Hot-rolled steel material for tanks and method for manufacturing the same

Abstract

The present invention provides a hot-rolled steel material for tanks, the hot-rolled steel material containing, in mass%, 18.0% to 20.0% of Cr and 8.0% to 13.0% of Ni, while having an Md30 expressed by formula (1) of 35°C or less. With respect to this hot-rolled steel material for tanks, the hydrogen (H) content in a surface layer part, which extends from the surface of the steel material to the position corresponding to 1 / 5 of the sheet thickness in the depth direction, is 3.5 ppm by mass or less; and the tensile strength is 600 MPa or more.　(1): Md30 (°C) = 413 - 462(C + N) -9.2Si - 8.1Mn - 13.7Cr - 9.5(Ni + Cu) -18.5Mo

Description

[Technical Field]

[0001] This invention relates to hot-rolled steel materials for tanks and methods for manufacturing the same. In particular, it relates to austenitic stainless steel hot-rolled steel materials and methods for manufacturing the same, which are used for tanks that carry low-temperature fuels. [Background technology]

[0002] Austenitic stainless steel is suitably used as a material with excellent corrosion resistance and toughness at low temperatures. Taking advantage of these properties, austenitic stainless steel is excellent as a steel material for low-temperature tanks. In austenitic stainless steel, corrosion resistance deteriorates due to precipitates during hot rolling; therefore, in conventional manufacturing methods, solution heat treatment is applied after hot rolling to reduce precipitates. As a result of solution heat treatment, the tensile strength is at most about 400 MPa.

[0003] To increase the strength of austenitic stainless steel, a method has been proposed that omits solution heat treatment and performs hot rolling under specific temperature conditions. For example, Patent Document 1 discloses a method for manufacturing austenitic stainless steel containing predetermined components, in which a predetermined homogenization heat treatment is applied to the steel billet before hot rolling, and a predetermined rapid cooling (controlled cooling) is performed after hot rolling, without solution heat treatment. This has resulted in a hot-rolled austenitic stainless steel material that has good seawater resistance and high strength while ensuring low-temperature toughness. The homogenization heat treatment before hot rolling diffuses segregation, and the controlled cooling after hot rolling suppresses the reprecipitation of intermetallic compounds and prevents the recovery of processing strain introduced during finish rolling. High strength can be maintained by omitting solution heat treatment. Steel materials manufactured by such a manufacturing method (thermo-mechanical control process) are also called TMCP materials (Thermo-Mechanical Control Process).

[0004] Using an austenitic stainless hot-rolled steel material, tanks, especially low-temperature tanks, are manufactured. Here, in order to manufacture a formwork tank, it is necessary to apply plastic processing to the steel material to manufacture a member called a platen. In the normal use of thick plates of austenitic stainless steel, strong plastic processing is not often applied, but the above-mentioned platen of the tank is characterized in that strong plastic processing is applied.

[0005] In the case of thin plates of austenitic stainless steel, which are different from the thick steel materials (especially thick plates) targeted by the present invention, taking advantage of the high workability of the same material, it is used as a material for drawing processing. Also, in order to obtain the gloss on the surface, bright annealing is performed at the stage of the steel strip. It is known that when drawing processing is performed on austenitic stainless steel, especially when bright annealing is performed, delayed fracture called time cracking occurs when the processing rate exceeds the limit. Hydrogen penetrates into the steel during bright annealing. It is said that the transformed martensite phase, residual stress, hydrogen in the steel, processing conditions, etc. after drawing processing are the main factors that govern the time cracking property (see Non-Patent Document 1).

[0006] Non-Patent Document 2 presents Md30 as an evaluation index for the austenite stability of austenitic stainless steel. Md30 is the temperature (°C) at which the structure transforms into a 50% martensite phase when a tensile true strain of 0.30 is applied to a sample of austenite single phase, and the higher this value is, the more unstable the material is. In Non-Patent Document 2, in addition to the formula for Md30 ((Formula (1))) presented by Angel as a function of the component composition of the steel, the formula for Md30 ((Formula (6))) proposed in the same document is presented.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Non-Patent Documents

[0008]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] When an austenitic stainless hot-rolled steel sheet that has not been subjected to solution heat treatment and has a thick plate with a tensile strength of 600 MPa or more is used as a material for the mirror plate of a tank, it has been found that delayed fracture occurs after plastic processing as a mirror plate. <00001​​​​​​​​​​​​​​​​​​Md30(°C) = 413 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 9.5(Ni + Cu) - 18.5Mo (1) In formula (1), the elemental symbols on the right side represent the content (mass%) of the corresponding element in the steel. Also, the "hot-rolled steel sheet" means a steel sheet that is as it is after hot rolling and has not been subjected to solution heat treatment after hot rolling. [2] The hot-rolled steel sheet for a tank according to [1], further containing one or two groups selected from the following Group A and Group B in place of a part of the Fe. [Group A] By mass%, one or more of Sn: 0.050% or less, W: 1.00% or less, Co: 1.00% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, REM: 0.10% or less. [Group B] By mass%, one or more of V: 0.30% or less, Nb: 0.060% or less, Ti: 0.030% or less, B: 0.0050% or less, Zr: 0.050% or less, Hf: 0.100% or less, Ta: 0.100% or less. [3] When the hydrogen content in the surface layer part from the surface of the steel sheet to a depth of 1 / 5 of the sheet thickness in the thickness direction is H H2O , , , O2 , C , , C , H2O , O2 , 2 , ,

[0012] , H2O , , S , S , O2 , and the hydrogen content at the center of the steel sheet thickness is H C , the hot-rolled steel sheet for a tank according to [1] or [2], characterized in that H S / H C ≤ 0.85.

[0012] [4] A method for manufacturing a hot-rolled steel sheet for a tank according to any one of [1] to [3], characterized in that a casting material with a thickness of T mm is heated for (T H2O / 140) minutes or more in an atmosphere where the water vapor partial pressure (p O2 ) and the oxygen partial pressure (p 2 ) satisfy the following formula (2), and then hot-rolled. LOG(p H2O / √(p O2 )) ≤ 1.00 (2) Here, LOG is the common logarithm to the base 10, and the units of p H2O and p O2 are (atm). [5] The cast material is hot-rolled to produce an intermediate-rolled material with a thickness of T1 mm, and this intermediate-rolled material is subjected to a water vapor partial pressure (p H2O ) and oxygen partial pressure (p O2 ) in an atmosphere that satisfies the following equation (2) (T1 2 A method for manufacturing hot-rolled steel for tanks according to any one of [1] to [3], characterized by being heated for 200 minutes or more and then hot-rolled. LOG(p H2O / √(p O2 ))≦1.00 (2) Here, LOG is the common logarithm of base 10, p H2O and p O2 The unit is (atm). [6] The method for manufacturing hot-rolled steel for tanks according to [4], characterized in that the hot rolling is carried out at a final finishing temperature of 1000°C or less, water cooling is performed from 600°C or higher after rolling, and solution heat treatment is not performed. [7] The method for manufacturing hot-rolled steel for tanks according to [5], characterized in that the hot rolling is carried out at a final finishing temperature of 1000°C or less, water cooling is performed from 600°C or higher after rolling, and solution heat treatment is not performed.

[0013] The present invention makes it possible to prevent delayed fracture when manufacturing tank end plates using hot-rolled austenitic stainless steel with a tensile strength of 600 MPa or more, without solution heat treatment. [Modes for carrying out the invention]

[0014] The hot-rolled steel material for tanks targeted by this invention is mainly thick plates. Plate thicknesses of 8 mm or more, and more preferably 12 mm or more, and 20 mm or more, are used.

[0015] The hot-rolled steel material for tanks of the present invention is an austenitic stainless steel having a tensile strength of 600 MPa or higher. In order to achieve a tensile strength of 600 MPa or higher, the material contains the component composition described below, and processing strain is introduced by rolling during the finishing rolling stage of hot rolling. Controlled cooling is performed to accelerate the cooling rate of the steel material after rolling, and solution heat treatment after rolling is omitted.

[0016] As mentioned above, when a thick plate made of austenitic stainless steel that has not undergone solution heat treatment and has a tensile strength of 600 MPa or more was used as the material for the end plate of a tank, it was found that delayed fracture occurred after plastic deformation to form the end plate.

[0017] Since cracks occurred in the plastically deformed portion and were the cause of delayed fracture, it is assumed that the transformed martensite phase generated by work-induced martensitic transformation due to plastic deformation is the initiation point of the cracks. In addition, residual stress associated with plastic deformation may be one of the driving forces for crack initiation.

[0018] As mentioned above, reducing the value of Md30 based on the component composition is effective in suppressing the formation of a work-induced martensite phase associated with plastic deformation. In this invention, the following formula was adopted for Md30, based on Angel's formula described in Non-Patent Document 1 mentioned above, with the Cu term added to it as having a contribution equivalent to that of Ni. Md30(℃)=413-462(C+N)-9.2Si―8.1Mn-13.7Cr-9.5(Ni+Cu)-18.5Mo (1) In equation (1), the element symbol on the right side represents the content (mass %) of that element in the steel.

[0019] Among austenitic stainless steels, it is known that when thin sheets are subjected to bright annealing at the strip stage, delayed fracture (timing cracking) is likely to occur when high-degree drawing is performed. Bright annealing is carried out in a hydrogen-containing atmosphere, so hydrogen penetrates the steel during annealing, increasing the hydrogen content in the steel, which is considered to be one of the causes of timing cracking. The present invention deals with thick steel materials, and bright annealing is not performed. On the other hand, in the manufacturing stage of the steel materials targeted by the present invention, if the concentration of hydrogen contained in the molten steel is high, it is expected that the hydrogen content of the final hot-rolled steel material will also be high. In particular, since solution heat treatment is not performed after hot rolling, the opportunity for hydrogen in the steel material to be reduced by diffusion is low, which may also be one of the reasons for the high hydrogen content in the steel material. It is estimated that as the thickness of the steel material after hot rolling increases, the degree of hydrogen release by diffusion decreases, and the amount of hydrogen remaining in the steel material also increases. Therefore, we conceived the idea that reducing the hydrogen content in the steel material could reduce cracking during the processing of end plates for tank steel.

[0020] When steel materials for tanks are processed into end plates, the hardness of the steel increases due to plastic deformation. The increase in hardness is particularly large in the thickness direction of the steel material, especially near the surface. Tensile residual strain due to plastic deformation also tends to be large at the surface of the steel material. Therefore, it is thought that the surface of the steel material is the starting point for cracking that occurs in end plates. Accordingly, in order to prevent cracking by reducing the hydrogen content in the steel material as described above, we decided to focus particularly on reducing the hydrogen content near the surface of the steel material. The surface layer was defined as the portion from the surface of the steel material to 1 / 5 of the plate thickness in the depth direction.

[0021] Steel (cast material before rolling, intermediate-rolled material during rolling, and steel material after rolling) is heated to a high temperature, and the hydrogen partial pressure (p H2By maintaining the steel in a low-temperature atmosphere, the hydrogen content can be reduced by hydrogen diffusion. In hydrogen diffusion, the hydrogen concentration near the surface decreases in the initial stages of heating, and then the hydrogen in the center of the steel decreases sequentially. If the occurrence of cracking can be suppressed by reducing the hydrogen content in the surface layer of the steel (the part from the surface of the steel material to 1 / 5 of the plate thickness in the depth direction), then dehydrogenation is not necessary in the center of the thickness of the steel material, and the heat treatment time for reducing hydrogen in the steel can be shortened.

[0022] Based on the above ideas, the hydrogen partial pressure (p) of the cast material before rolling and the intermediate-rolled material during the rolling process was determined. H2 The materials were subjected to a high-temperature, long-duration heating process in a low-temperature atmosphere. These materials were then hot-rolled and controlled-cooled to produce steel without solution heat treatment. The hydrogen (H) content of the surface layer of the steel was then analyzed from the surface down to 1 / 5 of the plate thickness.

[0023] Within the range of austenitic stainless steel, the composition of the steel material was changed, and the value of Md30 in equation (1) was varied. In addition, the heat treatment conditions of the cast material before rolling and the intermediate rolled material during rolling were changed to vary the hydrogen content of the surface layer of the steel material. The manufactured steel material was processed into tank end plates, and the occurrence of cracking was evaluated. As a result, it was found that if Md30 in equation (1) is 35°C or less and the hydrogen content of the surface layer of the steel material is 3.5 ppm by mass or less, the occurrence of cracking can be suppressed even after processing the end plates, thus completing the present invention.

[0024] The present invention will be described in detail below. First, the component composition of the steel material of the present invention will be described. Hereafter, % refers to mass%.

[0025] C: 0.08% or less To ensure the corrosion resistance of stainless steel, the content of carbon (C) is limited to 0.08% or less. While there is no lower limit for C, it is preferable to set it at 0.01% or higher to ensure strength.

[0026] Si: 0.05~1.0% Si is added at a concentration of 0.05% or more for deoxidation during steel melting. However, since adding more than 1.0% reduces toughness, the upper limit is limited to 1.0%.

[0027] Mn: 0.10~2.0% Mn is added at a concentration of 0.10% or more to reduce the deterioration of hot workability caused by sulfur. However, excessive Mn content can lead to deterioration of toughness and corrosion resistance, so the upper limit is limited to 2.0%.

[0028] P:0.045% or less Since P is an impurity that degrades the hot workability and toughness of steel, its upper limit is limited to 0.045%. Preferably, it is 0.035% or less.

[0029] S: 0.030% or less S is an impurity that degrades the hot workability, toughness, and corrosion resistance of steel, so its upper limit is limited to 0.030%. Preferably, it is 0.010% or less.

[0030] Cr: 18.0~20.0% To ensure the basic corrosion resistance of steel, chromium (Cr) should be included at a concentration of 18.0% or more. However, exceeding 20.0% reduces austenitic stability. Therefore, the Cr content should be between 18.0% and 20.0%.

[0031] Ni: 8.0~13.0% Ni is included in the alloy at a concentration of 8.0% or more to stabilize the austenitic structure of steel and improve its corrosion resistance and toughness against various acids. However, since Ni is an expensive alloy, its upper limit is limited to 13.0% from a cost perspective.

[0032] Mo: 1.0% or less Mo is a very effective element for additionally enhancing the corrosion resistance of stainless steel and can be included as needed. For this purpose, it is preferable to include 0.2% or more. However, because it is a very expensive element, the upper limit is limited to 1.0%. Mo may not be included at all.

[0033] Cu: 1.0% or less Cu is an element that further enhances the acid corrosion resistance of stainless steel and also improves its toughness, so it can be included as needed. For this purpose, it is preferable to include 0.3% or more. On the other hand, if added in excess, εCu precipitates and causes embrittlement, so the upper limit should be limited to 1.0%. Cu may not be included at all.

[0034] N: 0.03~0.10% N is included at a concentration of 0.03% or more to stabilize the austenite phase and to enhance the solid solution. On the other hand, if N is included in excess, Cr nitride precipitates and the corrosion resistance decreases, so the upper limit is limited to 0.10%.

[0035] Al: 0.10% or less Al is an important element for deoxidizing steel and can be included together with Si as needed to reduce oxygen in the steel. For this purpose, it is preferable to include 0.01% or more. On the other hand, Al has a relatively high affinity for N, and excessive addition produces AlN, which inhibits the toughness of the steel. The degree of this depends on the N content, but since the decrease in toughness becomes significant when Al exceeds 0.10%, the upper limit of its content is set at 0.10%. Al may not be included at all.

[0036] In the chemical composition of steel, the remainder consists of Fe and impurities. The steel material of the present invention may further contain trace elements such as B, Ti, Nb, Sn, V, W, Co, Ta, Ca, Mg, Zr, Hf, and REM as impurities in place of a portion of the Fe. The upper limits of the permissible content are as follows: B: 0.0050% or less, Ti: 0.030% or less, Nb: 0.060% or less, Hf: 0.100% or less, Sn: 0.050% or less, V: 0.30% or less, W: 1.00% or less, Co: 1.00% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, Zr: 0.050% or less, and REM: 0.10% or less. Since these selected elements are not essential, the lower limit of their content is 0.

[0037] Sn: 0.050% or less Sn, like Cu, is an element that additionally enhances the acid corrosion resistance of stainless steel. This effect can be obtained even with trace amounts, but to reliably obtain the effect, it is preferable to include 0.001% or more. On the other hand, Sn is an element that is detrimental to hot workability, and including more than 0.050% may impair hot workability. For this reason, the upper limit was set at 0.050%. The preferred content when adding Sn is 0.003 to 0.030%.

[0038] W: 1.00% or less W, like Mo, is an element that improves the corrosion resistance of stainless steel. This effect can be obtained even with trace amounts of addition, but to reliably obtain the effect, it is preferable to include 0.01% or more. On the other hand, adding large amounts increases costs, so the content in the steel material of this invention is limited to 1.00%. The preferred content when adding is 0.1 to 0.5%.

[0039] Co: 1.00% or less Co is an effective element for improving the toughness and corrosion resistance of steel. This effect can be obtained even with trace amounts, but to reliably obtain the effect, it is preferable to add 0.01% or more. If the content exceeds 1.00%, the effect will not justify the cost due to the high cost of the element, so the upper limit has been set at 1.00%. The preferred content when adding cobalt is 0.1 to 0.5%.

[0040] Ca: 0.0050% or less, Mg: 0.0030% or less, REM: 0.10% or less Ca, Mg, and REM are elements that improve the hot workability of steel and are added as needed. Here, REM refers to rare earth elements, and its content is the sum of the content of rare earth elements. These elements are also strong oxide-forming elements and have the effect of forming relatively large oxides in steel, thereby inhibiting the toughness of the steel. These effects can be obtained even with trace amounts of addition, but in order to reliably obtain the effects, it is preferable to use Ca: 0.0003~0.0050%, Mg: 0.0001~0.0030%, and REM: 0.005~0.10%, with Ca: 0.0010~0.004%, Mg: 0.0003~0.002%, and REM: 0.010~0.06% being more preferable.

[0041] V:0.30% or less V is an element that has an affinity for N and C and has the ability to form nitrides and carbides. Its nitride-forming effect is slightly weaker than that of other nitride-forming elements specified in this invention, and its content is permissible up to 0.30%. However, if the content exceeds 0.30%, a large amount of V nitrides and carbides may precipitate, potentially impairing toughness, so the upper limit has been set at 0.30%.

[0042] Nb: 0.060% or less Nb has a stronger affinity for N and C than V, and is an element that readily forms nitrides and carbides. Since exceeding 0.060% inclusion can lead to the precipitation of large amounts of Nb carbonitrides, potentially impairing toughness, the upper limit was set at 0.060%.

[0043] Ti: 0.030% or less Ti has a very strong affinity for N, forming Ti nitrides in steel. Therefore, if it is included, it must be in very small amounts. Since including more than 0.030% may impair toughness due to Ti nitrides, the upper limit has been set at 0.030%.

[0044] B: 0.0050% or less B is an element with a very strong affinity for N, and if it is included in large quantities, B nitrides may precipitate, potentially impairing toughness. For this reason, the upper limit of its content has been set at 0.0050%.

[0045] Zr: 0.050% or less, Hf: 0.100% or less, Ta: 0.100% or less Zr and Hf are elements with a strong affinity for N, while Ta is an element with a strong affinity for both N and C. Including large amounts of these elements may cause their carbonitrides to precipitate, potentially impairing toughness. Therefore, the upper limits for the content of Zr, Hf, and Ta were set at 0.050%, 0.100%, and 0.100%, respectively.

[0046] Regarding the component composition, as mentioned above, the Md30 in equation (1) is set to 35°C or lower. This suppresses processing-induced martensitic transformation when plastic deformation such as end plate processing is performed, and, in combination with the reduction of hydrogen in the surface layer of the steel material described later, can suppress the occurrence of cracking. Md30 is preferably 30°C or lower, more preferably 25°C or lower.

[0047] The steel material of this invention has a hydrogen (H) content of 3.5 ppm by mass or less in the surface layer up to 1 / 5 of the plate thickness in the depth direction from the surface of the steel material. A sample is cut out from the portion of the steel material up to 1 / 5 of the plate thickness in the depth direction from the surface of the steel material. By adjusting the dimensions of the sample in the width direction and length direction of the steel material so that the mass of the sample is 0.5 g or more, the accuracy of hydrogen analysis can be ensured. The hydrogen content of the cut-out sample is analyzed using the inert gas-column separation-thermal conductivity method.

[0048] Next, the method for manufacturing hot-rolled steel for tanks according to the present invention will be described.

[0049] First, we will describe a manufacturing method for reducing the hydrogen (H) content of the surface layer of the steel material to 3.5 ppm by mass or less in the depth direction from the surface to 1 / 5 of the plate thickness.

[0050] For either the cast material before rolling or the intermediate-rolled material during the rolling process, the hydrogen partial pressure (p H2 By performing a process of heating at high temperatures for a long period of time in a low-temperature atmosphere, hydrogen diffuses within the steel and is released from the steel surface, thereby reducing the hydrogen concentration in the steel, especially the hydrogen concentration near the surface of the material.

[0051] Partial pressure of hydrogen in the atmosphere (p H2 Regarding the hydrogen partial pressure (p), it is necessary to set it to a value such that the hydrogen concentration in the steel at equilibrium with the hydrogen partial pressure is sufficiently low. In this invention, if the hydrogen partial pressure in the atmosphere is set so that the hydrogen concentration in the steel at equilibrium is 2 ppm by mass or less, dehydrogenation from the steel can be promoted. The hydrogen partial pressure that satisfies these conditions is very low, so it is difficult to directly identify it by atmospheric analysis. On the other hand, from the equilibrium relationship of hydrogen, oxygen, and water vapor in the atmosphere, the hydrogen partial pressure (p H2 ), water vapor partial pressure (p H2O ) and oxygen partial pressure (p O2 During this period, the following relationship is satisfied: p H2 ∝p H2O / √(p O2 )

[0052] Therefore, the partial pressure of water vapor in the atmosphere (p H2O ) and oxygen partial pressure (p O2 If the hydrogen partial pressure (p) can be measured, then from those values, the hydrogen partial pressure (p) can be calculated. H2 It is considered possible to relatively estimate the degree of (p). And, as a result of various experiments, the water vapor partial pressure (p H2O ) and oxygen partial pressure (p O2 It was found that if heat treatment is performed in an atmosphere that satisfies equation (2) below, hydrogen can be released from the surface of the steel during heat treatment, thereby reducing the hydrogen content in the steel. LOG(p H2O / √(p O2 ))≦1.00 (2) Here, LOG is the common logarithm of base 10, p H2O and p O2 The unit is (atm). LOG(p H2O / √(p O2)) is preferably 0.90 or less. More preferably 0.50 or less.

[0053] In the present invention, in order to produce a product in which the hydrogen concentration in the surface layer of the steel material after hot rolling is 3.5 ppm by mass or less, the following manufacturing processes can be used. The first is a method in which a cast material (thickness T mm) is heated for a long time to release hydrogen from the surface of the cast material, and then hot-rolled. The second is a method in which the cast material is heated in the usual way before hot rolling, and then the cast material is hot-rolled to produce an intermediate-rolled material with a thickness of T 1 mm, this intermediate-rolled material is heated for a long time to release hydrogen from the surface of the intermediate-rolled material, and then hot-rolled further. These will be explained in order below.

[0054] The first method will be explained. A cast material with a thickness of T=140mm was prepared. Analysis of the hydrogen concentration in the cast material revealed that it was approximately 6 ppm by mass both near the surface and in the center of the plate thickness. Regarding this material, LOG(p H2O / √(p O2 )) = -0.178 In a heating furnace with an atmosphere of heat treatment temperature of 950-1250°C, the elapsed time from when the surface temperature of the steel material reached 950°C until extraction was defined as Δt1, and heat treatment was performed for Δt1 = 129 minutes. The surface temperature of the steel material at the time of extraction, i.e., the heat treatment temperature, was 1205°C. Subsequently, a steel plate with a thickness of 12 mm was formed by hot rolling. The hot rolling method employed controlled cooling, as described later, and solution heat treatment was not performed. Using the method described above, the hydrogen (H) content in the surface layer from the surface to 1 / 5 of the plate thickness was analyzed, and a result of 2.7 mass ppm was obtained (sample No. 2, described later). The hydrogen concentration in the center of the steel plate thickness was 3.4 mass ppm.

[0055] From the fundamental characteristics of the transient diffusion equation, when the thickness T of the cast material is changed, in order to maintain the same hydrogen distribution pattern in the thickness direction of the cast material at the same heat treatment temperature (i.e., the same hydrogen diffusion coefficient), the heat treatment time must be changed to T. 2It can be understood that it needs to increase in proportion to (T). As described above, when heat treatment was performed at a heat treatment temperature of 1192°C and Δt1 = 120 minutes at T = 140 mm, the hydrogen concentration in the surface layer of the steel material was well maintained at 2.9 mass ppm (sample No. 3 described later). Here, in order to obtain a result in which the hydrogen concentration in the surface layer of the steel material is 3.5 mass ppm or less at different casting thickness T, when the heat treatment temperature is 1000°C or higher, Δt1 should be (T 2 It was determined that the heat treatment temperature should be 1000°C or higher, and the heat treatment time should be (T 2 It was confirmed that by setting the time to 200 minutes or more, the hydrogen concentration in the surface layer of the steel material could be reduced to 3.5 ppm by mass or less.

[0056] I will now explain the second method. As shown in Sample No. 7 of Table 2 below, a cast material with a thickness T = 200 mm was prepared. Analysis of the hydrogen concentration in the cast material revealed that it was approximately 5.4 ppm by mass both near the surface and in the center of the plate thickness. This material was subjected to the usual pre-hot-rolling heating and then hot-rolled to produce an intermediate-rolled material with a thickness T1 = 120 mm. Regarding this intermediate-rolled material, LOG(p H2O / √(p O2 ))=-0.117 The steel was subjected to a heat treatment at a temperature of 1182°C for Δt1 = 80 minutes as defined above. Subsequently, it was hot-rolled to form a steel plate with a thickness of 20 mm. The hot-rolling method employed controlled cooling, as described later, and solution heat treatment was not performed. Using the method described above, the hydrogen (H) content in the surface layer of the steel plate, from the surface to 1 / 5 of the plate thickness, was analyzed and a result of 3.1 ppm by mass was obtained. The hydrogen concentration in the center of the steel plate thickness was 5.2 ppm by mass.

[0057] Similar to the first method described above, in the second method, when various thicknesses are used for the thickness T1 mm of the intermediate-rolled material, in order to obtain a result in which the hydrogen concentration in the surface layer of the steel material is 3.5 ppm by mass or less, when the heat treatment temperature is 1000°C or higher, the heat treatment time should be (T1 2It was determined that the heat treatment temperature should be 1000°C or higher, and the heat treatment time should be (T1 2 It was confirmed that by reducing the time to 300 minutes or more, the hydrogen concentration in the surface layer of the steel material could be reduced to 3.5 ppm by mass or less. As described above, the heat treatment time required for dehydrogenation can be shortened by reducing the thickness of the intermediate-rolled material, but since multiple rolling processes significantly impair productivity, intermediate rolling is not performed more than twice. Furthermore, even if left at room temperature for a long period of time, no change in hydrogen concentration that would have a practical impact is observed.

[0058] As is clear from the above explanation, in the first and second methods described above, when the heat treatment for reducing hydrogen from steel by diffusion is set to a predetermined temperature of 1000°C or higher, the same or better results can be achieved under the same conditions as described above. When the heat treatment temperature changes, the diffusion coefficient of hydrogen in the steel changes accordingly, and therefore the heat treatment time required to reduce hydrogen in the steel also changes. The required heat treatment time is proportional to the reciprocal of the diffusion coefficient. In the austenitic stainless steel targeted by the present invention, the diffusion coefficient of hydrogen D(cm) 2 It was confirmed that the ( / s) follows the following formula. Therefore, it is possible to calculate the degree of difference in the diffusion coefficient from the difference in heat treatment temperature and calculate the required heat treatment time at the changed heat treatment temperature. D = 7.4 × 10 -3 exp(-14400 / Rθ) Here, θ is the heat treatment temperature (K), and R is the gas constant. While there is no upper limit set for the heat treatment temperature, it is preferable to keep it below 1250°C, as high-temperature heat treatment can coarse the microstructure and reduce strength and toughness.

[0059] The conditions for hot rolling and controlled cooling after rolling are characterized by performing finish rolling at a final finishing temperature of 1000°C or less, and after rolling, cooling the steel material by water cooling from 600°C or higher, without performing solution heat treatment. This makes it possible to obtain steel materials that achieve both high strength by introducing strain below the recrystallization temperature and corrosion resistance by suppressing chromium carbide precipitation.

[0060] The hot-rolled steel material of the present invention has not undergone solution heat treatment after hot rolling. This can be confirmed by observing the microstructure of the hot-rolled steel material using EBSD (electron backscatter diffraction), which reveals the presence of microstructure regions with a KAM value of 1 degree or higher.

[0061] The hot-rolled steel material of the present invention, even if the casting contains a high concentration of hydrogen during the casting stage, reduces the hydrogen concentration in the surface layer by performing high-temperature heat treatment in a predetermined atmosphere during the casting or intermediate-rolled material stage, thereby diffusing hydrogen from the surface. Therefore, the hydrogen content in the surface layer from the surface of the steel material to 1 / 5 of the plate thickness in the depth direction is reduced. S The hydrogen content in the center of the steel thickness is H C When H S / H C The value is ≤0.85. [Examples]

[0062] Examples of the present invention will be described below. Table 1 shows the chemical composition, hydrogen content in molten steel, Md30, and slab thickness of the test material. Components other than those listed in Table 1 are Fe and impurity elements. Elements with a content of "0" in Table 1 indicate that they were not actively added. The hydrogen content in molten steel was analyzed using the inert gas-cooling-thermal conductivity method after the molten steel sample was water-cooled. These steels were melted in an actual refining furnace, sampled from slabs cast in a continuous casting apparatus, and evaluated for hydrogen content and tensile properties after hot rolling under the conditions shown in Tables 2 and 3. In Tables 2 and 3, Invention Examples No. 1-4, 16, 17, and Comparative Examples No. 28 and 29 undergo the heat treatment of the present invention at the casting stage. Invention Examples No. 5-15, 18-27, and Comparative Examples No. 30-34 undergo the heat treatment of the present invention at the intermediate-rolled material stage (intermediate-rolled material thickness shown in Tables 2 and 3). Regarding the heat treatment conditions, for a heat treatment atmosphere at temperature TA (°C) as shown in Table 1, Δt1 is defined as the elapsed time from when the surface temperature reaches 950°C until extraction, and Δt1 is shown in Tables 2 and 3. Furthermore, the p of the heat treatment atmosphere is also shown. H2O , p O2 , LOG(p H2O / √(p O2 The values ​​for heating the cast material and the intermediate-rolled material are summarized in Tables 2 and 3. The final finishing temperature for hot rolling and the water cooling start temperature are shown in Tables 2 and 3.

[0063] For determining the hydrogen content of the product, the area from the surface of the steel material to 1 / 5 of the plate thickness in the depth direction was defined as the "surface layer," and the area including the center of the plate thickness was defined as the "center." The dimensions of the sample in the width and length directions of the steel material were adjusted so that the mass of each sample was 0.5 g or more, and hydrogen analysis samples were taken. The hydrogen content of the cut samples was analyzed using the inert gas-column separation-thermal conductivity method.

[0064] For evaluating corrosion resistance, the JIS G0571 oxalic acid electrolytic etching standard was used, with A (stepped structure) being considered acceptable and all other results being unacceptable (B).

[0065] We evaluated delayed fracture (timing cracking) in steel plates after plastic deformation. For the evaluation of delayed fracture, we performed cold pressing to form end plates on steel materials with thicknesses of 20, 12, and 8 mm. For each thickness of steel material, we formed two circular dish-shaped end plates with inner diameters of 2500, 2500, and 1800 mm, and heights of 660, 660, and 470 mm. For the evaluation of timing cracking, we observed whether timing cracks occurred at the circumferential edges of the end plates after storing them at room temperature for one month after forming. If timing cracks were visually confirmed at even one location on the circumferential edge, it was judged as "timing cracking". Steel materials with a thickness of 40 mm were excluded from evaluation because they exceeded the size for cold pressing to form end plates. In the "timing cracking" column of Tables 2 and 3, no timing cracking was judged as pass (A), and all other cases were judged as fail (B).

[0066] [Table 1]

[0067] [Table 2]

[0068] [Table 3]

[0069] As shown in Table 2, the hot-rolled steel materials according to the present invention, which contain the component composition of the present invention and are manufactured using a preferred manufacturing method of the present invention, all have a hydrogen concentration of 3.5 ppm or less in the surface layer, as well as a tensile strength exceeding 600 MPa, good corrosion resistance and resistance to cracking, and exhibit excellent properties as materials for low-temperature tanks.

[0070] On the other hand, in Table 3, the steel samples No. 28 to 34, which do not fall under the present invention, had a hydrogen concentration in the surface layer exceeding 3.5 ppm or a tensile strength below 600 MPa. In samples No. 28 to 30, insufficient heating time or high hydrogen partial pressure during heating of the cast material or intermediate-rolled material resulted in a large amount of residual hydrogen in the steel. As a result, the delayed fracture evaluation results were poor, and cracking occurred during storage after tank fabrication. Sample No. 31 underwent solution heat treatment after hot rolling, and although the amount of hydrogen in the steel was reduced, the processing strain introduced during hot rolling recovered, resulting in a tensile strength below 600 MPa. In addition, in sample No. 32, the high final finishing temperature of hot rolling resulted in insufficient processing strain being introduced, and the tensile strength below 600 MPa. Sample No. 33 showed reduced corrosion resistance due to a low water cooling start temperature after hot rolling, which caused carbide precipitation and resulted in a dual rating in JIS G0571 oxalic acid electrolytic etching. Sample No. 34 had a high Md30, which led to excessive work-induced martensitic transformation during tank fabrication, resulting in a poor delayed fracture evaluation, and in fact, time cracking occurred within 48 hours of tank fabrication.

[0071] As can be seen from the above examples, the present invention clearly provides an austenitic stainless steel hot-rolled steel material having a tensile strength of 600 MPa or more, which does not experience delayed fracture after plastic deformation when used as a material for tank end plates, and a method for manufacturing the same.

Claims

1. The chemical composition, in mass%, consists of C: 0.08% or less, Si: 0.05-1.0%, Mn: 0.10-2.0%, P: 0.045% or less, S: 0.030% or less, Cr: 18.0-20.0%, Ni: 8.0-13.0%, Mo: 1.0% or less, Cu: 1.0% or less, N: 0.03-0.10%, Al: 0.10% or less, with the remainder being Fe and impurities. (1) The Md30 shown in equation (1) is 35°C or lower. The hydrogen (H) content in the surface layer of the steel material, from the surface to 1 / 5 of the plate thickness in the depth direction, is 3.5 ppm by mass or less. Hot-rolled steel material for tanks having a tensile strength of 600 MPa or more. Md30(℃)=413-462(C+N)-9.2Si-8.1Mn-13.7Cr-9.5(Ni+Cu)-18.5Mo (1) In equation (1), the element symbol on the right side represents the content (mass %) of that element in the steel. Furthermore, "hot-rolled steel" refers to steel that has been hot-rolled and has not undergone solution heat treatment after hot rolling.

2. The hot-rolled steel material for tanks according to claim 1, further comprising one or two of the following groups A and B in place of a portion of the Fe. [Group A] One or more elements in mass percent, such as Sn: 0.050% or less, W: 1.00% or less, Co: 1.00% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less. [Group B] In mass percent, one or more of the following elements are present: V: 0.30% or less, Nb: 0.060% or less, Ti: 0.030% or less, B: 0.0050% or less, Zr: 0.050% or less, Hf: 0.100% or less, Ta: 0.100% or less.

3. The hydrogen content in the surface layer of the steel material, from the surface to 1 / 5 of the plate thickness in the depth direction, is H Ｓ The hydrogen content in the center of the steel thickness is H Ｃ When H Ｓ / H Ｃ A hot-rolled steel material for tanks according to claim 1 or 2, characterized in that the value is ≤0.

85.

4. Water vapor partial pressure (p Ｈ２Ｏ ) and oxygen partial pressure (p Ｏ２ A cast material of thickness T mm is cast in an atmosphere that satisfies the following equation (2) (T ２ A method for manufacturing hot-rolled steel for tanks according to claim 1 or 2, characterized by being manufactured by hot-rolling after heating for 200 minutes or more. LOG(p Ｈ２Ｏ / √(p Ｏ２ ))≦1.00 (2) Here, LOG is the common logarithm of base 10, p Ｈ２Ｏ and p Ｏ２ The unit is (atm).

5. The cast material is hot-rolled to a thickness T １ A medium-sized intermediate-rolled material is manufactured, and this intermediate-rolled material is subjected to a water vapor partial pressure (p Ｈ２Ｏ ) and oxygen partial pressure (p Ｏ２ ) in an atmosphere that satisfies the following equation (2) (T １ ２ A method for manufacturing hot-rolled steel material for tanks according to claim 1 or 2, characterized by being manufactured by hot-rolling after heating for 300 minutes or more. LOG(p Ｈ２Ｏ / √(p Ｏ２ ))≦1.00 (2) Here, LOG is the common logarithm of base 10, p Ｈ２Ｏ and p Ｏ２ The unit is (atm).

6. The method for manufacturing hot-rolled steel for tanks according to claim 4, characterized in that the hot rolling is performed at a final finishing temperature of 1000°C or less, water cooling is performed from 600°C or higher after rolling, and solution heat treatment is not performed.

7. The method for manufacturing hot-rolled steel for tanks according to claim 5, characterized in that the hot rolling is performed at a final finishing temperature of 1000°C or less, water cooling is performed from 600°C or higher after rolling, and solution heat treatment is not performed.

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