Steel sheet manufacturing method

Abstract

To provide a production method for a steel sheet that can improve micro-segregation without performing heat treatment for a long time in a slab produced by continuous casting with a general slab thickness (a thickness of 150 mm or more).SOLUTION: A production method for a steel sheet is characterized in that when producing a steel sheet by bloom-rolling a slab with a thickness of 150 mm or more, which is produced by continuous casting and then heating and hot-rolling the slab, pre-hot-rolling heating is performed by securing a prescribed heating time within a range of a prescribed total rolling-reduction ratio R and a prescribed temperature T, wherein the total rolling-reduction ratio R during the bloom-rolling is 15% or more and less than 50%, and the temperature T of a 1 / 4 thick part of the slab is in a temperature range of 1,150°C or more and 1,300°C or less in the pre-hot-rolling heating.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a steel sheet having reduced microsegregation of contained elements. [Background technology]

[0002] In recent years, steel sheets for automobiles have been required to have high strength in order to reduce weight, and there is an increasing demand for high-tensile steel sheets to have further improved impact resistance. In addition, high-tensile steel sheets used in automobiles and the like are required to have formability such as ductility and hole expandability in order to obtain complex part shapes. Patent Document 1 discloses high-strength steel sheets and high-strength galvanized steel sheets that have excellent formability and impact resistance.

[0003] High-tensile steel sheets with large amounts of alloying elements such as C, Si, and Mn have been developed. With such high-tensile steel sheets, the deterioration of material properties due to segregation as the alloying elements become more highly alloyed has become an issue. In particular, microsegregation in the slab formed in the middle of the slab (near the 1 / 4t part of the slab with thickness t) causes band-like structures to form in the cross section of the steel sheet, and this band-like structure makes the structure non-uniform, causing the deterioration of the properties of the steel sheet. In plated steel sheets, plating streak patterns can also occur due to microsegregation of the steel sheet components.

[0004] In Patent Document 2, since the average cooling rate during casting is slow in the normal case of manufacturing from a continuous cast slab with a thickness of about 200 mm, and the microsegregation of Mn is large in the middle of the thickness of the slab, the thickness of the slab cast by continuous casting is thinned to 100 to 30 mm to increase the cooling rate during casting and improve the microsegregation of Mn. However, since the thickness of a slab in general continuous casting is about 200 mm to 600 mm, it is not possible to obtain the effect of increasing the cooling rate during solidification that is obtained when casting these thin slabs.

[0005] In addition, there is a method of subjecting the slab to a high-temperature, long-term heat treatment in order to promote the diffusion of microsegregation in the slab, but this method is not preferable because it can lead to reduced yields due to scale formation, surface defects caused by scale, increased heating costs, and low productivity.

[0006] A method is known in which the slab is bloomed at a predetermined reduction rate and then heated and held to reduce segregation in the slab. For example, Patent Document 3 discloses a method in which the slab is subjected to primary hot working with a cross-sectional area reduction rate of 20% or more, and then reheated at a predetermined temperature within a range of 1000 to 1300°C for a predetermined time. This improves central segregation in the center of the thickness of the slab and reduces cracks in the heat-affected zone (HAZ) when the steel plate is welded. The target is steel plate for thick plates. However, even if this method is applied to thin steel plates, it does not improve the deterioration of the properties of the steel plate caused by the non-uniformity of the band-shaped structure brought about by the microsegregation formed near the 1 / 4t part of the steel plate. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. WO2017 / 164346 [Patent Document 2] JP 2007-070659 A [Patent Document 3] Japanese Patent Application Publication No. 57-120614 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to provide a method for producing a steel plate capable of improving microsegregation in a slab having a general slab thickness (thickness of 150 mm or more) produced by continuous casting without requiring a long-term heat treatment. [Means for solving the problem]

[0009] That is, the gist of the present invention is as follows. [1] A method for producing steel plate, in which a slab with a thickness of 150 mm or more produced by continuous casting is bloomed, and then heated and hot rolled to produce a steel plate, characterized in that the heating time within a temperature range including the temperature T during heating before hot rolling is determined based on the total reduction rate R during blooming and the temperature T of the 1 / 4 thickness portion of the slab during heating before hot rolling. [2] The method for producing a steel sheet according to claim 1, characterized in that the relationship between the total reduction rate R during the blooming, the temperature T of the 1 / 4 thickness portion of the slab during the heating before hot rolling, and the heating time within a temperature range including said temperature T during the heating before hot rolling is such that the maximum heating temperature Tmax among the temperatures T is 1,300°C or less and that any one or more of the following (A), (B), and (C) are satisfied: (A) The total reduction ratio R during the blooming is 15% or more and less than 50%, and The heating is performed at a temperature T in the range of 1150° C. to 1300° C. for 1.5 hours to 2.0 hours. (B) The total rolling reduction R is 20% or more and less than 50%, and The heating is performed at a temperature T in the range of 1200° C. or more and 1300° C. or less for a period of more than 1.0 hour and not more than 2.0 hours. (C) The total rolling reduction R is 25% or more and less than 50%, and The heating is performed at a temperature T in the range of 1260° C. to 1300° C. for 0.5 hours to 2.0 hours. Effect of the Invention

[0010] The present invention provides a method for producing a steel sheet that improves microsegregation without long-term heat treatment, even in the case of a slab having a general slab thickness (150 mm or more) produced by continuous casting. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the influence of the presence or absence of blooming into a slab and the subsequent 1200° C. heat treatment time on the Mn segregation degree. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] In the present invention, the temperature of the slab during heating before hot rolling is defined as the temperature T of the 1 / 4 thickness part of the slab during heating before hot rolling. By evaluating the temperature T of the 1 / 4 thickness part of the slab, the behavior of microsegregation formed near the 1 / 4t part of the steel sheet can be accurately evaluated. Compared to the heating temperature (furnace temperature in the heating furnace soaking zone) normally used in heating before hot rolling, the above temperature T does not reach the heating temperature immediately, but is heated to the heating temperature over a period of time. The thermal history of temperature T varies depending on the slab thickness and heating conditions, but temperature T can be obtained by heat transfer simulation.

[0013] A continuous cast slab with a thickness of 200 mm and a major component composition of 0.20%C-2.60%Mn was rolled by blooming at a total reduction rate of 40.8%. A slab sample after blooming and a slab sample without blooming (both 25mm length x 25mm width x total thickness) were charged into a test heating furnace and heated so that the temperature T of the 1 / 4 thickness part of the slab sample was 1200°C. The residence time of the slab at the temperature in the test heating furnace (heat treatment time) was 0 hours (no heating), 1 to 20 hours, and the Mn microsegregation behavior of the slab sample after heating was evaluated. The 1 / 4t part of the slab sample was analyzed by EPMA (beam diameter 5μm) with the measurement area of ​​3mm length x 3mm thickness as the evaluation surface. The Mn analysis value obtained by EPMA was divided by the average Mn concentration of the sample to obtain the Mn segregation degree, and the area ratio of the region with a Mn segregation degree of 1.1 or more was evaluated.

[0014] The results are shown in Figure 1. It was found that the Mn segregation degree was significantly reduced by blooming compared to no blooming at a heat treatment time of 1 hour. The same results were obtained whether the cross section to be inspected was a cross section perpendicular to the casting direction (C cross section) or a cross section perpendicular to the width direction of the slab (L cross section). This shows that Mn is sufficiently diffused in the structure subjected to rolling even with short-time, low-temperature heating, reducing microsegregation. In addition, it is considered that high-speed diffusion occurs via the induced strain. Therefore, the present invention has been arrived at, which makes it possible to reduce microsegregation by blooming the slab after casting before heating for hot rolling, without the need for high-temperature, long-term heating before hot rolling.

[0015] Next, a continuous cast slab having a thickness of 280 mm and a composition of 0.18%C-2.6%Mn steel was used, and blooming was performed with a total reduction rate of 15% or more and less than 50%. After that, heating before hot rolling was performed, and a steel plate was manufactured by hot rolling. As a heating condition before hot rolling, the temperature T of the 1 / 4 thickness part of the slab at the time of heating before hot rolling was adjusted to various temperatures in the range of 1150 to 1300°C, and the heating time near the temperature T was variously changed in the range of 0.5 to 2 hours. The heated slab was hot rolled to obtain a steel plate with a plate thickness of 2.0 mm. The Mn concentration distribution of the 1 / 4t part of the steel plate was measured by EPMA analysis (beam diameter 1 μm) to calculate the standard deviation σ (mass%) of the Mn concentration distribution, and the value obtained by dividing the σ by the Mn concentration (mass%) of the steel plate was used as the microsegregation index (denoted as "σ / Mn"). It was determined that the microsegregation state was good if the σ / Mn value was 0.10 or less.

[0016] As a result, it was found that if blooming and heating before hot rolling are performed so as to satisfy one or more of the following conditions (A), (B), and (C), the σ / Mn value of the steel sheet after hot rolling will be 0.10 or less. (A) The total reduction ratio R during blooming is 15% or more and less than 50%, and The temperature T of the 1 / 4 thickness portion of the slab during heating before hot rolling is in the range of 1150° C. to 1300° C. for 1.5 hours to 2.0 hours. (B) The total rolling reduction R is 20% or more and less than 50%, and The heating is performed at a temperature T in the range of 1200° C. or more and 1300° C. or less for a period of more than 1.0 hour and not more than 2.0 hours. (C) The total rolling reduction R is 25% or more and less than 50%, and The heating is performed at a temperature T in the range of 1260° C. to 1300° C. for 0.5 hours to 2.0 hours.

[0017] Blooming is carried out after the continuous casting is completed and the slab is cut into pieces. Blooming may be carried out after continuous casting without reheating, or blooming may be carried out after reheating. When reheating is carried out, the reheating temperature is preferably in the range of 800°C to 1200°C. If it is less than 800°C, the load during blooming increases and cracks occur. If it is more than 1200°C, it is not preferable because of the yield loss due to scale generation, surface defects due to scale, increased heating costs, and low productivity.

[0018] If the total reduction rate of blooming is less than 15%, the segregation intervals in the slab after blooming are not fine, and this does not affect the diffusion of Mn. If the total reduction rate is 50% or more, the load during blooming is high, and productivity must be reduced. In addition, the reduction rate during rough rolling in the subsequent hot rolling process cannot be achieved, resulting in poor material quality.

[0019] In the pre-hot rolling heating, the maximum heating temperature Tmax of the temperature T has a lower limit and an upper limit. If the maximum heating temperature Tmax is less than 1150°C, the conditions (A) to (C) cannot be met, and sufficient Mn diffusion does not occur. If the maximum heating temperature Tmax exceeds 1300°C, surface defects due to scale growth occur.

[0020] In the case of pre-hot rolling heating, if the heating time exceeds two hours, the hot rolling production volume will decrease, leading to increased costs.

[0021] According to the present invention, the value of the Mn microsegregation index σ / Mn at the ¼t portion of the steel plate is 0.10 or less, so that surface defects caused by microsegregation can be prevented when the steel plate is used.

[0022] In the present invention, when performing continuous casting, it is preferable to use a soft reduction segment as a measure against center segregation, which can improve not only the micro-segregation of Mn in the 1 / 4t part of the steel plate, but also the center segregation in the 1 / 2t part.

[0023] In the steel sheet of the present invention, the microsegregation of Mn in the 1 / 4t portion of the steel sheet is improved, so that the elongation and hole expandability of the steel sheet can be improved. In addition, surface defects during press forming can be suppressed. Even in plated steel sheets in which a steel sheet is plated, surface defects caused by microsegregation can be suppressed.

[0024] The present invention can exert a particularly excellent effect in high-tensile steel sheets. The high-tensile steel sheet means a steel sheet having a tensile strength of 340 MPa or more. The thin steel sheet means a steel sheet manufactured through continuous hot rolling and, if necessary, cold rolling, or a steel sheet further subjected to a plating treatment.

[0025] The composition of the steel sheet to which the method for producing a steel sheet of the present invention is applied is not particularly limited, and can be applied to all steel sheets. Preferably, the composition is, in mass%, C: 0.075 to 0.400%, Si: 0.01 to 2.50%, Mn: 0.50 to 3.50%, P: 0.1000% or less, S: 0.0100% or less, Al: 2.000% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0.000 to 0.200%, Nb: 0.000 to 0.100%, V: 0.000 to 0.500%, Zr: 0.000 to 0.100%, Cr: 0.00 to 2.00%, Ni: It can be used in a steel sheet having a composition consisting of 0.00-2.00%, Cu: 0.00-2.00%, Mo: 0.00-1.00%, B: 0.0000-0.0100%, W: 0.00-2.00%, Ca: 0.0000-0.0100%, Mg: 0.0000-0.0100%, one or more of REM (Sc, Y, and elements with atomic numbers 57 to 71): 0.0000-0.0100% in total, and the balance: Fe and impurities. An element whose lower limit in the composition range is zero means that it is an optional component. EXAMPLES

[0026] In the test, slabs having the steel composition shown in Table 1 and the thickness shown in Table 2 were produced by continuous casting. In Table 2, values ​​outside the range of the present invention are underlined.

[0027] [Table 1]

[0028] [Table 2]

[0029] The continuous casting conditions were, for example, when the slab thickness was 280 mm, the slab width was 1600 mm, and the casting speed was 1.2 m / min. Except for Reference Example 18 in Table 2, the slabs were bloomed before hot rolling using blooming at a total reduction rate R shown in Table 2. The slabs cast by continuous casting were cut after solidification and bloomed as is or after reheating. The slab surface temperature before blooming was 900 to 1150°C. After blooming, the slabs were transported to a pre-hot rolling heating furnace.

[0030] In the pre-hot rolling heating furnace, the temperature T of the 1 / 4 thickness part of the slab was heated in various temperature patterns. The temperature T of the 1 / 4 thickness part of the slab during pre-hot rolling heating was obtained by heat transfer simulation. The "Maximum heating temperature Tmax" in the "Pre-hot Rolling Heating" column of Table 2 shows the maximum temperature value of the temperature T of the 1 / 4 thickness part of the slab during pre-hot rolling heating. In addition, in the "Pre-hot Rolling Heating Time of Each Region" column of the "Pre-hot Rolling Heating" column, the heating time in the temperature range of 1150°C to 1300°C during pre-hot rolling heating when the total reduction R during blooming is 15% to less than 50% is listed in column (A). When the total reduction R is 20% to less than 50%, the heating time in the temperature range of 1200°C to 1300°C is listed in column (B). When the total rolling reduction R was 25% or more and less than 50%, the heating time within the temperature range T of 1260°C or more and 1300°C or less was recorded in column (C).

[0031] After the pre-hot rolling heating, the steel sheet was hot rolled to a thickness of 2.0 mm at a finishing temperature of 850° C. and a coiling temperature of 600° C. The resulting steel sheet was a high-tensile steel sheet with a tensile strength of 590 MPa or more.

[0032] The microsegregation state of the steel sheet was investigated using an EPMA. The Mn concentration distribution in an area of ​​500 μm × 500 μm at 1 / 4 of the sheet thickness in the cross section parallel to the rolling direction and perpendicular to the width direction of the steel sheet was measured with a beam diameter of 1 μm. The standard deviation σ (mass%) of the obtained Mn concentration distribution was divided by the Mn concentration (mass%) of the steel material to obtain the microsegregation index (σ / Mn).

[0033] In the present invention examples (Test Nos. 1 to 8), the relationship between the total reduction ratio R during blooming performed before hot rolling and the temperature T at the 1 / 4 thickness part of the slab satisfied one or more of the above conditions (A) to (C). The microsegregation index σ / Mn of the hot-rolled steel sheets produced under these conditions was all 0.10 or less.

[0034] On the other hand, in the comparative examples, Test No. 9 (total rolling reduction rate R: 15%) and Test No. 13 (R: 18%), the heating time in region (A) was insufficient, in Test No. 10 and Test No. 14 (both R: 20%) the heating time in both regions (A) and (B) was insufficient, and in Test No. 11 (R: 25%) the heating time in both regions (A), (B), and (C) was insufficient, and the reduction in microsegregation was insufficient.

[0035] In test number 12, the total reduction rate R of the blooming was insufficient, and the reduction of microsegregation was insufficient.

[0036] In test No. 15, microsegregation was reduced, but the total reduction rate R of blooming was high at 55%, and the load during blooming was high, so productivity had to be reduced. In test No. 16, microsegregation was reduced, but the maximum heating temperature Tmax was high at 1310℃, and surface defects due to scale occurred.

[0037] Test No. 17 used a 140mm thick slab. When blooming was performed with a total reduction rate R of 15%, the slab thickness after blooming was only 119mm, which was thin, and the slab shape was deformed due to sagging between the skids of the hot rolling heating furnace, making it impossible to continue hot rolling. Although it is possible to deal with this by narrowing the gap between the skids, this is not desirable because it would increase equipment costs and would increase the number of skid marks (the slab part in contact with the skid cooling).

[0038] As a reference example, test number 18 was a condition in which blooming was not used and the heating time at 1260°C in the heating before hot rolling was a long time of 2.3 hours. Under these conditions, the segregation index can be made 0.10 or less, but the long heating time causes a decrease in hot rolling productivity and the occurrence of scale defects, resulting in an increase in manufacturing costs.

[0039] When the steel sheets manufactured in the present invention (Test Nos. 1 to 8) were subjected to press forming, the occurrence of surface defects due to microsegregation of the steel sheets could be suppressed. This is believed to be because the reduction in microsegregation led to a uniform internal structure of the steel sheets, which allowed for uniform deformation by the press forming process. In addition, the elongation and hole expandability of the steel sheets could be improved.

[0040] In conventional plated steel sheets using high ductility steel sheets, the alloying acceleration of plating varies depending on the concentration unevenness caused by Mn microsegregation in the steel sheet, and plating streak patterns, which are streaky patterns formed on the plated surface, sometimes occur. When the steel sheets manufactured in the present invention examples (Test Nos. 1 to 8) were plated to produce plated steel sheets, the plating streak patterns caused by the microsegregation of the steel sheet could be eliminated.

Claims

[Claim 1] A method for producing a steel plate in which a slab having a thickness of 150 mm or more produced by continuous casting is bloomed, and then heated and hot rolled to produce a steel plate, the method determining a heating time within a temperature range including the temperature T during heating before hot rolling based on a total reduction rate R during blooming and a temperature T of a 1 / 4 thickness portion of the slab during heating before hot rolling, a relationship between the total reduction rate R during the blooming, the temperature T of the ¼ thickness portion of the slab during the heating before hot rolling, and the heating time within a temperature range including said temperature T during the heating before hot rolling, such that a maximum heating temperature Tmax among said temperatures T is 1,300°C or less, and such a relationship satisfies any one or more of the following (A), (B), and (C): (A) The total reduction ratio R during the blooming is 15% or more and less than 50%, and The heating is performed at a temperature T in the range of 1150° C. to 1300° C. for 1.5 hours to 2.0 hours. (B) The total rolling reduction R is 20% or more and less than 50%, and The heating is performed at a temperature T in the range of 1200° C. or more and 1300° C. or less for a period of more than 1.0 hour and not more than 2.0 hours. (C) The total rolling reduction R is 25% or more and less than 50%, and The heating is performed at a temperature T in the range of 1260° C. to 1300° C. for 0.5 hours to 2.0 hours.

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