Spot welded joint
Provided is a spot welded joint ( 10 ) which includes at least one thin steel plate with a tensile strength of 750 MPa to 1850 MPa and a carbon equivalent Ceq of equal to or more than 0.22 mass % to 0.55 mass % and in which a nugget ( 3 ) is formed in an interface of the thin steel plates ( 1 A, 1 B). In a nugget outer layer zone, a microstructure consists of a dendrite structure in which an average value of arm intervals is equal to or less than 12 μm, an average grain diameter of carbides contained in the microstructure is 5 nm to 100 nm, and a number density of carbides is equal to or more than 2×10 6 /mm 2 .
1. A spot welded joint of a high-strength steel plate, comprising:
two or more thin steel plates which are spot welded to each other; and
a nugget formed in a joint surface of the thin steel plates,
wherein at least one of the two or more thin steel plates is a high-strength steel plate whose tensile strength is 750 MPa to 1850 MPa and whose carbon equivalent Ceq represented by formula (1) below is 0.22 mass % to 0.55 mass %, and
wherein, in a nugget outer layer zone being a zone except a similar figure zone which is 90% as large as an outer shape of the nugget in the nugget,
a microstructure consists of a dendrite structure in which an average value of arm intervals is equal to or less than 12 μm,
an average grain diameter of carbides contained in the microstructure is 5 nm to 100 nm, and a number density of the carbides is equal to or more than 2×10 6 /mm 2 ,
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] (1)
([C], [Si], [Mn], [P], and [S] each represent respective contents (mass %) of C, Si, Mn, P and S).
2. The spot welded joint of a high-strength steel plate according to claim 1 ,
wherein the high-strength steel plate contains:
C: 0.10 mass % to 0.40 mass %;
Si: 0.01 mass % to 2.50 mass %; and
Mn: 1.5 mass % to 3.0 mass %, and
wherein, in the high-strength steel plate,
P content is equal to or less than 0.03 mass %,
S content is equal to or less than 0.01 mass %,
N content is equal to or less than 0.0100 mass %,
O content is equal to or less than 0.007 mass %,
Al content is equal to or less than 1.00 mass %, and
the balance consists of Fe and an inevitable impurity.
3. The spot welded joint of a high-strength steel plate according to claim 2 , wherein the high-strength steel plate further contains at least one kind selected from the group consisting of:
Ti: 0.005 mass % to 0.10 mass %;
Nb: 0.005 mass % to 0.10 mass %; and
V: 0.005 mass % to 0.10 mass %.
4. The spot welded joint of a high-strength steel plate according to claim 3 , wherein the high-strength steel plate further contains at least one kind selected from the group consisting of:
B: 0.0001 mass % to 0.01 mass %;
Cr: 0.01 mass % to 2.0 mass %;
Ni: 0.01 mass % to 2.0 mass %;
Cu: 0.01 mass % to 2.0 mass %; and
Mo: 0.01 mass % to 0.8 mass %.
5. The spot welded joint of a high-strength steel plate according to claim 3 , wherein the high-strength steel plate further contains at least one kind selected from the group consisting of Ca, Ce, Mg, and REM within a range of 0.0001 mass % to 0.5 mass % in total.
6. The spot welded joint of a high-strength steel plate according to claim 2 , wherein the high-strength steel plate further contains at least one kind selected from the group consisting of:
B: 0.0001 mass % to 0.01 mass %;
Cr: 0.01 mass % to 2.0 mass %;
Ni: 0.01 mass % to 2.0 mass %;
Cu: 0.01 mass % to 2.0 mass %; and
Mo: 0.01 mass % to 0.8 mass %.
7. The spot welded joint of a high-strength steel plate according to claim 6 , wherein the high-strength steel plate further contains at least one kind selected from the group consisting of Ca, Ce, Mg, and REM within a range of 0.0001 mass % to 0.5 mass % in total.
8. The spot welded joint of a high-strength steel plate according to claim 2 , wherein the high-strength steel plate further contains at least one kind selected from the group consisting of Ca, Ce, Mg, and REM within a range of 0.0001 mass % to 0.5 mass % in total.