JP7442270B2 - Clad steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a clad steel plate and a method for manufacturing the same. In particular, it relates to a clad steel plate made of duplex stainless steel as a laminated material.

Duplex stainless steel contains large amounts of Cr, Mo, Ni, and N, and intermetallic compounds and nitrides are likely to precipitate therein. Therefore, a solution heat treatment is applied at 1000°C or higher to dissolve the precipitates into hot rolled steel. It was manufactured as. For this reason, when manufacturing clad steel sheets using duplex stainless steel as a laminated material, the base material must be carbon steel with a chemical composition that is designed to ensure mechanical properties through heat treatment at high temperatures of 1000°C or higher. has been proposed (Patent Document 1). Furthermore, a technique has also been proposed in which a duplex stainless clad steel sheet is manufactured by omitting heat treatment by controlling hot rolling conditions (Patent Document 2). Alternatively, a technique has been proposed in which precipitation is suppressed in the laminated material by reheating during hot rolling (Patent Document 3).

Furthermore, a clad steel plate of a dual-phase stainless steel with reduced alloying elements in which Ni, Mo, etc. are reduced has also been proposed (Patent Document 4). The precipitates that mainly affect the material quality of alloy element-saving duplex stainless steel are chromium nitrides.

Among duplex stainless steels, precipitates that affect duplex stainless steels with an Mo content in the range of 2.5 to 5.0% are intermetallic compounds represented by the sigma phase and chromium nitrides.
The sigma phase is an intermetallic compound with a high Cr content, and since Mo is an element that greatly accelerates the precipitation of the sigma phase, the precipitation start temperature of the σ phase becomes high in steel types with a high Mo content. When sigma phase precipitation occurs in duplex stainless steel, a chromium-deficient layer is formed around it, reducing the corrosion resistance of the steel. Similarly, even if chromium nitride precipitates, a chromium-deficient layer is formed around it, reducing the corrosion resistance of the steel. Among these, as a countermeasure against chromium nitride, a technique for controlling the precipitation temperature, hot rolling conditions, etc. has been proposed (Patent Document 4).

On the other hand, a clad steel plate is a steel material that can economically obtain composite properties by imparting corrosion resistance to the stainless steel used as a laminating material and imparting strength, toughness, and weldability to the base material. Clad steel plates are used in areas where stainless steel as a laminate and a base metal are structurally joined, and are generally thick and used in applications where strength and toughness are particularly required. Examples include seawater desalination equipment, tanks for transport ships, etc., and in many of these, austenitic stainless steel has conventionally been used as a laminating material. There is a growing trend of replacing the stainless steel used in these applications with inexpensive duplex stainless steel, and there is also a potential demand for even cheaper clad steel sheets using duplex stainless steel as the laminating material. When clad steel plates are used, the base material is responsible for strength and toughness, and the laminated material is responsible for corrosion resistance. In particular, the steel members of dams and water gates used in brackish water and seawater include sliding members such as doorstops and rails, and both sliding properties and corrosion resistance may be required. In addition, as the hulls of transport ships become larger, reducing weight by increasing the strength of steel materials is becoming important as it contributes to reducing transportation costs and environmental impact.

Japanese Patent Application Publication No. 7-292445 Japanese Patent Application Publication No. 2018-119186 Special Publication No. 6-36993 Japanese Patent Application Publication No. 2012-180567

ISIJ Vol. 58 (2018), p1181-1183

By the way, in the production of conventional duplex stainless steel hot-rolled steel sheets and clad steel sheets, solution heat treatment has become indispensable. This is because, as mentioned above, it is necessary to eliminate intermetallic compounds and nitrides that reduce corrosion resistance in duplex stainless steel. In particular, duplex stainless steel used for laminating clad steel sheets has the property that intermetallic compounds and nitrides are likely to precipitate in the hot working temperature range, and these precipitates are likely to precipitate after hot rolling. When dispersed in steel materials, corrosion resistance decreases. It is possible to eliminate precipitates in the laminated material by solution heat treatment, but solution heat treatment at 1000°C or higher will reduce the toughness of the base material, so it is preferable from the perspective of the use of clad steel sheets. There is no processing. In addition, in view of the demand for further cost reduction and the recent demand for reduction in energy usage, it is desired to omit the solution treatment to reduce the manufacturing cost of clad steel sheets and the energy required for manufacturing.

Further, there are various methods for increasing the strength of a common steel base material, but in rolling a solid material, TMCP (Thermo Mechanical Control Process), which combines rolling in a non-recrystallized region and cooling control, is mainly used. However, in ordinary carbon steel, the temperature in the non-recrystallized region is about 900°C at most, and in clad steel sheets made of steel types such as duplex stainless steel where the σ phase easily precipitates, the temperature at which the σ phase precipitates is Corrosion resistance decreases because the rolling temperatures overlap. Therefore, there has been a desire for a clad steel plate made of duplex stainless steel as a laminating material that achieves both the strength of the base material and the corrosion resistance of the laminated material.

An object of the present invention is to provide a clad steel plate that has both strength and corrosion resistance and can omit solution heat treatment, and a method for manufacturing the same.

In order to solve the above-mentioned problems, the present inventors discovered that intermetallic compounds were not present in the duplex stainless steel, which is the laminated material, in the step of joining the base material and the laminated material by hot rolling in the process of manufacturing clad steel sheets. It was thought that if nitrides do not precipitate, corrosion resistance will not be impaired even if the solution heat treatment, which is a post-process, is omitted. Therefore, we considered using duplex stainless steel, which can maintain high corrosion resistance even when the hot rolling temperature is lowered, for the clad steel plate lamination material. In order to obtain such duplex stainless steel, the chemical composition of the hot-rolled steel material without solution heat treatment, the hot working conditions, the amount of precipitation of sigma phase and chromium nitride, and the recovery and regeneration of the ferrite phase and austenite phase are required. We gained knowledge about the state of metallographic structure, including crystals, and the relationship between corrosion resistance of steel materials.
Based on this knowledge, the present invention was completed regarding a clad steel plate that uses duplex stainless steel as a laminating material and can omit solution heat treatment.
That is, the gist of the present invention is as follows.

[1]
A clad steel plate in which one side of a base steel plate is clad with a laminated material, the chemical composition of the laminated material being in mass%,
C: 0.03% or less,
Si: 0.05-1.0%,
Mn: 0.1 to 4.0%,
P: 0.05% or less,
S: 0.005% or less,
Cr: 20.0-28.0%,
Ni: 4.0 to 8.0%,
Mo: more than 2.5% to 5.0%,
Oxygen: 0.001-0.006%,
N: 0.08-0.30%,
W: 0-1.5%,
Co: 0-1.0%,
Cu: 0-3.0%,
V: 0-1.0%,
Nb: 0 to 0.2%,
Ta: 0-0.2%,
Ti: 0 to 0.03%,
Zr: 0 to 0.05%,
Hf: 0-0.10%,
B: 0 to 0.0050%,
Al: 0-0.05%,
Ca: 0-0.0050%,
Mg: 0 to 0.0050%,
REM: 0-0.10%,
Contains Sn: 0 to 0.10%,
The remainder is Fe and unavoidable impurities,
PREW determined by formula 4 is 35 or more and 45 or less,
The σ phase precipitation temperature Tσ (°C) determined by Formula 2 is 950°C or more and 1100°C or less,
The surface hardness of the laminated material is 1.3 times or less that of the solution heat treated state,
A clad steel sheet characterized in that the microstrain εα of the ferrite phase is equal to or less than ε _max determined by Equation 1.
ε _max =0.0035-Tσ*2.63*10 ^-6 (Formula 1)
Tσ=4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3)*(Mo-1.3)+5Si-6Mn-30N+569(℃) (Formula 2)
PREW=Cr+3.3(Mo+0.5W)+16N (Formula 4)
However, each element symbol in Formula 1, Formula 2, and Formula 4 indicates the content (mass%) of the element in the composite material, and if it is not contained, 0 is substituted.
[2]
The chemical composition of the steel plate serving as the base material is C: 0.03 to 0.15% in mass%,
Si: 1.0% or less,
Mn: 0.5-2.0%,
P: 0.05% or less,
S: 0.05% or less,
Nb: 0.005-0.10%,
Ti: 0.005 to 0.10% or less,
Cu: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
Al: 0-0.30%,
V: 0-0.40%,
Ca: 0-0.0050%,
B: 0 to 0.0030%,
REM: Contains 0 to 0.010%,
The clad steel plate according to [1], wherein the remainder consists of Fe and unavoidable impurities, and the yield strength of the base metal is 315 MPa or more.
[3]
Cladding is achieved by sandwich assembly in which two slabs in which the laminating material described in [1] or [2] is laminated to one side of a steel plate serving as a base material are stacked together so that the laminated material is placed on the inside to form an integrated slab. A method for manufacturing a steel plate, the method comprising:
Perform hot rolling to achieve a finishing temperature TF that satisfies equation 3,
A method for manufacturing a clad steel sheet, which comprises cooling from TF to 650°C at an average cooling rate of 1°C/s or more.
TF≧Tσ−50 (℃) (Formula 3)
Tσ=4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3)*(Mo-1.3)+5Si-6Mn-30N+569(℃) (Formula 2)
However, each element symbol in Formula 2 indicates the content (mass %) of the element in the composite material, and if it is not contained, 0 is substituted.
[4]
In the production of the sandwich assembled cladding,
The method for producing a clad steel sheet according to [3], wherein the cooling stop temperature of the cooling is less than 650°C.

According to the present invention, it is possible to obtain an inexpensive clad steel material that requires less energy to manufacture and has excellent surface corrosion resistance and strength characteristics. As a result, the contribution to industry and the environment is extremely large.

An embodiment of the present invention will be described below. Unless otherwise specified, "%" regarding components indicates mass % in steel.

As mentioned above, in order to solve the above-mentioned problems, the inventors of the present invention solved the problem by using duplex stainless steel, which is the laminated material, in the step of joining the base material and the laminated material by hot rolling in the process of manufacturing clad steel sheets. It was thought that if intermetallic compounds and nitrides do not precipitate during the process, corrosion resistance will not be impaired even if the subsequent solution heat treatment is omitted. Therefore, we considered using duplex stainless steel, which can maintain high corrosion resistance even when the hot rolling temperature is lowered, for the clad steel plate lamination material. In order to obtain such duplex stainless steel, the chemical composition of the hot-rolled steel material without solution heat treatment, the hot working conditions, the amount of precipitation of sigma phase and chromium nitride, and the recovery and regeneration of the ferrite phase and austenite phase are required. The following experiments were conducted to investigate the relationship between the state of the metal structure, including crystals, and the corrosion resistance of steel materials.

The sigma phase precipitation temperature Tσ is introduced as an index regarding the precipitation of the sigma phase, and using steel materials with different sigma phase precipitation temperatures Tσ, the heating temperature of hot rolling is set to 1150 to 1250°C, and the final finishing pass of hot rolling is The entrance temperature TF was set at 700 to 1000°C, and the accelerated cooling start temperature TC after hot rolling was set at 950°C or lower to obtain hot rolled steel materials with plate thicknesses of 10 mm to 35 mm. The strength and impact properties of the obtained hot-rolled steel materials and solution heat-treated steel materials were evaluated, as well as the metallographic structure and corrosion resistance of the surface layer and the center of the plate thickness.

Next, based on the knowledge of duplex stainless steel obtained from the above experiment, in order to obtain ordinary steel components and manufacturing conditions that can obtain high strength even under rolling conditions under which duplex stainless steel has good corrosion resistance, it is necessary to The following experiment was conducted based on the belief that it is necessary to increase the recrystallization temperature by adjusting the steel composition and to control cooling below the σ phase precipitation temperature range. Slabs were created by pasting laminated materials on the surfaces of common steel of various compositions, and two of the slabs were assembled using electron beam welding using a sandwich-type cladding material with laminated materials placed inside. Clad steel plates having a laminated material thickness of 3 mm and a total thickness of clad steel plates ranging from 20 mm to 50 mm were obtained by hot rolling, and their strength, impact properties, metallographic structure, and corrosion resistance were evaluated.
Through the above experiments, we have completed the present invention, which is a clad steel plate that uses duplex stainless steel as a bonding material and can omit solution heat treatment.

First, the chemical composition of the laminated material will be explained.

In order to ensure the corrosion resistance of stainless steel, the content of C is limited to 0.03% or less. If the content exceeds 0.03%, Cr carbide will be generated during hot rolling, resulting in deterioration of corrosion resistance and toughness.

Si is contained in an amount of 0.05% or more for deoxidation. The content is preferably 0.20% or more. However, if the content exceeds 1.00%, the toughness will deteriorate. Therefore, the upper limit is limited to 1.00%. The content is preferably 0.70% or less.

Mn has the effect of increasing the austenite phase and improving toughness, and is contained in an amount of 0.1% or more in order to ensure the toughness of the base metal and the welded part, and also to have the effect of lowering the nitride precipitation temperature TN. Preferably, the content is 0.2% or more. However, if the content exceeds 4.0%, corrosion resistance will deteriorate. Therefore, the upper limit is limited to 4.0%. The content is preferably 2.5% or less, more preferably 2.0% or less.

P is an element that is unavoidably mixed in from raw materials, and since it deteriorates hot workability and toughness, it is better to have less P, so it is limited to 0.05% or less. Preferably, the content is 0.03% or less.

S is also an element that is unavoidably mixed in from raw materials, and it also deteriorates hot workability, toughness, and corrosion resistance, so it is better to have less S than 0.005%. Preferably it is 0.003% or less.

Cr is contained in an amount of 20.0% or more to ensure basic corrosion resistance. Preferably, the content is 23.0% or more. On the other hand, if the content exceeds 28.0%, the ferrite phase fraction increases, impairing the toughness and corrosion resistance of the weld. Therefore, the Cr content was set to 28.0% or less. The content is preferably 26.0% or less.

Ni is contained in an amount of 4.0% or more in order to stabilize the austenite structure, improve corrosion resistance against various acids, and further improve toughness. By increasing the Ni content, it becomes possible to lower the nitride precipitation temperature. The content is preferably 4.5% or more. On the other hand, Ni is an expensive alloy, and in the steel of the present invention intended for alloy-saving duplex stainless steel, the content is set to 8.0% or less from the viewpoint of cost. The content is preferably 6.5% or less.

Mo is a very effective element that improves the corrosion resistance of stainless steel, and is contained in an amount exceeding 2.5%. Preferably, the content is 3.0% or more. In order to improve corrosion resistance, it is better to include a large amount of element, but since it is an element that promotes the precipitation of sigma phase, the upper limit of its content is preferably 5.0%. Preferably, the content is 4.5% or less.

N (nitrogen) is an effective element that dissolves in the austenite phase to improve strength and corrosion resistance. For this purpose, the content is made to be 0.08% or more. The content is preferably 0.15% or more. The solid solubility limit increases with the Cr content, but in the steel of the present invention, if the content exceeds 0.30%, the nitride precipitation temperature TN increases and Cr nitrides precipitate during hot rolling. Since this impairs toughness and corrosion resistance, the upper limit of the content was set at 0.30%. Preferably, the content is 0.22% or less.

O (oxygen) is an unavoidable impurity and an element constituting an oxide that is a representative nonmetallic inclusion, and excessive content impairs toughness. Further, the generation of coarse clustered oxides causes surface flaws. Therefore, the upper limit was set at 0.006%. On the other hand, excessive deoxidation increases costs, so the lower limit was set at 0.001%.

The remainder is Fe and unavoidable impurities. Unavoidable impurities are impurities that are unintentionally mixed into the steel during the manufacturing process and remain without being completely removed.
Furthermore, it contains one or more of the following elements (W, Co, Cu, V, Nb, Ta, Ti, Zr, Hf, B, Al, Ca, Mg, REM, Sn) in place of Fe. You may. Since these elements do not need to be contained, the content range also includes 0%.

Like Mo, W is an element that improves the corrosion resistance of stainless steel, and may be included. On the other hand, since it is an expensive element, its content is preferably 1.50% or less. Preferably it is 1.00% or less. When contained, the preferable content is 0.05% or more.

Co is an element effective for increasing the toughness and corrosion resistance of steel, and is selectively contained. If the content exceeds 1.00%, the effect commensurate with the cost will not be exhibited because it is an expensive element, so the upper limit was set at 1.00%. When contained, the preferable content is such that the lower limit is 0.03% and the upper limit is 0.50%.

Cu is an element that additionally increases the acid corrosion resistance of stainless steel and has the effect of improving toughness, so it can be included. If the content exceeds 3.0%, εCu will precipitate beyond the solid solubility during hot rolling and cause embrittlement, so the upper limit was set at 3.0%. When containing Cu, the lower limit is preferably 0.3% and the upper limit is 2.0%.

V, Nb, and Ta are elements that form carbides and nitrides in steel, and can be contained in trace amounts to additionally improve corrosion resistance. On the other hand, if a large amount of V, Nb, or Ta is contained in the steel of the present invention containing N, carbonitrides will be generated and the toughness will be impaired, so the upper limit is regulated. Taking into consideration the magnitude of the effect of V, Nb, and Ta and the alloy cost, the respective upper limits were determined to be 1.00%, 0.20%, and 0.20%. The preferred ranges for inclusion are lower limits of 0.03%, 0.01%, and 0.01%, and upper limits of 0.20%, 0.10%, and 0.10%, respectively.

Ti, Zr, and Hf are elements that form nitrides and carbides in steel, and can be included in trace amounts for the purpose of refining the crystal structure. Since the ability of Ti, Zr, and Hf to form nitrides is very strong, large amounts of Ti, Zr, and Hf in the steel of the present invention containing N will generate coarse nitrides and impair toughness. The upper limit is regulated. Taking into consideration the magnitude of the effect of Ti, Zr, and Hf and the alloy cost, the respective upper limits were determined to be 0.030%, 0.050%, and 0.10%. The preferred ranges for inclusion are 0.003%, 0.005%, and 0.01% as lower limits, and 0.020%, 0.030%, and 0.05% as upper limits, respectively.

B is an element that forms nitrides and carbides in steel, and has a low solid solubility in steel and is an element that tends to segregate at grain boundaries. Its effect is to improve hot workability. On the other hand, excessive content forms coarse nitrides, which impairs the toughness of steel. For this reason, the upper limit of the content was set at 0.0050%. When included, the preferable content has a lower limit of 0.0005% and an upper limit of 0.0035%.

Al can be included to deoxidize the steel. Further, since it is an element that forms nitrides, and excessive content forms coarse nitrides and impairs toughness, the upper limit is set at 0.050%. When included, the preferable content has a lower limit of 0.003% and an upper limit of 0.030%.

Ca and Mg can be contained in order to improve the hot workability of steel. Since excessive content will adversely affect hot workability, the upper limit was set at 0.0050%. A suitable content range has a lower limit of 0.0005% and an upper limit of 0.0035%.

REM can be included to improve the hot workability of the steel. Since excessive content will adversely affect hot workability, the upper limit was set at 0.10%. A suitable content range has a lower limit of 0.01% and an upper limit of 0.08%. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

Sn is an element that additionally increases the acid corrosion resistance of steel, and can be included for this purpose. On the other hand, since excessive content impairs the hot workability of steel, the upper limit was set at 0.10%. When included, the preferable content has a lower limit of 0.01% and an upper limit of 0.08%.

PREW is an index for the pitting corrosion resistance of stainless steel, and is calculated by Formula 4 using the contents (%) of alloying elements Cr, Mo, W, and N. If the PREW of duplex stainless steel is less than 35, it will not be able to exhibit corrosion resistance in brackish water or seawater environments, and if it contains alloying elements exceeding 45, the cost will increase, so the PREW range is set to 35 or more. It is best to set it to 45 or less.
PREW=Cr+3.3(Mo+0.5W)+16N (Formula 4)
However, each element symbol in Formula 4 indicates the content (mass%) of the element, and if it is not contained, 0 is substituted.

σ phase precipitation temperature: Tσ (°C) is an index determined by the chemical composition of the composite material, and represents the temperature at which the sigma phase begins to precipitate in equilibrium, and is a value determined by thermodynamic calculation for the equilibrium phase diagram of the metal material. be. Thermodynamic calculations can be performed using commercially available software called Thermocalc@ and a thermodynamic database (FE-DATA version 6, etc.). This calculation was performed for various duplex stainless steels. The sigma phase is an intermetallic compound containing Fe, Cr, Mo, and W as main elements, and Mo and W promote precipitation in duplex stainless steel in which the Fe and Cr contents are within a certain numerical range. Cr is a main element that precipitates the sigma phase, and Tσ changes depending on the amount of Cr. For this reason, the present inventors created a formula (Formula 2) for determining the value of Tσ that is applicable within the composition range of the invention steel.

In the production of clad steel sheets by hot rolling, which is the object of the present invention, the lower limit of Tσ (Formula 2) is set to 950°C, for the purpose of controlling the precipitation of the sigma phase during hot rolling and obtaining the desired clad steel sheets. The upper limit was set to 1100°C. When Tσ is less than 950°C, precipitation of sigma phase is suppressed, but the steel has low Mo and Cr contents, making it difficult to obtain the desired corrosion resistance.On the other hand, when Tσ exceeds 1100°C, the surface layer of the clad steel sheet Since it becomes difficult to suppress the precipitation of sigma phase, the above numerical range was determined. Preferably, Tσ is 1050°C or less.
Tσ=4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3)*(Mo-1.3)+5Si-6Mn-30N+569(℃) (Formula 2)
However, each element symbol in Formula 2 indicates the content (mass%) of the element, and if it is not contained, 0 is substituted.

The surface hardness of the laminated material is a property that influences the surface properties of the clad steel plate targeted by the present invention, and is preferably high. The steel sheet of the present invention is a product that is cooled after hot rolling and is applied without solution heat treatment, and is characterized by having higher hardness than solution heat treated material. For this reason, it was specified that the surface hardness of the laminated material should be 1.3 times or less that of the solution heat treated state. Although it is preferable for this hardness to be large, the large strain introduced and left during hot rolling promotes the precipitation of intermetallic compounds, impairing the corrosion resistance of the clad steel plate surface, so this upper limit should be increased by 1.3 times. It was determined that Here, the state of solution heat treatment refers to the surface hardness when the same laminated material is solution heat treated.

To measure this hardness magnification, the Vickers hardness is measured by polishing the target surface layer (approximately 0.1 to 0.5 mm below the skin) position (sample A) of the composite material made of duplex stainless steel. At the same time, the hardness of this sample was similarly measured for the material (sample B) subjected to solution heat treatment at 1050°C, and the surface hardness of the composite material was the ratio of the solution heat treated state (hardness of sample A/sample The hardness of B) is quantified. Preferably, the surface hardness is 1.05 to 1.30 times the surface hardness after solution heat treatment. Although sample A and sample B are explained as being the same sample, they do not necessarily have to be the same, and if they are made of the same steel material, they can be separate samples (for example, separate samples cut from steel material). It's okay.

The microstrain of the ferrite phase: εα is an important characteristic value that defines the laminated material of the steel sheet of the present invention. Duplex stainless steel is composed of a ferrite phase and an austenite phase, but the microstructural change behavior during hot working is significantly different. The strain introduced by hot working becomes dislocations inside the material, and these dislocations decrease through the process of recovery and recrystallization. The rate of decrease in dislocation density in the austenite phase is low, and the rate of decrease in dislocation density in the ferrite phase is high. Based on such general knowledge, the present inventors created target duplex stainless steel clad steel plates under various hot rolling conditions and observed the metallographic structure of the surface layer of the laminated material. conducted research.

As a result, the present inventors found that if duplex stainless steel is not subjected to appropriate hot rolling, the reduction in dislocation density in the ferrite phase will be suppressed, and strain will remain in the clad steel sheet laminated material. During the subsequent cooling process, intermetallic compounds that are difficult to observe with an optical microscope begin to precipitate in the ferrite phase, resulting in a corresponding decrease in corrosion resistance and an increase in microstrain in the ferrite phase during this precipitation process. I found out that. By organizing this knowledge, desired characteristics can be obtained by controlling the microstrain of the ferrite phase in the surface layer of the laminated material to be smaller than ε _max determined by Equation 1.
ε _max =0.0035-Tσ×2.63×10-6 (Formula 1)
However, each element symbol in Formula 1 indicates the content (mass%) of the element, and if it is not contained, 0 is substituted.

Here, the value of microstrain is a value that can be determined by an X-ray diffraction method, and the unit is dimensionless. The specific measurement method is as follows. At the time of sample preparation, the position (sample A) of the target surface layer (area 0.1 mm or more and 0.5 mm or less below the surface in the thickness direction) of the composite material made of duplex stainless steel was machined and electrolytically polished. After finishing the product to dimensions of approximately 3 mm thick x 20 mm wide x 20 mm long so that no distortion remains, X-ray diffraction was performed using a radiation source such as CuKα rays, and the diffraction intensity profiles of each diffraction surface of the ferrite phase and austenite phase were determined. Measure A. As a comparative material, the above laminated material was subjected to solution heat treatment at 1050°C to remove the strain introduced by hot working (sample B), and a similar sample for X-ray diffraction was prepared and analyzed for X-ray diffraction. and measure a distortion-free diffraction intensity profile B. In a sample with large residual strain, the diffraction intensity profile has a spread (half-width) with respect to the diffraction angle 2θ, and by comparing A and B, the amount of increase in half-width can be calculated for each diffraction surface and numerically processed. The microstrains of the ferrite and austenite phases are quantified. There is a certain relationship between the microstrain obtained in this way and the dislocation density inside the material, and for the relationship between the two regarding the ferrite phase, refer to Non-Patent Document 1.

Next, the chemical composition of the steel plate serving as the base material will be explained. In the clad steel plate according to the present invention, the chemical composition of the steel plate serving as the base material is not particularly limited, but it is desirable that the chemical composition is as described below.

C is an element that improves the strength of steel, and by containing it in an amount of 0.03% or more, sufficient strength is developed. However, if it exceeds 0.15%, weldability and toughness will deteriorate. Therefore, the amount of C is set to 0.03 to 0.15%. Preferably, the content is 0.10% or less.

Si is an element that is effective in deoxidizing and improves the strength of steel. However, if the content exceeds 1.0%, the surface quality and toughness of the steel will deteriorate. Therefore, the Si content is set to 1.0% or less. The content is preferably 0.5% or less.

Mn is an element that increases the strength of steel, and its effect is manifested when it is contained in an amount of 0.5% or more. However, if it exceeds 2.0%, weldability will be impaired and the alloy cost will also increase. Therefore, the Mn content is set to 0.5 to 2.0%. Preferably, the content is 0.9% or more, or 1.6% or less.

P is an unavoidable impurity in steel, and when the content exceeds 0.05%, toughness deteriorates. Therefore, the P content is set to 0.05% or less. The content is preferably 0.02% or less.

S is an unavoidable impurity in steel, and when the content exceeds 0.05%, toughness deteriorates. Therefore, the S content is set to 0.05% or less. The content is preferably 0.01% or less.

Nb is an element that increases the recrystallization temperature, and its effect is manifested when the content is 0.005% or more. However, if it exceeds 0.100%, weldability will be impaired and the alloy cost will also increase. Therefore, the amount of Nb is set to 0.005 to 0.100%. The content is preferably 0.010% or more, or 0.070% or less. More preferably, the content is 0.030% or more, or 0.050% or less.

Ti is an element that refines crystal grains and increases strength, and this effect is achieved when the content is 0.005% or more. However, if it exceeds 0.1%, weldability will be impaired and the alloy cost will also increase. Therefore, the Ti content is set to 0.005 to 0.1%. The content is preferably 0.010% or more, or 0.020% or less.

If the yield strength of the base metal is 315 MPa or more, it is higher in strength than ordinary mild steel and can be made thinner. The higher the yield strength, the thinner the plate thickness of the structural member can be, so no upper limit is set. Therefore, the yield strength of the base material was set to be 315 MPa or more. More preferably it is 355 MPa or more.

The remainder is Fe and unavoidable impurities. Unavoidable impurities are impurities that are unintentionally mixed into the steel during the manufacturing process and remain without being completely removed.
Furthermore, in place of Fe, one or more of the following elements (Cu, Cr, Ni, Mo, Al, V, Ca, B, REM) may be contained. Since these elements do not need to be contained, the content range also includes 0%.

Cu is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if the content exceeds 0.50%, weldability and toughness will deteriorate. Therefore, when containing Cu, the Cu content is 0.50% or less. Preferably it is 0.30% or less.

Cr is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if the content exceeds 0.50%, weldability and toughness will deteriorate. Therefore, when containing Cr, the Cr content should be 0.50% or less. The content is preferably 0.30% or less.

Ni is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if the content exceeds 1.0%, weldability and toughness will deteriorate. Therefore, when Ni is contained, the amount of Ni is 1.0% or less. The content is preferably 0.5% or less.

Mo is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if the content exceeds 0.50%, weldability and toughness will deteriorate. Therefore, when Mo is contained, the Mo content is set to 0.50% or less. The content is preferably 0.30% or less.

Al is an element that is effective in deoxidizing steel. However, if the content exceeds 0.30%, the toughness of the weld zone will deteriorate. Therefore, when containing Al, the Al content should be 0.30% or less. The content is preferably 0.10% or less.

V increases the strength of steel by forming carbonitrides. However, if the content exceeds 0.40%, weldability and toughness will deteriorate. Therefore, when containing V, the V content should be 0.40% or less. The content is preferably 0.20% or less.

Ca refines the structure of the weld heat affected zone and improves toughness. However, if the content exceeds 0.0050%, coarse inclusions will be formed and the toughness will deteriorate. Therefore, when Ca is contained, the Ca content should be 0.0050% or less. The content is preferably 0.0030% or less.

B is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if the content exceeds 0.0030%, weldability and toughness will deteriorate. Therefore, when B is contained, the B content should be 0.0030% or less. The content is preferably 0.002% or less.

REM refines the structure of the weld heat affected zone and improves toughness. However, if the content exceeds 0.010%, coarse inclusions will be formed and the toughness will deteriorate. Therefore, when REM is contained, the REM content is preferably 0.010% or less. The content is preferably 0.005% or less. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

The manufacturing method will be explained.

Sandwich assembly defined in the present invention refers to two slabs made by laminating a laminated material having the above chemical composition on one side of a steel plate as a base material, and stacking them together so that the laminated material is placed on the inside. This method is to assemble it as a slab. Although the chemical composition of the steel plate serving as the base material is not particularly limited, it is preferable that the steel plate has the above-mentioned chemical composition. In this way, the slab consisting of the base material, laminated material, laminated material, and base material is heated in accordance with a conventional method, and after being extracted from the heating furnace, hot rolling is performed to produce a rolled clad steel plate.

The finishing temperature TF is defined as the steel surface temperature at the entrance of the final pass of hot rolling. While conducting the above-mentioned experiments, the present inventors conducted hot rolling in sandwich assembly rolling with the relationship between Tσ and TF shown in equation 3, and determined that the TF of the clad steel plate after hot rolling was ~650°C. The average cooling rate is 1° C./s or more. Furthermore, it is preferable that the cooling stop temperature is less than 650°C. Preferably, the cooling stop temperature is set to less than 600°C. In this case, it is preferable that the average cooling rate from hot rolling to 600° C. be 1° C./s or more.
TF≧Tσ−50 (℃) (Formula 3)
Tσ is the σ phase precipitation temperature obtained from Equation 2 above.

The reason for specifying the finishing temperature TF to be Tσ - 50°C or higher is that if rolling is performed at a temperature that is supercooled below the sigma phase precipitation temperature, the sigma phase will precipitate depending on the degree of supercooling and the strain introduced during rolling. However, even if the cooling rate after rolling is increased, the corrosion resistance may deteriorate. In sandwich rolling, since the stainless steel is located at the center of the plate thickness, the additional shear strain on the surface layer and the effect of heat removal by the rolls are reduced, and hot rolling is performed in a state where the temperature of the laminated material is higher than the surface temperature of the steel material. As a result of experimental evaluation of this effect, it was found that when the finishing temperature TF is set to Tσ-50°C or higher, corrosion resistance does not deteriorate.

The reason why the average cooling rate from TF to 650°C is specified as 1°C/s or more is because the temperature range in which intermetallic compounds such as sigma phase precipitates in the metal structure after hot rolling of the steel of the present invention is Tσ ~ 700°C. This is because the cooling rate in this temperature range needs to be increased since it exists at about .degree. In order to achieve a cooling rate of 1° C./s or more for a clad steel plate hot rolled with a large plate thickness using the sandwich assembly method, it is preferable to perform water cooling after hot rolling. If the plate thickness is small, air cooling or forced air cooling may be used.

The reason why the cooling stop temperature is set to be less than 650°C is that if cooling is stopped at 650°C or higher, the structure of ordinary steel becomes coarse and sufficient yield strength cannot be obtained. More preferably it is less than 600°C.

The clad steel plate of the present invention is obtained by setting the hot rolling temperature to a specific temperature or higher and limiting the cooling rate to 1° C./s or higher depending on the sigma phase precipitation temperature of the duplex stainless steel that is the laminated material. Therefore, the base material of the clad steel plate is not particularly limited and can be selected from the group consisting of ordinary steel (carbon steel) and alloy steel excluding stainless steel. It can be selected and used as appropriate depending on the intended use. Examples of alloy steel include, but are not limited to, low-alloy steel, nickel steel, manganese steel, chromium-molybdenum steel, high-speed steel, etc., and even steel containing one or more elements in ordinary steel. Good.
Although an example of the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment.

Examples are described below. Table 1 shows the chemical composition of the laminated material, and Table 2 shows the chemical composition of the steel plate serving as the base material. Note that components other than those listed in Tables 1 and 2 are Fe and inevitable impurities. Further, for the components shown in Table 1, the parts whose contents are not listed indicate impurity levels, REM means lanthanoid rare earth elements, and the content indicates the total of these elements.
Further, the sigma phase precipitation temperature in Table 1 was calculated using the Tσ calculation formula (Formula 2) obtained by the above-mentioned thermodynamic calculation.

Clad steel plates are made of duplex stainless steel with the chemical composition shown in Table 1, carbon steel shown in Table 2 as the base material, and made of a specified thickness of steel. A laminated material was pasted on one side to form a slab with a thickness of 130 mm. Thereafter, two 130 mm thick slabs were assembled by welding (sandwich assembly) with the laminated material on the inside to form a 260 mm thick slab. In addition, as for the open method, the slab laminated with the above laminated material was used as it was as a raw material for hot rolling.

In hot rolling using the sandwich assembly method, a 260 mm thick slab was heated to a predetermined temperature of 1150 to 1220°C, and then a clad steel plate was produced using a two-high rolling mill in the laboratory. As for the hot rolling conditions, rolling is repeated 8 to 12 times, and finish rolling is performed at a rolling reduction rate of 0 to 40% below Tσ and a TF of 900 to 1100°C so that the final plate thickness is 20 to 50 mm. The sample was cooled to a cooling stop temperature of 670 to 550°C at an average cooling rate of 0.6 to 10°C/s from TF to 600°C, and then transferred to a cooling bed for cooling. After cooling, it was peeled off at the center of the plate thickness and separated into two clad steel plates. In this way, a clad steel plate was obtained in which the thickness of the laminated material was 3 mm and the total plate thickness was 10 to 25 mm. A part of this steel plate was subjected to solution heat treatment at 1050°C, and samples for X-ray diffraction and pitting potential measurement were collected to evaluate the metal structure of the surface layer of the laminated material before and after the heat treatment.

The micro-strain on the surface of the laminated material was measured using a 2.5 mm x 20 w x 25 L X-ray diffraction sample, which was finished by wet polishing with emery paper and electrolytic polishing on the surface 0.3 mm below the surface of the steel material so that no distortion from test piece processing remained. was sampled and subjected to X-ray diffraction measurement using a CuKα radiation source to measure the diffraction profiles of the ferrite phase and austenite phase, and calculate the half values of each steel material before and after solution heat treatment (A and B). The microstrain of both phases of sample A before solution heat treatment was determined from the width data. Among these, the microstrain values of the ferrite phase are shown in Table 3.

The surface hardness of the laminated material was measured at a surface 0.3 mm below the skin of the steel material under a Vickers load of 5 kgf. The steel materials before and after solution heat treatment (i.e., sample A without solution heat treatment and sample B with solution heat treatment) were measured at n = 3, respectively, the average value was determined, and the ratio of the average values (= sample Table 3 shows the values of hardness of sample A/hardness of sample B.

To measure the pitting corrosion potential of the laminated material, polarization was performed on a surface 0.3 mm below the skin of the steel material in a 1 molar NaCl solution at 80°C in accordance with the method specified in JIS G0577, and the current density was 100 μA/cm2. The potential (VC'100) corresponding to was measured. Steel materials before and after solution heat treatment (i.e., sample A without solution heat treatment and sample B with solution heat treatment) were measured at n = 3, and the average value was determined and used as the pitting potential of that sample. The pitting corrosion potentials of Sample A and Sample B and the difference therebetween are shown in Table 3. The pitting potential of sample A is 0.3V or more, and the difference in potential of the clad steel plate with respect to the potential of the solution heat treated material (difference in pitting potential between sample A and sample B) is 0.1V or less. And so.

The strength of the base material was determined by taking a JIS No. 13 B test piece with a thickness of 5 to 10 mm from 1/4 part of the thickness and conducting a tensile test according to the method specified in JIS Z 2247. Each sample was measured with n=3, and the average value of yield strength was determined. The average values are shown in Table 3. A yield strength of 315 MPa or more was considered to be a pass.

The examples shown in Table 3 are examples of laminated materials of clad steel plates assembled and hot rolled in a sandwich method using the steels shown in Table 1 as the laminated materials and the steels shown in Table 2 as the base materials.
As described above, it was confirmed that the clad steel plate according to the embodiment of the present invention has a high surface hardness of the laminated material, and its corrosion resistance is comparable to that of the solution heat-treated material, with a difference of 0.1 V or less. Note that Comparative Example 23 and Comparative Example 27 were rolled using an open sand method, and thus were rolled in a state where the temperature of the surface layer of the laminated material was lowered by heat removal due to roll contact and roll cooling water. Therefore, the microstrain in the surface layer of the composite material increased, and σ phase precipitation was promoted, resulting in corrosion resistance of 0.1 V or more compared to the solution heat treated material. Furthermore, in Comparative Example 31, the laminated material I exceeds the lower limit of Mo, and as a result, although the difference in corrosion resistance is 0.1 V or less compared to the solution heat treated material, the corrosion resistance of the steel material before and after solution heat treatment is It was less than 0.3V and had poor corrosion resistance.

Since the present invention has both strength and corrosion resistance, it can be used for all kinds of industrial equipment and structures such as seawater desalination equipment, tanks of transport ships, and various containers.

Claims (3)

A clad steel plate in which one side of a base steel plate is clad with a laminated material,
The chemical composition of the steel plate serving as the base material is in mass%,
C: 0.03-0.15%,
Si: 1.0% or less,
Mn: 0.5-2.0%,
P: 0.05% or less,
S: 0.05% or less,
Nb: 0.005-0.10%,
Ti: 0.005 to 0.10%,
Cu: 0 to 0.50%,
Cr: 0 to 0.50%,
Ni: 0-1.0%,
Mo: 0 to 0.50%,
Al: 0-0.30%,
V: 0 to 0.40%,
Ca: 0-0.0050%,
B: 0 to 0.0030%,
REM: Contains 0 to 0.010%,
The remainder consists of Fe and unavoidable impurities,
The yield strength of the base material is 315 MPa or more,
The chemical composition of the laminated material is in mass%,
C: 0.03% or less,
Si: 0.05-1.00%,
Mn: 0.1 to 4.0%,
P: 0.05% or less,
S: 0.005% or less,
Cr: 20.0-28.0%,
Ni: 4.0 to 8.0%,
Mo: more than 2.5% to 5.0%,
Oxygen: 0.001-0.006%,
N: 0.08-0.30%,
W: 0-1.50%,
Co: 0-1.00%,
Cu: 0-3.0%,
V: 0-1.00%,
Nb: 0 to 0.20%,
Ta: 0 to 0.20%,
Ti: 0 to 0.030%,
Zr: 0 to 0.050%,
Hf: 0-0.10%,
B: 0 to 0.0050%,
Al: 0 to 0.050%,
Ca: 0-0.0050%,
Mg: 0 to 0.0050%,
REM: 0-0.10%,
Contains Sn: 0 to 0.10%,
The remainder is Fe and unavoidable impurities,
The laminated material is
PREW determined by formula 4 is 35 or more and 45 or less,
The σ phase precipitation temperature Tσ (°C) determined by Formula 2 is 950°C or more and 1100°C or less,
The surface hardness of the laminated material is 1.3 times or less that of the solution heat treated state,
A clad steel sheet characterized in that the microstrain εα of the ferrite phase is equal to or less than ε _max determined by Equation 1.
ε _max =0.0035−Tσ×2.63×10 ⁻⁶ (Formula 1)
Tσ=4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3)×(Mo-1.3)+5Si-6Mn-30N+569(℃) (Formula 2)
PREW=Cr+3.3(Mo+0.5W)+16N (Formula 4)
However, each element symbol in Formula 1, Formula 2, and Formula 4 indicates the content (mass%) of the element in the composite material, and if it is not contained, 0 is substituted. A method for manufacturing a clad steel plate according to claim 1, comprising:
Two slabs in which a laminated material having the chemical composition according to claim 1 is bonded to one side of a steel plate serving as a base material having the chemical composition according to claim 1 , such that the laminated material is placed inside. A method for manufacturing a clad steel plate by sandwich assembly in which the slabs are stacked together to form an integral slab, the method comprising:
Perform hot rolling to achieve a finishing temperature TF that satisfies formula 3,
A method for manufacturing a clad steel sheet, which comprises cooling from TF to 650°C at an average cooling rate of 1°C/s or more.
TF≧Tσ−50 (℃) (Formula 3)
Tσ=4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3)×(Mo-1.3)+5Si-6Mn-30N+569(℃) (Formula 2)
However, each element symbol in Formula 2 indicates the content (mass %) of the element in the composite material, and if it is not contained, 0 is substituted. In the production of the sandwich assembled cladding,
The method for manufacturing a clad steel sheet according to claim 2, wherein the cooling stop temperature of the cooling is less than 650°C.