JP7348463B2 - steel material - Google Patents

Description

The present invention relates to steel materials.

The smoke exhaust equipment and gasification melting furnaces of boilers that burn various fuels, wastes, sewage sludge, etc. are exposed to the combustion exhaust gas atmosphere and may be exposed to sulfuric acid dew point corrosion, hydrochloric acid dew point corrosion environments, or aqueous solutions of sulfuric acid or hydrochloric acid. It becomes a close environment. Steel materials that are exposed to such severe corrosive environments are required to have excellent acid resistance over a long period of time.

To address these problems, sulfuric acid/hydrochloric acid dew point corrosion resistant steels and highly corrosion resistant stainless steels have been proposed (see, for example, Patent Documents 1 to 5). Among these, Patent Documents 1 to 4 propose steel materials that have added Cu, Sb, Co, Cr, etc. and have excellent sulfuric acid dew point corrosion resistance. Furthermore, Patent Document 5 proposes highly corrosion-resistant stainless steel to which Cr, Ni, etc. are added.

Japanese Patent Application Publication No. 2001-164335 Japanese Patent Application Publication No. 2003-213367 Japanese Patent Application Publication No. 2007-239094 JP2012-57221A Japanese Patent Application Publication No. 7-316745

Steel materials containing Cu, Sb, Cr, etc. have excellent sulfuric acid dew point corrosion resistance, and exhibit excellent corrosion resistance in sulfuric acid corrosive environments such as boilers and exhaust gas chimneys of incineration facilities. However, further improvement in corrosion resistance is expected in order to extend the lifespan of boilers, incineration equipment, etc.

In addition, steel materials used in heat exchangers, gas-gas heaters, desulfurization equipment, electrostatic precipitators, and the like are required to have not only acid resistance but also weldability.

In view of these circumstances, it is an object of the present invention to provide a steel material with excellent manufacturability, long-term corrosion resistance in acid corrosion environments such as sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion, and excellent weldability. be.

The present inventors focused on oxides and nitrides, which are the starting point of corrosion, and conducted studies to improve the corrosion resistance of steel materials in acid corrosion environments. Furthermore, by setting Si/Al, which is the mass-based ratio between the Si content and the Al content, to a range of 7.4 to 15.0, the generation of oxides that become the starting point of corrosion is suppressed. I understand.

It was also found that when the content of Cr and N, which are effective in improving acid resistance, increases, nitrides containing Cr become corrosion starting points. Therefore, we further investigated Cr and N, and found that by setting the value of acid corrosion resistance index BI determined by the following formula (1) to 34.0 to 85.0, corrosion resistance that exceeds expectations can be expressed. Obtained.

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

Furthermore, focusing on Cu and S, we investigated ways to improve corrosion resistance by forming a CuS film on the surface of the steel sheet and suppressing inclusions that tend to become corrosion starting points. As a result, it was found that when Cu and S are contained simultaneously, CuS is generated on the surface of the steel sheet and the acid resistance is improved. However, since S produces inclusions that become corrosion starting points, it has an adverse effect on corrosion resistance. Therefore, we further investigated Cu and S, and found that by setting the value of the acid corrosion resistance index CI determined by the following equation (2) to 17.0 to 224.0, corrosion resistance that exceeds expectations is achieved. Obtained.

CI=(Cu/64)/(S/32)... Formula (2)

Furthermore, it has been found that excellent acid resistance and weldability can be obtained by setting Ceq determined by the following formula (3) to 0.180 to 0.390.

Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15... (3)

The present invention has been made based on such knowledge, and the gist thereof is as follows.
[1] In mass%,
C: 0.090-0.20%,
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%,
P: 0.050% or less,
S: 0.030% or less,
N: 0.002 to 0.010%, and
O: 0.0035% or less,
, with the remainder consisting of Fe and impurities, and Si/Al, which is the ratio based on mass of Si content and Al content, is
Si/Al: 7.4-15.0
satisfies, the BI determined by the following formula (1) is 34.0 to 85.0, the CI determined by the following formula (2) is 17.0 to 224.0, and the following formula (3 ) A steel material characterized by having a Ceq of 0.180 to 0.390.
BI=(Cr/52)/(N/14)...Equation (1)
CI=(Cu/64)/(S/32)... Formula (2)
Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15...Formula (3)
Here, in the formula, Cr, N, Cu, S, C, Mn, Ni, Mo, and V represent the content based on mass % of each element, and are 0 when not contained.
[2] 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 [1], characterized in that it contains one or more selected from the group consisting of:
[3] Furthermore, in mass%,
Ti: 0.15% or less,
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] 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 [1] to [3], characterized in that it contains one or more selected from the group consisting of:

According to the present invention, excellent corrosion resistance is exhibited in acid corrosive environments such as sulfuric acid corrosive environments such as boilers and exhaust gas chimneys of incinerators, and hydrochloric acid corrosive environments such as exhaust gas chimneys of garbage incinerators, etc., and productivity and weldability are improved. It will also be possible to provide superior steel materials. Therefore, the present invention has an extremely significant contribution to industry.

Below, preferred embodiments of the present invention will be described in detail. The inventors have found that in order to improve the acid resistance of acid-resistant steel containing Cu and Cr, it is effective to form a film on the steel surface and to suppress oxides and nitrides, which tend to become corrosion starting points. I understand.

First, the present inventors have found that it is effective to utilize Si, which has weaker oxidizing power than Al, in order to suppress the generation of oxides, and that the Si/Al ratio is important.

Moreover, when the content of Cr, which is effective in improving acid resistance, is excessive, it tends to form nitrides, which can become a starting point for corrosion. As a result of studies conducted by the present inventors, it was found that the balance between the contents of Cr and N is important, and that it is necessary to set the acid corrosion resistance index BI, which is determined by the following formula (1), within an appropriate range. .

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

Furthermore, Cu and S are necessary to form a CuS film on the surface of the steel material. However, since S tends to form inclusions and become a starting point for corrosion, the balance between Cu and S is important, and the value of the acid corrosion resistance index CI determined by the following formula (2) must be within an appropriate range. was found to be necessary.

CI=(Cu/64)/(S/32)... Formula (2)

Furthermore, it has been found that weldability can be ensured without impairing manufacturability by setting the Ceq value determined by the following formula (3) within an appropriate range. By adjusting these values to appropriate ranges, we succeeded in obtaining a steel material with excellent acid resistance, weldability, and strength.

Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15...Formula (3)

The components of the steel material according to this embodiment will be explained. Note that the notation of % means mass % unless otherwise specified.

(C: 0.090-0.20%)
C is an element that improves strength, and needs to be contained in an amount of 0.090% or more. Preferably, the amount of C is 0.010% or more, more preferably 0.011% or more. On the other hand, if the amount of C exceeds 0.20%, carbides increase and acid resistance deteriorates, so the amount of C is set to 0.20% or less. Preferably, the amount of C is 0.18% or less, more preferably 0.17% or less.

(Si: 0.040-0.70%)
Si is an element that contributes to deoxidation and improvement of strength, and in order to control the form of oxides, it is necessary to contain 0.040% or more. Preferably, the amount of Si is 0.050% or more, more preferably 0.10% or more. On the other hand, if Si exceeding 0.70% is contained, oxides increase and acid resistance is impaired, so the amount of Si is set to 0.70% or less. Preferably, the amount of Si is 0.50% or less, more preferably 0.45% or less.

(Mn: 0.20-1.00%)
Mn is an element that improves strength and toughness, and is contained in an amount of 0.20% or more. Preferably, the amount of Mn is 0.30% or more, more preferably 0.35% or more. On the other hand, if Mn is contained in an amount exceeding 1.00%, coarse MnS is generated and corrosion resistance and mechanical properties are deteriorated, so the Mn amount is set to 1.00% or less. Preferably, the amount of Mn is 0.90% or less, more preferably 0.85% or less.

(Cu: 0.10-1.00%)
Cu is an extremely important element that exhibits remarkable corrosion resistance against sulfuric acid and hydrochloric acid. In order to ensure corrosion resistance in an acidic environment, it is necessary to make the amount of Cu 0.10% or more. Preferably, the amount of Cu is 0.15% or more, more preferably 0.20% or more. On the other hand, if the Cu content exceeds 1.00%, hot workability will decrease and manufacturability will be impaired, so the Cu content is set to 1.00% or less. Preferably, the amount of Cu is 0.90% or less, more preferably 0.80% or less.

(Al: 0.005-0.10%)
Al is a deoxidizing agent and needs to be contained in an amount of 0.005% or more. Preferably, the amount of Al is 0.015% or more, more preferably, the amount of Al is 0.020% or more. On the other hand, if excessive Al is contained, acid resistance will be impaired due to an increase in inclusions, so the amount of Al is set to 0.10% or less. Preferably, the amount of Al is 0.050% or less. More preferably, the amount of Al is 0.040% or less.

(Cr: 0.30-3.00%)
Cr is an element that improves corrosion resistance like Cu. In particular, by containing Cr at the same time as Cu and Ti, excellent corrosion resistance is exhibited in an acidic environment at high temperature and high concentration. Therefore, from the viewpoint of ensuring corrosion resistance, it is necessary to contain 0.30% or more of Cr. Preferably, the amount of Cr is 0.40% or more, more preferably 0.50% or more. On the other hand, if excessive Cr is contained, acid resistance will be impaired due to an increase in nitrides, which become a starting point for corrosion, so the amount of Cr is set to 3.00% or less. Preferably, the Cr content is 1.60% or less, more preferably 1.00% or less.

(P: 0.050% or less)
P is an impurity and reduces the mechanical properties and manufacturability of the steel material, so the amount of P is set to 0.050% or less. Preferably, the amount of P is 0.045% or less, more preferably 0.040% or less. There is no lower limit to the amount of P, but from the viewpoint of cost, the amount of P may be 0.001% or more.

(S: 0.030% or less)
S is an impurity and reduces hot workability and mechanical properties of the steel material, so the amount of S is set to 0.030% or less. Preferably, the amount of S is 0.025% or less, more preferably 0.020% or less. Although the lower limit of the amount of S is not limited, the amount of S may be 0.001% or more since S improves corrosion resistance in an acidic environment when contained simultaneously with Cu. Preferably, the amount of S is 0.004% or more, more preferably 0.008% or more.

(N: 0.002-0.010%)
N is an element that forms nitrides, and when Cr is contained, corrosion resistance decreases if the amount of N is excessive, so the amount of N is set to 0.010% or less. Preferably, the amount of N is 0.008% or less, more preferably 0.006% or less. On the other hand, since fine nitrides are effective in improving mechanical properties and the like, the amount of N is set to 0.002% or more. Preferably, the amount of N is 0.003% or more. More preferably, the amount of N is 0.004% or more.

(O: 0.0035% or less)
O is an impurity and an element that generates oxides. In order to suppress the formation of coarse oxides that become a starting point for corrosion in an acidic environment, the amount of O is set to 0.0035% or less. More preferably, the amount of O is 0.0030% or less, and even more preferably 0.0025% or less. From the viewpoint of cost, the amount of O is preferably 0.0005% or more, more preferably 0.0010% or more.

Furthermore, in order to improve 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. Note that since these elements are not necessarily essential in steel materials, the lower limit of their content is 0%.

(Mo: 0.10% or less)
Mo is an element that improves corrosion resistance in an acidic environment by containing it simultaneously with Cu and Cr. In particular, in order to improve the corrosion resistance against hydrochloric acid, the amount of Mo can be set to 0.005% or more, for example. Preferably, the amount of Mo is 0.02% or more, more preferably 0.03% or more. On the other hand, since Mo is an expensive element, from the viewpoint of cost, the content of Mo is set to 0.10% or less. More preferably, the amount of Mo is 0.08% or less, and even more preferably 0.05% or less.

(W: 0.10% or less)
Like Mo, W is an element that improves corrosion resistance in an acidic environment by containing it simultaneously with Cu and Cr. In particular, in order to improve the corrosion resistance against hydrochloric acid, the amount of W may be set to 0.005% or more, for example. Preferably, the amount of W is 0.007% or more, more preferably 0.008% 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 amount of W is 0.08% or less, and even more 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 exhibits the effect of improving manufacturability. Cu has a great effect on improving corrosion resistance, but it tends to segregate, and when 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 included, in addition to suppressing cracking of slabs caused by Cu segregation, occurrence of local corrosion caused by segregation is also suppressed, so that the effect of improving corrosion resistance is significantly exhibited. Preferably, the Ni amount is 0.02% or more, more preferably 0.10% or more, and still more preferably 0.20% or more. On the other hand, since Ni is an expensive element, the amount of Ni is set to 1.00% or less from the viewpoint of cost. More preferably, the amount of Ni is 0.80% or less.

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

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

(As: 0.30% or less)
Although the effect of As is not as remarkable as that of Sb and Sn, it is an effective element for improving corrosion resistance in an acidic environment, and for example, it may be contained in an amount of 0.01% or more. Preferably, the amount of As is 0.02% or more, and more preferably the amount of As is 0.05% or more. On the other hand, since hot workability decreases if As is contained excessively, the amount of As is set to 0.30% or less. More preferably, the amount of As is 0.20% or less, and still more preferably 0.10% or less.

(Co: 0.30% or less)
Although the effect of Co is not as remarkable as that of Sb and Sn, it is an element that improves corrosion resistance in an acidic environment, and for example, it may be contained in an amount of 0.01% or more. Preferably the amount of Co is 0.02% or more, more preferably the amount of Co is 0.05% or more. On the other hand, if Co is contained excessively, the economical efficiency decreases, so the Co amount is set to 0.30% or less. More preferably, the amount of Co is 0.20% or less, and even more preferably 0.10% or less.

Furthermore, in order to improve mechanical properties etc., it is preferable to contain one or more selected from the group consisting of Ti, Nb, V, Zr, Ta and B. Note that since these elements are not necessarily essential in steel materials, the lower limit of their content is 0%.

(Ti: 0.15% or less)
Ti is an element that improves corrosion resistance. In particular, by containing Ti at the same time as Cu and Cr, excellent corrosion resistance is exhibited in an acidic environment. Therefore, in order to further improve corrosion resistance, the amount of Ti is preferably set to 0.01% or more. More preferably, it is 0.03% or more. On the other hand, if Ti is contained excessively, acid resistance may be impaired due to an increase in nitrides that become a starting point for corrosion, so the amount of Ti is set to 0.15% or less. More preferably, the amount of Ti is 0.07% or less, and even more preferably, the amount of Ti is 0.05% or less.

(Nb: 0.10% or less)
Like Ti, Nb is an element that forms nitrides, and may be contained in an amount of 0.001% or more, for example, for the purpose of making crystal grains finer and improving strength. Preferably, the amount of Nb is 0.005% or more, more preferably 0.010% or more. On the other hand, if more than 0.10% of Nb is contained, the mechanical properties may deteriorate, so the amount of Nb is set to 0.10% or less. More preferably, the amount of Nb is 0.050% or less, still more preferably 0.030% or less, even more preferably 0.020% or less.

(V: 0.10% or less)
Like Ti and Nb, V is an element that forms nitrides, and may be included mainly to improve strength through precipitation strengthening. In order to obtain the effect of precipitation strengthening, the amount of V is preferably 0.003% or more. More preferably, the V amount is 0.010% or more. On the other hand, if more than 0.10% of V is contained, the mechanical properties may deteriorate, so the amount of V is set to 0.10% or less. More preferably, the V amount is 0.050% or less, still more preferably 0.030% or less, even more preferably 0.020% or less.

(Zr: 0.050% or less)
Like Nb and V, Zr is an element that forms nitrides, and can be included mainly to improve strength by precipitation strengthening. In order to obtain the effect of precipitation strengthening, it is preferable that the amount of Zr is 0.005% or more. More preferably, the amount of Zr is 0.010% or more. On the other hand, Zr is an expensive element, and if more than 0.050% of Zr is contained, mechanical properties may deteriorate, so the amount of Zr is set to 0.050% or less. More preferably, the Zr content is 0.040% or less, still more preferably 0.030% or less, even more preferably 0.020% or less.

(Ta: 0.050% or less)
Ta is an element that contributes to improving strength, and may be contained in an amount of 0.001% or more, for example. Furthermore, although the mechanism is not necessarily clear, it has been found that Ta also contributes to improving corrosion resistance. Therefore, the amount of Ta is preferably 0.005% or more. More preferably, the amount of Ta is 0.010% or more. On the other hand, since the cost increases if Ta is contained excessively, the amount of Ta is set to 0.050% or less. More preferably, the amount of Ta is 0.040% or less, still more preferably 0.030% or less, even more preferably 0.020% or less.

(B: 0.010% or less)
B is an element that improves hardenability and increases strength. In order to obtain the effect of improving strength by improving hardenability, the amount of B is preferably 0.0003% or more. More preferably, the amount of B is 0.0005% or more. On the other hand, even if B exceeds 0.010%, the effect may be saturated and the toughness of the base metal and HAZ (heat affected zone) may decrease. % or less. More preferably, the amount of B is 0.0050% or less, still more preferably 0.0030% or less, even more preferably 0.0020% or less.

Furthermore, for the purpose of deoxidation and control of inclusions, it is preferable to contain one or more selected from the group consisting of Ca, Mg, and REM (rare earth elements). Note that since these elements are not necessarily essential in steel materials, the lower limit of their content is 0%.

(Ca: 0.010% or less)
Ca is an element mainly used to control the form of sulfides, and may be contained in an amount of, for example, 0.0005% or more in order to form fine oxides. Preferably, the amount of Ca is 0.001% or more, more preferably 0.002% or more. On the other hand, if Ca content exceeds 0.010%, mechanical properties may be impaired, so the amount of Ca is set to 0.010% or less. More preferably, the amount of Ca is 0.005% or less.

(Mg: 0.010% or less)
For example, Mg may be contained in an amount of 0.0001% or more in order to form a fine oxide. Preferably, the Mg amount is 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 set to 0.010% or less. More preferably, the Mg amount is 0.005% or less, and even more preferably 0.003% or less.

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

REM specifically includes Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and one of these may be contained alone or in combination of two or more. When containing two or more types of rare earth elements, for example, a mixture or compound of rare earth elements such as a mixed rare earth element (misch metal) before separation and purification or a didymium alloy (an alloy consisting of Nd and Pr) may be used. . Furthermore, in the case of containing two or more types of rare earth elements, the above REM amount means the total amount of all rare earth elements.

The remainder of the chemical composition of the steel material according to this embodiment is iron (Fe) and impurities. Impurities refer to components that are mixed in from raw materials such as ore, scrap, and other factors during the industrial production of steel materials, and are allowed within the range that does not adversely affect the steel materials according to this embodiment. do. However, among the impurities, it is necessary to limit the upper limit of P, S, N, and O as described above.

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

(Si/Al: 7.4-15.0)
The Si/Al ratio (mass ratio) is an important index for suppressing oxides that tend to become corrosion starting points on the steel surface. In order to suppress the formation of oxides, it is effective to utilize Si, which has a weaker oxidizing power than Al, and by increasing the Si/Al ratio to 7.4 or more, acid resistance is significantly improved. On the other hand, even if the Si/Al ratio exceeds 15.0, the effect is saturated, and as the amount of Al decreases, deoxidation becomes insufficient, and acid resistance may decrease due to oxides. Therefore, the Si/Al ratio is set to 7.4 to 15.0. Si/Al is preferably 8.0 or more, more preferably 8.5 or more. On the other hand, Si/Al is preferably 13.0 or less, more preferably 11.0 or less.

(BI:34.0-85.0)
The acid corrosion resistance index BI is the ratio of the number of Cr atoms to 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, respectively. The acid corrosion resistance index BI is important for suppressing nitrides that tend to become corrosion starting points on the steel surface. Although Cr is effective in improving acid resistance, if its content is excessive, it tends to form nitrides that become the starting point of corrosion. Therefore, in order to significantly improve acid resistance, the acid resistance corrosion index BI must be increased. It is necessary to set it to 85.0 or less. Preferably, BI is 75.0 or less, more preferably 70.0 or less, still more preferably 50.0 or less. On the other hand, if Cr is insufficient, the effect of improving acid resistance becomes insufficient, so it is necessary to set the acid resistance corrosion index BI to 34.0 or more. Preferably, BI is 37.0 or more, more preferably 40.0 or more.

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

(CI:17.0-224.0)
The acid corrosion resistance index CI is the ratio of the number of Cu atoms to the number of S atoms, as shown in the following formula (2). That is, Cu/64 and S/32 are terms obtained by dividing the content of Cu and S by the atomic weight of each element, respectively. The acid corrosion resistance index CI is an important index for forming a CuS film on the surface of a steel sheet while suppressing inclusions that tend to become a starting point for corrosion. Cu and S are necessary to form a film, but if S is included in excess, it will form inclusions that become a starting point for corrosion, so in order to significantly improve acid resistance, it is necessary to balance Cu and S. becomes important. When CI exceeds 224.0, it becomes difficult to form a CuS film on the surface of the steel material, and sufficient corrosion resistance cannot be obtained. On the other hand, if CI is less than 17.0, inclusions tend to become corrosion starting points, and corrosion resistance may deteriorate. Therefore, the CI value is set to 17.0 to 224.0. In order to improve acid resistance, the acid corrosion index CI is preferably 21.0 or more, more preferably 30.0 or more. Further, in order to further improve corrosion resistance, the acid corrosion resistance index CI is preferably 200.0 or less, more preferably 100.0 or less.

CI=(Cu/64)/(S/32)... Formula (2)

(Ceq: 0.180-0.390)
Ceq is an index indicating deterioration of weldability due to increase in hardness, and in order to ensure weldability, Ceq is set to 0.390 or less, preferably 0.380 or less, more preferably 0.375 or less. on the other hand. If Ceq is too low, the mechanical properties will be insufficient, so it is set to 0.180 or more, preferably 0.200 or more, more preferably 0.250 or more. Ceq is an index shown in the following formula (3).

Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15...Formula (3)

In the above formulas (1) to (3), Cr, N, Cu, S, C, Mn, Ni, Mo, and V indicate the content based on the mass% of each element, and if not contained, 0 Calculate as.

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

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

When manufacturing a steel pipe, a steel plate may be formed into a tubular shape and welded, and UO steel pipes, electric resistance welded steel pipes, forge welded steel pipes, spiral steel pipes, etc. can be made. This embodiment also includes a seamless steel pipe manufactured by subjecting a steel billet to hot extrusion or piercing rolling.

The steel material according to this embodiment burns fossil fuels such as heavy oil and coal, gas fuels such as liquefied natural gas, general wastes such as municipal waste, industrial wastes such as waste oil, plastics, and used tires, and sewage sludge. Can be used for boiler smoke exhaust equipment. Specifically, the flue duct, casing, heat exchanger of the smoke exhaust equipment, gas-gas heater consisting of two heat exchangers (heat recovery device and reheater), desulfurization device, electric precipitator, and induced blower. It can be suitably used for basket materials, heat transfer element plates, etc. of rotary regenerative air preheaters.

Hereinafter, the technical contents of the present invention will be further explained while giving examples of the present invention. Note that the conditions in the examples shown below are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. Further, the present invention can adopt various conditions without departing from the gist of the present invention.

Steel having the chemical composition shown in Tables 1 and 2 was melted, and the steel ingot was heated at 1150° C. for 2 hours, then hot rolled and air cooled to produce a steel plate with a thickness of 20 mm. Note that the remainder of the chemical composition of each steel shown in Tables 1 and 2 is iron and impurities.

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

Based on Comparative Example AA, corrosion resistance has improved by 30% or more (corrosion loss has decreased to 70% or less of Comparative Example AA). A case where the corrosion loss was less than 30% (the corrosion loss was more than 70% of Comparative Example AA) was marked as a *.

In addition, a y-type weld cracking test was conducted based on JIS Z 3158:2016. A 20 mm thick test piece was used, and after welding from both sides at a current of 170 A, the presence or absence of cracks on the surface and cross section was checked after a predetermined period of time had passed. Further, a tensile test piece was prepared in accordance with JIS Z 2241:2011, and a tensile test was performed to determine the tensile strength. Those with a tensile strength of 400 MPa or more were rated as ○, and those with a tensile strength of less than 400 MPa were rated as ×. The results are shown in Table 3.

As shown in Table 3, the composition, Si/Al, BI value, CI value, and Ceq value of test materials A to Z are within the range of the present invention, and the corrosion resistance to hydrochloric acid and sulfuric acid, weldability, and strength are all good. be. On the other hand, test materials AA to AK have any of the components, Si/Al, BI value, CI value, Ceq value, Cu, or Cr outside the scope of the present invention, and have poor corrosion resistance, weldability, and strength against hydrochloric acid and sulfuric acid. Something is declining.

Test material AA is a steel that was used as the standard for evaluation in the hydrochloric acid corrosion test and the sulfuric acid corrosion test, but since the Si/Al content is lower than the lower limit specified in the present invention, it is less susceptible to hydrochloric acid than the steel of the present invention. and corrosion resistance to sulfuric acid has decreased. Since the Si/Al ratio of sample material AB exceeds the upper limit defined by the present invention, the corrosion resistance against hydrochloric acid and sulfuric acid is lower than that of the steel of the present invention. Test material AI, which has a small amount of Cu, and test material AJ, which has a Cr content below the lower limit defined by the present invention, also have lower corrosion resistance to hydrochloric acid and sulfuric acid than the steel of the present invention.

Sample material AC has a BI value exceeding the upper limit defined by the present invention, and sample material AD has a BI value below the lower limit defined by the present invention, so compared to the steel of the present invention. Corrosion resistance against hydrochloric acid and sulfuric acid has decreased. The CI value of sample material AE is below the lower limit defined by the present invention, and the CI value of sample material AF exceeds the upper limit defined by the present invention, so compared to the steel of the present invention. Corrosion resistance to hydrochloric acid and sulfuric acid is reduced.

Sample material AH has a small Ceq and insufficient strength. On the other hand, the weldability of test material AG with a large Ceq is decreased. In addition, the test material AK has lower corrosion resistance because it exceeds the upper limit of Cr specified in the present invention, and has decreased weldability because it exceeds the upper limit of Ceq specified in the present invention. .

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to such examples. It is clear that a person with 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 stated in the claims. It is understood that these also naturally fall within the technical scope of the present invention.

The steel material of the present invention is suitable for use in 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. Can be used for smoke exhaust equipment. Specifically, the flue duct, casing, heat exchanger of the smoke exhaust equipment, gas-gas heater consisting of two heat exchangers (heat recovery device and reheater), desulfurization device, electric precipitator, and induced blower. It can be suitably used for basket materials, heat transfer element plates, etc. of rotary regenerative air preheaters.

Claims (4)

In mass%,
C: 0.090-0.20%,
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%,
P: 0.050% or less,
S: 0.030% or less,
N: 0.002 to 0.010%, and
O: 0.0035% or less,
, with the remainder consisting of Fe and impurities, and Si/Al, which is the ratio based on mass of Si content and Al content, is
Si/Al: 7.89 ~15.0
satisfies the following, BI determined by the following formula (1) is 34.0 to 85.0, CI determined by the following formula (2) is 21.4 to 95.0 , and the following formula (3 ) A steel material characterized by having a Ceq of 0.289 to 0.390.
BI=(Cr/52)/(N/14)...Equation (1)
CI=(Cu/64)/(S/32)... Formula (2)
Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15...Formula (3)
Here, in the formula, Cr, N, Cu, S, C, Mn, Ni, Mo, and V represent the content based on mass % of each element, and are 0 when 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%,
Ti: 0.15% or less,
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, characterized in that it contains 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, characterized in that it contains one or more selected from the group consisting of: