WO2016177420A1 - Flat steel product and method for the production thereof - Google Patents

Abstract

The invention relates to a flat steel product, having a tensile strength R_m ≥ 950 MPa, a yield strength ≥ 800 MPa and an elongation at break A₅₀ ≥ 8%. According to the invention, the flat steel product consists of steel which contains (in 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 inevitable impurities, wherein: 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, AI or Si content of the steel. At the same time the flat steel product has a structure which consists of (in area percentage) ≤ 5% bainite, ≤ 5% polygonal ferrite, ≥ 90% martensite and ≤ 2% by volume residual austenite, at least half of the martensite being tempered martensite. The invention also relates to a method for producing such a flat steel product.

Description

 Flat steel product and process for its production

The invention relates to a flat steel product having an optimized combination of strength and elongation.

Likewise, the invention relates to a method for producing such a product.

When it comes to flat steel products, this refers to steel strips, sheets or sheet metal blanks derived therefrom, such as sinkers.

Unless otherwise stated, are in

present text and in the claims, the contents of certain alloying elements each in wt .-% and the proportions of certain microstructural constituents in area% indicated.

CA 2 734 976 A1 (WO 2010/029983 A1) discloses a steel with good toughness and deformability, which should have a tensile strength of at least 980 MPa. The steel contains in addition to 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 of the Al and Si contents should be at least 0.7%. At the same time, in each case in relation to the totality of all

Microstructure components, the proportion of arsenic in the microstructure of the steel is 10 - 90%, the proportion of retained austenite in the range of 5 - 50%, and the proportion of ferritic bainite derived from "upper bainite" is at least 5%, the term "upper bainite" being a bainite, in The fine carbide grains are evenly distributed, as they are not found in "lower bainite"

Upper bainite contents of 17% and more are considered to be beneficial to achieving the desired high levels

To generate retained austenite contents in the microstructure.

From EP 2 524 970 AI is further a

Steel flat product is known, which has a tensile strength R _m of at least 1200 MPa and consists of a steel, in addition to Fe and unavoidable impurities (in

% By weight) C: 0.10-0.50%, Si: 0.1-2.5%, Mn: 1.0-3.5%, Al: up to 2.5%, P: to 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 - Contains 0.05%. The following applies for the sum I (V, Ti, Nb) of the contents of V, Ti and Nb: Z (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 structure with (in area%) less than 5% ferrite, less than 10% bainite, 5-70% unangelassenten martensite, 5 - 30%

Retained austenite and 25-80% tempered martensite, with at least 99% of the martensite tempered

contained iron carbides have a size of less than 500 nm. Due to its minimized share of

Superior martensite has such a nature

procured steel flat product an optimized

Deformability on. Similarly, from EP 2 524 970 AI a method for

Production of a flat steel product of the type described above known. In this method, first a flat steel product with the above

Composition with a heating rate θ _Η ι, θ _Η 2 of at least 3 ° C / s heated to above the A ₃ temperature of the steel of the flat steel product and at most 960 ° C amount austenitizing temperature T _HZ . There, the flat steel product over a

Austenitisierungsdauer t _H z of 20 - kept s, to be then cooled to a cooling stop temperature 180th This is greater than the martensite stop temperature and less than the martensite start temperature, wherein the cooling takes place at a cooling rate which is at least equal to one depending on the

Alloy contents of the steel determined

Minimum cooling rate is. Then that will be

Flat steel product held for 10 - 60 s at the cooling stop temperature, then at a heating rate of

2 - 80 ° C / s to a 400 - 500 ° C amounts

 Partitioning temperature to be heated. This can be an isothermal hold the steel flat product in the

Partitioning temperature for up to 500 s.

Subsequently, the flat steel product with a

 3 - 25 ° C / s cooled cooling rate.

In the above-described known method is by the heating and optionally in addition

Holding at the partitioning temperature of the retained austenite in the structure of the flat steel product with

Carbon enriched from the oversaturated Martensxt. This process is also known in the jargon as "Partitioning the Carbon" or "Partitioning". Partitioning can already be done during the

Heating up as a so-called "Ramped Partitioning", by the holding carried out after heating in the

Partitioning temperature (so-called "isothermal")

Partitioning) or by a combination of isothermal and ramped partitioning. The Ramped

Partitioning compared to the isothermal partitioning desired slower heating rate allows a particularly accurate control of each predetermined partitioning temperature with reduced energy use.

The steels obtained and processed in the above-mentioned manner belong to the so-called "AHSS steels" (Advanced High Strength Steel).

Modern variants of these steels and steel flat products produced therefrom have a very high strength combined with high elongation and are therefore particularly suitable for the production of safety-relevant components of automobile bodies, which in the event of a crash

To absorb deformation energy. However, it is found in practice that high retained austenite contents in the structure of such steels, while their uniaxial elongation can improve by the known TRIP effect, but that they do not reliably succeed in achieving in all directions equally good deformability, as for example a good hole expansion behavior is characterized.

Against this background, the task of creating a flat steel product that is not just one

optimized combination of high strength and elongation but with improved performance properties, such as good weldability, surface finish and suitability for coating with a metallic

Protective cover, also has a structure, the one

ensures optimized ductility regardless of the orientation of the deformation.

Likewise, a method for producing such a flat steel product should be specified.

With respect to the flat steel product, the invention has achieved this object in that a

Flat steel product has at least the features specified in claim 1.

With regard to the method, the solution according to the invention of the abovementioned object consists in that during the production of a flat steel product according to the invention at least the steps mentioned in claim 7 are completed.

Advantageous embodiments of the invention are set forth in the dependent claims and will become hereafter as the general inventive concept in detail

explained.

An inventive flat steel product is characterized by a tensile strength R _m of at least

950 MPa, a yield strength of at least 800 MPa and one according to DIN EN ISO 6892, sample form 1, determined

Elongation at break A ₅₀ of at least 8%. In this case, a flat steel product according to the invention consists of a steel, in addition to iron and unavoidable impurities (in% by weight)

C: 0.05-0.20%,

 Si: 0.2-1.5%,

 AI: 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 "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

 contains

 being for the ratio

 ψ = (% C +% Mn / 5 +% Cr / 6) / (% A1 +% Si)

 with% C: respective C-content of the steel

 % Mn: respective Mn content of the steel

 % Cr: respective Cr content of the steel

 % A1: respective Al content of the steel

 % Si: respective Si content of the steel

 applies

1.5 <ψ <3 and wherein the flat steel product has a structure consisting of

 - not more than 5% bainite,

 - not more than 5% by area of polygonal ferrite,

- not more than 2% by volume of retained austenite, and

 - at least 90% martensite by area, with at least half of the martensite being tempered martensite.

The invention is based on the recognition that by choosing a suitable alloy, a flat steel product can be obtained in which a high strength is achieved by a structure comprising at most minimal retained austenite contents and by a high proportion of tempered martensite and by finely divided non-tempered martensite is paired with a very good formability.

Typical tensile strengths Rm of inventive

Flat steel products are at 950 - 1300 MPa with a yield strength of at least 800 MPa and can reach to the respective tensile strength. The elongation A ₅₀ of flat steel products according to the invention lies

typically at 8 - 20%. At the same time achieves an inventive flat steel product in

Perforation test according to ISO 16630 regularly

Hole spreading ratios of at least 30%.

These property combinations succeed according to the

Invention by the exact addition of

inexpensive alloy components. These are coordinated so that the desired

mechanical properties can be achieved safely and the resulting flat steel product at the same time shows good weldability and coatability. Of paramount importance here is the setting of a suitable ratio between the elements which influence the austenite formation and hardenability of the steel and the elements which suppress the carbide formation. This ratio is in an inventive

Alloy adjusted by the factor ψ, in which the respective C, Mn, Cr, AI and Si content of the steel flows. The factor ψ should not be less than 1.5. Excessive levels of silicon or aluminum would negatively affect the coatability (silicon) or castability of the steel. at

Insufficient levels of carbon, manganese or chromium would not reach the required strength. Too much carbon and manganese can lead to increased Austenitgehalt content, which in turn in a

lower formability would result. This is avoided by setting the upper limit for the range in which the ψ-factor of a steel according to the invention is set to 3.0.

Carbon has several important functions in the steel of the present invention. On the one hand, the C content plays a major role in the formation of austenite and

Setting the A ₃ temperature. A sufficient C content allows full austenitization already at

Temperatures of less than 930 ° C. During subsequent quenching, the retained austenite is stabilized by carbon. This stabilization can be assisted by an additional heat treatment step, as provided by the invention in the process according to the invention. The strength of martensite is also strongly influenced by the C content of the steel. On the other hand, the martensite start temperature increases with increasing C content shifted to ever lower temperatures, which leads to production challenges. Out

For these reasons, the invention sees in steel

steel flat product according to the invention a C content of 0.05 to 0.2 wt .-%, in particular at least 0.065 wt .-% C, before, in practice, the positive effect of C in the steel according to the invention can then be used with particular certainty, if the C content 0.07 - 0.19 wt .-%

is.

For each specific dimensioning of the respective C content within the limits provided by the invention, the so-called carbon equivalent "CE" can also be used.

whose value is decisively influenced by the C content. To calculate the

Carbon equivalent CE has American Welding

Society 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 should be CE

not more than 1.1 wt .-% to ensure good weldability. A particularly good weldability can be ensured by the fact that the CE value is limited to at most 1.0 wt .-%. However, the CE value should not be less than 0.254 wt% in order to calculate the effect of the present invention of the carbon equivalent CE

To obtain alloying elements.

By presence of silicon in the steel one

According to the invention flat steel product is the production of cementite suppressed, would be bound by the carbon, which then no longer for the stabilization of the

Restaustenits would be available, and by which the stretching would be worsened. The same effect can also be achieved by alloying AI. However, a minimum of 0.2% by weight should be present in the steel provided by the invention. However, Si contents of more than 1.5% by weight would have a negative effect on the surface quality of a flat steel product according to the invention. Therefore, in a steel flat product of the present invention, the Si content is 0.2-1.5 wt%, and Si contents of at least 0.25 wt% or at most 0.95 wt% are practical for the practice have been found favorable.

Aluminum is added to the steel of a flat steel product according to the invention in the steelmaking process for deoxidizing and for setting any nitrogen present. AI can also be used for the suppression of cementite. However, in the presence of higher levels of Al, the austenitizing temperature also increases. Therefore, the Al content is one for an invention

Steel flat product provided steel limited to 0.01 to 1.5 wt .-%. If low austenitizing temperatures are to be ensured, it may be expedient to limit the Al content to a maximum of 0.1% by weight.

To any negative impact of Si and Al in the

Exclude steel flat product according to the invention, can - -

PCT / EP2015 / 059968 the sum of the contents of AI and Si in the steel of a

According to the invention flat steel product are limited to at most 1.7 wt .-%, with upper limits of at most 1.5 wt .-% have proven to be particularly favorable, especially with regard to an optimization of the weldability.

Manganese is important for the hardenability of the steel of a flat steel product according to the invention and also prevents the formation of unwanted perlite during the process

Cooling. The presence of Mn thus allows the formation of one for the formation of the invention

prescribed structure of appropriate initial structure

(Martensite and retained austenite). Too high an Mn concentration would negatively affect the elongation and weldability of the steel. Therefore, for the Mn content according to the invention a range of 1.0 to 3.0 wt .-%, in particular at least 1.5 wt .-% or at most 2.4 wt .-% provided.

Phosphorus has an unfavorable effect on the weldability of a flat steel product according to the invention. The P content should be as low as possible, in any case not exceed 0.02 wt .-%, in particular less than 0.02 wt .-% or less than 0.018 wt .-% amount.

The presence of effective levels of sulfur in the steel of a flat steel product according to the invention would lead to the formation of sulphides, in particular MnS or (Mn, Fe) S, which would have a negative effect on the elongation. To avoid this, the S content of the steel should be kept as low as possible, but not higher than 0.005 wt .-%, in particular less than 0.005 wt .-% or less than 0.003 wt .-% amount.

To the formation of nitrides, which are harmful to the

Forming should be avoided, the N content of the steel of a flat steel product according to the invention is limited to at most 0.008 wt .-%. Advantageously, the N content should be avoided to avoid any negative

Influence below 0.008 wt .-%, in particular less than 0.006 wt .-% amount.

Chromium in amounts of up to 1.0 wt .-% can in

Steel provided according to the invention can optionally be used as an effective inhibitor of perlite and also contributes to the strength. At contents of more than 1.0% by weight Cr, there is the danger of pronounced grain boundary oxidation. In order to be able to use the positive effect of Cr, at least 0.05 wt .-% are required. The presence of Cr in the steel has a particularly favorable effect

in accordance with the present invention, when at least 0.15% by weight of Cr is present, with optimum effect being achieved at levels of up to 0.8% by weight.

Optionally, the steel of a

In addition flat molybdenum product also contain molybdenum in amounts of 0.05 to 0.2 wt .-%. Mo in these grades

also suppresses the formation of unwanted perlite particularly effective.

The steel of a flat steel product according to the invention may further optionally contain levels of one or more

Contain micro-alloying elements to aid in the strength through the formation of very finely divided carbides. For this purpose, contents of Ti and Nb have proven particularly suitable.

Ti contents of at least 0.005 wt .-% and Nb contents of at least 0.001 wt .-% lead each alone or in combination with each other to freeze the grain and

Phase boundaries during heat treatment, the one

Inventive flat steel product in his

undergoes preparation according to the invention. Ti may also be used to set the nitrogen present in the steel to allow for the action of other alloying elements, particularly boron. Too high

However, concentration of micro-alloying elements would lead to oversized carbides, which could initiate cracks at high forming levels. Therefore, the Ti content of the steel of a present invention

Limited to a maximum of 0.2 wt .-% and its Nb content to a maximum of 0.05 wt .-%, wherein it has been found to avoid adverse effects of the presence of micro-alloying advantageous if the sum of the contents of Nb and Ti does not exceed 0.2% by weight.

The boron also optionally present in the steel of a flat steel product according to the invention segregates on the

Phase boundaries and slows their movement. This leads to a fine-grained structure, which has an advantageous effect on the mechanical properties. In order to make use of the effect of B, Ti may be added to the steel as mentioned above. In order to be able to use the positive effect of B, the steel provided according to the invention must contain at least 0.0001% by weight of B. At salaries of more than 0.005 wt .-%, no increase in the positive effect of B can be found more.

To protect it against corrosive attacks, the flat steel product according to the invention can be provided with a metallic protective coating. This can be applied in particular by hot dip coating. In particular, coatings based on Zn are suitable for a flat steel product according to the invention.

The method according to the invention for producing a high-strength steel flat product comprises the following

Steps: a) providing an uncoated flat steel product according to any of claims 1 to 5; b) heating the flat steel product to above the A _C 3 temperature of the steel of the flat steel product

 lying at a maximum of 950 ° C

Austenitizing temperature T _H z, wherein the heating up to a 200 - 400 ° C amounting inflection point temperature T _w with a heating rate θ _Η ι of 5 - 25 K / s and then to Austenitisierungstemperatur T _HZ with a heating rate Θ _Η 2 of at least 2 - 10 ° K / s takes place; c) holding the flat steel product in the

Austenitizing temperature T _H z over a

Austenitizing time t _H z of 5 - 15 s; d) first cooling the flat steel product over a

Cooling time t _k from 50 - 300 s to one

Intermediate temperature T _K of not less than 680 ° C; e) Quenching of the flat steel product starting from the intermediate temperature T _K with a cooling rate of more than 30 K / s to one

Cooling stop temperature T _Q for which applies:

(T _MS - 175 ° C) <T _Q <T _MS

 with TMS = martensite start temperature of the steel making up the flat steel product; f) holding the flat steel product on the

Cooling stop temperature T _Q for a holding period t _Q of

10 - 60 s; g) treating the to the cooling stop temperature T _Q

 Quenched flat steel product, g.l) wherein the flat steel product via a

Total treatment time t _B of 10 - 1000 s at a treatment temperature T _B , which is at least equal to the cooling stop temperature T _Q and not higher than 550 ° C, in particular not higher than 500 ° C, is maintained. or g.2) the flat steel product starting from the

Cooling stop temperature T _{Q is} heated to a 450 - 500 ° C treatment temperature T _B , the steel flat product then optionally at this treatment temperature T _B over a

Holding period t _{B is} held isothermally, wherein the heating to the treatment temperature T _B with a heating rate θ _Β ι of less than 80 ° K / s and takes place as the sum of the heating time required for the heating t _BR and the Holding time t _B i formed total treatment time t _B T is 10 - 1000 s, and wherein the

Steel flat product is passed through a melt bath after being treated to coat it with a Zn-based metallic protective coating; h) from the treatment temperature T _B outgoing cooling at a cooling rate Θ _Β 2 of more than 5 K / s.

The principle of the procedure according to the invention is illustrated in the diagram attached as FIG.

In step a) becomes a flat steel product

provided, which consists of a steel having the above-described composition. In which

provided flat steel product may be

in particular a cold rolled flat steel product. However, it is also conceivable to use a hot rolled flat steel product according to the invention

to process .

For heating the flat steel product on the

Austenitizing temperature T _H z (step b)) are basically two interruption-free successive steps possible, the flat steel product in the first step with a heating rate Θ _Η ι of

5 - 25 K / s is heated to a turning point temperature T _w , which is 200 - 400 ° C. Subsequently, the heating in the second step with a

Heating rate Θ _Η 2 of 2 - 10 K / s continued until the Austenitisierungstemperatur T _H z is reached. Since the heating rates Θ _Η ι, Θ _Η 2 - -

PCT / EP2015 / 059968 overlap, the heating to the austenitizing temperature can also be in a course with a 5-10 K / s constant amounts

Heating rate done. The

Heating rates θ _Η ι and Θ _Η 2 in step b) are then the same.

The austenitizing temperature T _H z must be above the A ₃ temperature. The A ₃ temperature is

analytic-dependent and can be combined with the following

estimate empirical equation (alloy contents used in% by weight):

A ₃ [° C] = 910 - 203V% C - 15, 2% +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 alloy of the steel selected according to the invention makes it possible to limit the austenitizing temperature T _H z to a maximum of 950 ° C., and thus allows for the

Implementation of the method according to the invention

to keep operating costs limited.

In order to prevent the formation of large austenite grains, which adversely affects the formability

Austenitizing the time t _HZ , over which the flat steel product in step c) at Austenitisierungstemperatur T _H z is maintained, on

5 - 15 seconds limited, with the Austenitizing t _H z may be less than 15 s to avoid any unwanted grain growth.

In step d) follows one of the

Austenitizing time t _H z outgoing controlled and slow cooling of the flat steel product. These

Cooling can be about 50 - 300 seconds and extend must end at an intermediate temperature T _K, which is not lower than 680 ° C in order to avoid the undesirable formation of ferrite. Up is the

Intermediate temperature T _K typically at 775 ° C

limited, since at higher intermediate temperatures T _K, the cooling power required for the subsequent cooling disproportionately high and, consequently, the

Profitability of the process is called into question.

After the slow cooling in step d), the steel flat product is quenched in step e) with a high cooling rate 0 _Q to an analysis-dependent cooling stop temperature T _Q. The high cooling rate 9 _Q can be achieved, for example, with a modern Gasj etkühlung.

The minimum cooling rate _Q 9, which is necessary in order to avoid the ferritic and bainitic transformation, is more than 30 K / s. The area where the

Cooling stop temperature T _Q is, is up by the martensite start temperature T _MS , and down by a 175 ° C below the martensite start temperature T _M s temperature (T (T _MS -175 ° C) <T _Q <T _MS ).

The martensite start temperature can be estimated by the following equation (alloy contents

used in% by weight): T _MS (° C) = 539 ° C + (-423% C - 30.4% Mn - 7.5% Si + 30% A1) ° C / wt% with% C: C content of the steel,

 % Mn: n content of the steel,

 % Si: Si content of the steel,

 % A1: Al content of the steel.

In step f), the flat steel product over a holding time t _Q of 10 - 60 seconds on the

Cooling stop temperature T _Q held to the structure

adjust. In the course of this step will be a

martensitic structure with up to 30% retained austenite

receive. How much martensite is formed in this step is essentially dependent on the cooling stop temperature is as much below the martensite start temperature T _MS.

In contrast to the prior art presented at the outset, the invention does not seek to stabilize

Retained austenite to room temperature. Rather, in step g) completed heat treatment of

A flat product of a controlled redistribution of carbon to the target that the microstructure of the flat steel product obtained after completion of the process in

Essentially consists of two different types of martensite, namely tempered martensite and unannealed martensite.

According to the invention, step g) comprises two

Process variants gl) and g.2), of which the first variant gl) to an uncoated steel flat product according to the invention and the second variant g.2) to a with a Zn coating provided inventive

Flat steel product leads.

The temperature control in both variants g.l), g.2) of step g) is in each case selected such that the retained austenite present in the structure with carbon from the supersaturated martensite is enriched. The

Formation of carbides and the decomposition of retained austenite is limited by the limitation of the invention

Total treatment time t _B deliberately suppressed. This is 10 - 1000 seconds to a sufficient

Redistribution of carbon.

For the first variant of the method gl) treating the flat steel product in the step g) comprises an over the total processing time t _B T extending holding the flat steel product at a treatment temperature T _B which is at least equal to the cooling stop temperature T _Q, and not higher than 550 ° C, wherein a cooling stop temperature T _Q of at most 500 ° C has proven to be particularly favorable. In this case, in the variant gl), the treatment temperature T _{B may} also be higher than the cooling stop temperature T _Q. In this case, the steel flat product, starting from the cooling stop temperature T _Q to the respective

Treatment temperature T _B heated, wherein the heating amounts to less than 80 K / s

Heating rate Θ _Β1 should be done.

According to the second alternative of step g), however, the flat steel product with a

Heating rate 0 _Bi of less than 80 K / s brought to a treatment temperature T _B of 400 - 500 ° C to the residual austenite with carbon from the supersaturated To enrich martensite. The formation of carbides and the decomposition of retained austenite is targeted by the inventive limitation of the total treatment time t _BT

suppressed, which in this variant g.2) of

Step g) from the time required for the heating heating time t _B R and the holding time t _B i composed over which the flat steel product isothermally in the

Temperature T _{B is} maintained. At a sufficiently slow rate of heating Θ _Β ι the isothermal holding can also be omitted, so the holding time t _B i be equal to "0".

In the second variant g.2) of the step g), the flat steel product passes through the following

Warming and optional holding at the

Treatment temperature T _B a hot-dip coating in which it is coated with a Zn coating. For this purpose, the treatment temperature T _{B can} be chosen so that it corresponds to the inlet temperature with which the

Steel flat product is to enter into the respective melt bath. Typically, this is the

Treatment temperatures T _B in the range of 450 - 500 ° C.

Regardless of which variant has been selected, the steel flat product after completion of step g) for re-production of martensite with a

Cooling rate Θ _Β2 cooled more than 5 K / controlled, the cooling rates are typically at most 50 K / s.

The process according to the invention can be carried out in continuous flow in this customary manner

conventional annealing plants or coil coating plants are performed. The flat steel product according to the invention has a structure that

at least, in particular more than 90 area% of martensite, of the at least, in particular more than 50 area%, tempered martensite from the first

 Cooling step (step f)),

 - at most, in particular less than 5% by area of bainite,

 - at most, in particular less than 2% by volume of retained austenite and

 - at most, in particular less than 5 area% of polygonal ferrite

consists .

The microstructure of a flat steel product according to the invention is very fine with an average particle size of less than 2 μπ and can hardly be assessed by means of conventional optical microscopy. Therefore, a judgment by means of scanning electron microscopy (SEM) and a

Minimum magnification of 5000x recommended.

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

As a measure of the quality of the mechanical properties of a flat steel product according to the invention, the distortion of the crystal lattice can also be used. 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 the electron backscatter diffraction (EBSD). With the EBSD method, a sample in the SEM

scanned dot-shaped, wherein at each measuring point a

Diffraction pattern is taken, from which the

determine crystallographic orientation. Details on the measurement and the various evaluation methods can be found in the manuals. A useful EBSD evaluation method is the so-called Kernel Average

Misorientation (KAM - further description in the manual "OIM Analysis v5.31" of the EDAX Inc., 91 McKee Drive, Mahwah, NJ 07430, USA), whereby the orientation of a

Measuring point is compared with the neighboring points.

Below a threshold, typically 5 °, adjacent points belong to the same (deformed) grain. Above the threshold, the neighboring dots belong to different (sub-) grains. Because the texture is so fine, we recommend a maximum step size of 100 nm for the EBSD. For the assessment of the structure of

Steel flat products according to the invention, the KAM is evaluated by the third neighbor points. An inventive flat steel product must have a KAM average of a

Measuring range of at least 75 μιη x 75 pm of more than 1.20 °, preferably more than 1.25 °.

The invention is based on

Embodiments explained in more detail.

To test the invention, Samples 1-43 are conventionally produced steel sheets

which consisted of steels A - G having the compositions given in Table 1. Table 1 additionally shows for each of the steels A-G the factor ψ and the carbon equivalent CE, which according to the above-explained formulas ψ = (% C +% Mn / 5 +% Cr / 6) / (% Al +% Si) and

CE =% C + (% Si +% n) / 5 + (% Cr +% Mo) / 6 with% C the respective C-, with% Si the respective Si, with% Mn the respective Mn-, with% Cr the respective Cr, with% Mo the respective Mo and with% A1 the respective Al content of the steels A - G have been calculated.

The steels E, F and G therefore did not fulfill the requirements for the coordination of the alloying elements and austenitic alloy elements which are determined by the factor ψ according to the invention.

The samples 1 - 43 made of the steels A - G

have the procedure shown in Figure 1

completed. They are first with a

Heating rate θ _Η ι on a

Turning point temperature T _w and then with a

Rate of heating Θ _Η 2 on one

Austenitizing temperature T _H z has been heated, the

each above the A ₃ temperature of the respective steel, but lower than 950 ° C. The thus heated samples 1 - 43 are then over a

Austenitizing time t _H z at the

Austenitization temperature T _H z held and then over a cooling time t _K to an intermediate temperature T _K

cooled. Upon reaching the intermediate temperature T _K has an accelerated cooling with a

Cooling rate 9 _{Q were} used in which the samples 1 - 43 have been cooled to a cooling stop temperature T _Q , which was lower by up to 175 ° C than the martensite TMS of the respective steel A - G of the samples 1 - 43. At the cooling stop temperature T _Q , the samples 1 - 43 have been held for a holding period t _Q of 10 - 60 s, followed by a

Heating rate θ _Β ι over a heating time t _B R to be heated to a treatment temperature T _B on which they have been held in some experiments for an additional holding time t _B i. Subsequently

Cooling took place to room temperature with a cooling rate Θ _Β 2 ·

The above, in the experiments

applied parameters are given in Table 2. Of the samples 1 - 32 consisting of steels A - D according to the invention, the samples 3 (6 _Q <30 K / s), 11 (T _HZ <A ₃ ), 18 (TQ> 500 ° C.), 19 (G _Q <30 K / s), 28

(T _HZ <A ₃ ) and 29 (9 _Q <30 K / s) have not been treated according to the invention.

In the course of the last cooling, samples 1-43 would have been in cases where the treatment temperature T _B was one for entry into a Zn melt bath

sufficient level of about 450 ° C, a

Can pass through the melt bath. In the context of the experiments, however, this has been omitted, without this having influenced the results of the investigation. On the samples 1 - 43 obtained after the heat treatment, the mechanical properties are yield strength R _p o, 2 _/

Tensile strength R _m , the ratio R _p o, 2 / Rnw the

Elongation at break A ₅₀ (according to DIN EN ISO 6892, sample form 1), the product Rm * A ₅₀ , and the hole spreading ratios λΐ, λ2 (according to ISO 16630) have been determined. Likewise, the microstructural fractions of ferrite "F", tempered martensite "AM", retained austenite "RA", unannealed martensite "M" and bainite "B", and the kernel average misorientation value "KAM" were determined. The respective property values are given in Table 3 for each of Samples 1-43.

The mechanical properties achieved in the annealed material with a quantification of the microstructure can be found in Table 3. In the samples, both the specifications of the invention with respect to the alloy of the respective steel, as well as the inventive

Conditions of heat treatment meet

regularly strain limits R _p o, 2 of more than 800 MPa,

Tensile strengths R _m of more than 950 MPa,

Elongation at break A50 of more than 8% at

Hole spreading ratios λ1, λ2 of more than 30% regularly achieved.

On the other hand, Comparative Examples Bll and D28 illustrate the effect of insufficient

Austenitizing temperature T _H z In these examples, the microstructure has not been completely austenitized, so that too much ferrite forms in the microstructure. This leads to extremely localized damage and premature failure during forming. Comparative Example D29 shows, as does too high austenitization at high temperatures

Formability can adversely affect.

Comparative Examples E33-E38 show a

Exceeding the desired yield strength and

Strength, indicating the non-inventive

Composition and too high ferrite content in the structure obtained is due.

Finally, Comparative Examples F39-F42 show the effects of a too low ψ-factor, which also leads to deviations from the desired microstructure. The

Minimum strength has been achieved in part, but the yield strength and the hole widening are not in the target area here.

With the comparison example G43 it becomes clear that an excessively high ψ factor leads to excessively high retained austenite parts and a reduced formability, which manifests itself in poor hole expansion values λ1, λ2.

 Data in wt .-%, balance Fe and unavoidable impurities

 Underlined and bold values indicate 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,% AI = Al content

Table 1

Ser.

Steel © _Η ι Tw © H2 A ₃ THZ t _H z T _K tK OQ TQ TS to Θ _Β i fei T _B Q _B 2 No. [K s] t ° C] [K / s] [° q [° c ] [s] [° C] [s] [K / sl [° C] ra [s] [K / s] [s] [s] [° CJ [K / s]

A 1 10 300 5 820 860 10 760 105 -31 310 425 50 3 46.7 0 450 -11

A 2 11 270 4 820 860 12 760 100 -47 310 425 50 3 46.7 0 450 -11

A 3 11 270 4 820 860 12 760 100 -16 370 425 40 2 40.0 0 450 -11

A 4 5 270 5 820 860 10 775 100 -42 350 425 50 3 33,3 0 450 -10

A 5 5 270 5 820 860 10 775 100 -39 370 425 50 1, 75 45.7 0 450 -9

A 6 5 270 5 820 860 12 775 120 -36 370 425 50 1, 75 45.7 15 450 -20

A 7 5 270 5 820 860 12 775 120 -36 370 425 50 1 55.0 20 425 -20

A 8 10 270 4 820 840 12 740 120 -22 300 425 90 -0.03 333.3 0 290 -20

A 9 11 300 5 820 840 12 740 120 -20 325 425 90 -0.09 388.9 0 290 -20

A 10 5 270 5 820 860 12 740 120 -21 325 425 90 -0.09 388.9 26 290 -20

B 11 5 300 2 809 780 8 760 135 -21 350 410 15 3 33.3 15 450 -10

B 12 5 300 2 809 840 10 760 110 -35 290 410 12 2 80.0 15 450 -12

B 13 5 300 2 809 840 12 760 105 -30 350 410 12 2.5 40.0 15 450 -12

B 14 7 300 2 809 860 10 800 120 -35 230 410 10 5 44.0 15 450 -20

B 15 7 300 2 809 840 14 700 135 -30 275 410 15 3 58.3 15 450 -10

B 16 8 300 2 809 860 10 740 120 -32 300 410 12 25 7,6 15 490 -15

B 17 5 300 2 809 840 12 740 120 -45 325 410 10 4 31, 3 15 450 -15

C 18 9 255 3 807 860 10 740 105 -32 510 411 10 -1 60.0 16 450 -20

C 19 9 255 3 807 860 12 740 105 -15 350 411 10 3 33.3 0 450 -20

C 20 20 295 3 807 860 10 740 105 -49 290 411 50 3 53.3 22 450 -20

C 21 5 270 5 807 860 14 760 95 -42 350 411 50 3 33.3 0 450 -20

C 22 14 310 5 807 860 14 715 125 -39 350 411 50 3 33.3 0 450 -10

C 23 10 270 3 807 860 12 700 125 -39 350 411 50 1, 5 50.0 0 425 -10

C 24 10 270 3 807 840 10 760 100 -22 300 411 90 -0.03 333.3 25 290 -10

C 25 15 290 5 807 840 10 780 80 -20 325 411 90 -0.09 388.9 0 290 -10

C 26 5 270 5 807 860 12 750 140 -21 325 411 90 -0.09 388.9 0 290 -16

C 27 20 300 3 807 860 12 775 135 -32 350 411 90 -0.2 300.0 0 290 -16

D 28 5 270 5 796 775 10 750 120 -32 290 400 11 3 45.0 0 425 -10

D 29 5 270 5 796 840 25 750 120 -44 290 400 10 3 53.3 25 450 -10

D 30 5 270 5 796 840 10 750 135 -38 250 400 12 3.5 57.1 0 450 -10

D 31 5 270 5 796 840 12 700 70 -50 350 400 15 3.5 28.6 0 450 -10

D 32 5 270 5 796 840 12 710 120 -45 200 400 10 3 83.3 0 450 -10

E 33 5 270 5 828 860 10 700 120 -54 300 454 50 3 50.0 5 450 -12

E 34 11 270 3 828 860 12 685 140 -49 300 454 50 3 50.0 5 450 -12

E 35 11 270 3 828 860 12 700 165 -42 350 454 50 3 33.3 5 450 -20

E 36 5 270 5 828 840 14 700 135 -22 300 454 90 -0.03 333.3 4 290 -10

E 37 5 270 5 828 840 14 700 35 -20 325 454 90 -0.09 388.9 0 290 -10

E 38 5 270 5 828 860 8 735 135 -21 325 454 90 -0.09 388.9 0 290 -10

F 39 5 270 4 820 860 12 700 120 -31 310 407 50 3 46.7 5 450 -16

F 40 5 270 5 820 860 10 685 125 -33 310 407 20 3 46.7 0 450 -16

F 41 10 300 3 820 840 10 720 140 -22 350 407 180 -0.1 600.0 0 290 -9

F 42 8 270 4 820 840 12 720 80 -22 300 407 90 -0.03 333.3 0 290 -9

G 43 5 340 4 771 850 10 720 100 -21 325 358 25 8 40.0 25 465 -1

Underlined and bold values indicate values outside the specifications of the invention

Table 2

 Underlined and bold values indicate values outside the specifications of the invention

 Table 3

Claims

PATENT APPLICATIONS 1. Flat steel product having a tensile strength R _m of has at least 950 MPa, a yield strength of at least 800 MPa and an elongation at break A ₅₀ of at least 8%, the flat steel product consisting of a steel other than iron and unavoidable  Impurities (in% by weight) C: 0.05-0.20%,  Si: 0, 2 - 1.5%,  AI: 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 of the group "Cr, Mo, Ti, Nb, B" in the following  Held :  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%  contains being for the ratio ψ = (% C +% Mn / 5 +% Cr / 6) / (% A1 +% Si) with% C: respective C content of the steel  % Mn: respective Mn content of the steel  % Cr: respective Cr content of the steel  % A1: respective Al content of the steel  % Si: respective Si content of the steel  applies 1, 5 <ψ <3 and wherein the flat steel product has a structure consisting of  - not more than 5% bainite,  - not more than 5% by area of polygonal ferrite,  - not more than 2% by volume of retained austenite,  and  - at least 90% martensite by area, with at least half of the martensite being tempered martensite. 2. Flat steel product according to claim 1, d a d u r c h  e n c i o n e s, s e s for the  Carbon equivalent 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 applies 0.254 <CE <1.1% by weight. 3. Flat steel product according to claim 2, d a d u r c h  g e n c i n e s, the s  Carbon equivalent CE is at most 1.0% by weight. 4. Flat steel product according to one of the preceding  Claims, in which the sum of its contents of Si and Al is not more than 1.7% by weight. 5. Flat steel product according to one of the preceding  Claims, in which the sum of the contents of Ti and Nb is at most equal to 0.2% by weight. 6. Flat steel product according to one of the preceding  It claims that it is provided with a metallic protective coating applied by hot-dip coating. 7. A method for producing a high strength, optionally provided with a Zn-coated steel flat product, comprising the following steps: a) providing one according to one of claims 1  up to 5 trained, uncoated Flat steel product; b) heating the flat steel product to a above the A ₃ temperature of the steel of the Austenitizing temperature T _H z lying at a maximum of 950 ° C., the heating being up to 200-400 ° C. amounting inflection point temperature T _w with a heating rate θ _Η ι of 5 - 25 K / s and then to Austenitisierungstemperatur T _H z with a heating rate Θ _Η 2 of at least 2 - 10 ° K / s takes place; c) holding the flat steel product in the Austenitizing temperature T _H z over a Austenitizing time t _HZ of 5 - 15 s; d) first cooling the steel flat product over a cooling time t _k of 50-300 s to one Intermediate temperature T _K of not less than 680 ° C; e) from the intermediate temperature T _K outgoing Quenching the steel flat product with a cooling rate 0 _Q greater than 30 K / s to a cooling stop temperature T _Q for which: (T _MS ~ 175 ° C) <T _Q <T _MS  with TMS = martensite start temperature of the steel making up the flat steel product; f) holding the flat steel product on the Cooling stop temperature T _Q for a holding period t _Q of 10 - 60 s; g) treating the chilled to the cooling stop temperature T _Q quenched flat steel product, gl) wherein the flat steel product via a Total treatment time t _B of 10 - 1000 s at a treatment temperature T _B , which is at least equal to the cooling stop temperature T _Q and not higher than 550 ° C is maintained,  or g.2) wherein the steel flat product starting from the cooling stop temperature T _Q amounts to a 450 - 500 ° C Treatment temperature T _{B is} heated, the steel flat product is then optionally held isothermally at this treatment temperature T _B over a holding period t _B i, wherein the heating on the Treatment temperature T _B with a heating rate θ _Β ι of less than 80 K / s is carried out and the sum of the time required for the heating heating time t _B R and the holding time t _B i formed Total treatment time t _Β τ 10-1000 s, and wherein the steel flat product is passed through a melt bath after being treated to coat it with a Zn-based metallic protective coating; h) starting from the treatment temperature T _B Cooling at a cooling rate Θ _Β2 of more than 5 K / s. 8. The method of claim 7, d a d u r c h  g e n c e n c e s e s in the Step b) the heating rates θ _Η ι and Θ _Η 2 are the same. 9. The method according to claim 7 or 8, d a d u r c h  g e n c i n e s, the s  Flat steel product in step g.l) of the Cooling stop temperature T _Q with a Heating rate θ _Β ι of less than 80 K / s to the treatment temperature T _{B is} heated.