Clad steel plate for structures, method for producing same, and structure

Abstract

To provide a clad steel plate, exhibiting excellent resistance to ammonia stress corrosion cracking and excellent bonding property, with minimal plate warpage, being suitable for liquid ammonia transportation, storage tanks and the like, and to provide a production method for the clad steel plate and a structure using the clad steel plate.SOLUTION: A clad steel plate for structures having a base material and a cladding material, wherein the Ceq of the cladding material as represented by formula (1) is 0.20 or less, and wherein, after bead-on-plate welding, the maximum Vickers hardness of a heat-affected zone within a range of 10 mm from a boundary between weld metal formed during bead-on-plate welding and the cladding material toward the cladding material side at a position 1 mm below the surface of the cladding material is 210 HV10 or less. Ceq=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+V / 14 (1), where [X] denotes the content (mass%) of the element X in the steel.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a clad steel plate suitable for structural members such as tanks used in a liquid ammonia environment. The present invention also relates to a method for manufacturing such a clad steel plate and a structure using the clad steel plate.

Background Art

[0002] In a liquid ammonia environment, carbon steel is liable to stress corrosion cracking caused by liquid ammonia. Therefore, for structures such as carbon steel pipes, storage tanks, tank trucks, and line pipes that handle liquid ammonia, application of steel materials with excellent ammonia stress corrosion cracking resistance and operational measures to suppress ammonia stress corrosion cracking have been taken.

[0003] Ammonia stress corrosion cracking is known to be correlated with the strength and hardness of materials. In particular, it is known that ammonia stress corrosion cracking often occurs in the heat affected zone of welding. Therefore, when using high-strength steel in a liquid ammonia environment, measures such as performing post-weld heat treatment by overall annealing to adjust the hardness of the welded part are necessary.

[0004] On the other hand, in recent years, liquid ammonia has attracted attention as a clean energy because it does not generate CO2 even when burned, and large-scale demand is expected. Along with this, an increase in the size of facilities for transporting and storing liquid ammonia is required. Generally, when increasing the size of a tank, thinning of the steel material used is desired from the viewpoints of weight reduction and reduction of construction costs, and thus the use of high-strength steel is desired.

[0005] As methods for achieving both such high strength and excellent ammonia stress corrosion cracking resistance, Patent Documents 1 to 4 are disclosed.

[0006] Among these, Patent Document 1 describes a method for softening the surface of a steel material.

[0007] In addition, Patent Documents 2 and 3 describe methods for manufacturing clad steel sheets having a mild steel layer on one side.

[0008] Also, stainless steel is known as a steel material that suppresses ammonia stress corrosion cracking, but there is a problem that its price is high. Therefore, Patent Document 4 discloses a stainless clad steel sheet that is economically superior by using expensive stainless steel as a clad material and relatively inexpensive plain steel or low alloy steel as a base material.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, in the method described in Patent Document 1 above, the thickness of the softened layer on the surface layer is as thin as 500 μm, and there is a problem that it is difficult to uniformly and stably form a softened layer over the entire surface of the steel sheet.

[0011] In addition, in the methods described in Patent Documents 2 and 3 above, the clad is manufactured by a casting method, a build-up welding method, or a continuous casting method, but there is an economic problem that the cost of equipment and energy therefor is large.

[0012] In addition, in the method described in Patent Document 4 above, there was an economic problem that the cost was high because stainless steel, which is more expensive than mild steel, was used as the clad material. Furthermore, since the clad material is a high-alloy compared to ordinary steel or low-alloy steel, which is the base material, in hot rolling performed to obtain joinability as a clad material, the deformation resistance during hot rolling in the high-temperature range that most affects joinability becomes extremely larger than that of ordinary steel or low-alloy steel, making it difficult to ensure joinability.

[0013] The present invention solves the above problems, and provides a clad steel sheet, a method for manufacturing the same, and a structure using the clad steel sheet, which are suitable for use in liquid ammonia transportation, storage tanks, etc., have excellent ammonia stress corrosion cracking resistance, excellent joinability, and further have a small plate warp.

Means for Solving the Problems

[0014] In order to achieve the above object, the present inventors have intensively studied various factors affecting the ammonia stress corrosion cracking resistance of steel sheets. As a result, the following findings were obtained.

[0015] That is, since ammonia stress corrosion cracking occurs inside the product (tank), the characteristics of the steel sheet surface layer on the inside of the product are dominant in the ammonia stress corrosion cracking resistance. Therefore, it was recalled that a clad steel sheet was made by using a steel sheet with excellent strength as the base material and further joining a steel sheet with a low hardness in the heat-affected zone of welding as the clad material to such a base material in order to improve the ammonia stress corrosion cracking resistance. And it was found that excellent ammonia stress corrosion cracking resistance can be obtained by using such a clad steel sheet. In addition, compared to a clad steel sheet using stainless steel as the clad material, since the clad material used in the present invention is carbon steel, the alloy cost can be significantly reduced, and the difference in deformation resistance between the base material and the clad material during hot rolling is small, resulting in excellent joinability. Therefore, the reduction in yield can be suppressed, and ultimately the manufacturing cost can be suppressed.

[0016] The present invention has been made based on the above findings, that is, the gist of the present invention is as follows. [1] In a clad steel plate having a base material and a facing material, the Ceq shown in the formula (1) of the facing material is 0.20 or less, and after bead-on-plate welding, the maximum value of the Vickers hardness of the heat-affected zone of the weld metal formed during bead-on-plate welding at a position 1 mm below the surface of the facing material and within a range of 10 mm toward the facing material side from the boundary between the weld metal and the facing material is 210 HV10 or less. A clad steel plate for a structure. Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + V / 14 ···(1) However, [X] represents the content (mass%) of element X in the steel. [2] The facing material contains, by mass%, C: 0.100% or less, Si: 0.01 to 0.50%, Mn: 0.01 to 1.10%, P: 0.100% or less, S: 0.0500% or less, Al: 0.001 to 0.100%, and has a component composition in which the balance is Fe and inevitable impurities. The clad steel plate according to [1]. [3] The component composition further contains, by mass%, Cu: 0.50% or less, Ni: 0.05% or less, Cr: 0.50% or less, Mo: 0.05% or less, V: 0.100% or less, Nb: 0.100% or less, Ti: 0.100% or less, Zr: 0.100% or less, B: 0.0005% or less, Ca: 0.0100% or less, Mg: 0.0100% or less and REM: 0.0200% or less The clad steel plate according to [2], which contains one or more selected from the above. [4] The clad steel sheet according to any one of [1] to [3], wherein the ratio of the Ceq of the facing material shown in formula (1) to the Ceq of the base material shown in formula (1) is 0.25 or more. Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + V / 14 ···(1) However, [X] represents the content (mass%) of element X in the steel. [5] A method for manufacturing a clad steel sheet, in which after combining the base material and the facing material according to any one of [1] to [4], heating is performed at 1000 to 1300 °C, and hot rolling is performed such that the reduction ratio at a steel sheet surface temperature of 850 °C or higher is 1.3 or more. Here, the reduction ratio is obtained by dividing the total plate thickness of the base material and the facing material before rolling by the total plate thickness of the base material and the facing material after rolling. [6] A structure having the clad steel sheet according to any one of [1] to [4]. [7] The structure according to [6], which is a tank.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a clad steel sheet that is excellent in ammonia stress corrosion cracking resistance and suitable for structural members such as tanks used in a liquid ammonia environment.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0019] The clad steel plate according to the present invention is a clad steel plate in which a bonding material made of carbon steel is joined to at least one surface of a base material. When using a clad steel plate with the bonding material joined only to one side of the base material, the bonding material may be disposed on either surface of the base material. However, when using the above clad steel plate in an ammonia environment, in order to suppress ammonia stress corrosion cracking, the bonding material is disposed on the side in contact with ammonia.

[0020] Hereinafter, embodiments of the present invention will be described more specifically. Note that “%” representing the content of the following component elements means “mass %” unless otherwise specified. (1) Component composition of the bonding material The most required performance for the bonding material is ammonia stress corrosion cracking resistance. However, since the bonding material is also one of the components of the clad member, it is desirable that it has high strength.

[0021] Ceq: 0.20 or less When the Ceq of the bonding material exceeds 0.20, the hardness of the heat affected zone of welding increases, leading to deterioration of ammonia stress corrosion cracking resistance. Therefore, the Ceq of the bonding material is set to 0.20 or less. Preferably, the Ceq of the bonding material is 0.15 or less. On the other hand, the lower limit is not particularly limited, but it is preferably 0.10 or more because a difference in volume change due to a phase change during cooling causes warping if it is too low compared to the Ceq of the base material. Note that Ceq follows the following formula (1). Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + V / 14 ···(1)

[0022] C: 0.100% or less C is an element that increases the hardness of the steel plate. The higher the hardness, the more the deterioration of ammonia stress corrosion cracking resistance is caused. Therefore, it is preferable to limit the C content to 0.100% or less. More preferably, the C content is limited to 0.060% or less. On the other hand, excessive reduction leads to an increase in refining cost. Therefore, the C content is preferably 0.001% or more.

[0023] Si: 0.01 - 0.50% Si is added for deoxidation. To obtain such an effect, it is preferable to limit the Si content to 0.01% or more. Further, it is preferably 0.03% or more. On the other hand, when the Si content exceeds 0.50%, it remains as non-metallic inclusions, and the ammonia stress corrosion cracking resistance deteriorates. Therefore, it is preferable to limit the Si content to 0.50% or less. More preferably, the Si content is limited to 0.40% or less.

[0024] Mn: 0.01 - 1.10% Mn is added for deoxidation. To obtain such an effect, it is preferable to limit the Mn content to 0.01% or more. More preferably, the Mn content is limited to 0.20% or more. On the other hand, when the Mn content exceeds 1.10%, the hardenability of the steel increases excessively and the hardness increases, leading to deterioration of the ammonia stress corrosion cracking resistance. Therefore, it is preferable to limit the Mn content to 1.10% or less. More preferably, the Mn content is limited to 1.00% or less.

[0025] P: 0.100% or less P is an impurity element, and when it is contained in an amount exceeding 0.100%, the ammonia stress corrosion cracking resistance deteriorates. Therefore, it is preferable that the P content is 0.100% or less. More preferably, the P content is 0.030% or less. On the other hand, excessive reduction leads to an increase in refining cost. Therefore, it is preferable that the P content is 0.001% or more.

[0026] S: 0.0500% or less S is an impurity element, and when it is contained in an amount exceeding 0.0500%, the ammonia stress corrosion cracking resistance deteriorates. Therefore, it is preferable that the S content is 0.0500% or less. More preferably, the S content is 0.0100% or less. On the other hand, excessive reduction leads to an increase in refining cost. Therefore, it is preferable that the S content is 0.0001% or more.

[0027] Al: 0.001 - 0.100% Al is added for deoxidation. To obtain such an effect, it is preferable to limit the Al content to 0.001% or more. More preferably, the Al content is 0.010% or more. On the other hand, when the Al content exceeds 0.100%, non-metallic inclusions increase and the ammonia stress corrosion cracking resistance deteriorates. Therefore, it is preferable to limit the Al content to 0.100% or less. More preferably, the Al content is limited to 0.060% or less.

[0028] The balance other than the above components in the composite material of the present invention is Fe and inevitable impurities. However, if necessary, one or more selected from the elements described below can be further contained as appropriate.

[0029] Cu: 0.50% or less Cu forms a protective film and has the effect of improving the ammonia stress corrosion cracking resistance. However, when it exceeds 0.50%, the effect saturates and the alloy cost increases. Therefore, when Cu is contained, the Cu content is set to 0.50% or less. More preferably, the Cu content is 0.40% or less. The lower limit is not particularly limited, but it is preferably 0.01% or more.

[0030] Ni: 0.05% or less Ni is an element having the effect of improving the strength of the steel sheet and can be optionally contained. On the other hand, when the Ni content exceeds 0.05%, the ammonia stress corrosion cracking resistance deteriorates. Therefore, when Ni is contained, the Ni content is set to 0.05% or less. More preferably, the Ni content is 0.03% or less. The lower limit is not particularly limited, but it is preferably 0.01% or more.

[0031] Cr: 0.50% or less Cr forms a protective film and has the effect of improving ammonia stress corrosion cracking resistance. However, when the Cr content exceeds 0.50%, the effect saturates and the alloy cost increases. Therefore, when Cr is contained, the Cr content should be 0.50% or less. It is more preferable that the Cr content is 0.30% or less. The lower limit is not particularly limited, but it is preferably 0.01% or more.

[0032] Mo: 0.05% or less Mo is an element that has the effect of improving the strength of the steel plate and can be optionally contained. On the other hand, when the Mo content exceeds 0.05%, the ammonia stress corrosion cracking resistance deteriorates. Therefore, when Mo is contained, it should be 0.05% or less. It is more preferable that the Mo content is 0.03% or less. The lower limit is not particularly limited, but it is preferably 0.001% or more. It is more preferably 0.01% or more.

[0033] V: 0.100% or less V is an element that has the effect of improving the strength of the steel plate and can be optionally contained. However, when the V content exceeds 0.100%, the hardness increases excessively, leading to deterioration of the ammonia stress corrosion cracking resistance. Therefore, when V is contained, the V content should be 0.100% or less. It is more preferable that the V content is 0.080% or less. The lower limit is not particularly limited, but it is preferably 0.001% or more.

[0034] Nb: 0.100% or less Nb is an element that has the effect of improving the strength of the steel plate and can be optionally contained. However, when the Nb content exceeds 0.100%, the hardness increases excessively, leading to deterioration of the ammonia stress corrosion cracking resistance. Therefore, when Nb is contained, the Nb content should be 0.100% or less. It is more preferable that the Nb content is 0.050% or less. The lower limit is not particularly limited, but it is preferably 0.001% or more.

[0035] Ti: 0.100% or less Ti is an element having the effect of improving the strength of the steel sheet and can be optionally contained. However, when the Ti content exceeds 0.100%, the hardness increases excessively, leading to deterioration of the ammonia stress corrosion cracking resistance. Therefore, when Ti is contained, the Ti content should be 0.100% or less. It is more preferable that the Ti content is 0.050% or less. The lower limit is not particularly limited, but it is preferably 0.001% or more.

[0036] Zr: 0.100% or less Zr is an element having the effect of improving the strength of the steel sheet and can be optionally contained. However, when the Zr content exceeds 0.100%, the hardness increases excessively, leading to deterioration of the ammonia stress corrosion cracking resistance. Therefore, when Zr is contained, the Zr content should be 0.100% or less. It is more preferable that the Zr content is 0.050% or less. The lower limit is not particularly limited, but it is preferably 0.001% or more.

[0037] B: 0.0005% or less B is an element having the effect of improving the strength of the steel sheet and can be optionally contained. On the other hand, when the B content exceeds 0.0005%, the ammonia stress corrosion cracking resistance deteriorates. Therefore, when B is contained, the B content should be 0.0005% or less. It is more preferable that the B content is 0.0003% or less. The lower limit is not particularly limited, but it is preferably 0.0001% or more.

[0038] Ca: 0.0100% or less Ca is an element that combines with S to control the morphology so that sulfide inclusions such as MnS that grow long in the rolling direction exhibit a spherical shape, and has the effect of improving the toughness of the heat affected zone of welding, and can be optionally contained. On the other hand, when the Ca content exceeds 0.0100%, the cleanliness of the steel decreases, leading to a decrease in toughness. Therefore, when Ca is contained, the Ca content should be 0.0100% or less. It is more preferable that the Ca content is 0.0050% or less. The lower limit is not particularly limited, but it is preferably 0.0001% or more.

[0039] Mg: Below 0.0100% Mg, similar to Ca, combines with S and controls the morphology so that sulfide inclusions such as MnS that elongate significantly in the rolling direction exhibit a spherical shape, and has the effect of improving the toughness of the heat affected zone of welding. It can be contained optionally. On the other hand, when the Mg content exceeds 0.0100%, the cleanliness of the steel decreases, leading to a decrease in toughness. Therefore, when Mg is contained, the Mg content should be 0.0100% or less. It is more preferable that the Mg content is 0.0050% or less. The lower limit is not particularly limited, but it is preferably 0.0001% or more.

[0040] REM: Below 0.0200% REM (rare earth metal), similar to Ca and Mg, combines with S and controls the morphology so that sulfide inclusions such as MnS that elongate significantly in the rolling direction exhibit a spherical shape, and has the effect of improving the toughness of the heat affected zone of welding. It can be contained optionally. On the other hand, when the REM content exceeds 0.0200%, the cleanliness of the steel decreases, leading to a decrease in toughness. Therefore, when REM is contained, the REM content should be 0.0200% or less. It is more preferable that the REM content is 0.0100% or less. The lower limit is not particularly limited, but it is preferably 0.0001% or more.

[0041] (2) Composition of the base metal The ratio of the Ceq of the filler metal shown in formula (1) to the Ceq of the base metal shown in formula (1) is 0.25 or more If the Ceq of the joining material shown in Formula (1) is too high or too low with respect to the Ceq of the base material shown in Formula (1) (Ceq of the joining material shown in Formula (1) / Ceq of the base material shown in Formula (1)), a difference will occur in the volume change due to the phase change during cooling, resulting in warping. That is, when the ratio of the Ceq of the joining material to the Ceq of the base material is 0.25 or more, warping is suppressed. For this reason, the ratio of the Ceq of the joining material to the Ceq of the base material is preferably 0.25 or more, and more preferably 0.30 or more. Although the upper limit of the ratio of the Ceq of the joining material to the Ceq of the base material is not particularly defined, it is preferably 4.00 or less. Note that the Ceq of the joining material shown in Formula (1) with respect to the Ceq of the base material shown in Formula (1) is Ceq of the joining material shown in Formula (1) / Ceq of the base material shown in Formula (1). Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + V / 14 ···(1) However, [X] represents the content (mass%) of element X in steel.

[0042] Note that the component composition of the base material is not particularly limited, and it is a carbon steel according to the required properties. For example, in terms of mass%, it is in the range of C: 0.02 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, P: 0.001 to 0.100%, S: 0.0001 to 0.0100%, and it can contain Al, Cu, Ni, Cr, Mo, V, Nb, Ti, Zr, B, Ca, Mg, and REM as necessary, and the balance is preferably Fe and inevitable impurities.

[0043] (3) Manufacturing conditions In the present invention, first, a base material steel sheet and a clad material steel sheet are manufactured. There are no particular restrictions on the manufacturing methods of these steel sheets, and conventionally known steel sheet manufacturing methods can be applied. That is, molten steel adjusted to the above-described preferred component composition is cast by a normal melting method (such as a converter method or an electric furnace method), and the obtained slab material is hot-rolled to obtain a steel sheet of a predetermined size by a normal casting method (such as a continuous casting method or an ingot method). Using these base material steel sheets and clad material steel sheets, an assembled slab for clad rolling (hereinafter referred to as "assembled slab") is assembled. Note that the base material steel sheet and the clad material steel sheet are the base material and the clad material described above, respectively.

[0044] Fig. 2 shows a view representing a cross section of the assembled slab. In Fig. 2, reference numeral 1 denotes a clad material, reference numeral 2 denotes a base material, reference numeral 10 denotes a release agent, and reference numeral 11 denotes an assembled slab. The assembled slab 11 can be manufactured using a known method, and a form in which two clad materials 1 are overlapped and sandwiched by the base material 2 from both sides is efficient in manufacturing. Also, considering the warpage during cooling, it is preferable that the thicknesses of the two base materials 2 are equal, and it is preferable that the thicknesses of the two clad materials 1 are equal. The assembled slab 11 for clad rolling that has been assembled is preferably temporarily fixed by performing electron beam welding in a vacuum chamber with adjusted pressure. Note that when assembled as described above, it is preferable to apply a release agent 10 such as Al2O3 or MgO between the two opposing clad materials. The film thickness of the release agent 10 after application is preferably 10 μm or more and 1 mm or less. Note that it is not necessary to be limited to the assembly method described above. After assembling the assembled slab 11 for clad rolling, it is heated and further hot-rolled is performed.

[0045] Heating temperature of the assembled slab: 1000°C or more and 1300°C or less From the perspective of the bondability of the clad steel plate, that is, the bondability between the base material (base steel plate) 2 and the facing material 1, it is preferable that the heating temperature of the assembled slab 11 is high, and the bondability is improved by heating at 1000°C or higher. Therefore, the heating temperature of the assembled slab 11 is 1000°C or higher. Preferably it is 1050°C or higher. However, if it is heated above 1300°C, it will cause a decrease in yield due to scale loss. Therefore, from the perspective of yield, the heating temperature should be 1300°C or lower. Preferably it is 1250°C or lower.

[0046] Heating time of the assembled slab: 30 minutes or more and 600 minutes or less (preferred conditions) From the perspective of the bondability of the clad steel plate, that is, the bondability between the base steel plate 2 and the facing material 1, it is preferable that the heating time of the assembled slab 11 is long. The important reason for long-time heating is that by reducing the temperature deviation within the assembled slab 11, the difference in deformation resistance between the facing material 1 and the base material 2 becomes smaller, and an ideal bonding interface can be obtained during rolling. The bondability is improved by heating for 30 minutes or more. Therefore, it is preferable that the heating time of the assembled slab 11 is 30 minutes or more. More preferably, it is 60 minutes or more. However, if it is heated for more than 600 minutes, it will cause a decrease in yield due to scale loss. Therefore, from the perspective of yield, it is preferable that the heating time is 600 minutes or less. More preferably, it is 300 minutes or less. The heating time refers to the total time of the time from room temperature to the heating temperature and the holding time at the heating temperature.

[0047] Reduction ratio at a steel plate surface temperature of 850°C or higher: 1.3 or more The clad steel plate ensures good bondability between the bonding material 1 and the base material 2 by rolling in a high-temperature range. The important reasons for rolling in a high-temperature range are that the difference in deformation resistance during rolling between the bonding material 1 and the base material 2 becomes small, so that an ideal bonding interface is formed by rolling, and that at high temperatures, elemental interdiffusion readily proceeds at the boundary between the bonding material 1 and the base material 2. On the other hand, in the present invention, since both the base material 2 and the bonding material 1 to be used are carbon steels, the difference in deformation resistance is small compared to the combination where the bonding material 1 is stainless steel and the base material 2 is carbon steel. Therefore, in the clad steel of the present invention, at a high temperature and at a steel plate surface temperature of 850 °C or higher, which is lower than the appropriate temperature conditions for stainless clad steel, bondability can be ensured by rolling with a reduction ratio (=(total plate thickness of the base material and the bonding material before reduction)÷(total plate thickness of the base material and the bonding material after reduction)) of 1.3 or more. Thus, in the present invention, rolling is carried out under the conditions of a reduction ratio of 1.3 or more at a steel plate surface temperature of 850 °C or higher. Preferably, the reduction ratio is 2.0 or more. The upper limit of the reduction ratio is not particularly limited, but in consideration of rolling efficiency, it is preferably 20.0 or less.

[0048] In order to ensure the desired mechanical properties of the base material, the clad steel plate of the present invention may further be subjected to controlled rolling or accelerated cooling after the above hot rolling. The average cooling rate from 800 °C to 500 °C at that time is preferably 2 °C / s or more and 50 °C / s or less. Further tempering may be carried out. When tempering is carried out, the tempering temperature is preferably 400 °C or higher and 700 °C or lower for the reason of ensuring toughness while avoiding excessive reduction in strength. Alternatively, after cooling to room temperature after hot rolling, reheat quenching and further tempering may be carried out.

[0049] In addition, in the manufacturing method according to the present invention, for items not described in this specification, any conventional methods can be used.

[0050] The clad steel plate of the present invention has a bonding interface located between the base material 2 and the cladding material 1, and from the viewpoint of bondability, the shear strength of the bonding interface is preferably 300 MPa or more in accordance with the shear strength test described in the test method for clad steels in JIS G0601:2012. A shear strength of 350 MPa or more is more preferable.

[0051] (4) Properties of the heat-affected zone after welding The maximum Vickers hardness of the weld heat affected zone is 210HV10 or less After bead-on-plate welding, the maximum hardness of the weld heat-affected zone (HAZ) in the range 10 mm from the boundary between the weld metal formed during bead-on-plate welding and the cladding material, located 1 mm below the surface of the cladding material, shall be 210 HV10 or less. The presence of a high-hardness region in the surface layer of a clad steel plate promotes ammonia stress corrosion cracking. Ammonia stress corrosion cracking, in particular, occurs in the weld heat-affected zone. The weld heat-affected zone (HAZ) refers to the region in the cladding material or base material surrounding the weld metal formed during bead-on-plate welding after bead-on-plate welding, where the structure of the base material or cladding material has changed due to the heat from welding. The region where the structure of the base material or cladding material has changed discolors due to corrosion with nital, and the discolored region can be visually confirmed after corrosion with nital. That is, if the maximum hardness of the weld heat affected zone of the cladding material exceeds 210 HV10, the desired ammonia stress corrosion cracking resistance cannot be obtained. Therefore, the maximum hardness of the weld heat affected zone of the cladding material is set to 210 HV10 or less. The Vickers hardness of the weld heat affected zone can be measured by the method described in the examples below.

[0052] The clad steel plate of the present invention can be used for structures, such as tanks, and is suitable for use as tanks for transporting and storing liquid ammonia. [Example]

[0053] Table 1 shows the component composition of the clad material (the balance is Fe and inevitable impurities). In the table, clad materials No. 1 to 19 and 26 are compatible steels belonging to the scope of the present invention. On the other hand, clad materials No. 20 to 25 are comparative steels in which Ceq is outside the scope of the invention or the component composition is outside the scope.

[0054] Using clad material 1 with the component composition shown in Table 1 and base material 2 with a component composition of C: 0.080%, Si: 0.25%, Mn: 1.22%, P: 0.007%, S: 0.0009%, Al: 0.028%, Cr: 0.38%, and the balance being Fe and inevitable impurities (base material No. is A) or C: 0.095%, Si: 0.41%, Mn: 1.84%, P: 0.009%, S: 0.0008%, Al: 0.025%, Cu: 0.56%, Ni: 3.55%, Cr: 1.71%, Mo: 0.387%, and the balance being Fe and inevitable impurities (base material No. is B), clad steel plates (clad Nos. 1 to 26) were manufactured. Also, the thickness of the clad material was 2 mm and the thickness of the base material was 28 mm. The size of the clad steel plate was 200 mm in width and 500 mm in length. Clad steel plates were obtained under the conditions shown in Table 2. For the obtained clad steel plates, measurement of the Vickers hardness of the heat-affected zone of the clad material welding, evaluation of ammonia stress corrosion cracking resistance in a liquid ammonia environment, and measurement of shear stress were respectively carried out. Each test method is as follows.

[0055] [Measurement of Hardness of Weld Heat-Affected Zone] From the clad steel, specimens containing clad material 1 were taken and bead-on-welding was carried out. The size of the welding specimen was 20 mm t × 75 mm w × 200 mm L, and the welding surface was the surface of the clad material. The welding method was shielded metal arc welding, and the welding material was TB-43 manufactured by Kobe Steel, Ltd., with a wire diameter of 4.0 mm. There was no preheating, the current was 170 A, the voltage was 24 V, and the welding speed was 150 mm / min. The welding length was 125 mm. Five specimens for cross-sectional macro observation were taken from the welding specimen. Figure 2 is a diagram showing a schematic diagram of the specimen for cross-sectional macro observation taken after welding. The collected specimens for cross-sectional macroscopic observation were mirror-polished and then corroded in a 3% nital solution so that the boundary between the weld metal 3 and cladding material 1 could be observed. The Vickers hardness (HV10) of cladding material 1 was then measured. In Figure 2, reference numeral 1 denotes the cladding material, reference numeral 2 denotes the base metal, reference numeral 3 denotes the weld metal, reference numeral 4 denotes the weld heat-affected zone, reference numeral 5 denotes the boundary between the weld metal and the weld heat-affected zone, and reference numeral 6 denotes the hardness measurement position, 1 mm below the surface. The measurement point was 1 mm below the surface of cladding material 1 at position 6, and 20 points on each side of the boundary between the weld metal 3 and cladding material 1 were measured at 0.5 mm intervals on the cladding material 1 side, for a total of 40 points. The hardest value among the measurement results was taken as the hardness of the weld heat-affected zone 4.

[0056] [Ammonia stress corrosion cracking resistance] The ammonia stress corrosion cracking resistance in the present invention was evaluated by an accelerated test in which a four-point bending test was carried out in a test solution and constant-potential anodic electrolysis was carried out to accelerate corrosion. Specifically, the following procedure was carried out.

[0057] Test pieces measuring 5 mm thick x 15 mm x 115 mm were taken from the surface of cladding material 1, ultrasonically degreased in acetone for 5 minutes, and subjected to four-point bending to a stress equal to the yield strength of each test piece. The test cell containing the four-point bending test pieces was filled with a solution containing 12.5 g of ammonium carbamate and 1 L of liquid ammonia. A voltage of +2.0 V vs. Pt was applied to the test pieces using a potentiostat, and the pieces were immersed at room temperature (25°C). If no cracks were observed after 720 hours of immersion, the ammonia stress corrosion cracking resistance was judged to be good and marked with a circle. On the other hand, if cracks were present, the test piece was judged to be poor and marked with an X.

[0058] The evaluation results are shown in Table 2.

[0059] As can be seen from Table 2, all of the invention examples have excellent ammonia stress corrosion cracking resistance, while the comparative examples have poor ammonia stress corrosion cracking resistance.

[0060] [Evaluation of the bondability between base material and cladding material] The bondability of the base material and cladding material in the present invention was measured in accordance with the shear strength test described in the test method for clad steel of JIS G0601: 2012. The evaluation criteria were that a shear strength of 300 MPa or more was considered to be good bondability.

[0061] [Warp evaluation] The method for evaluating warpage is shown in Figure 3. After rolling the assembled slab 11, the two overlapping clad steel plates 12 were peeled off to separate each clad steel plate 12. Reference plates 13 were placed at two ends of the clad steel plate 12 in the width direction on the surface of the clad steel plate 12, and the amount of displacement from the reference plate 13 at the center position in the width direction was measured with a dial gauge 14 and evaluated as warpage 15. The evaluation criteria were that a warpage of 5.0 mm or less was considered to be good.

[0062] [Table 1]

[0063] [Table 2] [Explanation of symbols]

[0064] 1. Cladding material 2 Base material (base material steel plate) 3 Weld metal 4. Weld heat affected zone 5 Boundary between weld metal and weld heat affected zone 6. Hardness measurement position 1 mm below the surface 10. Stripping Agent 11 Assembled slab 12 Clad steel plate 13 Reference board 14 Dial Gauge 15 Warp

Claims

1. In a clad steel plate having a base material and a cladding material, the Ceq shown in the formula (1) of the cladding material is 0.20 or less, and after bead-on-plate welding, the maximum value of the Vickers hardness of the heat-affected zone of the weld metal formed during bead-on-plate welding at a position 1 mm below the surface of the cladding material and within a range of 10 mm toward the cladding material side from the boundary between the weld metal and the cladding material is 210 HV10 or less. A clad steel plate for a structure. Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + V / 14... (1) However, [X] represents the content (mass%) of element X in the steel.

2. The cladding material, in mass%, C: 0.100% or less, Si: 0.01 to 0.50%, Mn: 0.01 to 1.10%, P: 0.100% or less, S: 0.0500% or less, Al: 0.001 to 0.100% is contained, The clad steel plate according to claim 1, having a component composition in which the balance is Fe and unavoidable impurities.

3. The component composition further includes, in mass%, Cu: 0.50% or less, Ni: 0.05% or less, Cr: 0.50% or less, Mo: 0.05% or less, V: 0.100% or less, Nb: 0.100% or less, Ti: 0.100% or less, Zr: 0.100% or less, B: 0.0005% or less, Ca: 0.0100% or less, Mg: 0.0100% or less and REM: 0.0200% or less The clad steel plate according to claim 2, containing one or more selected from the above.

4. The ratio of the Ceq of the cladding material shown in the formula (1) to the Ceq of the base material shown in the formula (1) is 0.25 or more. The clad steel plate according to any one of claims 1 to 3. Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + V / 14... (1) However, [X] represents the content (mass%) of element X in the steel.

5. A method for manufacturing a clad steel plate, in which after combining the base material with the cladding material according to any one of claims 1 to 3, heating is performed at 1000 to 1300 °C, and hot rolling is performed such that the reduction ratio at a steel plate surface temperature of 850 °C or higher is 1.3 or more. Here, the reduction ratio is obtained by dividing the total thickness of the base material and the cladding material before rolling by the total thickness of the base material and the cladding material after rolling.

6. A method for manufacturing a clad steel plate, in which after combining the base material with the cladding material according to claim 4, heating is performed at 1000 to 1300 °C, and hot rolling is performed such that the reduction ratio at a steel plate surface temperature of 850 °C or higher is 1.3 or more. Here, the reduction ratio is obtained by dividing the total thickness of the base material and the clad material before reduction by the total thickness of the base material and the clad material after reduction.

7. A structure having the clad steel plate according to any one of Claims 1 to 3.

8. A structure having the clad steel plate according to Claim 4.

9. The structure according to Claim 7, which is a tank.

10. The structure according to Claim 8, which is a tank.

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