JP7670971B2 - 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 corrosion environments such as exhaust gas chimneys. However, ash sweeping blades in cooling towers are used in high-temperature environments exceeding 350°C. Furthermore, because the gas contains sulfur oxides, when the temperature drops, the steel materials will be used in a sulfuric acid dew-point corrosion environment. Materials used in such environments need to have high sulfuric acid dew-point corrosion resistance as well as high yield stress at room temperature.

The present invention aims to solve the above problems and provide a steel material that has excellent corrosion resistance in a sulfuric acid dew-point corrosion environment and has high yield stress.

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: 0.050% or less,
N: 0.0015-0.0100%,
O: 0.0035% or less,
The balance is Fe and impurities.
The steel material contains TiN, the number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm is 0.5/ ^mm2 or more, the number density of TiN having a maximum length of 4.0 μm or more is less than 15/ ^mm2 , and the ratio of the number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm to the number density of TiN having a maximum length of 1.0 μm or more is 0.20 or more;
Steel.

(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.

(5) The yield stress at 25°C is 310 MPa or more.
The steel material according to any one of (1) to (4) above.

The present invention makes it possible to provide a steel material that has excellent corrosion resistance in a sulfuric acid dew-point corrosion environment and has high yield stress.

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

The pinning effect of TiN is effective in increasing the yield stress. However, coarse TiN can act as a starting point for corrosion, degrading corrosion resistance. In particular, it has been found that when gas containing sulfur oxides is cooled and an extremely severe sulfuric acid dew-point corrosion environment is created, the impact of the deterioration of corrosion resistance due to TiN in steel becomes even greater. For this reason, it has been discovered that by setting the size of TiN within an appropriate range, it is possible to improve the corrosion resistance while maintaining the yield stress of the steel.

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 morphology 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: 0.050% or less S is an impurity that reduces the mechanical properties and productivity of steel materials. Therefore, the upper limit of the S content is set to 0.050% or less. The S content is preferably 0.040% or less, and more preferably 0.030% or less. It is preferable to reduce the S 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 S content may be 0.001% or more, or 0.005% or more.

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 generates 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.

(B) Inclusions The steel material according to the present invention contains TiN in the steel material. The number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm is 0.5/ ^mm2 or more, and the number density of TiN having a maximum length of 4.0 μm or more is less than 15/ ^mm2 . In addition, the ratio of the number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm to the number density of TiN having a maximum length of 1.0 μm or more is 0.20 or more.

Note that TiN with a maximum length of less than 1.0 μm has almost no effect on the corrosion resistance and yield stress of steel materials, so in this invention, the inclusions with a maximum length of 1.0 μm or more are the subject of the present invention.

As described above, in the steel material of the present invention, the yield stress can be improved by forming TiN. However, coarse TiN becomes the starting point of corrosion, and the corrosion resistance in a sulfuric acid dew-point corrosion environment deteriorates. Therefore, the number density of TiN having a maximum length of 4.0 μm or more is set to less than 15/ ^mm2 . The number density of TiN having a maximum length of 4.0 μm or more is preferably 12/ ^mm2 or less, and more preferably 10/ ^mm2 or less.

On the other hand, in order to improve the yield stress, it is important that the number density of TiN is high. In order to increase the number density of TiN, it is effective to make TiN finer. Therefore, in the present invention, the number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm is 0.5/mm ² or more. The number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm is preferably 0.7/mm ² or more, more preferably 1.0/mm ² or more. In addition, the ratio of the number density of TiN having a maximum length of 1.0 μm or more to the number density of TiN having a maximum length of 1.0 μm or more is 0.20 or more. The above ratio is preferably 0.25 or more, more preferably 0.30 or more.

The number density of TiN is 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 TiN detected in the field of view is measured. The number of TiN particles with a maximum length of 1.0 μm or more, the number of TiN particles with a maximum length of 4.0 μm or more, and the number of TiN particles with a maximum length of 1.0 μm or more but less than 2.0 μm are counted, and the number density and the ratio of number densities are calculated by dividing by the area of the field of view.

TiN is identified by EDS, and inclusions with a combined Ti and N content of 90% or more by mass are determined to be TiN.

(C) Plate Thickness The thickness of the steel material according to the present invention is not particularly specified, but is preferably 0.3 to 10.0 mm, and more preferably 0.5 to 5.0 mm.

(D) Yield Stress The yield stress of the steel material according to the present invention is preferably 310 MPa or more at 25° C., and more preferably 320 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, further cold rolling and heat treatment.

The steel material according to this embodiment is manufactured by melting steel in a conventional manner, adjusting the composition, and casting the resulting steel slab, which is then hot-rolled and then cold-rolled. TiN is already precipitated at the steel slab stage, but by heating to a relatively high temperature during hot rolling, Ti is dissolved, reducing the number density of coarse TiN. Furthermore, TiN can be finely precipitated in the process described below. For this reason, a temperature of 1220 to 1400°C is specifically preferred.

If the heating temperature before hot rolling exceeds 1400°C, energy will be wasted and manufacturing costs will increase. Therefore, it is preferable to set the heating temperature before hot rolling to 1400°C or less.

On the other hand, by setting the heating temperature before hot rolling to 1220° C. or higher, it is possible to melt coarse TiN and reduce the number density of coarse TiN. Also, by setting the heating temperature before hot rolling to 1220° C. or higher, it is possible to make the number density of TiN having a maximum length of 4.0 μm or more less than 15/mm ^2. Furthermore, in order to make the number density of TiN having a maximum length of 4.0 μm or more 12/mm ² or less, it is more preferable to set the heating temperature before hot rolling to 1250° C. or higher.

The hot-rolled steel sheet after hot rolling is subjected to the next process such as coil winding. At that time, the temperature of the steel sheet is lowered, but it is preferable that the time from the completion of hot rolling to reaching 400°C is 4 hours or more. By cooling under such conditions and generating ultrafine TiN of less than 1.0 μm in the steel sheet, the number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm can be made 0.5/ ^mm2 or more, and the number density ratio of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm can be made 0.20 or more by performing heat treatment after cold rolling.

After hot rolling, the steel sheet is cold rolled to obtain a cold-rolled steel sheet. After cold rolling, heat treatment is further performed. By performing heat treatment in a temperature range of 750 to 850°C after cold rolling, the maximum length of TiN can be adjusted, and the corrosion resistance can be improved while maintaining the yield stress of the steel sheet. By setting the heat treatment temperature after cold rolling to 750°C or higher, the number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm can be set to 0.5/ ^mm2 or more, and the desired yield stress can be obtained.

On the other hand, by setting the heat treatment temperature after cold rolling to 850°C or less, it is possible to suppress the increase in TiN with a maximum length of 2.0 μm or more and less than 4.0 μm, which does not contribute to increasing strength. Therefore, the ratio of the number density of TiN with a maximum length of 1.0 μm or more and less than 2.0 μm to the number density of TiN with a maximum length of 1.0 μm or more can be set to 0.20 or more, and the desired yield stress can be obtained. Therefore, it is preferable that the heat treatment temperature after cold rolling be 750 to 850°C.

When manufacturing steel pipes from the obtained steel plate, the steel plate can be formed into a tubular shape and welded, for example, to produce electric resistance welded steel pipes, forged welded 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 W) 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, the steel was cooled to simulate coiling, and then cold-rolled to produce cold-rolled steel sheets with a thickness of 13 mm.

Test pieces for SEM observation were cut out 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 TiN detected in the field of view was measured, and the number of TiN with a maximum length of 1.0 μm or more, the number of TiN with a maximum length of 4.0 μm or more, and the number of TiN with a maximum length of 1.0 μm or more but less than 2.0 μm were counted, and the number density and the number density ratio were calculated by dividing by the field of view area.

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 40% sulfuric acid aqueous solution at 60° 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 20.0 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 310 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 21, which satisfy all the provisions of the present invention, achieved excellent results in all performance evaluation tests. In contrast, Test Nos. 22 to 31, 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 for ash sweeping impellers of cooling towers, etc.

Claims (5)

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: 0.050% or less,
N: 0.0015-0.0100%,
O: 0.0035% or less,
The balance is Fe and impurities.
The steel material contains TiN, the number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm is 0.5/ ^mm2 or more, the number density of TiN having a maximum length of 4.0 μm or more is less than 15/ ^mm2 , and the ratio of the number density of TiN having a maximum length of 1.0 μm or more and less than 2.0 μm to the number density of TiN having a maximum length of 1.0 μm or more is 0.20 or more;
Steel. 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. The yield stress at 25°C is 310 MPa or more.
The steel material according to any one of claims 1 to 4.