METHOD OF MANUFACTURING A STEEL ARTICLE AND ARTICLE

The invention relates to a method of air-hardening a steel, e.g. for manufacturing an article such as a formed product or part.

High strength car components are often manufactured out of hot rolled or cold rolled steel strip material by shape forming such as deep drawing, hydro-forming or other forming methods.

For cold shape forming various new concepts of high strength steels were developed and enhanced in the last years. Multiphase steels such as complex phase (CP), dual phase (DP), ferritic bainite (FB), transformation induced plasticity (TRIP), and martensite (MS) can be named as well as the evolving TWIP steels, which show an extraordinary formability at high strength levels.

Press-hardening, also known as hot-forming, is an alternative forming technology, which is increasingly used in industrial production. Steels for press-hardening are usually cost-effective manganese-boron heat treatable steels (22MnB5). In press hardening, sheets are heated above the austenitising temperature and subsequently formed and quenched in a single step by using a cooled forming die. By this, spring- back effects during forming can be avoided and parts of very high strength can be produced.

Both the steels for cold shape forming and the steels for press-hardening show some draw-backs. For cold shape forming the use of higher strength steels leads to lowered formability which complicates if not precludes the production of complex parts. As the strength of the steel sheets is increased, shape forming becomes more difficult. Additionally, the spring-back effect increases with higher strength, which limits possible tolerances. Spring-back and forming-forces are no problem for press-hardening steels, but press-hardening brings disadvantages such as the requirement of complex tools and the removal of scale from the surfaces of the hot-formed parts.

Further, the steels for cold shape forming and for press-hardening have a common disadvantage regarding their weldability. Usual welding processes lead to hardening and annealing effects, which are caused by different time-temperature gradients in the welding area. The high-strength cold shape formable steels have higher amounts of strengthening elements, like C, Mn, and Cr, and show a substantial local hardness increase in the welding joint. Hot-formed (press-hardened) parts are affected by the heat affected zones and show a local decrease in hardness. The resulting hardness gradients are undesired for dynamically loaded parts because of the so called notching effect which leads to a comparatively early failure. To solve the above problems, another solution is that shape forming such as stamping is carried out when the steel is soft and better formable in the as supplied hot rolled state and high strength is developed by a post forming heat treatment. Air hardening steels were developed to fit this purpose. This type of steel is also designated as PFHS (post-forming heat treatable steel).

US20050006011 discloses a steel that contains (in wt%) 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 - 0.25, the balance iron and incidental impurities. The difference between this prior art and the steel of the present invention is that the steel needs the expensive element Mo to reach the required properties.

US20090173412A1 discloses a steel that contains (in wt%) C 0.07 - 0.15, Si 0.15 - 0.30, Mn 1.60 - 2.10, P £ 0.02, S £ 0.01, N 0.0030 - 0.0150, Cr 0.50 - 1.00, Mo 0.30 - 0.60, Ti 0.0010 - 0.050, Al £ 0.05, V 0.12 - 0.20, B 0.0015 - 0.0040, remainder iron including incidental steel accompanying elements. Again, Mo is a compulsory element to meet the demanded properties.

EP0576107 discloses a steel that consists of (in wt%) C 0.15 - 0.30, Si 0.50 - 0.80, Mn 2.05 - 3.35, P at most 0.03, S at most 0.03, Cr 0.50 - 1.00, Mo at most 0.60, Al at most 0.05, Ti 0.01 - 0.05, B 0.0015 - 0.0030, N 0.002 - 0.015 with the proviso that the following relationships are met: Ti (%): N (%) ³ 3.4% Mn (%) + Cr (%) + Mo (%) + Si (%) ³ 3.3%, remainder iron including incidental steel accompanying elements. The steel has a drawback of the limited welding capability as a result of the relativity high contents of C and Mn.

US8404061 discloses a steel that contains (in wt%) C < 0.20, Al < 0.08, Si < 1.00, Mn 1.20 - 2.50, P < 0.020, S < 0.015, N < 0.0150, Cr 0.03 - 1.5, Mo 0.10 - 0.80, Ti 0.01 - 0.050, V 0.03 - 0.20, B 0.0015 - 0.0060, with the remainder being iron including the usual elements present in steel. A component is produced by heating a hot or cold rolled steel sheet or steel tube section to a temperature of © _blank = 800°C to 1050°C and then forming the sheet or tube into a component in a forming tool. After removal from the tool, the component is cooled down in air while the component still has a temperature above © _rem ovai = 200°C and below 800°C. In this application, the component is actually cooled in the forming tools when the removal temperature is in the lower end.

US 20200071789A1 discloses a steel that contains (in wt%) C 0.005 - 0.08, Al 0.005

- 0.10, Si 0 - 0.6, Mn 1.30 - 2.30, Cr 0 - 0.75, Ti 0.03 - 0.20, V 0 - 0.30, B 0.0002 - 0.0065, with the remainder being iron including the usual elements present in steel. The steel has been processed by isothermal annealing to obtain cementite free bainite microstructure. JP63111159 A2 discloses a steel that contains (in wt%) 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 - 0.0030, with the remainder being iron including the usual elements present in steel. The steel does not make best use of the advantages of Ti for hot rolled products.

JP5239589 A2 discloses a steel that contains (in wt%) 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, with the remainder being iron including the usual elements present in steel. The steel does not contain Al.

An objective of the present invention is to provide a method and a steel composition which are suitable for air-hardening directly after hot working, i.e. after hot rolling, optionally followed by hot shape forming.

Another objective is to provide a method of manufacturing an air-hardened steel article with a tensile strength of higher than 780 MPa after air cooling and/or tempering, for lightweight design and construction to ensure outstanding automotive crash properties.

Still a further objective is to provide a method of manufacturing an air-hardened steel article suitable for the construction of welded components subjected to high static and dynamic stresses for load bearing and safety-relevant components in the automotive industry.

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

To maintain the geometry of an article during and after the heat treatment process, the steels according to the invention are alloyed to possess excellent hardenability so that the microstructure is comprised of martensite and bainite after cooling from austenitising temperature in slow cooling media such as air. The steel in question is designed to get its air-hardening capabilities mainly by a fine-tuned balance of Mn, Cr, Ti, V and B and to avoid Mo and Ni. The steels also have good resistance to tempering. The steel has good welding properties in both the soft and air-hardened state. It is thus entirely feasible not only to weld soft to soft and air-hardened to air-hardened but also soft to air-hardened. The steel also responds well to coating using standard coating methods before and after post-forming heat treatment. The part can be heat treated in a furnace in protective gas atmosphere (austenitised) and then hardened and tempered by natural cooling in air or in a protective gas.

The advantages of articles manufactured according to the invention in summary are:

• Excellent weldability due to the low carbon content;

• High toughness at low temperatures due to low carbon content;

• Increased strength in the heat affected zone due to air-hardening and resistance to tempering;

• Improved weld fatigue properties due to air-hardening; • No hardness decrease over weld seams due to self-quenching and tempering- effects;

• Good ductility and crash resistance for chassis components;

• Stable regarding heat treatments up to 600°C (e.g.: tempering, batch galvanising, etc.);

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

• No expensive elements, like Mo and Ni, are needed.

An important feature of this invention is freedom from the need for quenching treatment or any particular cooling control technique in the manufacture of tough, high strength steel articles. Advantages include reduction in defects such as shape changing, cracking and decarbonization due to quenching, avoidance of the costs of special production equipment, and savings in time and energy costs.

As the name suggests, air-hardening can be performed by cooling in air, which leads to the steel having a microstructure comprising mainly bainite and martensite but no ferrite and pearlite after air cooling. The critical cooling rate, which is defined as the minimum cooling rate guaranteeing the absence of pro-eutectoid ferrite and/or pearlite, depends mainly on the chemical composition of the steel. The steel composition should be designed to ensure the critical cooling rate of the steel is smaller than the cooling rate of a product in air.

The cooling rate in air depends also on the dimension of a product and the surrounding atmosphere which may be e.g. flowing air. The relationship between the air cooling rate and the effective thickness of a steel strip (a maximum cross-section thickness in a plane normal to the length of the strip) in a temperature range between 500°C and 800°C in a static atmosphere is given by the article “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”: logy = -1.072 log/i + 1.625, wherein V is air cooling rate in °C/s, h is the effective thickness of the steel strip in mm.

According to calculations from this equation, the air cooling rate is about 42.2, 13.0 and 6.2 °C/s for a product with an effective thickness of 1 , 3 and 6 mm, respectively. In this invention, the steel composition is designed to have a minimum cooling rate guaranteeing no formation of pro-eutectoid ferrite of 6 °C/s, that corresponds to a part with an effective thickness of 6 mm. For parts with an effective thickness higher than 6 mm, intensified cooling must be used to reach the required cooling rate.

The steels used in this invention are free of high cost alloying elements such as Mo and W, and, in addition to limited amounts of carbon, consist essentially only of the low cost elements silicon, manganese, boron, titanium and vanadium in amounts and proportions carefully balanced so as to provide a mixture of bainitic and martensitic microstructure exhibiting a good combination of strength and toughness after cooling in air from a temperature in the range of 850°C to 1100°C.

Mn, Cr and B are used to ensure hardening ability to obtain bainite and/or martensite during air cooling. V and Ti are used to produce nano-scale precipitation during tempering to increase tempering resistance. According to the invention the article is directly air- hardened from the hot working conditions.

The composition of steels used in this invention is, in weight percent (wt %),

- C 0.025 - 0.150;

- Mn 1.30 - 3.00;

- Cr 0.05 - 1.50;

- Si 0.02 - 1.50;

- Al 0.01 - 0.50;

- B 0.0003 - 0.0050;

- V less than 0.350;

- Ti 0.030 - 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 up to 0.0030;

Mn + Cr + Cu + Ni ³ 2.00;

- rest Fe and unavoidable impurities.

In this embodiment, the steel is used to produce articles directly air-hardened from hot working, i.e. directly after hot rolling or subsequent optional hot shape forming.

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

Preferably an article made according to the invention has undergone a tempering treatment at a temperature from 200°C to 650°C after air-hardening. This tempering treatment can further increase the impact toughness of the steel.

The function of each element is now described: C: 0.025 - 0.150 %. C is an element that increases the strength of the 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 these effects, the C content needs to be not less than 0.025 %. On the other hand, if the content exceeds 0.150 %, polygonal ferrite is easy to form, which leads to a lower strength. Thus, the C content is limited to be in the range of 0.025 to 0.150 %, preferably 0.030 to 0.120 %.

Mn: 1.30 - 3.00 %.

Mn is a matrix strengthener in steels and it also contributes strongly to the hardenability. A minimum amount of 1.30 % Mn is needed to achieve the necessary hardenability and high strength. However, it can cause severe centre line segregation in continuously cast steels, which is harmful to formability of steels when it is too high, so an upper limit of 3.00 % Mn is imposed. Preferably, Mn is the range of 1.50 to 2.50 %.

Cr: 0.05 - 1.50 %,

Cr increases the strength of steel through the improvement of hardenability and facilitates the formation of a bainite phase thereby improving the desired microstructure. In order to obtain these effects, the Cr content is preferably not less than 0.05 %. If the content exceeds 1.50 %, however, alloying cost is unnecessarily increased and the toughness of the steel and its HAZ are lowered. Thus, the Cr content is limited to be in the range of 0.05 to 0.15 %, preferably 0.20 - 1.2 %. The sum of Mn + Cr + Ni + Cu ³ 2.00 % is needed to avoid the formation of polygonal ferrite and pearlite during air cooling in the invented steel. In certain embodiments, Cr may be omitted from the composition if the Mn content is high enough.

B: 0.0003 - 0.0050 %.

B is an element whose addition is effective in increasing the hardenability of the steel by inhibiting the formation of ferrite. 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 too high, higher than 0.0050 %, coarse boron carbides may be formed at grain boundaries, adversely affecting toughness. Accordingly, the concentration of B in the composition may range between about 0.0003 to 0.0050 %, preferably, 0.0010 to 0.0030 %.

Si: 0.02 - 1.50 %.

Si is an element that contributes to increasing the strength of steel by being dissolved in the steel. Si also works as a deoxidizer. In order to obtain these effects, the Si content needs to be not less than 0.02 %. On the other hand, if the Si content is too high, the surface quality and coatability may deteriorate. Thus, the Si content is limited to be in the range of 0.02 to 1.50 %, preferably, 0.10 to 1.20 %.

Ti: 0.030 - 0.200 %. Ti and V play a very important role in the invented steel. Ti combines with N, S and C to form nitrides, carbon sulphides and/or carbides depending on the specific chemical composition of the steel. Ti is useful in increasing the effectiveness of B in the steel, by fixing nitrogen impurity as TiN and inhibiting the formation of boron nitrides. Ti also contributes to the size reduction of austenite grains which leads to a fine microstructure of the finally obtainable steel sheet. The inventors have observed that a large portion of Ti is dissolved in the austenitic matrix during reheating and hot rolling, which can significantly increase the hardenability of the invented steel. During air cooling subsequent to hot rolling, Ti-C clusters or fine TiC precipitates can form, thus contributing to an increase in strength. The Ti-C cluster is a configuration in which Ti captures C, although precipitates of TiC are not easily formed. Since Ti captures C, precipitation of cementite which normally occurs at a temperature within a range of 440°C to 560°C can be suppressed. The presence of dissolved Ti can suppress the progression of cracks through the formation of TiC or Ti and C clusters which is easily induced by stress or deformation in a deformation-concentrated region at a tip of a crack generated during stretch flange formation. TiC, together with VC precipitates may also form during tempering treatment or during galvanizing and alloying process to produce the second hardening effect. In order to obtain these effects, the Ti content needs to be not less than 0.030 %. On the other hand, the addition of titanium in excess of 0.200 % is encountered with a saturation of the above effects, causes an increase of coarse precipitates, and results in deteriorations in friability and toughness. Thus, the Ti content is up to 0.200, preferably in a range from 0.050 to 0.150 %.

V: Less than 0.350 %.

V is added to give precipitation strengthening, by forming fine VC particles in the steel during air cooling after austenization and on tempering. Unlike TiC, VC has a much lower melting point and it can completely dissolve in the austenite during post-forming heat treatment. When dissolved in austenite, V has a strong beneficial effect on hardenability. Thus V will be effective in maintaining the strength in a high strength steel. V is especially useful in the inventive steels requiring highest strength and toughness as cold shape formed products, such as automotive parts, for example axles and connecting rods, which, after air cooling, may be subjected to a tempering treatment resulting in the precipitation of fine vanadium carbides and/or nitrides, which provides the second hardening effect. The addition of V in excess of 0.35 % is encountered with a saturation of the above effects and causes an increase in cost. Therefore, for steels used as cold shape formed products, the concentration of V may up to about 0.350 %, preferably up to about 0.300 %. V may be omitted from the composition when enough Ti is present, also to save cost. Nb: Less than 0.100 %.

Nb is an alloying addition which may be used to refine the austenitic grain size of the composition. Nb may further enhance the effects of boron on hardenability and provide precipitation hardening. It will give additional strengthening on tempering through the formation of Nb(C,N) precipitates. However, too much niobium will be harmful to the weldability and HAZ toughness. In certain embodiments, Nb may be omitted from the composition. In other embodiment, the concentration of Nb may range up to about 0.100 %.

Al: 0.01 - 0.50 %

Al is an element that works as a deoxidizer and is effective for increasing the cleanliness of steel. Al also produces solution hardening effect. In order to obtain these effects, the Al content needs to be not less than 0.01 %. On the other hand, adding Al in an excessively large amount exceeding 0.50 % causes a marked increase in the amounts of oxide inclusions and causes the generation of defects in steel sheets. Thus, the Al content is limited to be in the range of 0.01 - 0.50 %, preferably 0.02 - 0.10 %.

Cu: Less than 0.50 %.

Cu is not needed in the invented steel, but may be present. In some embodiments, depending on the manufacturing process, the presence of Cu may be unavoidable. Cu increase the hardenability of the invented steel. Cu can also provide precipitation strengthening on tempering the steel by forming fine copper particles in the steel matrix. Too much copper makes the steel more prone to surface cracking during hot rolling, so the maximum Cu content may be about 0.50 % or less.

Ni: Less than 0.50 %.

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

Ca: Up to 0.0030 %; REM: Up to 0.0030 %.

Calcium and/or a rare earth metal (REM), which may be added optionally, are elements that have effects of controlling the morphology of sulfides to a spherical shape and improving formability. In order to obtain these effects, it is recommended that Ca or a REM is added preferably at not less than 0.0003 %. However, adding these elements at contents exceeding 0.0030 % for each causes an increase in the amounts of inclusions and the like and increases the probability of the frequent occurrence of surface defects and internal defects. Thus, when these elements are added, the Ca content or the REM content is preferably limited to be in the range of 0.0003 to 0.0030 %. Here, examples of the rare earth element used in the present invention include Sc, Y, and lanthanide.

S, P, N and the like are impurities and their concentrations are preferably kept as low as possible. In certain embodiments, the concentration of each of S, P, and N may be independently provided as: S less than about 0.010 %, P less than about 0.030 %, and N less than about 0.0080 %.

Now two methods will be discussed according to which an article can be manufactured from a steel of the invention, referring also to Figures 1a and 1b.

In both methods the steel of the invention is in the usual way smelted in e.g. an oxygen-blown converter or in an electric furnace and cast into a cast product such as a slab or ingot or rod in a continuous caster or by an ingot pouring practice. These processes are not particularly restricted and may be carried out according to conventional methods.

For manufacturing an article air-hardened directly from the hot working condition, the cast product is subjected to a reheating operation into the austenitic range, up to a temperature of about 1200°C to 1300°C, preferably about 1250°C to dissolve all TiC particles. Thereafter, hot working (being hot rolling and optionally hot shape forming) is applied before air-hardening.

In the embodiment of the method 1a depicted in Figure 1a, the hot strips or rods are directly formed into articles (3), e.g. into a tubular bar or pipe in a hot working operation directly following the hot rolling (2). The hot rolled and hot shape formed products so manufactured from the steel are then cooled to room temperature after hot working by air cooling (4).

In another embodiment, the method depicted in Figure 1b, the cast product such as a slab or ingot is reheated (1), hot rolled (2) and subsequently air cooled (4) to room temperature directly 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.

For both processes depicted in Figure 1a and 1b, the finish hot working temperature, which is the start cooling temperature for air-hardening, affects the microstructure and properties after air-hardening. The finish hot working temperature should be above the Ar3 point (the temperature at which ferrite starts to form during cooling). A suitable finish 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, which reduces the strength of the steel. If it is too low, ferrite may form during hot working and TiC or VC precipitates may form in an excess amount, which reduces the ductility of the steel. Preferably, the finish hot working temperature is from 800°C to 950°C. Tempering (6) may be further performed upon air cooling. Selection of particular time and temperature conditions for tempering the steels of the invention depends upon the composition and dimensions of the article being tempered, and upon the desired properties as affected by the tempering operation. Tempering of articles of the invented steels is optional, for added toughness, since not all such articles require this heat treatment and corresponding enhancement of toughness over that of the untempered steels. Tempering may be performed by heating to temperatures ranging between about 400 to 700°C, holding at the tempering temperature for a selected duration for between about 5 to 60 minutes, and air cooling from the tempering temperature to about room temperature.

The final microstructure of the invented steel composition following air cooling, without any tempering, may comprise a mixture of bainite and martensite. The thinner the effective thickness is, the higher the amount of martensite is. Granular bainite might be the dominant microstructure of the steel. In certain embodiments, the microstructure may comprise no more than about 10 % of polygonal ferrite, preferably no more than 5 % of polygonal ferrite, most preferably, the microstructure comprises no more than 2 % of polygonal ferrite.

The invented steel articles can be further subjected to coating processes known to a person skilled in the art, for example hot dip galvanizing or galvannealing or applying an Al-Si based coating at a temperature between 420°C and 650°C. The coating may be applied separately or incorporated into the tempering treatment. The coating process has no negative impact on the strength of the air-tempered or air-hardened steel according to the invention and is a step that is from a heat treatment perspective comparable to tempering.

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

The invention will now be explained further by means of the following non-limiting figures and examples. Explanations and features disclosed in or in connection with the figures (only) can be extracted separately and combined with any other feature as far as not explicitly excluded herein.

In the schematic drawings:

Figures 1a and 1b show the process routes for manufacturing articles directly air- hardened from hot working conditions. In Figure 1a, forming is conducted at hot working temperatures followed by air-hardening. In Figure 1b, forming is conducted at room temperature after air-hardening from hot working. Any tempering process indicated with dashed line is optional; and

In the figures the numbers denote the following process steps: - 1 reheat

- 2 hot rolling

- 3 hot shape forming

- 4 air-hardening

- 5 cold rolling and/or cold shape forming

- 6 tempering or coating (optional)

Examples

Examples are performed using laboratory cast ingots.

Steels A01 to A11 having compositions according to the invention, as shown in T able 1 , were cast into 25 kg ingots of 200 mm x 110 mm x 110 mm in dimensions using vacuum induction. Then, the following process schedule was used to manufacture hot rolled strips of 3.5 mm thickness to mimic the manufacturing process.

- Reheating of the ingots at 1225°C for 2 hours;

- Rough rolling of the ingots from 140 mm to 35 mm;

- Reheating of the rough rolled ingots at 1200°C for 30 min;

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

- Cooling the steel strips in air;

- Tempering the samples at 650°C for 15 min.

Tensile tests: JIS-5 test pieces (gauge length = 50 mm; width = 25 mm) were machined from the obtained hot rolled sheets such that the tensile direction was parallel to the rolling direction. Room temperature tensile tests were performed in a Schenk TREBEL testing machine following NEN-EN10002-1:2001 standard to determine tensile properties (yield strength YS (MPa), ultimate tensile strength UTS (MPa), uniform elongation Ag (%) and total elongation A50 (%). For each condition, three tensile tests were performed and the average values of mechanical properties are reported.

Table 1. Compositions of the cast steels (in wt %)

* A10 is a comparative example. Any value underlined is outside the invention.

Table 2 shows the tensile properties of the invented steels after hot rolling to 3.5 mm in thickness and air cooling to room temperature. It can be seen that all the steels except example A09 have a tensile strength higher than 780 MPa. Steel A09 has a lower tensile strength because the steel does not contain boron and therefore a high amount of ferrite is obtained during air cooling. After tempering at 600°C for 15 min, the strength is generally increased due to the second hardening effect produced by TiC or VC.

Table 2. Tensile properties of the air cooled hot rolled strips with 3.5 mm in thickness