Method for manufacturing steel materials, and steel materials

Abstract

The present invention provides a method for manufacturing steel materials that exhibit excellent hydrogen-induced cracking properties. [Solution] This invention relates to a method for manufacturing steel materials, comprising a continuous casting process for continuously casting molten steel. The method for manufacturing steel materials comprises an oxygen concentration acquisition step for obtaining the oxygen concentration of the molten steel, and a Ca-containing substance addition step for adding a Ca-containing substance to the molten steel according to a target ACRS value determined by the concentrations of Ca, O, and S in the molten steel. The method for manufacturing steel materials of the present invention comprises a Ca-containing substance addition step for adding a Ca-containing substance to the molten steel according to a target ACRS value determined by the concentrations of Ca, O, and S in the molten steel. This ensures that a sufficient amount of Ca is available in the molten steel to fix MnS, which is the starting point for central segregation. Therefore, the occurrence of HIC in steel materials due to central segregation can be suppressed.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a steel material having a continuous casting process for continuously casting molten steel, and to a steel material.

Background Art

[0002] Natural gas pipelines, oil well pipes, etc. are used as lines for transporting gases containing hydrogen sulfide. In a sour environment exposed to gases containing hydrogen sulfide, hydrogen-induced cracking resistance (hereinafter also referred to as HIC resistance) is required as a performance of steel materials.

[0003] Hydrogen-induced cracking is one of the problems that occur in steel materials in a sour environment. Hydrogen-induced cracking is known to occur as follows. That is, when a steel material is exposed to a sour environment, hydrogen atoms generated by corrosion penetrate into the steel. The hydrogen atoms become hydrogen gas around non-metallic inclusions in the steel. The generated hydrogen gas causes cracks in the steel due to its pressure.

[0004] Conventionally, efforts have been made to improve the HIC resistance of steel materials. For example, in Patent Document 1, ACRM (%) is used as an index for quantifying the effect of controlling the morphology of MnS by Ca, and by adjusting ACRM within a predetermined range, the occurrence of HIC is suppressed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Patent Document 1 requires that ACRM (%) be 1.0 or higher. However, even if the requirement of ACRM (%) being 1.0 or higher is met, if the oxygen concentration of the molten steel is low, there is a problem in that sufficient Ca is not secured to suppress the generation of HIC that occurs due to central segregation, making it difficult to suppress the generation of HIC that occurs due to central segregation.

[0007] This invention has been made in view of the above-mentioned problems, and aims to provide a method for manufacturing steel materials with excellent hydrogen-induced cracking properties, and steel materials. [Means for solving the problem]

[0008] To solve the above problems, the present invention has the following features.

[0009] [1] A method for manufacturing steel materials having a continuous casting process for continuously casting molten steel, An oxygen concentration acquisition step for obtaining the oxygen concentration of the molten steel, A method for manufacturing steel, comprising: a step of adding a Ca-containing substance to the molten steel according to a target value of ACRS determined by the concentrations of Ca, O, and S in the molten steel. [2] The method for producing steel according to [1], wherein in the step of adding the Ca-containing substance, the Ca-containing substance is added in a component ratio in which CaO and 3CaO·Al2O3 are preferentially produced in the molten steel. [3] The method for manufacturing steel according to [1] or [2], wherein the molten steel has a PHIC value of 1.15 or less, which is determined by the concentrations of C, Mn, Cu, Ni, Cr, Mo, V, and P. [4] The oxygen concentration of the molten steel is 0.0015% by mass or less. A method for manufacturing steel according to any one of [1] to [3], wherein the target value of ACRS is set to 1.8 or higher in the Ca-containing substance addition step. [5] The process includes an RH vacuum degassing step in which the molten steel is subjected to RH vacuum degassing. The method for manufacturing steel according to any one of [1] to [4], wherein the Ca-containing substance addition step is performed in the RH vacuum degassing step. [6] It is made of steel, The component composition is expressed in mass percent. C: 0.020% or more and 0.080% or less Si: 0.01% or more and 0.50% or less Mn: 1.00% or more and 2.50% or less P: 0.010% or less, S: 0.0010% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.005% or more and 0.020% or less, Ca: 0.0003% or more and 0.0060% or less, N: 0.0080% or less, O: 0.0015% or less, Cu: 1.00% or less, Ni: 1.50% or less, Cr: 1.00% or less, Mo: 1.00% or less V: 0.100% or less, Includes, The remainder consists of iron and unavoidable impurities. Steel material in which the PHIC value, determined by the concentrations of C, Mn, Cu, Ni, Cr, Mo, V, and P, is 1.15 or less. [7] It is made of steel, The component composition is expressed in mass percent. C: 0.020% or more and 0.080% or less Si: 0.01% or more and 0.50% or less Mn: 1.00% or more and 2.50% or less P: 0.010% or less, S: 0.0010% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.005% or more and 0.020% or less, Ca: 0.0003% or more and 0.0060% or less, N: 0.0080% or less, O: 0.0015% or less, comprising the balance consisting of iron and inevitable impurities, a steel material in which the value of ACRS determined by the concentrations of Ca, O, and S in the molten steel is 1.8 or more.

Advantages of the Invention

[0010] According to the method for producing the steel material of the present invention, a Ca-containing substance addition step of adding a Ca-containing substance to the molten steel according to the target value of ACRS determined by the concentrations of Ca, O, and S in the molten steel is provided. Thereby, it is possible to sufficiently secure the amount of Ca for fixing MnS, which is the starting point of central segregation, in the molten steel. Therefore, the occurrence of HIC in the steel material due to central segregation can be suppressed.

Brief Description of the Drawings

[0011] [Figure 1] It is a flowchart showing a method for producing a steel material. [Figure 2] It is a graph showing the equilibrium composition of CaO-Al2O3 composite inclusions. [Figure 3] It is a graph showing the crack generation rate at the position of half the thickness of the steel material with respect to ACRM. [Figure 4] It is a graph showing the crack generation rate at the position of half the thickness of the steel material with respect to ACRS.

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described based on the drawings. FIG. 1 shows a flow for producing a steel material. As shown in FIG. 1, in the method for producing a steel material, first, an oxygen concentration acquisition step of acquiring the oxygen concentration of the molten steel is performed (step S01).

[0013] The oxygen concentration in the oxygen concentration acquisition step S01 is measured, for example, by analyzing the composition of a sample taken from molten steel. The oxygen concentration of the molten steel is preferably 0.0015% by mass or less. The inventors have found that, in particular, when the oxygen concentration of the molten steel is 0.0015% by mass or less, it is difficult to suppress the generation of HIC due to central segregation. The present invention can suppress the generation of HIC due to central segregation even in such harsh environments.

[0014] Furthermore, the molten steel used in the oxygen concentration acquisition step S01 should preferably have a PHIC value of 1.15 or less, determined by the concentrations of C, Mn, Cu, Ni, Cr, Mo, V, and P, and more preferably 0.960 or less. If the PHIC is 0.960 or less, the hardness of the central segregation can be reduced to 250 Hv or less, ensuring excellent HIC resistance.

[0015] PHIC(%) is a parameter that indicates the degree of hardening of central segregation. The higher the PHIC(%) value, the harder the central segregation area becomes. Furthermore, the harder the central segregation area becomes, the higher the frequency of HIC occurrence at the center in the thickness direction of the steel material.

[0016] PHIC(%) can be calculated using the following formula. PHIC(%)= 4.46C+2.37Mn / 6+(1.74Cu+1.7Ni) / 5+1.18Cr+(1.95Mo+1.74V) / 15+22.36P Here, the units of C, Mn, Cu, Ni, Cr, Mo, V, and P in the formula are ppm.

[0017] A PHIC (%) of 1.15 or less results in lower hardness in the central segregation area, leading to a lower frequency of HIC occurrence in the center of the steel.

[0018] Next, an RH vacuum degassing process is preferably performed on the molten steel (step S02). The RH vacuum degassing process in step S02 may be performed at any time before the continuous casting process described later.

[0019] Next, a Ca-containing substance addition step is performed (step S03), in which a Ca-containing substance is added to the molten steel according to the target value of ACRS determined by the concentrations of Ca, O, and S in the molten steel. The Ca-containing substance addition step in step S03 is not particularly limited, but is performed, for example, when the RH vacuum degassing step in step S02 is being carried out.

[0020] By performing the Ca-containing substance addition step S03 while the RH vacuum degassing step S02 is taking place, the uneven distribution of Ca components in the molten steel can be eliminated, resulting in a more uniform composition.

[0021] ACRS is a value that can be calculated using the following formula. ACRS= [Ca-(0.41Ca+0.40O-1.46)] / (1.25S) Here, the units of Ca, O, and S in the formula are ppm.

[0022] The target value for ACRS is preferably 1.8 or higher because it increases the amount of Ca that fixes S. On the other hand, if the target value for ACRS is too high, there will be an excess of Ca, which will significantly reduce the HIC resistance properties of the steel surface. For this reason, the target value for ACRS is more preferably 3.0 or lower.

[0023] Ca-containing substances are not particularly limited as long as they contain Ca as an element, but examples include Ca-containing alloys such as Ca-Si alloys, Ca-Fe alloys, and Ca-Al alloys. Other examples of Ca-containing substances include Ca compounds (calcium compounds) such as CaC2 (calcium carbide), CaCN2 (calcium cyanamide), and CaCl2 (calcium chloride).

[0024] Finally, the molten steel, whose composition has been adjusted in this manner, is cast in a continuous casting machine, and the continuous casting process is carried out (step S04). The steel material formed in the continuous casting process of step S04 is processed as appropriate and shipped.

[0025] Figure 2 shows the equilibrium composition of CaO-Al2O3 composite inclusions when the sulfur concentration in molten steel is 0.0005 mass%. The horizontal axis of Figure 2 represents the oxygen concentration (ppm) in the molten steel. The vertical axis of Figure 2 represents the calcium concentration (ppm) in the molten steel. In the graph of Figure 2, the solid line indicates the boundary, and in the region to the left, CaO and 3CaO·Al2O3 are preferentially formed in the molten steel.

[0026] In step S03, the Ca-containing substance addition step, it is preferable that the Ca-containing substance be added in a component ratio such that CaO and 3CaO·Al2O3 are preferentially formed in the molten steel. That is, CaO and 3CaO·Al2O3 are compositions that exist stably in the molten steel. By the preferential formation of CaO and 3CaO·Al2O3 in the molten steel, a sufficient amount of Ca can be secured to fix MnS, which is the starting point of central segregation. Therefore, the occurrence of HIC in the steel due to central segregation can be suppressed.

[0027] Figure 3 is a graph showing the crack occurrence rate at the halfway point of the steel thickness for ACRM. Figure 4 is a graph showing the crack occurrence rate at the halfway point of the steel thickness for ACRS. In both Figures 3 and 4, the oxygen concentration of the molten steel is 0.0015 mass% or less. Also, 1 / 2tCAR shown on the vertical axis of Figures 3 and 4 represents the crack occurrence rate (%) at the halfway point of the steel thickness.

[0028] As shown in Figure 3, when ACRM, an index shown in Patent Document 1, is 1.0 or higher, the crack occurrence rate is shown to be up to approximately 13%. In contrast, as shown in Figure 4, when ACRS is 1.8 or higher, the crack occurrence rate is shown to be at most less than 5%.

[0029] Thus, when the oxygen concentration of molten steel is 0.0015% by mass or less, an ACRS of 1.8 or higher ensures a sufficient amount of Ca to fix MnS, which is the starting point for central segregation.

[0030] The steel produced in the manner described above has a component composition in mass %, C: 0.020% or more and 0.080% or less Si: 0.01% or more and 0.50% or less Mn: 1.00% or more and 2.50% or less P: 0.010% or less, S: 0.0010% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.005% or more and 0.020% or less, Ca: 0.0003% or more and 0.0060% or less, N: 0.0080% or less, O: 0.0015% or less, Cu: 1.00% or less, Ni: 1.50% or less, Cr: 1.00% or less, Mo: 1.00% or less V: 0.100% or less, Including, The remainder consists of iron and unavoidable impurities.

[0031] C: 0.020% or more and 0.080% or less Carbon (C) is an essential element for ensuring the strength of steel. The required strength can be achieved by including 0.02% or more C in the steel. However, as the C content increases, the segregation of manganese (Mn) in central segregation intensifies, significantly reducing the steel's resistance to high-intensity carbon (HIC). By keeping the C content of the steel below 0.080%, the decrease in HIC resistance can be suppressed.

[0032] Si: 0.01% or more and 0.50% or less Si acts as a deoxidizing agent, strengthening the solid solution and increasing the strength of the steel. The required strength can be achieved by maintaining a Si content of 0.01% or more in the steel. However, increasing the Si content in the steel reduces its toughness. By keeping the Si content below 0.50%, the decrease in toughness can be suppressed.

[0033] Mn: 1.00% or more and 2.50% or less Mn improves the strength and toughness of steel. The required strength and toughness can be obtained by setting the Mn content in steel to 1.00% or more. Furthermore, a Mn content of 1.60% or more is preferable, as it allows for the reduction of other components and thus lower costs. As the Mn content in steel increases, the weldability and HIC (High-Intensity Coating) properties of the steel decrease. By setting the Mn content in steel to 2.50% or less, the decrease in weldability and HIC properties can be suppressed.

[0034] P:0.010% or less P is an unavoidable impurity that reduces weldability and HIC resistance. The P content in steel materials should be 0.010% or less.

[0035] S: 0.0010% or less S forms MnS, which reduces HIC resistance. The S content in steel materials should be 0.0010% or less.

[0036] Al: 0.010 or more and 0.100% or less Al is a strong deoxidizing element. By keeping the Al content in steel materials at 0.010% or higher, molten steel can be stably deoxidized. As the Al content in steel materials increases, coarse Al2O3 is formed in clusters, reducing HIC resistance. By keeping the Al content in steel materials at 0.100% or lower, the decrease in HIC resistance can be suppressed.

[0037] Ti: 0.005 or more and 0.020% or less Ti forms fine oxides with Al in steel. By keeping the Ti content in the steel material at 0.005% or higher, these Ti and Al oxides can be formed. As the Ti content in the steel material increases, the toughness of the steel decreases due to Ti solid solution and TiC precipitation. By keeping the Ti content in the steel material at 0.020% or lower, the decrease in toughness of the steel material can be suppressed.

[0038] O: 0.0015% or less O forms nonmetallic inclusions by generating oxides. These nonmetallic inclusions reduce the cleanliness and HIC resistance of steel materials. By reducing the O content in steel materials to 0.0015% or less, the decrease in the cleanliness and HIC resistance of steel materials can be suppressed.

[0039] Ca: 0.0003% or more and 0.0060% or less Ca acts to control the morphology of sulfides and suppresses the formation of MnS. By keeping the O content in steel materials at 0.0003% or higher, the formation of MnS can be appropriately suppressed. When the Ca content in steel materials increases, CaO-Al2O3 nonmetallic inclusions with high CaO concentrations are formed. These nonmetallic inclusions reduce HIC resistance. By keeping the Ca content in steel materials at 0.0060% or lower, the decrease in HIC resistance can be suppressed.

[0040] N: 0.0080% or less N is an unavoidable impurity. By reducing the N content in steel to 0.0080% or less, the desired base material toughness and HIC performance can be obtained.

[0041] Cu: 1.00% or less Cu is an effective element for improving hardenability and increasing strength. To obtain this effect, it is preferable to include 0.01% or more Cu. Preferably, the Cu content is 0.05% or more, and more preferably 0.10% or more. Since toughness deteriorates when the Cu content exceeds 1.0%, it is good to keep it at 1.00% or less, preferably 0.50% or less, and more preferably 0.35% or less.

[0042] Ni: 1.50% or less Ni is an effective element for improving the strength and toughness of the base material and welded joints. To obtain this effect, the Ni content should preferably be 0.01% or more, more preferably 0.05% or more, and more preferably 0.10% or more. However, as the Ni content increases, the structural steel becomes extremely expensive. Therefore, from an economic standpoint, the Ni content should preferably be 1.50% or less, more preferably 1.00% or less, and more preferably 0.50% or less.

[0043] Cr:1.00% or less Cr is an effective element for improving strength. To obtain this effect, the Cr content should be 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. If the Cr content exceeds 1.00%, the HAZ toughness deteriorates. The Cr content should be 1.00% or less, preferably 0.50% or less, and preferably 0.35% or less.

[0044] Mo: 1.00% or less Mo is an effective element for improving the strength and toughness of the base material. To obtain this effect, the Mo content should be 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. However, if the Mo content exceeds 1.00%, the HAZ toughness and weldability deteriorate. Therefore, the Mo content should be 1.00% or less, preferably 0.50% or less, and more preferably 0.35% or less.

[0045] V:0.100% or less V is an element effective in improving strength. To obtain this effect, it is preferable to include V in a content of 0.003% or more, and more preferably 0.010% or more. On the other hand, if the V content exceeds 0.1%, the weldability and base material toughness deteriorate. Therefore, it is preferable to keep the V content at 0.100% or less, and more preferably at 0.080% or less.

[0046] The ACRS value, which is determined by the concentrations of Ca, O, and S in the steel, should ideally be 1.8 or higher. As mentioned above, by setting the ACRS value of the steel to 1.8 or higher, the amount of Ca that fixes S can be increased.

[0047] The steel material has a PHIC value determined by the concentrations of C, Mn, Cu, Ni, Cr, Mo, V, and P, and it is preferable that the PHIC value is 1.15 or less, and more preferably 0.960 or less. If the PHIC value is 0.960 or less, the hardness of the central segregation can be reduced to 250 Hv or less, ensuring excellent HIC resistance.

[0048] Furthermore, the above component composition may be that of steel material or that of molten steel. [Examples]

[0049] The HIC resistance of steel materials produced by varying the ACRS of molten steel was evaluated. The sulfur concentration of the molten steel used in the test was 0.0005% by mass.

[0050] Table 1 shows the composition of each steel material. As shown in Table 1, the test material was an example where the ACRS target value was 1.8 or higher and a Ca-containing substance was added to the molten steel, while the process material was an example where the ACRS target value was less than 1.8 and a Ca-containing substance was added to the molten steel.

[0051] [Table 1]

[0052] HIC resistance was evaluated by the crack occurrence rate (%) using samples taken from the center of the thickness of the steel plates manufactured in each test. Specifically, these samples were subjected to an HIC test with an immersion time of 96 hours using a method compliant with NACE (National Association of Corrosion Engineers) standard TM0284. The values ​​obtained are shown in Table 1 as 1 / 2tCAR (%), i.e., the crack occurrence rate (%) at the halfway point of the steel thickness.

[0053] The pass / fail status of HIC resistance in Table 1 was evaluated according to the following evaluation criteria. (HIC resistance evaluation) ◎: 1 / 2t CAR < 1.5% ○: 1.5% ≤ 1 / 2tCAR ≤ 2.0% ×: 1 / 2tCAR > 2.0%

[0054] For process materials that do not meet the ACRS ≥ 1.8 standard, the 1 / 2t CAR ratio generally exceeds 2.0%. On the other hand, for test materials that meet the ACRS ≥ 1.8 standard, the 1 / 2t CAR ratio is consistently 2.0% or less.

[0055] Since all of the process materials had a 1 / 2t CAR content generally exceeding 2.0%, they were used as comparative examples. In other words, the HIC resistance test results for all of the comparative examples were "×".

[0056] All test materials contained less than 2.0% 1 / 2tCAR, thus qualifying as examples of the invention. In other words, the HIC resistance test results for the examples of the invention were all "◎" or "〇".

Claims

1. A method for manufacturing steel materials having a continuous casting process for continuously casting molten steel, An oxygen concentration acquisition step for obtaining the oxygen concentration of the molten steel, A method for manufacturing steel, comprising: a step of adding a Ca-containing substance to the molten steel according to a target value of ACRS determined by the concentrations of Ca, O, and S in the molten steel.

2. In the Ca-containing substance addition step, CaO and 3CaO・Al are added. ２ O ３ The method for producing steel according to claim 1, wherein the Ca-containing substance is added in a component ratio that is preferentially produced in the molten steel.

3. The method for manufacturing steel according to claim 1, wherein the molten steel has a PHIC value of 1.15 or less, which is determined by the concentrations of C, Mn, Cu, Ni, Cr, Mo, V, and P.

4. The method for manufacturing steel according to claim 2, wherein the molten steel has a PHIC value of 1.15 or less, which is determined by the concentrations of C, Mn, Cu, Ni, Cr, Mo, V, and P.

5. The oxygen concentration of the molten steel is 0.0015% by mass or less. A method for manufacturing steel according to any one of claims 1 to 4, wherein in the step of adding the Ca-containing substance, the target value of ACRS is set to 1.8 or higher.

6. The process includes an RH vacuum degassing step in which the molten steel is subjected to RH vacuum degassing. The method for manufacturing steel according to any one of claims 1 to 4, wherein the Ca-containing substance addition step is performed in the RH vacuum degassing step.

7. The process includes an RH vacuum degassing step in which the molten steel is subjected to RH vacuum degassing. The method for manufacturing steel according to claim 5, wherein the Ca-containing substance addition step is performed in the RH vacuum degassing step.

8. It is made of steel, The component composition is expressed in mass percent. C: 0.020% or more and 0.080% or less Si: 0.01% or more and 0.50% or less Mn: 1.00% or more and 2.50% or less P: 0.010% or less, S: 0.0010% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.005% or more and 0.020% or less, Ca: 0.0003% or more and 0.0060% or less, N: 0.0080% or less, O: 0.0015% or less, Cu: 1.00% or less, Ni: 1.50% or less, Cr: 1.00% or less, Mo: 1.00% or less V: 0.100% or less, Includes, The remainder consists of iron and unavoidable impurities. Steel material having a PHIC value of 1.15 or less, which is determined by the concentrations of C, Mn, Cu, Ni, Cr, Mo, V, and P.

9. It is made of steel, The component composition is expressed in mass percent. C: 0.020% or more and 0.080% or less Si: 0.01% or more and 0.50% or less Mn: 1.00% or more and 2.50% or less P: 0.010% or less, S: 0.0010% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.005% or more and 0.020% or less, Ca: 0.0003% or more and 0.0060% or less, N: 0.0080% or less, O: 0.0015% or less, Includes, The remainder consists of iron and unavoidable impurities. A steel material having an ACRS value of 1.8 or higher, which is determined by the concentrations of Ca, O, and S in the molten steel.

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