Nickel-containing steel slab and method for producing nickel-containing steel slab

Abstract

Provided is a nickel-containing steel slab that contains 5–10% by mass of nickel and has little surface cracking. This nickel-containing steel slab contains 5–10% by mass of nickel. The nickel-containing steel slab includes at least 0.004% by mass and no more than 0.015% by mass of antimony. The grain boundary oxidation depth of a surface layer of the nickel-containing steel slab is no more than 200 μm. In this nickel-containing steel slab, antimony is present in greater concentration in the vicinity of crystal grain boundaries. It can be considered that antimony is oxidized with greater preference than grain boundaries. As a result, oxidation of the nickel steel slab can be equalized. As a result, the progression of local oxidation can be suppressed, the formation of wedges that can cause cracking can be suppressed, and the yield of the nickel steel slab can be increased. Due to this configuration, the time for repair processing to remove cracks in the surface of the steel slab can be reduced.

Description

Ni-containing steel slab and method for producing Ni-containing steel slab

[0001] The present invention relates to a Ni-containing steel slab containing 5 to 10 mass % Ni (nickel), and a method for producing the Ni-containing steel slab.

[0002] It is known that adding Ni to steel improves low-temperature toughness. Steel containing approximately 9 mass% Ni (hereinafter also referred to as Ni-containing steel) is also called 9% Ni steel. 9% Ni steel can withstand use at temperatures below -160°C, and is therefore widely used as steel for welded structures for low-temperature applications such as LNG tanks.

[0003] It is known that Ni-containing steels, including 9% Ni steel, are prone to surface defects. For example, after casting, a cast slab has numerous cracks on or near the surface (hereinafter also referred to as surface cracks).

[0004] It has been known for some time that surface cracks in Ni-containing steel slabs (hereinafter also referred to as Ni-containing steel slabs) occur along the grain boundaries of the coarse solidification structure. Specifically, it is believed that S (sulfur) and P (phosphorus) segregated at the grain boundaries during solidification embrittle the grain boundaries, and the grain boundaries are destroyed and cracked by stresses due to bending and straightening of the slab in the secondary cooling zone of a continuous casting machine and thermal stresses due to cooling. In particular, it has been reported that surface cracks are likely to occur when bending and straightening are performed in the temperature range of 600 to 800°C (referred to as the "high-temperature embrittlement temperature range").

[0005] When producing Ni-containing steel slabs by continuous casting, it is important to prevent the occurrence of surface cracks in the slabs. As described above, in order to prevent surface cracks during continuous casting of Ni-containing steel slabs, it is considered effective to avoid straightening in the high-temperature embrittlement temperature range.

[0006] Therefore, temperature control has been carried out in secondary cooling when casting Ni-containing steel slabs. For example, Patent Document 1 discloses that when continuously casting molten steel containing 5 to 10 mass % Ni, the cooling rate and the surface temperature of the slab are controlled in the secondary cooling zone.

[0007] Furthermore, Patent Document 2 discloses that when continuously casting Ni-containing steel containing 8 to 10 mass % of Ni, the reduction in area during casting is estimated and the secondary cooling intensity is controlled so that the reduction in area becomes 50% or more.

[0008] JP-A-8-10919 JP-A-8-33964

[0009] Although continuous casting methods have been proposed that suppress many surface cracks, it is difficult to completely suppress the occurrence of surface cracks. When surface cracks occur in a steel slab, they are removed by a so-called finishing process, such as grinding the surface with a grinder. Therefore, if the number of surface cracks increases, the area and time required for the finishing process also increase, resulting in a decrease in productivity and an increase in manufacturing costs.

[0010] The present invention has been made in view of the above problems, and an object of the present invention is to provide a Ni-containing steel slab that contains 5 to 10 mass % Ni and has few surface cracks, and a method for producing the Ni-containing steel slab.

[0011] In order to solve the above problems, the present invention has the following features.

[0012] [1] A Ni-containing steel slab containing 5 to 10 mass% Ni, containing 0.004 mass% or more and 0.015 mass% or less of Sb, and having a surface intergranular oxidation depth of 200 μm or less. [2] A Ni-containing steel slab containing 5 to 10 mass% Ni, containing 0.004 mass% or more and 0.015 mass% or less of a grain boundary oxidation inhibitor containing Sb, and having a surface intergranular oxidation depth of 200 μm or less. [3] In mass%, C: 0.01% or more and 0.10% or less, Si: 0.01% or more and 0.40% or less, Mn: 0.20% or more and 1.00% or less, P: 0.005% or less, S: 0.005% or less, Al: 0.020% or more and 0.040% or less, N: 0.001% or more and 0.005% or less, Cu: 0.0% or more and 0.5% or less, Cr: 0.0% or more and 0.5% or less, Mo: 0.0% or more and 0.5% or less, V: 0.00% or more and 0.05% or less, Nb: 0.00% or more and 0.05% or less, and the balance consisting of Fe and unavoidable impurities. [4] A method for producing a Ni-containing steel slab containing 5 to 10 mass% Ni, comprising: a mixing step of mixing 0.004 mass% or more and 0.015 mass% or less of Sb into raw materials for the Ni-containing steel slab; and a straightening step of straightening the Ni-containing steel slab in a secondary cooling zone of continuous casting, wherein the straightening step is performed at a surface temperature of 800°C or more and 1100°C or less. [5] A method for producing a Ni-containing steel slab containing 5 to 10 mass% Ni, comprising: a mixing step of mixing 0.004 mass% or more and 0.015 mass% or less of a grain boundary oxidation inhibitor containing Sb into a raw material for the Ni-containing steel slab; and a straightening step of straightening the Ni-containing steel slab in a secondary cooling zone of continuous casting, wherein the straightening step is performed at a surface temperature of the Ni-containing steel slab of 800°C or more and 1100°C or less.

[0013] The Ni-containing steel slab of the present invention contains 0.004 mass % or more and 0.015 mass % or less of Sb. Sb functions as a grain boundary oxidation inhibitor. That is, in the Ni-containing steel slab of the present invention, Sb is concentrated near the grain boundaries. It is believed that Sb oxidizes preferentially over the grain boundaries. As a result, oxidation of the Ni-containing steel slab can be leveled.

[0014] Therefore, it is possible to suppress the progression of local oxidation, suppress the formation of wedges that may lead to cracks, and increase the yield of Ni-containing steel slabs, which in turn reduces the processing time required for the treatment to remove cracks on the surface of the Ni-containing steel slabs, thereby improving productivity and reducing production costs.

[0015] The Ni-containing steel slab of the present invention (hereinafter also simply referred to as Ni-containing steel slab) contains 5 to 10 mass % Ni. The Ni-containing steel slab can be used, for example, as a low-temperature steel used in a temperature range lower than room temperature.

[0016] The Ni-containing steel slab of the present invention contains Sb. Sb functions as a grain boundary oxidation inhibitor. Hereinafter, Sb will also be referred to as the grain boundary oxidation inhibitor. The Ni-containing steel slab preferably contains 0.004 mass% to 0.015 mass% of the grain boundary oxidation inhibitor, preferably 0.005 mass% to 0.015 mass%, and more preferably 0.006 mass% to 0.010 mass%. If the content of the grain boundary oxidation inhibitor exceeds 0.01%, the toughness of the Ni-containing steel slab tends to decrease.

[0017] The Ni-containing steel slab of the present invention preferably has a surface grain boundary oxidation depth of 200 μm or less, preferably 180 μm or less, more preferably 50 to 100 μm, and even more preferably 55 to 80 μm.

[0018] The intergranular oxidation depth of the surface layer can be measured, for example, by observing a cross section including the entire width direction of the Ni-containing steel slab using an optical microscope. Specifically, the intergranular oxidation depth of the surface layer can be determined as the longest measured length from the surface layer to the tip of the intergranular oxidation portion. By making the intergranular oxidation depth of the surface layer of the Ni-containing steel slab 200 μm or less, it is possible to prevent cracks from occurring starting from the intergranular oxidation portion.

[0019] The Ni-containing steel slab of the present invention has, in mass %, C: 0.01% or more and 0.10% or less, Si: 0.01% or more and 0.40% or less, Mn: 0.20% or more and 1.00% or less, P: 0.005% or less, S: 0.005% or less, Ni: 5.0% or more and 10.0% or less, Al: 0.020% or more and 0.040% or less, N: 0.001% or more and 0.005% or less, Cu: 0.0% or more and 0.5% or less, Cr: 0.0% or more and 0.5% or less, Mo: 0.0% or more and 0.5% or less, V: 0.00% or more and 0.05% or less, Nb: 0.00% or more and 0.05% or less, The balance is Fe and unavoidable impurities.

[0020] The strength of the base material of a Ni-containing steel slab can be ensured by including C (carbon) as a component. In particular, by setting the C content of the Ni-containing steel slab to 0.01 mass% (hereinafter simply referred to as "%") or more, the strength of the base material can be improved. If the C content of the Ni-containing steel slab is excessive, suitable toughness may not be obtained. By setting the C content of the Ni-containing steel slab to 0.10% or less, suitable toughness can be obtained.

[0021] By including silicon (Si) in the composition of Ni-containing steel slabs, the deoxidation effect of removing oxygen contained in the Ni-containing steel slabs can be enhanced. By setting the Si content of the Ni-containing steel slabs to 0.01% or more, an excellent deoxidation effect can be obtained. Furthermore, by setting the Si content of the Ni-containing steel slabs to 0.40% or less, an increase in temper embrittlement susceptibility can be suppressed.

[0022] The Ni-containing steel slab contains Mn as a component, thereby ensuring the strength of the base material. By setting the Mn content in the Ni-containing steel slab to 0.20% or more, the strength of the base material can be improved. Furthermore, by setting the Mn content in the steel slab to 1.00% or less, suitable toughness can be ensured.

[0023] P (phosphorus) is an impurity element in Ni-containing steel slabs. When P (phosphorus) is included in the composition of Ni-containing steel slabs, P (phosphorus) tends to segregate at grain boundaries, resulting in a decrease in toughness. It is preferable that the Ni-containing steel slabs contain as little P (phosphorus) as possible in their composition. Note that, if refining costs are acceptable, the P (phosphorus) content should be 0.005% or less, and preferably 0.004% or less.

[0024] S (sulfur) is an impurity element in Ni-containing steel slabs. When S (sulfur) is included in the composition of Ni-containing steel slabs, S (sulfur) tends to segregate at grain boundaries, resulting in a decrease in toughness. It is preferable that Ni-containing steel slabs contain as little S (sulfur) as possible in their composition. Note that, if refining costs are acceptable, the S (sulfur) content should be 0.005% or less, and preferably 0.001% or less.

[0025] The Ni-containing steel slab preferably contains 5.0 to 10.0% Ni (nickel) as a composition, and more preferably 7.0 to 9.5% Ni.

[0026] By setting the Ni content of the Ni-containing steel slab to 5.0% or more, appropriate toughness can be obtained. From the viewpoint of cost, the Ni content of the Ni-containing steel slab is preferably set to 10.0% or less.

[0027] By including Al (aluminum) in the composition of Ni-containing steel slabs, the deoxidation effect of removing oxygen from the steel slabs can be enhanced. If the Al (aluminum) content is higher than 0.100%, cleanliness tends to be impaired. Furthermore, as the Al (aluminum) content increases, AlN, which reduces the toughness of the base material, is formed. Therefore, the Al (aluminum) content should be 0.040% or less.

[0028] Ni-containing steel slabs contain nitrogen (N) as a component, which forms TiN, refines the structure of the base material, and increases its strength and toughness. Furthermore, as the nitrogen (N) content increases, AlN tends to form, reducing the toughness of the base material. Therefore, the nitrogen (N) content should be 0.005% or less, and more preferably 0.003% or less.

[0029] The Ni-containing steel slab may contain Cu (copper) as a component. By containing Cu (copper) as a component, the Ni-containing steel slab can increase the strength of the base material. As the Cu (copper) content increases, the toughness of the Ni-containing steel slab tends to decrease. The Cu (copper) content of the Ni-containing steel slab is preferably 0.5% or less.

[0030] The Ni-containing steel slab may contain Cr (chromium) as a component. By containing Cr (chromium) as a component, the Ni-containing steel slab can increase the strength of the base material. As the Cr (chromium) content increases, the toughness of the Ni-containing steel slab tends to decrease. The Cr (chromium) content of the Ni-containing steel slab may be 0.5% or less.

[0031] The Ni-containing steel slab may contain Mo (molybdenum) as a component. By containing Mo (molybdenum) as a component, the Ni-containing steel slab can reduce temper embrittlement. As the Mo (molybdenum) content increases, the toughness and weldability of the Ni-containing steel slab tend to decrease. The Mo (molybdenum) content of the Ni-containing steel slab is preferably 0.5% or less.

[0032] The Ni-containing steel slab may contain Nb (niobium) as a component. By containing Nb (niobium) as a component, the Ni-containing steel slab can increase the strength of the base material. As the Nb (niobium) content increases, the toughness of the Ni-containing steel slab tends to decrease. The Nb (niobium) content of the Ni-containing steel slab is preferably 0.05% or less.

[0033] The Ni-containing steel slab may contain V (vanadium) as a component. By containing V (vanadium) as a component, the Ni-containing steel slab can increase the strength of the base material. As the V (vanadium) content increases, the toughness of the Ni-containing steel slab tends to decrease. The V (vanadium) content of the Ni-containing steel slab is preferably 0.05% or less.

[0034] The method for producing a Ni-containing steel slab described above will now be described. The method for producing a Ni-containing steel slab includes a mixing step of mixing a grain boundary oxidation inhibitor with raw materials for the Ni-containing steel slab, and a straightening step of straightening the steel slab in the secondary cooling zone of continuous casting.

[0035] Specifically, in the mixing step, the grain boundary oxidation inhibitor is mixed into the molten steel so as to have a concentration of 0.004 mass % or more and 0.015 mass % or less. The mixing step is performed on molten steel before continuous casting, such as molten steel after converter blowing or in a ladle during secondary refining, or molten steel in a tundish before continuous casting. In other words, since the grain boundary oxidation inhibitor does not become an inclusion in the molten steel and is not floated and separated, it may be added at any timing up to before casting.

[0036] The molten steel produced in the mixing process is turned into a steel slab using a continuous casting machine (not shown), which performs primary cooling, in which the molten steel is cooled in a mold, and secondary cooling, in which cooling water is poured onto the steel slab produced by the primary cooling.

[0037] In the secondary cooling zone, where secondary cooling is performed, the steel slab is bent and straightened to produce a steel slab. Hereinafter, the part of the continuous casting machine where the steel slab is bent will be referred to as the bending section, and the part where the steel slab is straightened will be referred to as the straightening section. This bending and straightening process is performed at a surface temperature of the steel slab of 800°C or higher and 1100°C or lower.

[0038] Specifically, the bending straightening process is performed at a maximum temperature of the surface of the steel slab in the bending portion and straightening portion of 800° C. to 1100° C. The maximum temperature of the surface of the steel slab in the bending portion and straightening portion can be, for example, the maximum value in the width direction of the steel slab.

[0039] The surface temperature of the steel slab in the bending straightening process can be measured, for example, by providing a temperature measuring device between the plurality of rolls arranged in the bending section and the straightening section.

[0040] By performing the bending straightening process at a maximum surface temperature of 800°C or higher, the straightening process can be performed at a temperature higher than the embrittlement region. Furthermore, at temperatures higher than 1100°C, a liquid phase is generated in the oxide scale on the surface of a Ni-containing steel slab. When this liquid phase infiltrates the base metal, grain boundary oxidation is promoted even if the base metal contains a grain boundary oxidation inhibitor as a component. Therefore, the effect of the grain boundary oxidation inhibitor in preventing cracking of the Ni-containing steel slab tends to be weakened.

[0041] Furthermore, the inventors conducted a cross-sectional investigation of the cracked portion on the slab surface and found that the cause of cracks occurring in conventional Ni-containing steel slabs is not solely due to the embrittlement of the grain boundaries of Ni-containing steel, which are the starting points for cracks, but rather due to segregated elements such as P and S. It was found that grain boundary oxidation also contributes to the grain boundary embrittlement of Ni-containing steel slabs, and that simply avoiding the high-temperature embrittlement temperature range is not enough to suppress grain boundary oxidation.

[0042] Between the surface oxide scale of a Ni-containing steel slab and the base steel, the Ni component, which is less oxidizable than the base steel, is concentrated, and the grain boundaries, through which oxygen atoms can easily move, are preferentially oxidized. Therefore, grain boundary oxidation tends to progress in this region, and the grain boundary oxidation tends to extend deep from the surface layer of the Ni-containing steel slab.

[0043] Specifically, when oxygen penetrates into the interior of a conventional Ni-containing steel slab, the grain boundaries are preferentially oxidized. When localized oxidation progresses in a Ni-containing steel slab, wedges that can trigger cracks are formed. As a result, it was found that conventional secondary cooling control alone cannot suppress cracks.

[0044] In contrast, the Ni-containing steel slab of the present invention contains a predetermined amount of a grain boundary oxidation inhibitor that inhibits grain boundary oxidation. The grain boundary oxidation inhibitor is concentrated near the grain boundaries of the Ni-containing steel slab. In other words, the grain boundary oxidation inhibitor is unevenly distributed near the grain boundaries. The grain boundary oxidation inhibitor is oxidized preferentially over the grain boundaries. As a result, oxidation of the Ni-containing steel slab can be leveled, and the formation of wedges due to the progression of localized oxidation can be suppressed.

[0045] In particular, grain boundary oxidation inhibitors have low solid solubility in steel, and are thought to concentrate (distribute unevenly) in the surface layer of a Ni-containing steel slab in the high temperature range, generating gas and thereby exhibiting a gas barrier function that inhibits the oxidation of other components.

[0046] Furthermore, it is presumed that the grain boundary oxidation inhibitor concentrates near the grain boundaries and inhibits the preferential growth of liquid-phase oxide scale, which is specific to Ni-containing steel slabs, on the grain boundaries. According to a simulation conducted by the inventors, when Sb is used as a grain boundary oxidation inhibitor, equilibrium calculations for the sample components predict the generation of SbO gas. Therefore, it is believed that the oxidation of Sb reduces the oxygen partial pressure at the grain boundaries, inhibiting the growth of scale.

[0047] Sb has been described as an example of the grain boundary oxidation inhibitor. However, the grain boundary oxidation inhibitor is not limited to Sb. For example, Sn, Se, and Te, which are elements of the fourth to sixth series of the periodic table, can be used to obtain the same effects as Sb. Furthermore, the grain boundary oxidation inhibitor can achieve the above-described effects as long as it contains one or more of Sb, Sn, Se, and Te.

[0048] As described above, the Ni-containing steel slab and the method for producing the Ni-containing steel slab of the present invention can provide Ni-containing steel slabs with few surface cracks. Furthermore, as described above, the progress of local oxidation can be suppressed, which can prevent the formation of wedges that can lead to cracks, thereby increasing the yield of Ni-containing steel slabs. This can reduce the processing time required for the treatment to remove surface cracks from the Ni-containing steel slabs, thereby improving productivity and reducing production costs.

[0049] Ni-containing steel slabs were produced and the number of surface cracks was investigated for Inventive Examples 1 to 15 and Comparative Examples 1 to 3. Detailed compositions of Inventive Examples 1 to 15 and Comparative Examples 1 to 3 are shown in Table 1.

[0050] (Production of Ni-containing steel slabs) The Ni-containing steel slabs of Examples 1 to 15 and Comparative Examples 1 to 3 were produced as follows. First, molten steel was produced using a converter and an RH vacuum degasser. This molten steel was cast in a vertical bending type continuous slab caster with a thickness of 250 mm and a width of 1900 mm.

[0051] Inventive Examples 1 to 15 and Comparative Examples 1 to 3 were all produced at a casting speed of 1.2 m / min under the same mold cooling conditions. Inventive Examples 1 to 15 included Sb as a grain boundary oxidation inhibitor, which was mixed with other raw materials. Comparative Examples 1 to 3 were produced without adding any grain boundary oxidation inhibitor. Table 1 shows the Sb contents of Inventive Examples 1 to 15 and Comparative Examples 1 to 3. The secondary cooling water volume was set under different conditions for Inventive Examples 1 to 15 and Comparative Examples 1 to 3 in order to compare the effect of the surface temperature of the steel slab at the straightening point.

[0052] In all of Inventive Examples 1 to 15 and Comparative Examples 1 to 3, the amount of cooling water in the bending and straightening sections was adjusted so that the surface temperature of the steel slab would be 800°C or higher, which is higher than the high-temperature embrittlement temperature range. Table 1 shows the maximum surface temperatures during bending and straightening of the steel slab during casting. The maximum surface temperatures in the bending and straightening sections of the steel slab refer to the maximum value of the surface temperature in the width direction of the upper surface of the steel slab in the bending and straightening sections.

[0053] (Measurement of the depth of intergranular oxidation in the surface layer) The depth of intergranular oxidation in the surface layer was measured by observing the surface of a cross-sectional sample across the entire width of the steel slab using an optical microscope. The longest measured value from the surface layer to the tip of the intergranular oxidation portion was taken as the depth of intergranular oxidation.

[0054] (Measurement of crack length) The crack length was measured as follows. The locations where cracks occurred were confirmed on the front and back surfaces of the Ni-containing steel slab. The length of each confirmed crack was measured. The length of each crack was added up and divided by the total area of ​​the front and back surfaces of the Ni-containing steel slab. The value obtained was used as the crack length (mm / m ２ )

[0055] The Ni-containing steel slab used for measuring the crack length had a thickness of 250 mm, a width of 1900 mm, and a length in the casting direction of 300 mm. To measure the crack length, the oxide films on the front and back surfaces were removed by shot blasting. Magnetic particle testing was used to confirm the location of the crack.

[0056]

[0057] Inventive Examples 1 to 13, the crack length (mm / m ２ ) was 0. In addition, in both invention examples 14 and 15, the crack length (mm / m ２ ) was 5 or less. In all of Comparative Examples 1 to 3, the crack length (mm / m ２ ) was 108 or more. Therefore, in Examples 1 to 15, the crack length (mm / m ２ ) was significantly lower. Thus, it was found that the Ni-containing steel slabs of Examples 1 to 15, which satisfied the requirements of the present invention, had fewer surface cracks.

Claims

1. A Ni-containing steel slab containing 5 to 10 mass % Ni, By mass% Sb: 0.004% or more, 0.015% or less, C: 0.01% or more, 0.10% or less, Si: 0.01% or more, 0.40% or less, Mn: 0.20% or more, 1.00% or less, P: 0.005% or less, S: 0.005% or less, Al: 0.020% or more, 0.040% or less, N: 0.001% or more, 0.005% or less, Cu: 0.0% or more, 0.5% or less, Cr: 0.0% or more, 0.5% or less, Mo: 0.0% or more, 0.5% or less, V: 0.00% or more, 0.05% or less, Nb: 0.00% or more, 0.05% or less, and the balance being Fe and unavoidable impurities; A Ni-containing steel slab having a surface layer with a grain boundary oxidation depth of 200 μm or less.

2. A method for producing a Ni-containing steel slab according to claim 1, comprising: A mixing step of mixing 0.004% by mass or more and 0.015% by mass or less of Sb into the raw material of the Ni-containing steel slab; and a bending straightening step of straightening the Ni-containing steel slab in a secondary cooling zone of continuous casting, The method for producing a Ni-containing steel slab, wherein the bending straightening step is performed at a surface temperature of the Ni-containing steel slab of 800°C or more and 1100°C or less.