JP2010270383A - Hot-rolled steel plate superior in formability and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel plate which has aging resistance, softness and high ductility, and also has improved ¾ &Delta;r ¾ without excessively coarsening the grains, and to provide a method for manufacturing the same. <P>SOLUTION: The hot-rolled steel plate includes, by mass%, 0.03-0.07% C, 0.1% or less Si, 0.05-0.5% Mn, 0.03% or less P, 0.03% or less S, 0.02-0.1% Sol. Al, 0.005% or less N, less than 0.005% Nb+Ti, 0.0003-0.0020% B and the balance Fe with unavoidable impurities; has an average grain size of 12-25 &mu;m; and satisfies ¾ &Delta;r ¾&le;0.25 and AI&le;20 MPa. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a hot-rolled steel sheet applied to materials such as automobiles and home appliances. The hot-rolled steel sheet has workability suitable for press molding and has excellent aging resistance and in-plane anisotropy. About.

  Hot-rolled steel sheets used in automobiles and electrical products are required to have high formability. In particular, hot-rolled steel sheets used for deep drawing applications such as compressor covers are energetically advanced. It has been. In addition, with the recent globalization of production bases, as the transportation time and storage time of products become longer, the stability of materials, especially aging resistance, is becoming more important.

  For hot-rolled steel sheets for deep drawing with excellent aging resistance, by limiting the upper limit of strengthening elements such as C and Mn to ensure soft and high ductility, by adding more than 0.001% B and fixing N in steel Patent Document 1 proposes a technique for improving aging resistance. However, in such a technique, since the B addition amount of the hot-rolled steel sheet is as high as 0.001% or more, there is a problem that | Δr | (anisotropy difference of r value), which is an index indicating the in-plane anisotropy of drawing, increases. Arise.

In response to the above-mentioned problems found in hot-rolled steel sheets with B added, Patent Document 2 proposes a technique to improve │Δr│ by finishing rolling at a high temperature of Ar ₃ point + 20 ° C or higher by specifying the amount of B added Has been. Patent Document 3 discloses a technique of performing finish rolling at Ar ₃ points or more in order to improve the | Δr | of the hot-rolled steel sheet with respect to the hot-rolled steel sheet that defines the P content but is not a hot-rolled steel sheet to which B is added. Proposed.

  However, as disclosed in Non-Patent Document 1, the hot-rolled steel sheet to which B is added tends to be coarse and has a large dependency on the hot-rolling conditions. Therefore, if the finish rolling is simply performed at a high temperature as proposed in Patent Document 2 and Patent Document 3, excessive graining occurs, and the surface after press molding becomes rough. Further, the hot-rolled steel sheet proposed in Patent Document 3 does not add B, but grain growth is enhanced by making it extremely low P (0.005% or less), so that it is the same as the hot-rolled steel sheet proposed in Patent Document 2 The occurrence of rough skin due to excessive coarsening becomes a problem.

  On the other hand, Patent Document 4 discloses a technique of adding Ti or Nb to a hot-rolled steel sheet for the purpose of suppressing excessive grain growth. However, Ti and Nb have remarkably strong recrystallization suppressing effects and grain growth suppressing effects. Therefore, in order to improve | Δr | of the hot-rolled steel sheet to which Ti or Nb is added and to ensure an appropriate crystal grain size, it is necessary to keep it in a high temperature range for a long time. In addition, in the technique disclosed in Patent Document 4, since B is not added, the occurrence of aging in the low C region cannot be avoided, and there is a problem that the aging resistance is inferior.

JP-A-52-125411 JP-A-2-209423 JP-A-2-209424 JP-A-2-30713

Satoshi Ito and three others, "About hot-rolled steel sheet KFN for press working", Kawasaki Steel Technical Report, Kawasaki Steel Corporation, 1973, Vol.5 No.2, Pg.224-234

  As described above, several technologies for improving │Δr│ have been disclosed for hot-drawn steel sheets for deep drawing, but these technologies cause problems such as the occurrence of rough skin and deterioration of aging resistance. There was room for improvement. The present invention has been made in view of such circumstances, and it goes without saying that it is excellent in formability and aging resistance, and a soft hot-rolled steel sheet with improved │Δr│ without causing excessive coarsening, It is intended to provide with its advantageous manufacturing method.

  In order to solve the above-mentioned problems, the present inventors have made extensive studies on the recrystallization behavior and the grain growth behavior of B-added aluminum killed steel. As a result, by controlling the component composition of steel, finish rolling temperature and rolling rate, cooling conditions after finish rolling, etc., a relatively large amount of P is added in particular, and the process is By carrying out a short cooling treatment after hot rolling is completed, in hot-rolled steel sheets with aging resistance and soft high ductility, without causing excessive coarsening that causes skin roughness, etc. We obtained new knowledge that Δr│ can be reduced.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) By mass%,
C: 0.03% to 0.07%,
Si: 0.1% or less,
Mn: 0.05% to 0.5%,
P: 0.01% or more and 0.03% or less,
S: 0.03% or less,
sol.Al: 0.02% to 0.1%,
N: 0.005% or less,
Nb + Ti: less than 0.005% and
B: 0.0003% or more and 0.0020% or less, the balance is Fe and inevitable impurities, the average crystal grain size is 12 μm or more and 25 μm or less, and | Δr | ≦ 0.25, AI ≦ 20 MPa Hot rolled steel sheet.

(2) In mass%,
C: 0.03% to 0.07%,
Si: 0.1% or less,
Mn: 0.05% to 0.5%,
P: 0.01% or more and 0.03% or less,
S: 0.03% or less,
sol.Al: 0.02% to 0.1%,
N: 0.005% or less,
Nb + Ti: less than 0.005% and
B: Steel strip containing 0.0003% or more and 0.0020% or less, with the balance being Fe and inevitable impurities, the final pass of hot rolling is the condition of rolling temperature: Ar ₃ point + 50 ° C or more and rolling rate: 15% or more After the hot rolling is completed, the mixture is allowed to cool for 0.5 to 10 seconds, then cooled to 700 ° C. or less at a cooling rate of 20 ° C./s or more, and wound at 590 to 700 ° C. A method for producing a hot-rolled steel sheet.

  According to the present invention, with respect to a hot-rolled steel sheet, sufficient aging resistance and soft high ductility can be imparted, and in-plane anisotropy can be reduced while maintaining a crystal grain size suitable for press working. Therefore, according to the hot-rolled steel sheet of the present invention, even a product that requires high processing, such as a compressor cover, which has been difficult to form with conventional hot-rolled steel sheets, is produced without causing quality deterioration due to rough skin during forming. It becomes possible to do.

The reasons for limiting the component composition and production conditions of the present invention will be described below. The unit of the element content in the steel sheet is “% by mass”, but hereinafter, it is simply indicated by “%” unless otherwise specified.
(1) Component composition range
C: 0.03% to 0.07%
If the C content is large, a large amount of carbide is generated, and the elongation of the hot-rolled steel sheet is reduced to impair the formability. Therefore, the content is set to 0.07% or less. On the other hand, since extremely low C leads to an increase in manufacturing cost, the lower limit is set to 0.03%.

Si: 0.1% or less
If Si is contained excessively, the strength is increased and the moldability is deteriorated, so the content is made 0.1% or less.

Mn: 0.05% or more and 0.5% or less
Mn fixes S as MnS and has an effect of improving hot ductility, so its content needs to be 0.05% or more. On the other hand, excessive addition causes hardening of the steel and deteriorates formability, so the upper limit of the content is made 0.5%.

P: 0.01% to 0.03%
P is a characteristic element in the present invention. That is, when the P content is low, the grain growth increases and the risk of rough skin due to coarsening increases. Therefore, in the present invention, at least 0.01% of P is contained. Nonetheless, P is a solid solution strengthening element. If it is excessively contained, the steel is hardened, so the upper limit is made 0.03%.

S: 0.03% or less
S is an element that hinders hot ductility and formability, and its content is preferably low. Further, S is fixed as MnS for the purpose of improving hot ductility and formability. However, if the amount of MnS is excessive, the elongation is lowered, so the upper limit of the S content is 0.03%.

sol.Al: 0.02% to 0.1%
Al is not only useful as a deoxidizing agent, but also has the effect of fixing N not fixed with B as AlN and improving the aging resistance, so the content of sol.Al needs to be 0.02% or more. On the other hand, excessive addition causes an increase in production cost, so the upper limit is made 0.1%.

N: 0.005% or less
N is an element that causes aging of the hot-rolled steel sheet, and its content is preferably as low as possible. However, excessive reduction causes a significant cost increase. In the present invention, B and Al are added to fix N, so if the N content is 0.005% or less, the adverse effect can be ignored, so the upper limit is made 0.005%.

Nb + Ti: Less than 0.005%
Since Nb and Ti are strong recrystallization suppressing elements, it is difficult to reduce | Δr | if these elements are contained excessively. Moreover, since these elements are also strong grain growth inhibiting elements, if they are contained excessively, the crystal grains of the hot-rolled steel sheet become fine and cause a decrease in elongation. In this respect, the amount of Nb + Ti is preferably as small as possible. However, if the combined content of Nb and Ti is less than 0.005%, the above-described adverse effects can be ignored, so the upper limit of Nb + Ti is made less than 0.005%.

B: 0.0003% to 0.0020%
B is an element that fixes N and improves aging resistance. Further, since it has an effect of appropriately coarsening the crystal grains and has an effect of suppressing a decrease in elongation due to the refinement of the crystal grains, the B content needs to be 0.0003% or more. On the other hand, since | Δr | becomes large when added excessively, the upper limit is made 0.0020%.
The balance other than the above components is Fe and inevitable impurities. As impurities, for example, Cu: 0.02% or less and Ni: 0.02% or less are allowed.

(2) Average crystal grain size The average crystal grain size must be 12 μm or more and 25 μm or less. If it is less than 12 μm, the yield strength becomes high and molding becomes difficult, and if it exceeds 25 μm, rough skin occurs during press molding. More preferably, it is 12 μm or more and 23 μm or less.

(3) | Δr | ≦ 0.25
If the anisotropic difference | Δr | of the average r value expressed by the following equation (1) exceeds 0.25, the yield at the time of drawing is reduced, so it is necessary to set it to 0.25 or less.
Record
│Δr│ = | (r ₀ + r ₉₀ −2r ₄₅ ) / 2 2 (1)
The average r value is preferably 0.80 or more from the viewpoint of reducing the molding load. The steel sheet of the present invention is a hot-rolled steel sheet, and the average r value is generally 1.0 or less.

(4) AI ≦ 20MPa
If the aging index AI exceeds 20 MPa, the material will change during transportation and coil storage, and it will be necessary to optimize press molding conditions, leading to an increase in product cost. On the other hand, if AI is 20 MPa or less, the material fluctuation is not a problem, so AI is 20 MPa or less, more preferably 10 MPa or less.

(5) Manufacturing process The steel pass obtained by melting and casting the steel adjusted to the above component composition range immediately after casting or after reheating, the final pass of hot rolling is the rolling temperature: Ar ₃ points +50 More than ℃ and rolling rate: 15% or more, after the hot rolling, after cooling for 0.5 seconds to 10 seconds, cooled to 700 ℃ or less at a cooling rate of 20 ℃ / s, Wind up at 590 ℃ to 700 ℃. The reheating temperature need not be specified, but it must be reheated under conditions that can ensure the finish rolling temperature, and is generally 1050 to 1300 ° C.

  In the hot-rolled steel sheet to which B is added as in the present invention, in order to reduce the in-plane anisotropy while maintaining the crystal grain size suitable for press working, the finish rolling temperature and rolling rate, cooling after finish rolling It is important to optimize the conditions and the winding temperature.

The present inventors investigated the correlation between | Δr | of the B-containing hot-rolled steel sheet having the above composition and the rolling temperature of the final pass of hot rolling. Here, the final pass of the hot rolling means the final pass in the finish rolling of the hot rolling, that is, the final reduction stand, and the rolling temperature (also referred to as the finish rolling temperature) is the outlet temperature of the reduction stand. . As a result, ￨Derutaaru￨ of hot-rolled steel sheet as the rolling temperature of the final pass to high temperature from the Ar ₃ point decreases, the Ar ₃ point + 50 ℃ or higher and ￨Derutaaru￨ find that substantially shows a constant value did. The reason is not clear, but when the rolling temperature of the final pass is in the vicinity of the Ar ₃ point, the r ₄₅ value in the equation (1) shows a larger value than the r ₀ value and the r ₉₀ value. As the rolling temperature of the steel increases, the r ₀ and r ₉₀ values increase, while the r ₄₅ value decreases. As a result, | Δr | in the equation (1) decreases, and this | Δr | It is estimated that saturation occurs at ₃ points + 50 ° C.

Therefore, in the present invention, in order to reduce | Δr |, finish rolling is performed at a high temperature at which the final pass is a rolling temperature: Ar ₃ point + 50 ° C. or higher. However, the finish rolling temperature is preferably 950 ° C. or less from the viewpoint of reducing the scale layer. Moreover, in this invention, it cools for a predetermined time after finish rolling. This is because the austenite processed structure formed by finish rolling is recrystallized in the austenite region, the texture is randomized, and | Δr | is reduced. Here, if the cooling after finish rolling is less than 0.5 seconds, recrystallization of the austenite processed structure is not sufficient, so in the present invention, the cooling time is set to 0.5 seconds or more.

As described above, if finish rolling is performed at a high temperature and then allowed to cool for a long time, │Δr│ will be reduced, but excessive coarse grain formation, which is characteristic of B-added steel, will be caused, resulting in rough skin of the press-formed product. . In order to solve such a problem, the final pass rolling rate (rolling rate in the final pass) is 15% or more, more preferably 20% or more to increase the number of recrystallized nuclei, and further the cooling time after finish rolling is set to 10%. The size of the crystal grains after the ferrite transformation was adjusted to an appropriate range by suppressing the austenite grain growth for 2 seconds or less, preferably less than 5 seconds.
The rolling rate is preferably less than 25% from the viewpoint of improving the steel plate shape.

  After a predetermined holding (cooling) time has elapsed, the sample is cooled to 700 ° C. or lower at a cooling rate of 20 ° C./sec or higher, and wound at 590 ° C. or higher and 700 ° C. or lower. In order to suppress grain growth during cooling, the cooling rate needs to be 20 ° C./sec or more. When the winding temperature exceeds 700 ° C, the surface properties deteriorate due to the scale generated during winding. When the winding temperature is less than 590 ° C, AI exceeds 20 MPa, which causes material fluctuations during transportation and storage. Therefore, the coiling temperature is set to 590 ° C or more and 700 ° C or less. Preferably it is 625 ° C or more. The upper limit of the cooling rate is approximately 500 ° C./s from the viewpoint of not accompanied by a large increase in manufacturing cost.

  The hot-rolled steel sheet according to the present invention does not change its performance whether it is pickled or blackened. Further, there is no problem even if hot dip galvanization is performed after pickling or with the black skin without pickling. There is no restriction on the conditions for temper rolling, but if the rolling rate is excessively high, the elongation decreases drastically, so the temper rolling elongation rate is preferably 2% or less.

  The hot-rolled steel sheet that is the subject of the present invention is a soft hot-rolled steel sheet that is suitably used for deep drawing applications with a yield stress of about 250 MPa or less and a sheet thickness of about 2 mm to 6 mm.

Example 1
Steel with the ingredients shown in Table 1 was melted, and a processed Formaster test piece with a diameter of 8 mm and a height of 12 mm was cut out, heated to 1200 ° C, cooled to 950 ° C at a cooling rate of 10 ° C / sec, and at 950 ° C After compressing at 30% strain, it was cooled to 200 ° C. at a cooling rate of 10 ° C./second. The Ar ₃ point was measured from the thermal expansion curve during cooling and found to be 823 ° C. This steel was heated to 1200 ° C. and hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 3.2 mm.

  The obtained hot-rolled steel sheet was pickled and then subjected to temper rolling with an elongation of 1%, and mechanical property evaluation and structure observation were performed. For mechanical properties, JIS Z 2201 (1998) No. 5 test piece was taken in the rolling direction (RD) and subjected to a tensile test based on JIS Z 2241 (1998).

  The r value was measured by collecting JIS Z 2201 (1998) No. 5 test pieces in the rolling direction (RD), the rolling perpendicular direction (TD), and the rolling 45 ° direction (DD), and applying 15% strain. In addition, | Δr | was determined according to the equation (1).

  AI collected JIS Z 2201 (1998) No. 5 test piece in the rolling direction (RD), applied pre-strain 7.5%, and then heat-treated at 100 ° C for 30 minutes to give the stress before heat treatment (7.5% pre-strain). The difference between the stress at the time of application) and the yield stress after the heat treatment was evaluated.

  Further, the cross-sectional structure in the rolling direction was subjected to nital corrosion at a thickness of 1/4 position, and the average crystal grain size was measured according to the cutting method described in JIS G 0552 (1998). The results are also shown in Table 2.

  According to the conditions B, C, G, and H of the inventive examples, the yield stress YP is 220 MPa or less, the breaking elongation El is 48% or more and soft and highly ductile, and the AI is 20 MPa or less, and | Δr | is 0.25 or less. A hot rolled steel sheet having sufficient aging resistance and low anisotropy can be obtained. Further, the average crystal grain size of the hot-rolled steel sheet produced under the conditions B, C, G and H of the examples of the present invention satisfies the range of 12 to 25 μm, and therefore there is no risk of rough skin during press forming. Furthermore, conditions B, C, and H, which are examples of the present invention, have a coiling temperature of 625 ° C. or higher, so the AI of the obtained hot-rolled steel sheet is 10 MPa or less, and it is found that the aging resistance is particularly excellent. .

  On the other hand, under conditions A where the cooling time is short, conditions E and K where the rolling temperature of the final rolling final pass is low, and conditions I where the rolling rate of the final rolling final pass is low, recrystallization in the austenite region does not proceed sufficiently. Therefore, the anisotropy of the hot-rolled steel sheet manufactured under these conditions is high, and | Δr | exceeds 0.25. Further, in the hot-rolled steel sheet manufactured under the condition D having a long cooling time and the condition J having a low cooling rate, the crystal grains become coarse. Further, the hot rolled steel sheet manufactured under the condition F in which the winding temperature is less than 590 ° C. has an AI exceeding 20 MPa.

(Example 2)
Steels having the components shown in Table 3 were melted, and Ar ₃ points were measured by the same method as in Example 1. Next, after heating these steels to 1200 ° C, the final pass of finish rolling in hot rolling is performed at a rolling rate of 22% and a temperature of 875 ° C. After the finish rolling, the steel is allowed to cool for 4.2 seconds and then cooled at 25 ° C / second. It cooled to 630 degreeC with the speed | rate, and wound up as it is.

  From the obtained hot rolled sheet, mechanical properties, r value, AI, and average crystal grain size were measured in the same manner as in Example 1. The results are also shown in Table 3.

  According to steel Nos. 3 and 4 satisfying the component composition of the present invention, the yield stress YP is 220 MPa or less, the breaking elongation El is 48% or more and soft and highly ductile, and AI is 20 MPa or less, and | Δr | is 0.25 or less. Thus, a hot-rolled steel sheet having sufficient aging resistance and low anisotropy can be obtained. Further, the hot rolled steel sheet of steel No. 3 has an average crystal grain size of 20 μm or less, so that there is less concern about rough skin.

  On the other hand, in steel No. 2 with a large amount of C, the average crystal grains of the hot-rolled steel sheet are fine and the yield strength is high. Steel No. 5 with a small amount of P results in coarse average grain size of the hot-rolled steel sheet. In Steel No. 6 with a large amount of Nb and Ti and Steel No. 8 with a large amount of B, recrystallization in the austenite region does not proceed sufficiently, so the anisotropy of the hot-rolled steel sheet is high, and | Δr | It will exceed. Moreover, in steel No. 7 with a small amount of B, the AI of the hot-rolled steel sheet exceeds 20 MPa.

  In accordance with the present invention, P is added in the range of 0.01 to 0.03% to the hot-rolled steel sheet for processing to which B is added, and further, the rolling rate: 15% or more, and after the final pass, 0.5 to 10 seconds of cooling treatment is provided. Thus, a hot-rolled steel sheet having aging resistance and soft high ductility and improved | Δr | without causing excessive coarsening can be obtained.

Claims (2)

% By mass
C: 0.03% to 0.07%,
Si: 0.1% or less,
Mn: 0.05% to 0.5%,
P: 0.01% or more and 0.03% or less,
S: 0.03% or less,
sol.Al: 0.02% to 0.1%,
N: 0.005% or less,
Nb + Ti: less than 0.005% and
B: 0.0003% or more and 0.0020% or less, the balance is Fe and inevitable impurities, the average crystal grain size is 12 μm or more and 25 μm or less, and | Δr | ≦ 0.25, AI ≦ 20 MPa Hot rolled steel sheet. % By mass
C: 0.03% to 0.07%,
Si: 0.1% or less,
Mn: 0.05% to 0.5%,
P: 0.01% or more and 0.03% or less,
S: 0.03% or less,
sol.Al: 0.02% to 0.1%,
N: 0.005% or less,
Nb + Ti: less than 0.005% and
B: Steel strip containing 0.0003% or more and 0.0020% or less, with the balance being Fe and inevitable impurities, the final pass of hot rolling is the condition of rolling temperature: Ar ₃ point + 50 ° C or more and rolling rate: 15% or more After the hot rolling is completed, the mixture is allowed to cool for 0.5 seconds to 10 seconds and then cooled to 700 ° C. at a cooling rate of 20 ° C./s or more and wound at 590 ° C. to 700 ° C. A method for producing a hot-rolled steel sheet.