Steel materials

Abstract

To provide a steel having corrosion resistance in an acid corrosive environment, and also excellent hot workability.SOLUTION: A steel has a chemical composition consisting of, in mass%, C: 0.0010-0.20%, Si: 0.10-0.80%, Mn: 0.50-1.00%, Cu: 0.10-0.50%, Ni: 0.010-0.80%, Cr: 0.005-0.070%, Co: 0.002-0.020%, Mo and / or W: 0.001-0.30% in total, Al: 0.01-0.10%, Ti: 0.001-0.067%, Ca: 0% (exclusive) to 0.010% or less, N: 0.0005-0.0050%, P: 0.050%, S: 0.00050-0.0200%, O: 0.0010-0.0045%, the balance Fe and impurities, with FI: 0.08-1.5. The number density of MnS with a maximum length of 2.0 μm or more included in the steel is less than 50 / mm2.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a steel material. [Background technology]

[0002] Boiler furnaces and incinerators at waste incineration facilities generate exhaust gases that contain water vapor, sulfur oxides, hydrogen chloride, etc. When this exhaust gas is cooled in the exhaust gas chimney, etc., it condenses into sulfuric acid and hydrochloric acid, which causes severe corrosion of the steel materials that make up the exhaust gas passage, a phenomenon known as sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion.

[0003] To address these problems, sulfuric acid / hydrochloric acid dew-point corrosion resistant steels and highly corrosion resistant stainless steels have been proposed. For example, Patent Documents 1 to 8 propose steel materials with excellent sulfuric acid dew-point corrosion resistance to which Cu, Sb, Co, Cr, etc. are added. Patent Document 9 proposes highly corrosion resistant stainless steel to which Cr, Ni, etc. are added. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2001-164335 A [Patent Document 2] JP 2003-213367 A [Patent Document 3] JP 2007-239094 A [Patent Document 4] JP 2012-57221 A [Patent Document 5] International Publication No. 2018 / 038195 [Patent Document 6] International Publication No. 2018 / 038196 [Patent Document 7] International Publication No. 2018 / 038197 [Patent Document 8] International Publication No. 2018 / 038198 [Patent Document 9] Japanese Patent Application Publication No. 7-316745 Summary of the Invention [Problem to be solved by the invention]

[0005] Steels containing Cu, Sb, Cr, etc. have excellent corrosion resistance in corrosive sulfuric acid environments such as exhaust gas stacks. However, further improvements in corrosion resistance are needed to extend the service life of boilers and incineration facilities.

[0006] In addition to exhaust gas chimneys, steel materials used in incinerator flues for gasification melting furnaces, heat exchangers, gas-gas heaters, desulfurization equipment, electric dust collectors, etc., are required to have not only corrosion resistance but also hot workability from the standpoint of workability and productivity.

[0007] An object of the present invention is to solve the above problems and to provide a steel material having excellent corrosion resistance in a sulfuric acid corrosive environment and a hydrochloric acid corrosive environment and excellent hot workability. [Means for solving the problem]

[0008] The present invention has been made to solve the above problems, and the gist of the present invention is the following steel material.

[0009] (1) Chemical composition, in mass%, C: 0.0010~0.20%, Si: 0.10 to 0.80%, Mn: 0.50-1.00%, Cu: 0.10-0.50%, Ni: 0.010~0.80%, Cr: 0.005~0.070%, Co: 0.002 to 0.020%, Mo and / or W: 0.001 to 0.30% in total, Al: 0.01 to 0.10%, Ti: 0.001 to 0.067%, Ca: more than 0% and not more than 0.010% N: 0.0005~0.0050%, P: 0.050% or less, S: 0.00050~0.0200%, O: 0.0010~0.0045%, The balance is Fe and impurities. The FI defined by the following formula (i) is 0.12 to 1.50, The number density of MnS with a maximum length of 2.0 μm or more contained in the steel material is 50 / mm 2 is less than Steel material. FI=1000×((Cr / 52)+(Ti / 48)) ···(i) In the above formula, the element symbols represent the contents (mass%) of the respective elements contained in the steel material.

[0010] (2) The chemical composition contains, in mass%, a part of the Fe replaced by B: 0.10% or less, Nb: 0.10% or less, and V: 0.10% or less, It contains at least one of the following: The steel material described in (1) above.

[0011] (3) The chemical composition contains, in mass%, a part of the Fe replaced by Sn: 0.30% or less, and Sb: 0.30% or less, It contains one or more selected from The steel material according to (1) or (2) above.

[0012] (4) The mass ratio Al / O of the Al content to the O content is 3.0 to 60.0; The steel material according to (1) or (2) above. Effect of the Invention

[0013] According to the present invention, it is possible to provide a steel material having good corrosion resistance in an acid corrosion environment and excellent hot workability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In order to achieve the above object, the present inventors have conducted detailed investigations into the corrosion resistance and hot workability of steel materials, and have come to the following findings.

[0015] It was found that adding Cr and Ti simultaneously improves corrosion resistance in high-temperature, high-concentration acid environments more than adding Cr and Ti alone, but adding too much of them increases the amount of oxides and nitrides that are the starting points for corrosion, deteriorating corrosion resistance. Therefore, it was found that the Cr content and Ti content must be within a certain range. Therefore, a more detailed study was conducted on the relationship between the Cr and Ti contents, and the relationship equation for FI, defined by the following equation (i), was derived. It was then discovered that excellent corrosion resistance can be obtained by setting FI within an appropriate range. FI=1000×((Cr / 52)+(Ti / 48)) ···(i)

[0016] Mn is an essential element for ensuring the strength of steel, but on the other hand, it forms MnS, which deteriorates the corrosion resistance in an acid corrosion environment. In the present invention, S has the effect of improving the corrosion resistance when contained together with Cu, so that an extreme reduction in the amount of S is not preferable.

[0017] As a result of further investigations conducted by the inventors to solve this problem, they discovered that the inclusion of Ca is essential, and that part or all of the S contained in the steel can be fixed as CaS, while the MnS can be refined and bonded with oxygen to form MnS oxide, rendering it harmless.

[0018] The present invention has been made based on the above findings. Each of the features of the present invention will be described in detail below.

[0019] (A) Chemical composition The reasons for limiting the content of each element are as follows. In the following description, "%" for the content means "mass %."

[0020] C: 0.0010~0.20% C is an element that improves the strength of steel. However, if C is contained in excess, carbides increase and corrosion resistance deteriorates. Therefore, the C content is set to 0.0010 to 0.20%. The C content is preferably 0.0050% or more, and more preferably 0.010% or more. In addition, the C content is preferably 0.15% or less, and more preferably 0.10% or less.

[0021] Silicon: 0.10 to 0.80% Silicon is an element that contributes to improving deoxidation and strength, and controls the form of oxides. However, if silicon is contained in excess, the amount of oxides increases, and corrosion resistance is impaired. Therefore, the silicon content is set to 0.10 to 0.80%. The silicon content is preferably 0.20% or more, and more preferably 0.30% or more. The silicon content is preferably 0.75% or less.

[0022] Mn: 0.50-1.00% Mn is an element that improves strength. However, if Mn is contained in excess, coarse MnS is generated, and corrosion resistance and mechanical properties deteriorate. Therefore, the Mn content is set to 0.50 to 1.00%. The Mn content is preferably 0.60% or more, and more preferably 0.70% or more. In addition, the Mn content is preferably 0.95% or less, and more preferably 0.90% or less.

[0023] Cu: 0.10-0.50% Cu is an element that exhibits significant corrosion resistance against sulfuric acid and hydrochloric acid. However, if Cu is contained in excess, hot workability decreases, impairing productivity. Therefore, the Cu content is set to 0.10 to 0.50%. The Cu content is preferably 0.20% or more, 0.25% or more, or 0.30% or more. The Cu content is preferably 0.45% or less, and more preferably 0.40% or less.

[0024] Ni: 0.010~0.80% Ni is an element that improves corrosion resistance in an acid corrosion environment, and also has the effect of improving manufacturability in steel containing Cu. Cu has a large effect of improving corrosion resistance, but it is easily segregated, and if it is contained alone, it may promote cracking after casting. On the other hand, Ni has the effect of reducing the surface segregation of Cu. By containing Ni, in addition to suppressing Cu segregation and cast cracking, the occurrence of local corrosion due to segregation is also suppressed, so that the effect of improving corrosion resistance is obtained. However, Ni is an expensive element, and the inclusion of a large amount leads to an increase in steelmaking costs. Therefore, the Ni content is set to 0.010 to 0.80%. The Ni content is preferably 0.050% or more, more preferably 0.100% or more, and even more preferably 0.150% or more. Moreover, the Ni content is preferably 0.70% or less, and more preferably 0.60% or less.

[0025] Cr: 0.005~0.070% Cr is an element that has the effect of improving the strength by increasing the hardenability and improving the sulfuric acid resistance. However, when Cr is contained in excess, it forms oxides that tend to become the starting point of corrosion on the surface of the steel material. It also reduces the hydrochloric acid resistance. Therefore, the Cr content is set to 0.005 to 0.070%. The Cr content is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. Moreover, the Cr content is preferably 0.060% or less, and more preferably 0.050% or less.

[0026] Co: 0.002 to 0.020% Co is an element that has the effect of improving acid resistance. In particular, when it is contained simultaneously with Cu, it exhibits excellent corrosion resistance in an acidic environment. However, if an excessive amount of Co is contained, the economic efficiency decreases. Therefore, the Co content is set to 0.002 to 0.020%. The Co content is preferably 0.005% or more, more preferably 0.010% or more, and further preferably 0.013% or more. Moreover, the Co content is preferably 0.017% or less, and more preferably 0.015% or less.

[0027] Mo and / or W: 0.001 to 0.30% in total Mo and W are elements that improve corrosion resistance in an acid corrosion environment when contained simultaneously with Cu. However, since Mo and W are expensive elements, excessive inclusion leads to a decrease in economic efficiency. Therefore, the total content of Mo and W is set to 0.001 to 0.30%. Mo and W may be contained alone or both may be contained simultaneously. The total content of Mo and W is preferably 0.005% or more, more preferably 0.010% or more. Moreover, the total content of Mo and W is preferably 0.25% or less, more preferably 0.20% or less.

[0028] Al: 0.01 to 0.10% Al is added as a deoxidizer. However, if an excessive amount of Al is contained, the corrosion resistance is impaired due to an increase in inclusions. Therefore, the Al content is set to 0.01 to 0.10%. The Al content is preferably 0.02% or more. Moreover, the Al content is preferably 0.05% or less.

[0029] Ti: 0.001 to 0.067% Ti is an element that improves corrosion resistance. In particular, by simultaneously containing Ti with Cu and Co, excellent corrosion resistance is exhibited in an acidic environment. However, if Ti is contained in excess, the corrosion resistance is impaired due to an increase in nitrides that cause corrosion. Therefore, the Ti content is set to 0.001 to 0.067%. The Ti content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more. Moreover, the Ti content is preferably 0.050% or less, and more preferably 0.040% or less.

[0030] Ca: More than 0% and less than 0.010% Ca is an element mainly used to control the morphology of sulfides, and also has the effect of forming fine oxides. However, if Ca is contained in excess, mechanical properties may be impaired. Therefore, the Ca content is set to more than 0% and not more than 0.010%. The Ca content is preferably 0.001% or more, 0.002% or more, or 0.003% or more, more preferably 0.004% or more, and even more preferably 0.005% or more. In addition, the Ca content is preferably 0.009% or less.

[0031] N: 0.0005~0.0050% N forms fine nitrides and contributes to improving the mechanical properties of steel materials. However, if N is contained in excess, it reduces the mechanical properties and productivity of steel materials. Therefore, the N content is set to 0.0005 to 0.0050%. The N content is preferably 0.0010% or more, and more preferably 0.0020% or more. In addition, the N content is preferably 0.0040% or less, and more preferably 0.0030% or less.

[0032] P:0.050% or less P is an impurity that reduces the mechanical properties and productivity of steel materials. Therefore, the upper limit of the P content is set to 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. It is preferable to reduce the P content as much as possible, that is, the content may be 0%, but an extreme reduction leads to an increase in steelmaking costs. Therefore, the P content may be 0.001% or more.

[0033] S: 0.00050~0.0200% S is generally an impurity that reduces the mechanical properties and productivity of steel materials. However, in the present invention, S has the effect of improving corrosion resistance in an acid corrosion environment when contained simultaneously with Cu. Therefore, the S content is set to 0.00050 to 0.0200%. The S content is preferably 0.00100% or more, 0.00500% or more, or 0.01000% or more. In addition, the S content is preferably 0.0180% or less, and more preferably 0.0150% or less.

[0034] O: 0.0010 to 0.0045% O is an element that has the effect of detoxifying MnS by bonding with MnS and preventing deterioration of corrosion resistance and mechanical properties. However, when O is contained in excess, it generates coarse oxides that become the starting point of corrosion in an acid corrosion environment. Therefore, the O content is set to 0.0010 to 0.0045%. The O content is preferably 0.0020% or more, and more preferably 0.0030% or more. In addition, the O content is preferably 0.0040% or less, and more preferably 0.0035% or less.

[0035] In the chemical composition of the steel of the present invention, in addition to the above elements, at least one of B, Nb and V may be further contained within the ranges shown below in order to improve mechanical properties, etc. Note that these elements are not necessarily essential for steel materials, so the lower limit of the content is 0%. The reasons for limiting each element will be explained below.

[0036] B: 0.10% or less Since B is an element that improves hardenability and increases strength, it may be contained as necessary. However, even if B is contained in excess, the effect is saturated and the toughness of the base material and HAZ may decrease. Therefore, the B content is set to 0.10% or less. The B content is preferably 0.05% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. In order to obtain the above effects, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more.

[0037] Nb: 0.10% or less Nb, like Ti, is an element that forms nitrides and contributes to the refinement of crystal grains and the improvement of strength, so it may be contained as necessary. However, if Nb is contained in excess, the nitrides become coarse and the mechanical properties deteriorate. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. In order to obtain the above effects more reliably, the Nb content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0038] V: 0.10% or less V, like Ti and Nb, is an element that forms nitrides and contributes to the refinement of crystal grains and the improvement of strength, so it may be contained as necessary. However, if V is contained in excess, the nitrides become coarse and the mechanical properties deteriorate. Therefore, the V content is set to 0.10% or less. The V content is preferably 0.070% or less, more preferably 0.050% or less, and even more preferably 0.020% or less. In order to obtain the above effects more reliably, the V content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0039] In the chemical composition of the steel of the present invention, in addition to the above elements, one or more elements selected from Sn and Sb may be contained within the ranges shown below in order to improve corrosion resistance. Note that these elements are not necessarily essential in steel materials, so the lower limit of the content is 0%. The reasons for limiting each element will be explained.

[0040] Sn: 0.30% or less Sn is an element that improves corrosion resistance in an acid corrosion environment when contained simultaneously with Cu, so it may be contained as necessary. However, if Sn is contained in excess, hot workability decreases. Therefore, the Sn content is set to 0.30% or less. The Sn content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less. In order to obtain the above effects more reliably, the Sn content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0041] Sb: 0.30% or less Sb is an element that improves corrosion resistance against sulfuric acid and hydrochloric acid when contained simultaneously with Cu, so it may be contained as necessary. However, if Sb is contained in excess, hot workability decreases and productivity is impaired. Therefore, the Sb content is set to 0.30% or less. The Sb content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less. In order to obtain the above effects more reliably, the Sb content is preferably 0.01% or more, and more preferably 0.02% or more.

[0042] In the chemical composition of the steel material of the present invention, the balance is Fe and impurities. Here, the impurities refer to components that are mixed in due to raw materials such as ores and scraps or other factors during industrial production of the steel material, and are permissible within a range that does not adversely affect the steel material of the present invention.

[0043] FI: 0.12~1.50 FI is an index derived to suppress the formation of oxides and nitrides that are the starting points of corrosion when Cr and Ti are contained in the steel, and in order to obtain excellent corrosion resistance, FI is set to 0.12 to 1.50. FI is preferably 0.20 or more, more preferably 0.30 or more, and even more preferably 0.50 or more. FI is preferably 1.40 or less, more preferably 1.30 or less, and even more preferably 1.20 or less.

[0044] FI is composed of the sum of the number of Cr atoms and the number of Ti atoms, as defined by the following formula (i): Cr / 52 and Ti / 48 are terms obtained by dividing the contents of Cr and Ti by the mass numbers of the respective elements. FI=1000×((Cr / 52)+(Ti / 48)) ···(i)

[0045] Al / O: 3.0 to 60.0 As described above, Al and O affect the corrosion resistance of steel materials. When the O content is excessive, the Al / O ratio becomes small, and when the Al content is excessive, the Al / O ratio becomes large, and in either case, oxides are generated and tend to become corrosion starting points. Therefore, Al / O is preferably 3.0 to 60.0. Al / O is more preferably 5.0 or more, and even more preferably 10.0 or more. Moreover, Al / O is more preferably 50.0 or less, and even more preferably 40.0 or less.

[0046] (B) Inclusions In the steel material according to the present invention, the number density of MnS having a maximum length of 2.0 μm or more contained in the steel material is 50 / mm 2 is less than.

[0047] Incidentally, since MnS having a maximum length of less than 2.0 μm has almost no effect on the corrosion resistance of steel materials, the present invention targets inclusions having a maximum length of 2.0 μm or more.

[0048] MnS is a starting point for corrosion and deteriorates corrosion resistance in an acid corrosion environment. On the other hand, extreme reduction in the Mn and S contents is not preferable from the viewpoint of improving the strength and corrosion resistance of the steel material of the present invention, so it is necessary to achieve both.

[0049] Therefore, as described above, in the steel material of the present invention, by containing Ca, part or all of the S contained in the steel material is fixed as CaS. In addition, MnS is rendered harmless by bonding with oxygen to form MnS oxide. When it becomes MnS oxide, it is rendered harmless and is less likely to become a starting point of corrosion.

[0050] As a result, the number density of MnS particles with a maximum length of 2.0 μm or more contained in the steel material is reduced to 50 / mm 2 In the following description, MnS with a maximum length of 2.0 μm or more is simply called MnS, and MnS oxide with a maximum length of 2.0 μm or more is simply called MnS oxide. The number density of MnS is 45 / mm 2 Preferably, it is 40 / mm or less. 2 More preferably, it is:

[0051] In order to sufficiently render MnS harmless, it is preferable that the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more, preferably 0.12 or more, more preferably 0.15 or more.

[0052] The number density of MnS and the number density of MnS oxide are measured by energy dispersive X-ray analysis (EDS) equipped in a scanning electron microscope (SEM). The measurement magnification is 1000 times, and the maximum length of MnS and MnS oxide detected in the field of view is measured. The number of inclusions with a maximum length of 2.0 μm or more is then counted and divided by the area of ​​the field of view to obtain the number density.

[0053] Inclusions are identified by EDS. Inclusions with a total Mn and S content of 90% by mass or more are determined to be MnS. Furthermore, inclusions in which an O peak is detected, the O content is 18% by mass or more, and the total Mn, S, and O content is 90% by mass or more are determined to be MnS oxides.

[0054] (C) Manufacturing method A method for manufacturing a steel material according to an embodiment of the present invention will be described. The steel material according to this embodiment includes steel plates, shaped steel, steel pipes, etc., which are manufactured by hot rolling and, if necessary, cold rolling. The steel plate is preferably a thick steel plate having a plate thickness of 3 mm or more, more preferably 6 mm or more.

[0055] The steel material according to the present embodiment is manufactured by melting steel in a conventional manner, adjusting the components, and then hot-rolling the resulting steel billet, and further performing cold rolling as necessary. In order to set the ratio of the number density of MnS and MnS oxide present in the steel material to the above-mentioned range, it is important to set the heating temperature before hot rolling to a relatively low temperature, and specifically, it is preferable to set it to 1000 to 1130°C. In addition, as long as the temperature range is 1000 to 1130°C, MnS does not coarsen even if it is held for a long time, so the holding time is not particularly limited, but in the conventional method, the holding time is generally 5 to 20 minutes.

[0056] By lowering the heating temperature before hot rolling, it is possible to suppress the growth of MnS and to refine it during rolling. The refined MnS has a relatively large surface area, so it is more likely to combine with oxygen and become MnS oxide. The number density of MnS is 45 / mm 2 In order to make the ratio of the number density of MnS oxides to MnS 0.12 or more, it is more preferable that the heating temperature before hot rolling is 1080° C. or less.

[0057] On the other hand, by setting the heating temperature before hot rolling to 1000° C. or higher, the burden on the rolling machine can be reduced. Therefore, the heating temperature before hot rolling is preferably set to 1000° C. or higher.

[0058] After hot rolling, the hot-rolled steel sheet is subjected to the next process, such as coiling. During this process, the temperature of the steel sheet drops, but it is desirable that the time from the completion of hot rolling until the temperature reaches 400°C is 4 hours or more. Exposure to this temperature range promotes the bonding of MnS and oxygen. After hot rolling, the steel sheet may be cold-rolled to obtain a cold-rolled steel sheet. Furthermore, heat treatment may be performed after cold rolling.

[0059] When producing a steel pipe from the obtained steel plate, the steel plate may be formed into a tubular shape and welded, and for example, a UO steel pipe, an electric resistance welded steel pipe, a forge welded steel pipe, a spiral steel pipe, etc. may be produced.

[0060] The present invention will be described in more detail below with reference to examples. Note that the conditions in the examples shown below are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. In addition, various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention. EXAMPLES

[0061] Steels (A1 to A14, B1 to B11) having the chemical compositions shown in Table 1 were melted and the steel ingots were hot rolled under the conditions shown in Table 2 to produce hot-rolled steel sheets with a thickness of 20 mm. The heating hold time was 15 minutes. After hot rolling, some of the steel sheets were cooled to simulate coiling, and then further cold-rolled to produce cold-rolled steel sheets with a thickness of 13 mm.

[0062] [Table 1]

[0063] [Table 2]

[0064] Test pieces for SEM observation were cut out from each steel sheet obtained, and the number density of inclusions was measured using the EDS equipped in the SEM. The measurement magnification was 1000 times, and the maximum length of MnS and MnS oxides detected in the field of view was measured, and the number of inclusions with a maximum length of 2.0 μm or more was counted and divided by the field area to obtain the number density.

[0065] Furthermore, the obtained steel sheets were subjected to various performance evaluation tests as described below.

[0066] <Sulfuric acid resistance, hydrochloric acid resistance> A test piece measuring 3 mm in thickness, 25 mm in width, and 25 mm in length was taken from the center of each steel plate and finished with wet #400 polishing to prepare a test piece for evaluating corrosion resistance. Corrosion resistance was evaluated by a sulfuric acid immersion test and a hydrochloric acid immersion test. In the sulfuric acid immersion test, the test piece was immersed in a 50% sulfuric acid aqueous solution at 70°C for 6 hours, and in the hydrochloric acid immersion test, the test piece was immersed in a 10% hydrochloric acid aqueous solution at 80°C for 5 hours.

[0067] Then, the corrosion rate was calculated from the corrosion weight loss of the test piece in the sulfuric acid immersion test and the hydrochloric acid immersion test. In this example, the corrosion rate in the sulfuric acid immersion test was 20.0 mg / cm 2 / h or less, it is judged to have excellent sulfuric acid resistance, and the corrosion rate in the hydrochloric acid immersion test is 15.0 mg / cm 2 / h or less, it was determined that the hydrochloric acid resistance was excellent.

[0068] <Hot workability> The surface of the hot-rolled material rolled under the above conditions was visually inspected for appearance, and the hot workability was evaluated by rating cracks as "X" and no cracks as "O."

[0069] <Tensile strength> Tensile test pieces were prepared in accordance with JIS Z 2241:2011, and tensile tests were conducted to determine the tensile strength. Test pieces with a thickness of 12 mm were taken from the 20 mm thick hot-rolled steel plate, and test pieces with a thickness of 3.4 mm were taken from the 13 mm thick cold-rolled steel plate, and were subjected to the tensile test. Test pieces with a tensile strength of 400 MPa or more were marked with ○, and those with a tensile strength of less than 400 MPa were marked with ×.

[0070] Table 3 shows the measurement results of the number density of inclusions, as well as the evaluation results of the sulfuric acid immersion resistance test, the hydrochloric acid immersion resistance test, the hot workability test, and the tensile test.

[0071] [Table 3]

[0072] As shown in Table 3, Test Nos. 1 to 15, which satisfy all the provisions of the present invention, showed excellent results in all performance evaluation tests. In contrast, Test Nos. 16 to 29, which are comparative examples, showed poor results in at least one of sulfuric acid resistance, hydrochloric acid resistance, and hot workability. [Industrial Applicability]

[0073] The steel material of the present invention can be used in smoke exhaust systems for boilers that burn fossil fuels such as heavy oil and coal, gas fuels such as liquefied natural gas, general waste such as urban waste, industrial waste such as waste oil, plastics, and exhaust tires, and sewage sludge, etc. Specifically, it can be suitably used for flue ducts, casings, and heat exchangers of smoke exhaust systems, gas-gas heaters consisting of two heat exchangers (a heat recovery unit and a reheater), desulfurization equipment, electric dust collectors, induced draft fans, basket materials and heat transfer element plates of rotary regenerative air preheaters, etc.

Claims

1. The chemical composition, in mass%, is C: 0.0010-0.20%, Si: 0.10-0.80%, Mn: 0.50-1.00%, Cu: 0.10-0.50%, Ni: 0.010-0.80%, Cr: 0.005-0.070%, Co: 0.002 to 0.020%, Mo and / or W: 0.001 to 0.30% in total; Al: 0.01-0.10%, Ti: 0.001 to 0.067%, Ca: more than 0% and not more than 0.010%; N: 0.0005-0.0050%, P: 0.050% or less, S: 0.00050-0.0200%, O: 0.0010-0.0045%, The balance is Fe and impurities. FI defined by the following formula (i) is 0.12 to 1.5, The number density of MnS with a maximum length of 2.0 μm or more contained in the steel material is 50 / mm ２ is less than Steel material. FI=1000×((Cr / 52)+(Ti / 48))...(i) In the above formula, the element symbols represent the contents (mass%) of the respective elements contained in the steel material.

2. The chemical composition is, in mass %, replacing a part of the Fe, B: 0.10% or less, Nb: 0.10% or less, and V: 0.10% or less, It contains at least one of the following: The steel material according to claim 1.

3. The chemical composition is, in mass %, replacing a part of the Fe, Sn: 0.30% or less, and Sb: 0.30% or less, It contains one or more selected from The steel material according to claim 1 or 2.

4. The mass ratio Al / O of the Al content to the O content is 3.0 to 60.0; The steel material according to claim 1 or 2.

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