A thin gauge 700bl girder steel without nb and a production method thereof

Abstract

The present application relates to the technical field of girder steel, and particularly relates to a thin-specification 700BL girder steel without Nb and a production method thereof; a continuous casting blank is sequentially subjected to the steps of heating, post-furnace high-pressure water descaling, rough rolling, hot coil box, flying shear, pre-rolling descaling, finishing rolling, laminar cooling and coiling to prepare the thin-specification 700MPa girder steel; the post-furnace high-pressure water descaling is provided with two groups, the pre-rolling descaling is provided with one group, and descaling is performed at the inlet of the first pass of rough rolling; the thickness of the thin-specification 700MPa girder steel is 2.5-3mm. The product performance and processing performance of the present application both meet the requirements. Through the design of the whole-line temperature process parameters and the reasonable setting of the multi-point distributed descaling process, the resonance problem of the finishing rolling mill is solved, and the production stability is improved.

Description

Technical Field

[0001] This invention relates to the field of beam steel technology, specifically to a thin-gauge 700BL beam steel without Nb and its production method. Background Technology

[0002] The rapid development of the automotive industry has brought about problems such as environmental pollution and energy consumption, necessitating the continuous development of environmentally friendly high-performance steel plates to achieve energy conservation and emission reduction. While the strength levels of hot-rolled automotive beam steel are constantly improving, it is also developing towards lower costs, a good combination of strength and toughness, and high surface quality.

[0003] Domestically produced hot-rolled 700MPa automotive frame steel products typically incorporate 0.03% to 0.06% nitrogen (Nb) in their alloy composition. For example, CN 109161806 A discloses a 700MPa grade automotive frame steel strip and its preparation method. The steel strip contains 0.045% to 0.055% Nb in its chemical composition, which enhances the steel's strength through grain refinement and precipitation strengthening. However, given the relative scarcity of Nb resources in my country, this method suffers from high alloy costs.

[0004] CN 115747643 A discloses a method for producing 1.2~2.0mm thin-gauge 700MPa grade hot-rolled beam steel and its preparation. It uses relatively abundant Ti and rare earth elements to replace Nb, and achieves the production of extremely thin gauge steel under low-cost composition design by matching the hot rolling process with the rolling capacity of the mill. However, this method suffers from the problem of mill resonance in production practice, affecting production stability.

[0005] One of the main causes of resonance in finishing mills is the low inlet temperature. To ensure the surface quality of the produced steel strip, existing thin-gauge 700MPa grade beam steel production lines often install two sets of high-pressure water descaling devices between the heating furnace and the roughing mill, at least two sets of descaling devices at the inlet of the roughing mill passes, and two sets of high-pressure water descaling devices before the finishing mill stand. Multiple descaling operations lead to a decrease in billet temperature, causing intermediate billets to enter the finishing mill at a lower temperature. This results in significant vibration of the finishing mill during operation, with vibration values ​​potentially reaching 8 mm / s or even 10 mm / s, significantly impacting production. Reducing the number of descaling operations can improve the vibration of the finishing mill stand, but due to the lack of suitable heating or controlled rolling and cooling processes, the final product surface often exhibits obvious quality defects, such as red rust or large areas of iron oxide scale indentation. Summary of the Invention

[0006] In view of the problem that the resonance of the finishing mill affects the production stability when rolling thin-gauge 700BL beam steel, and that reducing the number of descaling times leads to a decrease in the surface quality of the finished product, there is a lack of technical methods that can avoid the resonance of the finishing mill while ensuring the surface quality. This invention provides a Nb-free thin-gauge 700BL beam steel and its production method.

[0007] The technical solution of this invention is as follows:

[0008] In a first aspect, the present invention provides a method for producing thin-gauge 700BL beam steel without Nb, wherein the continuously cast billet is sequentially heated, descaled by high-pressure water after furnace, rough rolling, hot coil box, flying shear, descaling before finishing rolling, finishing rolling, laminar flow cooling, and coiling to prepare thin-gauge 700MPa beam steel;

[0009] The chemical composition of the continuously cast billet, by mass percentage, is C: 0.05%~0.08%, Si: 0.05%~0.15%, Mn: 1.15%~1.30%, P≤0.020%, S≤0.005%, Ti: 0.105%~0.12%, with the balance being Fe and unavoidable impurities;

[0010] The high-pressure water descaling after the furnace is set up in two groups, and the descaling before the finishing rolling is set up in one group. Descaling is carried out at the entrance of the first pass of the roughing rolling.

[0011] The thickness of thin-gauge 700MPa main beam steel is 2.5~3mm.

[0012] Furthermore, the chemical composition of the continuously cast billet, by mass percentage, is preferably C: 0.07%, Si: 0.12%, Mn: 1.18%, P: 0.017%, S: 0.002%, Ti: 0.11%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, the heating step involves heating the continuously cast billet in a heating furnace, which includes a first heating section, a second heating section, a third heating section, an upper soaking section, and a lower soaking section. The furnace gas temperature in the first heating section is 800~1050℃, the furnace gas temperature in the second heating section is 1050~1250℃, the furnace gas temperature in the third heating section is 1280±20℃, the furnace gas temperature in the upper soaking section is 1270±20℃, and the furnace gas temperature in the lower soaking section is 1290±20℃. The continuous casting billet is in the furnace for 200~240 minutes.

[0014] Furthermore, the roughing process involves five passes to obtain an intermediate billet with a thickness of 32-34 mm.

[0015] Furthermore, the roughing mill exit temperature is 1110±20℃, the finishing mill inlet temperature is 1070±20℃, and the final rolling temperature is 890±20℃.

[0016] Furthermore, laminar cooling adopts a front-end continuous cooling mode. The front-end continuous cooling mode sets the laminar cooling water supply as follows: groups 1-6 are the close-fitting section, with 4 manifolds in each group; groups 7-16 are the fine-fitting section, with 2 manifolds in each group. By controlling rolling and cooling, the elongation is improved while ensuring strength.

[0017] Furthermore, the winding temperature is 600±20℃.

[0018] Secondly, the present invention provides a thin-gauge 700BL beam steel without Nb prepared by the above-mentioned production method, the microstructure of which is ferrite + bainite + cementite.

[0019] Furthermore, the banded structure is grade 1.5 to 2, the ferrite grain size is grade 11.5, the tensile strength is 715 to 790 MPa, the elongation after fracture is 20.32% to 22.93%, and the yield strength is 660 to 735 MPa.

[0020] The beneficial effects of this invention are as follows:

[0021] This invention employs a low-cost alloy composition design and utilizes controlled rolling and cooling technology (TMCP) to produce 700BL beam steel, achieving satisfactory product performance and machinability. Through comprehensive temperature process parameter design and the rational setting of a multi-point distributed descaling process, the surface quality of the finished product is ensured to meet standards. Furthermore, the invention resolves the issue of mill resonance during the rolling of thin-gauge 700BL beam steel, thereby improving production stability. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention 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.

[0023] Figure 1 This is a process flow diagram of the production method of Nb-free thin-gauge 700BL beam steel according to a specific implementation method.

[0024] Figure 2 This is a surface photograph of the Nb-free thin-gauge 700BL beam steel product prepared in Example 1.

[0025] Figure 3 This is a surface photograph of the Nb-free thin-gauge 700BL beam steel product prepared in Comparative Example 1.

[0026] Figure 4 This is a surface photograph of the Nb-free thin-gauge 700BL beam steel product prepared in Comparative Example 2.

[0027] Figure 5 This is a surface photograph of the Nb-free thin-gauge 700BL beam steel product prepared in Comparative Example 3. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0029] Example 1

[0030] A method for producing thin-gauge 700BL beam steel without Nb includes the following steps:

[0031] (1) A continuous casting billet with a thickness of 210 mm was obtained according to the chemical composition design. The chemical composition of the continuous casting billet by mass percentage is C: 0.07%, Si: 0.12%, Mn: 1.18%, P: 0.017%, S: 0.002%, Ti: 0.11%, with the balance being Fe and unavoidable impurities.

[0032] (2) The continuously cast billet is fed into the heating furnace for heating. The heating furnace includes a heating section 1, a heating section 2, a heating section 3, an upper soaking section and a lower soaking section. The furnace gas temperature of the heating section 1 is set to 950℃, the furnace gas temperature of the heating section 2 is 1130℃, the furnace gas temperature of the heating section 3 is 1280℃, the furnace gas temperature of the upper soaking section is 1270℃, the furnace gas temperature of the lower soaking section is 1290℃, and the furnace time of the continuously cast billet in the furnace is 220min.

[0033] (3) Two sets of high-pressure water descaling devices are installed after the heating furnace to descale the continuous casting billets coming out of the heating furnace in sequence.

[0034] (4) After descaling, the continuous casting billet is rough rolled in 5 passes. Descaling is performed once at the entrance of the first pass, and the exit temperature of the rough rolling is controlled to fluctuate within the range of 1108~1120℃.

[0035] (5) After rough rolling, an intermediate billet with a thickness of 32mm is obtained. The intermediate billet is rolled and unrolled in a hot rolling box, and then cut off by flying shear.

[0036] (6) A set of high-pressure water descaling device is installed between the finishing mill stand and the flying shear, so that the intermediate billet undergoes descaling treatment before entering the finishing mill stand.

[0037] (7) The intermediate billet is finished rolled. There are a total of 7 finishing mill stands. The temperature at the entry point of the finishing mill is controlled to fluctuate within the range of 1063~1080℃, and the final rolling temperature fluctuates within the range of 883~895℃, to obtain a steel strip with a specification of 2.5*1500mm.

[0038] (8) After finishing, the steel strip is output from the finishing mill output roller table and the upper laminar flow manifold is turned on for cooling. The cooling adopts the front continuous cooling mode, of which the first to sixth groups are the dense adjustment section, with 4 manifolds in each group; the seventh to sixth groups are the fine adjustment section, with 2 manifolds in each group.

[0039] (9) The cooled steel strip is wound up, and the winding temperature is controlled to fluctuate within the range of 580~607℃.

[0040] Using the same chemical composition and condition parameters, and adjusting the rolling reduction rate, another steel strip with a thickness of 3.0 mm is produced.

[0041] During the finishing rolling process, the vibration values ​​of each stand were detected and recorded, and the results are shown in Table 1 below.

[0042] Table 1. Vibration detection results of the finishing mill in Example 1 (unit: mm / s)

[0043]

[0044] The microstructure of the two thicknesses of the prepared beam steel was observed to determine the banded structure and grain size grade. The tensile strength, yield strength and elongation after fracture of the beam steel were tested, and the results are shown in Table 2 below.

[0045] Table 2. Performance test results of the main beam steel in Example 1

[0046]

[0047] Observe the surface quality of the finished product of Example 1, such as Figure 2 As shown, the steel strip has good surface quality and no defects such as iron oxide scale being pressed in.

[0048] Example 2

[0049] A thin-gauge 700BL beam steel without Nb, with a thickness of 2.5 mm, is prepared according to the following production method:

[0050] (1) A continuous casting billet with a thickness of 210 mm was obtained according to the chemical composition design. The chemical composition of the continuous casting billet by mass percentage is C: 0.05%, Si: 0.05%, Mn: 1.30%, P: 0.015%, S: 0.001%, Ti: 0.12%, with the balance being Fe and unavoidable impurities.

[0051] (2) The continuously cast billet is fed into the heating furnace for heating. The heating furnace includes a heating section 1, a heating section 2, a heating section 3, an upper soaking section and a lower soaking section. The furnace gas temperature of the heating section 1 is 1050℃, the furnace gas temperature of the heating section 2 is 1250℃, the furnace gas temperature of the heating section 3 is 1300℃, the furnace gas temperature of the upper soaking section is 1290℃, the furnace gas temperature of the lower soaking section is 1310℃, and the continuous casting billet is in the furnace for 200 min.

[0052] (3) Two sets of high-pressure water descaling devices are installed after the heating furnace to descale the continuous casting billets coming out of the heating furnace in sequence.

[0053] (4) After descaling, the continuous casting billet is rough rolled in 5 passes. Descaling is performed once at the entrance of the first pass, and the exit temperature of the rough rolling is controlled to fluctuate within the range of 1090~1110℃.

[0054] (5) After rough rolling, an intermediate billet with a thickness of 34 mm is obtained. The intermediate billet is rolled and unrolled in a hot rolling box, and then cut off by flying shear.

[0055] (6) A set of high-pressure water descaling device is installed between the finishing mill stand and the flying shear, so that the intermediate billet undergoes descaling treatment before entering the finishing mill stand.

[0056] (7) The continuously cast billet is finished rolled. There are a total of 7 finishing mill stands. The temperature at the entry point of the finishing mill is controlled to fluctuate within the range of 1050~1065℃, and the final rolling temperature fluctuates within the range of 870~882℃, to obtain a steel strip with a specification of 2.5*1500mm.

[0057] (8) After finishing, the steel strip is output from the finishing mill output roller table and the upper laminar flow manifold is turned on for cooling. The cooling adopts the front continuous cooling mode, of which the first to sixth groups are the dense adjustment section, with 4 manifolds in each group; the seventh to sixth groups are the fine adjustment section, with 2 manifolds in each group.

[0058] (9) The cooled steel strip is wound up, and the winding temperature is controlled to fluctuate within the range of 595~612℃.

[0059] During the finishing rolling process, the vibration values ​​of each stand were detected and recorded, and the results are shown in Table 3 below.

[0060] Table 3. Vibration detection results of the finishing mill in Example 2 (unit: mm / s)

[0061]

[0062] The microstructure of the prepared beam steel was observed to determine the banded structure and grain size grade. The tensile strength, yield strength and elongation after fracture of the beam steel were tested, and the results are shown in Table 4 below.

[0063] Table 4. Performance test results of the main beam steel in Example 2

[0064]

[0065] Upon observation, the finished product of Example 2 showed good surface quality and no defects such as iron oxide scale being pressed in.

[0066] Example 3

[0067] A thin-gauge 700BL beam steel without Nb, with a thickness of 2.5 mm, is prepared according to the following production method:

[0068] (1) A continuous casting billet with a thickness of 210 mm was obtained according to the chemical composition design. The chemical composition of the continuous casting billet by mass percentage is C: 0.08%, Si: 0.15%, Mn: 1.15%, P: 0.012%, S: 0.0005%, Ti: 0.105%, with the balance being Fe and unavoidable impurities.

[0069] (2) The continuously cast billet is fed into the heating furnace for heating. The heating furnace includes a heating section 1, a heating section 2, a heating section 3, an upper soaking section and a lower soaking section. The furnace gas temperature of the heating section 1 is 850℃, the furnace gas temperature of the heating section 2 is 1050℃, the furnace gas temperature of the heating section 3 is 1260℃, the furnace gas temperature of the upper soaking section is 1250℃, the furnace gas temperature of the lower soaking section is 1270℃, and the continuous casting billet is in the furnace for 240 minutes.

[0070] (3) Two sets of high-pressure water descaling devices are installed after the heating furnace to descale the continuous casting billets coming out of the heating furnace in sequence.

[0071] (4) After descaling, the continuous casting billet is rough rolled in 5 passes. Descaling is performed once at the entrance of the first pass, and the exit temperature of the rough rolling is controlled to fluctuate within the range of 1015~1130℃.

[0072] (5) After rough rolling, an intermediate billet with a thickness of 34 mm is obtained. The intermediate billet is rolled and unrolled in a hot rolling box, and then cut off by flying shear.

[0073] (6) A set of high-pressure water descaling device is installed between the finishing mill stand and the flying shear, so that the intermediate billet undergoes descaling treatment before entering the finishing mill stand.

[0074] (7) The continuously cast billet is finished rolled. There are a total of 7 finishing mill stands. The temperature at the entry point of the finishing mill is controlled to fluctuate within the range of 1082~1090℃, and the final rolling temperature fluctuates within the range of 899~910℃, to obtain a steel strip with a specification of 2.5*1500mm.

[0075] (8) After finishing, the steel strip is output from the finishing mill output roller table and the upper laminar flow manifold is turned on for cooling. The cooling adopts the front continuous cooling mode, of which the first to sixth groups are the dense adjustment section, with 4 manifolds in each group; the seventh to sixth groups are the fine adjustment section, with 2 manifolds in each group.

[0076] (9) The cooled steel strip is wound up, and the winding temperature is controlled to fluctuate within the range of 608~620℃.

[0077] During the finishing rolling process, the vibration values ​​of each stand were detected and recorded, and the results are shown in Table 5 below.

[0078] Table 5. Vibration detection results of the finishing mill in Example 3 (unit: mm / s)

[0079]

[0080] The microstructure of the prepared beam steel was observed to determine the banded structure and grain size grade. The tensile strength, yield strength and elongation after fracture of the beam steel were tested, and the results are shown in Table 6 below.

[0081] Table 6. Performance test results of the main beam steel in Example 3

[0082]

[0083] Upon observation, the finished product of Example 3 showed good surface quality and no defects such as iron oxide scale being pressed in.

[0084] Comparative Example 1

[0085] Comparative Example 1 prepared a thin-gauge 700BL beam steel without Nb, with a steel strip thickness of 2.5 mm. The preparation method of Comparative Example 1 was basically the same as that of Example 1, except that the high-pressure water descaling step after the furnace was cancelled. The vibration values ​​of each stand during the finishing rolling process were detected and recorded, and the results are shown in Table 7 below.

[0086] Table 7. Vibration test results of the finishing mill in Comparative Example 1 (unit: mm / s)

[0087]

[0088] The microstructure of the prepared beam steel was observed to determine the banded structure and grain size grade. The tensile strength, yield strength and elongation after fracture of the beam steel were tested, and the results are shown in Table 8 below.

[0089] Table 8 Performance test results of the main beam steel in Comparative Example 1

[0090]

[0091] Observe the surface quality of the finished product of Comparative Example 1, such as Figure 3As shown, the steel strip has poor surface quality and the iron oxide scale is severely pressed in.

[0092] Comparative Example 2

[0093] Comparative Example 2 prepared a thin-gauge 700BL beam steel without Nb, with a steel strip thickness of 2.5 mm. The preparation method of Comparative Example 2 was basically the same as that of Example 1, except that the high-pressure water descaling after the furnace and the descaling before finishing rolling were cancelled. The vibration values ​​of each stand during the finishing rolling process were detected and recorded, and the results are shown in Table 9 below.

[0094] Table 9. Vibration test results of the finishing mill in Comparative Example 2 (unit: mm / s)

[0095]

[0096] The microstructure of the prepared beam steel was observed to determine the banded structure and grain size grade. The tensile strength, yield strength and elongation after fracture of the beam steel were tested. The results are shown in Table 10 below. It can be found that by eliminating the high-pressure water descaling after the furnace and the descaling before finishing rolling, i.e. reducing the controlled rolling effect, the ferrite grain size of the beam steel will decrease.

[0097] Table 10 Performance test results of the main beam steel in Comparative Example 2

[0098]

[0099] Observe the surface quality of the finished product of Comparative Example 2, such as Figure 4 As shown, the steel strip has poor surface quality and the iron oxide scale is severely pressed in.

[0100] Comparative Example 3

[0101] Comparative Example 3 prepared a thin-gauge 700BL beam steel without Nb, with a steel strip thickness of 2.5 mm. The preparation method of Comparative Example 3 was basically the same as that of Example 1. The main difference was that the temperature at the finishing mill entrance was controlled to fluctuate within the range of 970~990℃, and the vibration values ​​of each stand during the finishing mill were detected and recorded. The results are shown in Table 11 below. Under these circumstances, the vibration of the finishing mill was more severe and the mill had to be shut down.

[0102] Table 11 Vibration test results of the finishing mill in Comparative Example 3 (unit: mm / s)

[0103]

[0104] The microstructure of the prepared beam steel was observed to determine the banded structure and grain size grade. The tensile strength, yield strength and elongation after fracture of the beam steel were tested. The results are shown in Table 12 below. Under these conditions, the elongation after fracture of the steel strip is significantly reduced, which is not conducive to user processing.

[0105] Table 12 Performance test results of the main beam steel in Comparative Example 3

[0106]

[0107] Observe the surface quality of the finished product of Comparative Example 3, such as Figure 5 As shown, the steel strip has good surface quality and no defects such as iron oxide scale being pressed in.

[0108] By comparing the vibration values, performance, and surface quality of the finishing mill in the embodiments and comparative examples, it can be found that the present invention, through the rational design and balance of the temperature and descaling process of the entire line, not only ensures high surface quality, but also meets the performance requirements of the finished product, while solving the vibration problem of the finishing mill stand and improving the stability of production.

[0109] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for producing thin-gauge 700BL beam steel without Nb, characterized in that, The continuous casting billet is sequentially heated, descaled by high-pressure water after the furnace, rough rolling, hot coil box, flying shear, descaling before finishing rolling, finishing rolling, laminar flow cooling, and coiling to prepare thin-gauge 700MPa beam steel. The chemical composition of the continuously cast billet, by mass percentage, is C: 0.05%~0.08%, Si: 0.05%~0.15%, Mn: 1.15%~1.30%, P≤0.020%, S≤0.005%, Ti: 0.105%~0.12%, with the balance being Fe and unavoidable impurities; The high-pressure water descaling after the furnace is set up in two groups, and the descaling before the finishing rolling is set up in one group. Descaling is carried out at the entrance of the first pass of the roughing rolling. The thickness of thin-gauge 700MPa main beam steel is 2.5~3mm; The roughing mill exit temperature is 1110±20℃, the finishing mill inlet temperature is 1070±20℃, and the final rolling temperature is 890±20℃. The heating process involves heating the continuously cast billet in a heating furnace, which includes a first heating section, a second heating section, a third heating section, an upper soaking section, and a lower soaking section. The furnace gas temperature in the first heating section is 800~1050℃, the furnace gas temperature in the second heating section is 1050~1250℃, the furnace gas temperature in the third heating section is 1280±20℃, the furnace gas temperature in the upper soaking section is 1270±20℃, and the furnace gas temperature in the lower soaking section is 1290±20℃. The continuous casting billet is in the furnace for 200~240 minutes.

2. The production method as described in claim 1, characterized in that, The chemical composition of the continuously cast billet, by mass percentage, is C: 0.07%, Si: 0.12%, Mn: 1.18%, P: 0.017%, S: 0.002%, Ti: 0.11%, with the balance being Fe and unavoidable impurities.

3. The production method as described in claim 1, characterized in that, The roughing process involves five passes to obtain an intermediate billet with a thickness of 32-34 mm.

4. The production method as described in claim 1, characterized in that, The laminar flow cooling adopts a front-end continuous cooling mode. The front-end continuous cooling mode sets the laminar cooling water conditions as follows: Groups 1 to 6 are the tight adjustment section, with 4 manifolds in each group; Groups 7 to 16 are the fine adjustment section, with 2 manifolds in each group.

5. The production method as described in claim 1, characterized in that, The winding temperature is 600±20℃.

6. A Nb-free thin-gauge 700BL beam steel prepared by the production method described in any one of claims 1 to 5, characterized in that, The microstructure of thin-gauge 700BL beam steel consists of ferrite, bainite, and cementite.

7. The thin-gauge 700BL main beam steel as described in claim 6, characterized in that, The banded microstructure of the thin-gauge 700BL beam steel is grade 1.5 to 2, the ferrite grain size is grade 11.5, the tensile strength is 715 to 790 MPa, the elongation after fracture is 20.32% to 22.93%, and the yield strength is 660 to 735 MPa.

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