High-strength if steel hot-dip galvanized steel sheet and method for manufacturing the same

Abstract

This invention discloses a high-strength IF steel hot-dip galvanized sheet and its preparation method, belonging to the field of steel manufacturing technology. The mass percentage content of each component of the steel sheet includes: C: 0.003%~0.012%; Mn: 0.3%~1.0%; Si: ≤0.05%; P: ≤0.03%; S: ≤0.005%; Als: 0.02%-0.07%; N: ≤0.003%; B: 0.0003%~0.0008%; Ti: 0.02%~0.09%; wherein [Ti]-4*[C]-3.43*[N]≥0.01%, [P] / [B]≤90, and the balance is Fe. This invention also discloses a production method for the hot-dip galvanized steel sheet. By controlling the hot rolling, cold rolling, and continuous hot-dip galvanizing annealing processes, and by adding Ti and C microalloying elements to form TiC, fine grain strengthening and precipitation strengthening are achieved to obtain the mechanical properties of high-strength IF steel. The reduction of P and Si content and the reasonable P / B ratio improve the secondary processing embrittlement performance. At the same time, by controlling the dew point, the Mn element is changed from external oxidation to internal oxidation, which improves the surface quality of the coating. As a result, the high-strength IF steel hot-dip galvanized steel sheet has excellent mechanical properties, secondary processing embrittlement, and surface quality.

Description

Technical Field

[0001] This invention relates to the field of steel manufacturing technology, and in particular to a high-strength IF steel hot-dip galvanized sheet and its preparation method. Background Technology

[0002] Interstitial-free (IF) steel is a steel without interstitial atoms. It is produced by using vacuum degassing technology to control the content of C and N solid-solution elements to extremely low levels, and by adding a certain amount of alloying elements such as Ti or Nb to fix the remaining solid-solution atoms in the steel. IF steel has good formability and is produced using continuous annealing or continuous hot-dip galvanizing methods. It is widely used for complex formed parts in automobiles and home appliances.

[0003] While IF steel has excellent formability, it suffers from low strength, poor dent resistance, and weak deformation resistance. To address these issues, strengthening elements such as Si, Mn, and P are added to IF steel to improve its strength, resulting in high-strength IF steel, which has gained widespread application.

[0004] While increasing the strength of IF steel by adding Si, Mn, and P elements, it also brings adverse effects. On one hand, Si, Mn, and P are all easily oxidized elements, readily undergoing oxidation reactions during hot rolling and annealing. The accumulation of oxides on the steel surface affects the continuity and uniformity of the coating. On the other hand, IF steel is pure and has low grain boundary bonding strength, making it prone to grain boundary fracture during low-temperature, high-speed deformation. This characteristic is called secondary processing brittleness, and phosphorus segregation at grain boundaries exacerbates this phenomenon. When used in cold regions, this type of steel is highly susceptible to brittle fracture, potentially posing safety hazards during winter use in northern China and North America. Secondary processing brittleness is primarily measured by the secondary processing embrittlement temperature; a lower embrittlement temperature indicates better resistance to low-temperature brittleness. Summary of the Invention

[0005] This invention proposes a high-strength IF steel hot-dip galvanized sheet and its preparation method. By controlling the hot rolling process, cold rolling process, hot-dip galvanizing process, and finishing process, a method is obtained that is beneficial to improving the strength, secondary processing brittleness, and surface quality of the high-strength IF steel hot-dip galvanized sheet. The galvanized steel sheet invented has a yield strength greater than 230 MPa, a secondary processing embrittlement temperature ≤ -60℃, and good surface quality. It solves or partially solves the technical problems of surface quality and poor secondary processing brittleness of high-strength IF steel in the prior art.

[0006] In a first aspect, the present invention provides a high-strength IF steel hot-dip galvanized sheet, wherein the mass percentage of each chemical component of the high-strength IF steel hot-dip galvanized sheet is as follows: C: 0.003%~0.012%; Mn: 0.3%~1.0%; Si: ≤0.05%; P: ≤0.03%; S: ≤0.005%; Als: 0.02%~0.07%; N: ≤0.003%; B: 0.0003%~0.0008%; Ti: 0.02%~0.09%;

[0007] Where [Ti]-4*[C]-3.43*[N]≥0.01%, [P] / [B]≤90, and the balance is Fe;

[0008] [Ti] represents the mass fraction of Ti, [C] represents the mass fraction of C, [N] represents the mass fraction of N, [P] represents the mass fraction of P, and [B] represents the mass fraction of B.

[0009] Furthermore, the average diameter of TiC particles in the high-strength IF steel hot-dip galvanized steel sheet is 4nm to 10nm, and the average grain size of the obtained steel grade is 6μm to 10μm.

[0010] Secondly, the present invention also provides a method for preparing high-strength IF steel hot-dip galvanized steel sheet, characterized by comprising the following steps:

[0011] A cast billet is prepared, wherein the chemical composition of the cast billet by weight percentage is: C: 0.003%–0.012%; Mn: 0.3%–1.0%; Si: ≤0.05%; P: ≤0.03%; S: ≤0.005%; Als: 0.02%–0.07%; N: ≤0.003%; B: 0.0003%–0.0008%; Ti: 0.02%–0.09%;

[0012] The billet is subjected to hot rolling, coiling, pickling, cold rolling, annealing, continuous hot-dip galvanizing and finishing processes in sequence to obtain high-strength IF steel hot-dip galvanized steel sheet.

[0013] Furthermore, during the hot continuous rolling process, the heating temperature is 1200℃~1300℃.

[0014] Furthermore, the initial rolling temperature is 1050℃~1150℃, and the final rolling temperature is 880℃~950℃.

[0015] Furthermore, the winding temperature is 400℃~650℃.

[0016] Furthermore, during the cold continuous rolling process, the total reduction rate is 60% to 85%.

[0017] Furthermore, the heating section temperature of the continuous hot-dip galvanizing process is 720℃~800℃, the soaking section temperature is 720℃~800℃, the slow cooling section outlet temperature is 600℃~680℃, the rapid cooling section outlet temperature is 460±10℃, and the zinc pot inlet temperature is 460±5℃.

[0018] Furthermore, the dew point of the atmosphere inside the furnace is controlled at -40℃ to -10℃.

[0019] Furthermore, a constant elongation control mode is adopted during the finishing process.

[0020] Furthermore, the elongation rate is controlled between 0.3% and 2.0%.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] This invention provides a high-strength IF steel hot-dip galvanized sheet and its preparation method. By adding Ti and C microalloying elements to form TiC, fine grain strengthening and precipitation strengthening are achieved to obtain the mechanical properties of high-strength IF steel. The reduction of P and Si content and the reasonable P / B ratio improve the secondary processing embrittlement performance. At the same time, by controlling the dew point, the Mn element is changed from external oxidation to internal oxidation, which improves the surface quality of the coating. As a result, the high-strength IF steel hot-dip galvanized sheet has excellent mechanical properties, secondary processing embrittlement and surface quality. Attached Figure Description

[0023] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 Scanning electron microscope (SEM) images of the high-strength IF steel hot-dip galvanized sheet provided in the embodiments of the present invention.

[0026] Figure 2 Photographs of TiC precipitated particles in high-strength IF steel hot-dip galvanized steel sheets provided in embodiments of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0029] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0030] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0031] In a first aspect, the present invention provides a high-strength IF steel hot-dip galvanized sheet, wherein the mass percentage content of each component includes: C: 0.003~0.012%, Mn: 0.3%~1.0%, Si: ≤0.05%, P: ≤0.03%, S: ≤0.005%, Als: 0.02%~0.07%, N: ≤0.003%, B: 0.0003%~0.0008%, Ti: 0.02%~0.09%, wherein [Ti]-4*[C]-3.43*[N]≥0.01%, [P] / [B]≤90, and the balance is Fe; wherein [Ti] represents the mass fraction of Ti, [C] represents the mass fraction of C, [N] represents the mass fraction of N, [P] represents the mass fraction of P, and [B] represents the mass fraction of B.

[0032] In some embodiments, the average diameter of TiC particles in the high-strength IF steel hot-dip galvanized steel sheet is 4nm to 10nm, and the average grain size of the obtained steel grade is 6μm to 10μm.

[0033] The design principles of the chemical element ratios in this technical solution are explained below:

[0034] Carbon (C) is fundamental to improving steel strength. It can directly strengthen the matrix through solid solution strengthening, or it can form precipitates with microalloying elements in the steel to enhance strength. However, as the carbon content increases, the r-value decreases, affecting formability. In this invention, the C content is selected between 0.003% and 0.012%, which is higher than that of conventional IF steel. This allows it to form fine TiC particles with Ti in the steel, improving the precipitation strengthening effect and achieving the strength level of high-strength IF steel.

[0035] Manganese (Mn) not only reacts with sulfur (S) to form manganese sulfide, eliminating the brittleness of S, but the addition of Mn can also refine the grain size, improving the strength and toughness of the steel. Furthermore, Mn is easily oxidized, affecting the surface quality of the coating. Therefore, in this invention, the Mn content is selected at 0.3% to 1.0%, ensuring that the surface quality of the coating is not affected while eliminating the brittleness of S and improving the strength and toughness of the steel.

[0036] Silicon (Si) is one of the conventional steel strengthening factors. An appropriate Si content can improve the strength of steel. However, Si is prone to forming oxides on the surface of steel plates, which affects the surface quality of the coating. Therefore, this invention limits the Si content to a very low range: Si: ≤0.05%.

[0037] For traditional high-strength IF steel, phosphorus (P) is an important solid solution strengthening element, and its content is generally controlled above 0.03%. However, because P is prone to oxidation, it affects the plating properties of the steel sheet. In addition, P is also an important element that exacerbates the deterioration of secondary processing brittleness, and it needs to be strictly controlled. This invention controls the P content to below 0.03% by combining it with other elements.

[0038] Generally, sulfur (S) is an impurity element in steel, affecting its plasticity. Furthermore, S reacts with Ti and C to form TiS or Ti4S4C2, reducing the total amount of TiC precipitated, thus affecting the steel's strength. Therefore, this invention strictly controls the S content: S ≤ 0.005%.

[0039] Aluminum is a strong deoxidizer and can inhibit the formation of other oxides. The aluminum (Als) in this invention is selected in the range of Als: 0.02% to 0.07%.

[0040] Nitrogen (N) exists as a residual element in steel, which affects the steel's plasticity. Lower content is better. Furthermore, N consumes Ti content, thereby reducing the amount of Ti that combines with C and decreasing the amount of TiC formed, thus affecting strength. In this invention, the nitrogen (N) content is selected to be ≤0.003%.

[0041] Boron (B) competes with phosphorus (P) for precipitation at grain boundaries, which can reduce the brittleness of the finished steel. Too little boron (B) will not lower the embrittlement temperature, but too much boron (B) will also impair the formability of the steel. In this invention, the boron (B) content is selected at 0.0003% to 0.0008%, and the ratio of [P] to [B] is limited to ≤90, where [P] represents the mass fraction of P and [B] represents the mass fraction of B. Under these conditions, the overall performance of the prepared steel is optimal.

[0042] Titanium (Ti) can combine with carbon (C) in steel to form TiC precipitates with a size within 10 nm, exhibiting a significant precipitation strengthening effect and thus improving the strength of the steel. Furthermore, interstitial C and N atoms have a significant impact on formability, significantly reducing the material's r-value. Sufficient Ti is essential to ensure the steel's formability and improve its strength. In this invention, the titanium (Ti) content is selected to be 0.02%–0.09%, and the effective Ti* content must satisfy the following relationship: [Ti] - 4*[C] - 3.43*[N] ≥ 0.01%, where [Ti] represents the mass fraction of Ti, [C] represents the mass fraction of C, and [N] represents the mass fraction of N.

[0043] Secondly, the present invention also provides a method for manufacturing this type of automotive steel, the steps of which are as follows:

[0044] A cast billet is prepared, wherein the chemical composition of the cast billet by weight percentage is: C: 0.003%–0.012%; Mn: 0.3%–1.0%; Si: ≤0.05%; P: ≤0.03%; S: ≤0.005%; Als: 0.02%–0.07%; N: ≤0.003%; B: 0.0003–0.0008%; Ti: 0.02%–0.09%;

[0045] The billet is subjected to heating, hot rolling, coiling, pickling, cold rolling, annealing, continuous hot-dip galvanizing and finishing processes in sequence to obtain high-strength IF steel hot-dip galvanized steel sheet.

[0046] In some implementations, the billet heating temperature is controlled at 1200℃ to 1300℃ to ensure full austenitization of the billet and complete dissolution of coarse precipitates in the billet, so that they can reprecipitate during subsequent rolling and cooling processes, thereby improving the strength of the steel. At the same time, this heating temperature also prevents the billet from overheating and inhibits excessive grain growth.

[0047] In some embodiments, during the hot continuous rolling process, the initial rolling temperature is 1050℃~1150℃, the final rolling temperature is 880℃~950℃, and the coiling temperature is controlled at 400℃~650℃. These three key temperature parameters are mainly to ensure stable rolling and refine the grains. The coiling temperature also controls the size of the TiC precipitates. The higher the coiling temperature, the larger the TiC particles, which reduces the precipitation strengthening effect and is not conducive to improving strength. Although low-temperature coiling will inhibit the precipitation of TiC, it can precipitate during the subsequent annealing process.

[0048] In some embodiments, during the cold rolling process, the total reduction rate is 60% to 85%. After cold rolling, sufficient distortion energy is generated in the steel to produce work hardening, providing driving energy for subsequent continuous annealing and recrystallization. At the same time, within this cold rolling reduction rate range, the steel plate has the highest r-value and the best performance.

[0049] In some embodiments, the heating temperature of the annealing and continuous hot-dip galvanizing process is 720℃~800℃, the soaking temperature is 720℃~800℃, the outlet temperature of the slow cooling section is 600℃~680℃, the outlet temperature of the rapid cooling section is 460±10℃, and the temperature of the zinc pot is 460±5℃. The entire annealing and continuous hot-dip galvanizing process ensures complete recrystallization and texture development, thereby achieving a fine and uniform equiaxed grain structure and good formability, while maintaining a large number of fine and dispersed second-phase particles to obtain high strength. Finally, the average diameter of TiC particles obtained in the IF steel is between 4nm and 10nm, and the average grain size of the obtained steel grade is 6μm~10μm.

[0050] In some implementations, to improve the easily oxidized properties of Si, Mn, and P elements, the water vapor content in the hot-dip galvanizing furnace is increased to change the external oxidation of oxides accumulating on the steel plate surface into internal oxidation, thereby improving the surface quality of the coating; this is known as dew point control. However, in traditional high-strength IF steel, the dew point temperatures for the internal oxidation reactions of Si, Mn, and P are different, making it impossible to simply control the dew point to induce an internal oxidation reaction. This invention retains only a certain amount of Mn element, thus improving surface quality by controlling the dew point of the furnace atmosphere between -40°C and -10°C.

[0051] In some embodiments, the finishing process employs a constant elongation control mode, with the elongation controlled between 0.3% and 2.0%.

[0052] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0053] Table 1 shows the chemical composition of Examples 1-4 and Comparative Examples 1-3; Table 2 shows the corresponding production method parameters; Table 3 shows the corresponding mechanical properties and secondary processing embrittlement temperatures. The metallographic scanning electron microscope images of the high-strength IF steel hot-dip galvanized steel sheet provided in Example 1 are attached. Figure 1 As shown in the image, the TiC precipitated particles in the steel plate are as follows: Figure 2 As shown.

[0054] Table 1. Chemical composition (mass fraction, %) of the examples and comparative examples

[0055] C Si Mn P S Als N B Ti [P] / [B] [Ti]-4*[C]-3.43*[N] Example 1 0.003 0.005 0.42 0.015 0.002 0.035 0.002 0.0005 0.03 30 0.011 Example 2 0.006 0.01 0.36 0.024 0.005 0.025 0.002 0.0004 0.05 60 0.019 Example 3 0.008 0.006 0.53 0.014 0.004 0.045 0.003 0.0006 0.07 23 0.028 Example 4 0.01 0.008 0.63 0.014 0.003 0.05 0.0025 0.0008 0.07 17.5 0.021 Comparative Example 1 0.005 0.01 0.15 0.050 0.012 0.025 0.005 0.0004 0.06 125 0.023 Comparative Example 2 0.003 0.04 0.18 0.072 0.012 0.035 0.006 0.0007 0.07 102.8 0.037 Comparative Example 3 0.003 0.004 0.18 0.012 0.012 0.035 0.006 0 0.02 - -0.013

[0056] Table 2. Main parameters of the embodiments and comparative examples

[0057]

[0058] Table 3 Mechanical properties, brittle-to-brittle transition temperature after secondary processing, and coating quality of the examples and comparative examples

[0059] Serial Number Yield strength / MPa Tensile strength / MPa Elongation / % r value Secondary processing embrittlement temperature / ℃ Surface quality Example 1 246 379 37.0 1.85 -60 good Example 2 248 388 37.5 2.02 -60 good Example 3 235 386 38.5 1.93 -60 good Example 4 256 398 35.0 1.87 -60 good Comparative Example 1 242 388 37 1.83 -20 The zinc layer is uneven. Comparative Example 2 246 393 34.5 1.78 -25 The zinc layer is uneven. Comparative Example 3 145 305 45 2.65 -20 good

[0060] In summary, the high-strength IF hot-dip galvanized steel sheet provided in this embodiment of the invention strictly controls the chemical elements in the steel, maintaining a certain C content. This trace amount of C can combine with Ti to form fine precipitated second-phase particles, increasing strength. It can also pin grain boundaries, increasing the bonding force between grain boundaries, which is beneficial for improving the steel's secondary processing brittleness. Furthermore, the embodiment strictly controls the content of brittle elements P and S, and by introducing B to react with brittle elements, the effects of harmful elements are offset, thereby improving secondary processing brittleness. Additionally, only a small amount of Mn is retained, enabling the resolution of surface problems through internal oxidation by improving the dew point in the furnace area.

[0061] The production method for high-strength IF steel hot-dip galvanized sheet provided by this invention balances the temperatures of each stage of hot rolling and each stage of annealing. This method appropriately reduces the coiling temperature during hot rolling, preventing excessively coarse grains and refining the grains to form fine TiC precipitates. Maintaining a suitable homogenization temperature during the annealing process achieves synergistic control of formability and strength. Furthermore, by controlling the dew point in the furnace zone, internal oxidation of Mn elements is induced to improve surface quality.

[0062] The high-strength IF steel hot-dip galvanized steel sheet and its production method provided in this invention are optimized by controlling the chemical composition, hot rolling, cold rolling and continuous hot-dip galvanizing process. The produced steel sheet has the characteristics of high formability and high resistance to secondary processing brittleness, which improves the surface quality of the product and can bring considerable economic benefits.

[0063] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, 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 spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A high-strength IF steel hot-dip galvanized sheet, characterized in that, The mass percentage of each chemical component in the steel plate is as follows: C: 0.006%~0.012%; Mn: 0.36%~1.0%; Si: ≤0.05%; P：≤0.03%； S: ≤0.005%; Als: 0.02%~0.07%; N: ≤0.003%; B: 0.0003%~0.0008%; Ti: 0.02%~0.09%; Where [Ti]-4*[C]-3.43*[N]≥0.01%, [P] / [B]≤90, and the balance is Fe; The average diameter of TiC particles in the high-strength IF steel hot-dip galvanized steel sheet is 4nm-10nm, and the average grain size of the obtained steel grade is 6μm~10μm; the yield strength is greater than 230MPa, and the secondary processing embrittlement temperature is ≤-60℃; The preparation method of the high-strength IF steel hot-dip galvanized steel sheet includes the following steps: Preparation of casting billets, The billet is subjected to hot rolling, coiling, pickling, cold rolling, annealing, continuous hot-dip galvanizing and finishing processes in sequence to obtain high-strength IF steel hot-dip galvanized steel sheet. The winding temperature is 400℃~650℃; the dew point of the atmosphere inside the furnace is controlled at -40℃~-10℃.

2. A method for preparing high-strength IF steel hot-dip galvanized sheet as described in claim 1, characterized in that, Includes the following steps: Preparation of casting billets, The billet is subjected to hot rolling, coiling, pickling, cold rolling, annealing, continuous hot-dip galvanizing and finishing processes in sequence to obtain high-strength IF steel hot-dip galvanized steel sheet. The winding temperature is 400℃~650℃; the dew point of the atmosphere inside the furnace is controlled at -40℃~-10℃.

3. The preparation method according to claim 2, characterized in that, During the hot continuous rolling process, the heating temperature is 1200℃~1300℃.

4. The preparation method according to claim 2, characterized in that, During the hot continuous rolling process, the initial rolling temperature is 1050~1150℃, and the final rolling temperature is 880℃~950℃.

5. The preparation method according to claim 2, characterized in that, During the cold continuous rolling process, the total reduction rate is 60% to 85%.

6. The preparation method according to claim 2, characterized in that, The continuous hot-dip galvanizing process has a heating section temperature of 720℃~800℃, a soaking section temperature of 720℃~800℃, a slow cooling section outlet temperature of 600℃~680℃, a rapid cooling section outlet temperature of 450℃~470℃, and an entry temperature of 455℃~465℃ into the zinc pot.

7. The preparation method according to claim 2, characterized in that, During the finishing process, a constant elongation control mode is adopted, and the elongation is controlled between 0.3% and 2.0%.

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