Technical Field

The invention relates to the development of heat treatment for use in the production of steel sheet, steel plate, outer part of automobiles, inner part of automobiles, automobile chassis, white goods, ballistic armor, construction steel, rope and profile used in industries such as automobile, defence, white goods, construction, and a method of steel production.

In particular, the invention relates to a method of heat treatment for hot rolled steel sheet production, cold rolled steel sheet production or sheet forming and part production processes that provides high energy absorption by maintaining the Mn partitioning between phases in the structure as a result of rapid heating prior to quenching.

Prior Art

Steel alloys are formed in nature by the combination of Iron (Fe) with Carbon (C) in ratios ranging from 0.02 % to 2 %. The quality of the steel used in today's industry is largely determined by the amount of C in the alloy. Although the C atom has a significant effect on the classification of steel, elements such as chromium, manganese, Wolfram, and vanadium are also used in the conversion of steel into a Fe alloy. The alloy elements provide superior mechanical properties of steel compared to the vast majority of other commercial alloys through various hardening mechanisms by joining the iron cage/frame structure or forming other cage/frame structures.

Steel production can be made by using alloy elements in different proportions/ratios according to the area in which the resulting alloy will be used. The amount of alloying elements determines the mechanical properties used in the classification of steels such as yield strength, tensile strength, and ductility in a steel alloy. For example, according to the level of tensile strength, steels are classified as low - strength steels (LSS), high - strength steels (HSS), advanced high strength steels (AFISS), and ultra - high - strength steels (UFISS).

In the automotive industry of today, HSS type steels are used to increase passenger safety and reduce fuel consumption, while AHSS type steels are preferred because of their very high strength, excellent energy absorption during deformation and strain hardening. UHSS type complex phase (CP) and martensitic (MS) steels are used in structural parts such as columns and beams where deformation is not desired but the highest strength levels are targeted.

The history of AHSS begins with the use of dual - phase (DP) steels in automobile parts. While AHSS and UHSS sheets, which offer high mechanical properties, were not found in many vehicles 20 years ago, yet they account for about 40 % of the steels used in an ordinary car body today.

The DP Steels, the first example of AHSS type steels, are steels containing around 10 - 30 % martensite phases scattered in the form of islets in a soft ferrite matrix, and the term dual - phase is used because of the ferrite and martensite phases these steels have. In DP steels, the main alloying elements are C, Si, and Mn.. Depending on the proportions of these elements in the structure, while the strength increases, a decrease in elongation values is observed.

The DP steel structure is mainly produced by heating these said steels in the Fe - FesC phase diagram to any temperature in the A1 - A3 temperature range (from the ferrite - austenite region) and cooling them at speeds where austenite can turn into martensite.

In the current technique, DP steel production is made via three methods;

1 . Continuous annealing of hot or cold rolled sheet

2. Batch annealing of cold rolled sheet

In the current technique, some problems are encountered in steel sheet production. One of these is that the high strength and elongation relationship can be reached by the production of TRIP Steels, TWIP steels, and QP Steels, which are the new AHSS types that are starting to substitute DP Steels. In order to produce these said types of steel, cementite precipitation must be prevented. Therefore, high amounts of Si are used as an alloying element. The high Si content makes it difficult for the steel producer to apply the galvanizing process. The red scale formed on the steel surface due to Si makes both cold rolling and galvanizing problematic. This condition remains in the steel sheet as a quality error.

Another drawback is that TRIP and especially TWIP steels contain a high amount of Mn in their chemical composition. The alloying of steels with high amount of Mn makes both continuous casting and hot rolling difficult. In addition, despite the superior mechanical properties of TWIP Steels, problems such as breakage due to hydrogen embrittlement after cold forming cannot be fixed.

Although the composition of conventional TRIP steel is in the ranges of 0.15 % - 0.30 C, 1 .5 % - 2.0 Mn, 0.1 % - 1 .5 Si, 0.05 - 1 .8 Al, and 0.1 - 0.3 P, in some TRIP Steels, Mn amounts can be up to 10 %, especially to increase the amount of residual austenite. The combination of high strength and elongation thus obtained is indicative of a high level of absorption energy. The general microstructure of TRIP steels consists of bainite and residual austenite grains contained within the soft ferrite matrix. TRIP steels are essentially composite materials. Residual austenite is mechanically unstable at room temperature but thermally stable (meta-stable) and transforms into martensite during forming (deformation). TWIP steels are mechanically and thermally stable austenitic steels with higher manganese, and unlike other high strength steels, twinning is observed in plastic deformation.

In the current technique, there are applications related to steel production. One of these is the European patent application publication no. EP2855725B1 entitled as 'Low - density hot - or cold - rolled steel, the method for implementing same and use thereof.' The summary of the invention is as follows: "The present invention relates to a rolled steel sheet having a mechanical strength greater than or equal to 600 MPa and an elongation at break greater than or equal to 20 % and its manufacturing method. The rolled steel sheet with contents expressed by weight, comprising: 0.10 % < C £ 0.30 % and 6.0 % £ Mn £ 15.0 %, 6.0 % < Al < 15.0 %, and optionally one or more elements selected from among: Si £ 2.0 %, Ti < 0.2 %, V < 0.6 %, Nb £ 0.3 %; the remainder of the composition being composed of iron and unavoidable impurities resulting from the elaboration. The ratio of the weight of manganese to that of aluminum being as below: (I). The microstructure of the sheet consisting of ferrite, austenite and up to 5 % Kappa precipitates in surface fraction."

The application is related to TRIPLEX steels comprising of high Mn and Al, and the microstructure of the product obtained in the scope of the study consists of austenite, ferrite, and kappa precipitates. The heat treatment used for this purpose also differs.

Another of these is the European patent application with publication number EP2719788B1 , entitled 'Hot press molded article, method for producing same, and thin steel sheet for hot press molding.' The summary of the invention is as follows: "The present invention makes it possible to provide a hot press - formed product, including a thin steel sheet formed by a hot press - forming method, and having a metallic structure that contains martensite at 80 % to 97 % by area and retained austenite at 3 % to 20 % by area, the remainder structure of which is at 5% by area or lower, whereby balance between strength and elongation can be controlled in a proper range and high ductility can be achieved.

In the said application, the austenite zone known as Quench and Partitioning refers to the cooling between Ms - Mf temperatures, keeping some amount in this temperature range, and then applying the method in which the residual austenite is stabilized by C to a press hardening process, with austenite stabilization is performed by C. Also, the Si ratio is high to prevent the formation of cementite.

As a result, due to the drawbacks described above and the lack of available solutions regarding the subject, an improvement in the technical field related to the production of a steel alloy with a microstructure of martensite / bainite and ferrite mixture has been made necessary.

The Purpose of Invention

The present invention is related to the steel production method, which allows the production of steels which include austenite in their microstructure without the need for high Si and Mn content, and provides high energy absorption through Mn partitioning and rapid heating.

The main purpose of the invention is to develop steel having a microstructure of a mixture of austenite, martensite / bainite, and ferrite.

Another purpose of the invention is to produce steel containing 15 % Mn by max weight via continuous casting.

Another purpose of the invention is to produce steel comprising of the amount of Si, Cr, Al is less than 2 % by weight, while the amount of C is between 0.02 - 0.77 % by weight.

Another purpose of the invention is to convert the produced steel parts into hot or cold rolled steel sheets by means of homogenization, hot rolling, cold rolling, continuous annealing line, recrystallization annealing, etc. used in conventional steel production.

Another purpose of the invention is to produce multiphase steel with a low Mn and C ferrite phase and a high Mn and C phase during the heat treatment for at least 5 minutes at a temperature below A3 by batch annealing method after hot rolling. The micro component residue containing high Mn and C may be various Fe - containing carbides such as austenite, martensite, bainite or cementite.

Another purpose of the invention is to produce a steel sheet or steel parts in martensite + bainite + austenite and martensite + ferrite + bainite + austenite structure.

In order to fulfill the above - described purposes, the invention is a heat treatment method used in hot rolled steel sheet production, cold - rolled steel sheet production or sheet forming processes, which provides high energy absorption by Mn partitioning and rapid heating, wherein; comprising process steps of,

a) forming a steel microstructure having micro - components of the desired quantity and chemical composition by annealing at a given T1 pre - heat treatment temperature,

b) heating the steel sheet to a certain temperature T2 with a certain heating rate and keeping it at a certain temperature for a certain period of time, following the pre - heat treatment, c) cooling below Ms temperature with a specific cooling rate.

This method is based on the principle that the interphase element partitioning obtained at a temperature T1 using a suitable alloy is carried out in a second process at a given temperature T2, and ultimately forming different phases as a result of rapid cooling, achieving a multi - phase structure by forming different phases of regions containing high element and regions containing low element, and resulting in a multiphase structure.

Detailed Description of The Invention

In this detailed description, the heat treatment method which provides high energy absorption as a result of Mn partitioning and rapid heating heat treatment as the subject of invention and the use of the related steel in hot rolled steel sheet production, cold - rolled steel sheet production or sheet forming and part manufacturing processes are explained.

The invention relates to a steel production method having a microstructure of a mixture of austenite, martensite/bainite, and ferrite, and providing high energy absorption by Mn partitioning and rapid heating.

Mechanical behavior of austenite in the final microstructure provides energy absorption by transformation to martensite, creating a twinning or changing shape during cold deformation according to the alloying elements that it contains along with grain size and morphology of the austenite. The micro - component comprising of martensite + bainite is responsible from the high strength of the structure. Depending on the temperature of T2, some ferrite may be expected to form in the final microstructure.

According to the method of the invention, steel is produced by continuous casting with containing Mn between 2.3 - 15 %, C between 0.02 - 0.77 %, and Si, Cr, Al with a total amount less than 2 %.

The produced steel parts are turned into hot or cold rolled steel sheets by the methods such as, homogenization,

• hot rolling,

• cold rolling,

• continuous annealing line,

• recrystallization annealing,

used in conventional steel production.

Following the hot rolling, the steel sheet is subjected to a heat treatment for at least 5 minutes via the batch annealing method at a temperature below Ac3 temperature. At this stage, it will be produced as double phased such that it has a ferrite phase containing low Mn and C with a micro - component containing high Mn and C.

The conversion temperatures of Aci and Ac3, which belong to a steel and seen in heating, vary according to the amount of alloying elements contained in the steel, and the point at which austenite starts to form during heating starting from room temperature is the Aci and the point at which the structure is completely austenite is the Ac3.

The heat treatment method of the invention:

a) A steel microstructure having micro - components of the desired quantity and chemical composition is formed by annealing at a given T1 pre - heat treatment temperature,

b) Following the pre - heat treatment, the steel sheet is heated to a certain temperature T2 with a certain heating rate and kept at there for a certain period of time,

c) It is cooled below Ms temperature with a specific cooling rate.

The annealing process mentioned in the process step a heat treatment method of the invention is applied after hot or cold rolling in the batch annealing or continuous annealing line between 5 minutes and 42 hours.

Cooling can be applied between the process steps a and b of the heat treatment method and the cooling can be selected in the form of cooling in the said cooling batch annealing oven, water cooling, air cooling, gas cooling.

The Ms temperature mentioned in the c process step of the heat treatment method is the point at which the formation of the martensite phase begins during cooling. Here, the cooling time in the C process step is 10 minutes maximum.

When T1 temperature is applied between Aci and Ac3 temperatures, after pre - heating treatment, the microstructure composes of FCC (Face Centered Cubic) austenite micro component ( ₇ AI ) in a volume of (%) f AI and in a volume of (%) fF ferrite micro component (OF). Accordingly,

Cu _{n '} The ratio of Mn% by weight contained in the OF phase The ratio of Mn% by weight contained in the YAI phase

C _Mn The ratio of Mn% by weight contained in the alloy

C _c : The ratio of C% to the weight contained in the alloy,

C “ + fid = loo

5 % < f _A1 < 40 %

C _Mn < 15 %

5 % < C ^Y < 40 %

he equation is satisfied.

When T1 temperature is applied below Aci temperature, after pre - heating treatment, the microstructure composes of carbide composition ( e _Fe3C ) of FesC in a volume of (%) f _Fe 3c and ferrite micro component (OF) in a volume of (%) fF. Accordingly,

naF . Ratio of Mn % by weight contained in the aF phase

Q

Fe ³ c The ratio of Mn % by weight contained in the phase

^Mn The ratio of Mn % by weight contained in the alloy

C _c The ratio of C % to the weight contained in the alloy,

C “ + flesc) = 100

5 % < f _F ^e _e3C £ 12 %

C _Mn 15 %

5 % < Cf, _n £ 40 %

ation is satisfied.

The heating process mentioned in the process step b of the heat treatment method of invention can be applied as induction heating, resistance heating, heating by burning with natural gas, and by heating to a temperature T2 above the Ac3 temperature during the heating process, the pre - treated structure is transformed to the austenite phase ( _7A 2), which is transformed from ferrite, which is vol % fA2, to the _7A3 structure having vol % fA3, which is formed by the dissolution of an amount of ₇ AI . Accordingly, gA2: austenite phase having a FCC (Face Centered Cubic) crystal structure transformed from OF formed after the heating process mentioned in process step b.

jA3'. austenite phase having a FCC (Face Centered Cubic) crystal structure formed by dissolution of phase ₇ AI after the heating process mentioned in process step b.

fA2: it is the % by volume of the austenite phase (JA2) with the FCC (Face Centered Cubic) crystal structure formed after the heating process mentioned in the process step b and transformed from OF formed in process step a.

fA3: it is the% by volume of the austenite phase ( _7A3 ) having a FCC (Face Centered Cubic) crystal structure formed by dissolution of phase ₇ AI after the heating process mentioned in process step b.

The ratio of Mn % by weight contained in the JA2 phase Ratio of Mn % by weight contained in the gA3 phase The ratio of C % by weight contained in the gA2 phase

C^ ^A3 ·. The ratio of C% by the weight contained in the gA3 phase if _F ^a + fL) = ⁽ fL + fL) = loo

fL ³ fL

fp £ fL

5 % < C _m ^yA3 < 40 %

rYAl _{> r} YA3

L n — ^ϋ Mh uation is satisfied.

By raising the temperature T3 above the Ac3 temperature during the heating process mentioned in step b of the heat treatment method of the invention, the pre - heat treated structure is transformed into austenite phase ( _7A 2) which is transformed from ferrite having vol % fA2, to _7A3 , having vol% fA3 , which is formed by e _Fe3C dissolution of the structure. Accordingly,

YA2'. is the austenite phase having a FCC (Face Centered Cubic) crystal structure transformed from OF formed after the heating process mentioned in process step b.

jA3'. is the austenite phase having a FCC (Face Centered Cubic) crystal structure formed by dissolution of phase after the heating process mentioned in process step b 0 _Fe3C .

fA2: is the % by volume of the austenite phase (JA2) with the FCC (Face Centered Cubic) crystal structure formed after the heating process mentioned in the process step b and transformed from OF formed in process step a. †A3: is the % by volume of the austenite phase (gA3) having a FCC (Face Centered Cubic) crystal structure formed by dissolution of phase after the heating process mentioned in process step b 0 _Fe3C .

C^ ^A2 ·. The ratio of Mn % by weight contained in the gA2 phase c , ^A3 The ratio of Mn % by weight contained in the gA3 phase The ratio of C % by weight contained in the gA2 phase

C^ ^A3 ·. The ratio of C % by the weight contained in the gA3 phase fF + f ) = f + fL) = ¹⁰⁰

equation is satisfied.

During the heating process mentioned in step b of the heat treatment process of the present invention, by increasing the temperature of T2 between the Aci and Ac3 conversion temperatures, the pre - treated structure dissolves to the austenite phase (YAå) with the volume % fA2 by dissolution of the ferrite with volume % fF and to the ferrite phase aF2 by volume of f F1 , and by some dissolution of the ₇ AI structure, it is transformed into the _7A3 structure by the volume of fA3. Accordingly,

aF2: is the ferrite phase with BCC (BODY CENTERED CUBiC)BCC (Body Centered Cubic) crystal structure which remains in the structure after some dissolution of OIF phase after the heating process mentioned in the process step b.

YA2: is the austenite phase having a FCC (Face Centered Cubic) crystal structure transformed from aF formed after the heating process mentioned in process step b.

YA3 : is the austenite phase having a FCC (Face Centered Cubic) crystal structure formed by the dissolution of phase YAI after the heating process mentioned in process step b.

fF2: is the % by volume of the ferrite phase with BCC (Body Centered Cubic) crystal structure which remains in the structure after some dissolution of aF phase after the heating process mentioned in the process step b.

fA2: it is the% by volume of the austenite phase (YAå) with the FCC (Face Centered Cubic) crystal structure formed after the heating process mentioned in the process step b and transformed from OF formed in process step a. †A3: it is the % by volume of the austenite phase ( _7A3 ) having a FCC (Face Centered Cubic) crystal structure formed by dissolution of phase ₇ AI after the heating process mentioned in process step b.

The ratio of Mn % by weight contained in the _7A 2 phase

C^ ^A3 ·. The ratio of Mn % by weight contained in the JA3 phase The ratio of C % by weight contained in the _7A 2 phase

C^ ^A3 ·. The ratio of C % by weight contained in the JA3 phase

Cc ^F2 : The ratio of C % by weight contained in the O.F2 phase

C n The ratio of Mn % by the weight contained in the O.F2 phase

(/“ + fL) = ⁽ fL + fL + /¾) - 100

f _F ^a £ fl ₂ + f _F \

5 % < C _M ^YA3 40 %

rYAl ^ _r YA3

bMn — ^b Mn

C _M n - (C x f ) + (C x fl) + (¾ ^F n ² X fh) equation is satisfied.

During the heating process mentioned in step b of the heat treatment process of the present invention, by increasing the temperature of T2 between the Aci and Ac3 conversion temperatures, the pre - treated structure dissolves to the austenite phase ( _7A 2) with the volume % fA2 by dissolution of the ferrite with volume % fF and to the ferrite phase aF2 by volume of f F2, and e _Fe3C by some dissolution of the structure, it is transformed into the _7A3 structure by the volume of fA3. Accordingly,

(XF2: is the ferrite phase with BCC (Body Centered Cubic) crystal structure which remains in the structure after some dissolution of OF phase after the heating process mentioned in the process step b.

YA2. is the austenite phase having a FCC (Face Centered Cubic) crystal aF structure transformed from OIF formed after the heating process mentioned in process step b.

jA3. is the austenite phase having a FCC (Face Centered Cubic) crystal structure formed by the dissolution of phase ₇ AI after the heating process mentioned in process step b.

fF2: is the % by volume of the ferrite phase with BCC (Body Centered Cubic) crystal structure which remains in the structure after some dissolution of aF phase following the heating process mentioned in the process step b.

fA2: it is the % by volume of the austenite phase (YAå) with the FCC (Face Centered Cubic) crystal structure formed after the heating process mentioned in the process step b and transformed from OF formed in process step a. †A3: it is the % by volume of the austenite phase (gA3) having a FCC (Face Centered Cubic) crystal structure formed by dissolution of phase YAI after the heating process mentioned in process step b.

The ratio of Mn % by weight contained in the _7A 2 phase c ) ^A3 : The ratio of Mn % by weight contained in the yphase

C ^YA2 : The ratio of C % by weight contained in the _7A 2 phase

C ^YA3 : The ratio of C% by weight contained in the _7A3 phase

Cc ^F2 : The ratio of C% by weight contained in the O.F2 phase

C _MU The ratio of Mn% by the weight contained in the O.F2 f phase

(// + ffesc) = ⁽ JL + fL + /¾) - loo

/ _F “ > /; ₂ + / _F ¾ ffeSC £ fL

5 % < C _M ^Y 40 %

equation is satisfied.

As a result of the cooling process mentioned in process step c of the heat treatment method of the invention, a mixture of phases _7A 2 and _7A3 with a total amount by volume of fA2+ fA3 is formed by the heating process applied in step b, is transformed to phase QMB, which is a mixture of martensite and bainite by volume of ΪMB, and to phase _7A4 by volume% amount of fA4, however, the O.F2 ferrite phase with an amount by volume / _F2 remains in its structure. Accordingly,

fA4: it is the % amount by volume of the austenite phase ( _7A4 ), which is the remaining part of the _7A3 phase without transforming to another phase and has a FCC (Face Centered Cubic) crystal structure, following the cooling process mentioned in the process step c.

†MB: is the % amount by volume of the mixture of martensite and bainite phases (CXMB) which is transformed from _7A 2 phase after the cooling process mentioned in the process step c.

C _MU ⁸ · Ratio of Mn % by weight contained in the CXMB phase

The ratio of Mn % by the weight contained in the _7A4 phase

(// + fL) = C fL + fL + f&) = (/F“ ₂ +/M“ _B + fL) = 100 .

As a result of the cooling process mentioned in the process step c of the heat treatment method of the present invention, the _7A 2 phase formed by the heating process mentioned in the process step b is transformed to the QMB phase which is fA2 by volume and a mixture of martensite and bainite by volume of fMB, and the _7A3 phase which is fA3 by volume, to the _7A4 phase which is austenite with FCC (Face Centered Cubic) crystal structure of fA4 by volume. Accordingly,

†A4: it is the % amount by volume of the austenite phase (YA4), which is the remaining part of the _7A3 phase without transforming to another phase and has a FCC (Face Centered Cubic) crystal structure, following the cooling process mentioned in the process step c.

fMB: is the % amount by volume of the mixture of martensite and bainite phases which is transformed from _7A 2 phase after the cooling process mentioned in the process step c.

C _MU ⁸ '· The ratio of Mn% by weight contained in the QMB phase

C^ ⁴ : The ratio of Mn% by the weight contained in the _7A4 phase

(// + fL) = (fL + fL) = (fL + fu 1 oo

rYA3 . _r YA4

bMn — ^b Mn

5 % < C £ 40 %

J fA ^Y 1 > f ^Y > f ^Y

3 % < f £ 40 % and the equations of AC1 £T 1 <AC3, T 1 <T2, AC3£T2 are satisfied.

As a result of the cooling process mentioned in the process step c of the heat treatment method of the present invention, the _7A 2 phase formed by the heating process mentioned in the process step b is transformed to the OMB phase which is fA2 by volume and a mixture of martensite and bainite by volume of fMB, and the _7A3 phase which is fA3 by volume, to the _7A4 phase which is austenite with FCC (Face Centered Cubic) crystal structure of fA4 by volume. Accordingly, fA4: it is the % amount by volume of the austenite phase ( _7A4 ), which is the remaining part of the _7A3 phase without turning into another phase and has a FCC (Face Centered Cubic) crystal structure, following the cooling process mentioned in the process step c.

fMB: is the % amount by volume of the mixture of martensite and bainite phases (OMB) which is transformed from _7A 2 phase after the cooling process mentioned in the process step c.

C _MU ⁸ · The ratio of Mn% by weight contained in the QMB phase

ned in the _7A4 phase

3 % < f ₄ < 40 % and the equations T1 <AC1 , T1 <T2 and AC3£T2 are satisfied.

The heating rate mentioned in the process step b of the heat treatment method is to increase from room temperature to T2 temperature between 10 and 600 seconds. The heating to the temperature T2 mentioned in the process step b of the hot rolled steel sheet can be applied during or after hot rolling, while it may be applied to the steel plate during cold rolling or after cold rolling.

The heating mentioned in the process step b of the heat treatment process of the present invention is applied to the cold - rolled or hot - rolled steel sheet before the press forming process.

The cooling process mentioned in the process step c of the heat treatment method of the invention can be applied in the form of air cooling, water cooling, oil cooling, polymer cooling, gas cooling, mould cooling, immersion in a metallic liquid, immersion in an organic liquid, or immersion in an inorganic liquid.

After the cooling process mentioned in the process step c of the heat treatment method, the steel plate or shaped part may have a microstructure containing at least 3 % and at most 40 % austenite. The phases other than austenite may be a mixture of martensite, bainite, and ferrite.

The Mn element has a lower diffusion rate compared to the C element. Therefore, a heterogeneous austenite distribution is encountered during austenitization , in which Mn - rich (formed in the earlier heat treatment) and Mn - poor (formed fresh during austenitization ) are seen. When rapid cooling is applied to the heterogeneous austenite obtained as mentioned above, austenite with high Mn remains as residual austenite in the structure. As the newly formed austenite has a sufficient amount of C, depending on the cooling rate, it turns into a structure of martensite, bainite or a mixture of martensite - bainite. The resulting steel plate or shaped part consists of martensite + austenite, martensite + bainite + austenite or bainite + austenite structure. During the rapid heating process, an austenitic structure is formed. The austenitization time should be maximum 10 minutes and the austenitization temperature should be below 850 O. The amount of micro component remaining as austenite can be between 2 % and 40 % by volume.

After rapid cooling, the steel plate or shaped part may have a microstructure containing between 3 % and 40 % austenite. The phases other than austenite may be a mixture of martensite, bainite, and ferrite.

The composition of the steel plate to which the method of the invention is applied consists of carbon between 0.02 % and 0.77 %, Mn between 2.3 % and 15 %, up to 2 % Si, up to 2 % Cr, and up to 2 % Al. Apart from this, it may include B, Mo, Nb, Ti, Co, and W. While the steel plate may be preferred as uncoated, metallic coated with high Zn content, or metallic coated with a high content of Al, the plate thickness may vary between 0.4 mm and 12 mm.

If the method of the invention is to be applied by hardening process in the press during the production of parts from hot rolled steel sheet, after batch annealing process, steel sheet is processed via the following operations: sheet opening by the part manufacturer, cutting the sheets to desired dimensions, rapid heating (to 850‘C), conveying to press, pres s forming and cooling in the mold.

If the method of the invention is to be applied by hardening process in the press during the production of parts from cold - rolled steel sheet, after the batch annealing process, steel sheet is processed by the opening of the hot - rolled coils, cold - rolling it, and the application of the same temperature of the batch annealing in the continuous annealing line. The cold - rolled sheet should then be processed by the manufacturer of the part via the opening of the sheet, cutting the sheet in the desired sizes, rapid heating (up to 850 Ό temperature), conveying to pr ess, press forming and cooling in the mold.

One feature of the invention is that the micro components that make up the steel sheet can contain 5 to 45 % Mn and also contains metal - carbon, metal - nitride, metal - carbonitride. The said metal may be iron, chromium, tungsten, molybdenum, niobium, cobalt, vanadium, silicon, titanium, aluminum, or boron.

One feature of the invention is that the micro component forming the steel sheet may have an austenite phase in the cubic crystal structure centered on the surface.

The steel plate or shaped part obtained as a result of the method of the invention can have a tensile strength between 800 MPa and 2000 MPa and total elongation value between 5 % and 30 %.

Another purpose of the invention is that it is possible to produce a steel sheet or plate suitable for cold forming during the production of steel by means of the method of the invention, or the method can also be applied during press forming.

Steel parts and steel sheets obtained by the heat treatment method of the invention can be used in vehicles used in the civil or defence field, white goods sector, construction, and building industry.