Steel plate excellent in shape freezing property and method for production thereof

• US 6,962,631 B2
• Filed: 09/21/2001
• Issued: 11/08/2005
• Est. Priority Date: 09/21/2000
• Status: Active Grant

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel Corporation
• Inventors: Yoshida, Tohru, Takahashi, Manabu, Yoshinaga, Naoki, Sugiura, Natsuko
• Primary Examiner(s): Yee, Deborah
• Application Number:
  US10/380,844
• Publication Number:
  US 20030196735A1
• Time in Patent Office: 1,509 Days
• Field of Search: 148/320, 148330-336, 148/602, 148/603, 148/648, 148/650, 148/651, 148/654, 148/653, 148/661
• US Class Current: 148/320
• CPC Class Codes: C21D 1/185 from an intercritical temperature
• CPC Class Codes: C21D 2201/05 Grain orientation
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C22C 38/002 containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• CPC Class Codes: C22C 38/004 Very low carbon steels, i.e. having a carbon content of less than 0,01%
• 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: C22C 38/12 containing tungsten, tantalum, molybdenum, vanadium, or niobium

Abstract

A ferritic steel sheet wherein a mean value of X-ray random intensity ratios of a group of {100}<011> to {223}<110> orientations is 3.0 or more and a mean value of X-ray random intensity ratios of three crystal orientations of {554}<225>, {111}<112>, and {111}<110> is 3.5 or less and further at least one of the r values in a rolling direction and a direction at a right angle of that is 0.7 or less.

Claims

1. A thin ferritic steel sheet with a particular shape fixability, comprising:

at least one section having first and second mean values of x-ray random intensity ratios that are at least 3.0 and at most 3.5, respectively,

wherein the x-ray random intensity ratios of the first mean value are of a group of {100}<

011>

to {223}<

110>

orientations at least at ½

of a thickness of the sheet, and wherein the x-ray random intensity ratios of the second mean value are of orientations of {554}<

225>

, {111}<

112>

, and {111}<

110>

.

2. The thin ferritic steel sheet according to claim 1, wherein at least one of an r value in a rolling direction of the sheet and the r value in a direction at a right angle to the rolling direction is 0.7 or less.

3. The thin ferritic steel sheet according to claim 1, wherein the x-ray random intensity ratios of the first mean value are of {112}<

110>

orientation, and wherein the first mean value is at least 4.0.

4. The thin ferritic steel sheet according to claim 1, wherein the x-ray random intensity ratios of the second mean value are of {100}<

011>

orientation, and wherein the second mean value is at least 4.0.

5. The thin ferritic steel sheet according to claim 1, wherein an occupancy of iron carbide at grain boundaries of the sheet is at most 0.1, and wherein a maximum grain size of the iron carbide is at most 1 μ

m.

6. The thin ferritic steel sheet according to claim 1, wherein the at least one section includes a multi phase structure, wherein one of ferrite and bainite in the at least one section is the maximum phase in terms of a percent area, and wherein a sum of a percent area of pearlite, martensite, and residual austenite in the at least one section is at most 30%.

7. The thin ferritic steel sheet according to claim 1, wherein the steel sheet comprises;

in terms of weight

C;

0.001 to 0.3%,

Si;

0.001 to 3.5%,

Mn;

less than 3%,

P;

0.005 to 0.15%,

S;

less than 0.03%,

Al;

0.01 to 3.0%,

N;

less than 0.01%,

O;

less than 0.01%, and remainder Fe and unavoidable impurities.

8. The thin ferritic steel sheet according to claim 1, wherein the steel sheet contains at least one element selected from the group consisting of, in terms of weight %, Ti:

less than 0.20%, Nb;

less than 0.20%, V;

less than 0.20%, Cr;

less than 1.5%, B;

less than 0.007%, Mo;

less than 1%, Cu;

less than 3%, Ni;

less than 3%, Sn;

less than 0.3%, Co;

less than 3%, Ca;

0.0005 to 0.005%, and REM;

0.001 to 0.02%.

9. The thin ferritic steel sheet according to claim 7, wherein the steel sheet satisfies the following equations.

203√

C+15.2Ni+44.7Si+104V+31.5Mo+30Mn+11Cr+20Cu+700P+200A1<

30 

 

(1)

44.7Si+700P+200A1>

40 

 

(2).

10. The thin ferritic steel sheet according to claim 1, wherein the steel sheet is plated.

11. A method for producing a thin ferritic steel sheet with a particular shape fixability, comprising the steps of;

hot rolling, by one of reheating to a temperature range of 1000°

C. to 1300°

C. and without reheating, a cast slab which contains, in terms of weight %,

C;

0.001 to 0.3%,

Si;

0.001 to 3.5%,

Mn;

less than 3%,

P;

0.005 to 0.15%,

S;

less than 0.03%,

Al;

0.01 to 3.0%,

N;

less than 0.01%,

O;

less than 0.01 %, and remainder Fe and unavoidable impurities, with a total reduction rate of 25% or more at (Ar₃−

100) to (Ar₃+100)°

C.,

terminating the hot rolling step at (Ar₃−

100)°

C. or more, and

cooling the hot rolled steel sheet, and then coiling the cooled steel sheet so that the steel sheet has first and second mean values of x-ray random intensity ratios that are at least 3.0 and at most 3.5, respectively, wherein the x-ray random intensity ratios of the first mean value are of a group of {100}<

011>

to {223}<

110>

orientations at least at ½

of the sheet thickness, and wherein the x-ray random intensity ratios of the second mean value are of orientations of {554}<

225>

, {111}<

112>

, and {111}<

110>

.

14. The method according to claim 11, wherein the x-ray random intensity ratios of the first mean value are of {112}<

110>

orientation, and wherein the first mean value is at least 4.0.

15. The method according to claim 11, wherein the x-ray random intensity ratios of the second mean value are of {100}<

011>

orientation, and wherein the second mean value is at least 4.0.

16. The method according to claim 11, wherein the cast slab further contains at least one element selected from the group consisting of, in terms of weight %, Ti:

less than 0.20%, Nb;

less than 0.20%, V;

less than 0.20%, Cr;

less than 1.5%, B;

less than 0.007%, Mo;

less than 1%, Cu;

less than 3%, Ni;

less than 3%, Sn;

less than 0.3%, Co;

less than 3%, Ca;

0.0005 to 0.005%, and REM;

0.001 to 0.02%.

17. The method according to claim 11, wherein the steel sheet is coiled at a temperature To that is determined by the chemical composition of the steel sheet shown in the following equations:

To=−

650.4×

{C %/(1.82×

C %−

0.001}+B

where B is obtained from ingredients of the steel sheet expressed by massy

B=−

50.6×

Mneq+894.3

Mneq=Mn %+0.24×

Ni %+0.13×

Si %+0.38×

Mo %+0.55×

Cr %+0.16×

Cu %−

0.50×

Al %+−

0.45×

Co %+0.90×

V %.

18. The method according to claim 11, wherein the hot rolling step is controlled so that the effective strain ε

* that is calculated by the following equation is at least 0.4;

${\mathrm{\varepsilon <br><br>}}^{*}=\underset{j=1}{\overset{n-1}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\varepsilon <br><br>}}_j<br><br>\exp <br><br>\lfloor <br><br>-\underset{i-j}{\overset{n-1}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(\frac{t_i}{{\mathrm{\tau <br><br>}}_i}\right)}^{2/3}\rfloor <br><br>+{\mathrm{\varepsilon <br><br>}}_n$

wherein n is a number of rolling stands of finish hot rolling, ε

i is strain added at an i-th stand, ti is a traveling time(seconds) between the i-th to i+l-th stands, and ε

i is determinable by the following equation using a gas constant R(=1.987) and a hot rolling temperature Ti(K) of the i-th stand;

ri=8,46×

10<sup>−

9</sup>·

exp{43800/R/Ti}.

19. The method according to claim 11, wherein the hot rolling step is carried out with a friction coefficient of less than 0.2 for at least one pass in the hot rolling step.

20. The method according to claim 11, wherein the cooling step is controlled to an average cooling rate of more than 10°

C./sec from hot rolling terminating temperature to a critical temperature to determined by the chemical composition of the steel sheet, and the cooling step is carried out at a temperature less than To.

21. The method according to claim 11, wherein the hot rolled steel sheet is pickled by acid, wherein, after the steel sheet is pickled, the steel sheet is cold rolled at a reduction rate of less than 80%, wherein, after the steel sheet is cold rolled, the cold rolled steel sheet is reheated between 600°

C. and (Ac3+100)°

C., and then cooled.

22. The method according to claim 11, wherein the hot rolled steel sheet is pickled by acid, wherein, after the steel sheet is pickled, the steel sheet is cold rolled with a reduction rate of less than 80%, wherein, after the steel sheet is cold rolled, the steel sheet is annealed at a temperature between Ac₁, and Ac₃ transformation temperature, and then cooled to a temperature below 500°

C. at a cooling rate of 1 to 250°

C./sec.

12. A method for producing a thin ferritic steel sheet with a particular shape fixability, comprising the steps of;

hot rolling, by one of reheating to a temperature range of 1000°

C. to 1300°

C. and without reheating, a cast slab that contains, in terms of weight %,

C;

0.001 to 0.3%,

Si;

0.001to35%,

Mn;

less than 3%,

P;

0.005 to 0.15%,

S;

less than 0.03%,

Al;

0.01 to 3.0%,

N;

less than 0.01%,

O;

less than 0.01%, and remainder Fe and unavoidable impurities, with one of a total reduction rate of 25% and more at (Ar₃+50) to (Ar₃150)°

C., and continuing the hot rolling with reduction rates of 5 to 35% at (Ar₃−

100) to (Ar₃50)°

C.,

terminating the hot rolling at (Ar₃−

100) to (Ar₃+50)°

C., and

cooling the hot rolled steel sheet, and then coiling the cooled steel sheet so that the steel sheet has first and second mean values of x-ray random intensity ratios that are at least 3.0 and at most 3.5, respectively, wherein the x-ray random intensity ratios of the first mean value are of a group of {100}<

011>

to {223}<

110>

orientations at least at ½

of the sheet thickness, and wherein the x-ray random intensity ratios of the second mean value are of orientations of {554}<

225>

, {111}<

112>

, and {111}<

110>

.

23. The method according to claim 12, wherein the x-ray random intensity ratios of the first mean value are of {112}<

110>

orientation, and wherein the first mean value is at least 4.0.

24. The method according to claim 12, wherein the x-ray random intensity ratios of the second mean value are of {100}<

011>

orientation, and wherein the second mean value is at least 4.0.

25. The method according to claim 12, wherein the cast slab further contains at least one element selected from the group consisting of, in terms of weight %, Ti:

less than 0.20%, Nb;

less than 0.20%, V;

less than 0.20%, Cr;

less than 1.5%, B;

less than 0.007%, Mo;

less than 1%, Cu;

less than 3%, Ni;

less than 3%, Sn;

less than 0.3%, Co;

less than 3%, Ca;

0.0005 to 0.005%, and REM;

0.001 to 0.02%.

26. The method according to claim 12, wherein the steel sheet is coiled at a temperature To that is determined by the chemical composition of the steel sheet shown in the following equations:

To=−

650.4×

{C %/(1.82×

C %−

0.001}+B

where B is obtained from ingredients of the steel sheet expressed by massy

B=−

50.6×

Mneq+894.3

 

Mneq=Mn %+0.24×

Ni %+0.13×

Si %+0.38×

Mo %+0.55×

Cr %+0.16×

Cu %−

0.50×

A1%+−

0.45×

Co %+0.90×

V %.

27. The method according to claim 12, wherein the hot rolling step is controlled so that the effective strain ε

* that is calculated by the following equation is at least 0.4;

${\mathrm{\varepsilon <br><br>}}^{*}=\underset{j=1}{\overset{n-1}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\varepsilon <br><br>}}_j<br><br>\exp <br><br>\left[-\underset{i=j}{\overset{n-1}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(\frac{t_i}{{\mathrm{\tau <br><br>}}_i}\right)}^{2/3}\right]+{\mathrm{\varepsilon <br><br>}}_n$

wherein n is a number of rolling stands of finish hot rolling, ε

i is strain added at an i-th stand, ti is a traveling time(seconds) between the i-th to i+l-th stands, and ε

i is determinable by the following equation using a gas constant R(=1.987) and a hot rolling temperature Ti(K) of the i-th stand;

ri=8,46×

10<sup>−

9</sup>·

exp{43800/R/Ti}.

28. The method according to claim 12, wherein the hot rolling step is carried out with a friction coefficient of less than 0.2 for at least one pass in the hot rolling step.

29. The method according to claim 12, wherein the cooling step is controlled to an average cooling rate of more than 10°

C./sec from hot rolling terminating temperature to a critical temperature to determined by the chemical composition of the steel sheet, and the cooling step is carried out at a temperature less than To.

30. The method according to claim 12, wherein the hot rolled steel sheet is pickled by acid, wherein, after the steel sheet is pickled, the steel sheet is cold rolled at a reduction rate of less than 80%, wherein, after the steel sheet is cold rolled, the cold rolled steel sheet is reheated between 600°

C. and (Ac3+100)°

C., and then cooled.

31. The method according to claim 12, wherein the hot rolled steel sheet is pickled by acid, wherein, after the steel sheet is pickled, the steel sheet is cold rolled with a reduction rate of less than 80%, wherein, after the steel sheet is cold rolled, the steel sheet is annealed at a temperature between Ac₁, and Ac₃ transformation temperature, and then cooled to a temperature below 500°

C. at a cooling rate of 1 to 250°

C./sec.

13. A method for producing a thin ferritic steel sheet with a particular shape fixability, comprising the steps of;

roughing hot rolling, using one of a temperature range of 1000°

C. to 1300°

C. and without reheating, a cast slab that contains, in terms of weight %,

C;

0.001 to 0.3%,

Si;

0.001 to 3.5%,

Mn;

less than 3%,

P;

0.005 to 0.15%,

S;

less than 0.03%,

Al;

0.01 to 3.0%,

N;

less than 0.01%,

O;

less than 0.01%, and remainder Fe and unavoidable impurities, exceeding a transformation temperature of Ar₃,

completing the hot rolling step at a temperature below an Ar₃ transformation temperature,

terminating the hot rolling step at a temperature below the Ar₃ transformation temperature, and

cooling the hot roiled steel sheet, and then coiling the cooled steel sheet, so that the steel sheet has first and second mean values of x-ray random intensity ratios that are at least 3.0 and at most 3.5, respectively, wherein the x-ray random intensity ratios of the first mean value are of a group of {100}<

011>

to {223}<

110>

orientations at least at ½

of the sheet thickness, and wherein the x-ray random intensity ratios of the second mean value are of orientations of {554}<

225>

, {111}<

112>

, and {111}<

110>

.

32. The method according to claim 13, wherein the x-ray random intensity ratios of the first mean value are of {112}<

110>

orientation, and wherein the first mean value is at least 4.0.

33. The method according to claim 13, wherein the x-ray random intensity ratios of the second mean value are of {100}<

011>

orientation, and wherein the second mean value is at least 4.0.

34. The method according to claim 13, wherein the cast slab further contains at least one element selected from the group consisting of, in terms of weight %, Ti:

less than 0.20%, Nb;

less than 0.20%, V;

less than 0.20%, Cr;

less than 1.5%, B;

less than 0.007%, Mo;

less than 1%, Cu;

less than 3%, Ni;

less than 3%, Sn;

less than 0.3%, Co;

less than 3%, Ca;

0.0005 to 0.005%, and REM;

0.001 to 0.02%.

35. The method according to claim 13, wherein the steel sheet is coiled at a temperature To that is determined by the chemical composition of the steel sheet shown in the following equations:

To=−

650.4×

{C %/(1.82×

C %−

0.001}+B

where B is obtained from ingredients of the steel sheet expressed by massy

B=−

50.6×

Mneq+894.3

Mneq=Mn %+0.24×

Ni %+0.13×

Si %+0.38×

Mo %+0.55×

Cr %+0.16×

Cu %−

0.50×

A1%+−

0.45×

Co %+0.90×

V %.

36. The method according to claim 13, wherein the hot rolling step is controlled so that the effective strain ε

* that is calculated by the following equation is at least 0.4;

${\mathrm{\varepsilon <br><br>}}^{*}=\underset{j=1}{\overset{n-1}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\varepsilon <br><br>}}_j<br><br>\exp <br><br>\left[-\underset{i=j}{\overset{n-1}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(\frac{t_i}{{\mathrm{\tau <br><br>}}_i}\right)}^{2/3}\right]+{\mathrm{\varepsilon <br><br>}}_n$

wherein n is a number of rolling stands of finish hot rolling, ε

i is strain added at an i-th stand, ti is a traveling time(seconds) between the i-th to i+l-th stands, and ε

i is determinable by the following equation using a gas constant R(=1.987) and a hot rolling temperature Ti(K) of the i-th stand;

ri=8,46×

10<sup>−

9</sup>·

exp{43800/R/Ti}.

37. The method according to claim 13, wherein the hot rolling step is carried out with a friction coefficient of less than 0.2 for at least one pass in the hot rolling step.

38. The method according to claim 13, wherein the cooling step is controlled to an average cooling rate of more than 10°

C./sec from hot rolling terminating temperature to a critical temperature to determined by the chemical composition of the steel sheet, and the cooling step is carried out at a temperature less than To.

39. The method according to claim 13, wherein the hot rolled steel sheet is pickled by acid, wherein, after the steel sheet is pickled, the steel sheet is cold rolled at a reduction rate of less than 80%, wherein, after the steel sheet is cold rolled, the cold rolled steel sheet is reheated between 600°

C. and (Ac3+100)°

C., and then cooled.

40. The method according to claim 13, wherein the hot rolled steel sheet is pickled by acid, wherein, after the steel sheet is pickled, the steel sheet is cold rolled with a reduction rate of less than 80%, wherein, after the steel sheet is cold rolled, the steel sheet is annealed at a temperature between Ac₁, and Ac₃ transformation temperature, and then cooled to a temperature below 500°

C. at a cooling rate of 1 to 250°

C./sec.