JP7662941B2 - Steel - Google Patents

Description

The present invention relates to steel materials.

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 that makes up the exhaust gas flow path, a condition known as sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion.

To address these issues, sulfuric acid/hydrochloric acid dew-point corrosion resistant steels and highly corrosion resistant stainless steels have been proposed. For example, Patent Documents 1 to 4 propose steel materials with excellent sulfuric acid dew-point corrosion resistance that contain added elements such as Cu, Sb, Co, and Cr.

JP 2001-164335 A JP 2003-213367 A JP 2007-239094 A JP 2012-57221 A

Steel materials containing Cu, Sb, Cr, etc. exhibit excellent corrosion resistance in sulfuric acid corrosive environments such as exhaust gas chimneys. However, steel materials used in boiler air preheaters and the like may be used in sulfuric acid dew-point corrosion environments exceeding 100°C. Furthermore, in such sulfuric acid dew-point corrosion environments exceeding 100°C, extremely high concentrations of sulfuric acid exceeding 70% may be generated, and the steel materials used are required to have excellent corrosion resistance against high-temperature and high-concentration sulfuric acid.

The present invention aims to solve the above problems and provide a steel material that has excellent corrosion resistance in a high-temperature, high-concentration sulfuric acid corrosive environment.

The present invention was made to solve the above problems, and the gist of the present invention is the following steel material.

(1) Chemical composition, in mass%,
C: 0.010-0.20%,
Si: 0.04-1.00%,
Mn: 0.20-2.00%,
Cu: 0.10-1.00%,
Al: 0.005-0.10%,
Cr: 0.40-3.00%,
Ti: 0.010-0.50%,
Ni: 0.01-0.50%,
P: 0.050% or less,
S: 0.0005-0.050%,
N: 0.0015-0.0100%,
O: 0.0035% or less,
The balance is Fe and impurities.
The following formula (i) is satisfied:
The steel material contains MnS and MnTi composite sulfides, the number density of MnS having a maximum length of 2.0 μm or more is less than 50.0/ ^mm2 , and the ratio of the number density of MnTi composite sulfides 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.
Steel.
3.6<(Si+Al+Ca)/Ti<27.0...(i)
In the above formula, the element symbols represent the content (mass%) of each element contained in the steel material, and 0 is substituted when the element is not contained.

(2) The chemical composition is, in mass%, replacing a part of the Fe,
Mo: 0.10% or less,
W: 0.10% or less,
Sn: 0.30% or less,
Sb: 0.30% or less,
As: 0.30% or less,
Co: 0.30% or less; and Bi: 0.30% or less;
It contains one or more selected from
The steel material according to (1) above.

(3) The chemical composition contains, in mass%, a part of the Fe replaced by
Nb: 0.10% or less,
V: 0.10% or less,
Ta: 0.050% or less; and B: 0.010% or less;
It contains one or more selected from
The steel material according to (1) or (2) above.

(4) The chemical composition is, in mass%, replacing a part of the Fe,
Ca: 0.010% or less,
Mg: 0.010% or less; and REM: 0.010% or less;
It contains one or more selected from
The steel material according to any one of (1) to (3) above.

The present invention makes it possible to provide steel materials that have excellent corrosion resistance in high-temperature and high-concentration sulfuric acid corrosive environments.

In order to solve the above problems, the inventors conducted a detailed investigation into the corrosion resistance of steel materials, and came to the following findings.

Mn is an essential element for ensuring the strength and toughness of steel, but on the other hand, it forms MnS, which deteriorates corrosion resistance in an acid corrosion environment. In addition, in the present invention, S has the effect of improving corrosion resistance when contained together with Cu, so extreme reduction is not desirable. To solve this problem, the relationship between MnS and corrosion resistance was investigated in detail. As a result, it was found that MnTi composite sulfides, which contain Ti and Mn, are less likely to become the starting point of corrosion than MnS.

Further investigation into methods for producing MnTi composite sulfides revealed that once Ti forms an oxide, it becomes difficult to incorporate Ti into MnS. Therefore, it was discovered that by appropriately incorporating elements such as Si, which is more easily oxidized than Ti, into the steel and consuming the O in the steel through the formation of oxides of Si, etc., it is possible to suppress the formation of Ti oxides and incorporate Ti into MnS.

Furthermore, they discovered that MnS can also be rendered harmless by micronizing it.

The present invention was made based on the above findings. Each aspect of the present invention will be explained in detail below.

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

C: 0.010-0.20%
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 0.010 to 0.20%. When strength is required, the C content is preferably 0.050% or more. In addition, the C content is preferably 0.15% or less, and more preferably 0.10% or less.

Si: 0.04~1.00%
Silicon is an element that contributes to deoxidation and strength improvement, and controls the form of oxides. However, if silicon is contained in excess, the amount of oxides increases, impairing corrosion resistance. Therefore, the silicon content is set to 0.04 to 1.00%. The silicon content is preferably 0.10% or more, and more preferably 0.20% or more. The silicon content is preferably 0.70% or less, and more preferably 0.60% or less.

Mn: 0.20-2.00%
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 0.20 to 2.00%. The Mn content is preferably 0.50% or more, and more preferably 0.60% or more. In addition, the Mn content is preferably 1.70% or less, more preferably 1.50% or less, and even more preferably 1.30% or less.

Cu: 0.10-1.00%
Cu is an element that exhibits significant corrosion resistance against sulfuric acid. However, if Cu is contained in excess, hot workability decreases, impairing productivity. Therefore, the Cu content is set to 0.10 to 1.00%. The Cu content is preferably 0.15% or more, 0.20% or more, or 0.25% or more. In addition, the Cu content is preferably 0.85% or less, and more preferably 0.70% or less.

Al: 0.005-0.10%
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 0.005 to 0.10%. The Al content is preferably 0.010% or more. Moreover, the Al content is preferably 0.07% or less.

Cr:0.40~3.00%
Cr is an element that has the effect of increasing hardenability and improving strength, as well as improving sulfuric acid resistance. However, if Cr is contained in excess, it forms oxides that are likely to become corrosion initiation points on the steel surface. Therefore, the Cr content is set to 0.40 to 3.00%. The Cr content is preferably 0.50% or more, more preferably 0.60% or more, and even more preferably 0.65% or more. In addition, the Cr content is preferably 2.50% or less.

Ti: 0.010-0.50%
Ti is an element that forms nitrides and contributes to refining crystal grains and improving strength. However, if Ti is contained in excess, the increase in nitrides that cause corrosion impairs corrosion resistance. Therefore, the Ti content is set to 0.010 to 0.50%. The Ti content is preferably 0.013% or more, more preferably 0.015% or more, and even more preferably 0.020% or more. In addition, the Ti content is preferably 0.40% or less, and more preferably 0.30% or less.

Ni: 0.01~0.50%
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 is prone to segregation, and when contained alone, it may promote cracking after casting. In contrast, Ni has the effect of reducing surface segregation of Cu. By containing Ni, in addition to suppressing Cu segregation and cast piece 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 of Ni leads to an increase in steelmaking costs. Therefore, the Ni content is set to 0.01 to 0.50%. The Ni content is preferably 0.03% or more, more preferably 0.05% or more, and even more preferably 0.08% or more. In addition, the Ni content is preferably 0.40% or less, less than 0.29%, 0.25% or less, 0.20% or less, and more preferably 0.15% or less.

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 0.030% or less, and more preferably 0.025% 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.

S: 0.0005-0.050%
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 0.0005 to 0.050%. The S content is preferably 0.0010% or more, 0.0050% or more, or 0.0100% or more. In addition, the S content is preferably 0.040% or less, and more preferably 0.030% or less.

N: 0.0015-0.0100%
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 0.0015 to 0.0100%. The N content is preferably 0.0020% or more, and more preferably 0.0030% or more. In addition, the N content is preferably 0.0090% or less, and more preferably 0.0080% or less.

O: 0.0035% or less O is an impurity and forms coarse oxides that become the starting point of corrosion in an acid corrosion environment. Therefore, an upper limit is set for the O content, and the O content is set to 0.0035% or less. The O content is preferably 0.0030% or less, and more preferably 0.0025% or less. It is preferable to reduce the O 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 O content may be 0.0013% or more, or 0.0020% or more.

In addition to the above elements, the chemical composition of the steel of the present invention may contain one or more elements selected from Mo, W, Sn, Sb, As, Co, and Bi in the ranges shown below to further improve corrosion resistance. Note that these elements are not necessarily essential in steel materials, so the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

Mo: 0.10% or less Mo is an element that improves corrosion resistance in an acidic environment when contained simultaneously with Cu and Cr, so it may be contained as necessary. However, since Mo is an expensive element, excessive inclusion of Mo leads to a decrease in economic efficiency. Therefore, the Mo content is set to 0.10% or less. The Mo content is preferably 0.09% or less, and more preferably 0.08% or less. In order to obtain the above effect more reliably, the Mo content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

W: 0.10% or less W is an element that improves corrosion resistance in an acidic environment when contained simultaneously with Cu and Cr, similar to Mo, and may be contained as necessary. However, since W is also an expensive element, excessive inclusion leads to a decrease in economic efficiency. Therefore, the W content is set to 0.10% or less. The W content is preferably 0.09% or less, and more preferably 0.08% or less. In order to obtain the above effect more reliably, the W content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

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 addition, if it is desired to obtain the above effect 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.

Sb: 0.30% or less Sb is an element that improves corrosion resistance to sulfuric 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 addition, if it is desired to obtain the above effects more reliably, the Sb content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

As: 0.30% or less Although As does not have a significant effect compared with Sb and Sn, it is an element that is effective in improving corrosion resistance in an acid corrosion environment, so it may be contained as necessary. However, if As is contained in excess, hot workability decreases. Therefore, the As content is set to 0.30% or less. The As content is preferably 0.20% or less, and more preferably 0.10% or less. In addition, if it is desired to obtain the above-mentioned effect more reliably, the As content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

Co: 0.30% or less Co is an element that improves corrosion resistance in an acid corrosion environment, although it does not have a significant effect compared to Sb and Sn, so it may be contained as necessary. However, if Co is contained in excess, the economic efficiency decreases. Therefore, the Co content is set to 0.30% or less. The Co content is preferably 0.20% or less, and more preferably 0.10% or less. In order to obtain the above effects more reliably, the Co content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

Bi: 0.30% or less Bi does not have a significant effect compared to Sb and Sn, but since it is an element that improves corrosion resistance in an acidic environment, it may be contained as necessary. However, if Bi is contained in excess, hot workability decreases. Therefore, the Bi content is set to 0.30% or less. The Bi content is preferably 0.20% or less, and more preferably 0.10% or less. In order to obtain the above effects more reliably, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

In addition to the above elements, the chemical composition of the steel of the present invention may further contain one or more elements selected from Nb, V, Ta, and B in the ranges shown below in order to improve mechanical properties, etc. Note that these elements are not necessarily essential in steel materials, so the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

Nb: 0.10% or less Like Ti, 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 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 0.09% or less, more preferably 0.08% or less, and even more preferably 0.07% or less. In addition, if it is desired to obtain the above effects more reliably, the Nb content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.

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 0.08% or less, more preferably 0.06% or less, and even more preferably 0.04% or less. In addition, if it is desired to obtain the above effects more reliably, the V content is preferably 0.005% or more.

Ta: 0.050% or less Ta is an element that contributes to improving strength, and also contributes to improving corrosion resistance, although the mechanism is not necessarily clear, so it may be contained as necessary. However, Ta is an expensive element, and the inclusion of a large amount of Ta leads to an increase in steelmaking costs. Therefore, the Ta content is set to 0.050% or less. The Ta content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. In addition, if it is desired to obtain the above effects more reliably, the Ta content is preferably 0.001% or more, and more preferably 0.005% or more.

B: 0.010% or less B is an element that improves hardenability and increases strength, so 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 0.010% or less. The B content is preferably 0.008% or less, more preferably 0.006% or less, and even more preferably 0.004% or less. In addition, if it is desired to obtain the above effects more reliably, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more.

In addition to the above elements, the chemical composition of the steel of the present invention may further contain one or more elements selected from Ca, Mg, and REM within the ranges shown below for the purpose of deoxidization and inclusion control. Note that these elements are not necessarily essential in steel materials, so the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

Ca: 0.010% or less Ca is an element mainly used to control the form of sulfides, and may be contained as necessary to form fine oxides. However, if Ca is contained in excess, mechanical properties may be impaired. Therefore, the Ca content is set to 0.010% or less. The Ca content is preferably 0.005% or less. In order to more reliably obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.

Mg: 0.010% or less Mg may be contained as necessary to form fine oxides. However, excessive addition of Mg leads to increased steelmaking costs. Therefore, the Mg content is set to 0.010% or less. The Mg content is preferably 0.005% or less, and more preferably 0.003% or less. In order to more reliably obtain the above effects, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

REM: 0.010% or less REM (rare earth elements) are elements mainly used for deoxidation, and may be contained as necessary to form fine oxides. However, excessive addition of REM leads to increased steelmaking costs. Therefore, the REM content is set to 0.010% or less. The REM content is preferably 0.005% or less, and more preferably 0.003% or less. In order to more reliably obtain the above effects, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

Here, REM is a general term for 17 elements in total: Sc, Y, and lanthanides, and the REM content refers to the total amount of the above elements. Note that lanthanides are added industrially in the form of misch metals.

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

3.6<(Si+Al+Ca)/Ti<27.0...(i)
As described above, when Ti becomes an oxide, it becomes difficult to form MnTi composite sulfides. However, it is industrially difficult to reduce the O content extremely, and it is difficult to suppress the formation of oxides. Therefore, in order to suppress the generation of Ti oxides, it is effective to increase the total content of Si, Al, and Ca, which are more likely to become oxides than Ti, relative to the content of Ti. As a result, O in the steel is consumed, and Ti is contained in MnS without forming an oxide, so that MnTi composite sulfides are formed and the corrosion resistance is improved. On the other hand, if the total content of Si, Al, and Ca becomes excessive relative to the content of Ti, not only the effect of improving the corrosion resistance is saturated, but also coarse oxides that become the starting point of corrosion are formed, so that the corrosion resistance is rather reduced. Therefore, it is necessary to satisfy formula (i). However, the element symbols in formula (i) represent the content (mass%) of each element contained in the steel, and 0 is substituted if it is not contained.

The value in formula (i) is preferably 4.0 or more, and more preferably 6.0 or more. The value in formula (i) is preferably 25.0 or less, and more preferably 20.0 or less.

(B) Inclusions The steel material according to the present invention contains MnS and MnTi composite sulfides in the steel material. The number density of MnS having a maximum length of 2.0 μm or more is less than 50.0/ ^mm2 . In addition, the ratio of the number density of MnTi composite sulfides 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.

Note that since MnS with a maximum length of less than 2.0 μm has almost no effect on the corrosion resistance of steel, the present invention focuses on inclusions with a maximum length of 2.0 μm or more. In the following explanation, MnS with a maximum length of 2.0 μm or more will be simply referred to as MnS, and MnTi composite sulfides with a maximum length of 2.0 μm or more will also be simply referred to as MnTi composite sulfides.

As described above, in the steel material of the present invention, the formation of MnS is unavoidable. However, MnS is a starting point of corrosion and deteriorates the corrosion resistance in an acid corrosion environment. Therefore, the number density of MnS is set to less than 50.0/ ^mm2 . The number density of MnS is preferably 45.0/ ^mm2 or less, and more preferably 40.0/ ^mm2 or less.

On the other hand, extreme reduction in the Mn and S contents is not preferable in the steel material of the present invention from the viewpoint of improving the strength and corrosion resistance. In order to achieve both, it is necessary to render MnS harmless. MnS is rendered harmless when MnTi composite sulfides are formed, and it becomes difficult for it to become a starting point for corrosion. Although the detailed reason why the formation of MnTi composite sulfides improves corrosion resistance is unclear, MnS dissolves easily in sulfuric acid and becomes a starting point for corrosion, while MnTi composite sulfides have a lower solubility than MnS and are therefore thought to be difficult to become a starting point for corrosion.

For these reasons, in the present invention, the ratio of the number density of MnTi composite sulfides to the number density of MnS is set to 0.10 or more. The above ratio is preferably 0.12 or more, and more preferably 0.15 or more.

The number density of MnS and the number density of MnTi composite sulfides 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 MnTi composite sulfides 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 determine the number density and the number density ratio.

Inclusions are identified by EDS, and inclusions with a total Mn and S content of 90% or more by mass are determined to be MnS, and inclusions where a Ti peak is detected, the Ti content is 12% or more, and the total Mn, S, and Ti content is 90% or more by mass are determined to be MnTi composite sulfides. When MnS contains 12% or more Ti and becomes an MnTi composite sulfide, MnS is significantly rendered harmless.

(C) Plate Thickness The thickness of the steel material according to the present invention is not particularly specified, but is preferably 0.6 to 25.0 mm, and more preferably 1.0 to 20.0 mm.

(D) Yield Stress The yield stress of the steel material according to the present invention is not particularly specified, but is preferably 300 MPa or more at 25°C, and more preferably 310 MPa or more.

(E) Manufacturing Method A manufacturing method of a steel material according to an embodiment of the present invention will be described. The steel material according to this embodiment includes steel plates, steel sections, steel pipes, etc., which are manufactured by hot rolling and, if necessary, cold rolling.

The steel material according to this embodiment is manufactured by melting steel in a conventional manner, adjusting the composition, casting the resulting steel slab, hot rolling the resulting slab, and further cold rolling as necessary. The heating temperature before hot rolling is preferably 1220 to 1400°C. In this temperature range, MnTi composite sulfides are formed. Although the detailed mechanism is not clear, it is believed that Ti precipitates as TiS in this temperature range. It is believed that MnTi composite sulfides are then formed by the precipitation of MnS with the precipitated TiS acting as a nucleus.

If the heating temperature before hot rolling exceeds 1400°C, energy is wasted and manufacturing costs increase. On the other hand, by setting the heating temperature before hot rolling to 1220°C or higher, the nuclei of TiS are sufficiently precipitated, and as a result, the ratio of the number density of MnTi composite sulfides to the number density of MnS can be made 0.10 or more.

The holding time in the above temperature range is preferably 60 to 150 minutes. By holding the temperature range for 60 minutes or more, TiS and MnS can be sufficiently precipitated, and the ratio of the number density of MnTi composite sulfides to the number density of MnS can be set to 0.10 or more. On the other hand, by holding the temperature range for 150 minutes or less, coarsening of MnS that did not form MnTi composite sulfides can be suppressed.

After hot rolling, the hot-rolled steel sheet is subjected to the next process, such as coil winding. 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 formation of MnTi composite sulfides with TiS as the nucleus. 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.

When manufacturing steel pipes from the obtained steel plate, the steel plate is formed into a tubular shape and welded, and can be made into, for example, UO steel pipes, electric resistance welded steel pipes, forged steel pipes, spiral steel pipes, etc.

The present invention will be explained in more detail below with reference to examples. Note that the conditions in the examples shown below are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. Furthermore, 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.

Steels (A to AB) with 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. 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.

Test pieces for SEM observation were cut from each of the obtained steel plates, 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 MnTi composite sulfides 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 of view area to obtain the number density and the number density ratio.

Furthermore, the various performance evaluation tests shown below were conducted using each of the obtained steel plates.

<Sulfuric acid resistance>
A test piece having a thickness of 1 mm, a width of 25 mm, and a length of 25 mm was taken from the center of each steel plate and finished by wet polishing with #400 to prepare a test piece for evaluating corrosion resistance. The corrosion resistance was evaluated by a sulfuric acid immersion test in which the test piece was immersed in a 75% sulfuric acid aqueous solution at 125° C. for 6 hours.

Then, the corrosion rate was calculated from the corrosion weight loss of each test piece in the sulfuric acid immersion test. In this example, a corrosion rate of 10.0 mg/cm ² /h or less in the sulfuric acid immersion test was determined to be excellent in sulfuric acid resistance.

<Yield stress>
A tensile test piece having a thickness of 1 mm was prepared and subjected to a tensile test to determine the yield stress in accordance with JIS Z 2241:2011. A specimen having a yield stress of 300 MPa or more was determined to have excellent yield stress.

Table 3 shows the measurement results of the number density of inclusions, as well as the evaluation results of the sulfuric acid immersion test and the tensile test.

As shown in Table 3, Test Nos. 1 to 24, which satisfy all the provisions of the present invention, achieved excellent results in all performance evaluation tests. In contrast, Test Nos. 25 to 33, which are comparative examples, showed deterioration in at least one of sulfuric acid resistance and yield stress.

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 municipal waste, industrial waste such as waste oil, plastics, and used tires, and sewage sludge, etc. Specifically, it can be suitably used in air preheaters for boilers, etc.

Claims (4)

The chemical composition, in mass%, is
C: 0.010-0.20%,
Si: 0.04-1.00%,
Mn: 0.20-2.00%,
Cu: 0.10-1.00%,
Al: 0.005-0.10%,
Cr: 0.40-3.00%,
Ti: 0.010-0.50%,
Ni: 0.01-0.50%,
P: 0.050% or less,
S: 0.0005-0.050%,
N: 0.0015-0.0100%,
O: 0.0035% or less,
The balance is Fe and impurities.
The following formula (i) is satisfied:
The steel material contains MnS and MnTi composite sulfides, the number density of MnS having a maximum length of 2.0 μm or more is less than 50.0/ ^mm2 , and the ratio of the number density of MnTi composite sulfides 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.
Steel.
3.6<(Si+Al+Ca)/Ti<27.0...(i)
In the above formula, the element symbols represent the content (mass%) of each element contained in the steel material, and 0 is substituted when the element is not contained. The chemical composition is, in mass %, replacing a part of the Fe,
Mo: 0.10% or less,
W: 0.10% or less,
Sn: 0.30% or less,
Sb: 0.30% or less,
As: 0.30% or less,
Co: 0.30% or less; and Bi: 0.30% or less;
It contains one or more selected from
The steel material according to claim 1. The chemical composition is, in mass %, replacing a part of the Fe,
Nb: 0.10% or less,
V: 0.10% or less,
Ta: 0.050% or less; and B: 0.010% or less;
It contains one or more selected from
The steel material according to claim 1 or 2. The chemical composition is, in mass %, replacing a part of the Fe,
Ca: 0.010% or less,
Mg: 0.010% or less; and REM: 0.010% or less;
It contains one or more selected from
The steel material according to any one of claims 1 to 3.