A method of rolling a steel plate

Abstract

This invention provides a method for rolling steel plates, belonging to the field of medium and heavy plate manufacturing technology. The method includes: first heating a cast billet, followed by cooling to room temperature; second heating the cooled billet under set process parameters, including a heating temperature of 895℃~905℃ and a heating time of 600min~800min; and rolling the second-heated billet in a completely non-recrystallized zone at a set rolling temperature, followed by cooling. The rolling method designed in this application achieves high strength, good low-temperature toughness, excellent resistance to lamellar tearing, and minimal performance differences in the thickness direction for extra-thick plates. The yield strength difference at different locations of the thickness is ≤50MPa, the tensile strength difference is ≤30MPa, the impact value is ≥300J, and the reduction of area in the thickness direction is ≥50%. This solves the problem of poor performance in extra-thick steel plates prepared by current traditional rolling methods.

Description

Technical Field

[0001] This application relates to the field of medium and heavy plate manufacturing technology, and in particular to a method for rolling steel plates. Background Technology

[0002] Extra-thick plates typically refer to steel plates with a thickness greater than 100mm, and are widely used in bridges, offshore platforms, wind power, and other fields. With the increasing emphasis on high-quality and green development in manufacturing, the applications of extra-thick plates are becoming increasingly critical, thus placing higher demands on their performance. However, the manufacturing challenge of extra-thick plates lies in their large thickness, low compression ratio, and the difficulty in achieving deep rolling deformation within the steel core. This results in poor core toughness, low strength, and poor resistance to lamellar tearing.

[0003] Rolling is crucial for improving the performance of extra-thick plates. Traditional rolling methods cannot address the poor core properties and resistance to lamellar tearing in extra-thick plates, while also exhibiting significant differences in properties along the thickness direction. Furthermore, the prolonged heating process in traditional rolling methods leads to microstructure growth, negatively impacting the performance and rolling efficiency of extra-thick plates. Therefore, researching the production of high-performance extra-thick plates using continuously cast billets and improving their performance and rolling efficiency are urgent practical problems that need to be solved. Summary of the Invention

[0004] This application provides a method for rolling steel plates to solve the technical problem that the mechanical properties of extra-thick steel plates prepared by traditional rolling methods are poor due to insufficient recrystallization.

[0005] In a first aspect, this application provides a method for rolling a steel plate, wherein the thickness of the steel plate is greater than 100 mm, the method comprising:

[0006] The billet is first heated and then cooled to room temperature;

[0007] The cooled billet is then subjected to a second stage of heating under set process parameters, including a heating temperature of 895℃~905℃ and a heating time of 600min~800min.

[0008] The billet after the second heating section is rolled in a completely non-recrystallized zone at a set rolling temperature, and then cooled.

[0009] Optionally, the set rolling temperature includes: an initial rolling temperature of 860℃~880℃ and a final rolling temperature of 800℃~820℃.

[0010] Optionally, the temperature of the first stage of heating is 1160℃~1180℃, and the heating time of the first stage is 360min~500min.

[0011] Optionally, the final cooling temperature is 530℃~600℃, and the cooling rate is 7℃ / s~15℃ / s.

[0012] Optionally, the chemical composition of the cast billet includes: C, Si, Mn, Alt, Nb, Ti, Ni, P, S, and Fe; by mass fraction,

[0013] The content of C is 0.025%–0.035%, the content of Si is 0.40%–0.50%, the content of Mn is 1.45%–1.55%, the content of Alt is 0.02%–0.06%, the content of Nb is 0.05%–0.06%, the content of Ti is 0.01%–0.03%, the content of Ni is 0.70%–0.80%, the content of P is <0.008%, and the content of S is <0.003%.

[0014] And it satisfies the following relationship: 0.70≤[C]+[Si] / 5+[Mn] / 3+[Ni] / 7+2[P]≤0.75,

[0015] In the formula, [C] represents the mass fraction of C, [Si] represents the mass fraction of Si, [Mn] represents the mass fraction of Mn, [Ni] represents the mass fraction of Ni, and [P] represents the mass fraction of P.

[0016] Optionally, the width of the billet is 2000mm to 2400mm, the thickness of the billet is 400mm, and the center segregation of the billet is less than 0.5 for Class C.

[0017] Secondly, this application provides a steel plate, which is obtained by rolling the steel plate by the method described in any embodiment of the first aspect;

[0018] The microstructure of the extra-thick steel plate includes ferrite and bainite; wherein,

[0019] The proportion of ferrite is 80% to 95%, and the proportion of bainite is 5% to 20%.

[0020] Optionally, the steel plate at different locations with varying thicknesses shall satisfy at least one of the following properties: yield strength difference ≤ 50 MPa, tensile strength difference ≤ 30 MPa, impact value ≥ 300 J, and reduction of area in the thickness direction ≥ 50%.

[0021] The technical solutions provided in this application have the following advantages compared with the prior art:

[0022] This application employs a staged pre-rolling heating process. The first stage, high-temperature heating, eliminates columnar crystals and microstructure segregation in continuous casting, ensuring full solid solution of alloying elements. After the billet exits the furnace and cools to room temperature, the microstructure is refined through phase transformation. The second stage uses a low-temperature heating process to ensure complete austenitization of the billet, while simultaneously obtaining fine austenite. This avoids the long waiting period before recrystallization in traditional two-stage rolling, improving rolling efficiency and resulting in more uniform temperature across the thickness section.

[0023] This application increases the cumulative deformation through one-stage rolling in the completely non-recrystallized region, providing more nucleation points for phase deformation nuclei, refining the microstructure, promoting ferrite phase transformation, suppressing the transformation of bulk upper bainite and granular bainite phases, and improving the low-temperature toughness of the steel plate.

[0024] The rolling method designed in this application enables ultra-thick plates to simultaneously meet the requirements of high strength, good low-temperature toughness, excellent resistance to lamellar tearing, and small differences in properties in the thickness direction. The yield strength difference at different locations of the thickness is ≤50MPa, the tensile strength difference is ≤30MPa, the impact value is ≥300J, and the reduction of area in the thickness direction is ≥50%. Attached Figure Description

[0025] 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.

[0026] 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.

[0027] Figure 1 A schematic flowchart illustrating the method for rolling steel plates provided in this application embodiment;

[0028] Figure 2 The surface microstructure of the extra-thick steel plate provided in Embodiment 1 of this application;

[0029] Figure 3 The microstructure of the extra-thick steel plate provided in Embodiment 1 of this application is 1 / 4 the thickness.

[0030] Figure 4 The microstructure of the extra-thick steel plate provided in Embodiment 1 of this application is 1 / 2 the thickness.

[0031] Figure 5 The surface microstructure of the extra-thick steel plate provided in Embodiment 2 of this application;

[0032] Figure 6 The microstructure of the extra-thick steel plate provided in Embodiment 2 of this application is 1 / 4 the thickness.

[0033] Figure 7 The microstructure is half the thickness of the extra-thick steel plate provided in Embodiment 2 of this application. Detailed Implementation

[0034] 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.

[0035] 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.

[0036] In the description of this application, the terms "comprising," "including," etc., 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, 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" 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.

[0037] 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.

[0038] In a first aspect, this application provides a method for rolling a steel plate, wherein the thickness of the steel plate is greater than 100 mm, the method comprising:

[0039] S1. The billet is heated in the first stage and then cooled to room temperature;

[0040] In some embodiments, before step S1, the process includes molten iron pretreatment, smelting, refining, and continuous casting to obtain a cast billet.

[0041] In addition, the smelting process employs KR desulfurization and converter smelting, combined top and bottom blowing, and vacuum treatment using LF and VD furnaces to reduce the content of harmful gases such as O and H, as well as P and S.

[0042] In some embodiments, the specified chemical composition includes: C, Si, Mn, Alt, Nb, Ti, Ni, P, S, and Fe; by mass fraction,

[0043] The content of C is 0.025%–0.035%, the content of Si is 0.40%–0.50%, the content of Mn is 1.45%–1.55%, the content of Alt is 0.02%–0.06%, the content of Nb is 0.05%–0.06%, the content of Ti is 0.01%–0.03%, the content of Ni is 0.70%–0.80%, the content of P is <0.008%, and the content of S is <0.003%.

[0044] And it satisfies the following relationship: 0.70≤[C]+[Si] / 5+[Mn] / 3+[Ni] / 7+2[P]≤0.75,

[0045] In the formula, [C] represents the mass fraction of C, [Si] represents the mass fraction of Si, [Mn] represents the mass fraction of Mn, [Ni] represents the mass fraction of Ni, and [P] represents the mass fraction of P.

[0046] The C content can be 0.025%, 0.027%, 0.029%, 0.031%, 0.033%, 0.035%, etc.; the Si content can be 0.40%, 0.42%, 0.44%, 0.46%, 0.48%, 0.50%, etc.; the Mn content can be 1.45%, 1.47%, 1.49%, 1.51%, 1.53%, 1.55%, etc.; and the Alt content can be 0.02%, 0.03%, 0.04%, 0.05%, 0.0%, etc. The content of Nb can be 0.05%, 0.055%, 0.06%, etc.; the content of Ti can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.; the content of Ni can be 0.70%, 0.72%, 0.74%, 0.76%, 0.78%, 0.80%, etc.; the content of P can be 0.001%, 0.003%, 0.005%, 0.007%, etc.; and the content of S can be 0.001%, 0.002%, etc.

[0047] The positive effect of controlling [C]+[Si] / 5+[Mn] / 3+[Ni] / 7+2[P]≤0.75 is that it simultaneously achieves high strength and good low-temperature toughness in extra-thick steel plates. The value of [C]+[Si] / 5+[Mn] / 3+[Ni] / 7+2[P] can be 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, etc.

[0048] In some embodiments, the width of the billet is 2000mm to 2400mm, the thickness of the billet is 400mm, and the center segregation of the billet is less than 0.5 for Class C.

[0049] The width of the casting billet can be 2000mm, 2100mm, 2200mm, 2300mm, 2400mm, etc.

[0050] In some embodiments, the temperature of the first stage of heating is 1160°C to 1180°C, and the heating time of the first stage is 360 min to 500 min.

[0051] The positive effects of controlling the first-stage heating temperature to 1160℃~1180℃ and the first-stage heating time to 360min~500min are as follows: High-temperature heating allows for complete solid solution of alloying elements and eliminates columnar crystals and microstructure segregation in continuous casting. Excessive heating temperature leads to austenite growth, affecting the final properties; insufficient heating temperature prevents complete solid solution of alloying elements, reducing strength. Natural cooling of the billet to room temperature in air allows austenite to transform into ferrite and pearlite, resulting in a refined and homogenized microstructure. The first-stage heating temperature can be 1160℃, 1165℃, 1170℃, 1175℃, 1180℃, etc., and the heating time can be 360min, 370min, 380min, 390min, 400min, 410min, 420min, 430min, 440min, 450min, 460min, 470min, 480min, 490min, 500min, etc.

[0052] S2. The cooled billet is heated in a second stage under set process parameters, including: heating temperature of 895℃~905℃ and heating time of 600min~800min.

[0053] The positive effects of controlling the second-stage heating temperature to 895℃~905℃ and the second-stage heating time to 600min~800min are: complete austenitization of the steel billet, and further refinement and homogenization of the microstructure. Excessive heating temperature leads to grain coarsening and also results in excessively high final rolling temperatures, affecting performance; insufficient heating temperature results in incomplete austenitization of the microstructure and also results in excessively low final rolling temperatures, affecting performance and sheet shape. The second-stage heating temperature can be 895℃, 897℃, 899℃, 901℃, 903℃, 905℃, etc., and the heating time can be 600min, 620min, 640min, 660min, 680min, 700min, 720min, 740min, 760min, 780min, 800min, etc.

[0054] S3. The billet after the second heating section is rolled in the completely non-recrystallized zone at a set rolling temperature, and then cooled.

[0055] In some embodiments, the set rolling temperature includes: an initial rolling temperature of 860°C to 880°C and a final rolling temperature of 800°C to 820°C.

[0056] The positive effects of controlling the initial rolling temperature to 860℃~880℃ and the final rolling temperature to 800℃~820℃ are as follows: Rolling in the completely non-recrystallized zone increases the cumulative deformation, provides more nucleation points for phase deformation nuclei, refines the microstructure, promotes ferrite phase transformation, inhibits the transformation of bulk upper bainite and granular bainite, and improves the low-temperature toughness of the steel plate. The initial rolling temperature can be 860℃, 865℃, 870℃, 875℃, 880℃, etc., and the final rolling temperature can be 800℃, 805℃, 810℃, 815℃, 820℃, etc.

[0057] In some embodiments, the cooling is achieved by rapidly introducing laminar flow cooling into the steel plate for water cooling.

[0058] In some embodiments, the final cooling temperature is 530°C to 600°C, and the cooling rate is 7°C / s to 15°C / s.

[0059] The positive effects of controlling the final cooling temperature to 530℃~600℃ and the cooling rate to 7℃ / s~15℃ / s are as follows: Without water cooling or with an excessively high final cooling temperature, the microstructure becomes coarse, affecting the final properties; with an excessively low final cooling temperature, the proportion of bainite is high, affecting low-temperature toughness. The final cooling temperature after rolling can be 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, etc., and the cooling rate can be 7℃ / s, 8℃ / s, 9℃ / s, 10℃ / s, 11℃ / s, 12℃ / s, 13℃ / s, 14℃ / s, 15℃ / s, etc.

[0060] Compared with the traditional two-stage rolling process, the rolling method of this application adds a heating process and a cooling process. Both the heating and cooling processes involve phase transformation, which can significantly refine the microstructure. This avoids the long waiting period before recrystallization in the traditional two-stage rolling process, improves rolling efficiency, and results in more uniform temperature across the thickness section.

[0061] Traditional two-stage rolling: In the first stage, rolling in the recrystallization zone of extra-thick plates results in insufficient core deformation to meet recrystallization requirements, leading to poor core performance and significant differences in microstructure across the thickness section, resulting in inhomogeneous properties. The second stage, before recrystallization, requires a prolonged heating period, causing grain growth and impacting rolling efficiency. This prolonged heating leads to large temperature differences across the thickness section, and because the first stage consumes a certain amount of billet thickness, the cumulative deformation in the non-recrystallization zone is small, failing to provide sufficient nucleation sites for phase transformation, resulting in a coarse steel microstructure and poor performance. The rolling method proposed in this application solves the problem of poor performance in ultra-wide and extra-thick plates caused by insufficient recrystallization due to mill capacity limitations.

[0062] Secondly, this application provides a steel plate, which is obtained by rolling the steel plate by the method described in any embodiment of the first aspect;

[0063] The microstructure of the extra-thick steel plate includes ferrite and bainite; wherein,

[0064] The proportion of ferrite is 80% to 95%, and the proportion of bainite is 5% to 20%.

[0065] The positive effects of controlling the ferrite proportion to 80%–95% and the bainite proportion to 5%–20% include: refining the microstructure through improved rolling methods, promoting ferrite phase transformation, suppressing the transformation of bulk upper bainite and granular bainite, and improving the low-temperature toughness of the steel plate. The ferrite proportion can be 80%, 85%, 90%, 95%, etc., and the bainite proportion can be 5%, 10%, 15%, 20%, etc.

[0066] In some embodiments, the steel plate at different locations of its thickness satisfies at least one of the following properties: yield strength difference ≤ 50 MPa, tensile strength difference ≤ 30 MPa, impact value ≥ 300 J, and reduction of area in the thickness direction ≥ 50%.

[0067] Under the process conditions designed in this application, an extra-thick plate simultaneously achieves high strength, good low-temperature toughness, excellent resistance to lamellar tearing, and small differences in properties along the thickness direction. The yield strength difference can be 30MPa, 35MPa, 40MPa, 45MPa, 50MPa, etc.; the tensile strength difference can be 10MPa, 15MPa, 20MPa, 25MPa, 30MPa, etc.; the impact value can be 300J, 325J, 350J, 375J, 400J, etc.; and the reduction of area along the thickness direction can be 50%, 55%, 60%, 65%, 70%, etc.

[0068] The steel plate is made based on the above-mentioned method for preparing extra-thick steel plates. The specific steps of the preparation method can be referred to the above embodiments. Since the steel plate adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0069] 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.

[0070] The molten steel of Examples 1 and 2 and Comparative Examples 1 and 2 was prepared and cast into slabs. The chemical composition of the slabs was as follows: C content was 0.030%, Si content was 0.45%, Mn content was 1.51%, Alt content was 0.04%, Nb content was 0.056%, Ti content was 0.02%, Ni content was 0.75%, P content was 0.003%, S content was 0.001%, and [C]+[Si] / 5+[Mn] / 3+[Ni] / 7+2[P]=0.74.

[0071] A method for rolling steel plates, the method comprising:

[0072] S11. The billet is heated in the first stage and then cooled to room temperature;

[0073] S21. The cooled billet is subjected to a second stage of heating under set process parameters;

[0074] S31. The billet after the second heating section is rolled in the completely non-recrystallized zone at a set rolling temperature, and then cooled; please refer to Table 1 for specific rolling process parameters.

[0075] Table 1 Process parameters for rolled steel plates

[0076]

[0077]

[0078] The performance of each embodiment and comparative example is shown in Table 2.

[0079] Table 2 shows the properties of the extra-thick steel plates obtained in the examples and comparative examples.

[0080]

[0081] Table 2 shows that in Examples 1-2, extra-thick plates simultaneously achieved high strength, good low-temperature toughness, excellent resistance to lamellar tearing, and minimal differences in properties along the thickness direction. The yield strength difference at different thickness locations was ≤50 MPa, the tensile strength difference was ≤30 MPa, the impact value was ≥300 J, and the reduction of area along the thickness direction was ≥50%. In Comparative Example 1, a two-stage rolling process was used, resulting in significant differences in properties along the thickness direction, with differences in impact energy and reduction of area at the half-thickness point. In Comparative Example 2, a two-stage rolling process was used, along with a low-end cooling process, which improved the strength, but increased the differences in properties along the thickness direction, with differences in impact energy and reduction of area along the thickness direction. Both of these are outside the scope of the embodiments in this application.

[0082] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. 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 this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method of rolling a steel plate, the thickness of which is > 100 mm, characterized in that, The method includes: The billet is first heated and then cooled to room temperature; The cooled billet is then subjected to a second stage of heating under set process parameters, including a heating temperature of 895℃~905℃ and a heating time of 600min~800min. The billet after the second stage of heating is rolled in the completely non-recrystallized zone at a set rolling temperature, and then cooled. The chemical composition of the cast billet includes: C, Si, Mn, Alt, Nb, Ti, Ni, P, S, and Fe; by mass fraction, The content of C is 0.025%~0.035%, the content of Si is 0.40%~0.50%, the content of Mn is 1.45%~1.55%, the content of Alt is 0.02%~0.06%, the content of Nb is 0.05%~0.06%, the content of Ti is 0.01%~0.03%, the content of Ni is 0.70%~0.80%, the content of P is <0.008%, and the content of S is <0.003%. And it satisfies the following relationship: 0.70≤[C]+[Si] / 5+[Mn] / 3+[Ni] / 7+2[P]≤0.75, In the formula, [C] represents the mass fraction of C, [Si] represents the mass fraction of Si, [Mn] represents the mass fraction of Mn, [Ni] represents the mass fraction of Ni, and [P] represents the mass fraction of P.

2. The method of claim 1, wherein, The set rolling temperatures include: initial rolling temperature of 860℃~880℃ and final rolling temperature of 800℃~820℃.

3. The method of claim 1, wherein, The temperature of the first stage of heating is 1160℃~1180℃, and the heating time of the first stage is 360min~500min.

4. The method of claim 1, wherein, The final cooling temperature is 530℃~600℃, and the cooling rate is 7℃ / s~15℃ / s.

5. The method of claim 1, wherein, The width of the billet is 2000mm~2400mm, the thickness of the billet is 400mm, and the center segregation of the billet is less than 0.5 of Class C.

6. A steel sheet characterized by, The steel plate is obtained by rolling the method described in any one of claims 1-5; The microstructure of the steel plate includes ferrite and bainite; wherein, The proportion of ferrite is 80% to 95%, and the proportion of bainite is 5% to 20%.

7. Steel sheet according to claim 6, characterized in that, The steel plate at different locations with varying thicknesses must meet at least one of the following properties: yield strength difference ≤ 50 MPa, tensile strength difference ≤ 30 MPa, impact value ≥ 300 J, and reduction of area in the thickness direction ≥ 50%.

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