10 wherein a sum total of Si and Al is at most 1.7% by weight.

16. The flat steel product of claim 10 further comprising a metallic protective coat applied by melt dip coating.

17. A process for producing a flat steel product comprising:

providing a flat steel product that is uncoated and comprises a steel consisting of iron, unavoidable impurities, and in percent by weight 0.05%-0.20% C, 0.2%-1.5% Si, 0.01%-1.5% Al, 1.0%-3.0% Mn, up to 0.02% P, up to 0.005% S, and up to 0.008% N;
heating the flat steel product to an austenitization temperature THZ that is above an A3 temperature of the steel of the flat steel product and is not more than 950° C., wherein the heating is effected up to an inflection temperature TW of 200-400° C. at a first heating rate θH1 of 5-25 K/s and then up to the austenitization temperature THZ at a second heating rate θH2 of at least 2-10° K/s;
holding the flat steel product at the austenitization temperature THZ over an austenitization period tHZ of 5-15 seconds;
first cooling the flat steel product over a cooling period tk of 50-300 seconds to an intermediate temperature TK of not less than 680° C.;
quenching the flat steel product from the intermediate temperature TK at a cooling rate θQ of more than 30 K/s to a cooling finish temperature TQ where (TMS-175° C.)<TQ<TMS, wherein TMS is a martensite start temperature of the steel;
maintaining the flat steel product at the cooling finish temperature TQ for a holding period tQ of 10-60 seconds;
treating the flat steel product that has been quenched to the cooling finish temperature TQ, wherein the flat steel product, over a total treatment period tB of 10-1000 seconds, is kept at a treatment temperature TB at least equal to the cooling finish temperature TQ and not higher than 550° C., or the flat steel product, proceeding from the cooling finish temperature TQ, is heated to a treatment temperature TB of 450-500° C., wherein the flat steel product is kept under isothermal conditions at the treatment temperature TB over a holding period tBI, wherein the heating to the treatment temperature TB is effected at a heating rate θB1 of less than 80 K/s and a total treatment period tBT, formed as a sum total of the heating period tBR required for the heating and the holding time tBI, is 10-1000 seconds, wherein after the treatment the flat steel product is passed through a melt bath to overcoat the flat steel product with a metallic protective coating based on Zn; and
cooling, proceeding from the treatment temperature TB, at a cooling rate θB2 of more than 5 K/s.

18. The process of claim 17 wherein the steel further includes in percent by weight one or more of

0.05%-1.0% Cr;
0.05%-0.2% Mo;
0.005%-0.2% Ti;
0.001%-0.05% Nb; or
0.0001-0.005% B,
wherein a ratio ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si) conforms to 1.5≤ψ≤3, with % Mn being a respective Mn content of the steel, % Cr being a respective Cr content of the steel, % Al being a respective Al content of the steel, and % Si being a respective Si content of the steel.

19. The process of claim 18 further comprising coating the flat steel product with a Zn coating.

20. The process of claim 17 wherein the first heating rate θH1 is the same as the second heating rate θH2.

21. The process of claim 17 wherein the treatment step of keeping the flat steel product, over the total treatment period tB of 10-1000 seconds, at the treatment temperature TB at least equal to the cooling finish temperature TQ and not higher than 550° C. comprises heating the flat steel product from the cooling finish temperature TQ at a heating rate θB1 of less than 80 K/s to the treatment temperature TB.

Patent History
Publication number: 20190119774
Type: Application
Filed: May 4, 2016
Publication Date: Apr 25, 2019
Applicants: THYSSENKRUPP STEEL EUROPE AG (Duisburg), thyssenkrupp AG (Essen)
Inventors: Richard G. THIESSEN (Malden), Thomas HELLER (Duisburg), Karsten MACHALITZA (Mülheim), Roland SEBALD (Geldern)
Application Number: 15/571,379
Classifications
International Classification: C21D 9/46 (20060101); C21D 6/00 (20060101); C21D 1/22 (20060101); C22C 38/38 (20060101); C22C 38/28 (20060101); C22C 38/26 (20060101); C22C 38/22 (20060101); C22C 38/32 (20060101); C22C 38/06 (20060101); C22C 38/02 (20060101); C22C 38/00 (20060101); C23C 2/06 (20060101); C23C 2/02 (20060101); C23C 2/40 (20060101); C23F 17/00 (20060101);