A method of heat treating a steel sheet and the resulting steel sheet

Abstract

The application provides a steel plate heat treatment method and a steel plate obtained by the method, and the preparation method comprises the following steps: sequentially performing shot blasting, straightening and tempering on a hot-rolled steel plate to obtain a dust-free steel plate; in the shot blasting step, the shot blasting current is 40-50 A, the shot blasting speed is 5-10 m / min, and the shot diameter is 0.6-0.8 mm. The shot blasting step removes the oxide layer on the surface of the hot-rolled steel plate, so that the alloy part with better plasticity is exposed, so that the ductility of the surface of the steel plate is uniform in the subsequent straightening process, the generation of dust is reduced, on the other hand, the internal stress in the steel plate can be eliminated through the final tempering step, the ductility or toughness of the steel plate is improved under the condition that the hardness and strength of the steel plate are not reduced or are appropriately reduced, and the final use performance of the steel plate is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel plate heat treatment, and particularly relates to a steel plate heat treatment method and a steel plate obtained by the method. BACKGROUND

[0002] As an important procedure of steel plate deep processing, steel plate heat treatment has an important influence on the performance and surface appearance of the steel plate. Currently, the steel plate heat treatment process is generally as follows: steel plate -> open flame heat treatment furnace tempering -> straightening -> printing, packaging and warehousing. However, the steel plate produced by the process has a large amount of dust or loose iron oxide skin on the surface, which greatly affects the surface appearance of the steel plate and seriously affects the use, such as paint spraying and the construction of a dust-free workshop.

[0003] Therefore, there is an urgent need for a steel plate heat treatment method and a steel plate to solve the problems in the related art. SUMMARY

[0004] The main purpose of the present application is to provide a steel plate heat treatment method and a steel plate to solve the technical problem of serious oxidation of the surface of the steel plate in the related art.

[0005] To achieve the above-mentioned purpose, the present application provides a steel plate heat treatment method, comprising the following steps: sequentially performing shot blasting, straightening and tempering on a hot-rolled steel plate to obtain a dust-free steel plate.

[0006] In the shot blasting step, the shot blasting current is 40-50 A, the shot blasting speed is 5-10 m / min, and the shot diameter is 0.6-0.8 mm.

[0007] Preferably, in the straightening step, the steel plate straightening temperature is 20-40℃, the edge wave unevenness is ≤1mm / m, the middle wave unevenness is ≤2mm / m, the buckle head unevenness is ≤2mm / m, and the U-shaped unevenness is ≤2mm / m.

[0008] Preferably, the tempering step is heated by a heat radiation method in a nitrogen-protected gas atmosphere, and the oxygen content in the gas atmosphere is <1000ppm.

[0009] Preferably, in the tempering step, the hot nitrogen temperature is 180-220℃, and the furnace pressure is 15-20Pa.

[0010] Preferably, the tempering step further comprises a cooling step, and the cooling step adopts an up-and-down convection cooling method, the cooling speed is 15-20℃ / min, and the cooling time is 15-20min.

[0011] Preferably, the cooling step is installed with an axial flow fan on the cooling bed.

[0012] The present application has the following beneficial effects:

[0013] The present application firstly removes the oxide layer on the surface of the hot-rolled steel plate through the shot blasting step, so that the alloy part with better plasticity of the steel plate is exposed, thereby making the ductility of the surface of the steel plate consistent in the subsequent straightening process, reducing the generation of dust, on the other hand, the internal stress in the steel plate can be eliminated through the final tempering step, the ductility or toughness of the steel plate is improved without reducing or appropriately reducing the hardness and strength of the steel plate, and the final use performance of the steel plate is improved.

[0014] The present application also provides a steel plate prepared by the above steel plate heat treatment method.

[0015] Preferably, the oxide layer on the surface of the steel plate comprises Fe3O4+ eutectoid structure 90-95% and FeO 5-10% by mass percentage.

[0016] Preferably, the thickness of the oxide layer is 3-11 μm, and the porosity is 50-52%.

[0017] Preferably, the tensile strength of the steel plate is 780-800 MPa, and the yield strength is 730-750 MPa.

[0018] The present application has the following beneficial effects:

[0019] The present application provides a steel plate with no dust on the surface, which not only has good tensile strength and yield strength, but also has a dense oxide layer on the surface, stable performance, and is not easy to be corroded. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0021] It should be noted that all directional indications (such as up, down, etc.) in the embodiments of the present application are only used to explain the relative position relationship, movement condition, etc. between components in a certain posture, and if the certain posture changes, the directional indications also change accordingly.

[0022] In addition, the description of "first", "second" and the like in the present application is only for the purpose of description, and cannot be understood as indicating or implying the relative importance of the indicated technical features or implicitly indicating the number of the indicated technical features. Therefore, the features limited by "first", "second" can explicitly or implicitly include at least one of the features.

[0023] Moreover, the technical solutions among the various embodiments of the present application can be combined with each other, but it must be based on the fact that a person skilled in the art can realize the combination, and when the combination of the technical solutions appears to be contradictory or unachievable, it should be considered that the combination of the technical solutions does not exist and is not within the protection scope required by the present application.

[0024] The present application provides a steel plate heat treatment method, comprising the following steps: sequentially performing shot blasting, straightening and tempering on a hot-rolled steel plate to obtain a dust-free steel plate.

[0025] In the shot blasting step, the shot blasting current is 40-50 A, the shot blasting speed is 5-10 m / min, and the shot diameter is 0.6-0.8 mm.

[0026] In the present application, the order of the shot blasting step, the straightening step and the tempering step is very important. On the one hand, the shot blasting step removes the oxide layer on the surface of the hot-rolled steel plate, so that the alloy part with better plasticity is exposed, thereby making the ductility of the steel plate surface uniform during the subsequent straightening process, reducing the generation of dust. On the other hand, the internal stress in the steel plate can be eliminated through the final tempering step, thereby improving the ductility or toughness of the steel plate without reducing or appropriately reducing the hardness and strength of the steel plate, and improving the final use performance of the steel plate.

[0027] In some embodiments, in the straightening step, the steel plate straightening temperature is 20-40℃, the edge wave unevenness is ≤1mm / m, the middle wave unevenness is ≤2mm / m, the buckle head unevenness is ≤2mm / m, and the U-shaped unevenness is ≤2mm / m.

[0028] Through the straightening step, the original unevenness of the steel plate or the deformation of the steel plate caused by the shot blasting step can be removed, thereby improving the overall performance of the product.

[0029] In some embodiments, the tempering step is heated in a hot radiation mode (for example, using a radiant tube type heat treatment furnace) under a nitrogen protective gas atmosphere, and the oxygen content in the gas atmosphere is <1000ppm.

[0030] By using the hot radiation heating mode, the consistency of the overall temperature of the steel plate during tempering can be improved, the phenomenon of uneven heating of the steel plate caused by using an open flame mode can be avoided, and the phenomenon of serious oxidation of the steel plate during the tempering process can be avoided, thereby improving the performance of the steel plate. In addition, by using the nitrogen atmosphere protection, the oxidation phenomenon of the steel plate during the tempering step can be reduced or avoided.

[0031] In some embodiments, in the tempering step, the hot nitrogen temperature is 180-220℃, and the furnace pressure is 15-20 Pa (i.e. positive pressure, higher than atmospheric pressure by 15-20 Pa).

[0032] In some embodiments, the tempering step is followed by a cooling step, which employs an up-and-down convection cooling method, with a cooling speed of 15-20℃ / min and a cooling time of 15-20min.

[0033] The cooling step (up-and-down convection cooling method) after the tempering step causes the temperature of the steel plate to drop rapidly, so that a dense oxide layer mainly composed of Fe3O4+eutectoid structure and a small amount of FeO can be rapidly generated on the surface of the steel plate. This method not only improves the corrosion resistance of the steel plate surface, but also avoids the problem of a loose and large amount of dust in the oxide layer after oxidation of the steel plate in the related art.

[0034] It is noted that the cooling method employed in the present application is an up-and-down convection cooling method, which can improve the consistency of the overall cooling temperature of the steel plate and reduce or avoid the phenomenon of uneven surface properties and large changes in the thickness of the oxide layer caused by uneven temperature.

[0035] In some embodiments, the cooling step is performed with axial flow fans installed on the cooling bed.

[0036] By installing axial flow fans on the upper and lower parts of the cooling bed, the flowability of air is improved, and uneven cooling caused by the retention of high-temperature gas near the steel plate is avoided.

[0037] The present application also provides a steel plate prepared by the above-mentioned steel plate heat treatment method.

[0038] In some embodiments, the oxide layer on the surface of the steel plate comprises Fe3O4+eutectoid structure 90-95% and FeO 5-10% by mass percentage. It is noted that eutectoid structure refers to an organization in which two or more different phases that are in close proximity are simultaneously precipitated from the same parent phase when a solid metal is cooled from high temperature. In this application, it refers to different metallographic structures of iron.

[0039] In some embodiments, the thickness of the oxide layer is 3-11μm, and the porosity is 50%-52%.

[0040] In some embodiments, the tensile strength of the steel plate is 780-800MPa, and the yield strength is 730-750MPa.

[0041] The steel plate provided by the present application not only has good tensile strength and yield strength, but also has a dense oxide layer on the surface, stable performance, and is not easily corroded.

[0042] Example 1

[0043] A steel plate heat treatment method, comprising the following steps: sequentially performing the following steps on a hot-rolled steel plate:

[0044] Shot blasting: shot blasting current 40 A, shot blasting speed 8 m / min, shot diameter 0.6 mm;

[0045] Straightening: steel plate straightening temperature 30℃, edge wave unevenness ≤1 mm / m, middle wave unevenness ≤2 mm / m, buckling head unevenness ≤2 mm / m, U-shaped unevenness ≤2 mm / m;

[0046] Tempering: heating in a hot radiation mode (using a radiation tube type heat treatment furnace) under a nitrogen protective gas atmosphere, oxygen content in the gas atmosphere <1000 ppm, hot nitrogen temperature 180-220℃; furnace pressure 15-20 Pa;

[0047] Cooling: the cooling step uses an up and down convection cooling mode, cooling speed 15-20℃ / min, cooling time 15-20 min; both the upper and lower cold beds are equipped with axial flow fans;

[0048] A steel plate with a dust-free surface is obtained.

[0049] Example 2: Different from Example 1, in the shot blasting step, shot blasting current 50 A, shot blasting speed 10 m / min, shot diameter 0.7 mm.

[0050] Example 3: Different from Example 1, in the shot blasting step, shot blasting current 45 A, shot blasting speed 5 m / min, shot diameter 0.8 mm.

[0051] Comparative Example 1: Different from Example 1, in the shot blasting step, shot blasting current 60 A, shot blasting speed 4 m / min, shot diameter 0.7 mm.

[0052] Comparative Example 2: Different from Example 1, in the shot blasting step, shot blasting current 30 A, shot blasting speed 15 m / min, shot diameter 0.6 mm.

[0053] Comparative Example 3

[0054] A steel plate heat treatment method, comprising the following steps: sequentially performing the following steps on a hot-rolled steel plate:

[0055] Straightening: steel plate straightening temperature 30℃, edge wave unevenness ≤1 mm / m, middle wave unevenness ≤2 mm / m, buckling head unevenness ≤2 mm / m, U-shaped unevenness ≤2 mm / m;

[0056] Shot blasting: shot blasting current 40 A, shot blasting speed 8 m / min, shot diameter 0.6 mm;

[0057] Tempering: heating in a hot radiation mode (using a radiant tube heat treatment furnace) under a gas atmosphere with an oxygen content < 1000 ppm in a nitrogen atmosphere, hot nitrogen temperature 180-220℃; furnace pressure 15-20 Pa;

[0058] Cooling: the cooling step is performed in an up-and-down convection cooling mode, cooling speed 15-20℃ / min, cooling time 15-20 min; axial flow fans are installed on the cooling bed in an up-and-down manner;

[0059] A steel plate with a surface free of dust is obtained.

[0060] Comparative Example 4

[0061] A steel plate heat treatment method, comprising the following steps: sequentially performing the following steps on a hot-rolled steel plate:

[0062] Shot blasting: shot blasting current 40 A, shot blasting speed 8 m / min, shot diameter 0.6 mm;

[0063] Tempering: heating in a hot radiation mode (using a radiant tube heat treatment furnace) under a gas atmosphere with an oxygen content < 1000 ppm in a nitrogen atmosphere, hot nitrogen temperature 180-220℃; furnace pressure 15-20 Pa;

[0064] Straightening: steel plate straightening temperature 30℃, edge wave unevenness ≤1 mm / m, middle wave unevenness ≤2 mm / m, buckling head unevenness ≤2 mm / m, U-shaped unevenness ≤2 mm / m;

[0065] Cooling: the cooling step is performed in an up-and-down convection cooling mode, cooling speed 15-20℃ / min, cooling time 15-20 min; axial flow fans are installed on the cooling bed in an up-and-down manner;

[0066] A steel plate with a surface free of dust is obtained.

[0067] Comparative Example 5: different from Example 1 in that the cooling step is not included.

[0068] Comparative Example 6: different from Example 1 in that the shot blasting step is not included.

[0069] The steel plates obtained in Examples 1-3 and Comparative Examples 1-6 are subjected to tensile testing, corrosion resistance testing, and oxide layer detection, and the results are shown in Table 1: the tensile testing is performed in accordance with GB / T 228.1-2010; the corrosion resistance testing is performed in accordance with GB / T 19746, and the corrosion solution is a 2wt% NaCl solution, and the corrosion time is 400 h.

[0070] Table 1 Performance results of the steel plates obtained in Examples 1-3 and Comparative Examples 1-6

[0071] Table 1 Performance results of the steel plates obtained in Examples 1-3 and Comparative Examples 1-6

[0072]

[0073] From the data of Examples 1-3 and Comparative Examples 1-2 in Table 1, it can be found that in the process of preparing the dust-free steel plate, the parameter control of the shot blasting step is very important. When the shot blasting current is too large, although the oxide layer can be effectively removed, the hot-rolled steel plate is prone to deformation, and even after subsequent straightening, the influence cannot be completely eliminated, resulting in poor surface performance of the steel plate, and even leading to product degradation. When the shot blasting current is too small, the oxide layer on the surface of the hot-rolled steel plate remains, resulting in a thicker oxide layer, larger porosity and looser texture after the subsequent tempering step, and the prepared product has a larger amount of dust on the surface.

[0074] From the data of Examples 1 and Comparative Examples 3-6, it can be found that when the shot blasting step and the straightening step are exchanged in sequence, the surface unevenness of the obtained steel plate is 12 mm / m, which is much larger than that of the steel plate obtained in Example 1. When the straightening step and the tempering step are exchanged in sequence, since the straightening is performed after the tempering, on the one hand, during the straightening process, the steel plate is in contact with air to generate an oxide layer, and due to the slow cooling speed, the generated oxide layer is thick, has large porosity and poor corrosion resistance. On the other hand, under the action of straightening, the oxide layer is broken, resulting in a larger amount of dust on the surface of the final product. When the shot blasting step is omitted, since the hot-rolled steel plate has an oxide layer on the surface, the oxide layer is broken after the straightening step, resulting in a larger amount of dust on the surface of the final product. Since the oxide layer exists before the cooling step, a dense oxide layer (the content of FeO in the oxide layer is high, reaching 30%) cannot be formed on the surface of the steel plate, the porosity is large, and the corrosion resistance is poor. When the cooling step is omitted, the hot-rolled steel plate is slowly oxidized in air after tempering, the content of FeO in the oxide layer reaches 40%, the porosity is large, the corrosion resistance is significantly reduced, and the amount of dust reaches 20.3 g / m 2 The group with the largest amount of dust in Examples 1-3 and Comparative Examples 1-6.

[0075] In summary, in the shot blasting-straightening-tempering-cooling steel plate heat treatment method provided by the present application, each step has a strong synergistic effect, and the order of each step is crucial. The exchange of step order or the omission of a step has a great influence on the performance test results of the final product, such as the amount of dust, the composition of the oxide layer, the porosity, the corrosion rate and the unevenness.

[0076] In the above technical solution of the present application, the above is only a preferred embodiment of the present application, and does not limit the patent scope of the present application, and any equivalent structural transformation made by using the content of the present application specification, or direct / indirect application in other related technical fields is included in the patent protection scope of the present application.

Claims

1. A method of heat treatment of a steel sheet, characterized in that, The method comprises the following steps: The hot-rolled steel plate is sequentially subjected to shot blasting, straightening and tempering to obtain a dust-free steel plate; In the shot blasting step, the shot blasting current is 40-50 A, the shot blasting speed is 5-10 m / min, and the shot diameter is 0.6-0.8 mm; The tempering step is performed under a nitrogen-protected gas atmosphere, and the oxygen content in the gas atmosphere is less than 1000 ppm; The tempering step is followed by a cooling step, and the cooling step is performed by using an up-and-down convection cooling method, the cooling speed is 15-20 ℃ / min, and the cooling time is 15-20 min.

2. The steel sheet heat treatment method according to claim 1, characterized by, In the straightening step, the steel plate straightening temperature is 20-40 ℃, the edge wave unevenness is less than or equal to 1 mm / m, the middle wave unevenness is less than or equal to 2 mm / m, the buckling head unevenness is less than or equal to 2 mm / m, and the U-shaped unevenness is less than or equal to 2 mm / m.

3. The steel sheet heat treatment method according to claim 1, characterized by, In the tempering step, the hot nitrogen temperature is 180-220 ℃, and the furnace pressure is 15-20 Pa.

4. The steel sheet heat treatment method according to claim 1, characterized by, In the cooling step, axial flow fans are installed on the upper and lower cooling beds.

5. A steel sheet characterized by, The steel plate is prepared by using the steel plate heat treatment method according to any one of claims 1-4.

6. The steel sheet according to claim 5, characterized by The composition of the oxide layer on the surface of the steel plate comprises Fe3O4+eutectoid structure 90-95% and FeO 5-10% by mass percentage.

7. Steel sheet according to claim 6, characterized in that, The thickness of the oxide layer is 3-11 μm, and the porosity is 50-52%.

8. The steel sheet according to claim 5, characterized by The tensile strength of the steel plate is 780-800 MPa, and the yield strength is 730-750 MPa.

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