KR20220152532A - Steel products and their manufacturing methods - Google Patents

Abstract

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

Description

Steel products and their manufacturing methods

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 made from hot rolled or cold rolled steel strips by shape forming such as deep drawing, hydro-forming or other forming methods. strip) material.

Various new concepts of high-strength steels have been developed and improved over the past few years for cold forming. Multiphase steels such as complex phase (CP), dual phase (DP), ferritic bainite (FB), transformation induced plasticity (TRIP), and martensite (MS) 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. Press-hardening steels are generally cost-effective manganese-boron heat-treated steels (22MnB5). In press-hardening, steel sheet is formed and quenched in a single step using a forming die that is heated above the austenitizing temperature and subsequently cooled. In this way, spring-back effects during molding can be avoided and parts with very high strength can be produced.

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

In addition, steels for cold forming and press-hardening have a common disadvantage 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 steels have a high content of reinforcing elements such as C, Mn, and Cr, and show a significant increase in local hardness in welded joints. 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 has good formability in the supplied hot-rolled state, and high strength is developed by heat treatment after forming. Air-hardening steels were developed for this purpose. This type of steel is also referred to as post-forming heat treatable steel (PFHS).

US20050006011 is C: 0.09 ~ 0.13, Si: 0.10 ~ 0.50, Mn: 1.10 ~ 1.80, P: Max 0.02, S: Max 0.02, Cr: 1.00 ~ 2.00, Mo: 0.20 ~ 0.60, Al: 0.02 ~ 0.06, V: 0.10 to 0.25, the balance discloses a 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 is C: 0.07 ~ 0.15, Si: 0.15 ~ 0.30, Mn: 1.60 ~ 2.10, P: Max 0.02, S: Max 0.01, N: 0.0030 ~ 0.0150, Cr: 0.50 ~ 1.00, Mo: 0.30 ~ 0.60, Ti: 0.0010 to 0.050, Al: max. 0.05, V: 0.12 to 0.20, B: 0.0015 to 0.0040, the remainder being ancillary iron-containing steels. Again, Mo is an essential element to satisfy the required properties.

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

US8404061 is 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 rest 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 pipe into a part in a forming tool. After removal from the tool, if the part still maintains a temperature greater than θ _removal = 200°C but less than 800°C, the part is cooled in air. In this application, when the removal temperature is at the lower limit, the part is actually cooled in the forming tool.

US 20200071789A1 is C: 0.005 to 0.08, Al: 0.005 to 0.10, Si: 0 to 0.6, Mn: 1.30 to 2.30, Cr: 0 to 0.75, Ti: 0.03 to 0.20, V: 0 to 0.30, B in weight percent : 0.0002 to 0.0065, the remainder discloses a steel containing iron containing common elements present in steel. The steel was subjected to isothermal annealing to obtain a cementite-free bainitic microstructure.

JP63111159 A2 is C: 0.02 ~ 0.05, Al: 0.01 ~ 0.05, Si: 0.1 ~ 1.0, Mn: 1.00 ~ 3.50, Cr+Mn: 2.0 ~ 5.50, Ti: 0.005 ~ 0.015, V: 0.03 ~ 0.20 , B: 0.0003 to 0.0030, the rest discloses a steel containing iron including common elements present in steel. These steels do not fully exploit the advantages of Ti over hot rolled products.

JP5239589 A2 is C: 0.05 ~ 0.30, Si < 0.1, Cr < 0.2, Mn: 1.00 ~ 3.00, Ti: 0.01 ~ 0.10, V: 0.01 ~ 0.5, B: 0.0005 ~ 0.0035, the rest in steel It discloses a steel containing iron containing the common elements of The steel does not contain Al.

It is an object of the present invention to provide a method and steel composition suitable for air-hardening immediately after hot working, optionally followed by hot forming, ie after hot rolling.

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

Another object is to provide a method for producing air-hardened steel articles suitable for the construction of welded parts subjected to high static and dynamic stresses 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 steels according to the invention are alloyed to have good hardenability, the microstructure being martensite and bainite after cooling from the austenitizing temperature in an annealing medium such as air. consists of The steel is primarily designed to obtain air-hardening capability by a finely tuned balance of Mn, Cr, Ti, V and B and avoid Mo and Ni. This steel also has good resistance to tempering. The steel has excellent welding properties in both the soft state and the air-hardened state. Thus, welding between the soft state and the soft state, between the air-cured state and the air-cured state as well as between the soft state and the air-cured state is perfectly possible. The steel also responds well to coating using standard coating methods before and after post-forming heat treatment. The component is hardened and tempered by natural cooling in air or shield gas after heat treatment (austenitization) in a furnace in a shielding gas atmosphere.

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

- Excellent weldability due to low carbon content;

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

- the strength of the heat affected zone is increased due to resistance to air-hardening and tempering;

- weld fatigue properties are improved due to air-hardening;

- No reduction in hardness at welded joints due to self-quenching and tempering effects.

- Excellent ductility and crash resistance for chassis components;

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

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

- 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. It has advantages such as reduction of defects such as shape change by quenching, rb cracking, and decarbonization, cost reduction of special production facilities, and time and energy cost reduction.

As the name implies, air-hardening can be carried out by cooling in air, which after air-cooling gives a steel with a microstructure comprising mainly bainite and martensite, but no ferrite and pearlite. The critical cooling rate, defined as the minimum cooling rate that ensures the absence of pro-eutectoid ferrite and/or pearlite, 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 the plane perpendicular to the steel strip length) 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 (°C/s) , and h is the effective thickness of the steel strip (mm).

According to 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 pro-eutectoid 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 this invention is free of expensive alloying elements such as Mo and W, and is only made up of the essentially low-cost elements silicon, manganese, boron, titanium and vanadium in carefully balanced amounts and proportions, in addition to a limited amount of carbon. It is formulated to provide a mixture of bainite and martensitic microstructures that exhibits a good 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 precipitates during tempering to increase tempering resistance. According to the present invention, the article is air-cured directly from hot processing conditions.

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

- C: 0.025 ~ 0.150;

- Mn: 1.30 ~ 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: less than 0.350;

- Ti: 0.030 to 0.200;

- 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: max. 0.0030;

- Mn + Cr + Cu + Ni > 2.00;

- The rest is Fe and unavoidable impurities.

In this embodiment, the steel is used to make the air-hardened article directly from hot working, ie immediately after hot rolling or subsequent optional hot shape forming.

For this method, the finish hot working temperature is preferably in the range of 750°C to 1000°C, more preferably 800°C to 950°C.

Preferably, the article produced according to the present invention has undergone a tempering treatment at a temperature of 200° C. to 650° C. after air-curing. This tempering treatment can further increase the impact toughness of the steel.

Now, the function of each element is explained.

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 hardening by combining with Ti or V to form titanium carbide or vanadium carbide. 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. Therefore, the C content is limited to the range of 0.025 to 0.150%, preferably 0.030 to 0.120%.

Mn: 1.30 to 3.00%.

Mn is a matrix reinforcing agent for 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 continuously cast steel, which if too high is detrimental to the formability of the steel, 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 enhancement of hardenability and promotes the formation of bainite phase to improve the desired microstructure. In order to obtain these effects, it is preferable that the Cr content is 0.05% or more. However, if the content exceeds 1.50%, the alloy cost unnecessarily increases and the toughness and HAZ of the steel deteriorate. Therefore, the Cr content is limited to the range of 0.05 to 0.15%, preferably 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 condition: 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 element effective in suppressing the formation of ferrite to increase hardenability of steel. If the B content is less than 0.0003%, it may be difficult to obtain the desired hardenability of the steel. However, if the B content is too high than 0.0050%, coarse boron carbide may form at the 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, surface quality and coatability may deteriorate. Therefore, the Si content is limited to the range of 0.02 to 1.50%, preferably 0.10 to 1.20%.

Ti: 0.030 to 0.200%.

Ti and V play very important roles 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 fixing nitrogen impurities into TiN and suppressing the formation of boron nitride to increase the effectiveness of B in steel. Ti also contributes to a reduction in the size of the austenite grains, which gives the microstructure of the steel sheet finally obtained. The present inventors observed that a large portion of Ti is dissolved in the austenite matrix during reheating and hot rolling, which can greatly increase the hardenability of the present steel. During air cooling after hot rolling, Ti—C clusters or fine TiC precipitates are formed, which can contribute to strength improvement. The Ti-C cluster has a configuration in which Ti captures C, although TiC precipitates are not easily formed. Since Ti captures C, precipitation of cementite, which usually occurs at a temperature within 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 easily induced by stress or strain in the strain-concentrated region of the crack tip generated during elongation flange formation. TiC may form during the tempering process along with VC precipitates or during galvanizing and alloying processes to create a secondary hardening effect. In order to obtain this effect, the Ti content must be 0.030% or more. On the other hand, when titanium is added in excess of 0.200%, the above effects are saturated, increasing coarse precipitates, and as a result, crushability and toughness are lowered. Therefore, the Ti content is 0.200 or less, preferably in the range of 0.050 to 0.150%.

V: less than 0.350%.

V is added to provide precipitation strengthening by forming fine VC particles in the steel during air cooling after austenitization and upon tempering. Unlike TiC, VC has a much lower melting point and can completely dissolve into austenite during post-forming heat treatment. When dissolved in austenite, V has a strong beneficial 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 automobile 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 which causes fine precipitation of carbides and/or nitrides. Adding V in excess of 0.35% saturates the above effect and increases cost. Thus, in steels used as cold formed products, the concentration of V can be up to about 0.350%, preferably up to about 0.300%. When sufficient Ti is present, V can also be omitted from the composition to save cost.

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 increasing the cleanliness of steel. Al also produces a solution hardening effect. In order to obtain this effect, the Al content must be 0.01% or more. On the other hand, if Al is added in excess of 0.50%, the amount of oxide inclusions is significantly increased, resulting in defects in the steel sheet. Therefore, the Al content is limited to the range of 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 grains in the steel matrix. Since too much copper makes the steel prone to surface cracking during hot rolling, the maximum Cu content may be about 0.50% or less.

Ni: less than 0.50%.

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

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

Calcium and/or rare earth metal (REM), which may be selectively added, is an element having an effect of controlling the shape of the sulfide into a spherical shape and improving formability. In order to obtain these effects, it is preferable to add 0.0003% or more of Ca or REM. However, when each of these elements is added in a content 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 REM content is preferably limited to the range of 0.0003 to 0.0030%. Here, examples of rare earth elements used in the present invention include Sc, Y, and lanthanide elements.

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

Referring now to FIGS. 1A and 1B , two ways in which an article can be made from the steel of the present invention will be discussed.

In both methods, the steel of the present invention is smelted in a conventional manner, for example in an oxygen blown converter or electric furnace, and cast into cast products such as slabs, ingots or rods in a continuous casting machine or by ingot casting. These processes are not particularly limited and may be performed according to conventional methods.

To produce a direct air-hardened product in hot working conditions, the cast product is subjected to a reheat operation to a temperature in the austenitic range of about 1200° C. to 1300° C., preferably about 1250° C., to dissolve all TiC particles. Then, hot working (hot rolling and optionally hot forming) is applied before air-curing.

In the embodiment of the method shown in FIG. 1A , the hot steel strip or rod is directly formed into an article 3 , for example a tubular rod or pipe, in a hot working operation immediately after hot rolling 2 . The hot rolled and hot formed products thus produced from steel are then cooled to room temperature by air cooling (4) after hot working.

In another embodiment of the method shown in FIG. 1 b, the cast product, such as a slab or ingot, is reheated (1) and hot rolled (2), then air cooled (4) to room temperature immediately after hot rolling. Thereafter, cold forming operations such as cold rolling, pressing, drawing and cold forging (5) may be applied to form a more final product.

In the two processes shown in FIGS. 1A and 1B , the finish hot working temperature, which is the starting cooling temperature for air-curing, affects the microstructure and properties after air-curing. The finish hot working temperature must be higher than the Ar3 point (the temperature at which ferrite starts to form during cooling). A suitable finishing hot working temperature is in the range of 750°C to 1000°C. If this temperature is too high, some ferrite may form during air cooling, reducing the strength of the steel. If it is too low, ferrite is formed during hot working and TiC or VC precipitates are excessively formed, which can reduce the ductility of the steel. Preferably, the finish hot working temperature is 800°C to 950°C.

Tempering (6) may additionally be carried out during air cooling. The choice of specific time and temperature conditions for tempering the steels of this invention depends on the composition and dimensions of the article being tempered and the desired properties to be affected by the tempering operation. Tempering of the steel product of the present invention is optional for additional toughness. The reason is that not all of these articles require such heat treatment and a corresponding improvement in toughness compared to untempered steel. Tempering can 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 microstructures may include about 10% or less polygonal ferrite, preferably no more than 5% polygonal ferrite, and most preferably the microstructures include no more than 2% polygonal ferrite.

The steel article of the present invention may be subjected to a coating process known to those skilled in the art, for example hot dip galvanizing or galvannealing or an Al-Si based coating at a temperature of 420° C. to 650° C. The application of may be additionally performed. The coating can be applied separately or incorporated into the tempering treatment. The coating process is a step in terms of heat treatment comparable to tempering which does not adversely affect the strength of the air-tempered or air-hardened steel according to the invention.

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

The invention will now be further illustrated by the following non-limiting figures 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.

1A and 1B show a process route for producing air-cured articles directly from hot working conditions. In FIG. 1 a shaping is carried out at high processing temperatures followed by air-curing. In Figure 1b, molding is performed at room temperature after co-curing from hot working. 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 compositions 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. A 3.5 mm thick hot rolled strip was then produced using the following process schedule to mimic the manufacturing process.

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

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

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

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

- cooling the steel strip in air;

- Temper the samples at 650°C for 15 minutes.

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. A room temperature tensile test was performed on a Schenk TREBEL testing machine according to the NEN-EN10002-1:2001 standard to determine the tensile properties (yield strength YS (MPa), ultimate tensile strength UTS (MPa), uniform elongation Ag (%) and total elongation A50 (%)). )) 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 can be seen that the tensile strengths of all steels except Embodiment A09 are 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 usually increases due to the secondary hardening effect produced by TiC or VC.

The numbers in the figure represent the following process steps:
- 1: Reheat
- 2: hot rolled
- 3: hot forming
- 4: air-curing
- 5: cold rolling and/or cold forming
- 6: tempering or coating (optional)

Claims (5)

A method of making an air-hardened steel article comprising:
The steel is hot-rolled and/or hot-formed and air-hardened, and the steel article has a microstructure that is primarily a mixture of bainite and martensite, and a tensile strength higher than 780 MPa, wherein the steel has the following in weight percent: Method for producing air-hardened steel having the composition:
- C: 0.025 to 0.150;
- Mn: 1.30 ~ 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: less than 0.350;
- Ti: 0.030 to 0.200;
- 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: max. 0.0030;
- Mn + Cr + Cu + Ni >2.00;
- The rest is Fe and unavoidable impurities. According to claim 1,
A process for producing air-hardened steel, wherein the finish hot working temperature after hot rolling or after hot forming is in the range of 750°C to 1000°C, preferably 800°C to 950°C. An article produced by the method according to any one of claims 1 to 2 by subjecting to a 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,
wherein the steel is further subjected to a coating process, such as hot dip coating with a zinc-based coating such as galvanizing or galvannealing, or coating with an Al-Si-based coating. According to claim 3 or 4,
An article having an effective thickness of up to 6 mm and having the shape of a strip, elongated rod or tube, or any other shape.

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