ES2881667T3 - Method of producing steel product or steel component having excellent mechanical properties, steel product produced by the method and use of steel pipe made of strain-hardened steel - Google Patents

Abstract

A method of producing a steel product having excellent mechanical properties, such as a steel pipe, including the cold-forming method (1, 2), whereby a strain-hardened structure is formed in the steel, characterized in that the The method also comprises the following steps: steel having a strain-hardened structure is heated very rapidly (3), i.e. at a rate of RH >=100 ºC/s to a temperature of Ac3 ± 100 ºC, without softening annealing prior to rapid heating - the heated steel is cooled (4) such that the resulting microstructure formed in the steel product will comprise at least 30% martensite and/or vanite in volume percentages, whereby the steel product produced by the method comprises, in percentages by weight: C: 0.05 - 0.25% Si: 0 - 2.5% Mn: 0.8 - 3.0% Al: 0 - 2.5% Mo: <1 0.5%, preferably 0.1-0.8% Cr: <1.5%, preferably 0.2-1.5% Cu: <1.00% Ti: <0.15 % Nb: < 0.09 % V: < 0.17 % Ni: 0 - 0.07 % preferably 0.015 % < Ti+Nb+V < 0.22 % with the balance being Iron Fe and unavoidable residues, such that the hardenability index DI of the steel product is at least 3 mm, preferably at least 15 mm, or as far as the steel product produced by the method comprises, in percentages by weight: C: 0.10-0.40% , Si: 0 - 2.5% Mn: 0.2 - 3.0% 0 Al: 0 - 2.5% Cu: 0 - 1.0% B: 0.0005 - 0.009% Cr: 0 - 1, 5 % Mo: 0 - 1.5 % Ti: < 0.15 % Nb: < 0.09 % V: < 0.17 % Ni: 0 - 0.07 % being the rest iron Fe and unavoidable residues, in such a way such that the hardenability index DI of the steel product is at least 3 mm, preferably at least 15 mm.

Description

DESCRIPTION

Method of producing steel product or steel component having excellent mechanical properties, steel product produced by the method and use of steel pipe made of strain-hardened steel

Background of the invention

The invention relates to a method for preparing a steel product having excellent mechanical properties and to a steel product prepared by the method and used, in particular, to manufacture components, more precisely steel components, for the automotive industry, among other Furthermore, the invention relates to the use of a strain-hardened steel pipe and the manufacture of such a component.

It is known that higher strengths are sought in steel applications, which allow the preparation of lighter structures, so that the importance of lighter structures stands out particularly in vehicle applications. However, as the strength of the steel increases, typically the formability properties of the steel, such as deformation, are known to deteriorate. This poses restrictions for high strength steel applications. A combination of high strength and high ductility increases the usability of steels in many applications. However, heat treatments may not be a factor of delay in production, therefore it is also advantageous to make heat treatment methods faster.

It is commonly known that steel can be provided to have high strength combined with high ductility, for example, by Q&T treatments. However, a problem with this method is that the heat treatment involves two phases and requires a considerable amount of time. In recent years, rapid heat treatments have been developed, which make it possible to provide stronger and yet more extensible steels compared to conventional quench and quench treatments. A problem with these known rapid methods is, however, that the methods employ, after cold forming, a softening annealed steel, in other words the method employs a rapid heat treatment prior to the softening annealing and Again, the heat treatment part of the method involves two phases. Furthermore, due to the starting material previously used in the method, known methods including rapid heat treatments have a difficulty in achieving superior mechanical properties according to the invention which will be described later.

US 2006/169368 describes a low carbon alloy steel tube and a method for its manufacture, especially for a pressure vessel of a stored gas inflator, in which the steel tube consists essentially of, by weight: from about 0.06% to about 0.18% carbon, from about 0.3% to about 1.5% manganese, from about 0.05% to about 0.5% silicon, to about 0.015% sulfur, up to about 0.025% phosphorus, and at least one of the following: up to about 030% vanadium, up to about 0.10% aluminum, up to about 0.06% niobium, up to about 1% chromium, up to about 0.70% nickel, up to about 0.70% molybdenum, up to about 0.35% copper, up to about 0.15% residual elements, and the balance iron and occasional impurities.

After a high heating rate of about 100 ° C per second, a complete cooling of the steel pipe is rapidly applied in a water-based cooling solution at a cooling rate of about 100 ° C per second. Steel has a tensile strength of at least approximately 145 ksi and a high value of 220 ksi and exhibits ductility behavior at low temperatures of -100 ° C.

US 2010/132849 refers to a high tensile strength galvanized steel sheet including C: at least 0.05% but less than 0.12%, Si: at least 0.01% but less than 0 , 35%, Mn: 2.0% to 3.5%, P: 0.001% to 0.020%, S: 0.0001% to 0.0030%, Al: 0.005% to 0.1%, N: 0.0001% to 0.0060%, Cr: more than 0.5% but not more than 2.0%, Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080 %, Nb: from 0.010% to 0.080% and B: from 0.0001% to 0.0030%, the rest being Fe and unavoidable impurities, in which the galvanized steel sheet with high tensile strength has a microstructure that It contains 20% to 70% by volume of ferrite having an average grain size of 5 µm or less. High tensile strength galvanized steel sheet has a tensile strength of at least 980 MPa and excellent forming and welding ability.

EP 1,659,191 describes a high tensile cold rolled steel sheet consisting essentially of 0.04 to 0.13% C, 0.3 to 1.2% Si, 1.0 to 3.5 % Mn, 0.04% or less of P, 0.01% or less of S, 0.07% or less of Al, by weight, and the rest of Fe and unavoidable impurities, has a microstructure containing 50 % or more of percentage per area of ferrite and 10% or more of percentage per area of martensite, it has 0.85 to 1.5 martensite gap ratio in rolling direction to sheet thickness direction, and has 8 GPa or more of martensite nano-strength. The high tensile cold rolled steel sheet has a good strength-elongation balance and shows excellent shock resistance at approximately 10 s ^-1 strain rate. Therefore, the high tensile cold rolled steel sheet is suitable for reinforcing car dash and pillar members.

The object of the present invention is to provide such an improvement in known heat treatment and / or forming processes that gives a steel product with better mechanical properties than previously or makes possible a simplification of the process with respect to known techniques.

Brief description of the invention

The objects of the invention are achieved by the method according to claim 1.

Preferred embodiments of the method according to the invention are described in the dependent claims. Preferred embodiments of the steel product prepared by the method are described in claims 10-12.

Furthermore, the objects of the invention are achieved through the use of a strain-hardened steel pipe of independent claim 13.

The invention is based on the idea that in the microstructure of the steel to be heated very rapidly there is provided a strain hardened structure which is not subjected to softening annealing prior to rapid heating. In other words, the strain-hardened structure can refer to a structure that is not recrystallized after molding, that is, it can refer to a non-recrystallized structure. The strain-hardened structure, such as the complete hard structure, is provided by cold forming, such as cold rolling a steel tape, for example. Furthermore, the invention is characterized in that rapid heating is followed by immediate cooling. The microstructure provided in the steel comprises at least 30 percent martensite and / or green beans by volume. In other words, the steel is heated in an accelerated manner after obtaining a strain-hardened structure and is immediately hardened after rapid heating. Hardening can also be carried out by means of air cooling, if the composition of the steel is suitable for this. According to one embodiment, the cooling or hardening is provided in a mold in which molding is also carried out in the form of a component. The method of the invention provides an extremely fine microstructure in the steel and as a consequence the steel will have progressive mechanical properties.

The invention has significant advantages as regards the production line as well as the mechanical properties of the product. In the method of the invention, heat treatment can be put into practice very quickly, and consequently, when necessary, can be applied on some production lines of continuous operation, arranged in immediate connection with the formation of a steel product. The duration of the heat treatment step of the method can be less than one minute, even less than 30 seconds.

Particularly shaped products, such as steel pipes, are prepared by the method. Steel components can also be provided by the method. Where necessary, the method allows higher values of R ^m * A compared to steel products made by a conventional slow method. Furthermore, the production of a steel product does not require annealing after cold forming and therefore this step does not necessarily restrict the production volume.

Under certain conditions, by the method of the invention it is possible to provide even progressive properties typical of TRIP steel (Transformation Induced Plasticity) in terms of strength and ductility without slow heat treatments typical of TRIP steel fabrication, or advantageously even without expensive and high alloys that impair weldability.

According to one embodiment, by the method it is possible to provide strong and formable steel products, such as steel pipes, so that the use of a steel pipe prepared by tempering, still cold formable, is cost effective in the manufacture of castable components. cold because the final component made of steel pipe does not need to be hardened separately since the pipe material has already been hardened, at least partially.

Brief description of the figures

In the following, the present invention is described in more detail with reference to the accompanying drawings, in which

Figure 1 shows the main steps of the method according to an embodiment of the invention;

Figure 2 shows the main steps of the method according to a second embodiment of the invention;

Figure 3 shows, in principle, a temperature / time graph on the partial steps of a method according to the invention;

Figure 4 shows the strain-stress curves obtained from laboratory tests.

Description of reference numbers

1 cold forming

2 cold forming of a shaped product

3 rapid heating

4 cooling

Detailed description of the invention

Figure 1 shows the main stages of the method according to the invention, so that in the first stage a steel strip is cold formed 1, such as cold rolled, in such a way that a strain hardened structure will be formed in the steel of the steel tape. Cold forming 1 is performed on a suitable steel preform, such as a hot rolled or cast steel strip / plate / shaped product. In cold forming 1 the steel normally becomes thinner, at least partially.

The efficiency of cold forming can be approximated, for example, through reduction, which refers to a change in the thickness of the steel after cold forming, relative to the original thickness.

In the context of the present patent application, a strain-hardened structure refers to a steel structure whose reduction used in cold forming exceeds 15%. Preferably, the reduction used in cold forming is more than 30%, at most preferably more than 40%. According to one embodiment, the strain-hardened structure refers to a completely hard structure, in which the reduction used is at least 50%, such as 60%.

Preferably cold forming means that the steel is cold rolled to a thin sheet having a thickness of 0.4 to 5mm, for example. Advantageously, cold rolling is carried out to a thickness of 0.8 to 3 mm.

According to the embodiment of Figure 1, the steel having a work-hardened structure is cold formed to a shaped product 2. The cold forming step 2 of the shaped product may comprise cold forming a pipe and making a longitudinal joint weld to obtain a closed pipe profile. In cold forming the pipe, the degree of strain hardening of the steel is further increased. Thus, according to this embodiment, after step 2 a highly strain-hardened steel pipe is obtained, which is still unusable as such. According to the embodiment of Figure 1, in a next process step, the shaped steel product, such as steel pipe, having a strain-hardened structure, is heated 2 very rapidly, preferably at least at a rate RH> 100 ° C / s up to a temperature of Ac3 ± 100 ° C.

Thanks to the very rapid heating and the strain-hardened structure as shown in Figures 1 and 2, the granular structure of austenite is very fine due to heating 3. Figure 3 further illustrates the "rapid heating 3" and "" stages. cooling 4 "of the method according to the invention. According to the invention, the rapid heating 3 is preferably carried out at least at an average heating rate of 100 ° C / s. In this way, it is possible to ensure that during heating the grain size will not have time to grow excessively. On the other hand, high heating rates require a considerable amount of energy and can deteriorate the uniform quality of the mechanical properties of a steel product. According to one embodiment, heating 3 is carried out at a rate of RH> 160 ° C / s and, according to a second embodiment, heating 3 is carried out at a rate of RH = 160-500 ° C / s. Preferably, but not necessarily, the heating is implemented as induction heating, which can be carried out rapidly in closed steel pipes, for example. Other heating methods with a high heating rate are also possible.

The abbreviation HR stands for rate of heating.

The rapid heating 3 of the steel having a work-hardened structure is carried out at a temperature of Ac3 ± 100 ° C. Preferably, heating is carried out rapidly 3 above the Ac3 temperature. Preferably, but not necessarily, heating 3 begins substantially at room temperature.

It is important that due to heating, the steel does not remain excessively long above the Ac3 temperature or in its vicinity. In other words, the residence time At above the temperature Ac3, or in its vicinity, should be as short as possible, because in that case the grain size of the austenite remains small. Preferably, the cooling 4 starts in at least 10 seconds from the completion of the heating 3, most preferably the heating starts at least 2 seconds from the completion of the heating. In other words, the time spent in or near the austenite range should be as short as possible.

According to the embodiment of Figure 2, a rapidly heated steel pipe is hot formed, such as etched, immediately after the rapid heating 3, whereby it will no longer be necessary to form the cooled steel product. After fire etching, it is possible to carry out a cooling in a mold, preferably an annealing in a mold. The embodiment can provide an improvement in the mechanical properties of a fire-etched steel component, when the steel pipe to be heated and tempered is made of a strain-hardened steel tape. The steel pipe of the invention can also be more robust with respect to some parameters of the hot stamping process, due to the initial structure strongly hardened by deformation.

According to one embodiment, upon completion of heating 3, immediate cooling is started without a substantial constant temperature in the austenitic range. This makes it possible to minimize the time the steel is in or near the austenitic range.

Thanks to the very high heating rate and the deformation-hardened structure of the steel pipe to be heated, the density of appearance of small grains of austenite formed in rapid heating 3 is high, which gives rise to a microstructure very fine in a steel product of the invention, which has excellent mechanical properties. Thanks to the rapid heating 3, the grain size of the austenite will not have time to grow to a large size whereby, as a consequence of the heating 3, the microstructure comprises very densely formed small-sized austenite grains. The end result, after cooling, will be a stronger steel product, the tensile strain of which is still substantially on the same level as or higher than that of the steel product in rapid heating using a softening annealed steel.

In connection with the invention, the Ac3 temperature was found to be higher at high heating rates compared to Ac3 temperature at low heating rates. Therefore, it should be appreciated that in relation to this, the Ac3 temperature is understood to be a possibly elevated Ac3 temperature as a consequence of the higher heating rate.

Heating 5 from or near the austenitic range can be practiced in a variety of ways, in order to obtain a desired end result. However, it is essential that the cooling 5 is provided in a manner known per se, such that the microstructure formed in the steel structure comprises 30% by volume of martensite and / or vanite. In other words, in cooling 4, the steel is tempered in a way that is completely known per se. The remainder of the microstructure may consist of ferrite and possibly residual austenite.

The desired microstructure can be obtained in a manner known per se, for example such that the cooling 4 is carried out at an average cooling rate of at least 5 ° C / s. Furthermore, in a manner known per se, a steel is used whose hardenability is sufficient to provide 30% by volume of martensite and / or green bean on cooling.

The hardenability of steel can be determined in a number of ways, such as using a DI hardenability index that is based on a modification of the ASTM standard A255-89. The hardenability index DI is determined based on the composition of the steel according to the following formula:

DI = 13.0C x (1.15 2.48Mn 0.74Mn2) x (1 2.16Cr) x (1 3.00Mo) x (1 1.73V) x (1 0.36Ni) x (1 0, 70Si) x (1 0.37Cu)

In the formula, the composition is expressed in percentages by weight (%) and ID is millimeters (mm).

Preferably, the hardenability index DI of the steel product is at least 3 mm.

Most preferably, the hardenability index DI of the steel product is at least 15mm.

The cooling can be implemented in a manner known per se, for example, as steam or water cooling, depending particularly on the steel composition used and the thickness of the steel. An element for providing cooling can also be a mold.

Preferably, cooling 4 is provided such that the microstructure formed comprises at least 70% by volume martensite and / or vanite and / or the remainder being ferrite and possibly residual austenite. Preferably, the cooling is implemented as a steam or water cooling or as a cooling in a mold, so that the average cooling rate is at least 20 ° C / s, preferably at least 35 ° C / s . The final temperature of the accelerated cooling is preferably below the starting temperature of bean (temperature B ^s ), most preferably below the starting temperature of martensite (temperature M ^s ), that is, the temperature at which the transformation of green bean begins and, correspondingly, the transformation of martensite. In this way it can be ensured that the microstructure of the steel will be as desired.

After cooling 5, the steel product can be cut to the desired dimensions, when the steel product is a steel pipe.

The invention also relates to a steel product prepared by the method described below. The steel product obtained by the method has high strength and ductility. Therefore, the steel product has excellent mechanical properties. Preferably, the steel product is also highly formable and, thanks to the low alloy, its weldability is good.

In the steel product obtained by the method, the product of breaking strength and elongation at break is at least 12000, that is, R ^m * A> 12000. Preferably, R ^m * A> 15000 and most preferably R ^m * A> 18000. Furthermore, the breaking strength of the steel product obtained by the method is at least 800 MPa.

An advantage of the method is also that by using the method, with the same steel composition, it is possible to provide 1) a high-strength steel with excellent ductility (Rm> 1500 MPa and A> 10%) or 2) a strong steel. and particularly ductile (Rm> 800 MPa and A> 18%).

The products to be prepared by the method can be roughly divided into two categories: Category 1.

The steels in this category have a very high breaking strength and a well-retained elongation A at break, as follows: R ^m > 1500 MPa and A> 10%. Therefore, these steels are very strong, despite this the ductility is at a good level thanks to the method of the invention. This is indicated by Steels Examples 1, 4, 5, 10 and 11 in Table 2.

Surprisingly, the steel product prepared by the method also advantageously possesses a relatively high uniform elongation Ag> 3% despite the substantially hardened microstructure and high breaking strength of the steel prepared by the method.

Category 2

The steels in this category have a relatively high breaking strength (Rm> 800 MPa) and a very high elongation (A> 18%, preferably up to A> 18% and Ag> 8%). These steels are therefore relatively strong and the elongation has become very high, thanks to the method of the invention.

The chemical composition of the steel product prepared by the method of the invention is described below.

The steel product prepared by the method comprises in percentages by weight,

C: 0.05 - 0.25%

Yes: 0 - 2.5%

Mn: 0.8 - 3.0%

Al: 0 - 2.5%

Mo: <1.5%, preferably 0.1 - 0.8 %

Cr: <1.5%, preferably 0.2 - 1.5%

Cu: <1.00%

Ti: <0.15%

Nb: <0.09%

V: <0.17%

Ni: 0 - 0.07%

preferably 0.015% <ti Nb V <0.22%

the remainder being iron Fe and unavoidable residues, so that the hardenability index DI of the steel product is at least 3 mm, preferably at least 15 mm.

Alternatively, the steel product by the method comprises, in percentages by weight

C: 0.10-0.40%

Yes: 0 - 2.5%

Mn: 0.2 - 3.0%

Al: 0 - 2.5%

Cu: 0 - 1.0%

B: 0.0005 - 0.009%

Cr: 0 - 1.5%

Mo: 0 - 1.5%

Ti: <0.15%

Nb: <0.09%

V: <0.17%

Ni: 0 - 0.07%

the remainder being iron Fe and unavoidable residues, such that the hardenability index D of the steel product is at least 3 mm, preferably at least 15 mm.

The grain size of the pre-austenite formed on heating 3 of the steel product prepared by the method of the invention is very small, preferably less than 10 µm and most preferably less than 3 µm. Furthermore, the grain size of the ferrite and / or green bean possibly formed in the steel product on cooling 4 is also very small, preferably less than 5 µm and most preferably less than 3 µm. Also the martensite formed on hardening will be fine and hard thanks to the invention.

The thickness of the material that in the steel product can be 0.4 to 5mm, preferably 1 to 3mm. The steel product can be a cold formed shaped product. Most preferably, the steel product is a steel pipe used, for example, in the manufacture of components in the automotive industry. The steel product can thus have a hollow section, such as a circular or rectangular steel pipe. The steel product can also be a stamped steel component.

Examples

Table 1 shows the mechanical properties and process parameters of steel samples 10a and 8a produced under laboratory conditions by the method of the invention. Additionally, Table 1 shows as a reference the mechanical properties and process parameters of steel samples ref8a and ref10a whose heating speed is low, 4 ° C / s, that is, the heating speed is clearly lower than the heating speed used in the method of the invention. Furthermore, Table 1 shows for reference the mechanical properties of a ref test sample which has not been heat treated according to the method of the invention. For reasons of safety and clarity, the references are set out in groui in Table 1. Correspondingly, Figure 4 shows the corresponding ones whose stress-strain 10a, 8a, ref8a, ref 10a and ref. All the results in the Table and in the graph refer to strain-hardened CP800 steel, the composition of which is shown in Table 3. The results in Table and Figure 4 are not comparable with those of Table 2, but are mutually comparable.

In the laboratory test of Table 1, small test rods were rapidly heated to a temperature above A ^c3 , or in the vicinity, using a DC source whose current and / or voltage were modified depending on the sample and the desired heating rate. The temperature of the sample and its rate of rise was measured with a K-type thermocouple welded with wire spots in the middle of the sample. Immediately after heating, the samples were cooled with a water spray or in free air. Figure 3 shows the temperature-time graph of a laboratory test in which cooling is implemented as free-air cooling. In this example, the heating rate is approximately 1500 ° C / s and an average cooling rate of 50 ° C / s. A peak appearing in Figure 3 above the A ^c3 temperature is caused by a measurement error due to a high heating rate. In other words, the temperature peak that appears in Figure 3 is not part of the invention. After heat treatment, the test rods were machined and a tensile test was performed on their uniform parts (4 mm long, 2 mm wide, 2.5 mm thick. Laboratory test results show that the uniform elongation A ^g in the steel samples 10a and 8a prepared by the method of the invention is substantially improved compared to the untreated steel sample ref. In addition, the breaking strength R ^m increased substantially at the same time.

The results of the laboratory tests also show that due to the high heating rate, the elongation A of the water-cooled steel sample 10a increases compared to the ref 10a produced at a low heating rate. Furthermore, the elastic limit and the breaking strength (Rp ^0.2 and R ^m ) increased at the same time.

Correspondingly, the yield strength and breaking strength (Rp ^0.2 and R ^m ) of the air-cooled 8a steel sample increased significantly due to the high heating rate compared to the ref 8a steel sample produced at a low rate. heating. At the same time the uniform elongation Ag surprisingly remained the same and the elongation A at break only slightly weakened.

Table 1 Laboratory test results

Table 2 presents the mechanical properties and dimensions of steel products obtained in tests 1-15 at full scale, as well as the process parameters used. The table provides as reference values tests 3, 6, 9, 12 and 15, in which heat treatment did not use steel having a strain-hardened structure, that is, after cold forming, the steel was subjected to softening annealing in a known manner. X in column FH indicates that a steel that was not subjected to softening annealing was used in these tests. In addition, Table 2 presents as reference tests 16 to 21, in which a heating rate significantly lower than that of the invention was used. For the sake of clarity, the references are shown in gray in Table 2. In the tests, the diameter (D) of the pipe was 48 - 49 mm and the thickness of the material (T) was 1.7 - 2, 1 mm.

All the results in Table 2 refer to a strain-hardened B24 steel, the composition of which is shown in Table 3. The tensile tests were carried out in accordance with the EN 10002 pattern of tensile tests and the measurements used for the Tensile test rod were 50mm in gauge length and 10mm in width. The percent elongation A at break is determined by computer to correspond to a strain measured with a rod of ratio 5.65x (square root) S0.

Table 2 results of the tests

In the tests of Table 2, two different degrees of reduction were used, 50% and 20% with the degree of production representing the effective amount of cold forming and directly affecting the level of roll hardness. The magnitude of the reduction is indicated in the RR (%) column.

It is observed in Table 2 that both the strengths R ^p0.2 and R ^m as well as the elongation A at break of the steel products obtained in the tests (1, 7, 10 and 13) carried out by the method of the invention are substantially better than those of the steel products in the benchmark tests (3, 9, 12 and 15).

It can be seen in Table 2 that the product of elongation A at break and the resistance R ^m at break of the steel product improved by 15.8 to 116.5% in the test examples (1, 4, 7, 10, 13) using the method of the invention compared to tests (3, 6, 9, 12 and 15), that is, the improvement obtained by the invention is very significant.

It is observed in Table 2 that with a greater reduction (50%) a considerable increase was obtained both in the resistances R ^p0.2 and R ^m and in the uniform Ag elongation, in comparison with the corresponding tests carried out with a lower reduction ( twenty%).

Therefore, it is advisable to use a high reduction in the method, that is, the steel that is going to be heated rapidly comprises a strongly strain-hardened structure.

In the tests of Table 2, both high cooling effect water cooling and lower cooling effect steam cooling were used as the cooling method. This is indicated in column CR of Table 2. A higher product A * R ^m of elongation at break and resistance at break than in the reference test (3, 6, 9, 12, 15) was also achieved with steam, when the reduction is 50%.

The heating rate used is given in the HR column. It can be seen from the results of Table 2 that the tests (4, 5, 10, 11) carried out at the heating rate of the invention give rise to a higher elongation and a higher value of R ^m * A than the tests reference (16, 17, 19, 20) performed at a lower heating rate.

The heating rates of 300, 500 and 1000 ° C / s were used in the tests of the method according to the invention in Table 2. It is observed in the Table that the objects of the invention are achieved at the heating rate of 300 ° C / s. A lower heating rate is advantageous due to a lower energy requirement in the apparatus and, consequently, it is advisable to use a lower heating rate when sufficient. In many cases, a heating rate of at least 100 ° C / s will be sufficient to achieve the objectives of the invention.

However, advantageously, heating rates of at least 160 ° C / s are used, whereby it is sure that the grain size of the austenite formed on heating does not grow excessively and excellent mechanical properties will be achieved. Table 1 presents the results showing that the invention also works at heating rates of 192 and 197 ° C / s.

However, it should be appreciated that, under appropriate conditions, the heating rates used in stamping can be lower, even as low as 15 to 100 ° C / s, because the strain-hardened steel pipe improves the properties of its own. component mechanics to be achieved in said hot stamping process or is more robust to use said process than prior art solutions. Furthermore, a higher heating rate can provide advantages in this process as well.

Heating and cooling rates are average cooling rates.

However, it is desirable for the method to use heating rates, such as 160 to 500 ° C / s, because generating a lower heating rate requires less energy and can help control the uniformity of the results.

The heating temperature used is indicated on the HT column. The appropriate heating temperature depends on the composition of the steel, however it is selected so that the heating temperature is A ^c3 ± 100 ° C. Preferably, but not necessarily the heating is carried out at a temperature slightly above A ^c3 . Said temperature refers to a temperature at which the steel is completely austenitized. It is important, however, that, because the heating of the steel is not maintained for an excessive time above the temperature A ^c3 or in its vicinity. It is preferred to start cooling 4 in at least 10 seconds from the completion of heating 3, most preferably at least 2 seconds from the completion of heating.

Table 3 shows some steel compositions to which the method of the invention is applicable.

T l. n ni r n n r n n

In the foregoing, the invention is described by way of preferred embodiments and it is clear that the details of the invention can be practiced in a variety of ways within the scope of the above. appended claims, when sufficient. In many cases, a heating rate of at least 100 ° C / s will be sufficient to achieve the objects of the invention.

However, advantageously, heating rates of at least 160 ° C / s are used, whereby it can be ensured that the austenite grain size formed on heating does not grow excessively and excellent mechanical properties will be achieved. Table 1 presents the results showing that the invention also works at heating rates of 192 and 197 ° C / s.

It should be appreciated, however, that without proper conditions, the heating rates used in hot stamping may be lower, even 15 to 100 ° C / s, because the strain-hardened steel pipe itself improves the mechanical properties of the components to be achieved in said hot stamping process, or is more robust to use in said process than in prior art solutions. Furthermore, a higher heating rate can provide advantages in this process as well.

Heating and cooling rates are average cooling rates.

However, it is advisable for the method to use heating rates such as 160 to 500 ° C / s, because generating a lower heating rate requires less energy and can help control the uniformity of the results.

The heating temperature used is indicated on the HT column. An appropriate heating temperature depends on the composition of the steel, however, it is selected so that the heating temperature is Ac3 ± 100 ° C. Preferably, but not necessarily, the heating is carried out at a temperature slightly above Ac3. Said temperature refers to the temperatures at which the steel is completely austenitized. However, it is important that, due to the heating of the steel, it is not kept for an excessively long time above the Ac3 temperature or in its vicinity. It is preferred to start the cooling 4 in at least 10 seconds from the completion of the heating 3, most preferably at least 2 seconds from the completion of the heating.

Table 3 shows some compositions to which the method of the invention is applicable.

T l. n ni r n n r n n

In the foregoing, the invention is described by way of preferred embodiments, and it is clear that the details of the invention can be practiced in a variety of ways within the scope of the accompanying claims.

Claims (15)

1. A method of producing a steel product having excellent mechanical properties, such as a steel pipe, including the cold forming method (1, 2), whereby a strain-hardened structure is formed in the steel, characterized because the method also comprises the following steps: steel having a strain-hardened structure is heated very quickly (3), that is, at a rate of RH> 100 ° C / s up to a temperature of Ac3 ± 100 ° C, without softening annealing before rapid heating - the heated steel is cooled (4) in such a way that the resulting microstructure formed in the steel product will comprise at least 30% martensite and / or green beans in percentages by volume, therefore the steel product produced by the method comprises , in percentages by weight: C: 0.05 - 0.25% Yes: 0 - 2.5% Mn: 0.8 - 3.0% Al: 0 - 2.5% Mo: <1.5%, preferably 0.1 - 0.8% Cr: <1.5%, preferably 0.2 - 1.5% Cu: <1.00% Ti: <0.15% Nb: <0.09% V: <0.17% Ni: 0 - 0.07% preferably 0.015% <Ti + Nb + V <0.22% the remainder being iron Fe and unavoidable residues, such that the hardenability index DI of the steel product is at least 3 mm, preferably at least 15 mm, or so that the steel product produced by the method comprises, in percentages by weight: C: 0.10-0.40%, Yes: 0 - 2.5% Mn: 0.2 - 3.0% Al: 0 - 2.5% Cu: 0 - 1.0% B: 0.0005 - 0.009% Cr: 0 - 1.5% Mo: 0 - 1.5% Ti: <0.15% Nb: <0.09% V: <0.17% Ni: 0 - 0.07% the remainder being iron Fe and unavoidable residues, such that the hardenability index DI of the steel product is at least 3 mm, preferably at least 15 mm. The method of claim 1, characterized in that the cold forming (1) is implemented by cold rolling a steel strip. The method of claim 1 or claim 2, characterized in that heating (3) at a rate of RH> 160 ° C / s, most preferably at a rate of 160 to 500 ° C / s. 4. The method of any one of claims 1 to 3, characterized in that it is cooled at a rate of at least 5 ° C / s. The method of any one of claims 1 to 3, characterized in that it is cooled (4) at a rate of at least 20 ° C / s. The method of any one of claims 1 to 3, characterized in that the cooling (4) is provided in such a way that the microstructure formed in the steel product comprises at least 70% by volume of martensite and / or green bean, the remainder being ferrite and possibly residual austenite. The method of any one of the preceding claims, characterized in that the cooling (4) starts in at least 10 seconds, more preferably in at least 2 seconds, from the completion of the heating (3). The method of any one of the preceding claims, characterized in that the reduction used in cold forming (1, 2) is more than 15%, preferably more than 30% and most preferably more than 40%. The method of claim 8, characterized in that the strain-hardened structure is a complete hard structure whose reduction used in cold forming is at least 50%, such as 80%. The method of any one of the preceding claims, characterized in that the cooling or hardening is provided in a mold, in which the one-component forming is also carried out. 11. A steel product, such as a steel pipe, produced by the method of any one of claims 1 to 10, characterized in that the product of breaking strength and elongation at break Rm * A is al minus 12,000, preferably at least 15,000, and most preferably at least 18,000. 12. The steel product produced by the method of any one of claims 1 to 10, characterized in that the uniform elongation of the steel product is Ag> than 3%. The steel product prepared by the method of any one of claims 1 to 10, characterized in that the breaking strength of the steel product is Rm> 800 MPa. 14. Use of a steel pipe made from strain-hardened steel in a rapid heat treatment process according to claim 1. 15. Use according to claim 14, characterized in that the process comprises extremely rapid heating, at a rate of at least RH> 160 ° C / s at a temperature of Ac3 ± 100 ° C.