Hot-rolled austenitic stainless steel material and method for producing same

Abstract

The present invention relates to a highly economical steel material having excellent workability and weld joint characteristics, which can be applied to container applications, while enjoying high strength due to TMCP technology and alloying elements. This hot-rolled steel material is characterized by having a prescribed chemical composition, log ([Ti] * [N]) being-3.7 or more, and in that Md30 represented by Md30 = 413-462 ([C] + [N])-9.2 [Si]-8.1 [Mn]-13.7 [Cr]-9.5 ([Ni] + [Cu])-18.5 [Mo] is in the range of-30-+ 40, in that the microstrain measured by X-ray diffraction is 0.0005-0.0025 inclusive, the crystal grain size number is 7 or more, the tensile strength at room temperature is 690-880 MPa inclusive, and the elongation is 35% or more. In this connection, [X] represents the content of element X in mass%.

Description

Technical Field

[0001] The present invention relates to a hot-rolled austenitic stainless steel material that can be used as a liquid container, has high strength, excellent workability, and good weld joint characteristics, and a method for producing the same. Background Art

[0002] As a method for increasing the strength of austenitic stainless steel, a technique called thermal process control (hereinafter referred to as TMCP) for controlling the working and temperature in the hot rolling process can be applied. Patent Documents 1 to 5 disclose inventions of TMCP techniques for austenitic stainless steel sheets.

[0003] Furthermore, as a method of increasing the strength of stainless steel using alloying elements, nitrogen is commonly incorporated. Austenitic nitrogen-containing steels such as SUS304N1, 304N2, 304LN, and 316LN are standardized as JIS steel grades. Combining these two methods allows for the production of high-strength hot-rolled steel, as disclosed in, for example, Patent Document 6. Achieving high strength allows for structural designs that reduce steel weight by reducing wall thickness, thus providing economical materials to customers.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Publication No. 3-66368

[0007] Patent Document 2: Japanese Patent Publication No. 5-82453

[0008] Patent Document 3: Japanese Patent Publication No. 5-75809

[0009] Patent Document 4: Japanese Patent No. 3799179

[0010] Patent Document 5: Japanese Patent No. 3000860

[0011] Patent Document 6: Japanese Patent No. 6176208

[0012] Non-patent literature

[0013] Non-patent literature 1: ISIJ International, 2019, Vol. 59, No. 3, pp. 567-572 Summary of the Invention

[0014] Problems to be solved by the invention

[0015] To realize the application of high-strength austenitic stainless steel produced by the above-mentioned method in liquid-holding containers, the present inventors conducted a case study of containers for liquids. While there were examples of solution heat-treated materials in practical use, no examples of hot-rolled stainless steel sheets strengthened by TMCP technology being used as liquid containers were found within the scope of the study.

[0016] As an example of such containers, the manufacture of bag-type tanks requires plastic working of steel to create components called end plates. While heavy plastic working is rarely performed on thick austenitic stainless steel plates, the end plates of these tanks are characterized by the fact that they undergo such heavy plastic working.

[0017] Furthermore, these containers are manufactured by welding thick plates that have undergone plastic working. Thick austenitic stainless steel plates manufactured using TMCP have strain introduced by hot working. The heat of welding causes strain release and strength reduction in the base material near the weld metal. Excessive strength reduction near the heat-affected zone (HAZ) can affect the overall strength of the structure. Therefore, steel materials strengthened using TMCP technology require material design that minimizes the reduction in strength of the HAZ.

[0018] Against this background, the present invention aims to develop a highly economical steel material that can be used in containers while enjoying the increased strength achieved by TMCP technology and alloying elements and having excellent workability and weld joint properties.

[0019] Means for solving problems

[0020] To realize the application of hot-rolled austenitic stainless steel sheets produced by the aforementioned TMCP technology for container applications, the present inventors evaluated the room-temperature tensile properties and weld joint properties of steels obtained by varying the chemical composition and hot rolling method. This investigation revealed that the desired properties of the tensile strength and elongation of hot-rolled steels produced by the TMCP technology, as well as the hardness of the heat-affected zone of welded joints, can be achieved by comprehensively controlling the following four factors: (1) grain refinement of the steel by addition of grain-refining elements and controlled rolling from slab heating to the recrystallization temperature range, (2) the amount of microstrain remaining in the steel by controlled rolling in the non-recrystallization temperature range, (3) the stability of the martensitic transformation of the steel, and (4) the addition of alloying elements to suppress the reduction in hardness in the weld heat-affected zone.

[0021] The present invention has been further studied based on the above findings, and the gist of the research is as follows.

[0022] [1] A hot-rolled austenitic stainless steel material characterized in that it has the following chemical composition: containing, in mass%, C: 0.060% or less, Si: 1.50% or less, Mn: 0.10-3.00%, P: 0.045% or less, S: 0.030% or less, Cr: 16.0-20.0%, Ni: 6.0-10.0%, O: 0.0001-0.008%, N: 0.010% or more and less than 0.100%, V: 0.01-0.30%, Ti: 0.001-0.015%, Nb: 0.001-0.060%, Co: 0-0.30%, Cu: 0-2.0%, Mo: 0-2.0%, W: 0-1.0%, Al: 0-0 0.10%, B: 0-0.0040%, Sn: 0-0.050%, Ca: 0-0.005%, Mg: 0-0.003%, REM: 0-0.10%, Zr: 0-0.03%, Hf: 0-0.08%, Ta: 0-0.10%, the remainder being Fe and unavoidable impurities, log([Ti]×[N]) is -3.7 or greater, and Md30 expressed by the following formula 1 is within the range of -40 to +30, microstrain (which may also be referred to as microscopic strain) measured by X-ray diffraction is 0.0005 or greater and 0.0025 or less, the grain size number is 7 or greater, the tensile strength at room temperature is 600 MPa or greater and 780 MPa or less, and the elongation is 30% or greater.

[0023] Md30=413-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-9.5([Ni]+[Cu])-18.5[Mo]

[0024] (Formula 1)

[0025] Here, [X] is a value expressing the content of element X in mass %.

[0026] [2] A hot-rolled austenitic stainless steel material characterized in that it has the following chemical composition: containing, in mass%, C: 0.060% or less, Si: 1.50% or less, Mn: 0.10-3.0%, P: 0.045% or less, S: 0.030% or less, Cr: 16.0-20.0%, Ni: 6.0-10.0%, O: 0.0001-0.008%, N: 0.100-less than 0.250%, V: 0.01-0.30%, Ti: 0.001-0.015%, Nb: 0.001-0.060%, Co: 0-0.30%, Cu: 0-2.0%, Mo: 0-2.0%, W: 0-1.0%, Al: 0-0.10%, B: 0-0.0040%, Sn: 0-0.050%, Ca: 0-0.005%, Mg: 0-0.003%, REM: 0-0.10%, Zr: 0-0.03%, Hf: 0-0.08%, Ta: 0-0.10%, the remainder is Fe and inevitable impurities, log([Ti]×[N]) is -3.7 or greater, and Md30 expressed by the following formula 1 is in the range of -40 to +30, the microstrain measured by X-ray diffraction is 0.0005 or greater and 0.0025 or less, the grain size number is 7 or greater, the tensile strength at room temperature is 690 MPa or greater and 880 MPa or less, the elongation is 30% or greater, and the pitting potential at 30°C is 0.35 V or greater. Md30=413-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-9.5([Ni]+[Cu])-18.5[Mo]

[0027] (Formula 1)

[0028] Here, [X] is a value expressing the content of element X in mass %.

[0029] [3] A method for manufacturing a hot-rolled steel material, which is a method for manufacturing a hot-rolled steel material of austenitic stainless steel according to [1] or [2], characterized in that the manufacturing method comprises: heating a slab at a temperature T below T1 (°C) and above 1050°C obtained by the following formula 2; and performing the following controlled rolling: the thickness t0 (mm) of the slab and the thickness ts (mm) after rolling in the temperature range above Tc (°C) obtained by the following formula 3 satisfy log (t0 / ts) ≥ 0.3, and the cumulative reduction rate in the temperature range below Tc (°C) and above 800°C is above 10% and below 30%.

[0030] T1(℃)=13500 / (5.6-log([Ti]×[N]))-273 (Formula 2)

[0031] Tc(℃)=1030+100[N]+30[Mo] (Formula 3)

[0032] Here, [X] is a value expressing the content of element X in mass %.

[0033] Effects of the Invention

[0034] The austenitic stainless steel hot-rolled steel for containers obtained by the present invention has a high tensile strength of 600 to 880 MPa. Therefore, when used as a container, it can be economically designed, plastically processed, and welded with reduced wall thickness compared to existing solution heat-treated materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a graph showing the relationship between microstrain and tensile strength when the grain size is 7 to 10. DETAILED DESCRIPTION

[0036] The hot-rolled austenitic stainless steel material of the present invention will be described in detail. First, the chemical composition will be described. Hereinafter, "%" in the chemical composition means "mass %."

[0037] <Chemical Composition>

[0038] (C: 0.060% or less)

[0039] To ensure the corrosion resistance and toughness of stainless steel, the C content is set to 0.060% or less. To reliably suppress the deterioration of corrosion resistance and toughness due to the formation of Cr carbides during hot rolling, the C content can be set to 0.040% or less, 0.030% or less, or 0.020% or less.

[0040] (Si: 1.50% or less)

[0041] Si is added for deoxidation. Hot-rolled steel does not necessarily need to contain Si. To reliably achieve the deoxidation effect, the hot-rolled steel may contain 0.05% or more Si. The Si content can be set to 0.10% or more, or 0.20% or more. Considering the toughness of stainless steel, the Si content is set to 1.50% or less. The Si content can be set to 1.20% or less, 1.00% or less, or 0.80% or less.

[0042] (Mn: 0.10-3.00%)

[0043] Mn increases the austenite phase and improves toughness. Therefore, it is contained at a level of 0.10% or more. On the other hand, Mn reduces the corrosion resistance of stainless steel, so the upper limit of the Mn content is set to 3.00% or less. The Mn content can be set to 0.20% or more, 0.30% or more, or 0.50% or more. The Mn content can be set to 2.70% or less, 2.50% or less, or 2.00% or less.

[0044] (P: 0.045% or less)

[0045] P is an element inevitably mixed from the raw materials and deteriorates hot workability and toughness, so its content is set to 0.045% or less, preferably 0.040% or less, 0.035% or less, or 0.030% or less.

[0046] (S: 0.030% or less)

[0047] S is an element inevitably mixed from the raw materials. Since it deteriorates hot workability, toughness and corrosion resistance, the content is made 0.030% or less.

[0048] (Cr: 16.00~20.00%)

[0049] Cr is contained at 16.00% or more to ensure corrosion resistance. However, considering the decrease in hot workability and toughness caused by an increase in the ferrite phase fraction, the Cr content is set to 20.00% or less. The Cr content can be set to 16.50% or more, 17.00% or more, or 17.50% or more. The Cr content can be set to 19.50% or less, 19.00% or less, or 18.50% or less.

[0050] (Ni: 6.00-10.00%)

[0051] In order to stabilize the austenite structure, improve the corrosion resistance to various acids, and further improve the low-temperature toughness, nickel is contained at 6.00% or more. Since nickel is an expensive alloy, the content can be set to 10.00% or less from a cost perspective. The nickel content can be set to 6.50% or more, 7.00% or more, or 7.50% or more. The nickel content can be set to 9.50% or less, 9.00% or less, or 8.50% or less.

[0052] (O: 0.0001~0.008%)

[0053] O (oxygen) is an element that forms trace amounts of oxides and suppresses grain coarsening, so it should be contained at a level of 0.001% or more. On the other hand, it impairs the hot workability, toughness, and corrosion resistance of stainless steel, so its upper limit is 0.008%. Considering toughness, the O content can be set to 0.005%, 0.004%, or 0.003% or less.

[0054] (N: 0.010~0.250%)

[0055] Nitrogen is an effective element that dissolves in the austenite phase, enhancing the strength and corrosion resistance of the stainless steel of the present invention. To achieve this effect, the nitrogen content is set to 0.010% or higher. As the nitrogen content increases, elongation gradually decreases. Considering the reduction in toughness caused by the effects of dissolved nitrogen and tiny nitrides, and the reduction in corrosion resistance caused by the accelerated precipitation of chromium nitrides, the upper limit of the nitrogen content is set to 0.250%.

[0056] When the nitrogen content is within the range of 0.010% to less than 0.100%, a tensile strength of 600 MPa or greater can be achieved. Furthermore, high toughness at low temperatures can be achieved. Considering low-temperature toughness, the nitrogen content can be set to less than 0.100%, and can be set to 0.080% or less, 0.070% or less, or 0.050% or less.

[0057] To achieve a tensile strength of 690 MPa or higher and higher corrosion resistance, the N content should be 0.100% or higher. The N content can be set to 0.110% or higher, 0.120% or higher, or 0.140% or higher. In this case, considering the reduction in elongation, the upper limit of the N content is set to 0.250% or lower. The N content can be set to 0.220% or lower, 0.200% or lower, or 0.180% or lower.

[0058] (V: 0.01~0.30%)

[0059] V has an affinity with N and C, forming nitrides and carbides, and is an element that contributes to grain refinement. In addition, it is an element that suppresses the softening and hardness reduction of the hot-rolled steel of the present invention in the weld heat-affected zone. In order to achieve this effect, the V content is set to 0.01% or more. The nitride-forming effect of V is slightly smaller than that of other nitride-forming elements. Taking into account the reduction in toughness caused by the large amount of precipitation of V nitrides and carbides, the upper limit of the V content is set to 0.30%. The V content can be set to more than 0.02%, more than 0.05%, or more than 0.08%. The V content can be set to less than 0.25%, less than 0.20%, or less than 0.15%.

[0060] (Ti: 0.001-0.015%)

[0061] Ti and N have a very strong affinity, forming Ti nitrides in the steel. In the hot-rolled steel of the present invention, the fine dispersion of Ti nitrides (TiN) is effectively utilized by the method described below, and grain refinement is achieved through the action during slab heating and in the high-temperature zone of hot rolling. In order to achieve this effect, the Ti content is set to 0.001% or more. Taking into account the reduction in toughness caused by coarse Ti nitrides, the upper limit of the Ti content is set to 0.015%. The Ti content can be set to more than 0.002%, more than 0.003%, or more than 0.005%. The Ti content can be set to less than 0.012%, less than 0.010%, or less than 0.008%.

[0062] (Nb: 0.001-0.060%)

[0063] Nb has a stronger affinity for N and C than V, forming nitrides and carbides, and is an element that contributes to grain refinement. Furthermore, it is an element that suppresses softening and hardness reduction in the hot-rolled steel of the present invention in the weld heat-affected zone. To achieve this effect, the Nb content is set to 0.001%. Taking into account the reduction in toughness caused by the large amount of precipitation of Nb carbonitrides, the Nb content is set to 0.060% or less. The Nb content can be set to 0.002% or more, 0.005% or more, or 0.010% or more. The Nb content can be set to 0.050% or less, 0.040% or less, or 0.030% or less.

[0064] The hot-rolled austenitic stainless steel material of the present invention may contain, in addition to the above-mentioned elements, one or more of Co, Cu, Mo, W, Al, B, Sn, Ca, Mg, REM, Zr, Hf, and Ta as needed.

[0065] (Co: 0-0.30%)

[0066] Co is an effective element for improving the toughness and corrosion resistance of steel and can be added. The effect of adding Co can be achieved even in trace amounts. When Co is included, it is preferably set to 0.01% or more. Considering cost, the Co content is preferably set to 0.30% or less. The Co content can be set to 0.02% or more, 0.03% or more, or 0.05% or more. The Co content can be set to 0.25% or less, 0.20% or less, or 0.15% or less.

[0067] (Cu: 0-2.00%)

[0068] Cu is an element that additionally improves the corrosion resistance of stainless steel to acid and is an element that has the effect of improving toughness, so it can be added. The effect of adding Cu can be obtained even in a trace amount. When Cu is contained, it is preferably set to 0.01% or more. Taking hot workability into consideration, the Cu content is set to 2.00% or less. The Cu content can be set to 0.10% or more, 0.20% or more, or 0.30% or more. The Cu content can be set to 1.50% or less, 1.20% or less, or 1.00% or less.

[0069] (Mo: 0-2.00%)

[0070] Mo is an extremely effective element for improving the corrosion resistance of stainless steel and can be added. The effects of Mo addition can be achieved even in trace amounts. If Mo is present, the content is preferably set to 0.01% or more. Considering cost, the Mo content is preferably set to 2.00% or less. The Mo content can be set to 0.02% or more, 0.05% or more, or 0.10% or more. The Mo content can be set to 1.80% or less, 1.50% or less, or 1.00% or less.

[0071] (W: 0-1.00%)

[0072] Like Mo, W is an element that improves the corrosion resistance of stainless steel and can be added. The effects of W addition can be achieved even in trace amounts. If included, the W content is preferably set to 0.01% or greater. Considering cost, the W content is preferably set to 1.00% or less. The W content can be set to 0.02% or greater, 0.03% or greater, or 0.05% or greater. The W content can be set to 0.80% or less, 0.50% or less, or 0.30% or less.

[0073] (Al: 0-0.100%)

[0074] Al is an element used for deoxidation. It can be added to molten steel to reduce the oxygen content in steel, but hot-rolled steel does not have to contain Al. However, when adding Al, it is recommended to add an amount of Al to the hot-rolled steel to contain 0.001% or more. The content can be 0.003% or more, 0.005% or more, or 0.010% or more. Considering the reduction in toughness caused by the formation of Al nitrides, the Al content is set to 0.10% or less. The Al content can be set to 0.080% or less, 0.050% or less, or 0.030% or less.

[0075] (B: 0 to 0.0040%)

[0076] B is an element that improves hot workability and can be added. Considering the reduction in toughness caused by the precipitation of B nitrides, the B content is set to 0.0040% or less. The B content can be set to 0.0020% or less, 0.0015% or less, or 0.0010% or less.

[0077] (Sn: 0 to 0.050%)

[0078] Sn, like Cu, is an element that additionally improves the acid corrosion resistance of stainless steel. The effect of Sn addition can be achieved even in trace amounts. If Sn is present, it is preferably set to 0.001% or more. Considering the reduction in hot workability, the Sn content is set to 0.050% or less. The Sn content can be set to 0.002% or more, 0.003% or more, or 0.005% or more. The Sn content can be set to 0.040% or less, 0.030% or less, or 0.020% or less.

[0079] (Ca: 0~0.0050%, Mg: 0~0.0030%, REM: 0~0.10%)

[0080] 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 their content is the sum of the contents of all rare earth elements. The effects of adding these elements can be achieved even in trace amounts. If included, the content of each element is preferably set to 0.001% or more. Considering that these elements form relatively large oxides in steel, which reduces the toughness of the steel, the Ca content is set to 0.0050% or less, the Mg content to 0.0030% or less, and the REM content to 0.10% or less. The Ca content can be set to 0.0002% or more. The Ca content can be set to 0.0400% or less. The Mg content can be set to 0.0020% or less or 0.0010% or less. The REM content can be set to 0.01% or more, 0.02% or more, or 0.03% or more. The REM content can be set to 0.08% or less, 0.06% or less, or 0.05% or less.

[0081] (Zr: 0~0.030%, Hf: 0~0.080%, Ta: 0~0.100%)

[0082] Zr, Hf, and Ta have the effect of refining the crystal grains, so they can be added to the steel of the present invention. The effect of adding these elements can be obtained even in trace amounts. When they are contained, the content of each element is preferably set to 0.001% or more. Taking into account the reduction in toughness caused by the large amount of precipitation of carbonitrides of these elements, the content of Zr is set to 0.030% or less, the content of Hf is set to 0.080% or less, and the content of Ta is set to 0.100% or less. The content of Zr can be set to 0.020% or less, or 0.010% or less. The content of Hf can be set to 0.060% or less, or 0.050% or less, or 0.030% or less. The content of Ta can be set to 0.080% or less, or 0.050% or less, or 0.030% or less.

[0083] The remainder of the chemical composition other than the above-mentioned elements is Fe and inevitable impurities. Inevitable impurities herein refer to components that are introduced into raw materials such as ores and scrap iron during industrial production of steel sheets due to various factors in the manufacturing process. These impurities are contained within a range that does not adversely affect the properties of the hot-rolled steel material of the present invention, that is, within a range that allows desired properties such as workability required of the hot-rolled steel sheet of the present invention to be achieved.

[0084] (log([Ti]×[N]) is -3.7 or more)

[0085] In the hot-rolled steel of the present invention, the contents of Ti and N are further specified in such a manner that log([Ti]×[N]) is -3.7 or greater. Here, [X] (X is an arbitrary element symbol) represents a value representing the content of element X in mass % (hereinafter, the same applies to Formulas 1 to 3). Ti and N suppress grain growth in the hot rolling heating process of the present steel, thereby achieving grain refinement. In order to obtain this effect, a lower limit of this value is specified. Furthermore, as described later, in the hot rolling heating process of the manufacturing method of the present invention, by appropriately controlling the heating temperature according to the value of log([Ti]×[N]), the grain size of the slab before the start of hot rolling can be controlled to be smaller. log([Ti]×[N]) can be set to -3.70 or greater, -3.60 or greater, -3.55 or greater, or -3.50 or greater.

[0086] (Md30)

[0087] In the hot-rolled steel material of the present invention, when the N content is 0.010% or more and less than 0.100%, Md30 expressed by the following formula 1 is -30 to +40, and when the N content is 0.100% or more and 0.250% or less, Md30 expressed by the following formula 1 is -40 to +30.

[0088] Md30=413-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-9.5([Ni]+[Cu])-18.5[Mo]

[0089] (Formula 1)

[0090] Md30 is a value specified to control the stability of the hot-rolled steel of the present invention against work-induced martensitic transformation. Austenitic stainless steels containing a large amount of nitrogen tend to approach stable austenitic steel due to the nitrogen's austenite stabilization effect. However, in the hot-rolled steel of the present invention, the chemical composition is adjusted, including the contents of key elements such as Cr and Ni. If Md30 is low, work-induced transformation is suppressed, and the desired elongation cannot be achieved. On the other hand, if Md30 is high, work-induced transformation is promoted, resulting in excessively high strength and a tendency for elongation to decrease. When the nitrogen content is 0.010% or more and less than 0.100%, Md30 can be set to be above -25 or above -20. Md30 can also be set to be below 35 or below 30. When the nitrogen content is 0.100% or more and less than 0.250%, Md30 can be set to be above -35 or above -30. Md30 can also be set to be below 25 or below 20.

[0091] (Microstrain measured by X-ray diffraction is 0.0005 or more and 0.0025 or less)

[0092] Microstrain is an indicator that quantitatively represents the amount of dislocations introduced into the hot-rolled steel material of the present invention during the hot rolling process and is one of the factors that improve the strength of the steel material. Here, the factors that control the strength of the hot-rolled steel material of the present invention are described.

[0093] Strength-dominant factors are often explained by the additive law of solid solution strengthening, dislocation strengthening, grain refinement strengthening, and phase transformation strengthening. Microstrain corresponds to dislocation strengthening. Microstrain in steel is quantified by measuring the half-value width of X-ray diffraction reflections from multiple crystal planes of the austenite phase.

[0094] The unit of microstrain is dimensionless. The specific measurement method is as follows. After finishing the sample into a size of about 3 mm thick × 20 mm wide × 20 mm long from the 1 / 4t and 1 / 2t positions of the thick plate by mechanical processing and electrolytic grinding in a way that no residual strain is left when the sample is made, X-ray diffraction using a CuKα ray source is performed to measure the diffraction intensity distributions A1 and A2 of each diffraction surface of the austenite phase. As a comparative material, the test material is subjected to a 1050°C solution heat treatment to remove the strain introduced by the hot working (sample B), and the same X-ray diffraction sample is made and X-ray diffraction is performed to measure the diffraction intensity distributions B1 and B2 without strain. In the sample with large residual strain, the diffraction intensity distribution has a width (half-value width) relative to the diffraction angle 2θ. By comparing distributions A and B, the increase in half-value width is calculated for each diffraction surface and numerically processed, thereby quantifying the microstrain of the austenite phase. The microstrain thus obtained has a certain relationship with the dislocation density inside the material. The details of the analysis method are known from Non-Patent Document 1, which describes microstrain measurement of ferritic steel after cold rolling. In the Non-Patent Document, microstrain is defined by ε in Formula (1).

[0095] The following describes the concept of determining the lower limit and upper limit of microstrain.

[0096] Microstrain is a characteristic value of steel corresponding to the amount of dislocation strengthening introduced during hot rolling and is controlled to a value of 0.0005 or greater to achieve strengthening. Microstrain remains in the steel due to a balance between the amount of strain introduced during hot rolling and the reduction in internal strain caused by dynamic recovery, static recovery, and recrystallization. The reduction in internal strain caused by dynamic recovery, static recovery, and recrystallization is governed by the steel's primary chemical composition, the precipitation behavior of trace elements, and the hot rolling temperature range. Increasing the reduction rate in the low-temperature region below Tc for strengthening, resulting in excessive dislocation strengthening, can lead to reduced elongation of the steel and reduced weld joint properties. Here, reduced weld joint properties refer to a reduction in the strength or hardness of the weld heat-affected zone. Steel with excessive microstrain leads to a reduction in the hardness of the weld heat-affected zone, resulting in reduced joint properties when welding containers and structures. In the hot-rolled steel of the present invention, Nb and V are contained to suppress the hardness reduction of the weld heat-affected zone. However, if the microstrain exceeds 0.0025, the hardness reduction begins to become significant, so the upper limit is set to 0.0025. Microstrain does not depend on the grain size, while tensile strength depends on the grain size. When limited to a specific grain size, the correlation between microstrain and tensile strength is observed. This relationship is shown in Figure 1 The details of the microstrain control method will be described later. The microstrain can be set to 0.0007 or more, 0.0008 or more, or 0.0010 or more. The microstrain can be set to 0.0020 or less, 0.0018 or less, or 0.0015 or less.

[0097] (Grain size number is 7 or above)

[0098] By increasing the grain size number, grain refinement and strengthening are achieved in the hot-rolled steel of the present invention. To this end, the chemical composition and manufacturing method of the hot-rolled steel of the present invention are specified so that the grain size number measured in accordance with JIS G0551:2020 becomes larger. In the hot-rolled steel of the present invention, the tensile strength of the base material is controlled to a desired value by making the grain size number 7 or higher. The details of the control method are described later. The preferred range of the grain size number is 8 or higher, and more preferably 9 or higher.

[0099] (Tensile strength at room temperature)

[0100] The tensile strength at room temperature becomes the design benchmark when making containers and structures, so it is an important characteristic. In the hot-rolled steel of the present invention, the desired tensile strength is achieved by composite grain refinement strengthening and dislocation strengthening. That is, a controlled rolling technology is applied with the goal of adjusting the chemical composition for grain refinement, controlling the grain size in the recrystallization temperature range, and introducing strain in the non-recrystallization temperature range, and then by adjusting the chemical composition (Md30) related to the processing-induced phase transformation, the processing-induced phase amount is appropriately controlled to obtain the desired tensile strength. When the N content is 0.010% or more and less than 0.100%, the tensile strength is 600MPa or more and 780MPa or less. The tensile strength can be set to 750MPa or less. When the N content is 0.100% or more and less than 0.250%, the tensile strength is 690MPa or more and 880MPa or less. The tensile strength can be set to 850MPa or less.

[0101] (elongation)

[0102] In order to ensure the processability of hot-rolled steel, the elongation is set to 30% or more. Generally, the elongation decreases with increasing strength, and if the tensile strength is 690 MPa or more, it is difficult to ensure the elongation. In the hot-rolled steel of the present invention, processing-induced phase transformation is effectively utilized to economically achieve both high elongation and high strength with a small amount of alloy addition. When the nitrogen content is 0.010% or more and less than 0.100%, the tensile strength is 600 MPa or more and 780 MPa or less, and the elongation can be set to 35% or more.

[0103] (Pitting Potential)

[0104] The hot-rolled steel material of the present invention can achieve a pitting potential of 0.25 V or higher. When the nitrogen content is set to 0.100% or higher and 0.250% or lower, corrosion resistance can be further improved, enabling the pitting potential to be set to 0.35 V or higher. The pitting potential can be measured in accordance with JIS G0577:2014. The potential is based on Ag / AgCl.

[0105] Next, the method for producing the hot-rolled steel material of the present invention will be described.

[0106] (Slab heating temperature: T1 (°C) or lower obtained from the following formula 2 and 1050°C or higher)

[0107] The concentration product of Ti and N and the heating temperature of the slab are explained. In the hot-rolled steel of the present invention, fine TiN is dispersed in the steel during the hot rolling heating process, thereby finely controlling the grain size of the hot-rolled steel. The above-mentioned log ([Ti] × [N]) is an important indicator for grasping the precipitation amount and precipitation temperature range of TiN in the hot rolling heating process and controlling the grain size in this process. The present inventors investigated the solid solubility product of austenitic stainless steel with changed Ti and N contents relative to TiN precipitation and the grain refining ability based on heating experiments. As a result, it was found that by heating the slab at a temperature T below the temperature T1 shown in the following formula 2, the grain refining effect brought about by TiN can be exerted. The lower the heating temperature T, the more effective it is for grain refining, but if it is too low, the load during hot rolling increases, so from the perspective of making hot rolling easier, the lower limit is set to 1050°C.

[0108] T1(℃)=13500 / (5.6-log([Ti]×[N]))-273 (Formula 2)

[0109] (log(t0 / ts)≥0.3)

[0110] In the method for manufacturing a hot-rolled steel material of the present invention, the logarithm of the ratio of the slab thickness t0 to the thickness ts after rolling in the recrystallization region, log(t0 / ts), is set to be greater than 0.3. Log(t0 / ts) is an indicator of the total amount of strain introduced into the steel material in the recrystallization temperature region of hot rolling, and is specified in order to increase the grain size number of the hot-rolled steel material of the present invention to greater than 7. That is, the grain size number of the hot-rolled steel material of the present invention is dominated by the content of grain refining elements and hot rolling in a temperature region above the Tc temperature. If the value of log(t0 / ts) is less than 0.3, even if the method of Formula 2 is used to reduce the heating temperature of hot rolling according to the product of the Ti and N contents and finely control the grain size before hot rolling, it is difficult to increase the grain size number of the hot-rolled steel material of the present invention to greater than 7.

[0111] (The cumulative reduction rate in the temperature range of Tc (°C) or lower and 800°C or higher is 10% or more and 30% or less)

[0112] In the method for producing a hot-rolled steel material of the present invention, during hot rolling, the cumulative reduction ratio in the temperature range of Tc (°C) or lower and 800°C or higher obtained from the following formula 3 is 10% to 30%.

[0113] Tc(℃)=1030+100[N]+30[Mo] (Formula 3)

[0114] Tc is an indicator of the recrystallization temperature of the hot-rolled steel of the present invention. As shown in Formula 3, it becomes a high temperature of 1030°C or higher depending on the N and Mo contents. Let t be the thickness (mm) of the hot-rolled steel of the present invention, let ts be the thickness after rolling in the recrystallization region described in the previous item, and when the reduction rate at a temperature below Tc is expressed in units of %, it is expressed as 100(1-t / ts). In addition, controlled rolling in this temperature range can also be referred to as rolling in the non-recrystallization temperature range. If the controlled rolling temperature in the non-recrystallization temperature range is reduced to below 800°C, the steel will be hardened and the rolling efficiency will be reduced. Furthermore, if the reduction rate exceeds 30%, the microstrain introduced into the steel will increase, and the elongation will be significantly reduced. In addition, it will lead to a reduction in the properties of the weld joint. On the other hand, if the reduction rate is less than 10%, the reduction rate of controlled rolling is insufficient, the microstrain introduced into the steel will be small, and it will be difficult to achieve an increase in room temperature tensile strength. Therefore, the temperature range of the controlled rolling is set to below Tc and above 800°C, and the reduction rate is set to above 10% and below 30%. Here, the controlled rolling temperature is defined based on the average temperature of the steel based on the steel temperature measurement value at the entry side of each rolling pass. The lower limit of the temperature range of the controlled rolling is preferably above 850°C, and more preferably above 900°C. In hot rolling of thick materials with a thickness of 30mm or more during controlled rolling, the temperature difference between the surface temperature of the steel and the center of the steel has to be large. In this case, controlled rolling is performed by grasping the difference between the average cross-section temperature calculated by heat transfer calculation and the surface temperature of the steel.

[0115] Example

[0116] Examples are described below.

[0117] Table 1 shows the chemical composition of the test steels. These steels are obtained by hot rolling laboratory melt materials, or by cutting out a part of the slab of the actual manufacturing material and hot rolling. It should be noted that the remainder of the components recorded in Table 1 is Fe, including unavoidable impurity elements. In addition, with respect to the components shown in Table 1, the part without the content recorded represents the impurity level, REM represents lanthanide rare earth elements, and the content represents the total of these elements. On the right side of Table 1 are shown log([Ti]×[N]) calculated from the chemical composition of Ti and N, and Md30, T1, and Tc calculated by Formulas 1 to 3.

[0118]

[0119] Table 2 shows examples of hot-rolled steel products according to the present invention and comparative examples. Materials melted in the laboratory were hot forged and machined to produce rolling materials 60 mm thick, 110 mm wide, and 150 mm long. Similar rolling materials with thicknesses of 60 to 120 mm were produced by direct casting or hot rolling from slabs of actual production materials. These materials were heated at a heating temperature T of 1080 to 1250°C for 30 to 120 minutes. Hot rolling was initiated from a thickness of t0, with the hot pressing end temperature Tf set at 880 to 980°C and the accelerated cooling start temperature set at 700 to 850°C, to produce hot-rolled steel sheets with thicknesses t of 6 to 45 mm. This hot rolling process was divided into rolling in a temperature range higher than Tc (T to Tc) and rolling in a temperature range below Tc (T to Tf). The thickness of the rolled material at the time of this division was controlled as ts. The log(t0 / ts), corresponding to the magnitude of the reduction strain in the temperature range higher than Tc, is shown in the table. The specific conditions are shown in Table 2.

[0120] The seven items following GS No. in Table 2 represent the characteristics of the steel material.

[0121] GS No. After the microstructure of the microscopic specimen (L cross section) in the rolling direction was visualized with an etching solution, the average slice length was determined using the slice method, and the value converted into the grain size number is shown.

[0122] Microstrain was measured using X-ray measurement. X-ray specimens (Sample A) measuring 2 mm thick, 20 mm wide, and 20 mm long were prepared at 1 / 4 and 1 / 2 of the steel plate's thickness. The strain on the measurement surface was removed by electrolytic polishing, and CuKα radiation was used to measure reflections at 111, 200, 220, 311, 222, and 331 of the austenite phase, and the full width at half maximum (FWHM) was measured. Furthermore, solution heat-treated materials were prepared from these steel plates, which were solution-heat-treated at 1050°C for 5 minutes. Similarly, X-ray specimens (Sample B) were collected at 1 / 4 and 1 / 2 of the plate's thickness, and the full width at half maximum (FWHM) was measured. The solution heat-treated material (Sample B) was used as a reference material with zero microstrain, and the microstrain of the steel material (Sample A) being measured was calculated. The microstrains in Table 2 represent the values ​​calculated from the average values ​​at the 1 / 4 and 1 / 2 plate thickness sections.

[0123] The mechanical properties were evaluated as follows.

[0124] A tensile test at room temperature was carried out on steel plates with a thickness of 12 to 40 mm (n=2). The test pieces were all collected from the direction perpendicular to the rolling. The tensile test pieces were made under the conditions of 8mmφ×50mm in the parallel part and 40mm in the distance between the evaluation points. For materials with a thickness of less than 20mm, they were collected from the center of the plate thickness, and for materials with a thickness of more than 25mm, they were collected from the 1 / 4 plate thickness part. For steel plates with a thickness of 6mm, the tensile test at room temperature was a plate-shaped test piece of the total thickness. The tensile test was carried out using the tensile test method for metal materials (according to JIS Z2241:2022), and the impact test was carried out in the same way. As for the martensite amount, the ferrite phase amount was measured on the fracture part of the tensile test piece using a ferrite meter made by Fischer, which can measure the amount of ferrite phase by magnetism, and the amount of martensite was converted based on the ferrite amount and the conversion formula to the martensite amount. In this manner, the tensile strength TS, the elongation EL, and the amount of martensite were determined, and the values ​​are shown in Table 2.

[0125] In addition, an impact test at 20°C (n=3) was carried out according to the Charpy impact test method for metal materials (JIS Z2242:2018). For materials with a plate thickness of 12 mm or more and 20 mm or less, a JIS No. 4 full-size test piece was collected from the center of the plate thickness. For materials with a plate thickness of 25 mm or more, a JIS No. 4 full-size test piece was collected from the 1 / 4 plate thickness portion. For steel plates with a plate thickness of 6 mm, a small-size test piece with a plate thickness of 5 mm was made. Regarding the absorbed energy (J) obtained by the test, in the case of full size, vE 20 If the value is 200J or more, it is set as "○", and if it is less than this value, it is set as "×". In the case of small size, vE 20 The values ​​of 100 J or more were marked as "○", and those less than this value were marked as "×", which are shown in Table 2.

[0126] In addition, according to the Charpy impact test method for metal materials (JIS Z2242: 2018), an impact test at -196°C was carried out in the same manner as in the case of 20°C (n=3). The absorbed energy (J) obtained by the test was calculated as vE in the case of full size. -196 140J or more is set as "○", and less than this value is set as "△". In the case of small size, 70J or more is set as "○", and 25J or more and less than 70J is set as "△", which is shown in Table 2.

[0127] Corrosion resistance is evaluated by pitting potential. The pitting potential is measured according to JIS G0577:2014 by the following method. After cutting a 15mm×20mm test piece from the steel plate, it is wet-polished with #600. Then, the electrode surface (exposed part) of the test piece is 10mm×10mm, and the portion other than the electrode surface is insulated and covered with a silicone resin to obtain a test piece for pitting potential measurement. Next, the test piece for pitting potential measurement is immersed in a 3.5% NaCl solution at 30°C that has been fully degassed with Ar, and electric anodic polarization is performed at 20mV / min from the natural potential to measure the pitting potential. The pitting potential is set to a current of 100μA / cm 2 The potential during flow was shown in Table 2, with a potential of 0.35 V or higher being designated as "○" and a potential of 0.25 V or higher and less than 0.35 V being designated as "△" based on Ag / AgCl.

[0128] Evaluation of weld properties was performed as follows.

[0129] The width of the 120mm wide × 150mm long test steel plate was divided into two in the middle and a 60-degree V-groove was processed. FCAW welding with a welding line energy of 30 to 40 kJ / cm was performed using 309MoL welding materials. In the case of a plate thickness of 12mm, a weld joint can be produced in 2 to 3 passes. In this way, a macro corrosion specimen for cross-sectional hardness measurement perpendicular to the welding direction was processed. The hardness was measured with a 10kg Vickers hardness load at a 1mm interval from the melting boundary on both sides 2mm below the epidermis on the surface weld side to 15 to 20mm on the parent material side. Among them, the difference between the minimum hardness near the melting boundary and the average hardness of the parent material part was calculated. The case where the difference is less than 20 is evaluated as "○", and the case where it exceeds 20 is evaluated as "×", and is shown in Table 2.

[0130]

[0131] The embodiments shown in Tables 1 and 2 above indicate that the hot-rolled steel of the present invention has high tensile strength and an elongation of more than 30% through solid solution strengthening, grain refinement strengthening, dislocation strengthening, and phase transformation strengthening, and is a steel with little softening of the heat-affected zone of the weld.

[0132] The above examples clearly demonstrate that the present invention can provide a hot-rolled austenitic stainless steel material for containers that exhibits high strength, excellent workability, and excellent weld properties. Furthermore, it was confirmed that by setting the nitrogen content to 0.010% or more and less than 0.100% and Md30 to -30 to +40, excellent low-temperature toughness can be achieved. Furthermore, it was confirmed that by setting the nitrogen content to 0.100% to 0.250% and Md30 to -40 to +30, excellent corrosion resistance can be achieved.

[0133] Industrial applicability

[0134] The austenitic stainless steel hot-rolled steel obtained by the present invention has high tensile strength, good elongation and weld properties. Therefore, when used as a container, it can be economically designed and manufactured with reduced wall thickness compared to conventional solution heat-treated materials.

Claims

1. A hot-rolled austenitic stainless steel material, characterized in that: It has the following chemical composition: contains by mass% C: 0.060% or less, Si: 1.50% or less, Mn: 0.10~3.00%, P: 0.045% or less, S: 0.030% or less, Cr：16.00～20.00％、 Ni: 6.00~10.00%, O：0.0001～0.008％、 N: 0.010% or more and less than 0.100%, V：0.01～0.30％、 Ti: 0.001~0.015%, Nb: 0.001~0.060%, Co: 0-0.30%, Cu: 0-2.00%, Mo: 0-2.00%, W：0～1.00％、 Al：0～0.100％、 B：0～0.0040％、 Sn: 0~0.050%, Ca: 0~0.0050%, Mg: 0~0.0030%, REM: 0~0.10% Zr：0～0.030％、 Hf: 0~0.080%, Ta: 0~0.100%, The rest is Fe and inevitable impurities. log([Ti]×[N]) is -3.7 or greater, and Md30 expressed by the following formula 1 is in the range of -30 to +40. The microstrain measured by X-ray diffraction is 0.0005 or more and 0.0025 or less, The crystal grain size number is 7 or above. The tensile strength at room temperature is 600 MPa or more and 780 MPa or less. The elongation is more than 35%, Md30=413-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-9.5([Ni]+[Cu])-18.5[Mo] (Formula 1) wherein [X] is the value representing the content of element X in mass %.

2. A hot-rolled austenitic stainless steel material, characterized in that: It has the following chemical composition: contains by mass% C: 0.060% or less, Si: 1.50% or less, Mn: 0.10~3.00%, P: 0.045% or less, S: 0.030% or less, Cr：16.00～20.00％、 Ni: 6.00~10.00%, O：0.0001～0.008％、 N：0.100～0.250％、 V：0.01～0.30％、 Ti: 0.001~0.015%, Nb: 0.001~0.060%, Co: 0-0.30%, Cu: 0-2.00%, Mo: 0-2.00%, W：0～1.00％、 Al：0～0.100％、 B：0～0.0040％、 Sn: 0~0.050%, Ca: 0~0.0050%, Mg: 0~0.0030%, REM: 0~0.10% Zr：0～0.030％、 Hf: 0~0.080%, Ta: 0~0.100%, The rest is Fe and inevitable impurities. log([Ti]×[N]) is -3.7 or greater, and Md30 expressed by the following formula 1 is in the range of -40 to +30. The microstrain measured by X-ray diffraction is 0.0005 or more and 0.0025 or less, The crystal grain size number is 7 or above. The tensile strength at room temperature is 690 MPa or more and 880 MPa or less. The elongation is more than 30%, The pitting potential at 30°C is above 0.35V. Md30=413-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-9.5([Ni]+[Cu])-18.5[Mo] (Formula 1) wherein [X] is the value representing the content of element X in mass %.

3. A method for producing a hot-rolled steel material, which is a method for producing the hot-rolled steel material of the austenitic stainless steel according to claim 1 or 2, characterized in that: The manufacturing method comprises: The slab is heated at a temperature T of 1050°C or higher and not higher than T1 (°C) obtained by the following formula 2. The following controlled rolling is performed: the thickness t0 (mm) of the slab and the thickness ts (mm) after rolling in the temperature range above Tc (°C) obtained by the following formula 3 satisfy log (t0 / ts) ≥ 0.3, and the cumulative reduction rate in the temperature range below Tc (°C) and above 800°C is 10% to 30%. T1(℃)=13500 / (5.6-log([Ti]×[N]))-273 (Formula 2) Tc(℃)=1030+100[N]+30[Mo] (Formula 3) Here, [X] is a value expressing the content of element X in mass %.

Images