Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/12—Accessories for subsequent treating or working cast stock in situ
  • B22D11/124—Accessories for subsequent treating or working cast stock in situ for cooling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to Ni-containing steel slabs containing 5 to 10 mass% Ni (nickel) and a method for producing Ni-containing steel slabs.
• Ni-containing steel Steel containing around 9% Ni by mass (hereafter also referred to as Ni-containing steel) is also called 9% Ni steel.
• 9% Ni steel can withstand use at temperatures below -160°C, so it is widely used as a welded structural steel for low-temperature applications such as LNG tanks.
• Ni-containing steels including 9% Ni steel, are known to be prone to surface defects.
• a cast piece has numerous cracks on the surface or near the surface (hereinafter referred to as surface cracks).
• Ni-containing steel slabs (hereafter 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) that segregate to the grain boundaries during solidification embrittle the grain boundaries, which are then destroyed and lead to cracks by stress caused by bending and straightening the slab in the secondary cooling zone of the continuous casting machine and thermal stress caused by cooling. It has been reported that surface cracks are particularly likely to occur when bending, straightening, or other processes are performed in the temperature range of 600 to 800°C (referred to as the "high-temperature embrittlement temperature range").
• Ni-containing steel slabs When manufacturing Ni-containing steel slabs by continuous casting, it is important to prevent the occurrence of surface cracks on the slab. As mentioned above, it is believed that avoiding straightening in the high-temperature embrittlement temperature range is an effective way to prevent surface cracks during continuous casting of Ni-containing steel slabs.
• 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.
• Patent Document 2 also discloses that when continuously casting Ni-containing steel containing 8 to 10 mass% Ni, the reduction in area during casting is estimated and the secondary cooling intensity is controlled so that the reduction in area is 50% or more.
• the present invention was made in consideration of the above problems, and aims to provide a Ni-containing steel slab that contains 5 to 10 mass% Ni and has few surface cracks, and a method for manufacturing the Ni-containing steel slab.
• the present invention has the following features:
• a Ni-containing steel slab containing 5 to 10 mass % Ni Contains 0.004 mass% or more and 0.015 mass% or less of Sb, A Ni-containing steel slab having a surface layer with a grain boundary oxidation depth of 200 ⁇ m or less.
• a Ni-containing steel slab containing 5 to 10 mass % Ni Contains 0.004 mass% or more and 0.015 mass% or less of a grain boundary oxidation inhibitor containing Sb, A Ni-containing steel slab having a surface layer with a grain boundary oxidation depth of 200 ⁇ m or less.
• a method for producing a Ni-containing steel slab containing 5 to 10 mass % Ni 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; A bending straightening process 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.
• a method for producing a Ni-containing steel slab containing 5 to 10 mass % Ni comprising: a mixing step of mixing an intergranular oxidation inhibitor containing Sb in an amount of 0.004 mass% or more and 0.015 mass% or less into a raw material for the Ni-containing steel slab; A bending straightening process 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.
• 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, the oxidation of the Ni-containing steel slab can be leveled out.
• Ni-containing steel slab of the present invention contains 5 to 10 mass% Ni.
• the Ni-containing steel slab can be used, for example, as low-temperature steel used in a temperature range lower than room temperature.
• the Ni-containing steel slab of the present invention contains Sb.
• Sb functions as a grain boundary oxidation inhibitor.
• Sb is also referred to as a grain boundary oxidation inhibitor.
• the Ni-containing steel slab may contain 0.004 mass% or more and 0.015 mass% or less of the grain boundary oxidation inhibitor, preferably 0.005 mass% or more and 0.015 mass% or less, and more preferably 0.006 mass% or more and 0.010 mass% or less. If the content of the grain boundary oxidation inhibitor exceeds 0.01%, the toughness of the Ni-containing steel slab tends to decrease.
• the Ni-containing steel slab of the present invention has a surface grain boundary oxidation depth of 200 ⁇ m or less, preferably 180 ⁇ m or less, more preferably 50 to 100 ⁇ m, and even more preferably 55 to 80 ⁇ m.
• the depth of the grain boundary oxidation of the surface layer can be measured, for example, by observing a cross section including the entire width of the Ni-containing steel slab with an optical microscope. Specifically, the longest measured length from the surface layer to the tip of the grain boundary oxidation portion can be taken as the grain boundary oxidation depth of the surface layer.
• the Ni-containing steel slab of the present invention has, in mass%, 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, Ni: 5.0% or more, 10.0% 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, The balance is Fe and unavoidable impurities.
• the strength of the base material can be ensured by including C (carbon) in the composition of Ni-containing steel slabs.
• C carbon
• the strength of the base material can be made good. If the C content of the Ni-containing steel slabs is excessive, suitable toughness may not be obtained. By making the C content of the Ni-containing steel slabs 0.10% or less, suitable toughness can be obtained.
• the deoxidation effect of removing oxygen contained in the Ni-containing steel slabs can be enhanced.
• the Si content of the Ni-containing steel slabs 0.01% or more, an excellent deoxidation effect can be obtained.
• the Si content of the Ni-containing steel slabs 0.40% or less it is possible to prevent an increase in temper embrittlement susceptibility.
• Ni-containing steel slabs can ensure the strength of the base material.
• Mn content of the Ni-containing steel slab 0.20% or more the strength of the base material can be made good.
• Mn content of the steel slab 1.00% or less suitable toughness can be ensured.
• P (phosphorus) is an impurity element in Ni-containing steel slabs.
• P (phosphorus) tends to segregate at grain boundaries, reducing toughness. It is recommended that Ni-containing steel slabs contain as little P (phosphorus) as possible as part of their composition.
• the P (phosphorus) content should be 0.005% or less, and preferably 0.004% or less.
• S (sulfur) is an impurity element in Ni-containing steel slabs.
• S (sulfur) tends to segregate at grain boundaries, reducing toughness.
• Ni-containing steel slabs should contain as little S (sulfur) as possible in their composition. If refining costs are acceptable, the S (sulfur) content should be 0.005% or less, and preferably 0.001% or less.
• Ni-containing steel slabs should contain 5.0-10.0% Ni (nickel) in their composition, and preferably 7.0-9.5%.
• Ni content of Ni-containing steel slabs 5.0% or more, appropriate toughness can be obtained. Also, from the viewpoint of cost, it is preferable that the Ni content of Ni-containing steel slabs be 10.0% or less.
• Ni-containing steel slabs can enhance the deoxidation effect of removing oxygen contained in the steel slab. If the Al (aluminum) content is higher than 0.100%, cleanliness tends to be impaired. Also, as the Al (aluminum) content increases, AlN is formed, which reduces the toughness of the base material. For this reason, it is recommended that the Al (aluminum) content be 0.040% or less.
• Ni-containing steel slabs contain N (nitrogen) as a component, which forms TiN, refines the structure of the base material, and increases its strength and toughness. As the N (nitrogen) content increases, AlN tends to form, decreasing the toughness of the base material. For this reason, the N (nitrogen) content should be 0.005% or less, and more preferably 0.003% or less.
• Ni-containing steel slabs should contain Cu (copper) as part of their composition. By containing Cu (copper) as part of their composition, Ni-containing steel slabs can increase the strength of the base material. If the Cu (copper) content increases, the toughness of the Ni-containing steel slabs tends to decrease.
• the Cu (copper) content of Ni-containing steel slabs should be 0.5% or less.
• Ni-containing steel slabs should contain Cr (chromium) as part of their composition. By containing Cr (chromium) as part of their composition, Ni-containing steel slabs can increase the strength of the base material. As the Cr (chromium) content increases, the toughness of the Ni-containing steel slabs tends to decrease. The Cr (chromium) content of Ni-containing steel slabs should be 0.5% or less.
• Ni-containing steel slabs should contain Mo (molybdenum) as part of their composition. By containing Mo (molybdenum) as part of their composition, Ni-containing steel slabs can reduce temper embrittlement. If the Mo (molybdenum) content is high, the toughness and weldability of the Ni-containing steel slabs tend to decrease. The Mo (molybdenum) content of Ni-containing steel slabs should be 0.5% or less.
• Ni-containing steel slabs should contain Nb (niobium) as part of their composition. By containing Nb (niobium) as part of their composition, Ni-containing steel slabs can increase the strength of the base material. If the Nb (niobium) content increases, the toughness of the Ni-containing steel slabs tends to decrease.
• the Nb (niobium) content of the Ni-containing steel slabs should be 0.05% or less.
• Ni-containing steel slabs should contain V (vanadium) as part of their composition. By containing V (vanadium) as part of their composition, Ni-containing steel slabs can increase the strength of the base material. If the V (vanadium) content increases, the toughness of the Ni-containing steel slabs tends to decrease.
• the V (vanadium) content of Ni-containing steel slabs should be 0.05% or less.
• the manufacturing method of the Ni-containing steel slab described above includes a mixing process in which the grain boundary oxidation inhibitor is mixed with the raw material of the Ni-containing steel slab, and a bending straightening process in which the steel slab is bent and straightened in the secondary cooling zone of the continuous casting process.
• the grain boundary oxidation inhibitor is mixed into the molten steel so that the concentration is 0.004 mass% or more and 0.015 mass% or less.
• the mixing process is carried out on molten steel before continuous casting, such as molten steel in a ladle after converter blowing or during secondary refining, or molten steel in a tundish before continuous casting.
• molten steel before continuous casting such as molten steel in a ladle after converter blowing or during secondary refining, or molten steel in a tundish before continuous casting.
• the grain boundary oxidation inhibitor does not become an inclusion in the molten steel and does not float and separate, it may be added at any time up until before casting.
• the molten steel produced in the mixing process is turned into steel slabs using a continuous casting machine (not shown).
• a continuous casting machine primary cooling is performed to cool the molten steel in a mold, and secondary cooling is performed to cool the steel slabs produced in the primary cooling process by pouring cooling water over them.
• the steel slab is bent and straightened to produce a steel slab.
• the area in the continuous casting machine where the steel slab is bent is also referred to as the bending section, and the area where the steel slab is straightened is also 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.
• the bending straightening process is carried out at a maximum surface temperature of the steel slab in the bending and straightening sections of 800°C or more and 1100°C or less.
• the maximum surface temperature of the steel slab in the bending and straightening sections can be, for example, the maximum value in the width direction of the steel slab.
• the surface temperature of the steel slab during the bending and straightening process can be measured, for example, by installing a temperature measuring device between the bending section and the multiple rolls arranged in the straightening section.
• the straightening process can be performed at a temperature higher than the embrittlement zone. Furthermore, at high temperatures exceeding 1100°C, a liquid phase is generated in the oxide scale on the surface of Ni-containing steel slabs. When this liquid phase infiltrates the base material, grain boundary oxidation is promoted even if the base material contains a grain boundary oxidation inhibitor as an ingredient. For this reason, the effect of the grain boundary oxidation inhibitor in preventing cracking of Ni-containing steel slabs tends to be weakened.
• the inventors also conducted a cross-sectional survey of the cracked areas on the slab surface and found that the embrittlement of the grain boundaries of Ni-containing steel, which is the starting point of cracks in conventional Ni-containing steel slabs, is not caused solely by the segregated elements P and S. It was found that grain boundary oxidation also contributes to grain boundary embrittlement in cracks in Ni-containing steel slabs, and that grain boundary oxidation cannot be suppressed simply by avoiding the high-temperature embrittlement temperature range.
• Ni components which are less susceptible to oxidation than the base steel, are concentrated, and the grain boundaries, where oxygen atoms are more likely to move, are preferentially oxidized. For this reason, grain boundary oxidation is likely to progress in this area, and grain boundary oxidation tends to extend deep from the surface layer of the Ni-containing steel slab.
• 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, the oxidation of the Ni-containing steel slab can be leveled out, and the formation of wedges due to the progression of localized oxidation can be inhibited.
• grain boundary oxidation inhibitors have low solid solubility in steel, and are thought to concentrate (distribute unevenly) on the surface of Ni-containing steel slabs at high temperatures, generating gas that acts as a gas barrier to inhibit the oxidation of other components.
• the grain boundary oxidation inhibitor is concentrated near the crystal grain boundaries and inhibits the preferential growth of liquid phase oxide scale, which is specific to Ni-containing steel slabs, on the grain boundaries.
• the generation of SbO gas is predicted in equilibrium calculations for the sample components. Therefore, it is believed that the oxidation of Sb reduces the oxygen partial pressure at the grain boundaries, inhibiting the growth of scale.
• Sb has been described as an example of a grain boundary oxidation inhibitor.
• the grain boundary oxidation inhibitor is not limited to Sb, and for example, the same effect as Sb can be obtained even when Sn, Se, or Te, which are elements of the 4th to 6th series of the periodic table, are used. Furthermore, the grain boundary oxidation inhibitor can achieve the above-mentioned effects as long as it contains one or more of Sb, Sn, Se, and Te.
• the Ni-containing steel slab and the manufacturing method of the Ni-containing steel slab of the present invention make it possible to provide Ni-containing steel slabs with fewer surface cracks. Furthermore, as described above, it is possible to prevent the progression of localized oxidation, prevent the formation of wedges that can lead to cracks, and increase the yield of Ni steel slabs. This makes it possible to reduce the processing time required for the maintenance process to remove cracks on the surface of the Ni-containing steel slab, thereby improving productivity and reducing manufacturing costs.
• Ni-containing steel slabs were produced for invention examples 1 to 15 and comparative examples 1 to 3, and the number of surface cracks was investigated.
• Detailed compositions of invention examples 1 to 15 and comparative examples 1 to 3 are shown in Table 1.
• 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.
• 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 Sb was used as a grain boundary oxidation inhibitor and 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 content of Inventive Examples 1 to 15 and Comparative Examples 1 to 3.
• the amount of secondary cooling water 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.
• the amount of cooling water was adjusted in the bending and straightening sections so that the surface temperature of the steel slab was 800°C or higher, which is higher than the high-temperature brittle temperature range.
• Table 1 shows the maximum surface temperature during bending and straightening of the steel slab during casting.
• the maximum surface temperature of the bending and straightening sections of the steel slab was the maximum value of the surface temperature in the width direction of the top surface of the steel slab in the bending and straightening sections.
• the depth of the intergranular oxidation on the surface layer was measured by observing the surface layer of a cross-sectional sample of the entire width of the steel slab under an optical microscope. The longest measured length from the surface layer to the tip of the intergranular oxidation was taken as the intergranular oxidation depth.
• the crack length was measured as follows. The locations where cracks occurred on the front and back surfaces of the Ni-containing steel slab were confirmed. 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 to obtain the crack length (mm/m 2 ) of the Ni-containing steel slab.
• the Ni-containing steel slab used to measure the crack length was 250 mm thick, 1,900 mm wide, and 300 mm long in the casting direction.
• the oxide film on the front and back surfaces was removed by shot blasting. Magnetic particle testing was also used to confirm the location of the crack.
• Example 1 to 13 the crack length (mm/m 2 ) was 0. In all of 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. It was therefore confirmed that Examples 1 to 15 had significantly shorter crack lengths (mm/m 2 ) than Comparative Examples 1 to 3. In this way, it was found that the Ni-containing steel slabs of Examples 1 to 15, which satisfy the provisions of the present invention, had fewer surface cracks.

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