WO2024101317A1

WO2024101317A1 - Clad steel sheet and method for producing same

Info

Publication number: WO2024101317A1
Application number: PCT/JP2023/039921
Authority: WO
Inventors: 恭野安田; 和彦塩谷
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-11-08
Filing date: 2023-11-06
Publication date: 2024-05-16

Abstract

The present invention provides a high-strength clad steel sheet which has excellent ammonia SCC resistance and excellent toughness at low temperatures, and which is suitable for use for a tank that is used for transportation or storage of liquid ammonia, or the like. According to the present invention, CE_B/CE_C is 2.000 or more; with respect to the metal structure of the base material, the volume fraction of bainite is 90% or more and the average grain size of bainite is 25 µm or less; the cladding material has a Vickers hardness of 210 HV10 or less; t_C1 is 2.0 mm or more; t_C2 is 0.0 mm or more; and (t_C1 + t_C2)/(t_B + t_C1 + t_C2) is 0.30 or less.

Description

Clad steel plate and its manufacturing method

The present invention relates to a clad steel plate and a manufacturing method thereof. The present invention also relates to a high-strength clad steel plate with excellent low-temperature toughness and ammonia SCC resistance, which is particularly suitable for use as structural components for tanks and the like in low-temperature, liquid ammonia environments.

In a liquid ammonia environment, there is concern that carbon steel may suffer from liquid ammonia-induced stress corrosion cracking (hereafter referred to as "ammonia SCC"). For this reason, steel materials with low ammonia SCC susceptibility have been used for carbon steel structures that handle liquid ammonia, such as piping, storage tanks, tank cars, and line pipes, and operational measures have been taken to suppress ammonia SCC.

For example, ammonia SCC is known to be correlated with the strength and hardness of materials. In other words, when using carbon steel, it is considered desirable to use materials with a tensile strength of less than 600 MPa. Therefore, when using high-strength carbon steel (hereinafter also referred to as high-strength steel) in a liquid ammonia environment, measures such as adjusting the tensile strength by performing post-weld heat treatment through full annealing are necessary.

Incidentally, liquid ammonia does not generate _CO2 when burned. Therefore, in recent years, liquid ammonia has been attracting attention as a clean energy source, and large-scale demand is expected. Accordingly, there is a demand for larger facilities for transporting and storing liquid ammonia.
Generally, when making a tank larger, there is a preference to use thinner steel materials in order to reduce weight and construction costs, and therefore the use of high-strength steel is desirable.

Furthermore, to ensure efficient operation of transport and storage facilities, these facilities may be used for both liquid ammonia and LPG. Liquefied gases such as liquid ammonia and LPG are transported and stored at low temperatures. Therefore, the steel materials used for these applications require excellent low-temperature toughness.

For example, Patent Documents 1 to 3 disclose technologies related to steel materials used for the above applications. Of these, Patent Document 1 describes a method for softening the surface of steel materials. Patent Documents 2 and 3 describe methods for manufacturing clad steel plates with a mild steel layer on one side.

Japanese Patent Publication No. 55-30062 Japanese Patent Application Laid-Open No. 57-139493 Japanese Patent Application Laid-Open No. 8-269537

However, the method described in Patent Document 1 requires a long period of heat treatment to uniformly and sufficiently soften the surface layer, making it difficult to control the strength of the center of the steel sheet. This causes problems in terms of strength.

In addition, in the methods described in Patent Documents 2 and 3, clads are manufactured by a casting method, a build-up welding method, and a continuous casting method. However, it cannot be said that the clad steel plates manufactured by the methods described in Patent Documents 2 and 3 simultaneously achieve excellent low-temperature toughness, excellent ammonia SCC resistance, and high strength. In addition, the methods described in Patent Documents 2 and 3 have the economic problem of high costs for the equipment and energy required for their manufacture.

The present invention aims to solve the above problems and provide a high-strength clad steel plate with excellent ammonia SCC resistance and low-temperature toughness suitable for use in tanks for transporting and storing liquid ammonia, and a method for manufacturing the same.

In order to achieve the above object, the present inventors have conducted extensive research into various factors affecting the ammonia SCC resistance, low-temperature toughness, and tensile properties (strength properties) of steel plates, and have obtained the following findings.
That is, since ammonia SCC occurs inside the product (tank), the characteristics of the surface layer of the steel plate on the inside are dominant in ammonia SCC resistance. Therefore, the inventors conceived of a clad steel plate in which a base material and a clad material are bonded together, that is, a clad steel plate in which a steel plate excellent in strength and low-temperature toughness is used as the base material and a steel plate with low hardness is used as the clad material from the viewpoint of improving ammonia SCC resistance.
The present inventors have found that such a clad steel plate is excellent in ammonia SCC resistance, low-temperature toughness and tensile properties.

In addition, the inventors have found that if the thickness of the clad steel plate's clad material is too thin, corrosion will cause wear and tear, deteriorating the ammonia SCC resistance of the clad steel plate. On the other hand, the inventors have found that if the thickness of the clad steel plate's clad material is too thick, the strength of the clad steel plate will decrease.

The clad steel plate of the present invention can significantly reduce alloy costs and manufacturing costs compared to clad steel plates that use stainless steel or non-ferrous alloys as cladding materials.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A clad steel plate having a base material and a cladding material made of carbon steel on at least one side thereof,
The composition of the base material is, in mass%,
C: 0.010 to 0.200%,
Si: 0.01 to 1.00%,
Mn: 0.50 to 2.50%,
Al: 0.001 to 0.060%,
Contains P: 0.0200% or less and S: 0.0100% or less, with the balance being Fe and unavoidable impurities;
_CEB / _CEC is 2.000 or more, where _CEB and _CEC are the carbon equivalents of the base material and the cladding material, respectively;
In the metal structure of the base material, the volume fraction of bainite is 90% or more, and the average grain size of the bainite is 25 μm or less,
The Vickers hardness of the cladding material is 210HV10 or less,
A clad steel plate, wherein _tC1 is 2.0 mm or more, _tC2 is 0.0 mm or more, and ( _tC1 + _tC2 )/( _tB + _tC1 + _tC2 ) is 0.30 or less, where _tB is the plate thickness of the base material, _tC1 is the plate thickness of the clad material on one side of the base material, and _tC2 is the plate thickness of the clad material on the other side of the base material.

2. The composition of the base material is further, in mass%,
Cu: 1.00% or less,
Ni: 2.00% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 0.500% or less,
Ti: 0.100% or less,
Nb: 0.100% or less,
Ca: 0.0200% or less,
2. The clad steel plate according to 1 above, containing one or more selected from Mg: 0.0200% or less and REM: 0.0200% or less.

3. The component composition of the cladding material is, in mass%,
C: 0.100% or less and Mn: 0.01 to 1.50%
and further comprising
Cu: 0.01 to 0.50%,
Cr: 0.01 to 0.50%,
Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50%
and the balance being Fe and unavoidable impurities,
3. The clad steel plate according to claim 1 or 2, wherein a CR value calculated by the following formula (1) is 0.30 or more.
CR value = 2.3 [Cu] + 2.8 [Cr] + 7.3 [Sb] + 3.6 [Sn] ... (1)
Here, [X] indicates the content (mass %) of element X in the component composition of the cladding material.

4. A method for manufacturing a clad steel plate having a cladding material that is carbon steel on at least one surface of a base material, comprising the steps of:
A clad material steel plate obtained by overlapping a base material steel plate having the composition of the base material described in 1 or 2 with a clad material steel plate of the carbon steel described in 1 is heated to 1000 to 1250 ° C.,
Next, the clad material steel plate is subjected to hot rolling at a cumulative rolling reduction rate of 20% or more in the non-recrystallization temperature range of the base material steel plate and at a rolling end temperature of the _Ar3 transformation point or more to obtain a hot-rolled steel plate;
The hot-rolled steel sheet is then cooled to a cooling start temperature of at least the Ar ₃ transformation point, an average cooling rate of 20 to 120°C/s, and a cooling end temperature of 500°C or less.

5. The composition of the cladding material steel plate is, in mass%,
C: 0.100% or less and Mn: 0.01 to 1.50%
and further comprising
Cu: 0.01 to 0.50%,
Cr: 0.01 to 0.50%,
Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50%
and the balance being Fe and unavoidable impurities,
5. The method for producing a clad steel plate according to 4 above, wherein a CR value calculated by the following formula (1) is 0.30 or more.
CR value = 2.3 [Cu] + 2.8 [Cr] + 7.3 [Sb] + 3.6 [Sn] ... (1)
Here, [X] indicates the content (mass %) of element X in the chemical composition of the base steel sheet for cladding.

6. The method for manufacturing clad steel plate described in 4 or 5 above, in which the hot-rolled steel plate is tempered at a temperature of 650°C or less after the cooling.

The present invention provides a high-strength clad steel plate with excellent ammonia SCC resistance and low-temperature toughness, suitable for use in tanks for transporting and storing liquid ammonia. In addition, the clad steel plate of the present invention does not use stainless steel or non-ferrous alloys as cladding materials and has a simple manufacturing process, making it extremely advantageous in terms of cost.

A clad steel plate according to one embodiment of the present invention has a cladding material, which is carbon steel, on at least one surface of a base material.
Here, the clad steel plate according to one embodiment of the present invention has excellent ammonia SCC resistance and low-temperature toughness, and is therefore suitable for structural members such as tanks used in a liquid ammonia environment. The usage environment is not limited to liquid ammonia, and may be liquefied gas such as LPG or liquefied _CO2 . In this disclosure, when referring to ammonia, it means not only liquid ammonia but also LPG, liquefied _CO2 , and other liquefied gases in general.
In the clad steel plate according to one embodiment of the present invention, the surface having the cladding material may be on either side of the base material. However, when used in a tank or the like, the cladding material is placed on the surface expected to come into contact with ammonia or the like (hereinafter also referred to as the first surface or inner surface of the base material). This is because ammonia SCC resistance and low-temperature toughness can be obtained as a tank (structure). In this disclosure, the cladding material placed on the first surface of the base material is also referred to as the first cladding material. Also, the other surface of the first surface of the base material is referred to as the second surface or outer surface of the base material. The cladding material placed on the second surface of the base material is also referred to as the second cladding material. Here, the second cladding material is optional. That is, the clad steel plate according to one embodiment of the present invention includes a clad steel plate having a first cladding material on one surface of the base material and a clad steel plate having a first cladding material on one surface of the base material and a second cladding material on the other surface of the base material. Also, the thickness of the base material is t _B , the thickness of the first cladding material is t _C1 , and the thickness of the second cladding material is t _C2 .

The clad steel plate according to one embodiment of the present invention will be described in more detail below. Note that "%" representing the content of each element below means "mass %" unless otherwise specified.
(1) Composition of the base material C: 0.010-0.200%
C is the most effective element for increasing the strength of the clad steel plate according to one embodiment of the present invention. To obtain this effect, the C content is set to 0.010% or more. Furthermore, from the viewpoint of reducing the content of other alloy elements and manufacturing at a lower cost, the C content is preferably set to 0.030% or more. On the other hand, if the C content exceeds 0.200%, it will lead to deterioration of toughness and weldability. Therefore, the C content is set to 0.200% or less. Furthermore, from the viewpoint of toughness, the C content is preferably set to 0.170% or less.

Si: 0.01 to 1.00%
Si is added for deoxidation. To obtain this effect, the Si content is set to 0.01% or more. Furthermore, the Si content is preferably set to 0.03% or more. On the other hand, if the Si content exceeds 1.00%, it leads to deterioration of toughness and weldability. Therefore, the Si content is set to 1.00% or less. Furthermore, from the viewpoint of toughness, the Si content is preferably set to 0.40% or less.

Mn: 0.50 to 2.50%
Mn is an element that has the effect of increasing the hardenability of steel. That is, Mn is one of the important elements for obtaining high strength. In order to obtain such an effect, the Mn content is set to 0.50% or more. Furthermore, from the viewpoint of reducing the content of other alloy elements and manufacturing at a lower cost, the Mn content is preferably set to 0.70% or more. On the other hand, if the Mn content exceeds 2.50%, the toughness decreases. Therefore, the Mn content is set to 2.50% or less. Furthermore, from the viewpoint of suppressing the decrease in toughness, the Mn content is preferably set to 2.30% or less.

Al: 0.001 to 0.060%
Al acts as a deoxidizer. To obtain this effect, the Al content is set to 0.001% or more. On the other hand, if the Al content exceeds 0.060%, oxide-based inclusions increase and cleanliness decreases. Furthermore, toughness decreases. Therefore, the Al content is set to 0.060% or less. Furthermore, from the viewpoint of suppressing the decrease in toughness, the Al content is preferably set to 0.050% or less.

P: 0.0200% or less P is an element contained as an inevitable impurity. In addition, P has an adverse effect, such as reducing toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to reduce the P content as much as possible. However, the P content is acceptable if it is 0.0200% or less. The lower limit of the P content is not particularly limited and may be 0%. In addition, since P is usually an element that is inevitably contained in steel as an impurity, the P content may be more than 0% industrially. In addition, excessive reduction of P leads to an increase in refining costs. Therefore, the P content is preferably 0.0005% or more.

S: 0.0100% or less S is an element contained as an inevitable impurity. In addition, S is an element that exists in steel as sulfide-based inclusions such as MnS, and has an adverse effect such as becoming the origin of fracture and reducing toughness. Therefore, it is desirable to reduce the S content as much as possible. However, the S content is acceptable if it is 0.0100% or less. The lower limit of the S content is not particularly limited and may be 0%. In addition, since S is usually an element that is inevitably contained in steel as an impurity, the S content may be more than 0% industrially. In addition, excessive reduction of S leads to an increase in refining costs. Therefore, from the viewpoint of cost, it is preferable to set the S content to 0.0005% or more.

In addition, the composition of the base material of the clad steel plate according to one embodiment of the present invention may optionally contain the elements described below (hereinafter also referred to as optional added elements). In addition, in the composition of the base material of the clad steel plate according to one embodiment of the present invention, the remainder other than the above elements and the following optional added elements is Fe and unavoidable impurities.

Cu: 1.00% or less Cu is an effective element for improving the strength of clad steel plate. However, if the Cu content is less than 0.01%, the effect is poor. Therefore, when Cu is contained, the Cu content is preferably 0.01% or more. On the other hand, if the Cu content exceeds 1.00%, the toughness deteriorates. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less.

Ni: 2.00% or less Ni is not only effective in improving the strength of the clad steel plate, but also has the effect of improving the toughness. However, if the Ni content is less than 0.01%, the effect is poor. Therefore, when Ni is contained, the Ni content is preferably 0.01% or more. On the other hand, if the Ni content exceeds 2.00%, the effect is saturated and the alloy cost increases. Therefore, when Ni is contained, the Ni content is preferably 2.00% or less.

Cr: 1.00% or less Cr is an effective element for improving the strength of clad steel plate. However, if the Cr content is less than 0.01%, the effect is poor. Therefore, when Cr is contained, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 1.00%, the toughness deteriorates. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less.

Mo: 1.00% or less Mo is an effective element for improving the strength of clad steel plate. However, if the Mo content is less than 0.01%, the effect is poor. Therefore, when Mo is contained, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 1.00%, the toughness deteriorates. Therefore, when Mo is contained, the Mo content is preferably 1.00% or less.

V: 0.500% or less V is an element that has the effect of improving the strength of the clad steel plate. In order to obtain such an effect, when V is contained, the V content is preferably 0.005% or more. On the other hand, if the V content exceeds 0.500%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when V is contained, the V content is preferably 0.500% or less. More preferably, the lower limit of the V content is 0.010%. More preferably, the upper limit of the V content is 0.100%.

Ti: 0.100% or less Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing solute N. Therefore, the addition of Ti can improve the toughness of the base material and the welded part. In order to obtain such an effect, when Ti is contained, the Ti content is preferably 0.005% or more. Furthermore, the Ti content is more preferably 0.007% or more. On the other hand, when the Ti content exceeds 0.100%, the toughness is rather reduced. Therefore, when Ti is contained, the Ti content is preferably 0.100% or less. Furthermore, the Ti content is more preferably 0.090% or less.

Nb: 0.100% or less Nb is an element that has the effect of reducing the prior austenite grain size and improving toughness by precipitating as carbonitride. In order to obtain such an effect, when Nb is contained, the Nb content is preferably 0.005% or more. Furthermore, the Nb content is more preferably 0.007% or more. On the other hand, when the Nb content exceeds 0.100%, a large amount of NbC precipitates, and the toughness decreases. Therefore, when Nb is contained, the Nb content is preferably 0.100% or less. Furthermore, the Nb content is more preferably 0.060% or less.

Ca: 0.0200% or less Ca is an element that combines with S and has the effect of suppressing the formation of MnS and the like that elongates in the rolling direction. That is, by including Ca, the morphology of the sulfide-based inclusions is controlled to be spherical, and the toughness of the welded portion and the like can be improved. In order to obtain such an effect, when Ca is included, the Ca content is preferably 0.0005% or more. On the other hand, when the Ca content exceeds 0.0200%, the cleanliness of the steel decreases. The decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is included, the Ca content is preferably 0.0200% or less. The Ca content is more preferably 0.0020% or more. The Ca content is more preferably 0.0100% or less.

Mg: 0.0200% or less Like Ca, Mg is an element that combines with S and has the effect of suppressing the formation of MnS and the like that elongates in the rolling direction. That is, by including Mg, the morphology of the sulfide-based inclusions is controlled to be spherical, and the toughness of the welded part and the like can be improved. In order to obtain such an effect, when Mg is included, the Mg content is preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0200%, the cleanliness of the steel decreases. The decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is included, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0020% or more. The Mg content is more preferably 0.0100% or less.

REM: 0.0200% or less Like Ca and Mg, REM (rare earth metal) is an element that combines with S and suppresses the formation of MnS, etc., which elongates in the rolling direction. That is, by containing REM, the morphology of sulfide-based inclusions is controlled to be spherical, and the toughness of welds, etc. can be improved. In order to obtain such an effect, when REM is contained, the REM content is preferably 0.0005% or more. On the other hand, if the REM content exceeds 0.0200%, the cleanliness of the steel decreases. The decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is contained, the REM content is preferably 0.0200% or less. The REM content is more preferably 0.0020% or more. The REM content is more preferably 0.0100% or less.

(2) Composition of Cladding Material In a clad steel plate according to one embodiment of the present invention, a carbon steel having a CE _B /CE _C ratio of 2.000 or more is used for the cladding material.
Carbon steel is an alloy of iron (Fe) and carbon (C) and has a composition in which the C content is 2.2 mass% or less, the total content of elements other than C and Fe (such as Si, Mn, P, and S, and also including elements corresponding to unavoidable impurities) is 5.0 mass% or less, and the balance is Fe.
CE _B /CE _C : 2.000 or more CE _B /CE _C is set to 2.000 or more. Here, CE _B and CE _C are the carbon equivalent of the base material and the carbon equivalent of the cladding material, respectively. If CE _B /CE _C is less than 2.000, the carbon equivalent of the cladding material becomes too large and the hardness of the clad steel plate increases too much, resulting in deterioration of ammonia SCC resistance. CE _B /CE _C is preferably 2.050 or more, more preferably 2.100 or more. In addition, there is no particular upper limit for CE _B /CE _C. CE _B /CE _C is preferably 5.000 or less, for example.
It should be noted that CE _B and CE _C can be calculated by the following formulas.
CE _B = [C] _B + [Mn] _B /6 + [Si] _B /24 + [Ni] _B /40 + [Cr] _B /5 + [Mo] _B /4 + [V] _B /14
Here, [X] _B represents the content (mass%) of element X in the composition of the base material. Note that elements that are not contained may be calculated as "0". The same applies to the following formulas.
_CEC = [C] + [Mn]/6 + [Si]/24 + [Ni]/40 + [Cr]/5 + [Mo]/4 + [V]/14
Here, [X] indicates the content (mass %) of element X in the component composition of the cladding material.

The preferred composition of the clad steel plate according to one embodiment of the present invention is as follows:
C: 0.100% or less C is an element that increases the hardness of steel, and the higher the hardness, the higher the liquid ammonia SCC susceptibility. Therefore, the C content of the cladding material is preferably 0.100% or less. On the other hand, the lower the C content of the cladding material, the better, but excessive reduction of C leads to an increase in refining costs. Therefore, the C content is preferably 0.0005% or more.

Mn: 0.01 to 1.50%
Mn is an element that has the effect of increasing the hardenability of steel. Therefore, if the Mn content is too high, the hardness of the cladding material will increase too much. If the hardness of the cladding material increases too much, it will lead to deterioration of ammonia SCC resistance. Therefore, it is preferable to set the Mn content to 1.50% or less. The Mn content is more preferably 1.25% or less, and further preferably 1.00% or less. On the other hand, it requires a large cost to reduce Mn to less than 0.01%. Therefore, it is preferable to set the Mn content to 0.01% or more.

Cu, Cr, Sb and Sn can further improve the ammonia SCC resistance of the steel sheet. Therefore, it is preferable to contain one or more of these elements in the amounts described below and to set the CR value calculated by the following formula (1) to 0.30 or more.
The CR value is a formula devised for estimating ammonia SCC resistance from the content of each element, and the higher the CR value, the better the ammonia SCC resistance. Therefore, by making the CR value 0.30 or more, it becomes possible to effectively suppress stress corrosion cracking in a liquid ammonia environment.
CR value = 2.3 [Cu] + 2.8 [Cr] + 7.3 [Sb] + 3.6 [Sn] ... (1)
Here, [X] indicates the content (mass%) of element X in the composition of the cladding material. Since the composition of the cladding material and the composition of the cladding material steel plate are substantially the same, [X] can also be said to indicate the content (mass%) of element X in the composition of the cladding material steel plate. Similarly, the above-mentioned [X] _B can also be said to indicate the content (mass%) of element X in the composition of the base material steel plate.

Here, by using a low-hardness steel plate as the cladding material, it is possible to suppress ammonia SCC. However, if there are dents or scratches on the surface of the cladding material, stress concentration occurs at the location of the dents or scratches, and there is a concern that the ammonia SCC resistance will deteriorate. In this regard, by setting the CR value to 0.30 or more, it is possible to prevent the ammonia SCC resistance from deteriorating even if there are dents or scratches on the surface of the cladding material. Therefore, the CR value is preferably 0.30 or more. The CR value is more preferably 0.32 or more, and even more preferably 0.35 or more. Furthermore, there is no particular limit to the upper limit of the CR value. For example, a CR value of 1.50 or less is preferable.

In addition, Cu, Cr, Sb and Sn have the effect of quickly forming protective corrosion products in a liquid ammonia environment and suppressing stress corrosion cracking. In order to obtain such an effect, when Cu is contained, the Cu content is preferably 0.01% or more. When Cr is contained, the Cr content is preferably 0.01% or more. When Sb is contained, the Sb content is preferably 0.01% or more. When Sn is contained, the Sn content is preferably 0.01% or more.

On the other hand, excessive addition of Cu, Cr, Sb, and Sn deteriorates weldability and toughness. It is also disadvantageous from the viewpoint of alloy cost. Therefore, when Cu is contained, it is preferable that the Cu content be 0.50% or less. When Cr is contained, it is preferable that the Cr content be 0.50% or less. When Sb is contained, it is preferable that the Sb content be 0.50% or less. When Sn is contained, it is preferable that the Sn content be 0.50% or less. More preferably, the Cu content is 0.40% or less, the Cr content is 0.40% or less, the Sb content is 0.40% or less, and the Sn content is 0.40% or less.

In the preferred composition of the clad steel plate composite material according to one embodiment of the present invention, the remainder other than the above elements is Fe and unavoidable impurities.

(3) Metal structure [Base metal structure: Volume fraction of bainite is 90% or more, and the average grain size of bainite is 25 μm or less]
In order to satisfy the tensile properties and low-temperature toughness of the base material, the volume fraction of bainite in the metal structure of the base material needs to be 90% or more. In other words, if the volume fraction of bainite is less than 90%, the volume fractions of other elements such as ferrite, island martensite, martensite, pearlite, and austenite will increase, and sufficient strength and low-temperature toughness will not be obtained. The upper limit of the volume fraction of bainite is not particularly limited, and may be 100%.

Here, bainite includes structures called bainitic ferrite and granular ferrite, as well as structures obtained by tempering these structures. Here, the structures called bainitic ferrite and granular ferrite are structures that contribute to transformation strengthening and are formed during or after cooling after hot rolling.
The remaining structure, which has a volume fraction of 10% or less, may contain martensite in addition to ferrite, pearlite, and austenite. The volume fraction of each structure in the remaining structure does not need to be particularly limited, but it is preferable that the remaining structure is pearlite. The volume fraction of the remaining structure may be 0%.

Furthermore, in order to obtain excellent low-temperature toughness, the average grain size of bainite is set to 25 μm or less. That is, if the average grain size of bainite exceeds 25 μm, the crack propagation resistance decreases and sufficient low-temperature toughness cannot be obtained. The average grain size of bainite is preferably 22 μm or less. Furthermore, there is no particular lower limit for the average grain size of bainite. For example, the average grain size of bainite is preferably 1 μm or more.
Here, the volume fraction and average grain size of bainite can be measured by the method described in the examples below.

The metal structure of the cladding material is not particularly limited. The metal structure of the cladding material may be, for example, a conventionally known metal structure of carbon steel, such as ferrite and bainite.

[Vickers hardness of cladding material: 210HV10 or less]
The Vickers hardness of the cladding material is 210HV10 or less. If a high hardness region exists in the surface layer of the clad steel plate, ammonia SCC is promoted. In other words, if the Vickers hardness of the cladding material exceeds 210HV10, the desired ammonia SCC resistance cannot be obtained. The Vickers hardness of the cladding material is preferably 200HV10 or less. The lower limit of the Vickers hardness of the cladding material is not particularly limited. For example, the Vickers hardness of the cladding material is preferably 100HV10 or more.
The Vickers hardness can be measured by the method described in the Examples section below.
Furthermore, the Vickers hardness of the base material is not particularly limited, and may be 210 HV10 or less, or may be more than 210 HV10.

(4) Plate thickness of base material and cladding material [ _tC1 is 2.0 mm or more, _tC2 is 0.0 mm or more, and ( _tC1 + _tC2 )/( _tB + _tC1 + _tC2 ) is 0.30 or less]
_tC1 is 2.0 mm or more, _tC2 is 0.0 mm or more, and ( _tC1 + _tC2 )/( _tB + _tC1 + _tC2 ) (hereinafter also referred to as the "cladding ratio") is 0.30 or less, where _tB (mm) is the thickness of the base material, _tC1 (mm) is the thickness of the cladding material on one side of the base material, i.e., the thickness of the first cladding material, and _tC2 (mm) is the thickness of the cladding material on the other side of the base material, i.e., the thickness of the second cladding material.
If t _C1 is less than 2.0 mm, the low hardness region is worn away by corrosion, and the ammonia SCC resistance is deteriorated. t _C1 is preferably 2.1 mm or more, more preferably 2.2 mm or more. t _C1 is preferably 5.0 mm or less.
Also, if the clad ratio exceeds 0.30, the ratio of the low hardness clad material to the base material that provides the strength becomes too large, and sufficient strength cannot be obtained. The clad ratio is preferably 0.20 or less. The clad ratio is preferably 0.05 or more.
In addition, since the second cladding material may not be used (in other words, the second cladding material is optional), t _C2 may be 0.0 mm or more. t _C2 is preferably 0.5 mm or more, more preferably 1.0 mm or more. t _C2 is preferably 5.0 mm or less.
The total thickness of the clad steel plate, typically t _B +t _C1 +t _C2 , is preferably 8 mm or more, more preferably 15 mm or more. The total thickness of the clad steel plate is preferably 50 mm or less, more preferably 40 mm or less.

(5) Manufacturing method Next, a manufacturing method of a clad steel plate according to one embodiment of the present invention will be described. First, for example, a base material steel plate having the composition of the base material described above and a clad material steel plate which is a carbon steel (with CE _B /CE _C of 2.000 or more) described above, preferably a clad material steel plate having the composition of the clad material described above, are stacked together to form a clad material steel plate. The method of preparing the base material steel plate and the clad material steel plate is not particularly limited. For example, the base material steel plate and the clad material steel plate can be prepared using a conventionally known manufacturing method. Specifically, molten steel adjusted to a predetermined composition by a normal melting method (converter method, electric furnace method, etc.) is cast by a normal casting method (continuous casting method or ingot casting method) to form a slab material. Next, the obtained slab material is hot-rolled to form a base material steel plate. Next, a cladding material steel plate is superimposed on at least one surface of the base material steel plate, particularly on the surface expected to come into contact with ammonia, etc., to prepare a two-layer clad material steel plate. Alternatively, a cladding material steel plate is superimposed on both surfaces of the base material steel plate to prepare a three-layer clad material steel plate (a clad material steel plate in the order of cladding material steel plate/base material steel plate/cladding material steel plate).

The clad steel plate is then pressure welded and heat treated under predetermined conditions to control the structure.
That is, the clad steel plate is heated to 1000 to 1250°C,
Next, the clad material steel plate is subjected to hot rolling at a cumulative rolling reduction rate of 20% or more in the non-recrystallization temperature range of the base material steel plate and at a rolling end temperature of the _Ar3 transformation point or more to obtain a hot-rolled steel plate;
Next, the hot-rolled steel sheet is cooled to a cooling start temperature of the _Ar3 transformation point or higher, an average cooling rate of 20 to 120° C./s, and a cooling end temperature of 500° C. or lower. This allows the production of a clad steel sheet according to one embodiment of the present invention.
After the cooling, the hot-rolled steel sheet may be tempered at a temperature of 650° C. or lower.
The temperatures in each manufacturing condition are all temperatures at a 1/2 thickness position of the base material or base material steel plate. The temperature at the position may be measured directly. The temperature at the position may also be determined by performing a differential calculation using, for example, a process computer from the surface temperature of the clad steel plate or clad material steel plate measured by a radiation thermometer.

[Heating temperature: 1000 to 1250° C.]
If the heating temperature of the clad material steel plate (hereinafter also referred to as the heating temperature) is less than 1000°C, the solid solution of carbides is insufficient and the necessary strength cannot be obtained. In addition, from the viewpoint of the bondability between the base material steel plate and the cladding material steel plate, a high heating temperature is preferable. Therefore, the heating temperature is set to 1000°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains of the base material will become coarse, leading to a deterioration in toughness. Therefore, the heating temperature is set to 1250°C or lower.

[Cumulative rolling reduction in the non-recrystallization temperature range of base steel sheet: 20% or more]
By setting the cumulative reduction in the non-recrystallization temperature range of the base material steel plate (hereinafter also simply referred to as the cumulative reduction) to 20% or more, deformation bands that become nucleation sites are introduced into the austenite grains of the base material steel plate. This refines the bainite that is transformed and generated during cooling after hot rolling, improving the toughness of the clad steel plate. For this reason, the cumulative reduction is set to 20% or more. The cumulative reduction is preferably 30% or more, more preferably 40% or more. Moreover, the cumulative reduction is preferably 85% or less, more preferably 80% or less.
Here, the non-recrystallization temperature range of the base material steel sheet is a temperature range equal to or lower than Tnr (°C). Tnr (°C) can be calculated by the following formula.
Tnr(°C)=174×log([Nb] _B ×([C] _B +12/14[N] _B ))+1444
Here, [X] _B represents the content (mass%) of element X in the composition of the base material steel sheet, and log is common logarithm.
The cumulative rolling reduction can be calculated by the following formula.
[Cumulative rolling reduction (%)] = [Total thickness reduction (mm) of base material steel plate in the non-recrystallization temperature range of the base material steel plate] ÷ [Thickness (mm) of base material steel plate in clad material steel plate before the start of hot rolling] × 100
Whether or not each pass of hot rolling is in the non-recrystallization temperature range of the base material steel plate (in other words, whether or not the plate thickness reduction amount of the base material steel plate in each pass of hot rolling is included in the total plate thickness reduction amount of the base material steel plate in the non-recrystallization temperature range of the base material steel plate) is determined based on the outlet temperature of each pass.
The reason why the cumulative rolling reduction is based on the plate thickness of the base material steel plate among the clad material steel plates is that the toughness of the clad steel plate is particularly significantly affected by the toughness of the base material.

[Rolling end temperature: Ar ₃ transformation point or higher]
If the rolling end temperature of the hot rolling is lower than the _Ar3 transformation point, the generated ferrite is affected by processing, and the toughness is deteriorated. Therefore, the rolling end temperature is set to be equal to or higher than the _Ar3 transformation point. The upper limit of the rolling end temperature is not particularly limited. For example, the rolling end temperature is preferably equal to or lower than ( _Ar3 transformation point + 90°C).
The _Ar3 transformation point can be calculated by the following formula.
Ar ₃ transformation point (℃) = 910-310 [C] _B - 80 [Mn] _B - 20 [Cu] _B - 15 [Cr] _B - 55 [Ni] _B - 80 [Mo] _B
Here, [X] _B represents the content (mass%) of element X in the chemical composition of the base material steel sheet.

[Cooling start temperature: Ar ₃ transformation point or higher]
The hot-rolled steel sheet obtained after hot rolling is cooled from a temperature equal to or higher than the _Ar3 transformation point. If the cooling start temperature is lower than the _Ar3 transformation point, excessive ferrite is generated. The generated ferrite coexists with bainite and martensite, which have a large difference in strength from ferrite. As a result, insufficient strength and deterioration of toughness are caused. Therefore, the cooling start temperature after hot rolling is set to be equal to or higher than the _Ar3 transformation point. The upper limit of the cooling start temperature is not particularly limited. For example, the cooling start temperature is preferably equal to or lower than ( _Ar3 transformation point + 70°C).

[Average cooling rate: 20 to 120 ° C./s]
By setting the average cooling rate to 20°C/s or more, a clad steel plate with high strength and high toughness can be obtained. In particular, by cooling at a high rate, the effect of increasing strength due to transformation strengthening can be obtained. That is, if the average cooling rate is less than 20°C/s, the grain size of bainite becomes large. In addition, ferrite and pearlite are generated, which may lead to insufficient strength and deterioration of toughness. On the other hand, if the average cooling rate exceeds 120°C/s, the volume fraction of martensite becomes too large, and toughness decreases. Therefore, the average cooling rate is set to 20°C/s or more and 120°C/s or less.
The average cooling rate here is the average value of the cooling rate from the cooling start temperature to the cooling stop temperature, and is based on the temperature at 1/2 the thickness of the base material. For example, the temperatures at 1/2 the thickness of the base material at the start and end of cooling are calculated by a process computer using the surface temperatures at the start and end of cooling measured by a radiation thermometer. The average cooling rate can then be calculated by the following formula:
[Average cooling rate (℃/s)] = ([Temperature of base material at 1/2 thickness when cooling starts (℃)] - [Temperature of base material at 1/2 thickness when cooling stops (℃)]) ÷ [Cooling time (s)]

[Cooling stop temperature: 500°C or less]
By setting the cooling stop temperature at 500°C or less, it is possible to obtain a predetermined volume fraction of bainite in the metal structure of the base material. If the cooling stop temperature exceeds 500°C, ferrite and pearlite are excessively generated, resulting in insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is set to 500°C or less. On the other hand, the lower limit of the cooling stop temperature is not particularly limited. The cooling stop temperature may be, for example, room temperature, but is preferably 150°C or more from the viewpoint of production efficiency, etc.

[Tempering temperature: 650°C or less]
Tempering can be performed optionally for the purpose of recovering the toughness of the base material. Here, if the tempering temperature, i.e., the temperature of the clad steel plate during reheating by tempering (the temperature at the 1/2 plate thickness position of the base material) exceeds 650°C, dislocations may be restored and the strength of the base material may decrease. Therefore, when tempering is performed, the tempering temperature is set to 650°C or less. On the other hand, the lower limit of the tempering temperature is preferably 350°C from the viewpoint of recovering the toughness of the base material.

In the manner described above, the clad steel plate according to the embodiment of the present invention can be manufactured. The thus obtained clad steel plate according to the embodiment of the present invention has excellent tensile properties and toughness.
Here, excellent tensile properties mean that the yield strength YS (yield point YP when there is a yield point, 0.2% proof stress σ0.2 when there is no yield point) measured by a tensile test conforming to JIS Z 2241 (2022) is 490 MPa or more, and the tensile strength (TS) is 610 MPa or more. In addition, excellent toughness means that the fracture transition temperature (hereinafter also referred to as vTrs) measured by a Charpy impact test conforming to JIS Z 2242 (2018) is -30 ° C. or less. Details are as described in the examples described later.

　In addition, there are no particular restrictions on conditions other than those mentioned above, and standard procedures should be followed.

[Example 1]
Table 1 shows the composition of the base metal (the balance being Fe and unavoidable impurities). In the table, steel types A to P are suitable steels that satisfy the composition of the base metal of the clad steel plate according to one embodiment of the present invention. On the other hand, steel types Q to X are comparative steels that fall outside the range of the composition of the base metal of the clad steel plate according to one embodiment of the present invention.
Clad material steel plates were prepared by overlapping a base material steel plate having the chemical composition shown in Table 1 with a clad material steel plate having the CE _C shown in Table 2, and clad steel plates (Nos. 1 to 34) were manufactured under the conditions shown in Table 2. For the obtained clad steel plates, the volume fraction of bainite in the metal structure of the base material and the average grain size of bainite were measured, the Vickers hardness of the clad material was measured, and the tensile properties and toughness of the clad material were evaluated, as well as the ammonia SCC resistance in a liquid ammonia environment was evaluated. The test methods are as follows.

[Measurement of the volume fraction of bainite in the metal structure of the base material]
A sample was taken so that the center of the plate thickness (1/2 position of the plate thickness) of the base material of the clad steel plate was the observation surface. The taken sample was then mirror-polished and further subjected to nital etching. Next, a scanning electron microscope (SEM) was used to photograph an area of 10 mm x 10 mm of the sample at a magnification of 500 to 3000 times. The photographed image was analyzed using an image analyzer to determine the volume fraction of bainite in the metal structure of the base material. Note that when the anisotropy of the metal structure of the base material is small, the area fraction corresponds to the volume fraction, so here the area fraction was considered to be the volume fraction.

[Measurement of average grain size of bainite in metal structure of base material]
The same sample was used for measuring the average grain size of bainite as for measuring the volume fraction of bainite in the metal structure of the base material. First, the surface of the sample was mirror-polished. Next, an Electron Back-Scattering Pattern (EBSP) device attached to the SEM was used to measure the crystal orientation from an electron beam backscatter diffraction image. Specifically, the crystal orientation was measured at intervals of 0.3 μm within a 200 μm square region of the sample. Next, a region surrounded by grain boundaries in which the crystal orientation difference with adjacent grains is 15° or more was defined as one grain, and the circle-equivalent diameter of the grain determined to be bainite was calculated from the area of the grain. The average circle-equivalent diameter of the grains determined to be bainite was defined as the average grain size of bainite.
Among the crystal grains, crystal grains having elongated, lath-shaped ferrite were determined to be bainite.

[Measurement of Vickers hardness of cladding material]
A sample was taken from the clad steel plate so that a cross section perpendicular to the rolling direction, that is, a so-called T-section, was the measurement surface. The sample was then mirror-polished. Next, in accordance with JIS Z 2244 (2020), the Vickers hardness (HV10, measurement load: 10 kgf) was measured at 20 points at 1 mm intervals in the direction perpendicular to the rolling (direction perpendicular to the rolling direction and the plate thickness direction) at the plate thickness 1/2 position of the clad material. The average value of these values was taken as the Vickers hardness of the clad material. For example, when the Vickers hardness measured under the condition of a measurement load of 10 kgf is 210, it is usually expressed as 210HV10.

[Tensile properties]
A JIS Z 2241 (2022) No. 1B test piece was taken from the clad steel plate so that the direction perpendicular to the rolling was the longitudinal direction, and a tensile test was performed as described in JIS Z 2241 (2022) to measure the yield strength YS (yield point YP when there is a yield point, 0.2% proof stress σ0.2 when there is no yield point) and tensile strength (TS). Then, those with a yield strength of 490 MPa or more and a tensile strength of 610 MPa or more were evaluated as having excellent tensile properties. The initial strain rate was 1 × 10 ^-3 /s.

[Toughness]
A V-notch test piece according to JIS Z 2242 (2018) was taken from the base material of the clad steel plate so that the rolling direction was the longitudinal direction, and a Charpy impact test was performed according to the procedure of JIS Z 2242 (2018) to measure vTrs. Then, those with a vTrs of -30°C or less were evaluated as having excellent toughness.

[Ammonia SCC resistance]
The ammonia SCC resistance was evaluated by a four-point bending anodic electrolysis test according to the following procedure.
A 15 mm x 115 mm test piece with a thickness of 5 mm was taken from each clad steel plate so that the first surface (inner surface) of the clad steel plate was the evaluation surface. Next, ultrasonic degreasing was performed on the test piece in acetone for 5 minutes. Next, a stress equivalent to the yield strength of each test piece was applied to each test piece by four-point bending. Next, each test piece was placed in a test cell while the stress was applied. Next, a test solution in which 12.5 g of ammonium carbamate and 1 L of liquid ammonia were mixed was filled in the test cell. Next, the potential difference with the reference electrode was controlled by a potentiostat to +2.0 V vs Pt, and constant potential anodic electrolysis was performed. The temperature of the test atmosphere was set to room temperature (25°C). Then, in this state, it was held for 720 hours from the start of immersion (current application). Then, the test piece after holding was visually confirmed, and if no cracks were found in the test piece, it was judged to have excellent ammonia SCC resistance (passed). On the other hand, if a crack occurred in the test piece, it was judged as defective.
The evaluation results are shown in Table 2.

As can be seen from Table 2, all of the invention examples have a yield strength YS of 490 MPa or more and a tensile strength of 610 MPa or more. In addition, all of the invention examples have a vTrs of -30°C or less. Furthermore, all of the invention examples have excellent ammonia SCC resistance. In other words, all of the invention examples have excellent ammonia SCC resistance and low-temperature toughness, as well as high strength.

In contrast, in Comparative Example No. 17, the _CEB / _CEC and Vickers hardness of the clad material are outside the appropriate range. In Comparative Examples No. 18 and 19, the _tC1 and clad ratio are outside the appropriate range, respectively. In addition, in Comparative Examples No. 20 to 26, some of the manufacturing conditions are outside the appropriate range, so the desired metal structure of the base material is not obtained. As a result, these Comparative Examples are inferior in any one of the yield strength YS, tensile strength TS, low temperature toughness, and ammonia SCC resistance.

In addition, Nos. 27 to 34 have inferior yield strength YS, tensile strength TS, low-temperature toughness, and ammonia SCC resistance because some of the base metal composition is outside the appropriate range.

[Example 2]
A base material steel plate having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) and a clad material steel plate having the composition shown in Table 3 (the balance being Fe and unavoidable impurities) were overlapped to prepare a clad material steel plate, and clad steel plates (Nos. 1 to 17) were manufactured under the conditions shown in Table 4. For the obtained clad steel plates, the volume fraction of bainite in the metal structure of the base material and the average grain size of bainite were measured, the Vickers hardness of the clad material was measured, and the tensile properties and toughness were evaluated in the same manner as in Example 1. In addition, an evaluation of ammonia SCC resistance in a liquid ammonia environment was performed in the following manner.

[Ammonia SCC resistance]
As in Example 1, two test pieces measuring 15 mm x 115 mm and having a thickness of 5 mm were taken from each clad steel plate so that the first surface (inner surface) of the clad steel plate was the test surface. In one of the test pieces, a notch having a depth of 0.3 mm and a diameter of 0.2 mm was provided on the test surface of the test piece, assuming that a dent was present on the surface (first surface) of the clad steel plate. Hereinafter, the test piece without the notch is referred to as the first test piece, and the test piece with the notch is referred to as the second test piece. Next, a four-point bending anodic electrolysis test was performed using the first and second test pieces in the same manner as in Example 1. The test pieces after the test were visually inspected, and the ammonia SCC resistance was evaluated according to the following criteria.
Excellent (pass, particularly excellent): No cracks were observed in both the first test piece and the second test piece.
Pass (Excellent): No cracks observed on the first test piece, but cracks observed on the second test piece.
Poor: Cracks are observed in both the first and second test pieces.

As can be seen from Table 4, all of the inventive examples have a yield strength YS of 490 MPa or more and a tensile strength of 610 MPa or more. In addition, all of the inventive examples have a vTrs of -30°C or less. Furthermore, all of the inventive examples have excellent ammonia SCC resistance. In other words, all of the inventive examples have excellent ammonia SCC resistance and low-temperature toughness, as well as high strength. In particular, when the CR value of the cladding material was 0.30 or more, they had particularly excellent ammonia SCC resistance.

Claims

A clad steel plate having a cladding material which is carbon steel on at least one surface of a base material,
The composition of the base material is, in mass%,
C: 0.010 to 0.200%,
Si: 0.01 to 1.00%,
Mn: 0.50 to 2.50%,
Al: 0.001 to 0.060%,
Contains P: 0.0200% or less and S: 0.0100% or less, with the balance being Fe and unavoidable impurities;
CEB / CEC is 2.000 or more, where CEB and CEC are the carbon equivalents of the base material and the cladding material, respectively;
In the metal structure of the base material, the volume fraction of bainite is 90% or more, and the average grain size of the bainite is 25 μm or less,
The Vickers hardness of the cladding material is 210HV10 or less,
A clad steel plate, wherein tC1 is 2.0 mm or more, tC2 is 0.0 mm or more, and ( tC1 + tC2 )/( tB + tC1 + tC2 ) is 0.30 or less, where tB is the plate thickness of the base material, tC1 is the plate thickness of the clad material on one side of the base material, and tC2 is the plate thickness of the clad material on the other side of the base material.
The composition of the base material is further expressed in mass% as follows:
Cu: 1.00% or less,
Ni: 2.00% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 0.500% or less,
Ti: 0.100% or less,
Nb: 0.100% or less,
Ca: 0.0200% or less,
The clad steel plate according to claim 1, containing one or more selected from the group consisting of Mg: 0.0200% or less and REM: 0.0200% or less.
The component composition of the composite material is, in mass%,
C: 0.100% or less and Mn: 0.01 to 1.50%
and further comprising
Cu: 0.01 to 0.50%,
Cr: 0.01 to 0.50%,
Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50%
and the balance being Fe and unavoidable impurities,
The clad steel plate according to claim 1 or 2, wherein a CR value calculated by the following formula (1) is 0.30 or more.
CR value = 2.3 [Cu] + 2.8 [Cr] + 7.3 [Sb] + 3.6 [Sn] ... (1)
Here, [X] indicates the content (mass %) of element X in the component composition of the cladding material.
A method for manufacturing a clad steel plate having a base material and a cladding material which is carbon steel on at least one surface of the base material, comprising the steps of:
A clad material steel plate obtained by overlapping a base material steel plate having the composition of the base material according to claim 1 or 2 with a clad material steel plate of the carbon steel according to claim 1 is heated to 1000 to 1250 ° C.,
Next, the clad material steel plate is subjected to hot rolling at a cumulative rolling reduction rate of 20% or more in the non-recrystallization temperature range of the base material steel plate and at a rolling end temperature of the Ar3 transformation point or more to obtain a hot-rolled steel plate;
The hot-rolled steel sheet is then cooled to a cooling start temperature of at least the Ar 3 transformation point, an average cooling rate of 20 to 120°C/s, and a cooling end temperature of 500°C or less.
The composition of the cladding material steel plate is, in mass%,
C: 0.100% or less and Mn: 0.01 to 1.50%
and further comprising
Cu: 0.01 to 0.50%,
Cr: 0.01 to 0.50%,
Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50%
and the balance being Fe and unavoidable impurities,
The method for producing a clad steel plate according to claim 4, wherein a CR value calculated by the following formula (1) is 0.30 or more.
CR value = 2.3 [Cu] + 2.8 [Cr] + 7.3 [Sb] + 3.6 [Sn] ... (1)
Here, [X] indicates the content (mass %) of element X in the chemical composition of the base steel sheet for cladding.
The method for manufacturing clad steel plate according to claim 4, wherein after the cooling, the hot-rolled steel plate is tempered at a temperature range of 650°C or less.
The method for producing a clad steel plate according to claim 5, wherein after the cooling, the hot-rolled steel plate is tempered in a temperature range of 650°C or less.