A method for preparing high surface quality uncoated hot stamping steel

Abstract

The present application relates to the field of steel preparation, and in particular to a method for preparing high-surface-quality uncoated hot-stamped steel; the method comprises: obtaining a steel billet; cold-rolling the steel billet raw material to obtain a raw material plate; heating, heat-insulating, hot-forming and shot-blasting the raw material plate to obtain high-surface-quality uncoated hot-stamped steel; wherein the terminal temperature of the heating is 890°C to 950°C; by first heating the cold-rolled raw material plate and then heat-insulating it, combined with the composition of the steel plate, it is ensured that the steel plate is fully austenitized, thereby ensuring that the thickness ratio of iron oxide and ferrous oxide in the surface structure of the steel plate and the layered structure distribution are reasonable, forming an iron oxide scale with a refined structure, and realizing precise control of the refined structure of the iron oxide scale.

Description

Technical Field

[0001] The present application relates to the field of steel preparation, and in particular to a method for preparing uncoated hot stamped steel with high surface quality. Background Art

[0002] The steel plate used in hot stamping is a special boron alloy steel plate. It has low strength at room temperature and good plasticity and formability. After forming and quenching through the hot stamping process, the microstructure of the B-containing steel plate is transformed from the original ferrite and pearlite to martensite, thereby increasing the tensile strength of the steel plate by more than 2 to 3 times.

[0003] The existing hot stamping process is as follows: steel raw material sheet containing B → sheet is heated in a heating furnace → sheet is transferred to a hot forming press for loading → hot stamping and cooling → the formed steel sheet is shot blasted → finished product. However, during the hot stamping process of uncoated hot-formed steel, there are problems such as surface iron oxide scale easily falling off in the mold and surface iron scale residue during the subsequent steel plate shot blasting, which can easily cause damage to the shot blasting mold and poor surface quality of the finished product. To address the problem of iron oxide scale, either the raw material plate is pre-formed with an oxide layer to control the shedding of the iron scale, or antioxidant alloy elements are added to the raw material composition to reduce the thickness of the oxide layer. However, the structure of the iron oxide scale is not controlled, which leads to uneven shedding of the iron oxide scale, resulting in poor surface quality of the finished product.

[0004] Therefore, how to accurately control the fine structure of the iron oxide scale so that the iron oxide scale only peels off or loosens slightly in the mold without leaving the steel matrix is a technical problem that needs to be solved urgently. Summary of the Invention

[0005] The present application provides a method for preparing uncoated hot stamped steel with high surface quality to solve the technical problem in the prior art that the fine structure of iron oxide scale cannot be accurately controlled.

[0006] In a first aspect, the present application provides a method for preparing uncoated hot stamped steel with high surface quality, the method comprising:

[0007] Obtaining a steel billet;

[0008] cold rolling the steel billet raw material to obtain a raw sheet;

[0009] The raw material sheet is heated, kept warm, hot stamped and shot blasted to obtain a non-coated hot stamped steel with high surface quality;

[0010] Wherein, the terminal temperature of the heating is 890°C to 950°C.

[0011] Optionally, the chemical composition of the steel billet includes, by mass fraction: C: 0.18% to 0.35%, Mn: 1.0% to 2.0%, Si: 0.2% to 0.6%, Cr: 0.1% to 0.5%, B: 0.002% to 0.004%, and the remainder is Fe and unavoidable impurities.

[0012] Optionally, the steel billet further includes [Si]+[Cr]≥0.4% in terms of mass fraction, wherein [Si] is the mass fraction of the Si, and [Cr] is the mass fraction of the Cr.

[0013] Optionally, the terminal temperature of the heating is 915°C to 930°C.

[0014] Optionally, the insulation time is 240s to 300s.

[0015] Optionally, the temperature of the hot stamping is 810°C to 870°C.

[0016] Optionally, the roughness of the raw material sheet is 0.7 μm to 1.2 μm.

[0017] Optionally, the surface structure of the uncoated hot stamping steel includes a layered structure of an iron oxide layer and a ferrous oxide layer.

[0018] Optionally, the ratio of the thickness of the iron oxide layer to the thickness of the ferrous oxide layer is 3:1 to 4:1.

[0019] Optionally, the thickness of the iron oxide layer is 4 μm to 5 μm, and the thickness of the ferrous oxide layer is 1 μm to 3 μm.

[0020] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0021] The embodiment of the present application provides a method for preparing uncoated hot-stamped steel with high surface quality. By first heating the raw material sheet after cold rolling and then keeping it warm, combined with the composition of the steel plate, it is ensured that the steel plate is fully austenitized, thereby ensuring that the thickness ratio of iron oxide and ferrous oxide in the surface structure of the steel plate and the layered structure distribution are reasonable, forming a refined structure of iron oxide scale, and realizing precise control of the refined structure of the iron oxide scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0024] Figure 1 A schematic diagram of a process for the method provided in an embodiment of the present application;

[0025] Figure 2 A schematic diagram of a hot stamped steel product provided in Comparative Example 1 of this application;

[0026] Figure 3 A schematic diagram of the surface structure of the hot stamping steel provided in Comparative Example 2 of the present application;

[0027] Figure 4 A schematic diagram of the morphology of the surface structure of the hot stamping steel provided in Example 1 of the present application;

[0028] Figure 5 A schematic structural diagram of the surface structure of hot stamping steel provided in Comparative Example 3 of the present application;

[0029] Figure 6 A schematic structural diagram of the surface structure of hot stamping steel provided in Example 2 of the present application;

[0030] Figure 7 A schematic structural diagram of the surface structure of hot stamping steel provided in Comparative Example 4 of the present application; DETAILED DESCRIPTION

[0031] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0032] In one embodiment of the present application, Figure 1 As shown, a method for preparing uncoated hot stamping steel with high surface quality is provided, the method comprising:

[0033] S1. Obtain a steel billet;

[0034] S2. The steel billet raw material is cold rolled to obtain a raw sheet;

[0035] S3. The raw sheet is heated, kept warm, hot stamped and shot blasted to obtain a high surface quality uncoated hot stamped steel;

[0036] Wherein, the terminal temperature of the heating is 890°C to 950°C.

[0037] In the present application, the positive effect of the heating end temperature of 890°C to 950°C is that within this temperature range, the raw material plate can be austenitized, thereby obtaining an oxide scale structure that meets the expected target; when the temperature value is greater than the maximum value of the endpoint of the range, the adverse effect will be that the excessively high temperature will cause the steel plate to be completely austenitized, and will affect the thickness of the overall surface structure, thereby affecting the quality of the steel product; when the temperature value is less than the minimum value of the endpoint of the range, the adverse effect will be that the excessively low temperature will cause the steel plate to be unable to be austenitized, resulting in the surface structure of the steel plate being unable to be formed.

[0038] In some optional embodiments, the chemical composition of the steel billet includes, by mass fraction: C: 0.18% to 0.35%, Mn: 1.0% to 2.0%, Si: 0.2% to 0.6%, Cr: 0.1% to 0.5%, B: 0.002% to 0.004%, and the balance is Fe and unavoidable impurities, wherein the mass fraction of C can be 0.18%, 0.2%, 0.28% and 0.35%, the mass fraction of Mn can be 1.0%, 1.46%, 1.68% and 2.0%, the mass fraction of Si can be 0.2%, 0.23%, 0.32% and 0.6%, the mass fraction of Cr can be 0.1%, 0.12%, 0.32% and 0.5%, and the mass fraction of B can be 0.002%, 0.0023%, 0.0026% and 0.004%.

[0039] In the present application, the positive effect of the C mass fraction being 0.18% to 0.35% is that within this mass fraction range, it avoids excessively high carbon content reacting with oxygen during the steel plate's own oxidation stage to release carbon dioxide gas, thereby causing cracking and blistering of the surface iron oxide scale, while also avoiding excessively low carbon content that will not guarantee the strength of the steel plate and the thickness of the surface structure.

[0040] The positive effect of a mass fraction of Mn of 1.0% to 2.0% is that within this mass fraction range, since Mn needs solid solution strengthening to provide strength, and excessive Mn will directly react with the iron in the iron oxide scale, causing the iron oxide scale to peel off, the mass fraction of Mn is limited to 1.0% to 2.0%.

[0041] The positive effect of Si's mass fraction being 0.2% to 0.6% is that within this mass fraction range, it can ensure that Si / Cr oxides, together with Mn and Cr elements, are gathered on the surface of the steel at high temperature, affecting the formation of iron oxide scale on the surface of the steel, thereby affecting the bonding strength of the iron scale, thereby facilitating shot blasting in the subsequent hot stamping stage.

[0042] The positive effect of a Cr mass fraction of 0.1% to 0.5% is that within this mass fraction range, it can form Si / Cr oxides together with Mn and Si elements, thereby affecting the formation of iron oxide scale on the surface of the steel and affecting the bonding strength of the iron scale, thereby facilitating shot blasting in the subsequent hot stamping stage.

[0043] The positive effect of the B mass fraction of 0.002% to 0.004% is that within this mass fraction range, since the B element will reduce the oxidation melting points of Si, Cr and Mn, by adjusting the B element content, a reasonable Si-Mn-B ratio can be obtained, thereby making it easier to change the interface between the iron oxide scale and the steel substrate, obtain a suitable interface flatness, and obtain an iron oxide scale with good bonding strength and easy shot blasting.

[0044] In some optional embodiments, the steel billet further comprises [Si]+[Cr]≥0.4% by mass, wherein [Si] is the mass fraction of the Si, and [Cr] is the mass fraction of the Cr.

[0045] In the present application, the positive effect of [Si] + [Cr] ≥ 0.4% is that within this mass fraction range, it can ensure that Si / Cr oxides can fully penetrate the steel matrix interface, thereby increasing the bonding strength of the iron oxide scale, ensuring the degree of bonding between the surface structure and the steel matrix, and thus ensuring the shot blasting effect in the subsequent hot stamping stage.

[0046] In some optional embodiments, the terminal temperature of the heating is 915°C to 930°C, wherein the terminal temperature of the heating can be 915°C, 920°C, 925°C and 930°C.

[0047] In the present application, the positive effect of the heating end temperature of 915°C to 930°C is that within this temperature range, the raw material plate can be guaranteed to be austenitized, and the thickness ratio of iron oxide and ferrous oxide in the iron oxide scale can be further adjusted, so that the iron oxide scale structure that meets the expected target can be obtained; when the temperature value is greater than the maximum value of the endpoint of the range, the adverse effect will be that the excessively high temperature will cause the steel plate to be completely austenitized, and will affect the thickness of the overall surface structure and the balance of iron oxide and ferrous oxide, thereby affecting the quality of the steel product; when the temperature value is less than the minimum value of the endpoint of the range, the adverse effect will be that the excessively low temperature will cause the steel plate to be unable to be austenitized, resulting in the surface structure of the steel plate being unable to be formed.

[0048] In some optional embodiments, the insulation time is 240s to 300s, wherein the insulation time can be 240s, 260s and 300s.

[0049] In the present application, the positive effect of the holding time of 240s to 300s is that within this time range, the raw material sheet can be austenitized, and the thickness ratio of iron oxide and ferrous oxide in the iron oxide scale can be further adjusted, so that the iron oxide scale structure that meets the expected target can be obtained, and the overall process time consumption can be ensured to be reasonable.

[0050] In some optional embodiments, the temperature of the hot stamping is 810°C to 870°C, wherein the temperature of the hot stamping can be 810°C, 830°C and 870°C.

[0051] In this application, the positive effect of the hot stamping temperature being 810℃~870℃ is that when the temperature is lower than 810℃, the time for the material to transfer from the heating furnace to the hot forming process is longer, the thickness of the iron oxide scale will increase, and the plastic deformation during stamping will be affected, thereby affecting the quality of the product. When the temperature exceeds 870℃, FeO will not have time to decompose, and the proportion of Fe3O4 in the iron oxide scale will be greatly reduced, which will increase the risk of falling off during the hot stamping process.

[0052] In some optional embodiments, the roughness of the raw material sheet is 0.7 μm to 1.2 μm, wherein the roughness of the raw material sheet can be 0.7 μm, 1.0 μm and 1.2 μm.

[0053] In the present application, the positive effect of the roughness of the raw material sheet being 0.7 μm to 1.2 μm is that within this roughness range, the smooth production of the raw material sheet can be guaranteed. Values lower than or higher than the end points of this range will increase the difficulty of producing the raw material sheet.

[0054] In some optional embodiments, the surface structure of the uncoated hot stamping steel includes a layered structure of an iron oxide layer and a ferrous oxide layer.

[0055] In some optional embodiments, the ratio of the thickness of the iron oxide layer to the thickness of the ferrous oxide layer is 3:1 to 4:1.

[0056] In the present application, by limiting the ratio of the thickness of iron oxide to the thickness of ferrous oxide, the lubrication performance of the surface iron oxide scale structure can be ensured under the condition of a determined surface structure. During the hot forming process, the iron oxide scale will neither fall off easily and accumulate in the mold nor remain on the surface of the part during subsequent shot blasting.

[0057] In some optional embodiments, the thickness of the iron oxide layer is 4 μm to 5 μm, and the thickness of the ferrous oxide layer is 1 μm to 3 μm.

[0058] In the present application, the positive effect of limiting the thickness of the iron oxide layer and the ferrous oxide layer is that within this thickness range, the iron oxide layer and the ferrous oxide layer can be stably distributed in the surface structure in combination with the composition and process of the raw material sheet.

[0059] Example 1

[0060] like Figure 1 As shown, a method for preparing uncoated hot stamping steel with high surface quality comprises:

[0061] S1. Obtain a steel billet;

[0062] S2. The steel billet raw material is cold rolled to obtain a raw sheet;

[0063] S3. The raw sheet is heated, kept warm, hot stamped and shot blasted to obtain a high surface quality uncoated hot stamped steel;

[0064] The heating end point temperature is 925°C.

[0065] Calculated by mass fraction, the chemical composition of the steel billet includes: C: 0.28%, Mn: 1.68%, Si: 0.32%, Cr: 0.35%, B: 0.0026%, and the balance is Fe and inevitable impurities.

[0066] The end point temperature of the heating was 920°C.

[0067] The holding time is 260s.

[0068] The hot stamping temperature is 860°C.

[0069] The roughness of the raw material sheet is 1.2 μm.

[0070] The surface structure of uncoated hot stamping steel includes a layered structure of iron oxide layer and ferrous oxide layer.

[0071] The ratio of the thickness of the iron oxide layer to the thickness of the ferrous oxide layer is 3.6:1.

[0072] The thickness of the iron oxide layer is 4.4 μm, and the thickness of the ferrous oxide layer is 1.2 μm.

[0073] Example 2

[0074] Comparing Example 2 with Example 1, the difference between Example 2 and Example 1 is:

[0075] The end point temperature of the heating was 925°C.

[0076] The hot stamping temperature is 810℃

[0077] Comparative Example 1

[0078] The method of the present application is not adopted, and the process before the improvement is directly adopted.

[0079] Comparative Example 2

[0080] Comparing Comparative Example 2 with Example 1, the difference between Comparative Example 2 and Example 1 is:

[0081] Calculated by mass fraction, the chemical composition of the steel billet includes: C: 0.20%, Mn: 1.46%, Si: 0.23%, Cr: 0.12%, B: 0.0023%, and the balance is Fe and inevitable impurities.

[0082] Comparative Example 3

[0083] Comparing Comparative Example 3 with Example 2, the difference between Comparative Example 3 and Example 2 is:

[0084] The end point temperature of the heating was 945°C.

[0085] Comparative Example 4

[0086] Comparing Comparative Example 4 with Example 1, the difference between Comparative Example 4 and Example 1 is:

[0087] The hot stamping temperature is 890°C.

[0088] Related experiments:

[0089] The uncoated hot stamped steels obtained in Examples 1-2 and Comparative Examples 1-4 were collected and tested for performance. The results are shown in Table 1.

[0090] The thickness of the iron oxide layer and ferrous oxide was measured using a scanning electron microscope (SEM) instrument;

[0091] The ratio of iron oxide to ferrous oxide was determined using an X-ray diffraction analyzer.

[0092] Table 1

[0093]

[0094]

[0095] Table 1 Detailed analysis:

[0096] The ratio of iron oxide to ferrous oxide refers to the ratio of the thickness of iron oxide to ferrous oxide in the surface structure of the prepared stamped steel. When the ratio of iron oxide to ferrous oxide is closer to 3:1 to 4:1, the surface structure is more stable and refined.

[0097] From the data of Examples 1-2, it can be seen that:

[0098] When the chemical composition content and preparation process parameters of the present application are adopted, the ratio of the thickness of the iron oxide layer and the thickness of the ferrous oxide layer can be controlled within an appropriate range, thereby ensuring that an iron oxide scale surface layer with good surface structure morphology and refined structure is obtained.

[0099] From the data of Comparative Examples 1-4, we can see that:

[0100] When the method of the present application is not adopted, or the process parameter range of the present application is not adopted, the ratio of the thickness of the obtained iron oxide layer to the thickness of the ferrous oxide layer is not within an appropriate range, and the obtained iron oxide scale interface will have a high flatness, which is not conducive to the bonding of the iron oxide scale of the surface structure and the steel matrix interface.

[0101] One or more technical solutions in the embodiments of the present application may have at least the following technical effects or advantages:

[0102] (1) The method provided in the embodiment of the present application heats the raw material plate, keeps it warm, and then performs hot stamping. In combination with the composition of the steel plate, it is ensured that the steel plate is fully austenitized during heating and that the iron sheet has good bonding strength during hot stamping, thereby ensuring a reasonable distribution of iron oxide and ferrous oxide in the surface structure of the steel plate.

[0103] (2) The method provided in the embodiment of the present application has good lubricity between the iron oxide scale of the stamped steel obtained and the mold in hot stamping. The iron oxide scale will not easily fall off and accumulate in the mold, nor will it remain on the surface of the part during shot blasting. This solves the problem of the iron oxide scale in the surface structure falling off in the mold and the iron scale remaining on the surface of the steel plate after subsequent shot blasting.

[0104] (3) The method provided in the embodiment of the present application is simple, economical and efficient. It can improve the surface quality of hot-formed steel without increasing the difficulty of production line production and without affecting the performance of the finished product after hot-forming.

[0105] Explanation of the accompanying drawings:

[0106] Figure 2 This is a schematic diagram of a hot stamping steel product provided in Comparative Example 1 of this application, Figure 2 It can be seen that when the method of the present application is not adopted, the surface distribution of the obtained hot stamping steel is scattered and the surface structure is unstable.

[0107] Figure 3 A schematic diagram of the surface structure of the hot stamping steel provided in Comparative Example 2 of the present application;

[0108] Figure 4 This is a schematic diagram of the surface structure of the hot stamping steel provided in Example 1 of the present application.

[0109] Depend on Figure 3 and Figure 4 It can be seen that when the mass fraction of the alloying elements satisfies [Si]+[Cr]<0.4%, the interface of the iron oxide scale in the surface structure has a high flatness, which will cause poor bonding between the iron oxide scale and the steel matrix. When the mass fraction of the alloying elements satisfies [Si]+[Cr]≥0.4%, there is a certain serrated structure between the interface of the iron oxide scale and the steel matrix, so the iron oxide scale formed by the obtained interface flatness has better bonding strength.

[0110] Figure 5 A schematic structural diagram of the surface structure of hot stamping steel provided in Comparative Example 3 of the present application;

[0111] Figure 6 A schematic structural diagram of the surface structure of hot stamping steel provided in Example 2 of the present application;

[0112] Depend on Figure 5 and Figure 6 It can be seen that at different heat treatment temperatures, such as the different heating temperatures provided in Comparative Example 3 and Example 2, the ratios of the thickness of iron oxide to the thickness of ferrous oxide in the resulting iron oxide scale structure are 1.6:1 and 3:1, respectively. This indicates that the heating treatment temperature conditions provided in Example 2 meet the expected ratio of the thickness of iron oxide to the thickness of ferrous oxide.

[0113] Figure 7 A schematic structural diagram of the surface structure of hot stamping steel provided in Comparative Example 4 of the present application;

[0114] Depend on Figure 7 and Figure 4It can be seen that under the conditions of Comparative Example 4 and Example 1, the ratio of the thickness of iron oxide to the thickness of ferrous oxide in the obtained iron oxide scale structure is 1.8:1 and 3.6:1, respectively. Therefore, it is shown that the heating treatment temperature conditions provided in Example 1 meet the expected ratio of the thickness of iron oxide to the thickness of ferrous oxide.

[0115] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or device comprising the element.

[0116] The foregoing description is intended only to provide specific embodiments of the present invention, which will enable those skilled in the art to understand and implement the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not intended to be limited to the embodiments shown herein, but is intended to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing high surface quality uncoated hot stamping steel, characterized in that: The method comprises: Obtaining a steel billet; cold rolling the steel billet raw material to obtain a raw sheet; The raw material sheet is heated, kept warm, hot stamped and shot blasted to obtain a non-coated hot stamped steel with high surface quality; The terminal temperature of the heating is 915°C to 930°C; the temperature of the hot stamping is 810°C to 870°C; The chemical composition of the steel billet includes, by mass fraction, C: 0.18% to 0.35%, Mn: 1.0% to 2.0%, Si: 0.2% to 0.6%, Cr: 0.1% to 0.5%, B: 0.002% to 0.004%, and the balance is Fe and unavoidable impurities, [Si] + [Cr] ≥ 0.4%, wherein [Si] is the mass fraction of Si, and [Cr] is the mass fraction of Cr; The surface structure of the uncoated hot stamping steel includes a layered structure of an iron oxide layer and a ferrous oxide layer, the ratio of the thickness of the iron oxide layer to the thickness of the ferrous oxide layer is 3:1 to 4:1, the thickness of the iron oxide layer is 4 μm to 5 μm, and the thickness of the ferrous oxide layer is 1 μm to 3 μm.

2. The method according to claim 1, characterized in that The insulation time is 240s to 300s.

3. The method according to claim 1, characterized in that The roughness of the raw material plate is 0.7 μm to 1.2 μm.

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