ES2921013T3 - Flat steel product and process for its manufacture - Google Patents

Abstract

The invention relates to a bake hardening treatment of a very high strength flat steel product and to a method of producing such a flat steel product. The flat steel product consists of a steel consisting of (in wt%) 0.1-0.5% C, 1.0-3.0% mn, 0.5-2.0% Si, 0.01-1.5% Al, 0.001-0.008% N, up to 0.02 %P, up to 0.005%s and optionally one or more of the following elements: 0.01-1.0%cr, 0.01-0.2%mo, 0.001-0.01%b, and optionally a total of 0.005-0.2%V, Ti and Nb, La IT proportion is not more than 0.10%, and as the rest, iron and unavoidable impurities. The flat steel product has a microstructure consisting of not more than 15 area % ferrite, not more than 5 area % bainite, at least 5 volume % residual austenite and at least 80 area % martensite, of which at least 75% by area is quenched martensite. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Flat steel product and process for its manufacture

The invention relates to a high-strength flat steel product suitable for a furnace hardening treatment and to a process for the manufacture of said flat steel product.

Flat steel products suitable for a furnace hardening treatment (BH treatment) are also called furnace hardening flat steel products (BH flat steel products) and are commonly used in applications in the manufacture of automobiles, such as for body parts.

When talking about flat steel products in the present case, this means steel strips, steel sheets or blanks generated from these, such as slabs.

BH steel flat products exhibit a lower strength level before BH treatment than after BH treatment. This circumstance is used to carry out the shaping before the BH treatment in the case of flat steel products that are going to be shaped and therefore with lower elastic limits and better shaping capacity. The level of resistance increases with the BH treatment, in which the material is subjected to a heat treatment. The BH treatment is normally carried out for 3 to 40 minutes within a temperature range of 120 to 250 °C. The BH treatment stimulates diffusion of interstitially dissolved element atoms, where they can attach to dislocations. This makes it difficult for dislocations to move, leading to an increase in yield strength. This effect of increasing the yield strength is also called the oven hardening effect (BH effect) and the difference of the yield strengths before and after the BH treatment is also called the oven hardening value (BH value). The higher the BH value, the greater the increase in yield strength due to the BH treatment.

If a flat steel product has a pronounced elastic limit, the term elastic limit in the present case means a characteristic value called the upper elastic limit ReH. Before the BH treatment, in flat products made of BH steel, there is often no pronounced elastic limit, but only an elongation limit. If in the present case we speak of the increase in the yield strength or the BH value, this means the difference between the elongation limit RpO,2 before the BH treatment and the yield strength ReH after the BH treatment for flat steel products that do not have pronounced elastic limit, but rather an elongation limit.

A high BH value has a positive effect on the buckling resistance of components made with BH flat steel products. As a result, it is possible to reduce component thickness by using flat steel products, which have a high BH value, while maintaining component stiffness.

The BH effect has so far been used for soft steels, which often have a predominantly ferritic matrix, only low proportions of martensite, and tensile strengths below 700 MPa.

From US 2013/0240094 A1 cold-rolled oven-hardened sheets for automotive applications are known. The plates must be made of a steel that, in addition to iron and unavoidable impurities, contains 0.0010 - 0.0040% by mass of C, 0.005 - 0.05% by mass of Si, 0.1 - 0.8% in mass of Mn, 0.01 - 0.07 mass% of P, 0.001 - 0.01 mass% of S, 0.01 - 0.08 mass% of Al, 0.0010 - 0.0050 mass% of mass of N, 0.002 -0.02% by mass of Nb and 0.005 - 0.050% by mass of Mo, where the value of the ratio of the parts of Mn and P [Mn %]/[P %] must be between 1 .6 and 45, and the amount of carbon present in solid solution obtained from [C%]-(12/93)x[Nb%] must be between 0.0005 and 0.0025% by mass. Cold rolled sheets suitable for oven hardening must satisfy the equation X(222)/{X(119)+X(200)>3.0. In this respect, X(222), x(110), and X(200) are the integrated intensity of the X-ray diffractometry of the {222} plane, the {110} plane, and the {200} plane that are parallel to a plane that is located starting from the sheet area in % of the sheet thickness. The sheets must have a good ability to be deep drawn and have tensile strengths of 300 to 450 MPa.

High-strength steel flat products are commonly used to make reduced component thicknesses with good buckling resistance. High strength steels are characterized by a high proportion of martensite in the microstructure. Martensite is a carbon-rich microstructural component from which carbon can diffuse into other microstructural components when thermally activated. The higher the proportion of martensite in the microstructure, the more pronounced the BH effect will usually be. However, high proportions of martensite are accompanied by poor deformability.

High-strength hot-dip galvanized steel sheets are known from US 2017/009315 A1. Sheets must contain 0.05 - 0.30 mass % C, 0.5 - 3.0 mass % Si, 0.2 - 3.0 mass % Mn, up to 0.10 mass % of P, up to 0.010 mass % S, up to 0.010 mass % N and 0.001 - 0.10 mass % Al, the balance iron and unavoidable impurities. The microstructure must contain 50 - 85 area % martensite, less than 5 area % ferrite and the remainder bainite and a dislocation density of at least 5.0 x 1015 m'2 and at least 0.08 mass %. dissolved carbon. Veneers must be suitable for hardening in oven and present good bending properties and tensile strengths of 1180 MPa or more. The sheets are produced by conventional continuous casting, hot rolling and cold rolling. Cold-rolled sheets must be heated at annealing temperatures from Ac3+50 °C to 930 °C, held at this annealing temperature for 30 to 1200 s, then cooled at an average rate of 15 °C/s or more to a temperature cooling stop temperature between 450 °C to 550 °C, then immersing in a molten bath for 10 to 60 s at 480 to 525 °C within a maximum of 30 s from when the cooling stop temperature is reached, and then at 200 °C at an average cooling rate of 15 °C/s or more.

JP 2014034716 discloses a high-strength steel flat product with a tensile strength of 980 MPa or more, good drawability and good hole elongation properties, and a method for its manufacture. The flat steel product has a microstructure containing less than 80% per unit area of martensite, of which less than 75% per unit area is tempered martensite.

WO 2016/177763 A1 describes a flat steel product with an optimized combination of strength and elongation and a method for its manufacture. The residual austenite content in the microstructure of the flat steel product should be less than 2% by volume to improve the isotropy of deformability.

JP 2015224359 A describes a high-strength steel flat product with good processing properties and a tensile strength of 980 MPa and more, as well as a method for its manufacture. The reheating (partitioning) that is done after quenching to coolant temperature is done in very short times clearly less than 10 s to provide an efficient and cost-effective process. In this connection a low temperature transformation microstructure is obtained comprising tempered martensite, bainite and tempered bainite.

In this context, the object of the invention was to present a high strength steel flat product with optimized properties, in particular very good furnace hardening properties, and very good forming properties both before and after a BH treatment.

In addition, a method for the manufacture of said flat steel product should be indicated.

With regard to the flat steel product, the objective was achieved by means of a product that has at least the characteristics indicated in claim 1.

With regard to the process, the object was achieved in that at least the process steps specified in claim 10 are completed in the manufacture of a flat steel product according to the invention.

A flat steel product according to the invention is made up of a steel that is made up of (in % by weight)

0.1-0.5% C,

1.0 - 3.0% manganese,

0.9-2.0% Si,

0.02-1.5% Al,

0.001 - 0.008% N,

up to 0.02% P,

up to 0.005% S

and optionally one or more of the following elements

0.01-1.0% Cr,

0.01 - 0.2% Mo,

0.001 - 0.01%B

and optionally from a total of 0.005 - 0.2% of V, Ti and Nb, where the proportion of Ti is not greater than 0.10% and iron and unavoidable impurities as the rest, and where the flat steel product has a microstructure that is made up of

not more than 15% per unit area of ferrite,

- not more than 5% per unit area of bainite,

- at least 5% by volume of residual austenite and

- at least 80 % per unit area of martensite, of which

at least 75% per unit area is tempered martensite.

If we are talking about ferrite in the present case, then we are dealing with polygonal ferrite in each case. With respect to total proportion of martensite in the microstructure, at least 90 % of the martensite has a martensite lancet length of at most 7.5 |jm and a martensite lancet width of at most 1000 nm.

A flat steel product according to the invention is characterized in that, before a BH treatment, it has an elongation limit Rp0.2 greater than 700 MPa or an elastic limit ReH greater than 700 MPa, a tensile strength Rm of 950 - 1500 MPa as well as an A80 elongation of 7 - 25% and has a high oven hardening (BH) potential. The BH potential is expressed in that the flat steel product exhibits an increase in yield strength of at least 80 MPa after a BH treatment and an A80_BH elongation that is at least half of the A80 elongation before the BH treatment.

If information on the content and composition of alloys is given here, it refers to weight or mass, unless expressly stated otherwise.

The carbon content of the steel of a flat steel product according to the invention is 0.1 to 0.5% by weight. On the one hand, carbon contributes to the formation and stabilization of austenite. Especially during the first cooling after austenitization and during the subsequent split annealing, C contents of at least 0.1% by weight, preferably at least 0.12% by weight, help to stabilize the austenitic phase, whereby it is possible to guarantee in the flat steel product according to the invention a proportion of residual austenite of at least 5% by volume. Residual austenite can be stabilized particularly reliably if the C content is at least 0.14% by weight. On the other hand, the C content has a strong influence on the strength of martensite. This is true of both the strength of the martensite formed during the first quench and the strength of the martensite formed during the second quench beginning after partition annealing. To take advantage of the influence of carbon on the strength of martensite, the C content must be at least 0.1% by weight. Furthermore, a minimum content of 0.1% by weight is required to provide sufficient carbon atoms for a diffusion of the dislocations present in the material during a subsequent BH treatment and thus ensure a pronounced BH effect. Particularly high BH values are obtained when the C content is at least 0.14% by weight. However, with increasing C content, the Ms martensite starting temperature also shifts to lower temperatures. Therefore, a C content of more than 0.5% by weight could cause not enough martensite to form during quenching. The workability, in particular the weldability, is also impaired at higher C contents, whereby the C content should be at most 0.5% by weight, preferably at most 0.4% by weight.

As an alloying element, manganese is important for the hardenability of the steel as well as for preventing the formation of the pearlite microstructure component during early quenching. The Mn content of the steel of a flat steel product according to the invention is therefore at least 1.0% by weight, preferably at least 1.9% by weight, in order to provide a microstructure free of perlite for the following process steps after the first quench. However, as the Mn content increases, the weldability deteriorates and the risk of serious segregation increases. Segregations are chemical heterogeneities in the composition formed during the solidification process in the form of macroscopic or microscopic disintegrations. To reduce segregation and ensure good weldability, the Mn content of the steel in a flat steel product according to the invention is limited to a maximum of 3.0% by weight, preferably a maximum of 2.7% by weight. .

The Si content of the steel of a flat steel product according to the invention is limited to 0.9-2.0% by weight. Si as an alloying element helps to suppress cementite formation. Cementite is an iron carbide. Due to the formation of cementite, the carbon is bound in the form of iron carbide and is no longer available in atomic form for solution in the iron lattice. However, atomic carbon, which dissolves interstitially in the iron lattice, contributes significantly to the stabilization of residual austenite on the one hand and to the enhancement of the BH effect on the other. In turn, the residual austenite contributes to improving the formability, in particular the elongation, both before and after the BH treatment. A similar effect in terms of stabilization of residual austenite can also be achieved by alloy addition of aluminum. If the steel contains at least 0.2% by weight of Al, the minimum Si content that is at least necessary to obtain a flat steel product according to the invention can be reduced to 0.5% by weight. If the Al content is less than 0.2 wt%, the Si content should be at least 0.9 wt%. However, since a high Si content may have a negative effect on the surface quality of the flat steel product, the steel should not contain more than 2.0% by weight, preferably not more than 1.6% by weight. weight.

Aluminum is present in the steel of a flat steel product according to the invention in contents of 0.02-1.5% by weight. Al is added for deoxidation and grain refinement. Grain refinement occurs through the formation of AlN clusters and AlN precipitations, which in each case inhibit grain growth during austenitizing annealing, which is also known as austenitizing for short. By AlN groups in general, in this connection are meant clusters of aluminum and nitrogen atoms which, in contrast to AlN precipitations, do not have a defined phase boundary with respect to the matrix. To effectively inhibit austenite grain growth, the Al content should be at least 0.01 wt%. Therefore by fixing Al and N to the grid defects as well as their subsequent formation of cumulus or precipitation particularly fine grains of austenite can be produced. The finer austenite grain size leads to the formation of a fine martensite with a smaller lancet length than during the first quench. In cases where the austenitization time is to be shortened, high Al contents of at least 0.02% by weight are particularly advantageous. Another advantage for the formation of AlN clusters and AlN precipitations is a large number of lattice defects that are available during heating to the austenitization temperature (THZ). These lattice defects can be introduced into the material before austenitization, for example in the form of dislocations. For the flat steel products according to the invention, it has proven advantageous to introduce lattice defects by cold rolling with a degree of cold rolling of at least 37%. Aluminum, like silicon, contributes to the suppression of cementite formation. However, Al is not as effective as Si in suppressing cementite formation. However, since Si has a negative effect on the oxidation and coating ability and thus on the surface quality of flat steel products, Al can be used as a substitute for Si when selecting the composition of the steel. alloy. In this connection, Al contents of at least 0.1% by weight have proven to be particularly effective for the steel composition according to the invention. At lower Al contents, the influence of Al on cementite suppression is not significant. In addition, aluminum contributes to increasing the activity of carbon in the martensite. This is true both for the martensite formed after the first quench, which is performed after austenitizing, and for the martensite formed after the second quench, which is performed after partition annealing. In the martensite formed after the first quench, aluminum contributes to accelerate the partition of carbon from martensite to austenite during partition annealing. This can shorten the duration of the partition annealing. But the aging resistance of the final product is also improved, since a high carbon activity means that individual carbon atoms can already diffuse into dislocations during partition annealing and are no longer available for aging at room temperature afterwards.

The increase in carbon activity also shows a positive effect on the BH effect. In a BH treatment, the high carbon activity also increases the driving force for attachment of carbon atoms to dislocations, resulting in an increase in the BH value. Al contents of at least 0.02% by weight have proven to be particularly advantageous for increasing the carbon activity in martensite. Since aluminum increases the annealing temperature required for full austenitization and full austenitization is only possible with difficulty at Al contents above 1.5% by weight, the Al content of the steel of the flat steel product according to with the invention it is limited to at most 1.5% by weight. If a low austenitization temperature is to be set in order to improve energy efficiency, Al contents of at most 0.2% by weight have proven to be suitable.

According to the invention to improve the BH value, the sum of the Si content and half of the Al content is at least 0.9% by weight.

Then the following relationship holds:

%Si 0.5* %Al > 0.9% by weight

with % Si: respective Si content of the steel in % by weight

% Al: Al content of the steel in % by weight.

Values below 0.9 wt% increase the risk of cementite formation, through which carbon binds and is no longer available for diffusion into the residual austenite during partition annealing and thus is no longer available for residual austenite stabilization.

The content of N in the steel of a flat steel product according to the invention is limited to 0.001-0.008% by weight. In the steel of a flat steel product according to the invention, nitrogen forms nitrides, for example with aluminum or titanium. For effective grain refining by AlN clusters or AlN precipitations, the steel should contain at least 0.001 wt% N. To increase the thermodynamic driving force of precipitation formation and thus to stabilize the process , a preferred N content of at least 0.002% by weight can be set. Increased N contents tend to lead to larger precipitation formation. To avoid coarse precipitates, which can negatively affect deformability, the N content is limited to a maximum of 0.008% by weight.

Phosphorus has a negative effect on weldability in flat steel products according to the invention. For this reason, the P content must be as low as possible and, in particular, must not exceed 0.02% by weight.

Sufficiently high sulfur contents lead to the formation of sulfides such as MnS or (Mn,Fe)S. These sulfide precipitations impair the elongation of a flat steel product according to the invention, whereby the S content is limited to a maximum of 0.005% by weight.

Optionally, chromium can be present in the steel in contents of up to 1.0% by weight. Chromium is an effective inhibitor of perlite and contributes to strength. This is true in particular at Cr contents of at least 0.01% by weight. However, at Cr contents of more than 1.0% by weight, the risk of a pronounced oxidation of the integranular boundary increases, which leads to deterioration of surface quality.

Molybdenum may also optionally be contained in the steel of a flat steel product according to the invention in amounts of at least 0.01% by weight in order to avoid the formation of pearlite. For cost reasons, the Mo content is limited to contents of up to 0.2% by weight.

Boron may be contained as an optional alloying element in contents of 0.001 to 0.01% by weight in the steel of a flat steel product according to the invention. Boron segregates at phase boundaries and thus blocks its movement. This favors the formation of a fine-grained microstructure, which improves the mechanical properties of the flat steel product. When boron is alloyed, there should be enough aluminum available for AlN to be preferentially formed. In a preferred embodiment, an Al/B ratio of at least 10 is therefore established. However, no further improvement can be achieved by adding boron in excess of 0.01% by weight.

Optionally, the steels of flat steel products according to the invention may also contain one or more microalloying elements from the group Ti, Nb and V. The microalloying elements may form carbides, nitrides or carbonitrides with carbon or nitrogen. In the form of very finely distributed precipitation, these contribute to increased resistance. The sum of the microalloying elements should be at least 0.005% by weight, so that the precipitation of carbides, nitrides or carbonitrides can lead to freezing of intergranular and phase boundaries during austenitization and thus can counteract grain coarsening. At the same time, however, carbon, which in atomic form is favorable for the stabilization of residual austenite, is bound as carbide or carbonitride. To ensure sufficient stabilization of the residual austenite, the total concentration of microalloying elements should not exceed 0.2% by weight in total. To avoid coarse precipitation of titanium nitride, the concentration of titanium should not exceed 0.10%.

A flat steel product according to the invention has an elongation limit Rp02 of more than 700 MPa or an elastic limit ReH of more than 700 MPa, a tensile strength Rm of 950 - 1500 MPa and an elongation A80 of 7 - 25%, where the elongation limit Rp02 or the yield strength ReH, the tensile strength Rm as well as the elongation A80 are determined according to DIN EN ISO 6892:2009. At the same time, a flat steel product according to the invention has a high furnace hardening potential (Bh potential). A measure of the BH potential is the BH2 value, which is determined after 2% pre-deformation and tempering for 20 minutes at 170 °C according to DIN EN 10325:2006 and amounts to at least 80 MPa for products steel planes according to the invention. The elongation A80_BH which occurs after a BH treatment for 20 minutes at 170°C in flat steel products according to the invention preformed by 2% is in this case at least half of the elongation A80 before the BH treatment. The elongation values A80 and A80_BH are determined here in accordance with DIN EN ISO 6892:2009.

The flat steel product according to the invention has a structure that does not contain more than 15% per unit area of ferrite to ensure the high strengths required.

In addition, due to the way in which the process is carried out, the microstructure does not present more than 5% per unit area of bainite.

The microstructure of a flat steel product according to the invention contains at least 5% by volume of residual austenite. Residual austenite has a favorable effect on the formability and elongation of martensitic steels. Austenite, stabilized at room temperature, can be elongated more than other microstructural components using the TRIP effect with greater consolidation at the same time. With the limitation of austenite stabilizing alloying elements such as C and Mn for weldability reasons, a residual austenite ratio of greater than 20% by volume is not possible with the described manufacturing process.

Furthermore, the flat steel product according to the invention contains at least 80% by unit area of martensite, of which at least 75% by unit area is tempered martensite.

The martensite formed in the course of the process according to the invention after partitioning by the second quench in working step j) is also called untempered martensite. The martensite resulting from the first quench after austenitization, which undergoes partitioning, is also known as tempered martensite. All the total proportion of martensite present in the microstructure is composed of tempered and untempered martensite, where there is a possibility that there is no untempered martensite.

The total proportion of martensite, ie the sum of tempered and untempered martensite, should be at least 80% per unit area, preferably at least 90% per unit area. This high proportion of martensite contributes to a high strength of the flat steel product. Furthermore, martensite is a carbon-rich microstructural component. As such, the martensite serves as a source for carbon diffusion both during partition annealing and during the BH treatment. The residual austenite present is stabilized by diffusion of carbon from the martensite into the austenite during partition annealing, allowing a ratio of residual austenite of at least 5 % by volume. Carbon diffusion during BH treatment increases the BH effect, resulting in an increase in the BH value.

At least 75% of the martensite present in the flat steel product is tempered martensite, because only then is sufficient martensite available for sufficient residual austenite stabilization during partition annealing. In this regard, at least 90% of the martensite lancets have a martensite lancet width of at most 1000 nm. The reduced lancet width of at most 1000 nm leads to short diffusion paths during partition annealing, which enables a specific local stabilization of the residual austenite. The martensite lancet length is limited to a maximum of 7.5 pm to ensure good formability. Since the lancets grow with a defined length to width ratio, the width is limited, which has an advantageous effect on carbon diffusion.

Unless otherwise stated, information on microstructural parts for the microstructural components martensite, ferrite and bainite is based on % per unit area and for residual austenite on % by volume. Due to the fineness of the crystal structures, it is recommended to perform the microstructure tests, including the determination of the length and width of the martensite lancet, in a scanning electron microscope (SEM) at 5000x magnification. An X-ray diffraction (XRD) test according to ASTM E975 is recommended as a suitable procedure for the quantitative determination of residual austenite.

The process according to the invention for manufacturing a high-strength steel flat product suitable for the furnace hardening treatment comprises at least the following work steps:

a) provide a hot-rolled flat steel product, which is composed of a steel which, in addition to iron and unavoidable impurities, is composed of (in % by weight) 0.1 - 0.5% C, 1.0 - 3.0% Mn, 0.9 - 2.0% Si, 0.02 - 1.5% Al, 0.001 - 0.008% N, up to 0.02% up to 0.005% S, as well as optionally one or more of the following elements: 0.01-1.0% Cr, 0.01-0.2% Mo, 0.001-0.01% B and optionally a total of 0.005-0.2% of V, Ti and Nb, where the proportion of Ti does not exceed 0.10%;

b) pickling the hot-rolled flat steel product;

c) cold rolling the flat steel product with a degree of cold rolling of at least 37%;

d) heating the cold-rolled flat steel product to a holding zone temperature THZ, which is above the A3 temperature of the steel and amounts to at most 950 °C, whereby heating to an inflection temperature TW of 200 - 400 °C at a ThetaH1 heating rate of 5 - 50 K/s and above the inflection temperature TW with a ThetaH2 heating rate of 2 - 10 K/s; e) maintain the flat steel product for 5 -15 s at the holding zone temperature THZ; f) optionally cooling the flat steel product within 30-300 seconds from the holding zone temperature THZ to an intermediate temperature TLK amounting to at least 620 and at most 720 °C; g) cooling the flat steel product at a cooling rate ThetaQ of more than an average of 5 K/s to a cooling stop temperature TAB, which is between the initial martensite temperature TMS and a temperature that is up to 175 ° C lower than TMS;

h) holding the flat steel product at the cooling stop temperature TAB for 10-60 seconds; i) heating the flat steel product at a heating rate ThetaB1, amounting to 1 - 80 K/s, at a treatment temperature TB amounting to 350 - 500 °C and optionally isothermally maintaining the flat steel product at the treatment temperature TB, where the time for heating and optional isothermal holding totals 10 to 1000 seconds;

j) cooling the flat steel product to room temperature at a ThetaB2 cooling rate of more than 5 K/s and less than 500 K/s;

k) optionally coating the flat steel product in a molten bath either

k1) by means of dip-bath coating before cooling in work step j) or k2) by electrocoating after cooling in work step j).

In working step a), a hot-rolled flat steel product is provided, which consists of a steel of the composition mentioned in step a).

Hot rolled steel flat product is pickled before cold rolling. Pickling at the stage of work b) is done in a conventional way.

According to the invention, the cold rolling in work step c) must be carried out with a degree of cold rolling of at least 37%. In the present case, by degree of cold rolling KWG is meant the reduction in thickness that occurs as a result of the cold rolling of the flat steel product. The KWG can be described by the following relationship:

KWG = (h0 - h1) / h0

where h0 is the thickness of the flat steel product before cold rolling in mm and h1 is the thickness of the flat steel product after cold rolling in mm. If the flat steel product were to undergo several cold rolling processes or cold rolling passes after pickling and before heating in step d), KWG refers to the total degree of cold rolling, i.e. h0 is the thickness of the flat steel product before the first cold rolling process or cold rolling pass in mm, and h1 is the thickness of the flat steel product after the last cold rolling process or cold rolling pass in mm . Cold rolling with a KWG of at least 37% results in mechanical homogenization as well as a reduction in grain size and thus a fine-grained microstructure. Due to the high degree of cold rolling as well as precipitation processes and the resulting initial fine microstructure before annealing, a very fine grained austenite microstructure is already present before cooling. In this sense, grain boundaries act as an obstacle to the growth of martensite lancets, and the short distance between intergranular boundaries in a fine microstructure leads to shorter and narrower lancets. During quenching, a microstructure of the finest martensite lancets with residual austenite embedded in the middle is thus created. In the next treatment step, this leads to short diffusion paths, which means that a specific local stabilization of the residual austenite is possible.

It has been recognized that cold rolling grades of 37% or more provide many nucleation sites for austenite formation during austenitization annealing, resulting in a fine grained austenitic structure during austenitization. The grain size of the austenitic microstructure can be further reduced if the degree of cold rolling is at least 42%. For plant technology reasons, the degree of cold rolling is usually limited to 85%.

The heating of the cold-rolled steel flat product in work step d) to a holding zone temperature THZ is initially carried out until an inflection temperature ^{T w ,} which amounts to 200-400 °C, is reached, with a ThetaH1 heating rate of 5-50 K/s. Above the inflection temperature TW, heating is performed at a ThetaH2 heating rate of 2-10 K/s until the holding zone temperature THZ is reached. In this connection, the heating can also be carried out in one step, ie the heating rates ThetaH1 and ThetaH2 are set to the same value.

The flat steel product is heated to a holding zone temperature THZ, which is above the A3 temperature of the steel, to allow a full microstructure transformation to austenite. The A3 temperature is analysis dependent and can be estimated using the following empirical equation:

A3[ °C] = 910-203*V %C-15.2%Ni+44.7%Si+31.5%Mo-21.1%Mn

with %C=C steel content in % by weight, %Ni=Ni steel content in % by weight, %Si=Si steel content in % by weight, %Mo=Mo steel content in % by weight, % Mn= Mn content of the steel in % by weight.

The holding zone temperature THZ may also be referred to as austenitizing temperature and annealing in THZ may also be referred to as austenitizing. For economic reasons, the temperature in the holding area is limited to a maximum of 950 °C.

In working step e), the flat steel product is held at the holding zone temperature THZ for a holding time tHZ of at least 5 seconds to ensure complete austenitization. The retention time tHZ should not exceed 15 seconds to avoid the formation of a coarse austenitic grain and uneven growth of the austenitic grain. The objective of austenitization is to establish a fine and regular austenitic grain, since this structure has a favorable effect on the BH value.

Starting from the holding zone temperature THZ, the flat steel product can optionally first be slowly cooled in working step f) to an intermediate temperature TLK, which is 620 °C and at most 720 °C. TLK is not lower than 620 °C to avoid a phase transformation to ferrite. For the same reason, the THZ to TLK cooldown duration tLK is limited to 30 - 300 seconds.

After the optional slow cooling of the flat steel product in working step f) or already after holding the flat steel product at the holding zone temperature THZ in step e), the flat steel product is cooled in working stage g) at a cooling rate ThetaQ greater than the cooling rate in working stage f) of more than 5 K/s at a cooling stop temperature TAB. Due to the high quenching rate, such quenching is also called quenching or, to distinguish between quenching after partition annealing, quenching in working step g) is also called first quenching. The cooling rate from the intermediate temperature TLK to the cooling stop temperature TAB is greater than 5 K/s in order to avoid both transformation of austenite to ferrite and bainite for steel compositions according to the invention. This is even safer at higher cooling rates, so the ThetaQ cooling rate is preferably set to more than 20K/s. The ThetaQ cooling rate is limited in terms of plant technology to values of 500 K/s maximum, preferably 100 K/s maximum.

The cooling stop temperature TAB is between the initial martensite temperature TMS and a temperature that is up to 175 °C lower than TMS ((TMS-175 °C) < TAB < TMS). In this connection, the initial martensite temperature TMS is understood to mean the temperature at which the transformation of austenite to martensite begins. The initial temperature of martensite can be estimated using the following equation:

TMS[ °C] = 539 °C+(-423 %0-30.4 %Mn-7.5 %Si+30 %Al) °C/ % by weight

with %C=C steel content in % by weight, %Mn=Mn steel content in % by weight, %Si=Si steel content in % by weight, %l=Al steel content in % by weight.

Since the transformation from austenite to martensite does not take place suddenly but as a function of time, the degree of transformation, i.e. the proportion of martensite, can be controlled by the holding time tQ, with which the flat steel product is held. at the cooling stop temperature TAB. The holding time tQ in step h) is at least 10 seconds in order to ensure sufficient conversion of the austenite to martensite. With respect to the entire microstructure, the proportion of martensite produced by the first quench after austenitization must be at least 60% per unit area. The holding time tQ should not exceed 60 seconds to avoid a complete transformation to martensite and to ensure a residual austenite proportion of at least 5% by volume in the microstructure of the flat steel product at room temperature.

In stage i), the flat steel product is heated to a treatment temperature TB with a heating rate ThetaB1 and is optionally kept at TB to enrich the residual austenite present after working stage h) with carbon from the supersaturated martensite, which was formed by the first quench. The redistribution of carbon, which can also be called partitioning, takes place in this connection during the heating phase to TB. If the flat steel product is then also isothermally held at TB, additional partitioning also occurs during the optional isothermal holding. Heating to treatment temperature TB and subsequent optional holding at treatment temperature TB is also called partition or partition annealing. To allow sufficient redistribution of carbon, heating is done with a heating rate of at least 1 K/s and a maximum of 80 K/s. The treatment temperature TB amounts to 350 - 500 °C to avoid the formation of carbides and the decomposition of residual austenite. Furthermore, the total treatment time tBT is at least 10 and at most 1000 seconds, also to ensure sufficient carbon redistribution. The total treatment time tBT is made up of the time required for heating and, if applicable, the time used for the optional isothermal maintenance.

Subsequently, the flat steel product is cooled to room temperature in work step j) at a cooling rate ThetaB2. The ThetaB2 cooling rate is greater than 5 K/s, preferably greater than 20 K/s to allow martensite formation. This cooling stage can also be called quenching due to the high cooling rate. In order to distinguish it from the first quenching performed in the working step g), the quenching in the working step j) is also called the second quenching. The cooling rate of ThetaB2 is limited to values of at most 500 K/s, preferably at most 100 K/s in terms of plant technology.

The flat steel product can optionally additionally be subjected to a coating treatment (working step k)). The coating treatment can be carried out either as molten-dip coating (operation stage k1)) or as electrolytic coating (operation stage k2)). If hot dip coating (working stage k1) is performed, the flat steel product, after splitting in working stage i) and before cooling in working stage j), passes through a coating bath with a zinc-based molten bath composition. In this connection, the temperature of the molten bath is preferably 450-500°C.

As an alternative to applying a molten dip coating, the flat steel product can be electrolytically coated (work step k2)). In contrast to molten dip coating, electrolytic coating is not carried out in this respect before the flat steel product has cooled down in working stage j), but only after.

The coating treatment of working steps k1) or k2) is preferably carried out in a process continuous. A possible molten bath composition may consist of up to 1 % by weight of Al, remainder zinc and unavoidable impurities. Another possible molten bath composition may consist of 1-2 wt% Al, 1-2 wt% Mg, balance zinc and unavoidable impurities. By coating treatment, a corrosion protection coating is applied to the flat steel product on at least one side of the flat steel product. The coated flat steel product can also optionally be subjected to a galvanic annealing treatment.

The process according to the invention can be carried out continuously in annealing plants or strip coating plants which are usually provided for this purpose.

By proceeding according to the invention, in particular by maintaining the degree of cold rolling KWG, the quench rate ThetaQ after austenitization and the holding time tQ, a microstructure is produced which exhibits a very fine martensitic structure. This martensite structure is characterized by a particularly fine grain structure with a narrow lancet width. The high degree of cold rolling and precipitation of carbides and nitrides lead to an initial fine-grained microstructure for austenitizing annealing. With the process according to the invention, grain coarsening during austenitization is avoided, so that a very fine-grained microstructure is already present before the cooling following austenitization. The numerous grain boundaries of the fine microstructure hinder the growth of martensite lancets. The short distances between the grain boundaries of the fine-grained microstructure lead to short and narrow martensite lancets. Rapid cooling at ThetaQ cooling rates of greater than 5 K/s results in a microstructure of very fine martensite lancets with residual austenite embedded in the middle. Such a microstructure provides short diffusion paths for the carbon for the annealing process of the following working step i) and thus allows a specific local stabilization of the residual austenite.

At the same time, however, there is still enough carbon from the untempered martensite and strain-induced martensite formed during shaping for the subsequent BH treatment for dislocation fixation. Due to the fine microstructure present after partition annealing, the diffusion paths into the tempered martensite are short enough during the subsequent BH treatment to be able to achieve a high BH effect even at low BH temperatures and short BH treatment times.

The flat steel products made available by the present invention are particularly suitable for post-processing processes comprising a cold-forming process and post-heat treatment at temperatures below 300°C. The manufacture of components for automotive applications is mentioned here by way of example. Flat steel products are shaped in this regard to form components, for example, painted with cathodic dip paint (KTL) and then subjected to heat treatment in another process step, for example during paint drying baked The heat treatment is usually carried out as heating within a temperature range of typically 120 to 250°C for a period of typically 3 to 40 minutes. The flat steel products according to the invention in the present case are particularly suitable for such applications. However, the advantageous properties of the flat steel products according to the invention can also be used for products that have not been subjected to any prior deformation.

In the following, the invention is explained in more detail by means of embodiments:

For testing, five melts A-E of the compositions indicated in Table 1 were produced, from which 10 hot-rolled strips with a thickness of 1.8-2.5 mm were produced in a conventional manner. In this respect, melts C and E correspond to the specifications for the composition of the steel according to the invention, while melts A, B and D have excessively low Si contents.

The hot rolled strip was conventionally pickled and processed to form cold rolled strip with the cold rolling grades "KWG" indicated in Table 2a. Further production of the cold rolled strips took place according to the information given in table 2a and table 2b. In this connection, the cold-rolled strips were heated in each case with a faster first heating rate "ThetaH1" to a turning temperature "TW" and then with a slower second heating rate "ThetaH2" brought to the temperature from the maintenance zone "THZ", to which they were maintained for the duration "tHZ". The cold rolled strips from tests 1-9 initially cooled slowly to an intermediate temperature "TLK" within a period of time "tLK", then rapidly snap-cooled from the intermediate temperature "TLK" at a cooling rate " ThetaQ" to a cooling stop temperature. "TAB", in which they remained in a duration "tQ". The cold rolled strip from Test 10 was directly quenched to the quench stop temperature "TAB" with a quench rate "ThetaQ" without quenching and held at this temperature for duration "tQ". Next, the flat steel products were partitioned for a time "tBT", heating to partition temperature "TB" at a heating rate "ThetaB1". Finally, the flat steel products were quenched to room temperature with a "ThetaB2" cooling rate. 10 tests were carried out, of which tests 4, 8 and 10 comply with the specifications of the invention.

Samples were taken from trials 1-10, in which the microstructure was examined and the mechanical properties were tested. The results of the microstructure tests are indicated in Table 3 and the results of the tests of mechanical properties are indicated in Table 4. In this respect "MA" designates the proportion of tempered martensite in the entire structure, "M" the proportion of untempered martensite in the entire microstructure, "F" the proportion of ferrite, "B" the proportion of bainite, "RA" the proportion of residual austenite. The terms lancet length and lancet width refer to the structures of martensite.

The microstructure tests were carried out on cross sections in a 1/3t layer, that is, on sections that were taken at one third of the sheet thickness. Sections were prepared for scanning electron microscopy (SEM) examination and etched with 3% Nital. Due to the fineness of the microstructure structures, the microstructure was characterized by REM observation at 5000x magnification. Quantitative determination of residual austenite was performed by X-ray diffraction (XRD) according to ASTM E975.

The verification of the mechanical properties elongation limit "Rp02", tensile strength "Rm" and elongation "A80" was carried out according to DIN EN ISO 6892:2009 on samples that were not subjected to any BH treatment. To test the BH properties, samples of the same flat steel products were taken and subjected to 2% pre-strain and tempered at 170°C for 20 minutes. The oven hardening value "BH2" was tested according to DIN EN DIN EN 10325:2006. The test for the "A80_BH" elongation present after the BH treatment, which is also called residual elongation, was carried out in accordance with DIN EN ISO 6892:2009.

The tests show that the difference between the elongation limit Rp02 before the BH treatment and the yield strength after the BH treatment tends to increase with the increase in the strength of the flat steel product. This can be attributed to the higher proportion of martensite in the higher strength samples. With comparable strength and A80 elongation, samples 4 and 8 according to the invention have a higher BH value "BH2" and a significantly better residual elongation "A80_BH" than their comparison samples 5 and 9, which were not produced. according to the invention from the same melt.

Table 1

Table 2a

Table 2b

continuation

Table 3

Table 4

Claims (10)

1. High strength steel flat product suitable for a furnace hardening treatment, consisting of a steel which, in addition to iron and unavoidable impurities (in % by weight), is composed of C: 0.1-0.5% , Yes: 0.9 -2.0% , Mn: 1.0-3.0% , Al: 0.02 -1.5% , N: 0.001 - 0.008%, P: < 0.02%, S: < 0.005%, as well as optionally one or more of the following elements Cr: 0.01-1.0%, Mo: 0.01-0.2%, B: 0.001-0.01%, as well as optionally from a total of 0.005 - 0.2% of V, Ti and Nb, where the proportion of Ti is no more than 0.10%, where the flat steel product has a microstructure that is composed of - not more than 15% per unit area of ferrite, - not more than 5% per unit area of bainite, - at least 5% by volume of residual austenite and - at least 80% per unit area of martensite, of which at least 75% per unit area is tempered martensite, where with respect to the total proportion of martensite in the microstructure, for at least 90% of the martensite it has a martensite lancet length of at most 7.5 pm and a martensite lancet width of at most 1000 nm , measured as stated in the description, and where the flat steel product has an elongation limit Rp02 greater than 700 MPa or an elastic limit ReH greater than 700 MPa, a tensile strength Rm of 950 - 1500 MPa, an A80 elongation of 7 - 25% and a BH2 value which, after 2% pre-deformation and tempering for 20 minutes at 170 °C, is determined according to DIN EN 10325:2006, of at least 80 MPa . 2. Flat steel product according to claim 1, characterized in that the carbon content is at least 0.14% by weight. Flat steel product according to one of the preceding claims, characterized in that the Al content of the steel in the flat steel product is at least 0.2% by weight. Flat steel product according to one of the preceding claims. characterized in that it is provided with a zinc-based corrosion protection coating on at least one side. 5. Process for the manufacture of a high-strength steel flat product suitable for the furnace hardening treatment according to claim 1, comprising the following work steps: a) to provide a hot-rolled flat steel product, which is composed of a steel which, in addition to iron and unavoidable impurities, is composed of (in % by weight) 0.1 - 0.5% C, 1 0.0 - 3.0% Mn, 0.9 - 2.0% Si, 0.02 - 1.5% Al, 0.001 - 0.008% N, up to 0.02% P , up to 0.005% S as well as optionally one or more of the following elements: 0.01-1.0% Cr, 0.01-0.2% Mo, 0.001-0.01% of B, as well as optionally a total of 0.005-0.2% of V, Ti and Nb, where the proportion of Ti does not exceed 0.10%; b) pickling the hot-rolled flat steel product; c) cold rolling the flat steel product with a degree of cold rolling of at least 37%; d) heating the cold-rolled steel flat product to a holding zone temperature THZ, which is above the A3 temperature of the steel and amounts to at most 950 °C, where heating takes place up to an inflection temperature TW from 200 - 400 °C at a ThetaH1 heating rate of 5 - 50 K/s and above the inflection temperature TW at a ThetaH2 heating rate of 2 - 10 K/s; e) maintain the flat steel product for 5 -15 s at the holding zone temperature THZ; f) optionally cooling the flat steel product within 30-300 seconds from the holding zone temperature THZ to an intermediate temperature TLK amounting to at least 620 °C and at most 720 °C; g) cooling the flat steel product at a cooling rate ThetaQ of more than an average of 5 K/s to a cooling stop temperature TAB, which is between the initial martensite temperature TMS and a temperature that is up to 175 ° C lower than TMS; h) holding the flat steel product at the cooling stop temperature TAB for 10-60 seconds; i) heating the flat steel product at a heating rate ThetaB1, amounting to 1 - 80 K/s, at a treatment temperature TB amounting to 350 - 500 °C and optionally isothermally maintaining the flat steel product at the treatment temperature TB, where the time for heating and holding optional isothermal totals 10 to 1000 seconds; j) cooling the flat steel product to room temperature with a ThetaB2 cooling rate of more than 5 K/s and less than 500 K/s k) optionally coating the flat steel product in a molten bath either k1) by means of dip-bath coating before cooling in work step j) or k2) by electrocoating after cooling in work step j). 6. Process according to claim 5, characterized in that the degree of cold rolling in work step c) amounts to at least 42%. 7. Method according to claims 5 or 6, characterized in that the flat steel product passes through a zinc-based molten bath in working stage k). 8. Process according to claim 7, characterized in that the molten bath composition in working stage k) is composed of 1-2% by weight of Al, 1-2% by weight of Mg, the rest zinc and unavoidable impurities . Method according to one of Claims 5 to 8, characterized in that the optional coating treatment in work step k) is carried out in a continuous process. Method according to one of Claims 5 to 9, characterized in that the ThetaB2 cooling rate in working step j) does not exceed 20 K/s.