JP5699680B2 - High-strength galvannealed hot-rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention is a high-strength alloyed hot-dip galvanized hot-rolled steel sheet suitable for structural members used in the fields of automobiles and construction machinery, in particular, a plate having excellent bending workability and plating property and having a tensile strength TS of 1470 MPa or more. The present invention relates to a high-strength galvannealed hot-rolled steel sheet having a thickness of 6 mm or less and a manufacturing method thereof.

  In reducing the weight of automobiles, the use of high-strength steel sheets and the use of steel sheets with higher strength are being studied. Generally, increasing the strength of a steel sheet causes a decrease in the ductility of the steel sheet, that is, a decrease in workability. Therefore, an alloyed hot-dip galvanized steel sheet that has both high strength and high workability and excellent corrosion resistance is required. Has been. Among them, a high strength alloyed hot-dip galvanized hot-rolled steel sheet is desired for the structural member from the viewpoint of low cost.

As such a high-strength galvannealed hot-rolled steel sheet, a structure-strengthened steel sheet that has been strengthened by a hardened phase such as a martensite phase dispersed in a ferrite phase has been studied. For example, in Patent Document 1, thin steel sheets containing C: 0.005 to 0.15%, Mn: 0.3 to 2.0%, Cr: 0.03 to 0.8% by weight ratio are alloyed by hot dip galvanization using a continuous galvanizing line. In the method for producing a galvannealed high-strength steel sheet, a step of heating the steel sheet to a temperature between the Ac ₁ transformation point and the Ac ₃ transformation point, and a molten zinc in a temperature range of 450 to 550 ° C. during the cooling from the heating temperature a step of plating was further heated to a temperature range between 500 ° C. and Ac ₁ transformation point subjected to perform alloying process comprises a, a step of cooling to below the subsequently 300 ° C. after the alloying treatment, the Ac ₁ until subjected to hot-dip galvanizing than the heating temperature of between to Ac _3, and the cooling rate in the cooling step of cooling to 300 ° C. or less after the alloying treatment, logCR = -3.11Cr-1.93Mn + 4.61 is given at the critical cooling rate CR (° C / sec) or higher of alloyed galvanized high-tensile steel sheet with good workability Manufacturing methods have been proposed. Patent Document 2 includes, by weight, C: 0.04 to 0.1%, Si: 0.4 to 2.0%, Mn: 1.5 to 3.0%, B: 0.0005 to 0.005%, P: ≦ 0.1%, Ti> 4N and Ti ≤0.05%, Nb: ≤0.1% contained, the balance is Fe and unavoidable impurities steel plate surface layer with alloyed galvanized layer, Fe% in alloyed hot dip galvanized layer is 5-25% There has been proposed a high-strength galvannealed steel sheet excellent in formability and plating adhesion with a tensile strength of 800 MPa or more.

Japanese Patent Publication No.62-40405 JP 9-13147 A JP 2009-31269 A

  However, in the galvannealed high-tensile hot-rolled steel sheet manufactured by the method described in Patent Document 1, only 520 MPa TS is used, and in the high-strength galvannealed hot-rolled steel sheet disclosed in Patent Document 2, 920 MPa is used at most. However, there is a problem that the bending workability is inferior. Furthermore, the high-strength galvannealed steel sheet of Patent Document 2 has a problem that it is inferior in plateability.

  An object of the present invention is to provide a high-strength galvannealed hot-rolled steel sheet having a bending workability and a plating property and having a TS of 1470 MPa or more and a method for producing the same.

The present inventors diligently studied to achieve the above object, and as a result, the Si content was reduced, and the C and V contents were appropriately controlled to form V carbide (VC) having a size of 10 nm or less in the ferrite phase. It has been found that it is effective to obtain a uniformly precipitated microstructure. Here, the size of the VC is 2 ^1/2 × L (L: 1 of the square plate) in a square plate-like VC observed from the [001] orientation of the ferrite phase as a matrix by a transmission electron microscope (TEM). This is a value obtained by calculating the VC size that can be expressed by (length of side) for a plurality of VCs and arithmetically averaging them. 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.

  The present invention has been made based on such knowledge, in mass%, C: 0.26-0.35%, Si: 0.4% 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, Ti: 0.05% or less, V: 1.2-1.8%, the balance is composed of Fe and inevitable impurities, and the area ratio of the ferrite phase in the entire matrix Is 90% or more, and the ferrite phase has a microstructure in which V carbides (VC) are precipitated, and the ratio of the number of VCs having a size of less than 10 nm out of the total number of VCs is 85. The present invention provides a high-strength galvannealed hot-rolled steel sheet characterized by having a solid solution V content of 0.30% by mass or less.

  In the high-strength galvannealed hot-rolled steel sheet of the present invention, the mass% was further selected from Cr: 1% or less, B: 0.0030% or less, Mo: 0.5% or less, W: 1% or less. It is preferable to have a composition containing at least one element.

  The high-strength galvannealed hot-rolled steel sheet of the present invention is a steel having the above composition, after hot rolling at a finishing temperature of 900 ° C. or more, winding at a winding temperature of 580 to 680 ° C., pickling, It can be manufactured by a method of heating to 600 to 700 ° C. in a reducing atmosphere for 10 to 90 seconds, immersing in a galvanizing bath, performing a plating treatment, and alloying the plating layer at 460 to 550 ° C.

  According to the present invention, a high-strength galvannealed hot-rolled steel sheet having excellent bending workability and plating property and having a TS of 1470 MPa or more can be produced. The high-strength galvannealed hot-rolled steel sheet of the present invention is suitable for structural members of automobiles and construction machines.

  The inventors of the present invention are the reasons why the high-strength galvannealed steel sheets described in Patent Documents 1 and 2 are inferior in bending workability and the reasons why the high-strength galvannealed steel sheets described in Patent Document 2 are inferior in plateability In the former case, stress concentration is likely to occur at the interface between the hard martensite phase and the soft ferrite phase dispersed in the ferrite phase to increase the strength. It became clear that the main reason was the high amount. Therefore, in the present invention, in order to improve the bending workability, the matrix is a ferrite single phase, and fine VC having a size of 10 nm or less is uniformly deposited on the ferrite phase to prevent the occurrence of stress concentration and The strength is increased by strengthening. In particular, VC is an MC type carbide with the same structure as NaCl, and it is difficult to coarsen by heat treatment at 700 ° C or less, and it can maintain a fine state after alloying hot dip galvanizing treatment, so high strength alloyed hot dip galvanizing It can be said that this is an effective precipitate for producing a hot-rolled steel sheet. Further, in order to improve the plating property, the Si amount is set to 0.4% by mass or less.

  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: 0.26-0.35%
C precipitates in steel as VC and is an important element contributing to high strength. When the C content is less than 0.26%, a TS of 1470 MPa or more cannot be obtained, and when it exceeds 0.35%, the ductility decreases. Therefore, the C content is 0.26 to 0.35%.

Si: 0.4% or less
Si is an element effective for adjusting the strength as a solid solution strengthening element, but if its amount exceeds 0.4%, the plating property deteriorates. Therefore, the Si content is 0.4% or less.

Mn: 1.0% or less
If the amount of Mn exceeds 1.0%, the ferrite transformation is delayed and a hard phase such as a bainite phase or a martensite phase is formed, or a band-like structure is formed due to the segregation, resulting in deterioration of bending workability. Therefore, the Mn content is 1.0% or less, preferably 0.8% or less, and more preferably 0.5% or less.

P: 0.03% or less
When the P content exceeds 0.03%, grain boundary segregation becomes prominent and bending workability deteriorates. Therefore, the P content is 0.03% or less.

S: 0.01% or less
When the amount of S exceeds 0.01%, precipitation of MnS becomes remarkable and bending workability deteriorates. Therefore, the S content is 0.01% or less.

Al: 0.07% or less
If the Al content exceeds 0.07%, surface nitriding is promoted during the hot rolling process, and bending workability may be deteriorated. Therefore, the Al content is 0.07% or less.

N: 0.01% or less
If the N content exceeds 0.01%, coarse TiN is formed, and bending workability deteriorates. Therefore, the N content is 0.01% or less.

Ti: 0.05% or less
When the Ti content exceeds 0.05%, coarse TiN of about 1 μm is formed, and bending workability is deteriorated. Moreover, TiC is formed and inhibits the formation of VC necessary for the present invention. Therefore, the Ti content is 0.05% or less, preferably 0.015% or less.

V: 1.2-1.8%
V is an important element that contributes to high strength by forming fine VC without degrading bending workability. When the amount of V is less than 1.2%, a sufficient amount of VC does not precipitate, so that TS of 1470 MPa or more cannot be obtained, and cementite and pearlite are generated and bending workability is deteriorated. On the other hand, if the amount of V exceeds 1.8%, the ductility decreases. Therefore, the V amount is set to 1.2 to 1.8%.

  The balance is Fe and inevitable impurities, but for the following reasons, it is further at least one selected from Cr: 1% or less, B: 0.0030% or less, Mo: 0.5% or less, W: 1% or less It is preferable that an element is contained.

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. In addition, since the formation of VC is suppressed by the formation of Cr-based carbide, the Cr content is 1% or less.

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 0.0030% or less.

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 0.5% or less.

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 1% or less.

2) Microstructure To ensure good bendability, the area ratio of the ferrite phase in the entire matrix should be 90% to avoid the generation of coarse hard phases such as bainite phase, martensite phase, cementite and pearlite as much as possible. % Or more, preferably 95% or more.

  However, a TS of 1470 MPa or more cannot be obtained simply by using a microstructure mainly composed of a ferrite phase. Therefore, in the present invention, high strength is achieved by precipitating fine VC that does not greatly affect the bending workability in the ferrite phase. That is, the ratio of the number of VCs having a size of less than 10 nm out of the total number of VCs is 85% or more, and the amount of solute V in the steel is 0.30% by mass relative to the total steel composition being 100% by mass. Below, a sufficient number of fine VCs are deposited to ensure excellent bending workability and TS of 1470 MPa or more. If the amount of solute V in steel exceeds 0.30% by mass or the ratio of the number of VCs having a size of less than 10 nm is less than 85%, a TS of 1470 MPa or more cannot be obtained. In the present invention, the VC may contain a very small amount of Ti.

Here, the area ratio of the ferrite phase is that the 1/4 thickness portion of the cross section in the rolling direction of the base hot-rolled steel sheet is finished by electrolytic polishing, and the reflected electron image is obtained at an acceleration voltage of 10 kV and 2000 times by a scanning electron microscope (SEM). The area occupied by the observation field is extracted by extracting a region that is clearly different from the matrix observed in the backscattered electron image (the pearlite and hard phases can be determined by surface irregularities), and the remainder is determined as the ferrite phase. It is the value obtained by calculating the rate and arithmetically averaging over 5 fields of view. Further, the size of VC was obtained by the above method. Furthermore, the amount of solute V in the steel was determined according to the method described in Patent Document 3. That is, since the target precipitate is very fine, accuracy is not obtained in the general method for obtaining the solid solution amount by subtracting the extracted precipitate from the total addition amount, so here the sample is placed in a non-aqueous solvent electrolyte. Then, the electrolytic solution was used as an analysis solution, and the concentrations of V and Fe as a comparative element were measured using ICP mass spectrometry. Based on the obtained concentration, the concentration ratio of V to Fe was calculated, and further, the amount of solid solution V (% by mass) was obtained by multiplying by the amount of Fe (% by mass) in the sample. The amount of Fe (% by mass) in the sample can be obtained by subtracting the total of composition values other than Fe from 100% by mass.

  Whether or not the measured precipitate was VC was confirmed by electron diffraction using TEM.

3) Production conditions The high-strength galvannealed hot-rolled steel sheet of the present invention is a steel having the above composition, after hot rolling at a finishing temperature of 900 ° C or higher, and then winding at a winding temperature of 580 to 700 ° C. After pickling, it can be manufactured by a method of heating to 600 to 700 ° C. in a reducing atmosphere, dipping in a galvanizing bath to perform plating, and alloying the plating layer at 460 to 550 ° C.

Hot rolling finishing temperature: 900 ° C. or more When 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 bending workability deteriorates. Therefore, the finishing temperature is 900 ° C. or higher.

  In order to set the finishing temperature to 900 ° C. or higher, it is necessary to heat the steel to the austenite single phase region before hot rolling. At that time, VC in the slab is easily dissolved in the austenite phase. Heating at 1150 ° C. or higher is preferable to ensure dissolution. In the case of TiC and NbC, even if the same carbide is used, it is impossible to obtain a TS of 1470 MPa or more because it is necessary to heat it to 1300 ° C or more or until the steel is melted.

  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.

Winding temperature: 580 ~ 680 ℃
If the coiling temperature is less than 580 ° C, it will be difficult to deposit fine VC with a size of less than 10nm, and pearlite transformation will occur, and if it exceeds 680 ° C, the VC will become coarse, which hinders high strength. Is done. Therefore, the coiling temperature is 580 to 680 ° C. In addition, after winding, it is necessary to remove the scale by pickling for plating treatment.

Heating conditions in a reducing atmosphere before plating treatment: Heating at 600 to 700 ° C. for 10 to 90 seconds Before dipping in a galvanizing bath and performing plating treatment, for example, reducing atmosphere of 20 vol.% Hydrogen + 80 vol.% Nitrogen It is necessary to purify the steel sheet surface by heating to 600 ° C or higher for 10 seconds or more, but in order to prevent VC from becoming coarse and to achieve high strength, the upper limit of heating temperature and time is 700 ° C respectively. 90 seconds.

  Plating treatment: The plating treatment may be performed by immersing in a normal condition, that is, in a zinc plating bath containing Al at around 480 ° C.

  Alloying treatment of plating layer: After the plating treatment, the plating layer may be alloyed under normal conditions, that is, 460 to 550 ° C. The alloying treatment time is usually 60 seconds or less.

  In addition, the coarsening of VC does not occur in the plating process where the processing temperature is 700 ° C. or less or the alloying process of the plating layer.

After melting the steels A to I having the composition shown in Table 1, hot rolled steel sheets having a thickness of 2.5 mm were produced under the hot rolling conditions shown in Table 2. After removing the scale by pickling, this hot-rolled steel sheet was heated at 600 ° C for 60 seconds in a reducing atmosphere of hydrogen 20vol.% + Nitrogen 80vol.%, Then cooled to 500 ° C and maintained at 480 ° C: After immersing in a galvanizing bath containing 0.1% by mass and performing plating treatment, alloying treatment is performed at 550 ° C for 30 seconds, and cooling is performed at an average cooling rate of 10 ° C / second. Samples 1 to 15 were produced. The basis weight of Zn at this time is 40 g / m ² .

Then, the following investigation was performed on each sample.
Microstructure:
a) Area ratio of ferrite phase: From each sample, the area ratio of the ferrite phase was determined by the above method.
b) Ratio of VC number less than 10nm in size to the total number of VC: Samples were taken from the 1 / 4t region by mechanical grinding and wet polishing from the above samples, respectively, and thin film samples for TEM were prepared by electrolytic polishing method. The ratio of the number of VCs having a size of less than 10 nm to the total number of VCs was determined by the method.
c) Solid solution V amount in steel: After the sample was cut to an appropriate size, the hot dip galvanized layers on the front and back surfaces were removed by mechanical grinding. The steel plate from which the plating layer had been removed was subjected to constant current electrolysis at about 0.2 g at a current density of 20 mA / cm ² in a 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol). The amount of solute V in the steel was determined from the 10% AA electrolyte solution after electrolysis by the above method.
Strength and elongation: JIS No. 5 tensile test specimens were collected parallel to the rolling direction, and subjected to a tensile test at a crosshead speed of 10 mm / min in accordance with JIS Z 2241 to obtain TS and elongation El.
Bending workability: Specimens with a width of 50 mm and a length of 100 mm are taken in parallel with the rolling direction, and the ridgeline part is visually observed when bent 90 °, and the bending workability is excellent when the number of cracks is less than 5 ( ○) When the number of cracks was 5 or more, bending workability was judged as poor (x).
Plating property: The presence or absence of unplating was examined visually, and the plating property was evaluated with no plating (◯) and with no plating (×).

  The results are shown in Table 2. In the examples of the present invention, TS of 1480 to 1610 MPa, that is, 1470 MPa or more, El of 10% or more is obtained, and it can be seen that the bending workability and the plating property are also excellent.

Claims (3)

In mass%, C: 0.26-0.35%, Si: 0.4% 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, Ti: 0.05 %, V: 1.2 to 1.8%, the balance is composed of Fe and inevitable impurities, the area ratio of the ferrite phase occupying the entire matrix is 90% or more, V carbide in the ferrite phase (VC) has a microstructure in which the number of VCs with a size of less than 10 nm is 85% or more of the total number of VCs, and the amount of solute V in the steel is 0.30 mass High strength alloyed hot-dip galvanized hot-rolled steel sheet characterized by being less than or equal to%; where VC is the size of a square plate observed from the [001] orientation of the ferrite phase as a matrix by a transmission electron microscope In this VC, the VC size that can be expressed by 2 ^1/2 × L (L: length of one side of the square plate) is obtained for a plurality of VCs, and is an arithmetic average value.   Further, it is characterized by having a composition containing at least one element selected from Cr: 1% or less, B: 0.0030% or less, Mo: 0.5% or less, W: 1% or less in mass%. Item 2. The high-strength galvannealed hot-rolled steel sheet according to Item 1. The steel having the composition according to claim 1 or 2, after hot rolling at a finishing temperature of 900 ° C. or higher, winding at a winding temperature of 580 to 680 ° C., pickling, and 600 to 700 ° C. in a reducing atmosphere. The area ratio of the ferrite phase occupying the entire matrix is 90 , characterized in that it is heated for 10 to 90 seconds, immersed in a zinc plating bath, plated, and alloyed with a plating layer at 460 to 550 ° C. The ferrite phase has a microstructure in which V carbides (VC) are precipitated, and the ratio of the number of VCs having a size of less than 10 nm out of the total number of VCs is 85% or more. A method for producing a high-strength galvannealed hot-rolled steel sheet in which the amount of solute V in steel is 0.30% by mass or less .
Here, the size of VC is 2 ^1/2 × L (L: the length of one side of the square plate) in a square plate-like VC observed from the [001] orientation of the ferrite phase as a matrix by a transmission electron microscope. This is a value obtained by arithmetically averaging the VCs that can be expressed by a plurality of VCs.