JP7357761B2 - Clad steel plate and its manufacturing method and welded structure - Google Patents

Description

The present invention relates to a clad steel plate with excellent hydrogen embrittlement resistance on joint surfaces, a method for manufacturing the same, and a welded structure manufactured using the clad steel plate through a manufacturing process including welding or gouging using a hydrogen-containing gas.

Stainless steel and Ni-based alloys have excellent corrosion resistance, so they are suitable materials in severe corrosive environments. Examples of the above-mentioned corrosive environments include oil well environments, high chloride environments such as those exposed to seawater and brackish water, plant equipment and chemical tankers exposed to various acid solutions, and the like. In such a corrosive environment, stainless steel and Ni-based alloys are used in seawater desalination plants, flue gas desulfurization equipment, chemical storage tanks, structural components such as oil country pipes, pumps and valves, and heat exchangers. There is.

On the other hand, stainless steel and Ni-based alloys contain large amounts of alloying elements such as Cr, Ni, and Mo to ensure corrosion resistance, and compared to carbon steel and low alloy steel, they have lower material costs as well as processing and welding costs. It's also expensive. It is also conceivable that the price will fluctuate significantly due to a rise in the price of alloying elements. Therefore, its use may be limited mainly due to cost.

As mentioned above, when considering cost, it is effective to use a clad steel plate as a material from the viewpoint of processing, welding, etc. Clad steel plate is a material made by bonding two or more different metals. Further, a steel plate that is not bonded together will be referred to as a "solid steel plate" hereinafter. Compared to solid steel plates made only of high-alloy steel, clad steel plates can reduce the amount of high-alloy steel used, lowering material costs, and reducing dissimilar metal welding, which reduces the amount of weld metal used during welding. Costs can also be reduced.

In addition, in a clad steel plate in which two types of metals are bonded together, one metal is described as a "base material" and the other metal (material) bonded to the base material is described as a "laminated material". By laminating a material (laminated material) with excellent properties to a base material, it is possible to obtain the excellent properties of both the laminate material and the base material.

For example, a case can be considered in which the laminate is made of high-alloy steel that has the properties required in the environment in which it will be used, and the base material is made of carbon steel or low-alloy steel that has the toughness and strength required in the environment in which it is used. . In such a case, it is possible not only to reduce costs as described above, but also to ensure properties equivalent to solid steel plates and strength and toughness equivalent to carbon steel and low alloy steel. Therefore, both economy and functionality can be achieved.

Due to the above-mentioned circumstances, the need for clad steel sheets using stainless steel or Ni-based alloys has been increasing in various industrial fields in recent years. However, when using a clad steel plate, it is important to prevent peeling at the joint between the cladding material and the base material. If the laminate and base material separate during use, desired properties such as corrosion resistance and strength may not be obtained. Furthermore, it is also possible that there may be a risk of the structure being punctured or collapsing.

In clad steel sheets made of stainless steel or Ni-based alloys, during heating during rolling, Cr and Ni diffuse from the laminate to the base metal, and C diffuses from the base metal to the laminate, causing the bond to bond with the base metal. An elemental diffusion layer is generated at the interface of the laminated material (hereinafter simply referred to as "interface"). In the diffusion layer, the concentration of each element changes gradually, but depending on the element concentration, the temperature at which martensitic transformation starts is high, and the critical cooling rate at which martensitic transformation occurs is slow. Metamorphosis may occur.

In the normal usage of clad steel plates, martensite at the interface does not affect interfacial delamination, but when welding is performed using hydrogen as the welding gas, for example, hydrogen enters the martensite and causes stress due to the structure and during welding. It is assumed that stress is generated at the interface due to deformation of the weld, transformation of the base material near the weld, etc., and the combined effect of these causes hydrogen embrittlement.

Patent Document 1 discloses a technique for suppressing sensitization near the interface of a duplex stainless clad steel plate by controlling the thickness of the carbon diffusion layer at the interface. However, there is no description regarding the martensitic phase at the interface.

Patent Document 2 discloses a technique for preventing delayed fracture of martensite at the interface by regulating the temperature and time of tempering after rolling with respect to an austenitic stainless clad steel plate. However, this technique is for preventing delayed fracture during manufacturing, and there is no disclosure of a prevention technique for welded structures.

Furthermore, Non-Patent Document 1 evaluates the hydrogen embrittlement susceptibility of martensite at the interface for SUS316L and Inconel 625 cladding.

JP2013-209688A Japanese Patent Application Publication No. 6-7803

Kushida et al., Tetsu to Hagane, Vol. 75 (1989), p1508

As a result of intensive studies, the inventors of the present invention discovered the following problems to be solved.
Patent Document 2 discloses a technique for preventing delayed fracture of martensite at an interface. However, an increase in the number of tempering steps leads to an increase in cost, so in practice there is a need for a technology to improve the hydrogen embrittlement resistance of martensite at the interface without tempering, but there is no disclosure or suggestion of a solution.
Non-Patent Document 1 describes a method for evaluating the hydrogen embrittlement susceptibility of martensite at an interface. However, in actual clad steel, the width of the diffusion layer is presumed to vary depending on the heating temperature and rolling reduction ratio, but there is no description or suggestion of the relationship between the width of the diffusion layer and the susceptibility to hydrogen embrittlement.
The present inventor found that the higher the hardness of martensite, the higher the susceptibility to hydrogen embrittlement, and furthermore, the larger the width of martensite in the diffusion layer, the greater the risk that small hydrogen embrittlement will lead to large interfacial delamination. I realized that. Furthermore, the inventor found that in order to suppress interfacial peeling of the clad due to hydrogen embrittlement during welding, it is necessary to control the hardness and width of martensite at the interface, the hydrogen concentration in the steel, and the stress applied to martensite. It was found that this is an issue that needs to be solved.

In view of the above-mentioned problem recognition, an object of the present invention is to provide a clad steel plate having excellent hydrogen embrittlement resistance with good hydrogen embrittlement resistance of a joint surface, a method for manufacturing the same, and a welded structure.

The present invention has been made to solve the above problems, and its gist includes the following clad steel plate, method for manufacturing the same, and welded structure.
[1] A clad steel plate comprising a base material and a laminate joined to the base material,
The base material is made of carbon steel or low alloy steel,
The cladding material is made of a corrosion-resistant alloy,
A clad steel plate characterized in that the width in the plate thickness direction of a region having nanohardness of 7 GPa or more is 5 μm or less at an interface between a base material and a laminate material of the clad steel plate.
[2] In the clad steel plate according to [1] , the base material is C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, The cladding according to claim 1, which contains P: 0.050% or less , S: 0.050% or less, and has a Ceq of 0.20 to 0.40, with the remainder consisting of Fe and impurities. steel plate. Here, Ceq is defined by the following equation (1).
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...Formula (1)
In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass %) of each element in the component composition of the base material . For elements that are not included, add 0.
[3] The component composition of the base material is, in mass%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0, in place of a part of the Fe. .01-0.50%, Cu: 0.01-1.00%, Co: 0.01-0.50%, Se+Te: 0.01-0.10%, V: 0.001-0.100 %, Ti: 0.001-0.200%, Nb: 0.001-0.200%, Al: 0.005-0.300%, Ca: 0.0003-0.0050%, B: 0. The clad steel plate according to [2], containing one or more selected from 0003 to 0.0030% and REM: 0.0003 to 0.0100%.
[4] According to any one of [1] to [3], wherein the cladding steel plate is stainless steel or a nickel-based alloy containing Cr: 10% or more by mass%. clad steel plate.

[5] In the method for manufacturing a clad steel plate according to any one of [1] to [4], the base material and the laminate are laminated so that the crimped surface is in a vacuum, and the four circumferences of the crimped surface are sealed by welding. The maximum heating temperature T (°C) in the heating furnace for the clad rolled material made of the clad material, or the clad rolled material assembled from two or more of the clad materials, and the maximum heating temperature in the heating furnace. The time t (minutes) from the time when the heating temperature reaches T-20°C until extraction from the heating furnace, and the rolling reduction ratio r calculated by material thickness/product thickness, d calculated by formula (2) is 1 or more and 9 or less. Perform certain heating and hot rolling, and after rolling, perform cooling at an average cooling rate of 2°C/s or more in the interval from T _A3 (°C) to 650°C, calculated by formula (3), to improve the interface between the base material and the laminate. The method for manufacturing a clad steel sheet according to any one of claims 1 to 4, characterized in that the width in the thickness direction of the region where the nanohardness is 7 GPa or more is 5 μm or less.
d=2.2×10 ⁵ ×√(exp(-3.2×10 ⁴ /(T+273))×t)/r...Formula (2)
T _A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3)
In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass %) of each element in the component composition of the base material . For elements that are not included, add 0.

[6] A welded structure using the clad steel plate according to any one of [1] to [4].
[7] The clad steel plate according to any one of [1] to [4], wherein the clad steel plate is used for welding using hydrogen as a welding gas.

According to the present invention, it is possible to obtain a clad steel plate with excellent hydrogen embrittlement resistance at the joint surface.

FIG. 3 is a diagram showing the influence of the value d of formula (2) and the cooling rate CR after hot rolling on hydrogen embrittlement resistance.

The present inventors conducted the following study to solve the above problem. Specifically, in clad steel sheets made of various stainless steels and Ni-based alloys, we investigated the elemental diffusion and metallographic structure at the interface by varying the heating temperature, heating time, rolling reduction ratio, and post-rolling cooling rate. , and the relationship with the hydrogen embrittlement resistance of the interface was evaluated. As a result, the following findings (a) to (c) were obtained.

(a) The thinner the region where the nanohardness of the interface of the clad steel plate is 7 GPa or more, the lower the hydrogen embrittlement susceptibility of martensite tends to be. Therefore, it is effective to reduce the area of 7 GPa or more to 5 μm or less.

(b) In the rolled material of the clad steel plate, carbon steel or low alloy steel serving as the base material is in contact with stainless steel or Ni-based alloy serving as the mating material. The profile of alloying elements at the interface could be organized according to the material heating temperature, time, and rolling reduction ratio. Furthermore, when using a composite material containing 10% or more of Cr by mass, it was confirmed that the diffusion width of Cr corresponded to the width of the martensitic phase. This is because Cr is the element that diffuses the fastest among the main alloying elements and further increases hardenability, so martensitic transformation occurs in a region where only the Cr content is high and the content of austenite stabilizing elements such as Ni is low. It's for a reason.

(c) The hardness of martensite at the interface is affected by the cooling rate after rolling. This mechanism can be considered as follows. If the cooling rate is slow during cooling after rolling and expulsion and diffusion of carbon occurs due to austenite → ferrite transformation or austenite → ferrite + pearlite transformation, the carbon that was solidly dissolved in the austenite phase will contain a large amount of Cr. Carbon is concentrated on the side of the laminate with a low activity coefficient. At this time, if the laminate side is in an austenite phase, the degree of concentration will be greater. Due to this mechanism, if the cooling rate after rolling is slow, a region with high carbon concentration will occur near the interface, and if this region overlaps with the region where martensitic phase can be formed, a hard martensitic phase will form at the interface of the clad steel sheet. is formed, reducing the hydrogen embrittlement resistance of the interface.

Therefore, in order to obtain a clad steel sheet with excellent hydrogen embrittlement resistance at the joint surface, it is necessary to control Cr diffusion during heating and C diffusion during cooling after rolling. The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be explained in detail.

1. Configuration of the present invention A clad steel plate according to the present invention includes a base material and a laminate joined to the base material. The base material is made of carbon steel or low alloy steel, which will be described later. Further, the cladding material is made of a corrosion-resistant alloy, and examples of the corrosion-resistant alloy include stainless steel containing 10% or more of Cr and a Ni-based alloy. Furthermore, the width of the region having nanohardness of 7 GPa or more at the interface between the base material and the laminated material is 5 μm or less.

2. Properties of Clad Interface The interface properties of the clad steel plate related to the present invention will be explained. In order to obtain a clad steel sheet with good hydrogen embrittlement resistance at the joint surface, it is necessary to suppress the formation of a hard martensitic phase at the clad interface.

2-1. Nanohardness of cladding interface The width of the region where the nanohardness is 7 GPa or more at the cladding interface is 5 μm or less. If the width in the sheet thickness direction in the area where the nanohardness is 7 GPa or more is more than 5 μm, the martensite area is large and is hard and highly susceptible to hydrogen embrittlement, so the interface will peel off when welding is performed with hydrogen in the welding gas. There are cases. Preferably it is 3 μm or less, more preferably 1 μm or less. No lower limit is set because the smaller the region where the nanohardness is 7 GPa or more, the lower the susceptibility to hydrogen embrittlement.
Here, nanohardness means the hardness of a material evaluated in accordance with the instrumented indentation hardness test (also referred to as nanoindentation test) specified in ISO 14577.

3. Chemical composition of the base material The base material consists of carbon steel or low alloy steel. In addition, the preferable chemical composition of the base material is, in mass %, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, The steel plate contains S: 0.050%, Ceq is 0.20 to 0.40, and the remainder is Fe and impurities. Here, Ceq is defined by the following equation (1).
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...Formula (1)
In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass %) of each element in the component composition of the base material.

C is an element that improves the strength of steel, and by containing it in an amount of 0.020% or more, sufficient strength is developed. However, if it exceeds 0.200%, weldability and toughness will deteriorate. Therefore, the amount of C is set to 0.020 to 0.200%. Preferably it is 0.040% or more, more preferably 0.050% or more. On the other hand, the upper limit is preferably 0.100% or less, more preferably 0.080% or less. A more preferable range is 0.040% to 0.100%, and an even more preferable range is 0.050% to 0.080%.

Si is an element that is effective in deoxidizing and improves the strength of steel. However, if it exceeds 1.00%, the surface quality and toughness of the steel will deteriorate. Therefore, the amount of Si is set to 1.00% or less. Preferably it is 0.50% or less. Si may not be contained. The lower limit of the preferable content of Si is 0.01%.

Mn is an element that increases the strength of steel, and its effect is manifested when it is contained in an amount of 0.10% or more. However, if it exceeds 3.00%, weldability will be impaired and the alloy cost will also increase. Therefore, the Mn content is set to 0.10 to 3.00%. Preferably the lower limit is 0.50% and the upper limit is 2.00%. More preferably, the lower limit is 0.90% and the upper limit is 1.60%.

P is an impurity in steel, and when the content exceeds 0.050%, toughness deteriorates. Therefore, the amount of P is set to 0.050% or less. Preferably it is 0.020% or less.

S is an impurity in steel, and when the content exceeds 0.050%, toughness deteriorates. Therefore, the amount of S is set to 0.050% or less. Preferably it is 0.010% or less.

Ceq (carbon equivalent) is a value used to estimate hardness and weldability from the chemical composition of steel, and is calculated using equation (1). The higher the Ceq, the higher the hardness and the worse the weldability. If Ceq is less than 0.20, sufficient strength as a structure cannot be obtained. Therefore, Ceq is set to 0.20 or more. Preferably it is 0.23 or more. When Ceq exceeds 0.40, weldability deteriorates and welding costs increase, such as requiring inter-pass temperature control and post-heat treatment. Therefore, Ceq is set to 0.40 or less. Preferably it is 0.35 or less.
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...Formula (1)
In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass %) of each element in the component composition of the base material.

In addition to the component composition of the base material, in place of a part of the Fe, in mass%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0. .50%, Cu: 0.01-1.00%, Co: 0.01-0.50%, Se+Te: 0.01-0.10%, V: 0.001-0.100%, Ti: 0.001-0.200%, Nb: 0.001-0.200%, Al: 0.005-0.300%, Ca: 0.0003-0.0050%, B: 0.0003-0. 0030% and REM: 0.0003 to 0.0100%.

Ni is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if it exceeds 1.00%, weldability and toughness will deteriorate. Therefore, when Ni is contained, the amount of Ni is 1.00% or less. Preferably it is 0.50% or less, more preferably 0.30% or less. A preferable lower limit of Ni content is 0.01%.

Cr is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if it exceeds 1.00%, weldability and toughness will deteriorate. Therefore, when containing Cr, the amount of Cr is 1.00% or less. Preferably it is 0.50% or less, more preferably 0.30% or less. A preferable lower limit of Cr content is 0.01%.

Mo is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if it exceeds 0.50%, weldability and toughness will deteriorate. Therefore, when Mo is contained, the amount of Mo is 0.50% or less. Preferably it is 0.30% or less, more preferably 0.1% or less. A preferable lower limit of Mo content is 0.01%.

Cu is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if it exceeds 1.00%, weldability and toughness will deteriorate. Therefore, when containing Cu, the amount of Cu is 1.00% or less. Preferably it is 0.50% or less, more preferably 0.30% or less. A preferable lower limit of Cu content is 0.01%.

Co is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, when it exceeds 0.50%, hot workability is impaired and productivity is reduced. Therefore, when containing Co, the amount of Co should be 0.50% or less. Preferably it is 0.30% or less, more preferably 0.1% or less. A preferable lower limit of Co content is 0.01%.

Se and Te suppress elements that are easily oxidized such as Mn, Si, and Al in the steel sheet from being diffused to the surface of the steel sheet and form oxides, thereby improving the surface quality and plating properties of the steel sheet. However, this effect is saturated when the total amount exceeds 0.10%. Therefore, when adding Se and Te, the total amount of Se and Te should be 0.10% or less. More preferably, it is 0.05% or less. A preferable lower limit of Se+Te content is 0.01%.

Al is an element that is effective in deoxidizing steel. However, if it exceeds 0.300%, the toughness of the weld zone will deteriorate. Therefore, when containing Al, the amount of Al should be 0.300% or less. Preferably it is 0.100% or less. A preferable lower limit of Al content is 0.005%.

V increases the strength of steel by forming carbonitrides. However, if it exceeds 0.100%, weldability and toughness will deteriorate. Therefore, when containing V, the amount of V should be 0.100% or less. Preferably it is 0.050% or less. A preferable lower limit of V content is 0.001%.

Ti is an element that refines crystal grains and increases strength, and its effect is manifested when it is added in an amount of 0.001% or more. However, if it exceeds 0.200%, weldability will be impaired and the alloy cost will also increase. Therefore, the amount of Ti is set to 0.001 to 0.200%. Preferably the lower limit is 0.005% and the upper limit is 0.100%. More preferably, the lower limit is 0.010% and the upper limit is 0.050%.

Nb is an element that increases the recrystallization temperature, and its effect is manifested when it is added in an amount of 0.001% or more. However, if it exceeds 0.200%, weldability will be impaired and the alloy cost will also increase. Therefore, the amount of Nb is set to 0.001 to 0.200%. Preferably the lower limit is 0.005% and the upper limit is 0.100%. More preferably, the lower limit is 0.010% and the upper limit is 0.050%.

Ca refines the structure of the weld heat affected zone and improves toughness. However, when it exceeds 0.0050%, coarse inclusions are formed and the toughness is deteriorated. Therefore, when Ca is contained, the amount of Ca is 0.0050% or less. Preferably it is 0.0030% or less. A preferable lower limit of Ca content is 0.0003%.

B is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if it exceeds 0.0030%, weldability and toughness will deteriorate. Therefore, when B is contained, the amount of B is 0.0030% or less. Preferably it is 0.0020% or less. The preferable lower limit of B content is 0.0003%.

REM refines the structure of the weld heat affected zone and improves toughness. However, if it exceeds 0.0100%, coarse inclusions will be formed and the toughness will deteriorate. Therefore, when containing REM, the amount of REM should be 0.0100% or less. Preferably it is 0.005% or less. A preferable lower limit of REM content is 0.0003%.

Here, REM is a general term for 17 elements including 15 elements of lanthanoids plus Y and Sc. One or more of these 17 elements can be contained in the steel material, and the REM content means the total content of these elements.

In the chemical composition of the base material of the present invention, the remainder is Fe and impurities. Here, "impurities" are components that are mixed in during industrial manufacturing of steel materials due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are allowed within the range that does not adversely affect the present invention. means something that

4. Corrosion-resistant alloy is stainless steel or nickel-based alloy containing 10% or more of Cr The cladding material of the present invention is made of a corrosion-resistant alloy. As mentioned above, corrosion-resistant alloys contain a large amount of Cr, and the diffusion of Cr increases the hardenability of the cladding interface, making it easier to transform into martensite. At the same time, carbon from the base metal side diffuses into the mating material side, and the base metal A hard martensitic phase is formed at the side interface, which causes a decrease in the hydrogen embrittlement resistance of the joint surface. That is, the effects of the present invention are exhibited when a corrosion-resistant alloy containing a large amount of Cr is used. If the Cr content of the laminated material is 10% or more, the effects of applying the present invention will be noticeable. If the Cr content is 15% or more, the effect can be more pronounced.

The present invention is a technology for a clad steel plate that has excellent hydrogen embrittlement resistance on the joint surface by controlling the joint interface structure, and a method for manufacturing the same.The steel type of the cladding material is not particularly specified, but an example of the cladding material is stainless steel. Alternatively, a nickel-based alloy can be exemplified. Stainless steels include austenitic stainless steels, ferritic stainless steels, and duplex stainless steels, and nickel-based alloys have various alloy components under trade names such as Inconel, Incoloy, and Hastelloy.

5. Production method
A method for manufacturing a clad steel plate according to the present invention will be explained. As mentioned above, it is necessary to control the metal structure in order to obtain good hydrogen embrittlement resistance of the joint surface, but such a metal structure can be achieved by combining the chemical composition of the steel and appropriate manufacturing conditions. .
In the above-mentioned clad steel plate, the base material and the cladding material are laminated so that the crimped surface is in a vacuum, and the four circumferences of the crimped surface are sealed by welding to obtain a clad material. One or more clad materials are assembled to form a clad rolled material. For the assembled clad rolled material, the maximum heating temperature in the heating furnace T (°C), the time t (minutes) from the time when the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until extraction from the heating furnace, Heating and hot rolling are performed so that d calculated by formula (2) is 1 or more and 9 or less according to the rolling reduction ratio r calculated by material thickness / product thickness, and after rolling T _A3 ( A clad steel plate is produced by performing cooling at an average cooling rate of 2°C/s or more in the range from 2°C to 650°C.
d=2.2×10 ⁵ ×√(exp(-3.2×10 ⁴ /(T+273))×t)/r...Formula (2)
T _A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3)
In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass %) of each element in the component composition of the base steel plate.

5-1 Clad material The clad material is manufactured by the method described below. Specifically, after melting carbon steel and low-alloy steel as base materials and corrosion-resistant alloys as bonding materials using known methods such as converter furnaces, electric furnaces, and vacuum melting furnaces, continuous casting or ingot-making is performed. Create a slab using the blooming method. The obtained slab is hot-rolled under commonly used conditions to produce a laminate and a base material, which are hot-rolled plates. The obtained hot-rolled sheet may be subjected to annealing, pickling, polishing, etc., if necessary.
The above-mentioned laminate and base material are laminated so that the crimped surface is in a vacuum, and the 4 circumferences of the crimped surface are sealed by welding to assemble the cladding material. In order to improve adhesion and interfacial corrosion resistance, an insert material such as Ni foil may be inserted between the laminate material and the base material. There are no particular regulations regarding the method of vacuuming the crimping surface, but there are two methods: electron beam welding in a vacuum, or by drilling a vacuum hole in advance and welding four circumferences with arc welding or laser welding in the atmosphere, then using a vacuum pump. An example is a method of vacuuming. If the degree of vacuum (absolute pressure) is 0.1 Torr or less, a good bonding interface with less oxides at the interface can be obtained, and it is more preferably 0.05 Torr or less, and the higher the degree of vacuum (the lower the absolute pressure), the better the ) Since there is a tendency for the bonding interface to become good, no lower limit is set.
The obtained clad material may be subjected to hot rolling as it is as a clad rolled material, or it may be assembled by applying a release agent between two clad materials and stacked, and then subjected to hot rolling as a clad rolled material. It's okay. When two pieces are stacked, it is desirable that the base materials and the laminated materials have the same thickness to reduce warpage during cooling. Of course, it is not necessary to limit the assembly method to the one described above.

5-2. Hot rolling Next, the obtained clad rolled material was heated at the maximum heating temperature T (℃) in the heating furnace, and from the time when the heating temperature in the heating furnace reached the maximum heating temperature T - 20℃ until extraction from the heating furnace. Heating and hot rolling are performed for a time t (minutes) and a rolling reduction ratio r calculated as material thickness/product thickness so that d calculated by equation (3) is 1 or more and 9 or less. When d is more than 9, the element diffusion distance becomes long at the product interface, so the width of the region where martensitic transformation can occur becomes large, and the hydrogen embrittlement resistance of the interface decreases. Preferably d is 7 or less. When d is less than 1, elemental diffusion at the interface is too small and sufficient bonding strength cannot be obtained. Preferably d is 3 or more.
d=2.2×10 ⁵ ×√(exp(-3.2×10 ⁴ /(T+273))×t)/r...Formula (2)

Calculated using the maximum heating temperature T (℃) in the heating furnace, the time t (minutes) from the time when the heating temperature in the heating furnace reaches the maximum heating temperature T - 20℃ until extraction from the heating furnace, and material thickness / product thickness. The reduction ratio r to be applied may be determined as appropriate so that d falls within the above range, but preferred ranges are exemplified below from the viewpoint of properties other than the hydrogen embrittlement resistance of the interface and manufacturability.
The maximum heating temperature T in the heating furnace is preferably 1050 to 1250°C. When the maximum heating temperature T is less than 1050°C, hot workability deteriorates and bonding strength also deteriorates. Therefore, the maximum heating temperature T is preferably 1050°C or higher, more preferably 1100°C or higher. On the other hand, if the maximum heating temperature T exceeds 1250° C., the steel billet is likely to be deformed in the heating furnace, flaws are likely to occur during hot rolling, and diffusion at the interface becomes faster. Therefore, the maximum heating temperature T is preferably 1250°C or lower, more preferably 1220°C or lower.
The shorter the time t (minutes) from the time when the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until extraction from the heating furnace, the shorter the element diffusion distance at the interface, so there is no particular lower limit set. In order to make the temperature uniform throughout the thickness of the plate, it is desirable to heat the plate for 30 minutes or more.
The reduction ratio r calculated as material thickness/product thickness is preferably 3 or more and 15 or less. If the rolling ratio r is less than 3, the interfacial bonding due to rolling may be insufficient and the shear strength of the interface may become low. More preferably it is 5 or more. Further, when the rolling reduction ratio is more than 15, the rolling time becomes longer and the rolling cost increases. More preferably it is 10 or less.

As mentioned above, the size of the martensitic phase region at the interface is mainly influenced by the diffusion of Cr. Although Cr diffusion occurs at temperatures above several hundred degrees Celsius, the diffusion distance increases exponentially as the temperature increases, so substantial diffusion occurs while the material is heated near the maximum temperature. occurs in Furthermore, during rolling and cooling, the plate temperature drops quickly, so diffusion is negligibly small. Therefore, it can be considered that the Cr diffusion distance of the product is the diffusion distance caused during heating, which becomes smaller by the ratio of the rolling reduction ratio. The authors measured the size of the martensitic phase region by thin film TEM observation at the interface of clad products with various heating temperatures, times, and rolling reduction ratios, and found that the maximum temperature T (℃) in the heating furnace, The value d calculated using equation (2) from the time t (minutes) from the time when the heating temperature reaches the maximum heating temperature T - 20°C until extraction from the heating furnace and the reduction ratio r is the size of the martensitic phase region. We have confirmed that it corresponds with accuracy.

5-3. Cooling after rolling The average cooling rate in the range from T _A3 (°C) to 650°C calculated from equation (3) after rolling is preferably 2°C/s or more. At a cooling rate of less than 2°C/s, carbon diffuses and becomes concentrated in the austenite region that can become martensite at the interface due to austenite → ferrite transformation or austenite → ferrite + pearlite transformation, so the nanohardness becomes 7 GPa or more. The width of the area increases. Preferably it is 4°C/s or more. Although there is no particular upper limit, if the cooling rate is fast, the martensitic structure will become the main structure and the strength of the base material will become too high or the toughness will deteriorate, so it is desirably 10° C./s or less.
T _A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3)
In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass %) of each element in the component composition of the base steel plate.

According to the present invention, it is possible to obtain a clad steel plate with excellent hydrogen embrittlement resistance at the joint surface. The clad steel plate according to the present invention and the welded structure using the clad steel plate of the present invention do not require measures against peeling during welding or additional heat treatment. By adjusting the hardness and width of the martensite at the interface according to the present invention, it is freed from controlling the hydrogen concentration in the steel or the stress applied to the martensite. Moreover, the above-mentioned clad steel plate is not limited in its intended use and can be applied to structural members for which solid steel plates have conventionally been used. Therefore, the above clad steel plate greatly contributes to cost reduction. A welded structure using the clad steel plate of the present invention can be a welded structure manufactured by a manufacturing process including welding or gouging using a gas containing hydrogen.

Since the clad steel plate of the present invention has excellent hydrogen embrittlement resistance, hydrogen embrittlement does not occur even when used for welding using hydrogen as the welding gas.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

The composite material with the chemical composition shown in Table 1 and the base material with the chemical composition shown in Table 2 are melted into steel slabs, and through the steps of hot rolling, annealing, and pickling, the composite material has a thickness of 30 mm, and the base material has a thickness of 30 mm. A steel plate with a thickness of 130 mm was manufactured. Using the obtained laminate and base material as raw materials, the base material and laminate were laminated so that the crimping surface was in a vacuum, and the four circumferences of the crimping surface were sealed by welding to create a cladding material. The two cladding materials were stacked together in the following manner: base material - laminate material - release agent - laminate material - base material, with a release agent applied between the laminate materials, and assembled as a clad rolled material. The obtained clad rolled material was hot rolled under the hot rolling conditions shown in Table 3 and then peeled off at the release agent part to form a clad steel plate with a thickness of 53 mm (rolling ratio 3) to 12 mm (rolling ratio 13). Manufactured.

The conditions listed in Table 3 were varied during rolling of the clad steel plate, and each characteristic value was investigated. The manufacturing condition items in Table 3 will be explained below. In Table 3, T indicates the maximum heating temperature (℃) in the heating furnace before rolling, and t indicates the time from the time when the heating temperature in the heating furnace reaches the maximum heating temperature T - 20℃ until extraction from the heating furnace. (minutes). r indicates the rolling reduction ratio calculated as material thickness/product thickness. d indicates a value calculated from the above T, t, and r using equation (2). T _A3 indicates a value (°C) calculated from the chemical composition of the base material using equation (3). CR indicates the average cooling rate (°C/s) from T _A3 (°C) to 650°C. L indicates the width (μm) of a region near the interface where the nanohardness is 7 GPa or more. Hydrogen resistance is the result of a hydrogen embrittlement resistance evaluation test, where A indicates good hydrogen embrittlement resistance and X indicates poor hydrogen embrittlement resistance.
d=2.2×10 ⁵ ×√(exp(-3.2×10 ⁴ /(T+273))×t)/r...Formula (2)
T _A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3)
In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass %) of each element in the component composition of the base steel plate.

The measurement of nanohardness is based on the instrumented indentation hardness test specified in ISO 14577, and the nanohardness is measured at a pitch of 0.5μm in a range of 10μm from the interface to the plate thickness direction on the laminate side and base metal side. did. The conditions for nano-hardness measurement may be selected as appropriate, but for example, measurements are performed three times at each position with a load of 1000 μN, 5 seconds to the specified pushing load, 0 seconds for holding, and 5 seconds for return, and the average value is taken as the nano-hardness. can be exemplified. The range in which the nanohardness was 7 GPa or more was read and designated as L. Note that when an insert material such as Ni foil is inserted between the laminate material and the base material, it is sufficient to measure the interface between the laminate material and the insert material, and the interface between the insert material and the base material, respectively.

The following test was conducted to evaluate hydrogen embrittlement resistance. In order to ensure the length of the test piece in the plate thickness direction, the same steel type as the cladding material is welded to the cladding material side of the clad steel plate, and the same steel type as the base material is welded to the base metal side, so that the parallel part including the cladding interface is A round bar specimen measuring 4φ×20 mm was made with a notch of 60° and ρ=0.1 mm at the cladding interface to make it 3φ. In order to suppress the thermal effects of welding, we selected electron beam welding as the welding method, which has a small heat input and can reduce the width of the weld metal, and carried out grinding after welding. In addition, cross-sectional observation of the test piece was carried out, and it was confirmed that the weld metal was separated from the interface by 2 mm or more.
Before pulling the prepared test piece, it was cathode charged with a current density of 10 (A/m ² ) x 72 (hr) in an aqueous solution of 3% NaCl + 3 g/L NH ₄ SCN, and then charged with a current density of 10 (A/m 2 ) x 72 (hr). The specimen was pulled to break at a strain rate of 1×10 ⁻³ (1/s) in the parallel portion while being cathode charged at 10 (A/m ² ) in a ₄ SCN aqueous solution. A separate test was conducted in which the material was pulled without cathode charging before and during tension, and the strokes until breakage were compared. If the stroke of the charged material/stroke of the non-charged material was 0.25 or more, it was evaluated as good. "A" was written in the "Hydrogen resistance" column of Table 3, and if it was less than 0.25, it was evaluated as poor and "X" was written in the "Hydrogen resistance" column of Table 3.

The manufacturing conditions and the above results are summarized in Table 3 and FIG. 1. In Figure 1, the horizontal axis is the value d of formula (2), and the vertical axis is the average cooling rate CR from T _A3 (℃) to 650℃ after hot rolling. Marks indicate defects.

Sample numbers 1 to 41 are examples of the present invention, which satisfy the preferred manufacturing conditions, have a width L of a region where the nanohardness is 7 GPa or more and are 5 μm or less, and have good hydrogen embrittlement resistance of the joint surface. Sample numbers 42 to 47 are comparative examples and do not satisfy the preferred manufacturing conditions, the width L of the region where the nanohardness is 7 GPa or more is more than 5 μm, and the hydrogen embrittlement resistance of the joint surface is poor.

As described above, in the example of the present invention, good hydrogen embrittlement resistance of the joint surface was obtained. On the other hand, in the comparative example, the preferred manufacturing conditions were not satisfied, and the width of the region where the nanohardness was 7 GPa or more deviated from the specifications of the present invention, so the hydrogen embrittlement resistance of the joint surface was poor.

According to the present invention, it is possible to obtain a clad steel plate with good hydrogen embrittlement resistance on the joint surface, which is extremely useful industrially. If a corrosion-resistant alloy is used as the laminating material, the clad steel sheet of the present invention can be used in corrosive environments such as high chloride environments such as seawater, and plant equipment exposed to acid solutions such as phosphoric acid or sulfuric acid. Possible application to corrosive environments, etc. Specifically, these include seawater desalination plants, flue gas desulfurization equipment, chemical storage tanks, structural members such as oil country tubular goods, pumps and valves, and heat exchangers.

Claims (7)

A clad steel plate comprising a base material and a laminate joined to the base material,
The base material is made of carbon steel or low alloy steel,
The cladding material is made of a corrosion-resistant alloy,
A clad steel plate characterized in that the width in the plate thickness direction of a region having nanohardness of 7 GPa or more is 5 μm or less at an interface between a base material and a laminate material of the clad steel plate. In the clad steel plate according to claim 1 , the base material has a mass percentage of C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, and P: 0. .050% or less, S: 0.050% or less , Ceq is 0.20 to 0.40, and the remainder is Fe and impurities. Here, Ceq is defined by the following equation (1).
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...Formula (1)
In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass %) of each element in the component composition of the base material . For elements that are not included, add 0. Further, in place of a part of the Fe, the component composition of the base material is, in mass%, Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Cu: 0.01-1.00%, Co: 0.01-0.50%, Se+Te: 0.01-0.10%, V: 0.001-0.100%, Ti : 0.001-0.200%, Nb: 0.001-0.200%, Al: 0.005-0.300%, Ca: 0.0003-0.0050%, B: 0.0003-0 The clad steel plate according to claim 2, containing one or more selected from .0030% and REM: 0.0003 to 0.0100%. The clad steel plate according to any one of claims 1 to 3, wherein the cladding material of the clad steel plate is stainless steel or a nickel-based alloy containing Cr: 10% or more in mass %. . In the method for manufacturing a clad steel plate according to any one of claims 1 to 4, the base material and the cladding material are laminated so that the crimped surface is in a vacuum, and the 4 circumferences of the crimped surface are sealed by welding to form the cladding. The maximum heating temperature T (℃) in the heating furnace for the clad rolled material made of the above-mentioned clad material, or the clad rolled material assembled from two or more of the above-mentioned clad materials, and the heating temperature in the heating furnace is the maximum heating temperature T. Time t (minutes) from the time the temperature reaches -20°C until extraction in the heating furnace, and heating in which d, which is calculated by formula (2) using the rolling ratio r calculated from material thickness/product thickness, is 1 or more and 9 or less. Hot rolling is performed, and after rolling, cooling is performed at an average cooling rate of 2°C/s or more in the interval from T _A3 (°C) to 650°C, which is calculated by formula (3), to improve nano-hardness at the interface between the base material and the laminate. The method for manufacturing a clad steel sheet according to any one of claims 1 to 4, characterized in that the width in the thickness direction of the region where the thickness is 7 GPa or more is 5 μm or less.
d=2.2×10 ⁵ ×√(exp(-3.2×10 ⁴ /(T+273))×t)/r...Formula (2)
T _A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3)
In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass %) of each element in the component composition of the base material . For elements that are not included, add 0. A welded structure using the clad steel plate according to any one of claims 1 to 4. The clad steel plate according to any one of claims 1 to 4, wherein the clad steel plate is used for welding using hydrogen as a welding gas.

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