KR20220149776A - Steel article and method for manufacturing the same - Google Patents

Abstract

A method for producing a steel article having a microstructure, mainly a mixture of bainite and martensite, and a tensile strength greater than 780 MPa is provided.

Description

Steel article and method for manufacturing the same

FIELD OF THE INVENTION The present invention relates to a method for air-hardening steel for the manufacture of articles such as, for example, molded articles or parts.

High-strength automotive parts are often hot rolled or cold rolled steel strips ( strip) material.

For cold forming, the development and improvement of various new concepts of high-strength steels have been made over the past few years. Multiphase steels such as complex phase (CP), dual phase (DP), ferritic bainite (FB), transformation induced plasticity (TRIP), and martensite (MS: martensite) and evolving TWIP steels that exhibit excellent formability at high strength levels.

Press-hardening, also known as hot-forming, is an alternative forming technique increasingly used in industrial production. The steel for press-hardening is generally a cost-effective manganese-boron heat treated steel (22MnB5). In press-hardening, the steel sheet is heated above the austenitizing temperature and subsequently formed and quenched in a single step using a cooled forming die. In this way, spring-back effects during molding can be avoided and parts with very high strength can be produced.

Both cold forming and press-hardening steels have some drawbacks. In the case of cold forming, the use of high-strength steel reduces formability, making the production of complex parts difficult, if not impossible. As the strength of the steel sheet increases, forming becomes more difficult. Also, the springback effect increases with higher strength, limiting possible tolerances. Springback and forming force are not a problem for press-hardened steel, but press-hardening has disadvantages such as complex tools and the need to descale the hot formed part surface.

In addition, cold forming and press-hardening steels have common disadvantages with respect to weldability. A typical welding process leads to hardening and annealing effects, which are caused by different time-temperature gradients in the weld zone. High-strength cold formable steel has a high content of reinforcing elements such as C, Mn, and Cr, and shows a significant increase in local hardness at the weld joint. Hot-formed (press-hardened) parts are affected by the heat affected zone and show a local decrease in hardness. The resulting hardness gradients are undesirable for dynamically loaded parts due to the so-called notching effect, which leads to relatively premature failure.

In order to solve the above problems, another solution is to perform shape forming such as stamping when the steel is soft and formability is good in the supplied hot rolling state, and high strength is expressed by heat treatment after forming. Air-hardened steels have been developed for this purpose. This type of steel is also referred to as post-forming heat treatable steel (PFHS).

US20050006011 in weight % C: 0.09 to 0.13, Si: 0.10 to 0.50, Mn: 1.10 to 1.80, P: 0.02 to maximum, S: 0.02 to maximum, Cr: 1.00 to 2.00, Mo: 0.20 to 0.60, Al: 0.02 to 0.06, V: 0.10 to 0.25, the remainder discloses steel containing iron and incidental impurities. The difference between this prior art and the steel of the present invention is that the expensive element Mo is required in the steel to reach the required properties.

US20090173412A1 in weight % C: 0.07 to 0.15, Si: 0.15 to 0.30, Mn: 1.60 to 2.10, P: 0.02 to max, S: 0.01 max, N: 0.0030 to 0.0150, Cr: 0.50 to 1.00, Mo: 0.30 to 0.60, Ti: 0.0010 to 0.050, Al: 0.05 at most, V: 0.12 to 0.20, B: 0.0015 to 0.0040, the remainder discloses iron containing steel containing ancillary elements of steel. Again, Mo is an essential element to meet the required properties.

EP0576107 in weight % C: 0.15 to 0.30, Si: 0.50 to 0.80, Mn: 2.05 to 3.35, P: 0.03 max, S: 0.03 max, Cr: 0.50 to 1.00, Mo: 0.60 max, Al: 0.05 max, Ti: 0.01 to 0.05, B: 0.0015 to 0.0030, N: 0.002 to 0.015, the remainder discloses a steel consisting of iron containing ancillary elements of steel, provided that the following relationship is satisfied: Ti(%): N(%) ≥ 3.4% Mn(%) + Cr(%) + Mo(%) + Si(%) ≥ 3.3%. The steel has a disadvantage in that the weldability is limited as a result of the relatively high content of C and Mn.

US8404061 in weight % C < 0.20, Al < 0.08, Si < 1.00, Mn: 1.20 to 2.50, P < 0.020, S < 0.015, N < 0.0150, Cr: 0.03 to 1.5, Mo: 0.10 to 0.80, Ti: 0.01 to 0.050, V: 0.03 to 0.20, B: 0.0015 to 0.0060, the remainder discloses a steel containing iron containing common elements present in steel. A part is produced by heating a hot-rolled or cold-rolled sheet or steel pipe section to a temperature of θ _blank = 800° C. to 1050° C. and then forming the steel sheet or steel pipe into a part in a forming tool. After removal from the tool, the part is cooled in air if the part still maintains a temperature of θ _removal = greater than 200°C and less than 800°C. In this application, when the removal temperature is at its lower limit, the part is actually cooled in the forming tool.

It is an object of the present invention to provide a method and a steel composition suitable for air-hardening after reheating followed by winding, cooling, cold rolling and/or cold forming.

Another object is a method of manufacturing an air-hardened steel article having a tensile strength greater than 780 MPa after air cooling and/or tempering for a lightweight design and construction to ensure good automotive crash properties. is to provide

Another object is to provide a method for the production of air-hardened steel articles suitable for the construction of welded parts subjected to high static and dynamic stress for load-bearing and safety-related parts in the automotive industry.

Embodiments of the invention are characterized by the features of the claims.

In order to maintain the geometry of the article during and after the heat treatment process, the steel according to the invention is alloyed with good hardenability, the microstructure of which is martensitic and bainitic after cooling from the austenitizing temperature in an annealing medium such as air. is composed of The steel is designed primarily to gain air-hardenability and avoid Mo and Ni by a fine-tuned balance of Mn, Cr, Ti, V and B. This steel also has good resistance to tempering. The steel has excellent welding properties both in the soft state and in the air-hardened state. Thus, welding between the soft state and the soft state, not only between the air-hardened state and the air-hardened state, but also between the soft state and the air-hardened state is completely possible. The steel also responds well to coatings using standard coating methods before and after heat treatment after forming. The parts are hardened and tempered by heat treatment (austenitization) in a furnace in a protective gas atmosphere followed by natural cooling in air or protective gas.

In summary, the advantages of articles made according to the present invention are:

- low carbon content, good weldability;

- high toughness at low temperatures due to low carbon content;

- increased strength in the heat-affected area due to resistance to air-hardening and tempering;

- improved welding fatigue properties due to air-hardening;

- No decrease in hardness at the weld seam due to self-quenching and tempering effect.

- Excellent ductility and impact resistance for chassis components;

- stable against heat treatment up to 600°C (eg tempering, batch galvanizing, etc.);

- Available in cold rolled and hot rolled strips, Electric Resistance Welded (ERW) and seamless tubing;

- Expensive elements such as Mo and Ni are not required.

An important feature of the present invention is that no quenching treatment or any specific cooling control technique is required in the manufacture of tough high strength steel articles. There are advantages such as reduction of defects such as shape change due to quenching, rb cracking and decarbonization, cost reduction of special production facilities, and time and energy cost reduction.

As the name suggests, air-hardening can be carried out by cooling in air, which after air cooling gives a steel with a microstructure containing mainly bainite and martensite but not ferrite and pearlite. The critical cooling rate, defined as the minimum cooling rate that ensures the absence of pro-eutectoid ferrite and/or perlite, depends primarily on the chemical composition of the steel. The steel composition should be designed such that the critical cooling rate of the steel is less than the cooling rate of the product in air.

The rate of cooling in air also depends on the dimensions of the product and the surrounding atmosphere (eg, airflow). The relationship between the air cooling rate and the effective thickness of the steel strip (maximum cross-sectional thickness in a plane perpendicular to the length of the steel strip) in the temperature range between 500 °C and 800 °C in a static atmosphere is given by: "C. Liu at al ., in Proceedings of Thermec 2000 International Conference on Processing and Manufacturing of Advanced Materials, edited by T. Chandra, Elsevier Science Ltd; 2001": log V = -1.072 log h + 1.625, where V is the air cooling rate in °C/s. and h is the effective thickness (mm) of the steel strip.

According to the calculations from this equation, the air cooling rates are about 42.2, 13.0 and 6.2 °C/s for products with effective thicknesses of 1, 3 and 6 mm, respectively. In the present invention, the steel composition is designed to have a minimum cooling rate that does not guarantee the formation of proeutectoid ferrite of 6° C./s, which corresponds to a part with an effective thickness of 6 mm. For parts with an effective thickness greater than 6 mm, enhanced cooling must be used to reach the required cooling rate.

The steel used in the present invention is free of expensive alloying elements such as Mo and W, and only contains carefully balanced amounts and proportions of the essentially low cost elements silicon, manganese, boron, titanium and vanadium, in addition to limited amounts of carbon. It provides a mixture of bainitic and martensitic microstructures that exhibit an excellent combination of strength and toughness after cooling in air at temperatures ranging from 850°C to 1100°C.

Mn, Cr and B are used to ensure hardenability to obtain bainite and/or martensite during air cooling. V and Ti are used to create nanoscale precipitation during tempering to increase the tempering resistance. In other embodiments, air-cured directly from hot processing conditions or air-cured after post cold forming heat treatment.

The composition of the steel used in the present invention in weight % (wt%) is as follows:

- C: 0.025 to 0.150;

- Mn: 1.30 to 3.00;

- Cr: 0.05 to 1.50;

- Si: 0.02 to 1.50;

- Al: 0.01 to 0.50;

- B: 0.0003 to 0.0050;

- V: 0.050 to 0.350;

- Ti: less than 0.050;

- N: less than 0.0080;

- P: less than 0.030;

- S: less than 0.010;

- One or more of the following optional elements:

- Nb: less than 0.100;

- Cu: less than 0.50;

- Ni: less than 0.50;

- Ca and/or REM: up to 0.0030;

- Mn + Cr + Cu + Ni ≥ 2.00;

- The remainder is Fe and unavoidable impurities.

According to the present invention, post-molding heat treatment is used to produce air-cured articles after cold rolling and/or cold forming. Steels for this purpose include:

- V: 0.050 to 0.350;

- Ti: less than 0.050

In this method, the austenitizing temperature in the post-forming heat treatment is preferably in the range of 850°C to 1050°C, more preferably 900°C to 1000°C.

Preferably the articles made according to the invention have been subjected to a tempering treatment at a temperature between 200° C. and 650° C. after air-curing. This tempering treatment can further increase the impact toughness of the steel.

The function of each element is now described.

C: 0.025 to 0.150%.

C is an element that increases the strength of steel, promotes the formation of bainite, and contributes to precipitation strengthening by combining with Ti or V to form titanium carbide or vanadium carbide. In order to obtain this effect, the C content must be 0.025% or more. On the other hand, when the content exceeds 0.150%, polygonal ferrite is easily formed and the strength is lowered. Accordingly, the C content is limited in the range from 0.025 to 0.150%, preferably from 0.030 to 0.120%.

Mn: 1.30 to 3.00%.

Mn is a matrix reinforcing agent of steel and also contributes greatly to hardenability. A minimum amount of 1.30% Mn is required to achieve the required hardenability and high strength. However, this can lead to severe center line segregation in continuous cast steel, which is detrimental to the formability of the steel if too high, so 3.00% Mn is the upper limit. Preferably, Mn is in the range of 1.50 to 2.50%.

Cr: 0.05 to 1.50%;

Cr increases the strength of steel through improvement of hardenability and promotes the formation of bainite phase to improve the desired microstructure. In order to obtain such an effect, it is preferable that the Cr content is 0.05% or more. However, if the content exceeds 1.50%, the alloy cost increases unnecessarily, and the toughness and HAZ of the steel deteriorate. Accordingly, the Cr content is limited in the range from 0.05 to 0.15%, preferably from 0.20 to 1.2%. In order to avoid the formation of polygonal ferrite and pearlite during air cooling in the invented steel, the sum of the contents of Mn, Cr, Ni and Cu must satisfy the following conditions: Mn + Cr + Ni + Cu ≥ 2.00%. In certain embodiments, Cr may be omitted from the composition if the Mn content is sufficiently high.

B: 0.0003 to 0.0050%.

B is an effective element for suppressing the formation of ferrite to increase the hardenability of steel. If the B content is less than 0.0003%, it may be difficult to obtain a desired hardenability of the steel. However, if the B content is too high than 0.0050%, coarse boron carbide may be formed at grain boundaries, which may adversely affect toughness. Accordingly, the concentration of B in the composition may range from about 0.0003 to 0.0050%, preferably from 0.0010 to 0.0030%.

Si: 0.02 to 1.50%.

Si is an element that dissolves in steel and contributes to increasing the strength of steel. Si also acts as a deoxidizer. In order to obtain this effect, the Si content must be 0.02% or more. On the other hand, if the Si content is too high, the surface quality and coatability may deteriorate. Accordingly, the Si content is limited to 0.02 to 1.50%, preferably 0.10 to 1.20%.

Ti: less than 0.050%.

Ti and V play a very important role in the invented steel. Ti combines with N, S and C to form nitrides, carbon sulfides and/or carbides depending on the specific chemical composition of the steel. Ti is useful for improving the efficiency of B in steel by fixing nitrogen impurities to TiN and suppressing the formation of boron nitride. Ti also contributes to the reduction of the size of the austenite grains, which provides the microstructure of the finally obtainable steel sheet. The inventors observed that a large proportion of Ti was dissolved in the austenite matrix during reheating and hot rolling, which could greatly increase the hardenability of the inventive steel. During air cooling after hot rolling, Ti-C clusters or fine TiC precipitates are formed, which can contribute to strength improvement. Ti-C clusters have a configuration in which TiC traps C, although precipitates of TiC are not easily formed. Since Ti captures C, precipitation of cementite, which normally occurs at a temperature in the range of 440°C to 560°C, can be suppressed. The presence of dissolved Ti can inhibit crack propagation through the formation of TiC or Ti and C clusters, which are easily induced by stress or strain in the strain-concentrated region of the crack tip generated during elongated flange formation. However, if Ti is added in excess, coarse TiC precipitates may be formed during winding after hot rolling and during reheating for post-forming heat treatment 10 . When Ti is present in the form of coarse carbides, it greatly reduces the hardenability of the steel and does not contribute to its strength. Therefore, the Ti content is limited in the range of 0.005 to 0.050%.

V: 0.050 to 0.350%.

V is added to provide precipitation strengthening by forming fine VC grains in the steel upon tempering and during air cooling after austenitization. Unlike TiC, VC has a much lower melting point and can be completely dissolved in austenite during heat treatment after forming. When dissolved in austenite, V has a strong advantageous effect on hardenability. Therefore, V is effective in maintaining the strength of high-strength steel. V is particularly useful in the steels of the present invention which require very high strength and toughness as cold formed products such as automotive parts (eg axles and connecting rods), which, after air cooling, provide a secondary hardening effect of vanadium. It may be subjected to a tempering treatment that causes fine precipitation of carbides and/or nitrides. In order to obtain this effect, the V content needs to be 0.050% or more. On the other hand, adding V exceeding 0.35% saturates the above effect and increases the cost. Thus, the concentration of V may be from 0.05 to about 0.350%, preferably up to about 0.300%.

Nb: less than 0.100%.

Nb is an alloying additive that can be used to refine the austenite grain size of the composition. Nb can further enhance the effect of boron on hardenability and provide precipitation hardening. This provides additional strengthening upon tempering through the formation of Nb(C,N) precipitates. However, too much niobium can be detrimental to weldability and HAZ toughness. In certain embodiments, Nb may be omitted from the composition. In other embodiments, the concentration of Nb may range up to about 0.100%.

Al: 0.01 to 0.50%

Al is an element that acts as a deoxidizer and is effective in improving the cleanliness of steel. Al also creates a solution hardening effect. In order to obtain this effect, the Al content should be 0.01% or more. On the other hand, when Al is added excessively in excess of 0.50%, the amount of oxide inclusions is remarkably increased, thereby causing defects in the steel sheet. Accordingly, the Al content is limited to 0.01 to 0.50%, preferably 0.02 to 0.10%.

Cu: less than 0.50%.

Cu is not required in the invented steel, but may be present. In some embodiments, depending on the manufacturing process, the presence of Cu may be unavoidable. Cu increases the hardenability of the invented steel. Cu can also provide precipitation strengthening when tempering steel by forming fine copper particles in the steel matrix. If there is too much copper, the maximum Cu content can be about 0.50% or less, since the steel is prone to surface cracking during hot rolling.

Ni: less than 0.50%.

Ni is not required in the invented steel, but may be present. Ni is optionally added to counteract the deleterious effect of copper on surface cracking during hot rolling when Cu is present. Nickel is generally a beneficial element. For cost reasons the maximum amount is limited to 0.50% when added. In a preferred embodiment, the Ni content is one-quarter to one-half of the Cu content, preferably about one-third of the Cu content.

Ca: up to 0.0030%; REM: 0.0030% max.

Calcium and/or rare earth metal (REM), which may be optionally added, is an element having the effect of controlling the shape of the sulfide to a spherical shape and improving the formability. In order to obtain such an effect, it is preferable to add 0.0003% or more of Ca or REM. However, when each of these elements is added in an amount exceeding 0.0030%, the amount of inclusions and the like increases, and the probability of frequent occurrence of surface defects and internal defects increases. Therefore, when adding these elements, the Ca content or the REM content is preferably limited to the range of 0.0003 to 0.0030%. Here, examples of the rare earth element used in the present invention include Sc, Y, and a lanthanide element.

S: less than 0.010%; P: less than 0.030%; N: less than 0.0080%

S, P, N, etc. are impurities, and it is desirable to keep the concentration as low as possible. In certain embodiments, the concentrations of each of S, P, and N can be independently provided as follows: S is less than about 0.010%, P is less than about 0.030%, N is less than about 0.0080%.

Referring now to FIG. 1 , a discussion will be made of how an article may be made from the steel of the present invention.

In the above method for producing the article according to the invention, the steel is smelted in a conventional way, for example in an oxygen blown converter or an electric furnace, and cast into a cast product such as a slab, ingot or rod by a continuous casting machine or ingot pouring method. These processes are not particularly limited and may be performed according to a conventional method.

The steel of the present invention is suitable for air-hardening using post-forming heat treatment. A method of manufacturing the article is schematically illustrated in FIG. 1 . In this case, the cast products described above are processed using hot rolling after reheating at temperatures ranging from 1100°C to 1300°C. In general, hot rolling of slabs is carried out in 5 to 7 stands to final dimensions suitable for further cold rolling. The finish rolling is generally carried out in fully austenitic conditions above 800°C, advantageously above 850°C.

The hot-rolled strip obtained by hot rolling in this way is wound at a winding temperature of, for example, 550° C. or higher, preferably 600° C. or higher, thereby preventing the formation of very fine precipitates such as VC and TiC and in essentially soft conditions. Allow winding.

The hot rolled strip is then pickled and optionally cold rolled to obtain a cold rolled steel strip of suitable gauge suitable for forming the desired article.

The hot rolled strip is formed into an article in a cold state or is formed into a desired article after selective cold rolling. Thereafter, the cold rolled and/or cold formed product is heat treated after forming, for example, reheated to the austenite range at a temperature of 850° C. to 1050° C., held for 30 to 900 seconds, and then air cooled to room temperature. If the austenitization temperature is too high, the austenite grain size becomes too coarse and some ferrite is formed during air cooling, which may reduce the strength of the steel. If the austenitization temperature is too low, the coiled TiC or VC precipitates are not sufficiently dissolved in the austenite, which may reduce the hardenability of the steel. Preferably, the austenitization temperature is in the range from 900°C to 1000°C, and the holding time is from 300 to 600 seconds.

Tempering 6 may be additionally performed during air cooling. The choice of specific time and temperature conditions for tempering the steel of the present invention will depend on the composition and dimensions of the article being tempered and the desired properties effected by the tempering operation. Tempering of the steel product of the present invention is optional for additional toughness. This is because not all of these articles require this heat treatment and corresponding toughness improvement over untempered steel. Tempering may be performed by heating to a temperature in the range of about 400 to 700° C., holding at the tempering temperature for a selected time of about 5 to 60 minutes, and air cooling from the tempering temperature to about room temperature.

Without any tempering, the final microstructure of the steel composition of the present invention after air cooling may comprise a mixture of bainite and martensite. The thinner the effective thickness, the higher the martensite content. Granular bainite may be the dominant microstructure of the steel. In certain embodiments, the microstructure may comprise up to about 10% polygonal ferrite, preferably up to 5% polygonal ferrite, and most preferably the microstructure comprises no more than 2% polygonal ferrite.

The steel article of the invention may also be prepared by coating processes known to the person skilled in the art, for example hot dip galvanizing or galvannealing or Al-Si based at a temperature of 420° C. to 650° C. Application of the coating may further be carried out. The coating may be applied separately or incorporated into the tempering process. The coating process is a step in terms of heat treatment comparable to tempering without negatively affecting the strength of the air-tempered or air-hardened steel according to the invention.

The air-hardened steel according to the invention can be used as a starting material for chassis subframes, industrial machines and construction machines.

The invention will now be further illustrated by the following non-limiting drawings and embodiments. Descriptions and features disclosed in or in connection with the drawings may be separately extracted and combined with other features unless expressly excluded herein.

1 shows a process route for manufacturing an article formed in cold conditions after cold forming and before heat treatment is applied. The tempering process indicated by the dotted line is optional.

Embodiments were performed using laboratory cast ingots.

As shown in Table 1, steels A01 to A11 having a composition according to the present invention were cast into 25 kg ingots with dimensions of 200 mm x 110 mm x 110 mm using a vacuum induction furnace. Then, a 3.5 mm thick hot-rolled strip was fabricated using the following process schedule to mimic the manufacturing process.

- reheat the ingot at 1225° C. for 2 hours;

- to carry out rough rolling of the ingot from 140mm to 35mm;

- reheating the rough-rolled ingot at 1200° C. for 30 minutes;

- hot rolling from 35mm to 3mm (35 - 27 - 19 - 11 - 7 - 5 - 3.5mm) at a finish rolling temperature of about 900±40°C;

- winding the steel strip at 650°C;

- pickling the hot-rolled steel sheet in HCl at 85° C. to remove the oxide layer;

- Carry out cold rolling to 1.5mm;

- heated to 720°C at a heating rate of 15°C/sec and then to 920°C at a heating rate of 5°C/sec and held at 920°C for 3 minutes in a furnace with a protective atmosphere;

- cooling to room temperature in air;

- tempering the samples at 450°C, 550°C or 650°C for 15 minutes respectively.

Tensile test: A JIS-5 test piece (gauge length = 50 mm, width = 25 mm) was processed from the obtained hot-rolled steel sheet so that the tensile direction was parallel to the rolling direction. Tensile properties (yield strength YS (MPa), ultimate tensile strength UTS (MPa), uniform elongation Ag (%) and total elongation A50 (%) by performing room temperature tensile testing on a Schenk TREBEL testing machine according to the NEN-EN10002-1:2001 standard )) was determined. For each condition, the tensile test was performed three times, and the average value of the mechanical properties was reported.

Table 2 shows the tensile properties of the inventive steel after hot rolling to a thickness of 3.5 mm and air cooling to room temperature. It turns out that the tensile strength of all the steels except Embodiment A09 is higher than 780 MPa. Since steel A09 does not contain boron, its tensile strength is low and therefore a large amount of ferrite is obtained during air cooling. After tempering at 600° C. for 15 minutes, the strength generally increases due to the secondary hardening effect produced by TiC or VC.

Numbers in the figures indicate the following process steps:
- 1: reheat
- 2: hot rolled
- 5: Cold rolling and/or cold forming
- 6: Tempering or coating (optional)
- 7: coiling
- 8: heat treatment
- 9: Air-curing

Claims (5)

A method of making an air-hardened steel article, comprising:
wherein the steel is hot rolled, wound, cold rolled and/or cold formed and air-hardened, wherein the steel article has a microstructure mainly of a mixture of bainite and martensite, and a tensile strength greater than 780 MPa, wherein the steel comprises A method for producing an air-hardened steel having the following composition in weight percent:
- C: 0.025 to 0.150;
- Mn: 1.30 to 3.00;
- Cr: 0.05 to 1.50;
- Si: 0.02 to 1.50;
- Al: 0.01 to 0.50;
- B: 0.0003 to 0.0050;
- V: 0.050 to 0.350;
- Ti: less than 0.050;
- N: less than 0.0080;
- P: less than 0.030;
- S: less than 0.010;
- One or more of the following optional elements:
- Nb: less than 0.100;
- Cu: less than 0.50;
- Ni: less than 0.50;
- Ca and/or REM: up to 0.0030;
- Mn + Cr + Cu + Ni ≥ 2.00;
- The remainder is Fe and unavoidable impurities. According to claim 1,
The austenitizing temperature to which the article is reheated after cold forming is in the range from 850°C to 1050°C, preferably from 900°C to 1000°C. An article produced by the method according to any one of claims 1 to 2 by tempering treatment at a temperature of 200°C to 650°C. An article manufactured by the method according to any one of claims 1 to 3, comprising:
The article according to claim 1, wherein the steel is further subjected to a coating process such as hot dip coating by a zinc-based coating such as galvanizing or galvannealing or coating by an Al-Si-based coating. 5. The method of claim 3 or 4,
An article having an effective thickness of up to 6 mm and having the shape of a strip, an elongated rod or tube, or any other shape.

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