Anti-oxidation warm-formed high-strength steel, manufacturing method and forming method

Abstract

The present invention provides an oxidation-resistant warm-formed high-strength steel, a manufacturing method and a forming method. The chemical composition of the present invention is calculated by mass percentage as follows: C: 0.15-0.25%, Si≤0.1%, Mn: 7-10%, Al: 0.8-1%, Cr: 3-5%, P<0.020%, S<0.006%, and the balance is Fe and unavoidable impurities. The forming process includes: a heating process: heating the steel billet with qualified composition to 1050±50°C in a heating furnace and keeping it warm for a certain time; a rolling process: using ordinary rolling + laminar cooling, the hot rolling final rolling temperature is 660±30°C, and the coiling temperature is 620°C±30°C; a cold rolling process; a warm forming process: the steel plate is heated to 780-800°C, kept warm for a certain time, and air-cooled to room temperature after forming. The performance of the steel plate after warm forming of the present invention reaches a tensile strength greater than 1500MPa and an elongation of not less than 10%. Compared with conventional hot-formed steel plates, the surface oxidation is less.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and in particular to an oxidation-resistant warm-formed high-strength steel, a manufacturing method and a forming method. Background Art

[0002] As the national carbon neutrality policy is gradually implemented, the automotive industry is further developing in the direction of lightweight, low pollution and new energy. Lightweight is one of the current priority directions of automotive development. High-strength and ultra-high strengthening of steel materials, which are the main materials of automobiles, is one of the priority solutions for automobile lightweighting. However, as the strength of steel increases, cold forming becomes extremely difficult. There are no suitable molds and the service life of the molds drops sharply, which increases the cost of automobile manufacturers and greatly affects the promotion of cold-formed high-strength steel for automobiles. Therefore, hot-warm formed high-strength steel came into being, but the high-temperature heating of hot-formed steel before forming produces severe oxidation, which makes the surface quality of hot-formed steel parts not high. The galvanizing process is currently widely used to solve this problem, which increases the cost of steel plates and reduces productivity.

[0003] There are many shortcomings in the existing technology. For example, the "Method for preparing a hot-dip galvanized warm-formed high-strength medium manganese steel part" with application number CN201610784344.5 adopts a conventional medium manganese design, has no high-temperature oxidation resistance, and has insufficient plasticity after forming. The "High-toughness hot-formed steel with a tensile strength ≥ 1800MPa and its production method" with application number CN201910680367.5 adopts a multi-alloying scheme, which increases the cost, has no high-temperature oxidation resistance design, and has insufficient plasticity reserve after forming. Although the "Method for preparing a super-high-strength, high-plasticity, high-temperature oxidation-resistant hot stamping steel" with application number CN202110304149.9 takes oxidation resistance into consideration, the alloy system is complex, and expensive N and iMo elements are added, which greatly increases the cost. The "Method for preparing a super-high-strength hot-formed steel for automobiles" with application number CN201710560144.6 adopts a high-carbon design and has insufficient plasticity reserve. The "method for producing automobile safety parts by reverse transformation of austenite and sub-temperature forming" with application number CN201811019483.4 has a complex process. Although the plasticity is good, it does not take high-temperature oxidation resistance into consideration. Summary of the invention

[0004] In response to the technical problems raised above, an oxidation resistant warm-formed high-strength steel and a forming method are provided.

[0005] The technical means adopted by the present invention are as follows:

[0006] The invention discloses an oxidation-resistant warm-forming high-strength steel, whose chemical composition is calculated by mass percentage as follows: C: 0.15-0.25%, Si≤0.1%, Mn: 7-10%, Al: 0.8-1%, Cr: 3-5%, P<0.020%, S<0.006%, and the balance is Fe and unavoidable impurities.

[0007] Furthermore, the tensile strength of the steel plate after warm forming is greater than 1500 MPa, and the elongation is greater than 10%.

[0008] The selection principle and content design reasons of the chemical components of the present invention are as follows:

[0009] C: Carbon is the most important element in steel for solid solution strengthening and stabilizing austenite. A certain amount of solid solution carbon can ensure strength and suitable microstructure. Too high carbon content is not conducive to the forming and weldability of steel plates. Too low carbon content makes it difficult to ensure the strength of steel and control suitable microstructure. The optimal range of carbon in the present invention is 0.15-0.25%.

[0010] Si: Silicon can promote oxidation on the surface of a steel sheet. As a permanent element, its upper limit is set at 0.1%.

[0011] Mn: Manganese plays a role in solid solution strengthening and expansion of the austenite phase region, improving the hardenability of steel and increasing the residual austenite in the steel after forming, thereby improving plasticity. At the same time, AC3 is reduced by increasing the manganese content, thereby reducing surface oxidation when the steel plate is heated by lowering the austenitizing temperature. The manganese content in the present invention is 7-10%.

[0012] Al: Aluminum is an important element that resists high temperature oxidation, but too much aluminum will make continuous casting more difficult. The aluminum content of the present invention is 0.8-1%.

[0013] Cr: Cr is also an important anti-oxidation element, and at the same time improves the hardenability of steel and the strength of steel. The chromium content of the present invention is 3-5%.

[0014] P: Although phosphorus can improve the strength of the steel sheet, it will seriously deteriorate the cold forming of the steel, especially the secondary forming performance, so its upper limit is set at 0.020%.

[0015] S: Sulfur is a harmful element in steel. It will form manganese sulfide and reduce the performance of the steel plate. Therefore, the lower the content, the better. The upper limit is set at 0.006%.

[0016] To achieve the purpose of the present invention, the present invention also discloses a technical solution, namely a manufacturing method and a forming method of oxidation-resistant warm-formed high-strength steel, comprising the following steps:

[0017] Heating process: heat the steel billet with qualified composition to 1050±50℃ in a heating furnace and keep it warm for a certain period of time;

[0018] Rolling process: Ordinary rolling + laminar cooling, hot rolling final rolling temperature is 660±30℃, coiling temperature is 620℃±30℃;

[0019] The forming methods are as follows: warm forming process: the steel plate is heated to 780-800°C, kept warm for a certain period of time, and air-cooled to room temperature after forming.

[0020] Furthermore, during the heating process, the heat preservation time is 2±0.5 hours.

[0021] Furthermore, in the warm forming process, the holding time is 1-2 minutes.

[0022] Heating - The key to hot rolling is a final rolling temperature not exceeding 700°C and a coiling temperature not exceeding 650°C.

[0023] Temperature is an important parameter for oxidation. While ensuring austenitization, 780-800℃ is selected. A shorter holding time of 1-2 minutes is also beneficial to reduce high-temperature oxidation.

[0024] Compared with the prior art, the present invention has the following advantages: the present invention obtains warm (hot) formed steel original plate (hot rolled plate, cold rolled plate) through smelting, hot rolling (hot rolled plate can also be directly used for warm forming), and cold rolling. After warm forming, the steel plate has a tensile strength greater than 1500MPa and an elongation of not less than 10%. Compared with conventional hot formed steel plates, the surface oxidation is less. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0026] Figure 1 This is the microstructure of the steel after warm forming of the present invention.

[0027] Figure 2 It is the retained austenite content of the steel after warm forming of the present invention. DETAILED DESCRIPTION

[0028] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0029] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0031] Unless otherwise specifically stated, the relative arrangement of the parts and steps described in these embodiments, the numerical expressions and numerical values ​​do not limit the scope of the present invention. At the same time, it should be clear that, for ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship. The technology, methods and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be regarded as part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0032] In the description of the present invention, it is necessary to understand that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "lateral, vertical, perpendicular, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of the present invention: the directional words "inside and outside" refer to the inside and outside relative to the contours of each component itself.

[0033] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below their position devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0034] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0035] The embodiment of the present invention discloses an oxidation-resistant warm-forming high-strength steel, whose chemical composition is calculated by mass percentage as follows: C: 0.15-0.25%, Si≤0.1%, Mn: 7-10%, Al: 0.8-1%, Cr: 3-5%, P<0.020%, S<0.006%, and the balance is Fe and unavoidable impurities.

[0036] Furthermore, the tensile strength of the steel plate after warm forming is greater than 1500 MPa, and the elongation is greater than 10%.

[0037] To achieve the purpose of the present invention, the present invention also discloses a technical solution, namely a forming method of oxidation-resistant warm-formed high-strength steel, comprising the following steps:

[0038] Heating process: heat the steel billet with qualified composition to 1050±50℃ in a heating furnace and keep it warm for 2±0.5 hours;

[0039] Rolling process: Ordinary rolling + laminar cooling, hot rolling final rolling temperature is 660±30℃, coiling temperature is 620℃±30℃;

[0040] Cold rolling process; the reduction rate is set according to the thickness of the hot-rolled raw material and the cold-rolled specification requirements.

[0041] Warm forming process: The steel plate is heated to 780-800℃, kept warm for 1-2 minutes, and air-cooled to room temperature after forming.

[0042] The microstructure and retained austenite of the steel after warm forming are as follows: Figure 1 and Figure 2 As shown, the retained austenite in the steel after forming in the present application can improve plasticity, and the shorter holding time of 1-2 minutes in the warm forming process is also beneficial to reduce high-temperature oxidation.

[0043] Example 1

[0044] In this embodiment, 80kg of qualified ingots are cut off and heated to 1050±50℃, and kept warm for 2 hours. The hot rolling final rolling temperature is 660±30℃, the simulated coiling temperature is 620±30℃, kept warm for 6 hours, and cooled to room temperature with the furnace. The thickness of the hot rolled plate is 3.5mm. After pickling, the hot rolled plate is cold rolled to 1.5-2.0mm; the warm forming process is carried out to heat the steel plate to 780-800℃, keep warm for 1-2 minutes, and air cool after forming.

[0045] The specific composition, temperature regime and properties of the steel plates of the six embodiments of the present invention are shown in Tables 1 to 3.

[0046] Table 1 Chemical composition of the present invention (wt, %)

[0047]

[0048] Table 2 Hot rolling process, cold rolling reduction rate, warm forming process of the embodiment of the present invention

[0049]

[0050] Table 3 Mechanical properties parameters of the embodiments of the present invention

[0051]

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An oxidation-resistant warm-formed high-strength steel, characterized in that: The chemical composition is calculated by mass percentage as follows: C: 0.15%~0.25%, Si≤0.1%, Mn: 7%~10%, Al: 0.85%~1%, Cr: 3%~5%, P<0.020%, S<0.006%, and the balance is Fe and unavoidable impurities; The above oxidation resistant warm-formed high-strength steel is manufactured based on the following method: Heating process: heat the steel billet with qualified composition to 1050±50℃ in a heating furnace and keep it warm for a certain period of time; Rolling process: ordinary rolling + laminar cooling, hot rolling final rolling temperature is 660±30℃, coiling temperature is 620±30℃; Warm forming process: The steel plate is heated to 780-800℃, kept warm for a certain period of time, and air-cooled to room temperature after forming.

2. The oxidation resistant warm-formed high-strength steel according to claim 1, characterized in that: After warm forming, the tensile strength of the steel plate is greater than 1500MPa and the elongation is greater than 10%.

3. The oxidation resistant warm-formed high-strength steel according to claim 1, characterized in that: During the heating process, the holding time is 2±0.5 hours.

4. The oxidation resistant warm-formed high-strength steel according to claim 1, characterized in that: In the warm forming process, the holding time is 1-2 minutes.

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