SPOT WELDED JOINT AND SPOT WELDING METHOD

• US 20120141829A1
• Filed: 08/30/2010
• Published: 06/07/2012
• Est. Priority Date: 08/31/2009
• Status: Active Grant

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel & Sumitomo Metal Corporation (Nippon Steel Corporation)
• Inventors: Hamatani, Hideki, Azuma, Masafumi, Suzuki, Noriyuki, Watanabe, Fuminori, Maruyama, Naoki, Kawata, Hiroyuki, Oikawa, Hatsuhiko
• Granted Patent: US 8,962,149 B2
• Time in Patent Office: Days
• US Class Current: 428/683
• CPC Class Codes: B23K 11/115 by means of two electrodes placed opposite one another on both sides of the welded parts
• CPC Class Codes: B23K 11/16 taking account of the properties of the material to be welded
• CPC Class Codes: B23K 11/166 of galvanized or tinned materials
• CPC Class Codes: B23K 2101/18 Sheet panels
• CPC Class Codes: B23K 2101/34 Coated articles , e.g. plated or painted; Surface treated articles
• CPC Class Codes: B23K 2103/04 Steel or steel alloys
• CPC Class Codes: B32B 15/011 all layers being formed of iron alloys or steels
• CPC Class Codes: C22C 38/001 containing N
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/06 containing aluminium
• CPC Class Codes: Y10T 428/12347 Plural layers discontinuously bonded [e.g., spot-weld, mechanical fastener, etc.]
• CPC Class Codes: Y10T 428/12965 Both containing 0.01-1.7% carbon [i.e., steel]

Abstract

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 (1A, 1B). 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 2×10⁶/mm².

Claims

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 pm,

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×

10⁶/mm²,

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 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 %.

5. 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.

14. 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 %.

15. 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.

16. The spot welded joint of a high-strength steel plate according to claim 4, 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.

18. The spot welding method of a high-strength steel plate according to claim 16, comprising the step of, between the step of releasing pressurizing and the step of holding the thin steel plates at the heat treatment temperature after welding, cooling the thin steel plate thereby to reduce a surface temperature of a portion which the welding electrode of the thin steel plate having contacted to a temperature CT (°

C.) fulfilling a formula (10) below,

CT≦

150 

 

(10).

6. A spot welding method for forming a nugget in a joint surface of two or more thin steel plates,

at least one of the two or more thin steel plates being a high-strength steel plate whose tensile strength is 750 MPa to 1850 MPa and whose carbon equivalent Ceq represented by a formula (1) below being 0.22 mass % to 0.55 mass %,

the method comprising the steps of:

performing welding energization under a condition that a pressurizing force EF (N) of a welding electrode against the two or more thin steel plates fulfills a formula (2) below;

next, while holding the pressurizing force EF, performing post-heating energization to the welding electrode with a post-heating energization current PC (kA) fulfilling a formula (4) below prescribed with a welding current WC (kA) in the welding energization for a post-heating energization time Pt (ms) fulfilling a formula (5) below; and

next, releasing pressurizing at the pressurizing force EF,

wherein between the step of performing welding energization and the step of performing post-heating energization, the thin steel plates are cooled for a cooling time after welding Ct (ms) fulfilling a formula (3) below, while the pressurizing force EF is held, and

wherein a holding time Ht (ms) during which the pressurizing force EF is held between the step of performing post-heating energization and the step of releasing pressurizing at the pressurizing force EF fulfills a formula (6) below,

Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] 

 

(1)

1960×

h≦

EF≦

3430×

h 

 

(2)

16≦

Ct≦

300 

 

(3)

0.40×

WC≦

PC≦

0.95×

WC 

 

(4)

30≦

Pt≦

200 

 

(5)

0≦

Ht≦

240 

 

(6)

([C], [Si], [Mn], [P], and [S] each indicate respective contents (mass %) of C, Si, Mn, P, and S, and h indicates a thickness (mm) of the thin steel plate).

8. The spot welding method of a high-strength steel plate according to claim 6, comprising the step of, after the step of releasing pressurizing, holding the thin steel plates at a heat treatment temperature after welding AT (°

C.) fulfilling a formula (8) below for a heat treatment time after welding At (s) fulfilling a formula (9) below,

120≦

AT≦

220 

 

(8)

100≦

At≦

6000 

 

(9).

9. The spot welding method of a high-strength steel plate according to claim 8, comprising the step of, between the step of releasing pressurizing and the step of holding the thin steel plates at the heat treatment temperature after welding, cooling the thin steel plate thereby to reduce a surface temperature of a portion which the welding electrode of the thin steel plate having contacted to a temperature CT (°

C.) fulfilling a formula (10) below,

CT≦

150 

 

(10).

10. The spot welding method of a high-strength steel plate according to claim 6, 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,

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.

11. The spot welding method of a high-strength steel plate according to claim 10, 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 %.

12. The spot welding method of a high-strength steel plate according to claim 10, 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 %.

13. The spot welding method of a high-strength steel plate according to claim 10, 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.

7. A spot welding method for forming a nugget in a joint surface of two or more thin steel plates,

at least one of the two or more thin steel plates being 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 %,

the method comprising the steps of

performing welding energization under a condition that a pressurizing force EF (N) of a welding electrode against the two or more thin steel plates fulfills a formula (7) below;

next, while holding the pressurizing force EF, performing post-heating energization to the welding electrode with a post-heating energization current PC (kA) fulfilling a formula (4) below prescribed with a welding current WC (kA) in the welding energization for a post-heating energization time Pt (ms) fulfilling a formula (5) below; and

next, releasing pressurizing at the pressurizing force EF,

wherein between the step of performing welding energization and the step of performing post-heating energization, the thin steel plates are cooled for a cooling time after welding Ct (ms) fulfilling a formula (3) below, while the pressurizing force EF is held, and

wherein a holding time Ht (ms) during which the pressurizing force EF is held between the step of performing post-heating energization and the step of releasing pressurizing at the pressurizing force EF fulfills a formula (6) below,

Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] 

 

(1)

16≦

Ct≦

300 

 

(3)

0.40×

WC≦

PC≦

0.95×

WC 

 

(4)

30≦

Pt≦

200 

 

(5)

0≦

Ht≦

240 

 

(6)

1470×

h≦

EF<

1960×

h 

 

(7)

([C], [Si], [Mn], [P], and [S] each indicate respective contents (mass %) of C, Si, Mn, P, and S, and h indicates al thickness (mm) of the thin steel plate).

17. The spot welding method of a high-strength steel plate according to claim 7, comprising the step of, after the step of releasing pressurizing, holding the thin steel plates at a heat treatment temperature after welding AT (°

C.) fulfilling a formula (8) below for a heat treatment time after welding At (s) fulfilling a formula (9) below,

120≦

AT≦

220 

 

(8)

100≦

At≦

6000 

 

(9).

19. The spot welding method of a high-strength steel plate according to claim 7, 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,

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.

20. The spot welding method of a high-strength steel plate according to claim 19, 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 %.

21. The spot welding method of a high-strength steel plate according to claim 19, 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.