JP2012172234A - High-strength hot-rolled steel sheet having excellent bending workability, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel sheet that has a TS of around 1,180 MPa and stably shows an excellent bending workability, and to provide a method for producing the same.SOLUTION: The high-strength hot-rolled steel sheet excellent in bending workability has a component composition comprising, by mass, 0.16-0.25% C, ≤0.5% Si, ≤1.0% Mn, ≤0.03% P, ≤0.01% S, ≤0.07% Al, ≤0.01% N, 0.6-1.0% V and the balance being Fe and unavoidable impurities, and has a microstructure where a ferritic phase accounts for ≥95% of the area of the entire matrix. VC is dispersed and precipitated in the matrix and the volume ratio of the total volume of VC to the entire matrix is 0.01-0.02. Here, the average thickness t and the average diameter d of VC, determined based on the assumption that VC has a disk shape, satisfy the relation: (t+d)/2<10 nm.

Description

  The present invention is a high-strength hot-rolled steel sheet having a thickness of 6 mm or less suitable for structural members such as automobiles and construction machines, in particular, a high-strength hot-rolled steel sheet having a tensile strength TS of 1180 MPa class and excellent bending workability. And a manufacturing method thereof.

  In reducing the weight of automobiles, the use of high-strength steel sheets is being studied as the amount of high-strength steel sheets increases. In particular, steel sheets with a TS of 980 MPa or higher are not members with many irregularities due to overhanging, stretch flange processing, deep drawing, etc. applied to steel sheets with a TS of less than 980 MPa in order to take advantage of the high strength. In many cases, the material is processed into a member having many flat portions mainly composed of bending.

  However, DP (Dual Phase) steel sheets that have been developed as high-strength steel sheets with a TS of 980 MPa or more, and DP (Dual Phase) steel sheets composed of a martensite phase and martensite single-phase steel sheets are bent and the former is hard martensite. Starting from the interface between the soft ferrite phase and the soft phase, and the latter, starting from film-like cementite precipitated at coarse prior austenite grain boundaries, there is a problem that cracking is likely to occur and bending workability is poor.

  As technologies related to the bending workability of high-strength steel sheets, for example, in Patent Documents 1 and 2, hard carbides such as TiC and VC having a size of about 0.5 to 50 μm are dispersed and precipitated in a soft ferrite single-phase matrix. A wear-resistant steel sheet with excellent bending workability has been proposed. In Patent Document 3, the volume ratio of the ferrite phase is controlled to 70% or more by controlling the amounts of C, Ti, Nb, V, and Mo, and a large amount of fine carbides having a size of about 10 nm or less are precipitated in the ferrite grains. A hot-rolled steel sheet having excellent bending workability has been proposed.

JP 2007-197813 JP 2007-277590 A JP 2006-161111 A

  However, the wear-resistant steel sheets described in Patent Documents 1 and 2 are prone to cracking at bending radii of less than 1.0 t (t is the thickness of the steel sheet) and cannot be said to be sufficiently excellent in bending workability. In addition, 1180 MPa class TS is not always obtained. In the hot-rolled steel sheet described in Patent Document 3, 1180 MPa class TS cannot be obtained, and excellent bending workability cannot always be obtained.

  The present invention has been made to solve such problems, and has a 1180 MPa class TS, that is, a TS of 1180 MPa or more and less than 1470 MPa, and is capable of stably obtaining excellent bending workability. It aims at providing a steel plate and its manufacturing method.

  As a result of repeated studies on the above-described high strength hot-rolled steel sheet, the present inventors have found the following.

  i) A matrix mainly composed of a ferrite phase, which suppresses the formation of coarse precipitates such as TiN and AlN, and disperses and precipitates fine precipitates, thereby providing high strength and excellent bending workability. Can be applied stably.

  ii) For that purpose, it is effective to use VC which is easy to be refined as a precipitate, to make the size less than 10 nm and to set the total volume ratio of VC in the whole matrix to 0.01 to 0.02. Here, the size of VC means (t + d) / 2 of VC calculated from average thickness t and average diameter d assuming VC is a disk.

  The present invention has been made based on such knowledge, in mass%, C: 0.16-0.25%, Si: 0.5% or less, Mn: 1.0% or less, P: 0.03% or less, S: 0.01% or less Al: 0.07% or less, N: 0.01% or less, V: 0.6 to 1.0%, the balance is composed of Fe and inevitable impurities, and the area ratio of the ferrite phase in the entire matrix is 95% or more The matrix has a microstructure in which VC is dispersed and precipitated, and the total volume ratio of the VC in the entire matrix is 0.01 to 0.02, and the VC was obtained on the assumption of a disk. Provided is a high-strength hot-rolled steel sheet excellent in bending workability, characterized in that a relationship of (t + d) / 2 <10 nm is satisfied between the average thickness t and the average diameter d.

  The high-strength hot-rolled steel sheet of the present invention further includes, in mass%, Ti: 0.015% or less, Cr: 1% or less, B: 0.0030% or less, Mo: 0.5% or less, and W: 1% or less. It is preferable that at least one element selected from is contained individually or simultaneously.

  Moreover, the high-strength hot-rolled steel sheet of the present invention can be a steel sheet having a plating layer on the steel sheet surface.

  The high-strength hot-rolled steel sheet of the present invention is a steel having the above component composition, after hot rolling at a finishing temperature of 900 ° C. or higher, cooled to 700 ° C. or lower at an average cooling rate of 20 ° C./s or higher, 500 to It can be manufactured by a winding method at a winding temperature of 680 ° C.

  According to the present invention, it has a TS of 1180 MPa class, has a critical bend radius (ratio between the minimum bend radius at which cracks do not occur and the plate thickness of the steel sheet) of less than 1.0, and can stably obtain excellent bending workability. High-strength hot-rolled steel sheets can be manufactured. The high-strength hot-rolled steel sheet of the present invention is suitable for structural members such as automobiles.

  Details of the present invention will be described below. Note that “%” representing the content of each component element means “% by mass” unless otherwise specified.

1) Component composition
C: 0.16-0.25%
C precipitates in the steel as VC and is the most important element for increasing the strength of the steel. If the C content is less than 0.16%, 1180 MPa class TS cannot be obtained. On the other hand, if the C content exceeds 0.25%, coarse pearlite and cementite are likely to be produced, and the bending workability is deteriorated. Therefore, the C content is 0.16 to 0.25%.

Si: 0.5% or less
Since Si promotes the discharge of C from the austenite phase during the transformation from the austenite phase to the ferrite phase, it suppresses the formation of fine VC in the ferrite phase and inhibits the increase in strength and bending workability. Therefore, the upper limit of Si content is 0.5%. Preferably it is 0.1% or less.

Mn: 1.0% or less
If the amount of Mn exceeds 1.0%, the moving speed of the austenite-ferrite transformation interface is slowed down, resulting in coarsening of the VC precipitated in the ferrite, making it difficult to obtain a strength of 1180 MPa class. Since it facilitates formation, bending workability deteriorates. Therefore, the Mn content is 1.0% or less, preferably 0.8% or less. More preferably, it is 0.5% or less.

P: 0.03% or less
P segregates at the grain boundaries and significantly deteriorates workability. Therefore, the upper limit of P amount is 0.03%.

S: 0.01% or less
S forms MnS, promotes the generation of voids during bending, and degrades bending workability. Therefore, the upper limit of S content is 0.01%. More preferably, it is 0.005% or less.

Al: 0.07% or less
When the Al content exceeds 0.07%, coarse AlN is formed and bending workability deteriorates. Therefore, the upper limit of the Al content is 0.07%. Since Al is an element having a deoxidizing action of steel, its amount is preferably 0.015% or more.

N: 0.01% or less
N forms coarse nitrides, promotes the generation of cracks from the periphery during bending, and degrades bending workability. Therefore, the upper limit of the N amount is 0.01%, preferably 0.007%, more preferably 0.005%.

V: 0.6-1.0%
V is the most important element for forming fine VC and increasing the strength of steel. When the amount of V is less than 0.6%, a sufficient amount of VC does not precipitate, so that a 1180 MPa class TS cannot be obtained, and cementite and pearlite are generated, and bending workability deteriorates. On the other hand, if the amount of V exceeds 1.0%, VC tends to become coarse, and a 1180 MPa class TS cannot be obtained. Therefore, the V amount is set to 0.6 to 1.0%.

  The balance is Fe and inevitable impurities, but for the following reasons, Ti: 0.015% or less, Cr: 1% or less, B: 0.0030% or less, Mo: 0.5% or less, and W: 1% or less It is preferable that at least one element selected from the group consisting of:

Ti: 0.015% or less It is preferable to add a small amount of Ti in order to promote fine precipitation of VC and to easily obtain high strength. In this case, addition of 0.001% or more is preferable. On the other hand, if the Ti content exceeds 0.015%, coarse TiN exceeding about 1 μm is formed, and cracking is promoted from the interface between TiN and the matrix during bending, so that bending workability deteriorates. Therefore, the upper limit of Ti amount is 0.015%. More preferably, it is 0.01% or less.

Cr: 1% or less
Cr has the effect of suppressing scale formation exceeding 5 μm in thickness. However, if the Cr content exceeds 1%, the ferrite transformation is delayed and a hard phase such as a bainite phase or a martensite phase is formed, so that bending workability deteriorates. Therefore, the Cr content is preferably 1% or less. In order to obtain the above Cr effect, the Cr content is preferably 0.1% or more.

B: 0.0030% or less
B segregates at the ferrite grain boundary to strengthen the ferrite grain boundary and further improve the bending workability. However, if the B content exceeds 0.0030%, the ferrite transformation is delayed and a hard phase such as a bainite phase or a martensite phase is formed, so that the bending workability deteriorates. Therefore, the B content is preferably 0.0030% or less. In order to obtain the B effect as described above, the B content is preferably 0.0005% or more.

Mo: 0.5% or less
Mo has the effect of suppressing the formation of pearlite. However, if the Mo content exceeds 0.5%, the ferrite transformation is delayed and a hard phase such as a bainite phase or a martensite phase is formed, so that the bending workability deteriorates. Therefore, the Mo content is preferably 0.5% or less. In order to obtain the effect of Mo as described above, the Mo amount is preferably 0.03% or more.

W: 1% or less
W, like Mo, has the effect of suppressing the formation of pearlite. However, if the W content exceeds 1%, the ferrite transformation is delayed and a hard phase such as a bainite phase or a martensite phase is formed, so that the bending workability deteriorates. Therefore, the W amount is preferably 1% or less. In order to obtain the above W effect, the W amount is preferably set to 0.05% or more.

  In the high-strength hot-rolled steel sheet of the present invention, elements such as Cu, Ni, Sb and As may be mixed as inevitable impurities from scrap raw materials, but if the total of these elements is 0.2% or less, the present invention The effect of is not impaired.

2) Microstructure In order to ensure good bending workability, it is necessary to make a microstructure composed of a matrix composed mainly of a ferrite phase in which the formation of coarse hard phases such as bainite phase, martensite phase, cementite and pearlite is avoided as much as possible. There is. The main component of the ferrite phase is that the area ratio of the ferrite phase in the entire matrix is 95% or more, and more preferably 98% or more. This is because even if bainite phase other than ferrite phase, martensite phase, cementite, pearlite, etc. are present in an area ratio of 5% or less, preferably 2% or less, they will not hinder the effects of the present invention. is there.

Here, the area ratio of the ferrite phase occupying the entire matrix is determined by polishing the plate thickness section parallel to the rolling direction, corroding with nital, and observing with a scanning electron microscope (magnification: 5000 times). Thus, the area ratio of the ferrite phase occupying the 1 mm ² region of the portion having a thickness of 1/4 to 3/4 was obtained.

  However, a 1180 MPa class TS cannot be obtained simply by using a microstructure composed of a matrix mainly composed of a ferrite phase. Therefore, in the present invention, high strength is achieved by precipitating fine VC in the matrix that does not greatly affect the bending workability. That is, when the total volume ratio of the entire VC matrix having a size of less than 10 nm is 0.01 to 0.02, excellent bending workability and 1180 MPa class TS are ensured. If the size of the VC is 10 nm or more, or the total volume ratio of the entire VC matrix is less than 0.01, a 1180 MPa class TS cannot be obtained. Further, if the total volume ratio of the entire VC matrix exceeds 0.02, TS greatly exceeds 1180 MPa, and a desired strength level cannot be obtained.

  In addition, when steel contains Ti, Ti may be dissolved in VC and VC may be (V, Ti) C, but with a small amount of Ti addition as in the present invention, V in the carbide and The atomic concentration ratio of Ti (at%) is V / Ti> 0.5, and the same effect is obtained if the above-mentioned conditions for the size and volume ratio of VC are satisfied. ) And written as VC.

  Here, the size of VC means that since VC is in a disc shape, a square plate-like VC observed from the [001] orientation of the ferrite phase as a matrix by a transmission electron microscope is assumed to be a disc, and the average This is (t + d) / 2 of VC obtained from the thickness t and the average diameter d. The reason why the size of VC can be determined in this way is that a VC having a NaCl-type crystal structure has a specific orientation (Baker-Nutting) with the matrix.

  In addition, the total volume ratio of the entire VC matrix was determined by preparing a thin film sample from a portion having a thickness of 1/4 to 1/2 of the plate thickness cross section of the steel plate and observing it with a transmission electron microscope. After confirming that the object is VC by EDX (Energy Dispersive X-ray Spectroscopy) and EELS (Electron Energy-Loss Spectroscopy) attached to the electron microscope, the total volume of VC is obtained and obtained as a ratio to the volume of the entire observed sample. It was.

  Here, the volume of VC was obtained by calculating the volume of the sphere assuming a sphere equivalent volume, that is, a sphere having a diameter of (t + d) / 2, which is the size of the above-described disk-like VC. . In addition, the volume ratio of VC is the ratio of the total volume of the observed sample, calculated by calculating the total volume of VC in the field of view and converting it per 100 nm thickness based on the sample thickness obtained from the EELS measurement. As sought.

3) Manufacturing conditions Finishing temperature of hot rolling: 900 ° C or more If the finishing temperature is less than 900 ° C, large ferrite grains and small ferrite grains form a layer structure in a band shape, so that the bending workability deteriorates. Therefore, the finishing temperature is 900 ° C. or higher.

  In order to achieve a finishing temperature of 900 ° C or higher, it is necessary to heat the steel to the austenite single-phase region before hot rolling, but VC is more easily dissolved in the austenite phase than TiC, NbC, etc. It is sufficient that the heating temperature is 1150 to 1250 ° C. In the case of TiC or NbC, heating of 1300 ° C or higher is required to obtain TS of 980 MPa or higher. Further, in the present invention, direct feed rolling technology in which the steel after continuous casting is hot-rolled as it is can also be applied. At this time, in order to ensure a finishing temperature of 900 ° C. or higher, auxiliary heating can be performed before hot rolling.

Cooling conditions after hot rolling: Cooling to an average cooling rate of 20 ° C / s or more to 700 ° C or less After hot rolling, in order to suppress precipitation of coarse VC, 700 ° C at an average cooling rate of 20 ° C / s or more It is necessary to cool to the following. If the average cooling rate exceeds 20 ° C / s or cooling at this cooling rate is stopped at a temperature exceeding 700 ° C, VC becomes coarse and high strength is hindered.

Winding temperature: 500 ~ 680 ℃
When the coiling temperature is less than 500 ° C., it becomes difficult to precipitate fine VC having a size of less than 10 nm, and a hard phase such as a bainite phase is likely to be formed, resulting in a decrease in bending workability. Moreover, when it exceeds 680 degreeC, VC will coarsen and high intensity | strength will be inhibited. Therefore, the winding temperature is set to 500 to 680 ° C.

  The high-strength hot-rolled steel sheet of the present invention can be a steel sheet having a plating layer such as a Zn-based plating layer on the surface of the steel sheet. For example, the Zn-based plating layer can be obtained by forming a pure Zn or Zn alloy plating layer by a normal melting or electroplating method.

Steels 1 to 25 having the composition shown in Table 1 were melted to form slabs. These slabs were hot rolled under the hot rolling conditions shown in Table 2 to produce hot rolled steel sheets with a thickness of 2.0 mm. At this time, the steel sheet after hot rolling was cooled to the coiling temperature at an average cooling rate of 25 ° C./s. Further, the steel plate 25 was passed through a hot-dip Zn plating line to form a hot-dip zinc plating layer [adhesion amount 50 g / m ² (per side)] on the surface. In the Zn plating line, the annealing temperature was set to 720 ° C., and after immersing in a 470 ° C. Zn plating bath (0.1% Al—Zn), alloying was performed at 520 ° C. Then, the area ratio of the ferrite phase in the entire matrix, (t + d) / 2 of VC, and the total volume ratio of VC in the entire matrix were determined by the above method. Further, a JIS No. 5 tensile test piece was taken in parallel with the rolling direction, and a tensile test was performed at a crosshead speed of 10 mm / min in accordance with JIS Z 2241 to obtain TS and El. In addition, specimens with a width of 50 mm and a length of 100 mm were taken so that the rolling direction would be the length, and a V-bend test with a vertex angle of 90 degrees was performed. As measured.

  The results are shown in Table 2. In the example of the present invention, it is understood that TS of 1190 to 1395 MPa, that is, 1180 MPa class, is obtained, the bending limit radius is less than 1.0, and the bending workability is excellent.

Claims (5)

  In mass%, C: 0.16-0.25%, Si: 0.5% or less, Mn: 1.0% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.07% or less, N: 0.01% or less, V: 0.6 Containing 1.0 to 1.0%, the balance is composed of Fe and inevitable impurities, the area ratio of the ferrite phase in the entire matrix is 95% or more, and the microstructure in which VC is dispersed and precipitated in the matrix And the total volume ratio of the VC occupying the entire matrix is 0.01 to 0.02, and the average thickness t and the average diameter d obtained by assuming the VC as a disk is (t + d ) / 2 <10nm high-strength hot-rolled steel sheet with excellent bending workability, which satisfies the relationship.   2. The high-strength hot-rolled steel sheet having excellent bending workability according to claim 1, further comprising, by mass%, Ti: 0.015% or less.   Furthermore, it contains at least one element selected from Cr by mass%, Cr: 1% or less, B: 0.0030% or less, Mo: 0.5% or less, and W: 1% or less. Item 3. A high-strength hot-rolled steel sheet excellent in bending workability according to Item 1 or 2.   4. The high-strength hot-rolled steel sheet having excellent bending workability according to any one of claims 1 to 3, wherein the steel sheet surface has a plating layer.   The steel having the component composition according to any one of claims 1 to 3, after hot rolling at a finishing temperature of 900 ° C or higher, cooled to 700 ° C or lower at an average cooling rate of 20 ° C / s or higher, 500 A method for producing a high-strength hot-rolled steel sheet excellent in bending workability, characterized by winding at a winding temperature of ˜680 ° C.