A hot-formed steel and a method for producing the same

Abstract

The application discloses a hot forming steel and a preparation method thereof. The preparation method of the hot forming steel comprises the following steps: S1. heating a hot-rolled steel plate under the condition that the heating temperature is 20-30 DEG C lower than the Ac3 temperature of the hot-rolled steel plate, so that the hot-rolled steel plate is not completely austenitized; and S2. stamping and forming the hot-rolled steel plate treated in the step S1, and quenching, so that the hot forming steel is obtained. The preparation method of the hot forming steel controls the austenitization degree of the hot-rolled steel plate by controlling the heating temperature of the hot-rolled steel plate to be 20-30 DEG C lower than the Ac3 temperature of the hot-rolled steel plate, introduces the soft phase of ferrite, releases the second type of residual stress of the hot forming steel, reduces the driving force of hydrogen diffusion, and realizes the purpose of improving the hydrogen embrittlement resistance of the hot forming steel while maintaining the strength of the hot forming steel.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, and more specifically, to a hot-formed steel and its preparation method. Background Technology

[0002] With the rapid development of the automotive industry, the mechanical properties of automotive steel sheets have been continuously improving. Hot-formed steel, as the highest strength steel grade, has a global annual consumption of approximately 3 million tons in the automotive industry, indicating a broad market prospect. It can be used in protective components such as anti-collision beams, bumpers, and A / B pillars. With increasingly stringent requirements for lightweighting and collision safety, higher strength is an inevitable path for the development of hot-formed steel. However, higher strength steel sheets are more prone to hydrogen embrittlement, especially for high-strength steels above 1 GPa. A few ppm of hydrogen is sufficient to cause unexpected brittle fracture, posing a significant safety hazard. Hydrogen embrittlement has become a major bottleneck restricting further improvements in the strength of hot-formed steel.

[0003] Therefore, it is of great significance to develop a method for preparing hot-formed steel that can improve its resistance to hydrogen embrittlement while maintaining its strength. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a hot-formed steel and its preparation method.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] In a first aspect, the present invention provides a method for preparing hot-formed steel, the method comprising the following steps:

[0007] S1. Heating the hot-rolled steel plate at a temperature 20-30°C lower than the Ac3 temperature of the hot-rolled steel plate to prevent it from being fully austenitized;

[0008] S2. The hot-rolled steel sheet processed in step S1 is stamped and quenched to obtain hot-formed steel.

[0009] This invention regulates the austenitization degree of hot-rolled steel plates by controlling the heating temperature of the hot-rolled steel plates to be 20-30°C lower than their Ac3 temperature, introducing soft ferrite phases, thereby releasing the second type of residual stress in the hot-formed steel and reducing the driving force for hydrogen diffusion. This achieves the goal of improving the hydrogen embrittlement resistance of hot-formed steel while maintaining its strength.

[0010] In step S1, when the hot-rolled steel plate is heated at a temperature 20-30°C lower than the Ac3 temperature of the hot-rolled steel plate, the microstructure of the hot-rolled steel plate consists of a large amount of austenite and a small amount of ferrite. Subsequently, during the stamping process in step S2, the austenite in the hot-rolled steel plate undergoes a phase transformation into lath martensite, while the ferrite does not undergo a phase transformation, ultimately forming a hot-formed steel with lath martensite as the matrix and containing a small amount of ferrite.

[0011] In this invention, Ac3 temperature refers to the temperature at which all ferrite in a hot-rolled steel sheet transforms into austenite. In other words, heating a hot-rolled steel sheet at Ac3 temperature will completely austenitize it. The Ac3 temperature of a hot-rolled steel sheet can be measured using a thermal simulation testing machine (Gleeble-3500).

[0012] In a preferred embodiment of the method for preparing the hot-formed steel according to the present invention, the hot-formed steel in step S2 contains lath martensite and ferrite. More preferably, the volume fraction of ferrite in the hot-formed steel is 5-10%; the volume fraction of lath martensite in the hot-formed steel is 90-95%. The volume fractions of lath martensite and ferrite are obtained by metallographic analysis.

[0013] In a preferred embodiment of the method for preparing hot-formed steel according to the present invention, the heating time of the hot-rolled steel plate in step S1 is 3-5 minutes.

[0014] In a preferred embodiment of the method for preparing hot-formed steel according to the present invention, the cooling rate of quenching in step S2 is 50-100℃ / s.

[0015] In a preferred embodiment of the method for preparing hot-formed steel according to the present invention, the stamping process in step S2 needs to be carried out in a pure copper mold.

[0016] In a preferred embodiment of the method for preparing hot-formed steel according to the present invention, the hot-rolled steel plate in step S1 comprises the following components by mass fraction:

[0017] The composition is as follows: C 0.25-0.50%, Mn 0.30-0.50%, Cr 0.50-1.00%, Si 0.10-0.30%, Mo 0.10-0.20%, B 0.002-0.005%, P≤0.005%, S≤0.002%, with the balance being Fe and unavoidable impurities. A hot-rolled steel plate containing the above composition is designated as hot-rolled steel plate A. The Ac3 temperature of hot-rolled steel plate A, measured using a thermal simulation testing machine (Gleeble-3500), is 880-900℃.

[0018] In a preferred embodiment of the method for preparing hot-formed steel according to the present invention, the hot-rolled steel plate in step S1 comprises the following components by mass fraction:

[0019] C 0.25-0.50%, Mn 0.30-0.50%, X 0.10-0.60%, Si 0.10-0.30%, Mo 0.10-0.20%, B 0.002-0.005%, P≤0.005%, S≤0.002%, balance Fe and unavoidable impurities;

[0020] X is at least one of Ti and Nb. When X is Ti, its mass fraction is 0.10-0.30%; when X is Nb, its mass fraction is 0.10-0.30%; when X is both Ti and Nb, the mass fraction of Ti is 0.10-0.30% and the mass fraction of Nb is 0.10-0.30%. The hot-rolled steel plate containing the above components is designated as hot-rolled steel plate B. The Ac3 temperature of hot-rolled steel plate B, measured by a thermal simulation testing machine (Gleeble-3500), is 885-905℃.

[0021] In a preferred embodiment of the method for preparing hot-formed steel according to the present invention, the hot-rolled steel plate in step S1 comprises the following components by mass fraction:

[0022] C 0.25-0.50%, Mn 0.30-0.50%, Cr 0.50-2.00%, Z 0.10-0.60%, Si 0.10-0.30%, Mo 0.10-0.20%, B 0.002-0.005%, P≤0.005%, S≤0.002%, balance Fe and unavoidable impurities;

[0023] Z is at least one of Ti and Nb. When Z is Ti, its mass fraction is 0.10-0.30%; when Z is Nb, its mass fraction is 0.10-0.30%; when Z is both Ti and Nb, the mass fraction of Ti is 0.10-0.30% and the mass fraction of Nb is 0.10-0.30%. The hot-rolled steel plate containing the above components is denoted as hot-rolled steel plate C. The Ac3 temperature of hot-rolled steel plate C, measured by a thermal simulation testing machine (Gleeble-3500), is 870-890℃.

[0024] In a preferred embodiment of the method for preparing hot-formed steel according to the present invention, the method for preparing hot-rolled steel plate in step S1 includes the following steps:

[0025] The components used to prepare hot-rolled steel plates are melted, cast into ingots, heated and held at a certain temperature, and then hot-rolled to obtain hot-rolled steel plates.

[0026] More preferably, the heating temperature is 970-1030℃, the holding time is 1-2 hours, the hot rolling temperature is 950-1050℃, the hot rolling reduction is 85-90%, and the thickness of the hot-rolled steel plate is 1.5-2 mm.

[0027] Secondly, the present invention provides a hot-formed steel prepared by the above-described preparation method.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] The method for preparing hot-formed steel of the present invention regulates the austenitization degree of hot-rolled steel plates by controlling the heating temperature of the hot-rolled steel plates to be 20-30°C lower than their Ac3 temperature, introduces soft ferrite phase, thereby releasing the second type of residual stress of hot-formed steel and reducing the driving force of hydrogen diffusion, thus achieving the goal of improving the hydrogen embrittlement resistance of hot-formed steel while maintaining its strength.

[0030] The method for preparing hot-formed steel of this invention achieves the goal of improving the resistance to hydrogen embrittlement of hot-formed steel while maintaining its strength. This is done without optimizing the alloy composition of the hot-rolled steel sheet and / or the hot-formed steel, without adding extra processes, without improving the equipment conditions of existing hot-formed steel production lines, and without increasing production costs. Only a slight adjustment to the heating temperature of the hot-rolled steel sheet on existing hot-formed steel production lines is required. This method is applicable to all existing hot-formed steel production lines and can be used on a large scale in industry. Attached Figure Description

[0031] Figure 1 The images show the metallographic structures of the hot-formed steels of Example 1 and Comparative Example 1.

[0032] Figure 2 Figure 1 shows the engineering strain-engineering stress curves of the hot-formed steel of Example 1 and Comparative Example 1 before and after electrochemical hydrogen charging. Figure 2a shows the engineering strain-engineering stress curves of the hot-formed steel of Example 1 and Comparative Example 1 before electrochemical hydrogen charging, and Figure 3b shows the engineering strain-engineering stress curves of the hot-formed steel of Example 1 and Comparative Example 1 after electrochemical hydrogen charging. Detailed Implementation

[0033] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0034] Example 1

[0035] This embodiment provides a hot-formed steel, the preparation method of which includes the following steps:

[0036] S1. Heating the hot-rolled steel plate for 5 minutes at a temperature 20°C lower than the Ac3 temperature of the hot-rolled steel plate to prevent it from being fully austenitized;

[0037] S2. The hot-rolled steel sheet processed in step S1 is quickly placed in a pure copper mold for stamping and forming, and then quenched. The quenching cooling rate is 50℃ / s, thus obtaining hot-formed steel.

[0038] The hot-formed steel contains lath martensite and ferrite. Metallographic analysis shows that the volume fraction of ferrite in the hot-formed steel is 10%, and the volume fraction of lath martensite is 90%.

[0039] Based on mass fraction, the hot-rolled steel sheet in step S1 comprises the following components:

[0040] The composition is: C 0.34%, Mn 0.44%, Cr 0.50%, Si 0.20%, Mo 0.15%, B 0.002%, P 0.004%, S 0.001%, with the balance being Fe and unavoidable impurities. The hot-rolled steel plate containing the above composition is designated as hot-rolled steel plate A1. The Ac3 temperature of hot-rolled steel plate A1, measured using a thermal simulation testing machine (Gleeble-3500), is 900℃. Therefore, the phrase "20℃ lower than the Ac3 temperature of the hot-rolled steel plate" in step S1 refers to 880℃.

[0041] The method for preparing the hot-rolled steel plate in step S1 includes the following steps:

[0042] The components used to prepare hot-rolled steel plates are melted, cast into ingots, heated to 1000℃ and held for 2 hours, and then hot-rolled at 950℃ with a reduction of 90%. After air cooling, hot-rolled steel plates with a thickness of 1.5 mm are obtained.

[0043] Examples 2-3 and Comparative Examples 1-3

[0044] Examples 2-3 and Comparative Examples 1-3 provide different hot-formed steels and their preparation methods. The difference between them and Example 1 lies in the heating temperature of the hot-rolled steel sheet; otherwise, they are the same as Example 1, as shown in the table below:

[0045] Table 1. Temperature of hot-rolled steel plates in Examples 1-3 and Comparative Examples 1-3

[0046] Temperature of heating hot-rolled steel plate Example 1 880℃ (20℃ lower than the Ac3 temperature of hot-rolled steel plate) Example 2 875℃ (25℃ lower than the Ac3 temperature of hot-rolled steel plate) Example 3 870℃ (30℃ lower than the Ac3 temperature of hot-rolled steel plate) Comparative Example 1 920℃ (20℃ higher than the Ac3 temperature of hot-rolled steel plate) Comparative Example 2 900℃ (Ac3 temperature of hot-rolled steel plate) Comparative Example 3 860℃ (40℃ lower than the Ac3 temperature of hot-rolled steel plate)

[0047] Example 4

[0048] This embodiment provides a hot-formed steel, which differs from Embodiment 1 in that the composition of the hot-rolled steel plate in step S1 is different, as detailed below:

[0049] Based on mass fraction, the hot-rolled steel sheet in step S1 comprises the following components:

[0050] The composition is: C 0.30%, Mn 0.30%, Ti 0.10%, Nb 0.20%, Si 0.20%, Mo 0.10%, B 0.002%, P 0.004%, S 0.001%, with the balance being Fe and unavoidable impurities. The hot-rolled steel plate containing the above composition is designated as hot-rolled steel plate B1. The Ac3 temperature of hot-rolled steel plate B1, measured using a thermal simulation testing machine (Gleeble-3500), is 895℃. Therefore, the phrase "20℃ lower than the Ac3 temperature of the hot-rolled steel plate" in step S1 refers to 875℃.

[0051] Example 5

[0052] This embodiment provides a hot-formed steel, which differs from Embodiment 1 in that the composition of the hot-rolled steel plate in step S1 is different, as detailed below:

[0053] Based on mass fraction, the hot-rolled steel sheet in step S1 comprises the following components:

[0054] The composition is as follows: C 0.33%, Mn 0.49%, Cr 0.50%, Ti 0.10%, Nb 0.10%, Si 0.13%, Mo 0.15%, B 0.002%, P 0.004%, S 0.001%, with the balance being Fe and unavoidable impurities. The hot-rolled steel plate containing the above composition is designated as hot-rolled steel plate C1. The Ac3 temperature of hot-rolled steel plate C1, measured using a thermal simulation testing machine (Gleeble-3500), is 890℃. Therefore, the phrase "20℃ lower than the Ac3 temperature of the hot-rolled steel plate" in step S1 refers to 870℃.

[0055] Sample characterization

[0056] Figure 1 The images show the metallographic structures of the hot-formed steels from Example 1 and Comparative Example 1. (From...) Figure 1 It can be seen that the hot-formed steel prepared in Example 1 contains lath martensite and ferrite, while the hot-formed steel prepared in Comparative Example 1 contains only lath martensite and no ferrite.

[0057] Performance testing

[0058] The performance of the hot-formed steels in each embodiment and comparative example was tested, specifically:

[0059] (1) Mechanical property testing

[0060] According to the national standard GB / T228.1-2010, the tensile strength and elongation after fracture of the hot-formed steel in each embodiment and comparative example were tested.

[0061] (2) Hydrogen embrittlement resistance test

[0062] S1. Electrochemical hydrogen charging of the hot-formed steel in each embodiment and comparative example was performed using a three-electrode system: the reference electrode was a saturated calomel electrode, the counter electrode was a platinum sheet, and the working electrode was the hot-formed steel (10 mm long, 2.5 mm wide, and 1.5 mm high) in each embodiment and comparative example. The electrolyte solution consisted of a 30.93 g / L NaCl solution and a 0.22 g / L thiourea solution, and the hydrogen charging current density was 2 mA·cm⁻¹. -2 The time is 30 minutes;

[0063] S2. Perform a slow tensile test on the hot-formed steel treated in step S1 using an electronic universal testing machine, with a strain rate of 1×10⁻⁶. -4 s -1 ;

[0064] After the experiment, the hydrogen embrittlement susceptibility (I0.05) was quantified by the loss of elongation after fracture of the hot-formed steel before and after electrochemical hydrogen charging. HE ), I HE The lower the value, the lower the hydrogen embrittlement sensitivity of the hot-formed steel and the higher its resistance to hydrogen embrittlement; among which, I HE The formula for calculating the value is as follows:

[0065] I HE =(ε a -ε b ) / ε a

[0066] In the above formula, ε a The elongation after fracture of hot-formed steel before electrochemical hydrogen charging is described; ε b The elongation after fracture of hot-formed steel after electrochemical hydrogen charging is described.

[0067] The performance test results are shown below:

[0068] Table 3. Mechanical property test results of the hot-formed steels in each embodiment and comparative example.

[0069]

[0070]

[0071] Table 4. Test results of hydrogen embrittlement resistance of hot-formed steels in each embodiment and comparative example.

[0072] <![CDATA[ε a / %]]> <![CDATA[ε b / %]]> <![CDATA[Hydrogen embrittlement sensitivity (I HE )]]> Example 1 11.20 4.51 0.60 Example 2 12.12 5.21 0.57 Example 3 12.03 4.81 0.60 Example 4 10.63 4.55 0.57 Example 5 10.51 4.62 0.56 Comparative Example 1 11.88 2.61 0.78 Comparative Example 2 11.96 3.46 0.71 Comparative Example 3 11.11 2.94 0.74

[0073] Figure 2Figure 1 shows the engineering strain-engineering stress curves of the hot-formed steels of Example 1 and Comparative Example 1 before and after electrochemical hydrogen charging. Figure 2a shows the engineering strain-engineering stress curves of the hot-formed steels of Example 1 and Comparative Example 1 before electrochemical hydrogen charging, and Figure 3b shows the engineering strain-engineering stress curves of the hot-formed steels of Example 1 and Comparative Example 1 after electrochemical hydrogen charging. Figure 2 It can be seen that, compared with Comparative Example 1, under the same hydrogen charging conditions, the lower the temperature at which the hot-rolled steel plate in Example 1 is heated, the higher the elongation after fracture after hydrogen charging, which means that its resistance to hydrogen embrittlement is significantly improved.

[0074] Combining Tables 3 and 4 and Figure 2 As can be seen, the method for preparing hot-formed steel of the present invention regulates the degree of austenitization of the hot-rolled steel plate by controlling the heating temperature of the hot-rolled steel plate to be 20-30°C lower than its Ac3 temperature, introduces soft ferrite phase, thereby releasing the second type of residual stress of the hot-formed steel and reducing the driving force of hydrogen diffusion, thus achieving the goal of improving the hydrogen embrittlement resistance of the hot-formed steel while maintaining its strength.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing hot-formed steel, characterized in that, The preparation method includes the following steps: S1. Heating the hot-rolled steel plate at a temperature 20-30°C lower than the Ac3 temperature of the hot-rolled steel plate to prevent it from being fully austenitized; S2. The hot-rolled steel sheet processed in step S1 is stamped and quenched to obtain hot-formed steel. The hot-formed steel described in step S2 contains lath martensite and ferrite; The cooling rate for quenching in step S2 is 50-100℃ / s; The hot-rolled steel plate in step S1 comprises the following components by mass fraction: C 0.25-0.50%, Mn 0.30-0.50%, Ti 0.10-0.30%, Nb 0.10-0.30%, Si 0.10-0.30%, Mo 0.10-0.20%, B 0.002-0.005%, P≤0.005%, S≤0.002%, with the balance being Fe and unavoidable impurities; or, Based on mass fraction, the hot-rolled steel sheet in step S1 comprises the following components: C 0.25-0.50%, Mn 0.30-0.50%, Cr 0.50-2.00%, Ti 0.10-0.30%, Nb 0.10-0.30%, Si 0.10-0.30%, Mo 0.10-0.20%, B 0.002-0.005%, P≤0.005%, S≤0.002%, balance Fe and unavoidable impurities; or, Based on mass fraction, the hot-rolled steel sheet in step S1 comprises the following components: C 0.25-0.50%, Mn 0.30-0.50%, Cr 0.50-1.00%, Si 0.10-0.30%, Mo 0.10-0.20%, B 0.002-0.005%, P≤0.005%, S≤0.002%, balance Fe and unavoidable impurities.

2. The method for preparing hot-formed steel as described in claim 1, characterized in that, The heating time for the hot-rolled steel plate in step S1 is 3-5 minutes.

3. A hot-formed steel prepared by any one of the preparation methods described in claims 1-2.

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