High performance steel composition, steel product obtained from the composition and method for manufacturing the steel product

Abstract

This invention relates to high-performance steel compositions comprising, by weight percentage: C: 0.18 to 0.24; Mn: 2.25 to 2.50; Al: 2.00 to 3.00; Si: 1.00 to 1.75; Cu: 0.02 to 1.70; Ni: 0.03 to 1.50; P: not more than 0.01; S: not more than 0.03; Fe: balance. The compositions according to the invention allow for obtaining steel products exhibiting excellent yield strength, ultimate tensile strength, and elongation at break. The invention also relates to a method for obtaining said steel products starting from said compositions.

Description

Technical Field

[0001] This invention relates to steel production, and more particularly, to steel compositions exhibiting high performance in terms of structural strength and ductility.

[0002] The present invention also relates to steel products obtained from the said composition.

[0003] The present invention also relates to a method for manufacturing the steel product, including heat treatment. Background Technology

[0004] Industries such as the automotive industry are constantly seeking economical structural metal materials that offer a good combination of strength and deformation capacity. Steel has been used since the birth of the automotive industry, and today Advanced High Strength Steel (AHSS) is widely used in the automotive industry because it provides the strength-deformation capacity combination needed to provide good impact resistance.

[0005] Another highly desirable benefit that can be achieved with AHSS through alloy design is weight reduction, which plays an important role in energy conservation and environmental issues.

[0006] AHSS is a multiphase steel with a complex microstructure comprising different phases (e.g., austenite, bainite, ferrite, and martensite), the grain size, shape, and distribution of which contribute to the strength, ductility, and other properties of the specific steel.

[0007] In addition to iron, such steel contains many other elements, the selection and amount of which depend on the desired properties of the particular steel and therefore on the intended application.

[0008] In particular, manganese and carbon act as austenite stabilizers. As those skilled in the art know, the stability of austenite is an important aspect because its level determines a large number of final properties or behavioral changes during engineering use before the steel is put into engineering use. Manganese is also one of the most important alloying elements used in AHSS design because it stabilizes austenite and improves hardenability, ductility, and wear resistance.

[0009] AHSS can be produced in a combination of one, two or more conventional processing lines, such as a continuous casting line (CCL), a continuous annealing line (CAL), and a hot dip galvanizing line (HDGL).

[0010] In all these production lines, two-stage heat treatment can be easily performed, which allows for obtaining a sufficient amount of retained austenite (RA) with good stability.

[0011] In the first stage (known as critical zone annealing (IA)), the fraction of austenite and other micro-components can be tuned by controlling the heat treatment temperature and time. This stage also allows austenite stabilizers to diffuse into the austenite. The second stage (known as isothermal bainitic transformation (IBT)) improves the stabilization of the large amount of austenite produced by the first stage and also provides further enrichment of austenite stabilizers. The treatment involves a large number of variables (temperature, isothermal timeline or rate of temperature change, amount of deformation, deformation temperature), which also contributes to the extreme diversity of achievable results.

[0012] In addition to these conventional processing methods, many new hot working and thermomechanical processing strategies can be implemented to provide the maximum residual austenite content with good stability.

[0013] However, such a new processing strategy is very complex, and its implementation will require significant adaptation interventions to existing steel production plants.

[0014] Alloy design, particularly regarding the content of austenitic stabilizers, can serve as an alternative approach to complex processing strategies for developing high-performance AHSS.

[0015] However, even though austenite stabilization is an important aspect, stabilizers must be used in controlled amounts to avoid harmful effects.

[0016] By way of example, carbon is the most effective austenite stabilizer. However, its amount should be kept limited, as higher amounts of carbon in the alloy (e.g., above 0.3%) will be detrimental to the weldability of the steel.

[0017] Manganese is another effective austenite stabilizer. However, its content should also be kept limited (e.g., below 2.5%), because at higher levels, manganese segregation that occurs during dendritic solidification may not be easily eliminated by subsequent processes and may only be resolved by very high temperatures and very long homogenization heat treatments. Furthermore, banded structures resulting from manganese segregation can also degrade ductility and introduce texture during deformation.

[0018] Besides carbon and manganese, aluminum is a key component in the development of new alloys.

[0019] The addition of aluminum allows for a specific weight reduction because of its low atomic mass and the resulting lattice expansion. This is clearly crucial in manufacturing materials intended for use in the automotive industry, as weight reduction translates to lower energy consumption.

[0020] Aluminum acts as a ferrite stabilizer. The addition of aluminum also accelerates the precipitation of ordered perovskite carbides (κ-carbides), which, by optimizing their fraction, morphology, size, and distribution, can simultaneously improve strength and ductility, as recently recognized.

[0021] Numerous compositions have been proposed in the literature for next-generation AHSS candidate materials, which can be used to manufacture products with a combination of strength and ductility suitable for, for example, the automotive industry.

[0022] Examples are disclosed in the following literature: “Microstructure and mechanical properties of hot-rolled medium-Mn steel containing 3% aluminum”, C. Shao et al., Materials Science and Engineering: A, 682, January 13, 2017, pp. 45-53; “Effects of solutesegregation on tensile properties and serration behavior in ultra-high-strength high-Mn TRIP steels”, MC Jo et al., Materials Science and Engineering: A, 740-741, January 7, 2019, pp. 16-27; “Austenite stability and deformation behavior in medium Mn steel processed by cyclic quenching ARTheat treatment”, C. Liu et al., Materials 2021, 14, 7132; “Innovative processing of obtaining nanostructured bainite with high strength-high ductility combination in low-carbon-medium-Mn steel”. "Process-structure-propertyrelationship", YX Zhoua et al., Materials Science & Engineering: A, 718, March 7, 2018, pp. 267-276.

[0023] However, known compositions may only be transformed into the final product through complex and therefore expensive processing methods.

[0024] Furthermore, it contains medium or high levels of manganese, thus causing the aforementioned problems.

[0025] In addition, manganese itself is quite expensive, which also makes the cost of producing steel relatively high. Summary of the Invention

[0026] One object of the present invention is to provide an alloy for the production of a new generation of advanced high-strength steel that solves the above-mentioned problems of the prior art.

[0027] Another object of the present invention is to provide advanced high-strength steel compositions that allow for excellent performance and thus reduced costs with limited (or even minimal) amounts of alloying elements.

[0028] More specifically, another object of the present invention is to provide advanced high-strength steel compositions that allow for excellent performance with a limited amount of manganese.

[0029] Another object of the present invention is to provide an advanced high-strength steel composition that retains a large amount of well-stable retained austenite (RA) in the microstructure after two-stage heat treatment.

[0030] According to a first aspect, the present invention provides an AHSS steel composition comprising, by weight percentage:

[0031] C: 0.18 to 0.24;

[0032] Mn: 2.25 to 2.50;

[0033] A1: 2.00 to 3.00;

[0034] Si: 1.00 to 1.75;

[0035] Cu: 0.02 to 1.70;

[0036] Ni: 0.03 to 1.50;

[0037] P: Not greater than 0.01;

[0038] S: Not greater than 0.03;

[0039] Fe: Balance.

[0040] Copper and nickel are very effective austenite stabilizers, and their content in the composition is adjusted to ensure the necessary level of austenite stabilization. According to a preferred embodiment of the invention, copper and nickel are included in the composition in a preferred lower or higher percentage range, respectively.

[0041] Preferably, the lower percentage range of copper is 0.02% to 0.05%, and the higher percentage range of copper is 1.50% to 1.70%.

[0042] Preferably, the lower percentage range of nickel is 0.03 to 0.07, and the higher percentage range of nickel is 1.25% to 1.50%.

[0043] Copper and nickel can fall independently into their respective preferred lower or higher ranges.

[0044] The above ranges for carbon and manganese content meet the requirement of having a stable residual austenite content without producing large amounts of such elements, which would otherwise have the aforementioned harmful effects.

[0045] As mentioned above, the main reason for including aluminum is weight reduction.

[0046] Aluminum also helps prevent cementite formation during IBT and accelerates IBT kinetics. The upper limit of the selected range takes into account that, since aluminum acts as a ferrite stabilizer, an addition of more than 3% may lead to the formation of the δ-ferrite phase by solidification-expanding the ferrite phase field.

[0047] In addition, the relatively high aluminum content may be a key factor, as it has been reported that adding 1% by weight of aluminum will cause a 1.3% decrease in density.

[0048] Silicon also acts as a ferrite stabilizer. It not only increases the carbon activity coefficient in both austenite and ferrite, which is beneficial to austenite, but it also acts as an inhibitor of carbide precipitation, for example, during the IBT stage of two-stage heat treatment. Furthermore, silicon contributes to corrosion resistance.

[0049] The amount of silicon used is low enough to prevent the formation of a thick oxide layer that is not easily removed and adversely affects coatability during subsequent zinc plating processes. The amount of silicon can also be kept limited due to the presence of aluminum, which partially replaces silicon as a ferrite stabilizer.

[0050] As mentioned above, copper and nickel are also very effective austenitic stabilizers.

[0051] Because scrap steel is used in steelmaking, copper typically enters the steel, and its presence is difficult to avoid since removing it from the molten steel during the steelmaking process is impractical. Recognizing copper's austenitic stabilizing properties allows steel manufacturers to obtain a useful resource essentially at zero cost.

[0052] In addition to acting as an austenitic stabilizer, nickel also contributes to corrosion resistance. Nickel is usually added intentionally, and therefore it is more expensive than copper. Furthermore, it is quite expensive in itself, and therefore its quantity is kept as limited as possible.

[0053] When these two elements are included in steel in very low amounts (e.g., in amounts within the preferred lower percentage range of the compositions according to the invention), the austenitic phase of the steel becomes more reactive during thermomechanical processing.

[0054] Therefore, such steel allows for different thermomechanical treatment methods to improve the strength and deformability properties of the final steel product.

[0055] Meanwhile, the final product performed without any unexpected issues during its use.

[0056] On the other hand, when copper and nickel are included in steel in higher amounts (e.g., amounts within the preferred higher percentage range of the compositions according to the invention), the compositions allow for entirely different thermomechanical processing pathways to modulate the mechanical properties of the steel.

[0057] At this point, with the help of appropriate thermomechanical processing parameters, some reactive austenite can also be obtained at room temperature (i.e. in the final product), which determines, for example, energy absorption characteristics under impact conditions and additional deformation capabilities.

[0058] Sulfur and phosphorus are undesirable but unavoidable impurities.

[0059] Some examples of compositions according to the present invention are reported in Table 1 below.

[0060] [Table 1]

[0061]

[0062] As a final note regarding the composition, we would like to point out that while individual elements are important, the overall composition must be taken into account, as the various elements interact with each other in terms of specific effects, such as the variations caused by the selected thermomechanical processing parameters.

[0063] According to another aspect, the present invention provides steel products produced from the above composition.

[0064] For example, the product is a hot-rolled steel sheet or plate specifically designed for use in the automotive industry.

[0065] According to yet another aspect, the present invention provides a method for manufacturing steel products, comprising the following steps:

[0066] Melt the starting material having the composition given above;

[0067] A pre-product (referred to as an "ingot" hereinafter and in the claims) is formed from molten starting material;

[0068] The ingot is hot rolled in multiple passes at gradually decreasing temperatures, so that the hot rolling is completed at a temperature above the eutectoid temperature (Al temperature);

[0069] The hot-rolled ingot is subjected to a two-stage heat treatment, which includes a critical zone annealing stage and an isothermal bainite transformation stage.

[0070] The heat-treated product is cooled to room temperature in still air.

[0071] Multiple hot rolling passes are required because the quantity of the product to be produced from the ingot (e.g., sheets or plates) needs to be of an initial thickness that is typically too large to be deformed in a single pass. It should also be remembered that during continuous rolling operations, the temperature inevitably drops, and when this occurs, the steel gradually becomes more difficult to deform at lower temperatures. On the other hand, deformation not only alters the shape of the steel, but it also affects the stability of the austenite (within its possible temperature range).

[0072] This fundamental aspect gives rise to the possibility of altering the morphology of austenite and the morphology (grain size, shape, crystal orientation) of the phases that form thereafter (i.e., ferrite and carbides).

[0073] For example, if the deformation via rolling is completed well before the "normal" transformation temperature of austenite to ferrite (known as the A3 temperature), the steel product will have an equiaxed and relatively coarse-grained microstructure. Furthermore, this "normal" transformation temperature itself varies depending on the amount of deformation applied and the temperature level at which this deformation is introduced.

[0074] Alternatively, if the deformation is completed closer to the A3 temperature or is introduced more severely, the resulting microstructure will be very different, producing a different set of mechanical properties.

[0075] However, preferably, in this invention, all deformation passes are performed at temperatures below A3.

[0076] This completely altered the transformation kinetics and the distribution of ferrite-stabilizing and austenite-stabilizing elements, ultimately achieving the desired mechanical properties.

[0077] In short, as with the composition itself, the number of deformation steps, the temperature range of each step, the amount of deformation in each step, and their distribution in each rolling step will alter the resulting microstructure and final properties by primarily regulating the stability of austenite throughout the thermomechanical process.

[0078] In a preferred embodiment, hot rolling is carried out in six passes, the six passes being carried out at temperatures ranging from 1210°C to 1190°C, 1180°C to 1160°C, 1060°C to 1040°C, 980°C to 960°C, 960°C to 940°C and 860°C to 830°C, respectively.

[0079] The two-stage heat treatment achieves two objectives: the first stage adjusts the amount and stability level of austenite through the allocation of alloying elements, while the second stage aims to obtain austenite decomposition products in a controlled manner based on its duration. The concepts of austenite stabilization and austenite decomposition routinely performed in AHSS production have been discussed above.

[0080] The heat treatment is carried out in two stages, one in the first salt bath and the other in the second salt bath. The temperature of the first stage is selected as 50% of the austenite formation temperature in the first salt bath, while the temperature of the second stage is selected as the temperature at which the maximum residual austenite content is obtained in the second salt bath.

[0081] In a preferred embodiment of the method, the first salt bath is a BaCl2 bath, which must be maintained in the range of 740°C to 870°C, and the treatment duration is 10 minutes; and the second salt bath is a NaNO3+KNO3 (50%-50%) bath, which must be maintained in the range of 300°C to 375°C, and the treatment duration can be 10 minutes to 90 minutes, with longer treatment times producing better ductility levels.

[0082] By using the compositions disclosed above and carrying out the methods disclosed above, steel products with the following excellent properties can be obtained:

[0083] The yield strength level is not less than 584 MPa and is between 679 MPa;

[0084] The ultimate tensile strength (UTS) level is not less than 962 MPa to 1049 MPa;

[0085] The elongation at break is 22% to 28%.

[0086] Of course, while keeping the principles of the invention unchanged, the construction details may be extensively changed from what is described and shown by way of non-limiting examples only, without departing from the scope of the invention as defined in the appended claims.

[0087] For example, steel sheets or plate semi-finished products (e.g., intended for use in the automotive industry) can be produced via the methods described. However, parts can also be produced by, for example, a forging process, provided that the thermomechanical processing parameters are properly adjusted.

Claims

1. A composition for producing steel, characterized in that, The composition comprises, by weight percentage: C: 0.18 to 0.24; Mn: 2.25 to 2.50; A1: 2.00 to 3.00; Si: 1.00 to 1.75; Cu: 0.02 to 1.70; Ni: 0.03 to 1.50 P: Not greater than 0.01; S: Not greater than 0.03; Fe: Balance.

2. The composition according to claim 1, comprising: Cu: in the lower range of 0.02 to 0.05, or in the higher range of 1.50 to 1.

70.

3. The composition according to claim 1 or 2, comprising: Ni: in the lower range of 0.03 to 0.07, or in the higher range of 1.25 to 1.

50.

4. A steel product produced from the composition according to claim 1, 2 or 3.

5. The steel product according to claim 4, characterized in that: The yield strength level is not less than 584 MPa and is between 679 MPa; The ultimate tensile strength is not less than 962 MPa and is between 1049 MPa; The elongation at break is 22% to 28%.

6. A method for manufacturing the steel product according to claim 4, comprising the following steps: Melt the starting material having the composition according to claim 1, 2, or 3; Ingots are formed from molten starting material; The ingot is hot rolled in a plurality of passes at gradually decreasing temperatures, such that the hot rolling is completed at a temperature above the eutectoid temperature; The hot-rolled ingot is subjected to a two-stage heat treatment, comprising a critical zone annealing stage with given temperature and duration limitations and an isothermal bainitic transformation stage; and The heat-treated product is cooled to room temperature in still air.

7. The method of claim 5, wherein all hot rolling passes are performed at a temperature below A3 temperature.

8. The method according to claim 6 or 7, wherein hot rolling is carried out in six passes, the six passes being carried out at temperatures ranging from 1210°C to 1190°C, 1180°C to 1160°C, 1060°C to 1040°C, 980°C to 960°C, 960°C to 940°C and 860°C to 830°C, respectively.

9. The method according to any one of claims 6 to 8, wherein the two stages of the two-stage heat treatment are respectively carried out in a first salt bath and a second salt bath, and the temperature of the first stage is selected as 50% of the austenite formation temperature in the first salt bath, and the temperature of the second stage is selected according to the adjustment of the strength and ductility of the final steel in the second salt bath.

10. The method according to claim 9, wherein the first salt bath is a BaCl2 bath and the second salt bath is a NaNO3+KNO3 (50%-50%) bath.

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