JP2020111790A - Steel - Google Patents

Abstract

To provide a steel that has excellent manufacturability, and has long-term corrosion resistance and excellent weldability in an acid corrosive atmosphere such as sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion.SOLUTION: A steel comprises, in mass%, C: 0.010-0.20%, Si: 0.040-0.70%, Mn: 0.20-1.00%, Cu: 0.10-1.00%, Al: 0.005-0.10%, Cr: 0.30-3.00%, Ti: 0.01-0.15%, P: 0.050% or less, S: 0.030% or less, N: 0.0015-0.0100%, and O: 0.0035% or less, with the balance being Fe and impurities, BI obtained by the following formula (1) of 33.0-81.0, and Ceq obtained by the following formula (2) of 0.180-0.370. Formula (1): BI=(Cr/52)/(N/14). Formula (2): Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15.SELECTED DRAWING: None

Description

The present invention relates to steel materials.

Boiler smoke exhaust systems and gasification melting furnaces that burn various fuels, wastes, sewage sludge, etc. are exposed to combustion exhaust gas atmosphere and exposed to sulfuric acid dew point corrosion, hydrochloric acid dew point corrosion environment, or sulfuric acid or hydrochloric acid aqueous solution. It becomes a contact environment. A steel material exposed to such a severe corrosive environment is required to have excellent acid resistance for a long period of time.

For such problems, sulfuric acid/hydrochloric acid dew point corrosion steel and high corrosion resistance stainless steel have been proposed (see, for example, Patent Documents 1 to 5). Among them, Patent Documents 1 to 4 propose a steel material containing Cu, Sb, Co, Cr or the like and having excellent sulfuric acid dew point corrosion resistance. Further, Patent Document 5 proposes a highly corrosion resistant stainless steel to which Cr, Ni or the like is added.

JP, 2001-164335, A JP, 2003-213367, A JP 2007-239094A JP 2012-57221 A JP-A-7-316745

Steel materials containing Cu, Sb, Cr and the like are excellent in sulfuric acid dew point corrosion resistance and exhibit excellent corrosion resistance in a sulfuric acid corrosion environment such as an exhaust gas chimney of a boiler or an incinerator. However, further improvement in corrosion resistance is expected in order to prolong the life of boilers and incinerators.

Further, not only acid resistance but also weldability is required for steel materials used for applications such as heat exchangers, gas-gas heaters, desulfurizers, and electrostatic precipitators. Further, hot workability is also required from the viewpoint of manufacturability.

In view of such circumstances, the present invention is excellent in manufacturability, and in an acid corrosive environment of sulfuric acid dew point corrosion or hydrochloric acid dew point corrosion, long-term corrosion resistance, and an object of the invention is to provide a steel material having excellent weldability. is there.

The present inventors paid attention to a nitride that is a starting point of corrosion, and conducted a study for improving the corrosion resistance of a steel material in an acid corrosive environment. It was also found that when the content of Cr or N, which is effective for improving the acid resistance, increases, the nitride containing Cr becomes the starting point of corrosion. Therefore, by further examining Cr and N, and by setting the value of the acid corrosion resistance index BI obtained by the following formula (1) to 33.0 to 81.0, the finding that corrosion resistance exceeding the expectation is expressed Obtained.

BI=(Cr/52)/(N/14) (1)

Further, it was found that excellent acid resistance and weldability can be obtained by setting Ceq determined by the following formula (2) to 0.180 to 0.370.

Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15 (2)

The present invention has been made based on such findings, and the gist thereof is as follows.
[1] In mass%,
C: 0.010 to 0.20%,
Si: 0.040 to 0.70%,
Mn: 0.20 to 1.00%,
Cu: 0.10 to 1.00%,
Al: 0.005-0.10%,
Cr: 0.30 to 3.00%,
Ti: 0.01 to 0.15%,
P: 0.050% or less,
S: 0.030% or less,
N: 0.0015 to 0.0100%, and
O: 0.0035% or less,
Of which the balance is Fe and impurities, the BI calculated by the following formula (1) is 33.0 to 81.0, and the Ceq calculated by the following formula (2) is 0.180 to 0.370. A steel material characterized by being.
BI=(Cr/52)/(N/14)... Formula (1)
Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15 (2)
Here, in the formula, Cr, N, C, Mn, Cu, Ni, Mo, and V represent the contents based on the mass% of each element, and are 0 when they are not contained.
[2] Further, in mass%,
Mo: 0.10% or less,
W: 0.10% or less,
Ni: 1.00% or less,
Sn: 0.30% or less,
Sb: 0.30% or less,
As: 0.30% or less, and
Co: 0.30% or less,
The steel material according to [1], which contains one or more selected from the group consisting of:
[3] Further, in mass%,
Nb: 0.10% or less,
V: 0.10% or less,
Zr: 0.050% or less,
Ta: 0.050% or less, and
B: 0.010% or less,
The steel material according to [1] or [2], which contains one or more selected from the group consisting of:
[4] Further, in mass%,
Ca: 0.010% or less,
Mg: 0.010% or less, and
REM: 0.010% or less,
The steel material according to any one of [1] to [3], which contains one or more selected from the group consisting of:

According to the present invention, excellent corrosion resistance is exhibited in an acid corrosion environment such as a sulfuric acid corrosion environment such as an exhaust gas chimney of a boiler or an incinerator, or a hydrochloric acid corrosion environment such as an exhaust gas chimney of a refuse incinerator, etc. It also becomes possible to provide excellent steel materials. Therefore, the present invention makes a very significant industrial contribution.

Hereinafter, preferred embodiments of the present invention will be described in detail. From the study by the present inventors, it has been found that it is effective to suppress the nitride, which easily becomes a corrosion starting point on the surface of the steel material, in order to improve the acid resistance of the acid resistant steel containing Cu and Ti.

Cr, which is effective in improving acid resistance, tends to form a nitride when the content thereof is excessive, and may serve as a starting point of corrosion. As a result of studies by the present inventors, it has been found that the balance between the contents of Cr and N is important, and it is necessary to set the acid corrosion resistance index BI obtained by the following formula (1) within an appropriate range. ..

BI=(Cr/52)/(N/14)... Formula (1)

Further, it was found that by setting the Ceq value calculated by the following formula (2) within an appropriate range, the weldability can be ensured without impairing the manufacturability. By setting these in the proper range, we succeeded in obtaining a steel material having excellent acid resistance and weldability.

Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15 Equation (2)

The components of the steel material according to this embodiment will be described. In addition, the notation of% means mass% unless otherwise specified.

(C: 0.010 to 0.20%)
C is an element that improves the strength, and it is necessary to contain 0.010% or more. The C content is preferably 0.030% or more, and more preferably 0.080% or more. On the other hand, when the amount of C exceeds 0.20%, carbides increase and the acid resistance deteriorates, so the amount of C is made 0.20% or less. The C content is preferably 0.15% or less, more preferably 0.10% or less.

(Si: 0.040-0.70%)
Si is an element that contributes to deoxidation and improvement of strength, and it is necessary to contain Si in an amount of 0.040% or more in order to control the form of the oxide. The Si content is preferably 0.050% or more, and more preferably 0.10% or more. On the other hand, if the Si content exceeds 0.70%, the amount of oxides increases and the acid resistance is impaired, so the Si content is set to 0.70% or less. Preferably, the amount of Si is 0.50% or less. More preferably, the amount of Si is 0.30% or less.

(Mn: 0.20 to 1.00%)
Mn is an element that improves strength and toughness, and is contained at 0.20% or more. The Mn content is preferably 0.30% or more, and more preferably 0.60% or more. On the other hand, when Mn is contained in an amount of more than 1.00%, coarse MnS is generated and corrosion resistance and mechanical properties are deteriorated, so the Mn content is set to 1.00% or less. The Mn content is preferably 0.90% or less, more preferably 0.85% or less.

(Cu: 0.10 to 1.00%)
Cu is an extremely important element that remarkably exhibits corrosion resistance to sulfuric acid and hydrochloric acid. In order to secure corrosion resistance in an acidic environment, it is necessary to set the Cu content to 0.10% or more. The Cu content is preferably 0.15% or more, more preferably 0.20% or more. On the other hand, if the Cu content exceeds 1.00%, the hot workability deteriorates and the manufacturability is impaired, so the content is made 1.00% or less. The Cu content is preferably 0.90% or less, more preferably 0.80% or less.

(Al: 0.005-0.10%)
Al is a deoxidizing agent, and it is necessary to contain 0.005% or more. The Al amount is preferably 0.015% or more, and more preferably the Al amount is 0.019% or more. On the other hand, if Al is contained excessively, acid resistance is impaired by the increase of inclusions, so the Al content is made 0.10% or less. Preferably, the amount of Al is 0.050% or less. More preferably, the Al content is 0.040% or less.

(Cr: 0.30 to 3.00%)
Cr, like Cu, is an element that improves corrosion resistance. In particular, by containing Cr at the same time as Cu and Ti, excellent corrosion resistance is exhibited in an acidic environment where the temperature is high and the concentration is high. Therefore, from the viewpoint of ensuring corrosion resistance, it is necessary to contain 0.30% or more of Cr. The Cr content is preferably 0.40% or more, more preferably 0.50% or more. On the other hand, if Cr is excessively contained, the amount of Cr is 3.00% or less because the acid resistance is impaired due to an increase in the nitride that becomes the starting point of corrosion. The Cr content is preferably 1.60% or less, and more preferably the Cr content is 1.00% or less.

(Ti: 0.01 to 0.15%)
Ti is an element that improves corrosion resistance. Particularly, by containing Ti at the same time as Cu and Cr, excellent corrosion resistance is exhibited in an acidic environment. Therefore, in order to secure the corrosion resistance, it is necessary to set the Ti amount to 0.01% or more. The Ti content is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if Ti is contained excessively, the amount of Ti is set to 0.15% or less because the acid resistance is impaired due to an increase in the nitride that becomes the starting point of corrosion. The Ti content is preferably 0.07% or less, and more preferably the Ti content is 0.05% or less.

(P: 0.050% or less)
P is an impurity and reduces the mechanical properties and manufacturability of the steel material, so the P amount is made 0.050% or less. Preferably, the P content is 0.025% or less, and more preferably 0.010% or less. The lower limit of the amount of P is not limited, but the amount of P may be 0.001% or more from the viewpoint of cost.

(S: 0.030% or less)
S is an impurity and reduces the hot workability and the mechanical properties of the steel material, so the S content is made 0.030% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less. The lower limit of the amount of S is not limited and may be 0%. When S is contained together with Cu, the corrosion resistance in an acidic environment is improved, so the S content may be 0.001% or more. The S content is preferably 0.004% or more, and more preferably 0.008% or more.

(N: 0.0015 to 0.0100%)
N is an element that forms a nitride, and when Cr is contained, the corrosion resistance decreases when the N content becomes excessive, so the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0060% or less. On the other hand, fine nitrides are effective in improving mechanical properties and the like, so the N content is set to 0.0015% or more. The N content is preferably 0.0020% or more. More preferably, the amount of N is 0.0025% or more.

(O: 0.0035% or less)
O is an impurity and an element that forms an oxide. In order to suppress the formation of a coarse oxide that becomes a starting point of corrosion in an acidic environment, the amount of O is set to 0.0035% or less. The O content is more preferably 0.0030% or less, and further preferably 0.0025% or less. From the viewpoint of cost, the O content is preferably 0.0005% or more, and more preferably 0.0008% or more.

Further, in order to improve the corrosion resistance in an acidic environment, it is preferable to contain one or more selected from the group consisting of Mo, W, Ni, Sn, Sb, As and Co. In addition, since these elements are not necessarily essential in the steel material, the lower limit of the content is 0%.

(Mo: 0.10% or less)
Mo is an element that improves the corrosion resistance in an acidic environment by containing Cu and Cr at the same time. In particular, in order to improve the corrosion resistance to hydrochloric acid, the Mo content can be set to 0.005% or more. The Mo content is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, since Mo is an expensive element, the Mo content is set to 0.10% or less from the viewpoint of cost. More preferably, the Mo content is 0.08% or less, further preferably 0.05% or less.

(W: 0.10% or less)
W is an element that improves corrosion resistance in an acidic environment by including Cu and Cr at the same time as Mo. In particular, in order to enhance the corrosion resistance to hydrochloric acid, the W content may be 0.005% or more, for example. The W content is preferably 0.01% or more, more preferably 0.03% or more. On the other hand, since W is also an expensive element, the W content is set to 0.10% or less from the viewpoint of cost. More preferably, the W content is 0.08% or less, and further preferably 0.05% or less.

(Ni: 1.00% or less)
Ni is an element that improves corrosion resistance in an acidic environment, and when Cu is contained, Ni exerts an effect of enhancing manufacturability. Cu has a large effect of improving the 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 segregation of Cu. When Ni is contained, in addition to suppressing the cracking of the cast piece due to Cu segregation, the occurrence of local corrosion due to segregation is also suppressed, so that the effect of improving the corrosion resistance is remarkably exhibited. The Ni content is preferably 0.02% or more, more preferably 0.10% or more, still more preferably 0.20% or more. On the other hand, since Ni is an expensive element, the amount of Ni is 1.00% or less from the viewpoint of cost. More preferably, the Ni content is 0.80% or less.

(Sn: 0.30% or less)
Sn is an element that improves the corrosion resistance in an acidic environment, and may be contained, for example, in an amount of 0.01% or more. The Sn content is preferably 0.02% or more, and more preferably the Sn content is 0.05% or more. On the other hand, if Sn is contained excessively, the hot workability deteriorates, so the Sn content is made 0.30% or less. More preferably, the Sn content is 0.20% or less.

(Sb: 0.30% or less)
Sb is an element that improves acid resistance when contained together with Cu, and improves corrosion resistance in an acidic environment. Therefore, for example, Sb can be contained at 0.010% or more. The Sb content is preferably 0.050% or more, more preferably 0.080% or more. On the other hand, if the Sb content exceeds 0.30%, the hot workability deteriorates, so the Sb content is set to 0.30% or less. More preferably, the Sb content is 0.15% or less.

(As: 0.30% or less)
Although As is less effective than Sb and Sn, it is an element effective in improving corrosion resistance in an acidic environment, and may be contained in an amount of 0.01% or more. The As amount is preferably 0.02% or more, more preferably the As amount is 0.05% or more. On the other hand, if As is excessively contained, hot workability is deteriorated, so the As amount is set to 0.30% or less. More preferably, the As amount is 0.20% or less, further preferably 0.10% or less.

(Co: 0.30% or less)
Although Co is not so effective as Sb and Sn, it is an element that improves corrosion resistance in an acidic environment, and may be contained in an amount of 0.01% or more. The Co content is preferably 0.02% or more, and more preferably the Co content is 0.05% or more. On the other hand, if Co is contained excessively, the economical efficiency is lowered, so the Co content is made 0.30% or less. The Co content is more preferably 0.20% or less, and further preferably 0.10% or less.

Further, in order to improve mechanical properties and the like, it is preferable to contain one or more selected from the group consisting of Nb, V, Zr, Ta and B. In addition, since these elements are not necessarily essential in the steel material, the lower limit of the content is 0%.

(Nb: 0.10% or less)
Nb, like Ti, is an element that forms a nitride, and may be contained in an amount of 0.001% or more for the purpose of refining crystal grains and improving strength. The Nb content is preferably 0.003% or more, and more preferably 0.005%. On the other hand, if the content of Nb exceeds 0.10%, the mechanical properties may deteriorate, so the Nb content is set to 0.10% or less. The Nb content is more preferably 0.050% or less, still more preferably 0.030% or less, still more preferably 0.020% or less.

(V: 0.10% or less)
V is an element that forms a nitride, like Ti and Nb, and may be contained mainly for improving the strength by precipitation strengthening. In order to obtain the effect of precipitation strengthening, the V content is preferably 0.001% or more. More preferably, the V content is 0.003% or more. On the other hand, if V is contained in an amount of more than 0.10%, mechanical properties may deteriorate, so the V content is set to 0.10% or less. The V content is more preferably 0.050% or less, still more preferably 0.030% or less, still more preferably 0.020% or less.

(Zr: 0.050% or less)
Zr is an element that forms a nitride, like Ti, Nb, and V, and can be contained mainly for improving the strength by precipitation strengthening. In order to obtain the effect of precipitation strengthening, the Zr content is preferably 0.005% or more. More preferably, the Zr content is 0.010% or more. On the other hand, Zr is an expensive element, and if Zr of more than 0.050% is contained, mechanical properties may deteriorate, so the Zr content is set to 0.050% or less. The amount of Zr is more preferably 0.040% or less, further preferably 0.030% or less, still more preferably 0.020% or less.

(Ta: 0.050% or less)
Ta is an element that contributes to the improvement of strength, and may be contained in an amount of 0.001% or more, for example. Further, although the mechanism is not always clear, it was found that Ta also contributes to the improvement of corrosion resistance. Therefore, the Ta amount is preferably 0.005% or more. More preferably, the Ta amount is 0.010% or more. On the other hand, if Ta is contained excessively, the cost increases, so the Ta amount is made 0.050% or less. The Ta content is more preferably 0.040% or less, still more preferably 0.030% or less, still more preferably 0.020% or less.

(B: 0.010% or less)
B is an element that improves hardenability and strength. In order to obtain the strength improving effect by improving the hardenability, the B content is preferably 0.0003% or more. More preferably, the B content is 0.0005% or more. On the other hand, even if B is contained in an amount of more than 0.010%, the effect is saturated and the toughness of the base material and HAZ (heat affected zone) may decrease, so the B content is set to 0.010. % Or less. The B content is more preferably 0.0050% or less, still more preferably 0.0030% or less, still more preferably 0.0020% or less.

Further, for the purpose of controlling deoxidation and inclusions, it is preferable to contain one or more selected from the group consisting of Ca, Mg and REM (rare earth elements). In addition, since these elements are not necessarily essential in the steel material, the lower limit of the content is 0%.

(Ca: 0.010% or less)
Ca is an element mainly used for controlling the form of sulfide, and may be contained in an amount of 0.0005% or more for forming a fine oxide. The amount of Ca is preferably 0.001% or more, more preferably 0.002% or more. On the other hand, if Ca is contained in excess of 0.010%, mechanical properties may be impaired, so the amount of Ca is made 0.010% or less. More preferably, the Ca content is 0.005% or less.

(Mg: 0.010% or less)
In order to form a fine oxide, Mg may be contained in an amount of 0.0001% or more. The Mg content is preferably 0.0003% or more, more preferably 0.0005% or more. On the other hand, from the viewpoint of manufacturing cost, the amount of Mg is 0.010% or less. The Mg content is more preferably 0.005% or less, and further preferably 0.003% or less.

(REM: 0.010% or less)
REM (rare earth element) is an element mainly used for deoxidation, and in order to form a fine oxide, 0.0001% or more may be contained. The REM content is preferably 0.0003% or more, more preferably 0.0005% or more. On the other hand, from the viewpoint of manufacturing cost, the REM amount is set to 0.010% or less. The REM content is more preferably 0.005% or less, and further preferably 0.003% or less.

Specific examples of REM include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and one of these is used. Can be contained alone or in combination of two or more. In the case of containing two or more kinds of rare earth elements, for example, a mixed rare earth element (Misch metal) before separation and purification, or a mixture or compound of rare earth elements such as didymium alloy (alloy made of Nd and Pr) may be used. .. Further, when two or more kinds of rare earth elements are contained, the REM amount means the total amount of all rare earth elements.

The balance of the chemical composition of the steel material according to this embodiment is iron (Fe) and impurities. Impurity means a component that is mixed in by raw materials such as ores and scraps when industrially manufacturing steel products, and other factors, and is allowed as long as it does not adversely affect the steel products according to the present embodiment. To do. However, it is necessary to limit the upper limits of P, S, N, and O among the impurities as described above.

In the steel material according to this embodiment, it is necessary to control the relationship of specific elements in order to improve acid resistance, weldability and strength.

(BI: 33.0-81.0)
The acid corrosion resistance index BI is the ratio of the number of Cr atoms and the number of N atoms as shown in the following formula (1). That is, Cr/52 and N/14 are terms obtained by dividing the contents of Cr and N by the mass number of each element. The acid corrosion resistance index BI is important in order to suppress nitrides that are likely to become corrosion starting points on the surface of steel. Although Cr is effective in improving the acid resistance, if the content is excessive, it easily forms a nitride that becomes a starting point of corrosion. Therefore, in order to significantly improve the acid resistance, the acid corrosion index BI is It must be 81.0 or less. BI is preferably 64.0 or less, more preferably 55.0 or less, and further preferably 51.0 or less. On the other hand, if Cr is insufficient, the effect of improving the acid resistance becomes insufficient, so it is necessary to set the acid corrosion resistance index BI to 33.0 or more. The BI is preferably 36.0 or more, more preferably 40.0 or more.

BI=(Cr/52)/(N/14)... Formula (1)

(Ceq: 0.180 to 0.370)
Ceq is an index showing deterioration of weldability due to increase in hardness, and is set to 0.370 or less, preferably 0.360 or less, and more preferably 0.355 or less in order to secure weldability. on the other hand. If Ceq is too low, the mechanical properties will be insufficient, so 0.180 or higher, preferably 0.250 or higher, more preferably 0.330 or higher. Ceq is an index represented by the following formula (2).

Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15 Equation (2)

In the above formulas (1) and (2), Cr, N, C, Mn, Cu, Ni, Mo, and V indicate the content based on the mass% of each element, and if not contained, calculate as 0. To do.

Next, an example of the method for manufacturing a steel material according to this embodiment will be described. The steel material according to the present embodiment includes steel plates, shaped steels, steel pipes, etc. manufactured by hot rolling and, if necessary, cold rolling.

The steel material according to the present embodiment is manufactured by melting steel in a conventional method, adjusting the composition, hot rolling the steel piece obtained by casting, and further subjecting it to cold rolling as necessary. .. After hot rolling, it may be water-cooled as it is, or may be air-cooled and then reheated and quenched. After hot rolling, it may be wound into a coil. After hot rolling, cold rolling may be performed and further heat treatment may be performed.

When manufacturing a steel pipe, a steel plate may be formed into a tubular shape and welded, and a UO steel pipe, an electric resistance welded steel pipe, a forged steel pipe, a spiral steel pipe, or the like may be used. This embodiment also includes a seamless steel pipe manufactured by subjecting a steel slab to hot extrusion or piercing and rolling.

The steel material according to the present embodiment burns heavy oil, fossil fuel such as coal, gas fuel such as liquefied natural gas, general waste such as municipal waste, waste oil, plastic, industrial waste such as exhaust tire, and sewage sludge. It can be used for smoke exhaust equipment of boilers. Specifically, a flue duct of a smoke exhaust facility, a casing, a heat exchanger, a gas-gas heater consisting of two heat exchangers (heat recovery unit and reheater), a desulfurization device, an electric dust collector, and an induced blower. It can be preferably used as a basket material and a heat transfer element plate of a rotary regeneration air preheater.

Hereinafter, the technical contents of the present invention will be further described with reference to Examples of the present invention. The conditions in the following examples are one condition example adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. The present invention can employ various conditions without departing from the gist of the present invention.

Steels having the chemical compositions shown in Tables 1 and 2 were melted, the steel ingots were heated at 1150° C. for 2 hours, then hot-rolled and air-cooled to manufacture steel plates having a thickness of 20 mm. The balance of the chemical composition of each steel shown in Tables 1 and 2 is iron and impurities.

A test piece of 25×25×4 mm was taken from each of the obtained steel plates from the center of the plate thickness and finished by wet #400 polishing to obtain a test piece for corrosion resistance evaluation. The corrosion resistance was evaluated by a sulfuric acid immersion test and a hydrochloric acid immersion test. The sulfuric acid immersion test was conducted by immersing in a 50% sulfuric acid aqueous solution at 70° C. for 5 hours, and the hydrochloric acid immersion test was conducted by immersing in a 3.6% hydrochloric acid aqueous solution at 40° C. for 5 hours, and evaluated by corrosion weight loss.

With reference to Comparative Example AA, those having improved corrosion resistance of 30% or more (corrosion weight loss decreased to 70% or less of Comparative Example AA) were evaluated as ◯, and corrosion resistance of 50% or more improved (Corrosion weight loss of 50% of Comparative Example AA). (Reduced below) was marked with ⊚, and less than 30% (corrosion weight loss over 70% of Comparative Example AA) was marked with x.

Further, a y-type weld cracking test was conducted based on JIS Z 3158:2016. The test piece used was 20 mmt, and after welding at a current of 170 A from both sides, the presence or absence of cracks on the surface and cross section was confirmed after a lapse of a predetermined time. In addition, a tensile test piece was prepared according to JIS Z 2241:2011, and a tensile test was performed to determine the tensile strength. A sample having a tensile strength of 400 MPa or more was evaluated as ◯, and a sample having a tensile strength of less than 400 MPa was evaluated as x. The results are shown in Table 3.

As shown in Table 3, the test materials A to Z have components, BI values, and Ceq values within the ranges of the present invention, and have good corrosion resistance to hydrochloric acid and sulfuric acid, weldability, and strength. On the other hand, in the test materials AA to AG, any of the components, BI value, Ceq value, Cu, Cr or Ti is outside the scope of the present invention, and any one of corrosion resistance to hydrochloric acid and sulfuric acid, workability, weldability, and strength. Is falling.

The test material AA is a steel used as a standard for evaluation of the hydrochloric acid corrosion test and the sulfuric acid corrosion test, but since the BI value exceeds the upper limit defined by the present invention, hydrochloric acid and Corrosion resistance to sulfuric acid is reduced. Since the BI value of the test material AB is lower than the lower limit value specified in the present invention, the corrosion resistance to hydrochloric acid and sulfuric acid is lower than that of the steel of the present invention. The test materials AE and AF having lower limits of Cu and Cr defined in the present invention also have lower corrosion resistance to hydrochloric acid and sulfuric acid than the steel of the present invention. Further, the test material AG which exceeds the upper limit value of Ti defined in the present invention also has lower corrosion resistance to hydrochloric acid and sulfuric acid than the steel of the present invention.

The sample material AC has a small Ceq and insufficient strength. On the other hand, the weldability of the test material AD having a large Ceq is deteriorated.

The preferred embodiment of the present invention has been described above in detail, but the present invention is not limited to this example. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

The steel material of the present invention is used in boilers that burn heavy oil, fossil fuels such as coal, gas fuels such as liquefied natural gas, general waste such as municipal waste, waste oil, plastics, industrial waste such as exhaust tires, and sewage sludge. It can be used for smoke exhaust equipment. Specifically, a flue duct of a smoke exhaust facility, a casing, a heat exchanger, a gas-gas heater composed of two heat exchangers (heat recovery unit and reheater), a desulfurization device, an electric dust collector, and an induced blower. It can be preferably used as a basket material and a heat transfer element plate of a rotary regeneration air preheater.

Claims (4)

In mass %,
C: 0.010 to 0.20%,
Si: 0.040 to 0.70%,
Mn: 0.20 to 1.00%,
Cu: 0.10 to 1.00%,
Al: 0.005-0.10%,
Cr: 0.30 to 3.00%,
Ti: 0.01 to 0.15%,
P: 0.050% or less,
S: 0.030% or less,
N: 0.0015 to 0.0100%, and
O: 0.0035% or less,
Of which the balance is Fe and impurities, the BI calculated by the following formula (1) is 33.0 to 81.0, and the Ceq calculated by the following formula (2) is 0.180 to 0.370. Steel material characterized by being.
BI=(Cr/52)/(N/14)... Formula (1)
Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15 (2)
Here, in the formula, Cr, N, C, Mn, Cu, Ni, Mo, and V represent the contents based on the mass% of each element, and are 0 when they are not contained. Furthermore, in mass%,
Mo: 0.10% or less,
W: 0.10% or less,
Ni: 1.00% or less,
Sn: 0.30% or less,
Sb: 0.30% or less,
As: 0.30% or less, and
Co: 0.30% or less,
The steel material according to claim 1, containing one or more selected from the group consisting of: Furthermore, in mass%,
Nb: 0.10% or less,
V: 0.10% or less,
Zr: 0.050% or less,
Ta: 0.050% or less, and
B: 0.010% or less,
The steel material according to claim 1 or 2, containing one or more selected from the group consisting of: Furthermore, in mass%,
Ca: 0.010% or less,
Mg: 0.010% or less, and
REM: 0.010% or less,
The steel material according to any one of claims 1 to 3, containing one or more selected from the group consisting of: