Steel with a strength level of 460 MPa and its manufacturing method

Abstract

The present invention provides a steel with a strength level of 460 MPa and a manufacturing method thereof. The chemical composition of the steel plate in mass percentage is 0.05 wt% ≤ C ≤ 0.08 wt%, 0.15 wt% ≤ Si ≤ 0.30 wt%, 1.10 wt% ≤ Mn ≤ 1.30 wt%, P ≤ 0.02 wt%, S ≤ 0.005 wt%, 0.018 wt% ≤ Als ≤ 0.045 wt%, 0.025 wt% ≤ Ti ≤ 0.038 wt%, 0.015 wt% ≤ Nb ≤ 0.030 wt%, N ≤ 0.0055 wt%, 0.0015 wt% ≤ B ≤ 0.0025 wt%, Cr ≤ 0.30 wt%, and the balance is iron and other inevitable impurities. Compared with the prior art, the grains are uniform, the yield strength of the steel plate is < 500 MPa, the flatness of the plate shape is good, and the quality stability is high.

Description

Technical Field

[0001] The present invention relates to the field of metal casting, and in particular to a 460MPa strength grade steel and a manufacturing method thereof. Background Art

[0002] Steel with a strength grade of 460 MPa is a low-alloy structural steel with a relatively low material grade. It is primarily used for general structural components and has low quality requirements. However, with the increasing adoption of automation in downstream processing plants, such as laser cutting and automated welding, steel plates are now subject to high dimensional requirements, such as warping-free cutting and high dimensional accuracy when welded. To meet these requirements, the steel plates must exhibit low internal stress, excellent flatness, and stable mechanical properties to demonstrate both processability and aesthetic appeal. Therefore, this material demands stringent quality standards, both high quality and affordability. Steel with a strength grade of 460 MPa is a low-alloy structural steel used in large quantities, primarily for structural components. With the increasing adoption of automated production processes such as automated laser cutting, automated welding, and assembly in downstream processing plants, high quality requirements are being met during use, such as warping-free cutting, warp straightening, welded assembly, and high dimensional accuracy. To meet these requirements, low internal stress and excellent flatness are essential to ensure the steel plates meet the demands of downstream customers. Therefore, the material has stringent comprehensive quality requirements of high quality and low price.

[0003] However, due to the particularity of its production process, hot-rolled coil flattening steel plates have inherent disadvantages in product internal stress quality control, which is not as good as single-rolled steel plate rolled products; in addition, in order to control production costs, Ti microalloys are used to enhance mechanical properties, and the rolling and flattening processes pursue high efficiency, resulting in poor quality stability of the steel plates, which affects the control of internal stress in the material. Summary of the Invention

[0004] The main purpose of the present invention is to provide a 460MPa strength grade steel and a manufacturing method thereof, aiming to solve the problems of poor quality stability and high production cost of 460MPa strength grade steel produced by existing technology.

[0005] To achieve the above object, the present invention provides a steel with a strength level of 460 MPa. The chemical composition of the 460 MPa strength level steel in mass percentage is 0.05 wt% ≤ C ≤ 0.08 wt%, 0.15 wt% ≤ Si ≤ 0.30 wt%, 1.10 wt% ≤ Mn ≤ 1.30 wt%, P ≤ 0.02 wt%, S ≤ 0.005 wt%, 0.018 wt% ≤ Als ≤ 0.045 wt%, 0.025 wt% ≤ Ti ≤ 0.038 wt%, 0.015 wt% ≤ Nb ≤ 0.030 wt%, N ≤ 0.0055 wt%, 0.0015 wt% ≤ B ≤ 0.0025 wt%, Cr ≤ 0.30 wt%, and the rest is iron and other inevitable impurities; and the decarburized layer structure on the surface layer of the metallographic structure of the 460 MPa strength level steel is 30 - 50 μm, and the main microstructures are ferrite, pearlite and bainite.

[0006] The present invention also provides a manufacturing method of a steel with a strength level of 460 MPa, including the steps of: providing a hot-rolled material; the chemical composition of the hot-rolled material in mass percentage is 0.05 wt% ≤ C ≤ 0.08 wt%, 0.15 wt% ≤ Si ≤ 0.30 wt%, 1.10 wt% ≤ Mn ≤ 1.30 wt%, P ≤ 0.02 wt%, S ≤ 0.005 wt%, 0.018 wt% ≤ Als ≤ 0.045 wt%, 0.025 wt% ≤ Ti ≤ 0.038 wt%, 0.015 wt% ≤ Nb ≤ 0.030 wt%, N ≤ 0.0055 wt%, 0.0015 wt% ≤ B ≤ 0.0025 wt%, Cr ≤ 0.30 wt%, and the rest is iron and other inevitable impurities.

[0007] When the hot-rolled material is cooled to below 60°C, it is leveled and cut according to the thickness of the order specification to obtain the steel with a strength level of 460 MPa.

[0008] When the thickness a of the order specification ≤ 18 mm, the flatness ≤ 5 mm / 2m; when the thickness of the order specification 18 mm < a ≤ 25 mm, the flatness ≤ 8 mm / 2m; wherein, the width b of the order specification is 1800 - 2000 mm.

[0009] Furthermore, the reduction of the rough leveling inlet during the leveling and cutting process is -4 mm to -8 mm.

[0010] Furthermore, when the flatness does not meet the requirements, it also includes performing temper heat treatment on the steel plate and reusing it as the hot-rolled material for subsequent operation steps; the temperature of the temper heat treatment is 550°C; the holding time of the temper heat treatment is 1.8a min; wherein, a is the thickness of the order specification, with the unit of mm.

[0011] Further, the thickness of the hot-rolled material is 230 mm; and the center segregation and center porosity of the slab of the hot-rolled material do not exceed C1.5.

[0012] Further, before the cooling treatment of the hot-rolled material, it also includes successively performing descaling treatment, rough rolling treatment, finish rolling treatment and coiling treatment on the hot-rolled material; wherein, the temperature of the finish rolling treatment is 830 - 850 °C; the reduction ratio of the finish rolling treatment is not less than 50%.

[0013] Further, the rough rolling treatment uses a rough rolling mill with flat rolls; the finish rolling treatment uses a finish rolling mill with CVC rolls.

[0014] Further, between the finish rolling treatment and the coiling treatment, it also includes laminar cooling the rolled material after the refining treatment to a coiling temperature of 560 - 630 °C during the coiling treatment; wherein, the upper and lower water ratio of the laminar cooling is set to 1:1.1.

[0015] Further, the hot-rolled material is obtained by successively performing top and bottom combined blowing converter primary refining, LF refining and CC casting on the raw material to obtain the hot-rolled material.

[0016] Further, the process of cooling to below 60 °C is specifically to store the hot-rolled material in the warehouse for 24 hours for waiting to cool, and then naturally cool it to below 60 °C.

[0017] The beneficial effects achieved by the present invention:

[0018] The surface layer of the metallographic structure of the 460 MPa strength grade steel provided by the present invention has a decarburized layer structure of 30 - 50 μm, the main microstructures are ferrite, pearlite and bainite, the grains are uniform, the yield strength of the steel plate is <500 MPa, and the flatness of the plate shape is good and the quality stability is high.

[0019] The manufacturing method of the 460 MPa strength grade steel provided by the present invention enhances the hardenability of the 460 MPa strength grade steel by introducing B element and Cr element into the hot-rolled material and controlling 0.0015 wt% ≤ B ≤ 0.0025 wt% and Cr ≤ 0.30 wt%; combined with the leveling and cutting process for controlling flatness according to the order specification thickness, it stably produces a 460 MPa strength grade steel with good flatness of the plate shape, good control of warping of the laser-cut material and high forming stability; and the manufacturing method has low cost and is suitable for large-area popularization and application. Description of the Drawings

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the accompanying drawings required in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0021] Figure 1 Microscopic structure diagram of the material of the product obtained in Example 1;

[0022] Figure 2 Microscopic structure diagram of the material of the product obtained in Comparative Example 1.

[0023] The realization of the object of the present invention, its functional characteristics and advantages will be further described in conjunction with the embodiments with reference to the accompanying drawings. Specific Embodiments

[0024] The following will clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0025] It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other. It should also be understood that the terms used in the embodiments of the present invention are for the purpose of describing specific embodiments, rather than limiting the scope of protection of the present invention.

[0026] Unless otherwise defined, all technical and scientific terms used in the present invention are the same as those understood by those of ordinary skill in the art of the present technology and the records of the present invention. Any methods, devices and materials similar or equivalent to the methods, devices and materials described in the embodiments of the present invention can also be used to implement the present invention.

[0027] When the embodiments give a numerical range, it should be understood that unless otherwise stated in the present invention, both endpoints of each numerical range and any value between the two endpoints can be selected. The test methods without specific conditions indicated in the following embodiments are usually carried out under conventional conditions or according to the conditions recommended by each manufacturer. The materials or reagents required in the following embodiments are commercially available unless otherwise specified.

[0028] In order to solve the problems of poor quality stability and high production cost existing in the steel of 460 MPa strength level produced by the prior art, the present invention provides a steel of 460 MPa strength level. The chemical composition of the steel of 460 MPa strength level in mass percentage is 0.05 wt% ≤ C ≤ 0.08 wt%, 0.15 wt% ≤ Si ≤ 0.30 wt%, 1.10 wt% ≤ Mn ≤ 1.30 wt%, P ≤ 0.02 wt%, S ≤ 0.005 wt%, 0.018 wt% ≤ Als ≤ 0.045 wt%, 0.025 wt% ≤ Ti ≤ 0.038 wt%, 0.015 wt% ≤ Nb ≤ 0.030 wt%, N ≤ 0.0055 wt%, 0.0015 wt% ≤ B ≤ 0.0025 wt%, Cr ≤ 0.30 wt%, and the rest is iron and other inevitable impurities; and the surface layer of the metallographic structure of the steel of 460 MPa strength level has a decarburized layer structure of 30 - 50 μm, and the main microstructures are ferrite, pearlite and bainite.

[0029] The surface layer of the metallographic structure of the steel of 460 MPa strength level provided by the present invention has a decarburized layer structure of 30 - 50 μm, the main microstructures are ferrite, pearlite and bainite, the grains are uniform, the yield strength of the steel plate is < 500 MPa, the flatness of the plate shape is good, and the quality stability is high.

[0030] The present invention also provides a manufacturing method of a steel of 460 MPa strength level, including the steps: providing a hot-rolled material; the chemical composition of the hot-rolled material in mass percentage is 0.05 wt% ≤ C ≤ 0.08 wt%, 0.15 wt% ≤ Si ≤ 0.30 wt%, 1.10 wt% ≤ Mn ≤ 1.30 wt%, P ≤ 0.02 wt%, S ≤ 0.005 wt%, 0.018 wt% ≤ Als ≤ 0.045 wt%, 0.025 wt% ≤ Ti ≤ 0.038 wt%, 0.015 wt% ≤ Nb ≤ 0.030 wt%, N ≤ 0.0055 wt%, 0.0015 wt% ≤ B ≤ 0.0025 wt%, Cr ≤ 0.30 wt%, and the rest is iron and other inevitable impurities. Specifically, elements B and Cr improve the hardenability of the material, but experiments find that when the content of element B generally exceeds 30 ppm, the toughness of the material will be seriously deteriorated; when the content of element Cr is about 0.20 wt%, it can improve surface oxides, etc. Finally, it is determined that when 0.0015 wt% ≤ B ≤ 0.0025 wt% and Cr ≤ 0.30 wt%, a steel of 460 MPa strength level with good flatness of the plate shape, good control of warping of laser-cut materials, and high forming stability can be stably produced.

[0031] When the hot-rolled material is cooled to below 60 °C, it is leveled and cut according to the thickness of the order specification to obtain the steel of 460 MPa strength level.

[0032] When the thickness a of the order specification is ≤ 18 mm, the flatness is ≤ 5 mm / 2 m; when the thickness of the order specification is 18 mm < a ≤ 25 mm, the flatness is ≤ 8 mm / 2 m; among them, the width b of the order specification is 1800 - 2000 mm.

[0033] The manufacturing method of the 460 MPa strength grade steel provided by the present invention introduces B element and Cr element into the hot-rolled material, and controls 0.0015 wt% ≤ B ≤ 0.0025 wt%, Cr ≤ 0.30 wt%, preferably, Cr: 0.20 wt%. It enhances the hardenability of the 460 MPa strength grade steel and improves the surface oxide; combined with the leveler cutting process for controlling flatness according to the order specification thickness, it realizes the efficient and stable production of 460 MPa strength grade steel with good flatness and high quality stability; and this manufacturing method has a low cost and is suitable for large-scale popularization and application.

[0034] Furthermore, the reduction amount at the entrance of rough leveling in the leveler cutting process is -4 mm to -8 mm. When the reduction amount at the entrance of rough leveling in the leveler cutting process is -4 mm to -8 mm, the internal stress of the steel plate can be effectively released to improve the stability of the 460 MPa strength grade steel.

[0035] Furthermore, when the flatness does not meet the requirements, it also includes performing temper heat treatment on the steel plate and reusing it as a hot-rolled material for subsequent operation steps; the temperature of the temper heat treatment is 550 °C; the holding time of the temper heat treatment is 1.8a min; where a is the thickness of the order specification in mm. Specifically, when the flatness of the steel plate does not meet the above requirements, the steel plate that does not meet the flatness requirements should be subjected to temper heat treatment at 550 °C for 1.8a min, and after tempering and discharging from the furnace, it should be naturally cooled by rolling on the cooling bed to below 60 °C, and then the leveler cutting process should be carried out again according to the order specification thickness.

[0036] Furthermore, the thickness of the hot-rolled material is 230 mm; and the center segregation and center porosity of the hot-rolled material slab do not exceed C1.5.

[0037] Furthermore, before the cooling treatment of the hot-rolled material, it also includes successively performing descaling treatment, rough rolling treatment, finish rolling treatment and coiling treatment on the hot-rolled material; among them, the temperature of the finish rolling treatment is 830 - 850 °C; the reduction ratio of the finish rolling treatment is not less than 50%.

[0038] Furthermore, the rough rolling treatment uses a rough rolling mill with flat rolls; the finish rolling treatment uses a finish rolling mill with CVC rolls.

[0039] Further, between the finish rolling process and the coiling process, laminar cooling is also included to cool the rolled material after refining treatment to a coiling temperature of 560-630°C during coiling; wherein, the upper and lower water ratio of laminar cooling is set to 1:1.1. Specifically, when the upper and lower water ratio of laminar cooling is set to 1:1.1, the cooling of the upper and lower surfaces of the rolled material can be ensured to be uniform and consistent.

[0040] Further, the hot-rolled material is obtained by subjecting the raw material to top and bottom combined blowing converter primary refining, LF refining, and CC casting in sequence to obtain the hot-rolled material. Specifically, LF refining is slag-making and deoxidation, adjusting the composition of molten steel, and controlling the temperature of molten steel, finally forming molten steel with qualified composition, high cleanliness, and appropriate temperature. CC casting is to form a slab with good surface and internal quality.

[0041] Further, the process of cooling to below 60°C is specifically to store the hot-rolled material in the warehouse for 24 hours for waiting to cool, and then naturally cool it to below 60°C.

[0042] For a further understanding of the present invention, examples are given below for illustration:

[0043] Example 1

[0044] (1) Medium-package molten steel composition: C: 0.075wt%, Si: 0.183wt%, Mn: 1.25wt%, P: 0.0112wt%, S: 0.0032wt%, Als: 0.0296wt%, Ti: 0.0285wt%, Nb: 0.0185wt%, Cr: 0.255wt%, N: 0.0055wt%, B: 0.0018wt%, and the rest are iron and inevitable elements.

[0045] (2) Rolling process: finish rolling temperature 830°C - 850°C, coiling temperature 570°C, laminar water ratio 1:1.1 and laminar water height setting. The thickness of the hot-rolled material is 230mm, the slab heating furnace tapping temperature is 1210°C, and the total heating duration is 262min. After the slab is heated well, controlled rolling is carried out, two-stage rolling is adopted, the roughing mill uses flat rolls, and the finishing mill uses CVC rolls.

[0046] (3) According to the hot-rolling process for controlled rolling, the rolled steel coil is stored in the warehouse for 24 hours for waiting to cool. After the steel coil is naturally cooled to below 60°C, it is subjected to leveling and cutting production treatment. The production specification of the order thickness is 8mm, and the reduction at the entrance of the rough leveling is pressed down to -4mm. The flatness of the steel plate is controlled not to exceed 5mm / 2m. For the steel plate with flatness exceeding 5mm / 2m, heat treatment and tempering process are used for re-treatment.

[0047] (4) When the flatness of the steel plate exceeds 5mm / 2m, the steel plate is subjected to tempering treatment. The tempering process: temperature 550°C, holding time 36min. Subsequently, the steel plate is naturally cooled to room temperature by rolling on the cooling bed, and the flatness is 2mm / 2m.

[0048] Comparative Example 1

[0049] (1) The chemical composition of the molten steel in (1): C: 0.075 wt%, Si: 0.153 wt%, Mn: 1.32 wt%, P: 0.0132 wt%, S: 0.0022 wt%, Als: 0.0326 wt%, Ti: 0.0525 wt%, Nb: 0.0225 wt%, N: 0.0062 wt%, and the rest are iron and inevitable elements.

[0050] (2) The finish rolling temperature of the rolling process is 850°C to 870°C, the coiling temperature is 600°C, the laminar water ratio is 1:1.1 and the laminar water height is set. The thickness of the continuous casting slab is 230 mm, the tapping temperature of the continuous casting slab heating furnace is 1215°C, and the total heating duration is 246 min. After the slab is heated well, controlled rolling is carried out. Two-stage rolling is adopted. The roughing mill uses flat rolls, and the finishing mill uses CVC rolls.

[0051] (3) According to the hot rolling process for controlled rolling, the hot rolled coil is placed in the hot coil storage for more than 48 h. After the coil is naturally cooled to below 60°C, leveling and cutting are carried out. The reduction at the entrance of the rough straightening is pressed down to -5 mm, and the flatness of the steel plate is 8 - 15 mm / 2 m.

[0052] Analysis Example 1

[0053] (1) Comparative analysis of the mechanical properties of the products obtained in Example 1 and Comparative Example 1; the mechanical properties of the products obtained in Example 1 and Comparative Example 1 are shown in Table 1.

[0054] Table 1 Comparative analysis of the mechanical properties of the products obtained in Examples 1 - 2 and Comparative Examples 1 - 2

[0055]

[0056] According to Table 1, the yield and tensile mechanical properties of the material in Example 1 are about 30 MPa lower than those of the material in Comparative Example 1, and the elongation rate is increased by 4%. The material has good formability during processing.

[0057] (2) Microstructure observation of the metallographic structures of Example 1 and Comparative Example 1 was carried out respectively, and the micrographs of their materials are shown in Figure 1 、 Figure 2 respectively.

[0058] It can be seen from Figure 1 that there is a decarburized layer of 30 - 50 um on the outermost surface; the microstructure of the material is ferrite, pearlite and bainite, and the finish rolling and coiling temperatures refine the grain size of the material. The addition of element B and the reduction of element Ti achieve tissue strengthening instead of partial precipitation strengthening.

[0059] It can be seen from Figure 2 it can be seen that the microstructure of the material is ferrite, pearlite and bainite; according to Figure 1 and Figure 2 it is known that: Figure 1 in the outermost 30-50 μm decarburized layer, the microstructure is ferrite, pearlite and bainite, the grain size is finer, and it has good toughness. By using the hardenability characteristics of B and Cr elements, the addition of B and Cr elements and the reduction of Ti element, the strengthening of the structure is realized instead of precipitation strengthening, and the mechanical properties of the material are more stable.

[0060] In summary, in the above technical solutions of the present invention, the above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structural transformation made by using the content of the specification and drawings of the present invention under the technical concept of the present invention, or directly / indirectly applied to other related technical fields, is included in the patent protection scope of the present invention.

Claims

1. A steel with a strength level of 460 MPa, characterized in that, The chemical composition of the steel with a strength level of 460 MPa is 0.05 wt% ≤ C ≤ 0.08 wt%, 0.15 wt% ≤ Si ≤ 0.30 wt%, 1.10 wt% ≤ Mn ≤ 1.30 wt%, P ≤ 0.02 wt%, S ≤ 0.005 wt%, 0.018 wt% ≤ Als ≤ 0.045 wt%, 0.025 wt% ≤ Ti ≤ 0.038 wt%, 0.015 wt% ≤ Nb ≤ 0.030 wt%, N ≤ 0.0055 wt%, 0.0015 wt% ≤ B ≤ 0.0025 wt%, 0.20 wt% ≤ Cr ≤ 0.30 wt%, and the balance is iron and other inevitable impurities; And the surface layer of the metallographic structure of the steel with a strength level of 460 MPa has a decarburized layer structure of 30 - 50 µm, and the main microstructures are ferrite, pearlite, and bainite.

2. A manufacturing method for a steel with a strength level of 460 MPa, characterized in that, Including the steps: Provide a hot-rolled material; the chemical composition of the hot-rolled material is 0.05 wt% ≤ C ≤ 0.08 wt%, 0.15 wt% ≤ Si ≤ 0.30 wt%, 1.10 wt% ≤ Mn ≤ 1.30 wt%, P ≤ 0.02 wt%, S ≤ 0.005 wt%, 0.018 wt% ≤ Als ≤ 0.045 wt%, 0.025 wt% ≤ Ti ≤ 0.038 wt%, 0.015 wt% ≤ Nb ≤ 0.030 wt%, N ≤ 0.0055 wt%, 0.0015 wt% ≤ B ≤ 0.0025 wt%, 0.20 wt% ≤ Cr ≤ 0.30 wt%, and the balance is iron and other inevitable impurities; When the hot-rolled material is cooled to below 60 °C, level and cut the plate according to the order specification thickness to obtain the steel with a strength level of 460 MPa; the reduction of the rough leveling inlet for the level and cut plate treatment is -4 mm to -8 mm; When the order specification thickness a ≤ 18 mm, the flatness ≤ 5 mm / 2 m; when the order specification thickness 18 mm < a ≤ 25 mm, the flatness ≤ 8 mm / 2 m; where the order specification width b is 1800 - 2000 mm.

3. The manufacturing method according to claim 2, characterized in that, When the flatness does not meet the requirements, it also includes performing temper heat treatment on the steel plate and reusing it as the hot-rolled material for subsequent operation steps; The temperature of the temper heat treatment is 550 °C; the holding time of the temper heat treatment is 1.8a min; where a is the order specification thickness in mm.

4. The manufacturing method according to claim 2, wherein The thickness of the hot-rolled material is 230 mm; and the center segregation and center porosity of the slab of the hot-rolled material do not exceed C1.

5.

5. The manufacturing method according to claim 2, characterized in that, Before the cooling treatment of the hot-rolled material, it also includes successively performing descaling treatment, rough rolling treatment, finish rolling treatment, and coiling treatment on the hot-rolled material; Among them, the temperature of the finish rolling treatment is 830 - 850 °C; the reduction rate of the finish rolling treatment is not less than 50%.

6. The manufacturing method according to claim 5, characterized in that, The rough rolling treatment uses a rough rolling mill with flat rolls; the finish rolling treatment uses a finish rolling mill with CVC rolls.

7. The manufacturing method according to claim 5, characterized in that Between the finish rolling process and the coiling process, laminar cooling is also included to cool the rolled material after the finish rolling process to a coiling temperature of 560 - 630 °C during the coiling process; Among them, the ratio of upper water to lower water in the laminar cooling is set to 1:1.

1.

8. The manufacturing method according to claim 2, characterized in that, The hot-rolled material is obtained by subjecting the raw material to top and bottom combined blowing converter primary refining, LF refining, and CC casting in sequence to obtain the hot-rolled material.

9. The manufacturing method according to claim 2, characterized in that, The process of cooling to below 60 °C specifically is to store the hot-rolled material in the warehouse for 24 hours for waiting to cool, and then naturally cool it to below 60 °C.

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