WO2023223744A1

WO2023223744A1 - Sulfuric acid-resistant hot-rolled thick steel sheet and production method for same

Info

Publication number: WO2023223744A1
Application number: PCT/JP2023/015432
Authority: WO
Inventors: 貴之遠藤; 哲志田谷
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-05-20
Filing date: 2023-04-18
Publication date: 2023-11-23
Also published as: JPWO2023223744A1; JP7444338B1

Abstract

The purpose of the present invention is to provide a sulfuric acid-resistant hot-rolled thick steel sheet that has a sheet thickness of 6.0-14.0 mm and a tensile strength of not less than 490 MPa, and a production method therefor.　Provided is a sulfuric acid-resistant hot-rolled thick steel sheet that comprises a prescribed component composition, wherein: tensile strength in a tensile test at normal temperature is not less than 490 MPa; at a position at 1/4 of sheet thickness, the area ratio of ferrite is not less than 70%, the area ratio of bainite is 10-30%, and the combined area ratio of the remaining structure other than ferrite and bainite is less than 5%; and sheet thickness is 6.0-14.0 mm. <u />

Description

Hot-rolled thick sulfuric acid-resistant steel sheet and its manufacturing method

The present invention relates to a hot-rolled thick sulfuric acid-resistant steel sheet that has sulfuric acid-resistant properties and a tensile strength of 490 MPa or more in a tensile test at room temperature, and a method for producing the same.

Combustion exhaust gas emitted from thermal power plants, waste incineration plants, etc. mainly consists of water, sulfur oxides (sulfur dioxide, sulfur trioxide, etc.), hydrogen chloride, nitrogen oxides, carbon dioxide, nitrogen, oxygen, etc. has been done. When the ambient temperature in this exhaust gas becomes approximately 130° C. or lower, sulfur oxide and water react to produce and concentrate sulfuric acid. This sulfuric acid is highly corrosive, so dew point corrosion occurs due to sulfuric acid in smoke exhaust equipment such as flue ducts, casings, heat exchangers, desulfurization equipment, electrostatic precipitators, and induced blowers, to which combustion exhaust gas is introduced.

As a countermeasure for this, sulfuric acid dew point corrosion resistant steel (sulfuric acid resistant steel) as disclosed in Patent Document 1 and highly corrosion resistant stainless steel as disclosed in Patent Document 2 are used. However, since stainless steel inevitably contains large amounts of alloying elements such as Cu, Ni, and Mo, it has the problem of being extremely expensive.

In response to this demand, for example, Patent Document 3 describes an ultra-low carbon acid-resistant low-carbon steel that improves sulfuric acid resistance and hydrochloric acid resistance by adding Cu, Mo, etc. to steel with a C content of 0.03 mass% or less. Alloy steels are disclosed. Further, Patent Document 4 discloses a steel having excellent acid dew point corrosion resistance, which has secured sulfuric acid dew point corrosion resistance by adding Sn and/or Sb to a low sulfur steel.

In addition, Patent Document 5 proposes a sulfuric acid dew point corrosion resistant steel sheet that has good workability in addition to sulfuric acid resistance. The sulfuric acid-resistant steel sheet has an area ratio of 75% or more of the ferrite phase in the entire steel structure, an area ratio of the pearlite phase of less than 25%, and a total area ratio of the structures other than the ferrite phase and pearlite phase of less than 5%. It has a certain steel structure. In addition, the maximum Vickers hardness is 200 or less, and the average Vickers hardness is 80 or more.

Special Publication No. 43-014585 Japanese Patent Application Publication No. 07-316745 Special Publication No. 46-034772 Japanese Patent Application Publication No. 09-025536 International Publication No. 2018/038198

In recent years, garbage incineration plants have been designed to be large-scale facilities from the perspective of thermal efficiency. In large-scale plants, the ducts and chimneys are also larger than before, so thick steel plates are used in the design to support the weight.

Generally, sulfuric acid-resistant steel is used in dew point corrosive environments such as ducts of garbage incineration equipment. In a dew point corrosive environment, it is a relatively high temperature environment of 100 to 200 degrees Celsius, and the heavy equipment is large, so strength and plate thickness (6.0 to 14.0 mm) in hot conditions are required. Under certain circumstances.

The sulfuric acid-resistant steels and their manufacturing methods described in Patent Documents 1 to 5 have a plate thickness of up to about 6.0 mm, so they cannot support large equipment. Furthermore, since the usage environment is a dew point corrosive environment (temperature: 100 to 200°C), if the strength decreases in this temperature range, the locations where it can be used in plants will be limited. Note that "sulfuric acid-resistant steel" refers to steel that has corrosion resistance against sulfuric acid dew point corrosion.

The present invention has been made in view of the above-mentioned circumstances, and provides a hot-rolled material having sulfuric acid resistance and a tensile strength of 490 MPa or more even when the plate thickness is 6.0 mm or more and 14.0 mm or less. The purpose of the present invention is to provide a thick sulfuric acid-resistant steel plate and a method for manufacturing the same. In addition, in the present invention, "having sulfuric acid resistance" means the following cases. In other words, a test piece of plate thickness x 50 mm x 150 mm was immersed in a solution of sulfuric acid at a temperature of 70°C and a concentration of 50% by mass for 6 hours (hr), the weight change before and after immersion was measured, and the corrosion rate (g/ m ² hr), the corrosion rate is 500 (g/m ² hr) or less.

In order to achieve the above-mentioned problems, the present inventors conducted extensive studies on sulfuric acid-resistant steel with a plate thickness of 6.0 mm or more and 14.0 mm or less, and as a result, discovered the following.

In finish rolling, the total rolling reduction rate in the subsequent three passes including the final pass is 25% or more and 40% or less, and the cooling rate from immediately after finish rolling to the coiling temperature is within the range of 15°C/s or more and 25°C/s or less. And so. As a result, a bainite phase is generated in addition to a ferrite phase, making it possible to realize a hot-rolled thick sulfuric acid-resistant steel sheet having a tensile strength of 490 MPa or more even when the plate thickness is 6.0 mm or more and 14.0 mm or less.

The present invention has been made based on the above findings. That is, the gist of the present invention is as follows.
[1] In mass %, C: 0.010% or more and 0.170% or less, Si: 0.01% or more and 0.55% or less, Mn: 0.30% or more and 1.50% or less, P: 0. 05% or less, S: 0.03% or less, Cr: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 0.50% or less, Sb: 0.002% or more and 0.70% or less , Cu: 0.25% or more and 0.50% or less, and N: 0.001% or more and 0.010% or less, with the balance consisting of Fe and inevitable impurities, and the tensile test at room temperature. The tensile strength is 490 MPa or more, the area ratio of ferrite is 70% or more, the area ratio of bainite is 10% or more and 30% or less, and the total area ratio of the remaining structures other than ferrite and bainite at the 1/4 position of the plate thickness. is less than 5%, and the thickness is 6.0 mm or more and 14.0 mm or less.
[2] The component composition further includes, in mass %, W: 1.0% or less, Al: 0.5% or less, Ti: 0.1% or less, Sn: 0.5% or less, REM: 0. [1] ] Hot-rolled thick sulfuric acid-resistant steel sheet.
[3] The hot-rolled thick sulfuric acid-resistant steel sheet according to [1] or [2], which has a tensile strength of 490 MPa or more in a hot tensile test at 200°C.
[4] The method for producing a hot-rolled thick sulfuric acid-resistant steel sheet according to [1] or [2], wherein the molten steel having the above-mentioned composition is continuously cast into a slab, and the slab is heated at a temperature of 1000°C or higher and 1300°C or lower. The heated slab is finish-rolled at a finish-rolling temperature of 850°C or higher, and in the finish-rolling, the total rolling reduction ratio in the latter three passes including the final pass is 25% or more and 40% or less, and then , the hot rolled thick material is sulfuric acid resistant, wherein the average cooling rate from the finish rolling temperature to the coiling temperature is 15° C./s or more and 25° C./s or less, and the coiling temperature is 450° C. or more and 600° C. or less. Method of manufacturing steel plates.
[5] The method for producing a hot-rolled thick sulfuric acid-resistant steel sheet according to [3], wherein molten steel having the above-mentioned composition is continuously cast into a slab, the slab is heated to 1000°C or more and 1300°C or less, The heated slab is finish rolled at a finish rolling temperature of 850° C. or higher, and in the finish rolling, rolled at a total reduction rate of 25% or more and 40% or less in the latter three passes including the final pass, and then the finish rolling is continued. A method for producing a hot-rolled thick sulfuric acid-resistant steel sheet, wherein the average cooling rate from the temperature to the coiling temperature is 15°C/s or more and 25°C/s or less, and the coiling temperature is 450°C or more and 600°C or less. .

According to the present invention, it is possible to obtain a hot-rolled thick sulfuric acid-resistant steel plate that has sulfuric acid resistance and has a tensile strength of 490 MPa or more even when the plate thickness is 6.0 mm or more and 14.0 mm or less. Further, by applying the hot-rolled thick sulfuric acid-resistant steel sheet of the present invention to, for example, garbage incineration equipment, it can be made into heavy equipment, so it has extremely great industrial utility value.

Hereinafter, embodiments of the present invention will be described.

Hereinafter, the reasons for limiting the component composition in the present invention will be explained. In addition, the % display of each element means mass % unless otherwise specified.

C: 0.010% or more and 0.170% or less C is an element that increases the strength of steel, and in the present invention, it is contained in an amount of 0.010% or more in order to obtain the desired strength. On the other hand, a content exceeding 0.170% deteriorates sulfuric acid resistance and also deteriorates weldability and toughness of the weld heat affected zone. Therefore, in the present invention, C is set in a range of 0.010% or more and 0.170%. A preferable lower limit is 0.07% or more. A preferable upper limit is 0.120% or less, and a more preferable upper limit is 0.10% or less.

Si: 0.01% or more and 0.55% or less Si is a component contained as a deoxidizing agent, and also has the effect of increasing the strength of steel as a solid solution strengthening element, so in the present invention, Si is 0.01%. or more. However, a content exceeding 0.55% deteriorates the toughness of the steel. Furthermore, it becomes difficult to ensure the area ratio of bainite, making it difficult to ensure strength. Furthermore, it exists as a red scale on the surface and deteriorates the beauty of the surface. Therefore, Si is set in a range of 0.01% or more and 0.55% or less. Note that in an acidic environment, Si forms an anticorrosive film and contributes to improving sulfuric acid resistance. In order to obtain this effect of improving sulfuric acid resistance, the content is preferably 0.2% or more.

Mn: 0.30% or more and 1.50% or less Mn is an element that increases the strength of steel, and in the present invention, it needs to be contained at 0.30% or more in order to obtain the desired strength. On the other hand, Mn content exceeding 1.50% reduces the toughness and weldability of the steel. Therefore, in the present invention, Mn is set in a range of 0.30% or more and 1.50% or less. In addition, from the viewpoint of maintaining strength and suppressing the formation of inclusions that deteriorate sulfuric acid resistance, the upper limit is preferably 1.0% or less, and the more preferable upper limit is 0.7% or less.

P: 0.05% or less P is a harmful element that segregates at grain boundaries and reduces the toughness of steel. In particular, if it is contained in excess of 0.05%, the toughness will be markedly reduced. shall be 0.05% or less. Note that although it is desirable to reduce P as much as possible, reducing it to less than 0.005% causes an increase in manufacturing costs, so the lower limit of P is preferably about 0.005% or more.

S: 0.03% or less S is a harmful element that forms MnS, which is a nonmetallic inclusion, and becomes a starting point for local corrosion and reduces local corrosion resistance, and is preferably reduced. On the other hand, in the presence of Cu, it is an element that contributes to the formation of a Cu ₂ S film, suppresses corrosion reactions on the steel surface, and improves sulfuric acid dew point corrosion resistance. Therefore, in the present invention, in order to avoid a decrease in local corrosion resistance, the upper limit of S is set to 0.03% or less. Further, from the viewpoint of ensuring sulfuric acid resistance, it is preferable that the lower limit of S is 0.005 or more.

Cr: 0.01% or more and 1.00% or less Cr has little effect on improving sulfuric acid resistance at room temperature, but when the usage environment is at a high temperature of 120°C or higher, it maintains and improves sulfuric acid resistance. , which also has the effect of improving the mechanical strength of steel. However, if it is less than 0.01%, there is no effect, while if it exceeds 1.00%, the effect will be saturated and the cost will increase. Therefore, Cr is set in a range of 0.01% or more and 1.00% or less. In particular, when sulfuric acid resistance at high temperatures is required, the lower limit is preferably 0.5% or more.

Ni: 0.01% or more and 0.50% or less Ni has the function of suppressing deterioration of hot workability due to the inclusion of Cu and Sn. However, if it is less than 0.01%, it has no effect. On the other hand, if the content exceeds 0.50%, the effect will be saturated and the cost will increase. Therefore, the content should be in the range of 0.01% or more and 0.50% or less.

Sb: 0.002% or more and 0.70% or less Sb is an element that concentrates on the steel plate surface due to the combined effect with Cu and improves sulfuric acid resistance, and is an essential component. However, if it is less than 0.002%, the effect is small. On the other hand, if the content exceeds 0.70%, the effect will be saturated and the processability will deteriorate. Therefore, Sb is set in a range of 0.002% or more and 0.70% or less. A preferable lower limit is 0.02% or more. Moreover, a preferable upper limit is 0.30% or less.

Cu: 0.25% or more and 0.50% or less Cu is an element that improves sulfuric acid resistance in a corrosive environment caused by acid, and is an essential element for improving sulfuric acid resistance in the present invention. However, if the content is less than 0.25%, the effect is small. On the other hand, excessive content exceeding 0.50% causes deterioration of hot workability and leads to deterioration of surface properties. Therefore, Cu is set to 0.25% or more and 0.50% or less.

N: 0.001% or more and 0.010% or less N is an unavoidable impurity that deteriorates the toughness of steel in a solid solution state, and the lower the content, the better. From the viewpoint of ensuring toughness, a content of 0.010% or less is acceptable. On the other hand, it is technically difficult to completely remove N, and reducing it more than necessary will only increase steel manufacturing costs, so the lower limit of N is set to 0.001% or more.

In addition to the above-mentioned basic components, the hot-rolled thick sulfuric acid-resistant steel sheet of the present invention may contain the following components depending on the required level of sulfuric acid resistance.

W: 1.0% or less W is an element that improves sulfuric acid resistance. That is, WO ₄ ^2- ions formed in a corrosive environment exert a barrier effect against anions, and also form insoluble FeWO ₄ to suppress the progress of corrosion. Furthermore, the presence of W makes the rust layer (corrosion product) formed on the surface of the steel sheet in a sulfuric acid environment densified. However, it is known that the required strength and corrosion resistance properties can be obtained even without the presence of W, so no prescribed value is set as the lower limit. On the other hand, when the amount of W exceeds 1.0%, the above effects are saturated and the cost increases. Therefore, the amount of W is set in a range of 1.0% or less. A preferable upper limit is 0.300% or less. A preferable lower limit is 0.03% or more.

Al: 0.5% or less Al may be contained as a deoxidizing agent, but if it is contained in an amount exceeding 0.5%, the toughness of the steel will decrease. Therefore, if Al is contained, it should be in the range of 0.5% or less. A preferable lower limit is 0.01% or more. A more preferable lower limit is 0.02% or more. Further, a preferable upper limit is 0.06% or less.

Sn: 0.5% or less When contained in combination with W, Sn forms a dense rust layer and has the effect of suppressing corrosion in an acid environment. In particular, it is highly effective in maintaining and improving sulfuric acid resistance. On the other hand, a content exceeding 0.5% causes deterioration of hot workability and toughness. Therefore, when Sn is contained, it is preferably in a range of 0.5% or less. A preferable lower limit is 0.005% or more.

REM: 0.05% or less REM refers to rare earth components whose composition is Ce: 50 ± 5%, La: 25 ± 5%, Nd: 15 ± 5%, Pr: 10 ± 5%, and each Formation of sulfides and oxides in which elements are combined has the effect of improving sulfuric acid resistance. REM also has the effect of improving ductility and toughness by controlling the morphology of inclusions. On the other hand, if it exceeds 0.05%, the toughness deteriorates. Therefore, when REM is added, it is preferably within a range of 0.05% or less. From the viewpoint of ensuring sufficient sulfuric acid resistance, the preferable lower limit is 0.0005% or more, and the more preferable lower limit is more preferably 0.0050% or more.

Ca: 0.005% or less Ca, like REM, has the effect of improving ductility and toughness by controlling the morphology of inclusions, as well as improving sulfuric acid resistance. On the other hand, if it exceeds 0.005%, it causes a decrease in toughness, so when it is contained, it is preferably within the range of 0.005% or less. A preferable lower limit is 0.0002% or more.

Nb: 0.10% or less Nb is an element contained for the purpose of improving the strength of steel, but if it exceeds 0.10%, the toughness deteriorates, so if it is contained, it should be in the range of 0.10% or less. It is preferable to do so. A preferable lower limit is 0.005% or more.

V: 0.1% or less V is an element that is contained for the purpose of improving the strength of steel, but if it exceeds 0.1%, the toughness deteriorates, so if it is contained, it should be in the range of 0.1% or less. It is preferable to do so. A preferable lower limit is 0.005% or more.

Ti: 0.1% or less Ti is an element added for the purpose of improving the strength and toughness of steel, but if it exceeds 0.1%, the effect will be saturated, so if it is included, it should be 0.1% or less. It is preferable to set it as the range of. A preferable lower limit is 0.005% or more.

B: 0.001% or less B is an element contained for the purpose of improving the strength of steel, but if it exceeds 0.001%, the toughness deteriorates. Therefore, when it is contained, it is preferably 0.001% or less. A preferable lower limit is 0.0001% or more.

In the hot-rolled thick sulfuric acid-resistant steel sheet of the present invention, the remainder other than the above components is Fe and inevitable impurities. However, the inclusion of components other than those mentioned above is not prohibited as long as they do not impair the effects of the present invention.

Next, the steel structure of the high-strength steel plate of the present invention will be explained.

Ferrite area ratio: 70% or more Since sulfuric acid-resistant steel is designed for ducts and heat exchangers, it may be locally bent. Here, if the ferrite area ratio is less than 70%, cracks may occur during bending. Therefore, the area ratio of ferrite to the entire steel structure needs to be 70% or more. Note that the ferrite area ratio is allowed up to 90% as long as the strength can be maintained.

Bainite area ratio: 10% or more and 30% or less The bainite area ratio must not exceed 30% to avoid bending cracks during processing. At the same time, the bainite area ratio is set to 10% or more so that tensile strength can be maintained. It is preferable that the area ratio of bainite is 15% or more, since warm tensile strength can also be maintained.

Area ratio of residual structures other than ferrite and bainite: less than 5% The residual structures other than ferrite and bainite are mainly composed of pearlite and martensite. The area ratio needs to be less than 5%, which is a range that does not affect bending workability. Note that the lower limit of the area ratio of this remaining tissue may be 0%.

Tensile strength in a tensile test at room temperature: 490 MPa or more In order to apply sulfuric acid-resistant steel plates to large heavy equipment, in addition to the above sulfuric acid resistance and mechanical properties, the tensile strength at room temperature must be 490 MPa or more. is necessary. In addition, since cracks may occur during bending depending on the case, the upper limit is preferably 590 MPa or less. A more preferable upper limit is less than 560 MPa. Here, the tensile test at room temperature is measured by the method described in Examples.

Plate thickness: 6.0 mm or more and 14.0 mm or less In order to apply a sulfuric acid-resistant steel plate to large-scale heavy equipment, the plate thickness must be at least 6.0 mm or more. Further, if the length is preferably 7.0 mm or more, more preferably 8.0 mm or more, and most preferably 9.0 mm or more, there is an advantage that the locations of the equipment to which it can be applied are not limited. The upper limit of the plate thickness of the present invention is 14.0 mm or less. The upper limit of this plate thickness means the maximum thickness that can be wound into a coil shape in the later manufacturing method.

Tensile strength in hot tensile test at 200℃: 490MPa or more Since the usage environment may be a dew point corrosive environment (temperature 100 to 200℃), if the strength decreases in this temperature range, the plant equipment may The places where it can be used may be limited. In order to avoid this, the tensile strength in a hot tensile test at 200°C is preferably 490 MPa or more.

Next, the manufacturing method of the present invention will be explained.

A steel having the above-mentioned composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and is made into a slab by a conventional method such as a continuous casting method or an ingot-blowing method. Thereafter, it is heated and rolled, and after being water-cooled after rolling, the mechanical properties are measured. Note that the melting method and casting method are not limited to the methods described above.

Slab heating temperature: 1000° C. or higher and 1300° C. or lower If the heating temperature is lower than 1000° C., the carbide will not be completely dissolved and the solid solution carbon will be insufficient, resulting in a decrease in strength. On the other hand, when the heating temperature exceeds 1300°C, the structure becomes coarse and the toughness of the steel plate decreases. For this reason, the heating temperature of the steel material (slab) is set in a range of 1000°C or more and 1300°C or less. In addition, in order to maintain the finish rolling temperature, the lower limit is preferably 1100°C or higher, more preferably 1200°C or higher.

Finish rolling temperature: 850°C or higher Rolling is performed on a steel plate to suppress the generation of developed grains after rolling, and to make the grains fine and uniform, which contributes to strength. This effect becomes more pronounced as the finish rolling temperature increases. Therefore, the finish rolling temperature (finishing temperature) is 850°C or higher. In addition, if the finish rolling temperature is raised too much, the heating temperature will be raised, which may significantly reduce the rolling efficiency. Further, since powder scale may occur on the surface and peel off easily, the upper limit is preferably 1000°C or less.

In finish rolling, the total rolling reduction in the 3 subsequent passes including the final pass: 25% or more and 40% or less In finishing rolling, increasing the total rolling reduction in the latter stage increases the rolling strain, which causes bainite transformation even at relatively high temperatures. starts, the area ratio of bainite can be increased and the strength can be increased. On the other hand, as the total rolling reduction rate increases, the rolling load applied to each stand increases, so the upper limit was set to 40% or less. In addition, in consideration of the balance with the rolling load in the first stage of rolling finishing, the lower limit was set to 25% or more.

Average cooling rate from finish rolling temperature to coiling temperature: 15°C/s to 25°C/s Water cooling immediately after hot rolling (immediately after finish rolling). The purpose of this is to control the structure of the steel sheet so that the first phase is ferrite and the second phase is bainite. The cooling rate is in the range of 15°C/s or more and 25°C/s or less. If this average cooling rate cannot be maintained, the structure becomes ferrite-pearlite and the desired strength and hot tensile strength cannot be maintained. At the same time, increasing the cooling rate makes the crystal grains finer, making it possible to maintain hot tensile strength up to high temperatures.

Winding temperature: 450°C or more and 600°C or less The winding temperature is 450°C or more and 600°C or less. If the temperature is less than 450°C, the bainite structure ratio is high and workability deteriorates. If it exceeds 600°C, the desired bainite area ratio cannot be obtained and the desired strength cannot be maintained.

Examples of the present invention will be described below.

A slab having the component composition shown in Table 1, having a thickness of 250 mm, a width of 1300 mm, and a weight of 25 tons was heated to 1250°C in a hot rolling furnace. In addition, Tr in Table 1 means less than the lower limit of quantification when each element is analyzed by a conventional method.

Thereafter, the steel slab was heated to each heating temperature in a heating furnace of a hot rolling line, and rolled to a sheet bar thickness of 38 mm in a rough rolling process. Through a finish rolling process, each thickness shown in Table 2 was obtained, and the rolling was completed at each finishing temperature shown in Table 2. The total rolling reduction ratio of the latter three passes in the finish rolling including the final pass at that time is as shown in Table 2. Immediately after completion of finish rolling, water cooling was performed on a cooling table, and after cooling to each coiling temperature at the cooling rate shown in Table 2, the coils were entered into a winding device to manufacture HOT coils. After the target coil was air-cooled in a coil yard, samples were collected after cutting off the temperature-defected section.

(Tensile test at room temperature)
The tensile strength at room temperature was obtained by taking a JIS No. 1A plate specimen sample from 1/4 part of the steel plate in the width direction according to JIS Z2241:2022, and conducting a tensile test at room temperature. The results are summarized in Table 2. A pressure of 490 MPa or higher was determined to be an example compatible with the present invention. Note that room temperature here is 10° C. to 35° C. in accordance with JIS Z2241:2022.

(High temperature tensile test)
Regarding the tensile strength at a high temperature of 200°C, JIS Z 2241:2022 was obtained by soaking a test piece, which was taken from a point in contact with the place where the sample was taken in the tensile test at room temperature, and soaking it at 200°C. The test was conducted using the tensile test specified in 2007. The results are shown in Table 2. A suitable example is one having a pressure of 490 MPa or more.

(organizational observation)
Regarding the structure, after polishing the L cross section of the steel plate, 3vol. % nital, and 1/4 part of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) was observed in 10 fields at a magnification of 2000 times using an SEM. Ferrite has a gray structure, and bainite has a dark gray structure with many linear grain boundaries.

(Sulfuric acid resistance)
A test piece of board thickness x 50 mm x 150 mm was immersed in a sulfuric acid solution with a liquid temperature of 70°C and a concentration of 50% by mass for 6 hours (hr), and the weight change before and after immersion was measured, and the corrosion rate (g/m ² hr) was calculated. When the corrosion rate was 500 (g/m ² hr) or less, the sulfuric acid resistance was judged to be good, and when it exceeded 500 (g/m ² hr), it was judged as poor.

Claims

In mass%,
C: 0.010% or more and 0.170% or less,
Si: 0.01% or more and 0.55% or less,
Mn: 0.30% or more and 1.50% or less,
P: 0.05% or less,
S: 0.03% or less,
Cr: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 0.50% or less,
Sb: 0.002% or more and 0.70% or less,
Cu: 0.25% or more and 0.50% or less, and
N: Contains 0.001% or more and 0.010% or less,
having a component composition consisting of the remainder Fe and unavoidable impurities,
The tensile strength in a tensile test at room temperature is 490 MPa or more,
At the plate thickness 1/4 position,
The area ratio of ferrite is 70% or more,
The area ratio of bainite is 10% or more and 30% or less,
The total area ratio of residual structures other than ferrite and bainite is less than 5%,
A hot rolled thick sulfuric acid resistant steel plate having a thickness of 6.0 mm or more and 14.0 mm or less.
The component composition further includes, in mass%,
W: 1.0% or less, Al: 0.5% or less,
Ti: 0.1% or less, Sn: 0.5% or less,
REM: 0.05% or less, Ca: 0.005% or less,
Nb: 0.10% or less, V: 0.1% or less,
The hot-rolled thick sulfuric acid-resistant steel sheet according to claim 1, containing one or more selected from B: 0.001% or less.
The hot-rolled thick sulfuric acid-resistant steel sheet according to claim 1 or 2, which has a tensile strength of 490 MPa or more in a hot tensile test at 200°C.
A method for producing a hot-rolled thick sulfuric acid-resistant steel sheet according to claim 1 or 2, comprising:
Continuously casting molten steel having the above-mentioned composition into a slab,
heating the slab to a temperature of 1000°C or higher and 1300°C or lower;
Finish rolling the heated slab at a finish rolling temperature of 850°C or higher,
In the finish rolling, rolling at a total rolling reduction rate of 25% or more and 40% or less in the latter three passes including the final pass,
Subsequently, the hot rolled thick material is cooled at an average cooling rate of 15°C/s or more and 25°C/s or less from the finish rolling temperature to the coiling temperature, and the coiling temperature is 450°C or more and 600°C or less. Method for manufacturing sulfuric acid steel sheet.
A method for producing a hot-rolled thick sulfuric acid-resistant steel sheet according to claim 3,
Continuously casting molten steel having the above-mentioned composition into a slab,
heating the slab to a temperature of 1000°C or higher and 1300°C or lower;
Finish rolling the heated slab at a finish rolling temperature of 850°C or higher,
In the finish rolling, rolling at a total rolling reduction rate of 25% or more and 40% or less in the latter three passes including the final pass,
Subsequently, the hot rolled thick material is cooled at an average cooling rate of 15°C/s or more and 25°C/s or less from the finish rolling temperature to the coiling temperature, and the coiling temperature is 450°C or more and 600°C or less. Method for manufacturing sulfuric acid steel sheet.