Straightening method for cast slabs

Abstract

To provide a continuous cast slab that is a slab of high Al steel containing Al by 0.2 mass% or more, and has excellent crack resistance sensitivity, and to provide a method for continuous casting the slab.SOLUTION: Provided is a continuous cast slab of high Al steel containing Al by 0.2 mass% to 2.0 mass%, and when [Zr], [Ti], [B], [Al], and [N] are respectively the content (mass%) in the continuous cast slab, the Zr content and the Ti content satisfy the relation of [Zr] + 0.2×[Ti] + 0.8×[B]≥4 / 3×[Al]×[N], and B content is 0.0005 mass% or more and 0.0050 mass% or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention is particularly directed to the production of steels containing a large amount of Al. Straightening of cast slabs It concerns the method. [Background technology]

[0002] In recent years, alloy steels containing large amounts of Al have been widely produced as high-strength steel materials for thin plates to improve their mechanical properties. However, the more Al added, the more susceptible the steel is to transverse cracks on the surface of the slab during continuous casting, which is a problem in terms of both operation and product quality.

[0003] At the straightening point in a curved or vertical bending type continuous casting machine, a straightening stress is applied to the slab. It is known that transverse cracks occur along the prior austenite grain boundaries on the surface of the slab, and the transverse cracks occur when the straightening stress is concentrated on the austenite grain boundaries embrittled by the precipitation of AlN, NbC, etc., or on the film-like ferrite formed along the prior austenite grain boundaries. In addition, these transverse cracks tend to occur particularly in a temperature range slightly higher than the phase transformation region from austenite to ferrite, but transverse cracks also occur in non-transformation compositions. Therefore, a method is usually adopted to control the surface temperature of the slab so as to avoid the temperature range (embrittlement temperature range) where ductility decreases at the straightening point, thereby suppressing the occurrence of transverse cracks.

[0004] However, controlling the surface temperature of a slab to avoid the embrittlement temperature range is often difficult because it imposes significant constraints on operation. Therefore, Patent Document 1 discloses a technique in which Ti is added in an amount of more than 0.010 mass% and not more than 0.025 mass%, and the surface temperature of the slab in the upper part of the secondary cooling zone, where the solidified shell thickness of the slab is 10 mm to 30 mm, is made equal to or higher than the precipitation temperature of AlN. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6347164 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, high-Al steel containing 0.2 mass% or more of Al has been produced to further improve mechanical properties. As the Al concentration increases, AlN precipitates at a higher temperature, expanding the embrittlement temperature range. Therefore, when the steel contains 0.2 mass% or more of Al, the embrittlement temperature range expands significantly, making it almost impossible to perform bending and straightening while avoiding the embrittlement temperature range, and transverse cracks cannot be avoided.

[0007] Furthermore, the method described in Patent Document 1 targets low-carbon aluminum-killed steel with an Al concentration of 0.063 mass % to 0.093 mass %, and its effect on high-Al steel containing 0.2 mass % or more of Al is unclear.

[0008] In view of the above problems, the present invention provides a high-Al steel containing 0.2 mass % or more of Al. Straightening of cast slabs The present invention aims to provide a method. [Means for solving the problem]

[0009] The inventors of the present invention have focused on the fact that the high-temperature embrittlement of high-Al steel slabs is caused by the precipitation of large amounts of AlN, and have investigated the control of nitride precipitation. Specifically, they have investigated the high-temperature ductility of steels to which Zr, which has a higher N-fixing ability than Al, has been added. As a result, they have found that the high-temperature ductility is greatly improved by adding a small amount of Zr. Zr forms ZrN immediately after solidification and fixes N, so it has been found that this suppresses the precipitation of large amounts of AlN at grain boundaries and can drastically improve the high-temperature embrittlement of high-Al steels.

[0010] However, because Zr is an expensive metal, the inventors discovered that by keeping the amount of Zr added as low as possible and adding appropriate amounts of Ti and Zr, it is possible to suppress the precipitation of large amounts of AlN at grain boundaries without increasing costs too much.

[0011] On the other hand, since Zr and Ti inclusions precipitate at high temperatures immediately after solidification, adding a large amount of these elements tends to make the inclusions coarse in the cast slab, and the coarse inclusions may become the starting point of defects in the subsequent hot rolling process or cold rolling process, or may deteriorate material properties such as hole expansion property and elongation rate. Therefore, it is preferable to suppress the amount of Zr and Ti added to a certain extent. Therefore, the present inventors have conducted extensive research with the idea of ​​not only suppressing the precipitation of AlN, but also strengthening the grain boundary itself, which becomes embrittled at high temperatures.

[0012] The present inventors have found that by utilizing grain boundary strengthening by solute B, high-temperature embrittlement of high-Al steels can be improved and surface cracks can be prevented, and further cracks can be prevented by subsequent rolling. However, even if grain boundary strengthening is obtained simply by solute B, if the amount of precipitated AlN is large, high-temperature embrittlement cannot be avoided, and the effect of solute B cannot be obtained. In other words, it is necessary to add solute B to the amount of precipitated AlN, and further, the amount of precipitated AlN can be suppressed by adding Zr or Ti, and the effect of solute B can be stably obtained by adding these elements in combination. Since Zr and Ti have a stronger power to fix N than Al, nitrides are formed in steel from high temperatures in the order of ZrN, TiN, and AlN. In other words, by adding these elements in combination with B, the AlN at the origin of embrittlement is reduced, and B is stably segregated to the grain boundary as solute B, and ductility can be improved more effectively than when B is added alone.

[0014] Based on the above, the present invention is as follows. (1) C: 0.02% by mass ~ 0.50% by mass, Si: 0.2% by mass to 3.0% by mass, Mn: 0.5% by mass to 4.0% by mass, Al: 0.2% by mass to 2.0% by mass, and Zr: 0.05% by mass or less A method for bending and straightening a slab of high Al steel containing: wherein the Zr content, the Ti content, and the B content satisfy the following formula (1), and the B content satisfies the following formula (2), with the remainder being Fe and impurities, The cast piece Against The bending and straightening of the cast slab is performed at a surface temperature in the range of 800°C to 1000°C. Bending correction method . [Zr]+0.2×[Ti]+0.8×[B]≧4 / 3×[Al]×[N]···(1) 0.0005 mass%≦[B]≦0.0050 mass% (2) Here, [Zr], [Ti], [B], [Al], and [N] each represent the content (mass%) in the cast slab. Effect of the Invention

[0015] According to the present invention, it is possible to provide a continuously cast slab which is free from cracks due to corrective stress. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a graph showing changes in area shrinkage rate when the tensile temperature is in the range of 700° C. to 1100° C. [Diagram 2] FIG. 1 is a graph showing the relationship between [Al]×[N] and [Zr]+0.2×[Ti]+0.8×[B] at a tensile temperature of 900° C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The present invention will now be described with reference to the drawings. In order to produce high-Al steel containing 0.2 mass% or more of Al, it is necessary to prevent the occurrence of transverse cracks due to the straightening stress at the straightening point during continuous casting. Since it is difficult to keep the temperature at the straightening point outside the brittle temperature range, the present inventors have considered adding Zr and / or Ti to straighten the slab in a general temperature range at the straightening point.

[0018] On the other hand, the present inventors also focused on strengthening grain boundaries by solid solution B, investigated the addition of Zr, Ti and B, and carried out the following experiments to find conditions under which cracks would not occur.

[0019] (First experiment) First, a high-temperature tensile test was carried out to confirm the extent to which the addition of B improves high-temperature ductility. In this test, experiments were carried out with four types of steel, steel types A to D, shown in Table 1. All values ​​in Table 1 indicate mass %, and as shown in Table 1, steel type A contains only small amounts of Zr, Ti, and B, while steel type B contains a relatively large amount of Zr, but otherwise has almost the same composition as steel type A. Steel type C contains a relatively large amount of Ti, but otherwise has almost the same composition as steel type A. Meanwhile, steel type D is an example in which both Zr and B are contained in relatively large amounts. The remainder in all cases consists of Fe and impurities.

[0020] [Table 1]

[0021] Next, the tensile temperature was changed in the range of 700°C to 1100°C, and the area reduction (RA) (%) was obtained for these four types of steel. Specifically, each steel type was prepared by vacuum melting 25 kg, and then forged and stretched to φ15 to prepare a φ10 tensile test piece. In the high-temperature tensile test, a high-frequency induction heating type high-temperature tensile test device with a cold crucible was used, and the tensile test piece was melted and then cooled to a specified tensile temperature at a cooling rate of 1.0°C / s, and then held at the specified tensile temperature while straining at a rate of 3.3×10 -4 The specimen was subjected to tension at 1 / s until fracture. The cross-sectional area of ​​the specimen before the test was subtracted from the cross-sectional area of ​​the fractured surface of the tensile test specimen after the test to obtain the cross-sectional reduction ratio.

[0022] The tensile test results are shown in Figure 1. In Figure 1, the circle marks represent the area reduction ratio for steel type D, the triangle marks represent the area reduction ratio for steel type C, the diamond marks represent the area reduction ratio for steel type B, and the square marks represent the area reduction ratio for steel type A. As shown in Figure 1, it was found that adding appropriate amounts of both Zr and B increases the area reduction ratio and improves high-temperature ductility, especially in the temperature range of 800 to 1000 °C. Here, it can be considered that transverse cracks do not occur due to the straightening stress if RA is 50% or more. Since it is easy to pass the straightening point in the range of 800 to 1000 °C, it was found that transverse cracks can be prevented by adding Zr and B without performing temperature control to avoid the embrittlement temperature range.

[0023] (Second experiment) Next, a test was conducted to confirm how much Zr and / or Ti and B should be added to prevent transverse cracks. Specifically, the tensile temperature was set to 900°C, and a number of samples (No. 1 to No. 12) with different amounts of Al, Ti, N, Zr, and B were prepared as shown in Table 2, and a tensile test was conducted to determine the RA (%) for each sample. The specific method of the tensile test was the same as in the first experiment. The results of the tensile test are shown in Table 2.

[0024] [Table 2]

[0025] In Figure 2, as a guideline for when RA is thought to be unlikely to occur, a mark of O was given for 50% or more, and a mark of X was given for RA less than 50%. As a result, it was found that the contents of Zr, Ti, and B are correlated with the product of the Al content and the N content. In other words, if the value of Zr content + Ti content x 0.2 + B content x 0.8 is 4 / 3 times or more the product of the Al content and the N content, the RA will be 50% or more, and it was found that lateral cracks due to straightening stress can be prevented.

[0026] Based on the above experimental results, the chemical composition of the continuously cast slab according to the present invention will be described. The continuously cast slab according to this embodiment is a high Al steel containing 0.2 mass% to 2.0 mass% Al, and is mainly intended for use as a thin plate. From the above first and second experimental results, the continuously cast slab according to this embodiment contains Zr, Ti, and B in amounts that satisfy the following formula (1). [Zr]+0.2×[Ti]+0.8×[B]≧4 / 3×[Al]×[N] (1) Here, [Zr], [Ti], [B], [Al], and [N] each represent the content (mass%) in the continuously cast slab.

[0027] B segregates to the austenite grain boundaries in steel and strengthens the grain boundaries, thereby significantly improving the high-temperature ductility of high-Al steel. In particular, the effect of improving ductility is remarkable when added together with elements with high N fixing power such as Zr and Ti. On the other hand, when B exceeds 0.0050 mass%, the effect of B becomes saturated. For the above reasons, the B content of the continuously cast slab according to this embodiment is set to satisfy the following formula (2). Preferably, 0.0005 mass%≦B≦0.0030 mass%. 0.0005 mass%≦[B]≦0.0050 mass% (2)

[0028] Zr has a very high N fixing ability and a large effect of suppressing AlN precipitation, and its inclusion greatly improves the high-temperature ductility of high-Al steel. On the other hand, excessive Zr inclusions are generated, which become the starting point of defects in the rolling process and cause deterioration of material properties. Therefore, Zr in the steel is preferably 0.05 mass% or less. More preferably, Zr is 0.03 mass% or less. In addition, the lower limit of Zr is not particularly limited, and Zr may be 0 mass% as long as the above formulas (1) and (2) are satisfied.

[0029] Although Ti is inferior to Zr, it has a high nitrogen fixation ability and an effect of suppressing the precipitation of AlN. Therefore, especially when contained together with Zr and B, the hot ductility of high-Al steel is greatly improved. On the other hand, when Ti is contained in excess, coarse Ti inclusions are generated, which become the starting point of flaws in the rolling process and the cause of deterioration of material properties. Therefore, the Ti content in the steel is preferably 0.05% by mass or less. More preferably, the Ti content is 0.03% by mass or less. Also, the lower limit value of Ti is not particularly limited, and Ti may be 0% by mass as long as the above-mentioned equations (1) and (2) are satisfied.

[0030] N inevitably exists in the steel and forms nitrides, so it has an adverse effect on hot ductility, and the lower the N content, the better the ductility tends to be. Therefore, the N content in the steel is preferably 0% by mass or more and 0.0100% by mass or less. However, reducing the N that enters from the atmosphere causes a large load on the degassing process and an increase in cost. Also, when the N content is high, it is necessary to contain a large amount of the above-mentioned Zr, Ti, and B, which leads to an increase in cost. Therefore, more preferably, 0.0010% by mass ≤ N ≤ 0.0050% by mass. Also, in this embodiment, high-Al steel is targeted, but when the Al content exceeds 2.0% by mass, the Zr content, Ti content, and B content also increase according to equation (1), resulting in an unnecessary increase in cost. Therefore, the Al content is 0.2 to 2.0% by mass.

[0031] As described above, in the continuously cast slab according to this embodiment, the relationship between the contents of Zr, Ti, B, Al, and N satisfies the conditions of the above-mentioned equations (1) and (2). On the other hand, the contents of other elements are not particularly limited, but C, Si, and Mn are preferably contained in the following ranges.

[0032] <C: 0.02% by mass to 0.50% by mass> C is an element that improves the strength of steel. When the C content is less than 0.02% by mass, it does not meet the requirements for use as a high-strength steel plate. Also, when the C content exceeds 0.50% by mass, the hardness becomes too high and the required bendability cannot be ensured. Therefore, the C content is preferably 0.02% by mass to 0.50% by mass.

[0033] <Si: 0.2 mass% to 3.0 mass%> Si is an element that improves the strength of steel. If the Si content is less than 0.2 mass%, it does not meet the requirements for use as a high-strength steel plate. Also, if the Si content exceeds 3.0 mass%, it has an adverse effect on weldability. Therefore, the Si content is preferably 0.2 mass% to 3.0 mass%.

[0034] <Mn: 0.5 mass% to 4.0 mass%> Mn is an element that improves the strength of steel. If the Mn content is less than 0.5 mass%, it does not meet the requirements for use as a high-strength steel plate. Also, since Mn is a segregation element when the Mn content exceeds 4.0 mass%, it may cause uneven strength in the slab or steel plate. Therefore, the Mn content is preferably 0.5 mass% to 4.0 mass%.

[0035] Next, the continuous casting method of the above-mentioned continuous casting slab will be described. In this embodiment, since it is not necessary to avoid the embrittlement temperature range, a particularly common method can be used in continuous casting. From the results of the above-mentioned first experiment, when bending and straightening the slab, it is preferable to perform bending and straightening in a state where the surface temperature of the slab is 800°C to 1000°C because the effect is particularly remarkable.

Examples

[0036] Next, the examples of the present invention will be described. However, these conditions are one example of conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of these examples. The present invention can be implemented by various means without departing from the gist of the present invention and achieving the object of the present invention.

[0037] Sixteen types of molten steel with C content of 0.3 mass%, Si content of 1.5 mass%, Mn content of 2.0 mass%, and Al content, N content, Zr content, Ti content, and B content were prepared, poured into a mold, and continuously cast by a continuous casting machine. The continuous casting machine used was a vertical bending type continuous casting machine with a mold size of 250 mm thickness x 1200 mm width, and the casting speed was 1.2 m / min. In addition, the surface temperature of the cast piece was 850°C at the straightening point in all cases.

[0038] From each of the continuous cast slabs produced under the above conditions, a representative sample of about 10 m was cut out and the slab cracks were evaluated. First, the front and back surfaces of the slab were ground with a 0.7 mm grinder, and the presence or absence of cracks was visually confirmed. In addition, for slabs for which no cracks were found as a result of visual inspection, they were subjected to the next process of hot rolling without any maintenance (no crack checking with a grinder), and the occurrence of cracks thereafter was also confirmed. If there were no cracks in the slab and no cracks occurred after hot rolling, they were evaluated as "◎", and if there were no cracks in the slab but cracks occurred after hot rolling, they were evaluated as "△". If cracks were found in the slab but could be removed by minor maintenance (further grinding of 0.7 mm), they were evaluated as "×", and if cracks were found in the slab and could not be removed by minor maintenance and further heavy maintenance was required, they were evaluated as "XX". The experimental results are shown in Table 3. In addition, a tensile test was performed on some of the cast pieces at a tensile temperature of 900° C., and the RA (%) was calculated. The specific method of the tensile test was the same as in the first experiment. The results of the tensile test are also shown in Table 3.

[0039] [Table 3]

[0040] The underlined parts in Table 3 are examples that did not satisfy the conditions of the present invention. As shown in Table 3, when the conditions of formulas (1) and (2) were satisfied, no cracks were present in the slab, and no cracks occurred after hot rolling, regardless of the Al and N contents.

[0041] On the other hand, in Comparative Examples No. 8, 15, and 16, which satisfied only formula (1) and did not satisfy formula (2), grain boundary strengthening by solid solution B was insufficient, and cracks occurred in the slab or hot-rolled sheet. In particular, in Comparative Examples No. 15 and 16, although cracks were not confirmed on the surface of the slab, it is believed that there were cracks inside the slab or that the slab was in a state where cracks were likely to occur. Conversely, in Comparative Examples No. 9 to No. 14, which satisfied only formula (2) and did not satisfy formula (1), it is believed that a large amount of AlN remained, and cracks occurred in the slab.

Claims

[Claim 1] C: 0.02% by mass to 0.50% by mass, Si: 0.2% by mass to 3.0% by mass, Mn: 0.5% by mass to 4.0% by mass, Al: 0.2% by mass to 2.0% by mass, and Zr: 0.05% by mass or less A method for bending and straightening a slab of high Al steel containing: wherein the Zr content, the Ti content, and the B content satisfy the following formula (1), and the B content satisfies the following formula (2), with the remainder being Fe and impurities, The method for straightening a slab comprises bending and straightening the slab at a surface temperature in the range of 800°C to 1000°C. [Zr]+0.2×[Ti]+0.8×[B]≧4 / 3×[Al]×[N]...(1) 0.0005 mass%≦[B]≦0.0050 mass% (2) Here, [Zr], [Ti], [B], [Al], and [N] each represent the content (mass %) in the cast slab.

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