Clad steel plate and method of producing the same

1. A clad steel plate comprising:

a base steel; and

a cladding metal made of a corrosion resistant alloy bonded to one surface of the base steel;

wherein the base steel has a chemical composition containing, in mass %,

C: 0.020% or more and 0.100% or less,

Si: 0.05% or more and 0.50% or less,

Mn: 0.75% or more and 1.60% or less,

P: 0.010% or less,

S: 0.0010% or less,

Al: 0.010% or more and 0.070% or less,

Nb: 0.005% or more and 0.080% or less,

Ti: 0.005% or more and 0.030% or less,

N: 0.0010% or more and 0.0060% or less,

Ca: 0.0005% or more and 0.0040% or less,

O: 0.0030% or less, and

at least one selected from the group consisting of

Cu: 0.01% or more and 0.50% or less,

Cr: 0.01% or more and 0.50% or less,

Mo: 0.01% or more and 0.50% or less,

V: 0.010% or more and 0.100% or less, and

Ni: 0.01% or more and 0.50% or less,

so as to satisfy relations given by the following formulas (1) and (2), with the balance being Fe and inevitable impurities:

1.00≤ACR≤6.00  (1),

where ACR is defined by:

ACR=([Ca]−(0.18+130[Ca])×[O])/(1.25[S]),

where [Ca], [O], and [S] respectively represent the contents in mass % of Ca, O, and S in the base steel, and

P HIC <1.000  (2),

where P HIC is defined by:

P HIC =4.46[C]+2.37[Mn]/6+(1.18[Cr]+1.95[Mo]+1.74[V])/5+(1.74[Cu]+1.7[Ni])/15+22.36[P],

where [C], [Mn], [Cr], [Mo], [V], [Cu], [Ni], and [P] respectively represent the contents in mass % of C, Mn, Cr, Mo, V, Cu, Ni, and P in the base steel,

wherein the base steel has a steel microstructure in which

bainite is present in an area fraction of 94% or more at a ½ thickness position in a thickness direction of the base steel, and with an average crystal grain size of 25 μm or less,

wherein shear strength at a bonded interface between the base steel and the cladding metal is 300 MPa or more, and

wherein a crack area ratio obtained by HIC test in accordance with NACE Standard TM0284-2003 is 5.0% or less.

2. The clad steel plate according to claim 1 , wherein the corrosion resistant alloy is a nickel base alloy or austenitic stainless steel.

3. A method of producing the clad steel plate of claim 1 , the method comprising:

preparing a slab by stacking a base steel raw material having the chemical composition of the base steel as recited in claim 1 and a cladding metal raw material made of a corrosion resistant alloy, and heating the slab to a temperature range of 1050° C. to 1200° C. in surface temperature;

subjecting the slab to first rolling with a rolling reduction ratio at a temperature range of 950° C. or higher in surface temperature being 2.0 or more, followed by second rolling with an accumulated rolling reduction ratio at a temperature range of 900° C. or lower in surface temperature being 50% or more and a finish rolling temperature being at or above Ar 3 temperature in surface temperature, to thereby obtain a rolled plate formed of a base steel and a cladding metal;

then subjecting the rolled plate to accelerated cooling with a cooling start temperature being at or above the Ar 3 temperature in surface temperature, an average cooling rate being 5° C./s or higher, and a cooling stop temperature being 500° C. or lower at a ½ thickness position in a thickness direction of the base steel; and

then subjecting the rolled plate to tempering in a temperature range of 350° C. to 600° C. at the ½ thickness position in the thickness direction of the base steel,

wherein in the accelerated cooling, the average cooling rate is obtained by dividing the difference between the cooling start temperature and the cooling stop temperature at the ½ thickness position in the thickness direction of the base steel by a cooling time.

4. A method of producing the clad steel plate of claim 1 , the method comprising:

preparing a slab by stacking a base steel raw material having the chemical composition of the base steel as recited in claim 1 and a cladding metal raw material made of either austenitic stainless steel or a nickel base alloy of Alloy 825, and then heating the slab to a temperature range of 1050° C. to 1200° C. in surface temperature;

subjecting the slab to first rolling with a rolling reduction ratio being 1.5 or more in a temperature range of 950° C. or higher in surface temperature, followed by second rolling with an accumulated rolling reduction ratio being 50% or more in a temperature range of 900° C. or lower in surface temperature and a finish rolling temperature being at or above Ar 3 temperature in surface temperature, to thereby obtain a rolled plate formed of a base steel and a cladding metal;

then subjecting the rolled plate to accelerated cooling with a cooling start temperature being at or above the Ar 3 temperature in surface temperature, an average cooling rate being 5° C./s or higher, and a cooling stop temperature being 500° C. or lower at a ½ thickness position in a thickness direction of the base steel; and

then subjecting the rolled plate to tempering in a temperature range of 350° C. to 600° C. at the ½ thickness position in the thickness direction of the base steel,

wherein in the accelerated cooling, the average cooling rate is obtained by dividing the difference between the cooling start temperature and the cooling stop temperature at the ½ thickness position in the thickness direction of the base steel by a cooling time.

5. The method according to claim 3 , wherein the slab is prepared by stacking the base steel raw material and the cladding metal raw material in the following order: the base steel raw material/the cladding metal raw material/the cladding metal raw material/the base steel raw material.

6. The method according to claim 4 , wherein the slab is prepared by stacking the base steel raw material and the cladding metal raw material in the following order: the base steel raw material/the cladding metal raw material/the cladding metal raw material/the base steel raw material.