JP7733293B2 - Clad steel plate, its manufacturing method and welded structure - Google Patents

Description

The present invention relates to clad steel plates with excellent fracture toughness at the joining surface, their manufacturing method, and welded structures.

Stainless steel and Ni-based alloys have excellent corrosion resistance, making them suitable materials for severely corrosive environments. Examples of corrosive environments include oil well environments, high-chloride environments such as those exposed to seawater or brackish water, and plant equipment and chemical tankers exposed to various acid solutions. In such corrosive environments, stainless steel and Ni-based alloys are used in seawater desalination plants, flue gas desulfurization equipment, chemical storage tanks, structural components such as oil well tubular goods, pumps and valves, and heat exchangers.

On the other hand, stainless steels and Ni-based alloys contain large amounts of alloying elements such as Cr, Ni, and Mo to ensure corrosion resistance, and compared to carbon steels and low-alloy steels, not only are the material costs higher, but so are the costs of processing and welding. Furthermore, prices can fluctuate significantly due to rising prices of alloying elements. Therefore, their use is sometimes restricted, mainly due to cost considerations.

When considering costs as mentioned above, using clad steel plate as a material is effective from the perspective of processing and welding. Clad steel plate is a material made by bonding two or more different types of metal together. Compared to steel plate made only of high-alloy steel (hereinafter referred to as "solid steel plate"), clad steel plate allows for a reduction in the amount of high-alloy steel used, reducing material costs, and also reduces dissimilar material welding, thereby reducing welding material costs.

In addition, in clad steel plates, by laminating a material with excellent properties to the base material (hereinafter, the laminated material will be referred to as the "clad material"), it is possible to obtain the excellent properties of both the clad material and the base material.

For example, a high-alloy steel with the properties required for the intended use environment could be used for the cladding material, and a carbon steel or low-alloy steel with the toughness and strength required for the intended use environment could be used for the base material. In such cases, not only can costs be reduced as described above, but properties equivalent to those of solid steel plate and strength and toughness equivalent to those of carbon steel and low-alloy steel can be ensured. This allows for both economical efficiency and functionality.

For these reasons, the need for clad steel plates made from stainless steel or Ni-based alloys has been growing in recent years in various industrial fields. However, when using clad steel plates, it is important to prevent delamination at the joint between the clad material and the base material. If delamination occurs between the clad material and the base material during use, desired properties such as corrosion resistance and strength may not be achieved. Furthermore, there is a risk of holes being created in the structure or it may collapse.

In clad steel plates made from stainless steel or Ni-based alloys, Cr and Ni diffuse from the cladding material to the base material, and C diffuses from the base material to the cladding material during heating during rolling, creating a diffusion layer of elements at the interface. The concentration of each element gradually changes within the diffusion layer, but depending on the element concentration, martensitic transformation may occur during cooling after rolling in regions where the temperature at which martensitic transformation begins is high and the critical cooling rate for martensitic transformation is slow.

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

Patent Document 2 discloses a technology for austenitic stainless clad steel sheets that softens the martensite at the interface by specifying the temperature and time for tempering after rolling, thereby preventing delayed fracture. However, since adding a tempering process increases costs, there is a practical need for technology that improves the fracture toughness of the martensite at the interface without tempering, but no solution to this problem is disclosed or suggested.

Under normal use, the martensite at the interface does not affect interfacial delamination. However, if a defect occurs in the martensite at the interface, for example, due to welding, stress will be generated at the interface due to structural stress, deformation during welding, and transformation of the base material near the weld, and even if the defect is minute, the combination of these factors may result in large brittle cracks.

When strength is required at the bonding interface between the clad material and the base material to withstand such forces without causing delamination failure, strength evaluation tests are used for the bonding interface of clad steel plates.

The most basic test is the shear strength test specified in JIS standard G0601 (Testing methods for clad steel) (Non-Patent Document 1). This test is very simple and easy to perform, but a number of problems have been pointed out, including the fact that actual welded structures do not necessarily experience shear mode loading, the evaluation value obtained is insensitive to interface properties, and the test is strongly affected by deformation at the loading point (side of the clad material), making it an unreliable evaluation method.

The peel test described in the explanation of JIS standard G0601, for example, solves the problem of the loading method in the shear strength test mentioned above. In this test, if appropriate test specimen dimensions are used, it is possible to obtain the interfacial peel strength against a tensile load in the plate thickness direction. However, because shear can occur not only at the clad joint interface but also at the clad material itself, it is necessary to conduct tests with various dimensions in advance to find the appropriate test specimen dimensions.

Furthermore, both of the above JIS standard G0601 tests are strength tests in which defect-free material is pulled. However, actual welded structures may contain small weld defects that are overlooked as passing non-destructive testing. Furthermore, interfacial delamination may occur during welding construction. To prevent these construction defects from causing fatal damage during use, it is useful to manage allowable defects using fracture mechanics, which is where interfacial fracture toughness becomes necessary.

Crack Tip Opening Displacement (CTOD) evaluation is known as one test for determining the fracture toughness (fracture resistance) of materials containing cracks. CTOD tests determine the crack tip opening (critical CTOD value) at which unstable fracture occurs. One test for evaluating the fracture toughness of the bonded interface of clad steel plates, as shown in Non-Patent Document 2, involves attaching a supplementary member to each of the clad material and base material by multi-layer welding, and then extracting and evaluating a fracture toughness test piece, typically a CT test piece.

JP 2013-209688 A Japanese Patent Application Publication No. 6-7803

Japanese Industrial Standards, JIS G0601:2012 "Test methods for clad steel" Junichi Katsuta et al., "Fracture toughness and fatigue crack propagation characteristics of bonded interfaces of duplex stainless clad steel plates for chemical tankers," Pressure Technology, Vol. 57, No. 1, 2019, pp. 4-12

As a result of extensive research, the present inventors have found the following problems to be solved.
When CTOD evaluation is applied to evaluate the fracture toughness of the joint surface of clad steel plates using the method described in Non-Patent Document 2, the welding residual stress distribution introduced by multi-layer welding is large, and the length of the fatigue pre-crack introduced before the fracture toughness test varies significantly depending on the thickness position of the CT test specimen. Furthermore, because the test specimen has welding residual stress as an internal force, it is difficult to accurately calculate the fracture toughness value. Furthermore, fatigue pre-cracks aimed at the clad interface often deviate toward the base material, resulting in the fracture toughness of the base material rather than the clad interface. As a result, the true fracture toughness value of the clad interface cannot be obtained.

In order to use the joint interface between the clad steel plate and the base metal as the fracture toughness evaluation area, it is first necessary to weld a supplementary member, such as a CT test piece, to obtain the dimensions of the fracture toughness test piece. When multi-layer welding is performed using a conventional steel plate welding method (e.g., shielded metal arc welding, gas-shielded arc welding, submerged arc welding, TIG welding, etc.), a large welding residual stress distribution occurs around the welding joint. As a result, residual stress that varies greatly depending on the thickness position of the test piece is also generated in the fracture toughness evaluation area (the joint interface between the clad steel plate and the base metal), and the length of the fatigue pre-crack introduced before the fracture toughness test varies greatly depending on the thickness position of the test piece.
Furthermore, since the test piece has welding residual stress as an internal force, it is difficult to accurately calculate the fracture toughness value from the external load.
Therefore, it is necessary to join the clad steel plate and the supplementary member using a joining method that reduces the distribution of welding residual stress in the vicinity of the fracture toughness evaluation portion.

Furthermore, in clad steel plates, the hardness of the clad material and the base material generally do not match perfectly, resulting in differences in hardness at each location. In this case, even if a fatigue precrack is introduced through a mechanical notch at the clad joint interface, the fatigue precrack will deviate toward the softer side. In many cases, the carbon steel side of the base material is softer than the stainless steel side of the clad material, and the fatigue precrack deviates toward the carbon steel, resulting in the fracture toughness value of the carbon steel. Therefore, measures must be taken to prevent the fatigue precrack from deviating from the clad joint interface.
Therefore, first, we clarified the CTOD evaluation method for evaluating the fracture toughness of the bonding surface of clad steel plates.

The inventors then recognized that in the fracture toughness testing of the bonded surface clarified above, the lower the hardness of the martensite, the higher the fracture toughness value, and further recognized that the greater the width of the martensite in the diffusion layer, the greater the risk that microcracks will lead to large interfacial delamination. Furthermore, the inventors discovered that the challenge to be overcome in order to suppress interfacial delamination of the clad is to control the hardness and width of the martensite at the interface.

In light of the above-mentioned issues, the present invention aims to provide a clad steel plate with excellent fracture toughness at the joining surface, a manufacturing method thereof, and a welded structure.

The inventors recognized that in order to use the bond interface between the clad steel plate's clad material and base material as the fracture toughness evaluation area, it is necessary to avoid multi-layer welding when welding the supplementary member to the clad steel plate and use a single-pass full-penetration weld to minimize the residual stress distribution in the plate thickness direction of the fracture toughness test specimen, and to provide appropriate side grooves on the side of the specimen along the clad bond interface to prevent fatigue pre-cracks from deviating from the clad bond interface. Furthermore, when cutting a fracture toughness test specimen from the welded joint, it is preferable to reduce the thickness of the specimen in order to minimize the residual stress distribution in the thickness direction of the specimen.

The present invention has been made to solve the above-mentioned problems, and is summarized as follows: a clad steel plate, a manufacturing method thereof, and a welded structure.
[1] A clad steel plate comprising a base material and a clad material 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,
At the interface between the base material and the clad material of the clad steel plate (hereinafter referred to as the "clad interface"), the width in the plate thickness direction of the region where the nanohardness is 7 GPa or more is 5 μm or less,
A clad steel plate characterized in that, in evaluating the fracture toughness of the clad interface, a supplementary member is welded in one pass by electron beam or laser welding, and the CTOD value of the clad interface measured using a CT test piece in which a side groove is provided along the clad interface is 0.15 mm or more at 0°C.
[2] The clad steel plate according to [1], wherein the base material has a chemical composition containing, in mass%, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.001 to 0.200%, and N: 0.020% or less, and has a Ceq of 0.20 to 0.50, with the balance consisting of Fe and impurities. Here, Ceq is defined by the following formula (1):
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(1)
In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base steel plate.
[3] The chemical composition of the base metal further contains, in mass%, Ni: 0.01 to 3.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, W: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 0.50%, Se + Te: 0.01 to 0.10%, V: 0.001 to 0.100%, Ti: 0.00 1 to 0.200%, Al: 0.005 to 0.300%, Ca: 0.0003 to 0.0100%, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0100%, Zr + Hf + Ta: 0.0001 to 0.0100%, and REM: 0.0003 to 0.0100%. [2] The clad steel plate according to [2], containing one or more selected from the group consisting of: 0.1 to 0.200%, Al: 0.005 to 0.300%, Ca: 0.0003 to 0.0100%, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0100%, Zr + Hf + Ta: 0.0001 to 0.0100%, and REM: 0.0003 to 0.0100%.
[4] The clad steel plate according to any one of [1] to [3], characterized in that the clad steel plate is made of a stainless steel or nickel-based alloy containing 10% or more Cr by mass%.

[5] In the clad steel plate according to any one of [1] to [4], the base material and cladding material are laminated so that the bonding surface is vacuum, and the four periphery of the bonding surface is sealed by welding to form a clad material, and the clad rolled material assembled from one or more of the clad materials is subjected to heating and hot rolling such that d calculated by formula (2) is 1 or more and 9 or less, based on the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from the time the heating temperature in the heating furnace reaches the maximum heating temperature T-20 °C to the time of extraction from the heating furnace, and the rolling reduction ratio r calculated by the material thickness / product thickness, and the material is subjected to heating and hot rolling such that d calculated by formula (2) is 1 or more and 9 or less, and the material is cooled at an average cooling rate of 2 °C/s or more in the TA3 (°C) to 650 °C range calculated by formula (3), and the width in the plate thickness direction of the region where the nanohardness at the interface between the base material and the cladding material is 7 GPa or more is 5 μm or less, and the CTOD value at the clad interface is 0.15 mm or more at 0 °C.
d=2.2×10 ⁵ ×(√(exp(-3.2×10 ⁴ /(T+273))×t))/r ...Formula (2)
TA3 (℃)=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 chemical composition of the base steel plate.

[6] A welded structure made using the clad steel plate described in any one of [1] to [4].

The present invention makes it possible to obtain clad steel plates with excellent fracture toughness values at the bonding surface.

1A and 1B are diagrams showing the arrangement of the raw material (steel billet) of the test piece of the present invention, in which (A) is a side view and (B) is a front view. 1A and 1B are diagrams showing the position where one full-pass welding is performed in a welded joint, where FIG. 1A is a side view and FIG. 1B is a front view. 1A and 1B are diagrams showing positions at which test pieces (CT test pieces) of the present invention are cut out from a welded joint, where (A) is a side view and (B) is a front view. 1A and 1B are diagrams showing a CT test piece, in which (A) is a side view and (B) is a front view.

The inventors conducted the following study to address the above-mentioned issues. Specifically, they investigated element diffusion and metal structure at the interface of clad steel plates made from various stainless steels and Ni-based alloys by varying the heating temperature, heating time, reduction ratio, and cooling rate after rolling, and evaluated the relationship with the fracture toughness value of the interface. As a result, they obtained the following findings (a) to (c).

(a) The thinner the region of the clad steel plate where the nanohardness at the interface is 7 GPa or more, the higher the fracture toughness value tends to be. For this reason, it is effective to make the region of 7 GPa or more 5 μm or less.

(b) In the rolled material of clad steel plate, the base material, carbon steel or low-alloy steel, is in contact with the cladding material, stainless steel or Ni-based alloy. The profile of alloying elements at the interface could be organized by the material heating temperature and time and rolling reduction ratio. It was also confirmed that when cladding materials containing 10% or more Cr by mass were used, the Cr diffusion width corresponded to the martensite phase width. This is because, of the main alloying elements, Cr diffuses the fastest and is the element that enhances hardenability, so martensitic transformation occurs in regions where only the Cr content is high and the content of austenite-stabilizing elements such as Ni is low.

(c) The hardness of martensite at the interface is affected by the cooling rate after rolling. The mechanism behind this is thought to be as follows: If the cooling rate after rolling is slow and carbon is expelled and diffused during austenite-to-ferrite transformation or austenite-to-ferrite + pearlite transformation, the carbon that was dissolved in the austenite phase will concentrate on the cladding material side, which contains a lot of Cr and has a low carbon activity coefficient. In this case, the degree of concentration will be greater if the cladding material side is austenite. Due to this mechanism, if the cooling rate after rolling is slow, a region with high carbon concentration will form near the interface, and if this region overlaps with an area where martensite phase can form, a hard martensite phase will form at the interface of the clad steel plate, reducing the fracture toughness of the interface.

Therefore, in order to obtain clad steel plates with excellent fracture toughness at the bonding surface, it is necessary to control Cr diffusion during heating and C diffusion during cooling after rolling. The present invention was made based on the above findings. Each requirement of the present invention will be explained in detail below.

1. Configuration of the Present Invention The clad steel plate according to the present invention comprises a base material and a clad material bonded to the base material. The base material is made of a carbon steel or low-alloy steel as described below. The clad material is made of a corrosion-resistant alloy, such as a stainless steel containing 10% or more of Cr or a Ni-based alloy. Furthermore, the width of a region at the bonded surface (clad interface) between the base material and the clad material where the nano-hardness is 7 GPa or more is 5 μm or less, and the CTOD value of the clad interface at 0° C. is 0.15 mm or more.

2. Characteristics of the clad interface The interfacial characteristics of the clad steel plate according to the present invention will be explained below. In order to obtain a clad steel plate with good fracture toughness at the bonded surface, it is necessary to suppress the formation of a hard martensite phase at the clad interface.

2-1. Nanohardness of clad interface The width of the region at the clad interface where the nanohardness is 7 GPa or more is 5 μm or less. If the width in the plate thickness direction of the region where the nanohardness is 7 GPa or more exceeds 5 μm, the region of hard martensite with low fracture toughness is large, and when welding defects and stress in the welded structure are combined, the interface may peel off. It is preferably 3 μm or less, and more preferably 1 μm or less. The smaller the region where the nanohardness is 7 GPa or more, the higher the fracture toughness, so no lower limit is set.
Here, nanohardness means the hardness of a material evaluated in accordance with the instrumented indentation hardness test (also called nanoindentation test) specified in ISO 14577.

2-2. CTOD Value of Clad Interface In the present invention, the CTOD value of the clad interface shown below is used as an evaluation method for evaluating the fracture toughness value of the bonded surface.
The CTOD value at the clad interface is set to 0.15 mm or more at 0° C., preferably 0.20 mm or more, and more preferably 0.30 mm or more. The higher the CTOD value, the better the fracture toughness, so no upper limit is set.
Here, the CTOD value of the cladding interface is a value measured using a CT test piece of the following form when evaluating the CTOD (see FIGS. 1 to 4).

In order to minimize the residual stress distribution in the thickness direction of the fracture toughness test specimen (CT test specimen 12), when welding the supplementary members (4, 5) to the clad steel plate 1, multi-layer welding is avoided and a single-pass full-penetration weld is used. Specifically, electron beam welding or laser welding is used.

In addition, to prevent fatigue pre-cracks from diverging from the clad bond interface, appropriate side grooves (side grooves 8) are provided on the sides of the CT test specimen 12 along the clad interface 10. The side grooves 8 also serve to prevent shear-type fractures (shear cracks) from occurring on the side of the test specimen.

Furthermore, when cutting a fracture toughness test piece (CT test piece 12) from the welded joint, it is preferable to reduce the thickness of the test piece in order to reduce the residual stress distribution in the thickness direction of the test piece. However, an excessively thin test piece will be in a plane stress state, resulting in a shear-type fracture mode, and fracture toughness will not be obtained. It is preferable to use an appropriate test piece thickness so that the opening-type fracture mode becomes dominant. For this purpose, the test piece thickness (width W) is preferably 10 mm or more and 15 mm or less.

Figure 1 shows the layout of the material (steel piece) for the test specimen of the present invention. Clad steel plate 1 consists of clad material 2 of thickness H2 and base material 3 of thickness H3, which is cut to a length L x width W. A supplementary member 4 of height H4 made of the same material as base material 3 is butted against base material 3 with approximately the same area (length L x width W) as the surface of base material 3, and a supplementary member 5 of height H5 made of the same material as clad material is also butted against clad material 2 with approximately the same area (length L x width W). A single-pass weld is performed from the side to penetrate width W, resulting in a weld joint 13 as shown in Figure 2. Here, weld line 6 is the single-pass full-fill weld.

A fracture toughness test specimen is cut from the weld joint 13 along the CT test specimen cutting position 7 in Figure 3, and the CT test specimen 12 in Figure 4 is obtained. At this time, the center line of the mechanical notch 11 is cut along the clad interface 10. The thickness (width W) of the CT test specimen 12 is preferably set to 10 mm or more and 15 mm or less.

Furthermore, side grooves 8 are machined on both sides of the CT test piece 12 along the clad interface 10 to produce the CT test piece 12 shown in Figure 4. The dimensions of the side grooves 8 are, for example, an angle of 45 degrees, a radius of curvature at the bottom of 0.25 mm, and a depth of 1 mm.

Then, by introducing a fatigue pre-crack 9 from the bottom of the mechanical notch 11 along the clad interface 10, a fracture toughness test specimen (CT test specimen 12) for evaluating the clad steel plate bonding interface is obtained. The specific conditions for introducing the fatigue pre-crack 9 are not limited, but conditions conforming to ISO standard 15653 are preferred.

As described above, in the present invention, to evaluate the fracture toughness of the clad interface 10, the supplementary members (4, 5) were welded in one pass using electron beam or laser welding, and the CTOD value of the clad interface was measured using a CT test piece 12 in which a side groove 8 was provided along the clad interface 10.

3. Chemical Composition of Base Metal The base metal is made of carbon steel or low-alloy steel. The preferred chemical composition of the base metal is a steel sheet containing, by mass%, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.001 to 0.200%, and N: 0.020% or less, with Ceq of 0.20 to 0.50, and the balance consisting of Fe and impurities. Here, Ceq is defined by the following formula (1):
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(1)
In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass %) of each element in the composition of the base material.

C is an element that improves the strength of steel, and sufficient strength is achieved when it is contained at 0.020% or more. However, exceeding 0.200% leads to a deterioration in weldability and toughness. Therefore, the C content is set to 0.020-0.200%. Preferably, it is 0.040% or more, and even more preferably, it is 0.050% or more. On the other hand, the upper limit is preferably 0.100% or less, and even more preferably, it is 0.080% or less. A more preferable range is 0.040%-0.100%, and an even more preferable range is 0.050%-0.080%.

Si is effective for deoxidation and is an element that improves the strength of steel. However, if it exceeds 1.00%, it will deteriorate the surface quality and toughness of the steel. Therefore, the Si content should be 1.00% or less, and preferably 0.50% or less. Si may not be contained. The preferred lower limit of the Si content is 0.01%.

Mn is an element that increases the strength of steel, and this effect is realized when it is contained in an amount of 0.10% or more. However, if it exceeds 3.00%, weldability is impaired and alloy costs increase. Therefore, the Mn content is set to 0.10-3.00%. Preferably, it is 0.50-2.00%, and even more preferably, it is 0.90-1.60%.

P is an impurity in steel, and if the content exceeds 0.050%, toughness deteriorates. Therefore, the P content should be 0.050% or less, preferably 0.015% or less.

Sulfur is an impurity in steel, and if the content exceeds 0.050%, toughness deteriorates. Therefore, the S content should be 0.050% or less, and preferably 0.010% or less.

Nb is an element that raises the recrystallization temperature, and this effect is apparent when added at 0.001% or more. However, exceeding 0.200% impairs weldability and increases alloy costs. Therefore, the Nb content is set at 0.001-0.200%. Preferably, it is 0.005-0.100%, and even more preferably, it is 0.010-0.050%.

N is an impurity in steel, and if the content exceeds 0.020%, ductility and toughness deteriorate. Therefore, the N content must be 0.020% or less. The lower the N content, the better, so there is no particular lower limit for the content, but excessively low N content increases costs, so it is preferably 0.0005% or more.

Ceq (carbon equivalent) is a value used to estimate hardness and weldability from the chemical composition of steel, and is calculated using formula (1). The higher the Ceq, the better the hardness and the worse the weldability. If Ceq is less than 0.20, sufficient strength for a structure cannot be obtained. Therefore, Ceq is set to 0.20 or more, and preferably 0.23 or more. If Ceq exceeds 0.50, weldability deteriorates, and interpass temperature control and post-heat treatment become necessary, increasing welding costs. Therefore, Ceq is set to 0.50 or less, and preferably 0.40 or less, and more preferably 0.35 or less.
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(1)
In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass %) of each element in the chemical composition of the base material.

The composition of the base material further includes, in mass %, replacing a portion of the Fe, Ni: 0.01 to 3.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, W: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 0.50%, Se + Te: 0.01 to 0.10%, V: 0.001 to 0.100%, Ti: Contains one or more selected from the following: 0.001-0.200%, Al: 0.005-0.300%, Ca: 0.0003-0.0100%, B: 0.0003-0.0030%, Mg: 0.0003-0.0100%, Zr + Hf + Ta: 0.0001-0.0100%, and REM: 0.0003-0.0100%.

Ni is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 3.00%, it will cause a deterioration in weldability and toughness. Therefore, if Ni is contained, the Ni amount should be 3.00% or less. Preferably, it is 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. The preferred lower limit of the Ni content is 0.01%.

Cr is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 1.00%, it will cause a deterioration in weldability and toughness. Therefore, if Cr is contained, the Cr amount should be 1.00% or less. Preferably, it should be 0.50% or less, and more preferably, it should be 0.30% or less. The preferred lower limit of the Cr content is 0.01%.

Molybdenum is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 0.50%, it will cause a deterioration in weldability and toughness. Therefore, if Mo is contained, the Mo content should be 0.50% or less. Preferably, it is 0.30% or less, and more preferably, it is 0.1% or less. The preferred lower limit of the Mo content is 0.01%.

W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. If W exceeds 1.00%, hot workability deteriorates, resulting in reduced productivity. Therefore, if W is contained, the W content should be 1.00% or less. Preferably, it is 0.50% or less, and more preferably, it is 0.30% or less. The preferred lower limit for W content is 0.01%.

Cu is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 2.00%, it will cause a deterioration in weldability and toughness. Therefore, if Cu is contained, the Cu amount should be 2.00% or less. Preferably, it is 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. The preferred lower limit of the Cu content is 0.01%.

Co is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 0.50%, hot workability is impaired, resulting in reduced productivity. Therefore, if Co is contained, the Co amount should be 0.50% or less. Preferably, it is 0.30% or less, and more preferably, it is 0.1% or less. The preferred lower limit of the Co content is 0.01%.

Se and Te inhibit easily oxidizable elements in steel sheet, such as Mn, Si, and Al, from diffusing to the steel sheet surface and forming oxides, improving the surface quality and platability of the steel sheet. However, this effect saturates when the total exceeds 0.10%. Therefore, when Se and Te are added, the total amount of Se and Te should be 0.10% or less, and more preferably 0.05% or less. The preferred lower limit for the Se + Te content is 0.01%.

Al is an element that is effective in deoxidizing steel. However, if it exceeds 0.300%, it can cause a deterioration in the toughness of welds. Therefore, if Al is contained, the Al content should be 0.300% or less, preferably 0.100% or less. The preferred lower limit of the Al content is 0.005%.

V increases the strength of steel by forming carbonitrides. However, if it exceeds 0.100%, it causes deterioration of weldability and toughness. Therefore, if V is contained, the V content should be 0.100% or less, preferably 0.050% or less. The preferred lower limit of V content is 0.001%.

Ti is an element that refines crystal grains and increases strength, and this effect is apparent when added at 0.001% or more. However, exceeding 0.200% impairs weldability and increases alloy costs. Therefore, the Ti content is set to 0.001-0.200%. It is preferably 0.005-0.100%, and even more preferably 0.010-0.050%.

Ca is an element that refines the structure of the heat-affected zone and improves toughness. However, if it exceeds 0.0100%, it forms coarse inclusions, degrading toughness. Therefore, if Ca is contained, the Ca content should be 0.0100% or less. Preferably, it is 0.0050% or less, and more preferably, it is 0.0030% or less. The preferred lower limit for the Ca content is 0.0003%.

B is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 0.0030%, it will cause a deterioration in weldability and toughness. Therefore, if B is contained, the B content should be 0.0030% or less, preferably 0.0015% or less. The preferred lower limit of the B content is 0.0003%.

Mg is an element that improves ductility and toughness by controlling the morphology of sulfide inclusions. However, if it exceeds 0.0100%, the amount of non-metallic inclusions increases, reducing ductility and toughness. Therefore, if Mg is contained, it should be 0.0100% or less. Preferably, it is 0.0050% or less, and more preferably, it is 0.0030% or less. The preferred lower limit for Mg content is 0.0003%.

Zr, Hf, and Ta are elements that contribute to improving formability. If the total content of one or more of Zr, Hf, and Ta exceeds 0.01%, ductility may decrease. Therefore, when one or more of Zr, Hf, and Ta are contained, the total content should be 0.0100% or less. Preferably, it is 0.0070% or less. The preferred total content of one or more of Zr, Hf, and Ta is 0.0001%.

REM refines the structure of the weld heat-affected zone and improves toughness. However, if the content exceeds 0.0100%, coarse inclusions are formed, degrading toughness. Therefore, if REM is contained, the REM content should be 0.0100% or less, preferably 0.005% or less. The preferred lower limit for REM content is 0.0003%.

Here, REM is a collective term for 17 elements, including 15 lanthanide elements plus Y and Sc. One or more of these 17 elements can be contained in steel, and the REM content refers to the total content of these elements.

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

4. The corrosion-resistant alloy is a 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 described above, the corrosion-resistant alloy contains a large amount of Cr, and the diffusion of this Cr improves the hardenability of the cladding interface, facilitating transformation to martensite. At the same time, carbon on the base metal side diffuses to the cladding material side, forming a hard martensite phase at the base metal side interface, which reduces the fracture toughness of the joint interface. In other words, the effects of the present invention are achieved when a corrosion-resistant alloy with a large amount of Cr is used. When the Cr content of the cladding material is 10% or more, the effects of applying the present invention are significantly enhanced. When the Cr content is 15% or more, the effects are even more significantly enhanced.

This invention relates to technology for clad steel plates with excellent fracture toughness at the bonded interface through control of the bonded interface structure, and to a manufacturing method for such plates. While the type of clad material is not particularly specified, examples of clad materials include stainless steel and nickel-based alloys. Stainless steels include austenitic stainless steel, ferritic stainless steel, and duplex stainless steel, while nickel-based alloys come in a variety of alloy compositions and are marketed under trade names such as Inconel, Incoloy, and Hastelloy.

5. Manufacturing method
The method for manufacturing a clad steel plate according to the present invention will now be described. As mentioned above, in order to obtain a good fracture toughness value at the bonding surface, it is necessary to control the metal structure. Such a metal structure can be achieved by combining the chemical composition of the steel with appropriate manufacturing conditions.
In the clad steel plate described above, the base material and cladding material are laminated so that the bonding surface is vacuum, and the four peripheries of the bonding surface are sealed by welding to form a clad material. The clad rolled material assembled from one or more clad materials is subjected to heating and hot rolling such that d calculated by formula (2) is 1 or more and 9 or less, based on the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from the time the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until removal from the heating furnace, and the rolling reduction ratio r calculated by material thickness/product thickness. After rolling, the clad steel plate is manufactured by cooling the material at an average cooling rate of 2°C/s or more in the TA3 (°C) to 650°C range of the material calculated by formula (3).
d=2.2×10 ⁵ ×(√(exp(-3.2×10 ⁴ /(T+273))×t))/r ...Formula (2)
TA3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3)

5-1. Clad Material Clad materials are manufactured by the following method. Specifically, carbon steel and low-alloy steel, which serve as base materials, and a corrosion-resistant alloy, which serves as cladding material, are melted using a known method such as a converter, electric furnace, or vacuum melting furnace, and then slabs are produced by a continuous casting method or an ingot-blooming method. The resulting slabs are hot-rolled under commonly used conditions to produce hot-rolled sheets, which serve as cladding materials and base materials. The resulting hot-rolled sheets may be annealed, pickled, polished, or the like, as necessary.
The clad material is assembled by laminating the cladding material and the base material so that the bonding surface is vacuum, and then sealing the four periphery of the bonding surface by welding. An insert material such as Ni foil may be inserted between the cladding material and the base material to improve adhesion and interfacial corrosion resistance. The method for vacuuming the bonding surface is not particularly specified, but examples include electron beam welding in a vacuum, or a method in which a vacuum hole is pre-drilled, and the four periphery is welded in the atmosphere by arc welding or laser welding, followed by vacuuming with a vacuum pump. A vacuum level of 0.1 Torr or less results in a good bonded interface with little interfacial oxide, and is more preferably 0.05 Torr or less. Since a higher vacuum level tends to result in a better bonded interface, no lower limit is set.
The obtained clad material may be subjected to hot rolling as it is as a clad rolling material, or two clad materials may be stacked together with a release agent applied between them, and then the stacked clad material may be subjected to hot rolling. When stacking two clad materials, it is desirable that the base materials and clad materials have the same thickness to reduce plate warpage during cooling. Of course, the assembly method does not have to be limited to the one described above.

5-2. Hot Rolling Next, the obtained clad rolled material is heated and hot rolled such that d, calculated by equation (3) using the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from when the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until extraction from the heating furnace, and the rolling reduction ratio r calculated as the material thickness/product thickness, is 1 to 9. If d exceeds 9, the element diffusion distance at the product interface becomes long, increasing the width of the region where martensitic transformation can occur and reducing the fracture toughness of the interface. Preferably, d is 7 or less. If d is less than 1, element diffusion at the interface is too little, 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)

The maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from when the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until the material is removed from the heating furnace, and the reduction ratio r calculated by dividing the material thickness by the product thickness may be appropriately determined so that d falls within the above-mentioned ranges, but preferred ranges are shown below in terms of properties other than the fracture toughness of the interface and manufacturability.
The maximum heating temperature T in the heating furnace is preferably 1050 to 1250°C. If the maximum heating temperature T is less than 1050°C, the hot workability deteriorates and the bonding strength also deteriorates. Therefore, the maximum heating temperature T is preferably 1050°C or higher, and more preferably 1100°C or higher. On the other hand, if the maximum heating temperature T exceeds 1250°C, the steel billet may be deformed in the heating furnace, defects may easily occur during hot rolling, and diffusion at the interface may become rapid. Therefore, the maximum heating temperature T is preferably 1250°C or lower, and more preferably 1220°C or lower.
The shorter the time t (minutes) from when the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until the steel sheet is extracted from the heating furnace, the shorter the element diffusion distance at the interface, so no lower limit is set, but heating for 30 minutes or more is desirable in order to make the temperature uniform up to the center of the plate thickness.
The reduction ratio r, calculated by dividing the material thickness by the product thickness, is preferably 3 or more and 15 or less. If the reduction ratio r is less than 3, the interface bonding by rolling may be insufficient, resulting in a low shear strength at the interface. A reduction ratio of 5 or more is more preferable. Furthermore, if the reduction ratio exceeds 15, the rolling time becomes long, increasing the rolling cost. A reduction ratio of 10 or less is more preferable.

As mentioned above, the size of the martensitic phase region at the interface is primarily influenced by Cr diffusion. While Cr diffusion occurs at temperatures above several hundred degrees Celsius, the diffusion distance increases exponentially as the temperature increases, so substantial diffusion occurs during the heating period when the material is maintained near its maximum temperature. Furthermore, because the sheet temperature drops rapidly during rolling and cooling, diffusion is negligibly small. Therefore, the Cr diffusion distance in the product can be considered to be the diffusion distance generated during heating, reduced by a factor of the reduction ratio. The authors measured the size of the martensitic phase region at the interface using thin-film TEM observations of clad products with various heating temperatures, times, and reduction ratios. They confirmed that the value d calculated using equation (2) based on the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from when the heating temperature in the heating furnace reached the maximum heating temperature T-20°C to the time of extraction from the heating furnace, and the reduction ratio r, accurately corresponds to the size of the martensitic phase region.

5-3. Cooling after Rolling The average cooling rate of the material in the TA3 (°C) to 650°C range calculated from Equation (3) after rolling is desirably 2°C/s or higher. At a cooling rate of less than 2°C/s, carbon diffuses and concentrates in the austenite region at the interface that can become martensite due to the austenite to ferrite transformation or austenite to ferrite + pearlite transformation, increasing the width of the region where the nanohardness is 7 GPa or higher. A cooling rate of 4°C/s or higher is preferable. There is no particular upper limit, but a cooling rate of 10°C/s or lower is desirable because a martensite structure becomes dominant at a fast rate, resulting in the base material becoming too strong or its toughness deteriorating.
TA3 (℃)=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 chemical composition of the base steel plate.

The present invention makes it possible to obtain clad steel plates with excellent fracture toughness at the joining surface. The clad steel plates of the present invention and welded structures made using the clad steel plates of the present invention do not require measures to prevent peeling during welding or additional heat treatment. Furthermore, the clad steel plates are not limited in their applications and can be applied to structural components where solid steel plates have traditionally been used. Therefore, the clad steel plates make a significant contribution to cost reduction.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

A clad material with the chemical composition shown in Table 1 and a base material with the chemical composition shown in Table 2 were melted and formed into billets. These were then hot-rolled, annealed, and pickled to produce clad steel plates with a thickness of 30 mm and base steel plates with a thickness of 130 mm. Using the resulting clad material and base steel, the base steel and clad material were laminated so that the bonding surface was vacuum-sealed and the four edges of the bonding surface were sealed by welding to create a clad material. Two clad materials were stacked in the following order: base steel - clad material - release agent - clad material - base steel, with a release agent applied between them, and assembled as a clad rolling material. The resulting clad rolling material was hot-rolled under the hot-rolling conditions shown in Table 3, and then peeled at the release agent portion to produce clad steel plates with thicknesses of 53 mm (reduction ratio 3) to 12 mm (reduction ratio 13).

The rolling conditions listed in Table 3 were varied to examine each characteristic value during the rolling of clad steel plates. The manufacturing conditions in Table 3 are explained below. In Table 3, T indicates the maximum heating temperature (°C) in the heating furnace before rolling, and t indicates the time (minutes) from when the heating temperature in the heating furnace reached the maximum heating temperature T-20°C until extraction from the heating furnace. r indicates the reduction ratio calculated by the material thickness/product thickness. d indicates the value calculated from the above T, t, and r using formula (2). TA3 indicates the value (°C) calculated from the chemical composition of the base material using formula (3). CR indicates the average cooling rate (°C/s) from TA3 (°C) to 650°C. L indicates the width (μm) of the region near the interface where the nanohardness is 7 GPa or more. The CTOD value (mm) is the result of a CTOD test.
d=2.2×10 ⁵ ×(√(exp(-3.2×10 ⁴ /(T+273))×t))/r ...Formula (2)
TA3 (℃)=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 chemical composition of the base steel plate.

Nanohardness was measured in accordance with the instrumented indentation hardness test specified in ISO 14577. Nanohardness was measured at 0.5 μm intervals over a 10 μm range from the interface in the thickness direction on both the cladding and base metal sides. Nanohardness measurement conditions can be selected as appropriate, but an example is to perform measurements three times at each position using a load of 1000 μN, 5 seconds to the specified indentation load, 0 seconds of hold, and 5 seconds of return, and then calculate the average value to be the nanohardness. The range where the nanohardness was 7 GPa or greater was read and designated L. If an insert material such as Ni foil was inserted between the cladding material and base material, measurements should be taken at the interface between the cladding material and the insert material, and at the interface between the insert material and base material.

The following CTOD test was conducted to evaluate fracture toughness (see Figures 1 to 4). In Figures 1 to 4, components with the chemical composition of the clad material are indicated by dot hatching. The obtained clad steel plate 1 was cut to a length L of 170 mm and a width W of 15 mm. A supplementary member 5 with the same chemical composition as the clad material 2 was attached to the clad material side in the plate thickness direction, and a supplementary member 4 with the same chemical composition as the base material 3 was attached to the base material side, using a single electron beam penetration welding to create a welded joint 13. Electron beam welding was performed downward at a vacuum of 0.1 torr, a voltage of 60 kV, a current of 80 Am, an object distance of approximately 300 mm, and a speed of 90 mm/min.

A CT test piece 12 was taken from approximately the center of the above-mentioned welded joint 13 as a fracture toughness test piece. The cutout was made so that the center line of the machine notch 11 was aligned with the clad interface 10. The thickness of the CT test piece 12 was set to 10 mm. Furthermore, side grooves 8 were machined on both sides of the CT test piece 12 along the clad interface 10, with an angle of 45 degrees, a bottom radius of curvature of 0.25 mm, and a depth of 1 mm.

The CT test piece 12 was placed in a hydraulic servo-type fatigue testing machine (not shown), and a fatigue pre-crack 9 approximately 1.5 mm long was introduced from the bottom of the machine notch 11. The CT test piece 12 was cooled to 0°C and pulled between pins at a displacement rate of 0.01 mm/sec (not shown), and the fracture characteristics from the tip of the fatigue pre-crack (limiting CTOD (mm)) were obtained. Hereinafter, this will be referred to as the "CTOD value."

Samples No. 1 to 44 are examples of the present invention, satisfying the preferred manufacturing conditions, with the length L of the region where the nanohardness is 7 GPa or more being 5 μm or less, and the CTOD value of the clad interface being 0.15 mm or more. Samples No. 45 to 50 are comparative examples, not satisfying the preferred manufacturing conditions, with the length L of the region where the nanohardness is 7 GPa or more being greater than 5 μm, and the CTOD value of the clad interface being less than 0.15 mm.

As described above, the example of the present invention achieved good fracture toughness at the bonded surface. On the other hand, the comparative example did not meet the preferred manufacturing conditions, and the length of the region where the nanohardness was 7 GPa or greater fell outside the specifications of the present invention, resulting in a CTOD value at the clad interface that fell outside the specifications of the present invention.

The present invention makes it possible to obtain clad steel plates with excellent fracture toughness at the bonding surface, making them extremely useful industrially. By using a corrosion-resistant alloy as the cladding material, the clad steel plates of the present invention may be applicable to corrosive environments such as high-chloride environments exposed to seawater, and corrosive environments in plant equipment exposed to acid solutions such as phosphoric acid or sulfuric acid. Specific examples include seawater desalination plants, flue gas desulfurization equipment, chemical storage tanks, structural components such as oil well tubular goods, pumps and valves, and heat exchangers.

1: Clad steel plate 2: Cladding material 3: Base material 4: Supplementary material (material equivalent to base material)
5: Supplementary material (same material as cladding material)
6: Weld line 7: Cutting position of CT test piece 8: Side groove 9: Fatigue pre-crack 10: Cladding interface 11: Mechanical notch 12: CT test piece 13: Weld joint L: Length W: Width H2: Thickness of cladding material H3: Thickness of base material H4: Height of supplementary member 4 H5: Height of supplementary member 5

Claims (4)

A clad steel plate comprising a base material and a clad material joined to the base material,
the base material is made of carbon steel or low alloy steel,
The base metal has a chemical composition containing, in mass%, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.001 to 0.200%, and N: 0.020% or less, with Ceq being 0.20 to 0.50, and the balance being Fe and impurities;
the cladding material is made of a corrosion-resistant alloy,
The clad steel plate is made of a stainless steel or nickel-based alloy containing 10% or more Cr by mass%,
At the interface between the base material and the clad material of the clad steel plate (hereinafter referred to as the "clad interface"), the width in the plate thickness direction of the region where the nanohardness is 7 GPa or more is 5 μm or less,
A clad steel plate characterized in that, in evaluating the fracture toughness of the clad interface, a supplementary member is welded in one pass by electron beam or laser welding, and the CTOD value of the clad interface measured using a CT test piece in which a side groove is provided along the clad interface is 0.15 mm or more at 0°C.
Here, Ceq is defined by the following equation (1).
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base steel plate. The chemical composition of the base metal further contains, in mass %, Ni: 0.01 to 3.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, W: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 0.50%, Se+Te: 0.01 to 0.10%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, and 2. The clad steel plate according to claim 1, containing one or more selected from the group consisting of Al: 0.005 to 0.300%, Ca: 0.0003 to 0.0100%, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0100%, Zr+Hf+Ta: 0.0001 to 0.0100%, and REM: 0.0003 to 0.0100 % . 3. The clad steel plate according to claim 1 or claim 2 , wherein the base material and cladding material are laminated so that the bonding surface is vacuum, and the four periphery of the bonding surface is sealed by welding to form a clad material, and the clad rolled material assembled from one or more of the clad materials is subjected to heating and hot rolling such that d calculated by formula (2) is 1 or more and 9 or less, based on the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from the time the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C to the time of extraction from the heating furnace, and the rolling reduction ratio r calculated by the material thickness / product thickness, and the material is subjected to heating and hot rolling such that d calculated by formula (2) is 1 or more and 9 or less, and the material is cooled at an average cooling rate of 2°C/s or more in the TA3 (°C) to 650°C range calculated by formula (3), and the width in the plate thickness direction of the region where the nanohardness at the interface between the base material and the cladding material is 7 GPa or more is 5 μm or less, and the CTOD value at the clad interface is 0 °C or more .
d=2.2×10 ⁵ ×(√(exp(-3.2×10 ⁴ /(T+273))×t))/r ...Formula (2)
TA3 (℃)=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 chemical composition of the base steel plate. A welded structure made using the clad steel plate according to claim 1 or 2 .