JP2015528064A - Hot rolled flat steel product and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a hot rolled flat steel product in which the product of Rm and A80 is 18000 MPa *% or more, and the composition is C: 0.10-0.60% (by weight), Si: 0.4- 2.0%, Al: ≰ 2.0%, Mn: 0.4-2.5%, Ni: ≰ 1%, Cu: ≰ 2.0%, Mo: ≰ 0.4%, Cr: ≰ 2 %, Ti: 0.2%, Nb: 0.2%, V: 0.5%, the balance Fe and inevitable impurities, the microstructure is a microstructure occupied by two phases , One main component of the microstructure is bainite, the other main component of the microstructure is retained austenite, the microstructure is composed of bainite in an amount of 60% by volume or more and the remaining austenite as the balance, Includes (by volume) 5% or less ferrite and 10% or less mar Onsite well be those which occur in the microstructure, at least a portion of the residual austenite is present in block form, over 98% of the block of residual austenite has the following size 5 [mu] m. To manufacture a flat steel product, a slab, thin slab or cast strip having the above composition is provided, and the semi-finished product is hot rolled at a hot rolling finish temperature of 880 ° C. or more to form a hot strip. The hot strip is cooled to a coiling temperature between the martensite start temperature MS and 600 ° C. at a cooling rate of 5 ° C./s or more, and the hot strip is coiled to form a coil and cooled by the coil. The coil temperature is maintained between the bainite start temperature BS and the martensite start temperature MS until 60% by volume or more of the microstructure is composed of bainite. [Selection figure] None

Description

  The present invention relates to a hot rolled flat steel product having a product of tensile strength Rm and elongation A80 of at least 18000 MPa *%. This kind of flat steel product is distinguished by a combination of good elongation properties and very high strength and is therefore particularly suitable for parts products for automobile bodies.

  Similarly, the present invention relates to a method for producing a flat steel product according to the present invention.

  The term “flat steel product” is to be understood herein to mean a steel plate or steel strip produced by a rolling process and a sheet bar distinguished therefrom.

  In this specification, when the content of the alloy is simply described as “%”, it means “% by weight” unless explicitly specified otherwise.

  The product of tensile strength Rm and elongation A80 is also technically referred to as “quality”.

  EP 1466024B1 (DE60315129T2) discloses a method for producing flat steel products intended to have a tensile strength much higher than 1000 MPa. To achieve this, (by weight) 0.0005-1% C, 0.5-10% Cu, 2% Mn, 5% Si, 0.5% Ti, 0% A steel melt containing less than 5% Nb, less than 5% Ni, less than 2% Al and the balance iron and impurities inevitable for production reasons is produced. The melt is cast to form a strip, the thickness of which is up to 10 mm, and is rapidly cooled to a temperature of 1000 ° C. or less by sprinkling water or a mixture of water and air. Thereafter, the cast strip is hot rolled with a reduction of at least 10%. Hot rolling ends at an end temperature at which all of the copper is in solid solution in the ferrite and / or austenite matrix. The strip is then transferred to a rapid cooling step to retain the copper in a supersaturated solid solution in the ferrite and / or austenite solution. The cooled strip is finally wound to form a coil. Copper precipitation causes precipitation hardening, thereby achieving the desired strength level of the steel. At the same time, the copper content is intended to increase the corrosion resistance and embrittlement resistance of the steel through the formation of a protective oxide layer.

  A hot strip with a tensile strength higher than 1200 Mpa and an elongation of up to 10% and its production method are known from US 2009/0107588 A1. This known hot strip has, in addition to iron and unavoidable impurities, 0.10-0.25% C (by weight), 1-3% Mn, 0.015% or more Al, up to 1.985. % Si, up to 0.30% Mo, up to 1.5% Co, up to 0.005% B, where 1% ≦% Si +% Al ≦ 2% (% Al = Each Al content,% Si = each Si content), and% Cr + (3 ×% Mo) ≧ 0.3% (% Cr = each Cr content,% Mo = each Mo content) It is required to be. At the same time, the steel has a microstructure consisting of at least 75% bainite, at least 5% residual austenite, and at least 2% martensite. To manufacture the hot strip, a melt of the corresponding composition is cast to form a primary product or semi-finished product, which is then heated to above 1150 ° C. to keep the steel completely austenitic. Hot rolling is performed at the rolling end temperature. The resulting hot strip is then cooled in three steps. In the first step, cooling is achieved by transitioning from a temperature higher than the Ar3 temperature of the steel to a first intermediate temperature higher than 650 ° C at a cooling rate of at least 70 ° C / s. Thereafter, the cooling proceeds from the first intermediate temperature to the second intermediate temperature, and this second intermediate temperature includes the bainite start temperature at which bainite begins to form in the steel and the martensite start at which martensite begins to form in the steel. The lower limit temperature is 50 ° C. higher than the temperature. The cooling rate in this second step is 20-90 ° C./s. Thereafter, the process proceeds to a third cooling step in which the hot strip is cooled to room temperature. Here, the temperature at which the third cooling step is started is determined according to the respective cooling rates.

  Another method for producing high strength and easily deformable hot strips is likewise based on the strength-increasing action of Cu precipitation and is described in US 6,190,469 B1. In this method, 0.15-0.3% C, 1.5-2.5% Si, 0.6-1.8% Mn, 0.02-0.10% (by weight) Al, 0.6-2.0% Cu, 0.6-2.0% Ni and the balance iron and steel containing unavoidable impurities are cast to form a slab. The slab is rolled to form a hot strip, and the hot rolling end temperature is set to 750-880 ° C. The resulting hot strip is then cooled by water from an onset temperature of 680-740 ° C. to the coiling temperature, which is determined by the formula: 240 × (% Mn +% Ni) −140 where% Mn = Each Mn content,% Ni = each Ni content) is at least the same as the temperature calculated and less than 540 ° C. Thereafter, the hot strip cooled to the coiling temperature is rolled to form a coil. The resulting hot strip has a microstructure containing 5-20% residual austenite and 20-50% bainite, in addition to ferrite, which includes copper precipitation, which through precipitation hardening, Contributes to the strength of the resulting hot strip. The hot strips produced and provided in this way have a maximum elongation of 23% combined with a strength in the region of about 1000 MPa, and as a whole a high quality value of 20000 MPa *% or more has been achieved.

  Against the background of the prior art described above, the object of the present invention is to provide a hot rolled flat plate which can be produced in a simple and reliable manner and has an optimum combination of particularly high strength and good deformation performance. It was to provide steel products. In addition, its purpose was to provide a method for producing such flat steel products.

  With regard to hot strips, this object has been achieved by the present invention according to the hot rolled flat steel product indicated in claim 1.

  With respect to the method, the above-mentioned object is achieved by the present invention for producing the hot rolled flat steel product according to the present invention by performing at least the operating steps indicated in claim 8.

  Advantageous configurations of the invention are indicated in the dependent claims and will be described in detail below as a general concept of the invention.

In addition to iron and inevitable impurities, the hot-rolled flat steel product according to the present invention is (in wt%):
C: 0.10-0.60%,
Si: 0.4-2.0%,
Al: Up to 2.0%
Mn: 0.4-2.5%
Ni: up to 1%
Cu: up to 2.0%,
Mo: Up to 0.4%
Cr: up to 2%
Ti: maximum 0.2%,
Nb: maximum 0.2%,
V: Differentiated by including up to 0.5%.

  The flat steel product according to the present invention has a microstructure occupied by two phases, wherein one main component of the microstructure is bainite and the other main component of the microstructure is residual austenite. is there. Also, in addition to these two main components, a small proportion of martensite and ferrite may be present, but their content is too high to affect the properties of hot rolled flat steel products. Few. For this reason, the microstructure of the flat steel product according to the present invention has at least 50% by volume, in particular at least 60% by volume, in addition to any optionally present part of up to 5% by volume ferrite and up to 10% by volume martensite. % Bainite and the balance of retained austenite, at least a portion of the retained austenite is present in the form of blocks, and at least 98% of the blocks of retained austenite present in the form of blocks, Having an average diameter of less than 5 μm;

The method according to the invention for producing a flat steel product provided according to the invention comprises the following operational steps:
In addition to iron and inevitable impurities, 0.10-0.60% C, 0.4-2.0% Si, up to 2.0% Al, 0.4-2. 5% Mn, up to 1% Ni, up to 2.0% Cu, up to 0.4% Mo, up to 2% Cr, up to 0.2% Ti, up to 0.2% Nb and up to Providing a preliminary product containing 0.5% V in the form of a slab, thin slab or cast strip;
Hot rolling the semi-finished product in one or more rolling cavities to form a hot strip when the resulting hot strip leaves the last rolling cavities; Having a hot-rolling end temperature of at least 880 ° C .;
Accelerating cooling of the resulting hot strip to a coiling temperature in the range between the martensite start temperature MS and 600 ° C. with a cooling rate of at least 5 ° C./s;
Coiling the hot strip to form a coil;
A step of cooling the coil, the upper limit being the same as the bainite start temperature BS at which bainite begins to occur in the hot strip microstructure, and the martensite start temperature MS at which martensite begins to occur in the hot strip microstructure; In the temperature range with the same lower limit, the coil temperature is maintained during cooling to form bainite until at least 50% by volume, in particular at least 60% by volume, of the microstructure of the hot strip is composed of bainite. And steps.

  The present invention is based on the knowledge that when the retained austenite is present in the form of blocks, it will be due to the required properties of the hot rolled flat steel product as long as the diameter of the retained austenite block does not exceed 5 μm. is there. Retained austenite in the form of blocks has traditionally been considered a factor in the unstable nature of the microstructure and the associated tendency towards undesirable martensite formation. I thought it should be avoided. For this reason, up to now, as high a proportion of film-like retained austenite has always been sought in the microstructure of the type of steel in question here (ref. [HKD Bhadesia and DV Edmonds, “Bainite in silicon steels: new composition-property approach”, Metal Science Vol. 17, September 1983, pages 411-419 (“Part 1”) and pages 420-425. )]).

  In this specification, when the ratio of length / width of the microstructure component of retained austenite existing in the microstructure, that is, the ratio of the longest range / thickness is 1 to 5, what is called “blocked” retained austenite And In contrast, when the length / width ratio of the accumulated austenite existing in the microstructure is greater than 5 and the width of each microstructure component of the retained austenite is less than 1 μm, the retained austenite is expressed as “ It will be referred to as “film-like”. Film-like retained austenite typically appears as a finely distributed thin film.

  For this reason, the cost still required by the prior art to avoid residual austenite present in the block form is in the microstructure of the flat steel product obtained according to the invention in the production of the flat steel product according to the invention. It can be avoided by keeping the blocks of residual austenite present small, ie to the extent indicated by their average diameter being limited to less than 5 μm. In this respect it has surprisingly been found that residual austenite present in the form of blocks and whose diameter is smaller than 5 μm has a positive influence on the elongation properties of the type of steel provided according to the invention. . It has been found that residual austenite blocks present in this size are more stable than block residual austenite present in a coarser form. At the same time, they are not as stable as residual austenite present in filmy form, thus enabling the TRIP effect. The positive effect of residual austenite present in the block can be used particularly reliably when the size of the block retained austenite is at most 4 μm, in particular at most 3 μm. In this regard, in flat steel products having a composition according to the invention and manufactured according to the invention, the maximum residual austenite size present in the block is usually in the range of 1-3 μm, and the residual austenite block It has been found in practice that the maximum size is typically limited to an average of 2 μm. Surprisingly, complex and multi-stage temperature control during the production of flat steel products is not necessary for this purpose.

  For this reason, the hot-rolled flat steel product according to the present invention can be manufactured without special costs, while at the same time, by monitoring the parameters preset according to the present invention for the manufacturing method. it can. In particular, complex cooling techniques that are still considered inevitable in the prior art or cooling techniques that require high cooling power are no longer needed.

  The positive influence of the retained austenite component in the microstructure of the flat steel product according to the present invention is particularly sure when the amount of retained austenite is at least 10% by volume, and the amount of retained austenite is at least 15% by volume. In some cases, a particularly reliable and beneficial effect is expected.

  Hot rolled flat steel products produced in accordance with the present invention achieve a tensile strength Rm of typically higher than 1000 MPa, in particular at least 1200 MPa, with an elongation A80 usually higher than 17%, in particular higher than 19%. Thereby, the quality Rm * A80 of the hot strip according to the present invention is usually in the range of 18000 to 30000 MPa *%. In particular, it will usually be at least 20000 MPa *%. For this reason, the flat steel product according to the present invention has an optimum combination of very high strength and good deformation performance.

  Moreover, in the hot rolled flat steel product according to the present invention, the effect of increasing the strength of copper can also be used. In this regard, in the hot rolled flat steel product according to the present invention, a minimum Cu content of 0.15% by weight can be present.

  In the steel according to the present invention, carbon delays the transformation to ferrite / pearlite, lowers the martensite start temperature MS, and contributes to an increase in hardness. In order to take advantage of these positive effects, the C content of the flat steel product according to the invention can be set to at least 0.3% by weight.

  In steels treated according to the invention, Mn in amounts of up to 2.5% by weight, in particular up to 2.0% by weight, promotes bainite formation, as well as optionally present amounts of Cu, Cr and Ni. It contributes to the formation of bainite. Depending on the other components of the steel processed according to the invention, it may be advantageous here to limit the Mn content to a maximum of 1.6% by weight.

  Further, addition of arbitrary Cr can lower the martensite start temperature and suppress the tendency of bainite to transform into pearlite or cementite. Furthermore, at a content that maximizes the upper limit of at most 2% by weight as preset according to the present invention, Cr promotes ferrite transformation and is selective for the presence of Cr in the flat steel product according to the present invention. The effect occurs when the Cr content is limited to 1.5% by weight.

  The addition of optional Ti, V, or Nb can suppress the formation of a fine-grained microstructure and promote ferrite transformation. These microalloying elements also contribute to an increase in hardness through the formation of precipitates. The positive effect of Ti, V and Nb is that the flat steel product according to the present invention has a content of each of these elements in the range of 0.002 to 0.15% by weight, particularly not exceeding 0.14% by weight. Can be used particularly effectively.

  Through the presence of Si and Al, carbide formation in bainite can be suppressed, and concomitantly, retained austenite can be stabilized by dissolved carbon. Also, Si mainly contributes to solid solution solidification. In steel processed according to the present invention, Al can partially replace the Si component. For this, a minimum Al content of 0.4% by weight can be provided. This is especially true when the intention is to set the hardness or tensile strength of the steel to a relatively low value so that the addition of Al is advantageous for improving deformation performance.

  At the same time, the positive effect of the presence of Al and Si is that the Si and Al content within the limits pre-defined according to the invention is% Si + 0.8% Al> 1.2 wt%, or% Si + 0.8 Particularly effective when% Al> 1.5% by weight (where% Si is the content of each Si by weight%,% Al is the content of each Al by weight%) can do.

The formation of the microstructure according to the invention is in particular the amount of Mn, Cr, Ni, Cu and C of the steel treated according to the invention and the Mn, Cr, Ni, Cu and C of the flat steel product according to the invention. Can be ensured by satisfying the following conditions.
1 <0.5% Mn + 0.167% Cr + 0.125% Ni + 0.125% Cu + 1.334% C <2
Here,% Mn represents the content of each Mn in wt%,% Cr represents the content of each Cr in wt%, and% Ni represents the content of each Ni in wt%. ,% Cu indicates the content of each Cu in wt%, and% C indicates the content of each C in wt%.

  In order to produce a flat steel product according to the present invention, a semi-finished product cast from steel having the composition according to the present invention is first brought to a predetermined temperature or kept at that temperature. The predetermined temperature is a temperature sufficient to start from the temperature and end the hot rolling performed at the hot rolling end temperature, and the hot strip obtained at the hot rolling end temperature is completely recrystallized. It has an austenite microstructure and provides optimal preconditions for bainite formation. If the resulting hot strip has a hot rolling end temperature of at least 880 ° C. when leaving the last rolling hole mold, the hot rolling end temperature is set to at least 900 ° C. and 1100 ° C., in particular, If it does not exceed 1050 ° C., the method according to the invention can be carried out with a particularly high level of operational reliability. To achieve this goal, typically the semi-finished product is heated to a temperature in the range of 1100-1300 ° C. before hot rolling. When the hot rolling finish temperature falls below 900 ° C., a wide range of softening of austenite can be achieved by the main deformation of the hot strip occurring in the last rolling hole form. The hot strip thus obtained in the same way has a microstructure with a retained austenite ratio that satisfies the specifications according to the invention.

Following hot rolling, the hot strip is subjected to accelerated cooling to a coiling temperature in the range of 350-600 ° C. with a cooling rate of at least 5 ° C./s. Cooling is here preferably initiated when 50-60% of the austenite softens. For this purpose, in practice, a pause of, for example, 2 s maximum is given between the end of hot rolling and the start of cooling. The initial rest period tp can be calculated by the following empirical formula.
tp = 5 · 10 ^{+ 36} · T ^−12.5
Here, tp is a rest period in seconds after the last deformation, and T is a temperature in ° C. This formula gives the shortest time after 50-60% softened austenite appears. The rest period calculated from this formula is as follows.

  Cooling to the coiling temperature is here achieved in such a way that austenite transformation does not occur until coiling. This has the effect that bainite formation occurs over a sufficiently long time with only the coil. Once the hot strip cooled in the manner described above is wound to form a coil, for this purpose, the coil will have the same upper limit as the temperature at which bainite is formed from austenite, and martensite will be in the hot strip microstructure. It is cooled in a temperature range having a lower limit higher than the temperature at which it is formed. The period during which the coil is held in this temperature range is selected at this point so that the desired bainite ratio of at least 60% by volume is achieved in the present invention. In practice, a period of at least 0.5 h is usually sufficient for this purpose, and for longer periods a higher bainite content is set.

  Actual studies have shown that the end of hot rolling and coiling is particularly reliable when the cooling rate is at least 10 ° C./s and the actual cooling rate is at most 150 ° C./s, especially 10-50 ° C./s. It shows that the transformation of the microstructure in between can be avoided.

  Undesirable martensite formation can be avoided more reliably by making the lower limit of the coiling temperature at least 10 ° C., in particular at least 20 ° C., higher than the martensite start temperature.

  At the same time, the desired profile for bainite formation can be ensured, in particular, by setting the upper limit of the coiling temperature to 550 ° C.

The optimum profile of bainite formation occurring in the coil according to the present invention occurs when the coiling temperature corresponds at least to the temperature HTopt determined by the following equation:
HTMin = MS + (BS-MS) / 3

  Here, it goes without saying that compliance with this temperature is always subject to certain tolerances under the operating conditions, i.e. this temperature is generally not exactly what it is and is typical. Is monitored with a tolerance of +/− 20 ° C.

  In the following, the present invention will be described in more detail based on exemplary embodiments.

  Seven steels S1-S7 were melted. Their compositions are shown in Table 1.

  Corresponding steel melts were cast by conventional methods to form slabs and then similarly heated by conventional methods to reheat temperature OT.

  The heated slab was hot rolled in a similar conventional group of hot rolling stands to form hot strips W1-W10 having a thickness of 2.0 mm.

  The hot strips W1-W10 that emerged from the group of hot rolling stands were each set to the hot rolling end temperature ET, and were subjected to accelerated cooling from the temperature to the coiling temperature HT at the cooling rate KR. Hot strips W1-W10 were wound at this coiling temperature HT to form a coil.

  Thereafter, the coils were each cooled in a temperature range having an upper limit determined by each coiling temperature HT and a lower limit determined by the martensite start temperature MS determined for each steel S1-S7. The martensite start temperature MS is described here in the paper ("Thermodynamic Extrapolation and Martensite-Start-Temperature of Substitutively Alloyed Steels", H. Bhadeshia, Metalc. did.

  The period during which the coil is cooled in the temperature range defined by the above-described method is such that each of the hot strips thus obtained has a length having a microstructure composed of bainite and residual austenite, in which case the other microstructures The ratio of tissue components was present only at an ineffective amount of substantially “0” at the maximum.

  Table 2 shows operating parameters of the reheating temperature OT, the hot rolling end temperature ET, the cooling rate KR, the coiling temperature HT, and the martensite start temperature MS.

  Table 3 additionally shows the mechanical properties of the tensile strength Rm, the yield strength Rp, the elongation A80, the quality Rm * A80 and the respective retained austenite content RA determined for each hot strip. .

  It has been found that the desired tensile strength of at least 1200 MPa is not achieved in the case of a hot strip W3 made from steel S3 and having a relatively low Si content.

  In the case of the hot strip W5 made of steel S4 and not produced according to the invention due to the very low hot rolling end temperature ET, coarse martensite as well as up to 12% by volume of blocky coarse retained austenite Is present in the microstructure, whereby the elongation A80 is significantly reduced.

  On the other hand, a hot strip W4 manufactured in the same way from steel S4 but manufactured to monitor the specifications according to the invention was composed of coarse block residual austenite with an average size greater than 5 μm and a maximum of 1% by volume. The remaining retained austenite was present in film form and in finer block form, so that a high elongation A80 was achieved.

  In the case of the hot strip W7 manufactured from steel S5 and in the case of the hot strip W10 manufactured from steel S7, the desired minimum tensile strength of 1200 MPa was likewise not achieved. In those cases, the reason was the excessively high coiling temperature HT.

Claims (15)

A hot rolled flat steel product having a product of tensile strength Rm and elongation A80 of at least 18000 (MPa *%), in addition to iron and inevitable impurities (in weight%)
C: 0.10-0.60%,
Si: 0.4-2.0%,
Al: Up to 2.0%
Mn: 0.4-2.5%
Ni: up to 1%
Cu: up to 2.0%,
Mo: up to 0.4%,
Cr: up to 2%
Ti: maximum 0.2%,
Nb: maximum 0.2%,
V: including up to 0.5%
The flat steel product has a microstructure occupied by two phases, one main component of the microstructure is bainite, and the other main component of the microstructure is retained austenite, The microstructure of bainite in an amount of at least 50% by volume and residual austenite as the balance,
In the microstructure of the flat steel product, a maximum of 5% by volume of ferrite and a maximum of 10% by volume of martensite can be optionally present,
A hot rolled flat steel product characterized in that at least part of the retained austenite is present in block form and at least 98% of the blocks of retained austenite present in the form of this block have an average diameter of less than 5 μm. In the flat steel product according to claim 1,
A flat steel product, wherein the microstructure of the flat steel product contains at least 10% by volume of retained austenite. In the flat steel product according to claim 1 or 2,
The flat steel product has a Cu content of at least 0.15% by weight. In the flat steel product according to any one of claims 1 to 3,
A flat steel product, wherein the C content of the flat steel product is at least 0.3% by weight. In the flat steel product according to any one of claims 1 to 4,
The content of Mn, Cr, Ni, Cu and C in the flat steel product is
1 <0.5% Mn + 0.167% Cr + 0.125% Ni + 0.125% Cu + 1.334% C <2 is satisfied, where
% Mn: content of each Mn by weight%,
% Cr: content of each Cr by weight%,
% Ni: content of each Ni by weight%,
% Cu: content of each Cu by weight%,
% C: Flat steel product characterized by the content of each C by weight%. In the flat steel product according to any one of claims 1 to 5,
Si and Al content of the flat steel product,
% Si + 0.8% Al> 1.2% by weight, where:
% Si: Content of each Si by weight%,
% Al: Flat steel product characterized by the content of each Al by weight%. In the flat steel product according to any one of claims 1 to 6,
A flat steel product having a block retained austenite diameter of 1-3 μm. In the method for manufacturing a flat steel product according to any one of claims 1 to 7,
In addition to iron and inevitable impurities, 0.10-0.60% C, 0.4-2.0% Si, up to 2.0% Al, 0.4-2. 5% Mn, up to 1% Ni, up to 2.0% Cu, up to 0.4% Mo, up to 2% Cr, up to 0.2% Ti, up to 0.2% Nb and up to Providing a semi-finished product containing 0.5% V in the form of a slab, thin slab or cast strip;
Hot rolling the semi-finished product in one or more rolling perforations to form a hot strip, the resulting hot strip having a heat of at least 880 ° C. when leaving the last rolling perforation A step having an end rolling temperature, and
Accelerating cooling of the resulting hot strip to a coiling temperature in the range between the martensite start temperature MS and 600 ° C. with a cooling rate of at least 5 ° C./s;
Coiling the hot strip to form a coil;
A step of cooling the coil, the upper limit being the same as the bainite start temperature BS at which bainite begins to occur in the hot strip microstructure, and the martensite start temperature MS at which martensite begins to occur in the hot strip microstructure; The temperature of the coil is maintained during cooling to form bainite until at least 50% by volume of the microstructure of the hot strip is composed of bainite in a temperature range having a lower limit that is the same. A method characterized by. The method of claim 8, wherein
A method characterized in that the end temperature of hot rolling is at least 900 ° C. The method according to claim 8 or 9, wherein
A method characterized in that the cooling rate is at least 10 ° C./s. A method according to any one of claims 8 to 10,
A method characterized in that the cooling rate is 150 ° C./s at the maximum. 12. A method according to any one of claims 8 to 11,
A method characterized in that the cooling rate is at most 50 ° C./s. The method according to any one of claims 8 to 12,
A method in which the lower limit of the coiling temperature at which cooling starts in the coil is about 20 ° C. higher than the martensite start temperature MS. 12. A method according to any one of claims 8 to 11,
The upper limit of the coiling temperature at which cooling starts in the coil is 550 ° C. The method according to any one of claims 8 to 12,
The coiling temperature is
HTopt = MS + (BS-MS) / 3
The method corresponds to at least the temperature HTopt determined by the mathematical formula.

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