JP2018021231A - High strength steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet having tensile strength, yield ratio, TS×EL, LDR, hole-expansion rate, sheet thickness reduction rate of a breaking part during a tensile test and SW+ cross tensile at high level.SOLUTION: The high strength steel sheet containing C:0.15 mass% to 0.35 mass%, total of Si and Al:0.5 mass% to 3.0 mass%, Mn:1.0 mass% to 4.0 mass%, P:0.05 mass% or less, S:0.01 mass% or less and the balance Fe with inevitable impurities, and having a steel structure with ferrite fraction of 5% or less, total fraction of tempered martensite and tempered bainite of 60% or more and retained austenite amount of 10% or more, average size of MA of 1.0 μm or less, average size of retained austenite of 1.0 μm or less, retained austenite with size of 1.5 μm or more of 2% or more of all retained austenite, scattering intensity of 1.0 cmor less at q value in X ray small angle scattering of 1nm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high-strength steel sheet that can be used for various applications including automobile parts.

Steel sheets used for automobile parts and the like are required to achieve both strength improvement and impact resistance improvement in order to achieve both weight reduction and collision safety.
For example, in Patent Document 1, by heating a slab to 1210 ° C. or higher and controlling hot rolling conditions, fine TiN particles having a size of 0.5 μm or less are generated, and a particle size of 1 μm or more serving as a starting point for low-temperature fracture. A high-strength steel sheet that has improved impact resistance properties by suppressing the formation of AlN particles is disclosed.

In Patent Document 2, the C content is more than 0.45%, 0.77% or less, the Mn content is 0.1% or more, 0.5% or less, the Si content is 0.5% or less, and the addition amount of Cr, Al, N, O is specified. A high-strength steel sheet is disclosed in which the impact resistance is improved by forming a network structure in which 50% or more of the ferrite grain size is in contact with the hard phase.
In Patent Document 3, the amount of retained austenite is 10% or more by adding 3.5 to 10% Mn, and the average interval of retained austenite is 1.5 μm or less, thereby improving the impact resistance. A steel sheet is disclosed.

  Patent Document 4 discloses a high-strength steel sheet having a tensile strength of 980 to 1180 MPa and showing good deep drawability.

Japanese Patent No. 5240421 JP-A-2015-105384 JP 2012-251239 A JP 2009-203548 A

In order to realize further weight reduction of steel plates used for automobile parts, it is necessary to have sufficient strength and impact resistance characteristics while being thinner. That is, a steel sheet having higher tensile strength and excellent impact characteristics is required.
Also, in various applications including automotive parts, not only has high tensile strength and impact properties, but also has excellent strength-ductility balance, high yield ratio, excellent deep drawability and excellent hole expansion ratio. It is demanded.
Specifically, for each of the tensile strength, the strength-ductility balance, the yield ratio, the deep drawing characteristics, and the hole expansion ratio, the following are required.

  The tensile strength is required to be 980 MPa or more. In order to increase the stress that can be applied during use, it is necessary to have a high yield strength (YS) in addition to a high tensile strength (TS). In addition, from the viewpoint of ensuring collision safety and the like, it is also necessary to increase the yield strength of the steel sheet, and to suppress fracture at the time of deformation in order to stably develop the strength characteristics when a collision occurs. For this reason, specifically, with a yield ratio (YR = YS / TS) of 0.75 or more, an improvement in the thickness reduction rate of the fractured portion at the time of the tensile test is required as an evaluation index that substitutes the fracture characteristics. Moreover, the joint strength of a spot welded part is also calculated | required as basic performance of the steel plate for motor vehicles. Specifically, the cross tensile strength of the spot weld is required to be 6 kN or more.

  About strength-ductility balance, it is calculated | required that the product (TSxEL) of TS and total elongation (EL) is 20000 MPa% or more. Furthermore, in order to ensure the moldability at the time of component molding, it is also required that the LDR indicating deep drawability is 2.05 or more and the hole expansion ratio λ indicating hole expandability is 20% or more. .

  However, the high-strength steel plates disclosed in Patent Documents 1 to 4 are difficult to satisfy all of these requirements, and a high-strength steel plate that can satisfy all of these requirements has been demanded.

  The present invention has been made in order to meet such demands, and includes tensile strength (TS), yield ratio (YR), product of (TS) and total elongation (EL) (TS × EL), LDR, Provide high strength steel plate and method for manufacturing the same, all of which have high hole expansion rate (λ), plate thickness reduction rate (RA) of fractured portion during tensile test, and cross tensile strength (SW cross tension) of spot welds The purpose is to do.

Aspect 1 of the present invention
C: 0.15% by mass to 0.35% by mass,
Total of Si and Al: 0.5% by mass to 3.0% by mass,
Mn: 1.0% by mass to 4.0% by mass,
P: 0.05 mass% or less,
S: 0.01% by mass or less,
And the balance consists of Fe and inevitable impurities,
Steel structure
The ferrite fraction is 5% or less,
The total fraction of tempered martensite and tempered bainite is 60% or more,
The amount of retained austenite is 10% or more,
The average size of MA is 1.0 μm or less,
The average size of retained austenite is 1.0 μm or less,
Residual austenite having a size of 1.5 μm or more is 2% or more of the total retained austenite amount,
It is a high-strength steel sheet having a scattering intensity of 1.0 cm ⁻¹ or less when the q value in X-ray small angle scattering is 1 nm ⁻¹ .

  Aspect 2 of the present invention is the high-strength steel sheet according to Aspect 1, wherein the C content is 0.30% by mass or less.

  Aspect 3 of the present invention is the high-strength steel sheet according to aspect 1 or 2, wherein the Al amount is less than 0.10% by mass.

In aspect 4 of the present invention, C: 0.15% by mass to 0.35% by mass, Si and Al total: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% Including a mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, with the balance being made of Fe and inevitable impurities,
Heating the rolled material to a temperature of Ac ₃ point or higher to austenite;
After the austenitization, cooling is performed between 650 ° C. and 500 ° C. at an average cooling rate of 15 ° C./second or more and less than 200 ° C./second, and at a cooling rate of 10 ° C./second or less within a range of 300 to 500 ° C. for 10 seconds. As mentioned above, making it hold for less than 300 seconds,
After the residence, cooling at an average cooling rate of 10 ° C / second or more from a temperature of 300 ° C or more to a cooling stop temperature between 100 ° C and 300 ° C;
Heating from the cooling stop temperature to a reheating temperature in the range of 300 to 500 ° C. at an average heating rate of 30 ° C./second or more;
Holding the tempering parameter P defined by the formula (1) at 10,000 to 14500 and holding time 1 to 300 seconds at the reheating temperature;
After the holding, cooling from the reheating temperature to 200 ° C. at an average cooling rate of 10 ° C./second or more,
Is a method for producing a high-strength steel sheet.
P = T × (20 + log (t / 3600)) (1)
Here, T: temperature (K), t: time (second).

  A fifth aspect of the present invention is the manufacturing method according to the fourth aspect, wherein the retention includes holding at a constant temperature within a range of 300 to 500 ° C.

  Aspect 6 of the present invention is the manufacturing method according to aspect 4 or 5, wherein the tempering parameter is 11000 to 14000 and the holding time is 1 to 150 seconds.

  According to the present invention, tensile strength (TS), yield ratio (YR), product of (TS) and total elongation (EL) (TS × EL), LDR, hole expansion ratio (λ), fracture during tensile test It is possible to provide a high-strength steel sheet and a method for manufacturing the same, in which both the thickness reduction ratio (RA) (impact resistance) of the part and the cross tensile strength (SW cross tension) of the spot weld are high.

FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to the present invention, particularly heat treatment.

As a result of intensive studies, the present inventors have found that in steel having a predetermined component, the steel structure (metal structure) has a ferrite fraction of 5% or less, a total fraction of tempered martensite and tempered bainite: 60% or more, and remains. γ amount: 10% or more, average size of MA: 1.0 μm or less, average size of retained austenite: 1.0 μm or less, and residual austenite of size 1.5 μm or more: 2% or more of the total retained austenite amount, X-ray small angle Scattering intensity when the scattering q value is 1 nm ⁻¹ : 1.0 cm ⁻¹ or less, so that the product of tensile strength (TS), yield ratio (YR), (TS) and total elongation (EL) ( TS × EL), LDR, hole expansion rate (λ), plate thickness reduction rate (RA) (impact resistance) at the time of tensile test, and cross tensile strength (SW cross tension) of spot welds are all high High strength at level It was found that it is possible to obtain a steel plate.

1. Steel structure Details of the steel structure of the high-strength steel sheet of the present invention will be described below.
In the following description of the steel structure, a mechanism that can improve various properties by having such a structure may be described. It should be noted that these are the mechanisms considered by the present inventors based on the knowledge obtained at the present time, but do not limit the technical scope of the present invention.

(1) Ferrite fraction: 5% or less Although ferrite is generally excellent in workability, it has a problem of low strength. As a result, the yield ratio decreases when the amount of ferrite is large. Therefore, the ferrite fraction is set to 5% or less (5% by volume or less).
The ferrite fraction is preferably 3% or less, more preferably 1% or less.
The ferrite fraction can be obtained by observing with a light microscope and measuring a white region by a point calculation method. That is, the ferrite fraction can be obtained by an area ratio (area%) by such a method. And the value calculated | required by area ratio may be used as a value of volume ratio (volume%) as it is.

(2) Total fraction of tempered martensite and tempered bainite: 60% or more Both high strength and high hole expansibility are achieved by setting the total fraction of tempered martensite and tempered bainite to 60% or more (60% by volume or more). it can. The total fraction of tempered martensite and tempered bainite is preferably 70% or more.
The amount of tempered martensite and tempered bainite (total fraction) is measured by SEM observation of the cross-section subjected to nital corrosion, and the fraction of MA (that is, the sum of residual austenite and as-quenched martensite) is measured. It can be obtained by subtracting the above-mentioned ferrite fraction and MA fraction from the entire structure.

(3) Residual austenite amount: 10% or more Residual austenite causes a TRIP phenomenon that transforms into martensite by processing-induced transformation during processing such as press processing, and can obtain a large elongation. Further, the formed martensite has a high hardness. For this reason, the outstanding intensity-ductility balance can be obtained. By setting the amount of retained austenite to 10% or more (10% by volume or more), it is possible to realize an excellent strength-ductility balance with TS × EL of 20000 MPa or more.
The amount of retained austenite is preferably 15% or more.

In the high-strength steel sheet of the present invention, most of retained austenite exists in the form of MA. MA is an abbreviation for martensite-austenite constituent and is a composite (composite structure) of martensite and austenite.
The amount of retained austenite can be obtained by calculating the diffraction intensity ratio of ferrite (including tempered martensite and untempered martensite in X-ray diffraction) and austenite by X-ray diffraction. Co-Kα rays can be used as the X-ray source.

(4) Average size of MA: 1.0 μm or less MA is a hard phase, and the vicinity of the interface between the mother phase and the hard phase acts as a void formation site during deformation. The coarser the MA size, the more concentrated the strain on the matrix / hard phase interface, and the more likely the fracture starts from voids formed in the vicinity of the matrix / hard phase interface.
For this reason, the hole expansion ratio λ can be improved by making the MA size, particularly the MA average size as fine as 1.0 μm or less, and suppressing breakage.
The average size of MA is preferably 0.8 μm or less.

  The average size of MA is observed by observing three or more fields of view at 3000 times or more by SEM with a SEM, drawing a straight line of 200 μm or more at an arbitrary position in the photograph, and measuring a section length where the straight line and the MA intersect, It can be obtained by calculating an average value of the intercept lengths.

(5) Average size of retained austenite: 1.0 μm or less, and retained austenite having a size of 1.5 μm or more: 2% or more of the total amount of retained austenite The average size of retained austenite is 1.0 μm, and the size is 1.5 μm or more. It has been found that excellent deep drawability can be obtained when the ratio (volume ratio) of the retained austenite to the total retained austenite is 2% or more.

If the inflow stress of the flange portion is smaller than the tensile stress of the vertical wall portion formed at the time of deep drawing, drawing forming will easily proceed and good deep drawing properties will be obtained. As the deformation behavior of the flange portion, compressive stress is strongly applied from the board surface direction and the circumference, and therefore, the flange portion is deformed in a state where isotropic compressive stress is applied. On the other hand, since martensitic transformation is accompanied by volume expansion, martensitic transformation is less likely to occur under isotropic compressive stress. Therefore, the work-induced martensitic transformation of the retained austenite at the flange portion is suppressed and work hardening is reduced.
As a result, the deep drawability is improved. The larger the size of retained austenite, the greater the effect of suppressing martensitic transformation.

  In order to increase the tensile stress of the vertical wall formed by deep drawing, it is necessary to maintain a high work hardening rate during deformation. Unstable residual austenite that easily undergoes processing-induced transformation under relatively low stress and stable residual austenite that does not cause processing-induced transformation unless under high stress cause processing-induced transformation over a wide stress range. By doing so, a high work hardening rate can be maintained during deformation. For this purpose, we studied to obtain a steel structure containing a predetermined amount of coarse and unstable retained austenite and fine and stable retained austenite. Then, the inventors set the average size of retained austenite to 1.0 μm, and the ratio of the amount of retained austenite having a size of 1.5 μm or more to the total amount of retained austenite (volume ratio) is 2% or more. It has been found that a high work hardening rate can be maintained during deformation and an excellent deep drawability (LDR) can be obtained.

  Further, as described above, when the retained austenite undergoes processing-induced transformation, a TRIP phenomenon occurs and a large elongation can be obtained. On the other hand, the martensite structure formed by the process-induced transformation is hard and acts as a starting point for fracture. Larger martensite structures are more likely to be the origin of destruction. By setting the average size of retained austenite to 1.0 μm or less and reducing the size of martensite formed by processing-induced transformation, an effect of suppressing fracture can be obtained.

  The average size of retained austenite and the ratio of the amount of retained austenite with a size of 1.5 μm or more to the total austenite amount are prepared by using the EBSD (Electron Back Scatter Diffraction Patterns) method, which is a crystal analysis technique using SEM. Can be obtained. From the obtained Phase map, the area of each austenite phase (residual austenite) is determined, the circle equivalent diameter (diameter) of each austenite phase is determined from the area, and the average value of the determined diameters is the average size of the retained austenite. To do. Further, by integrating the area of the austenite phase having an equivalent circle diameter of 1.5 μm or more and determining the ratio to the total area of the austenite phase, the ratio of the retained austenite of size 1.5 μm or more to the total austenite can be obtained. . The ratio of the retained austenite having a size of 1.5 μm or more to the total austenite thus obtained is an area ratio, but is equivalent to a volume ratio.

(6) Scattering intensity when the q-value of X-ray small angle scattering is 1 nm ⁻¹ is 1.0 cm ⁻¹ or less X-ray small angle scattering is a method of irradiating a steel plate with X-rays and scattering X-rays transmitted through the steel plate. By measuring, the size distribution of fine particles (for example, cementite particles dispersed in the steel plate) contained in the steel plate can be obtained. In the steel sheet of the present invention, the size distribution of cementite particles, which are fine particles dispersed in tempered martensite, can be determined by X-ray small angle scattering. Specifically, in X-ray small angle scattering, the size and fraction of cementite particles can be analyzed using the q value and the scattering intensity.
The q value is an index of the size of particles (for example, cementite particles) in the steel sheet. “The q value is 1 nm ⁻¹ ” corresponds to a cementite particle having a particle diameter of about 1 nm. Scattering intensity is an index of the volume fraction of particles (for example, cementite particles) in a steel plate. The stronger the scattering intensity, the greater the volume fraction of cementite.

The scattering intensity at a certain q value semi-quantitatively indicates the volume fraction of cementite particles having a size corresponding to the q value. For example, the scattering intensity at a q value of 1 nm ⁻¹ is semiquantitatively shown as a volume fraction of fine cementite particles of about 1 nm.
That is, a large scattering intensity at a q value of 1 nm ⁻¹ indicates that the volume fraction of fine cementite particles of about 1 nm is large. In a steel sheet having a “q value of 1 nm ⁻¹ and a scattering intensity of 1.0 cm ⁻¹ or less”, the volume fraction of fine cementite particles of about 1 nm existing in the steel sheet is a predetermined value (scattering intensity of 1.0 cm). ^-1 ) or less). As will be described below, a steel sheet having a “q value of 1 nm ⁻¹ or less and a scattering intensity of 1.0 cm ⁻¹ or less” has a low volume fraction of about 1 nm of cementite, and thus has excellent collision resistance. It is thought that.

In high ductility steel containing residual γ, it is ideal that no cementite exists ideally in a state where carbon is gathered in residual austenite. Fine cementite having a particle diameter of about 1 nm dispersed in the steel material can hinder the movement of dislocations and reduce the deformability of the steel material. Therefore, in a steel material having a particle size of about 1 nm and a large volume fraction of cementite, the fracture at the time of deformation is promoted, and the collision resistance can be lowered.
In the steel sheet of the present invention, the volume fraction of fine cementite is kept low, more specifically, the scattering intensity at a q value of 1 nm ⁻¹ is 1 cm ⁻¹ or less, so that the tempered martensite lath is within the lath. The fine carbides formed are reduced to improve the deformability in martensite. Thereby, it is suppressed that a steel plate destroys at the time of a collision, and the impact resistance characteristic of a steel plate is improved.

X-ray small angle scattering was measured using a Nano-viewer and Mo tube manufactured by RIGAKU. As a sample, a disk-like sample having a diameter of 3 mm was cut out from a steel plate, and a sample having a thickness of 20 μm was cut out from around ¼ of the thickness. Data with a q value of 0.1 to 10 nm ⁻¹ were collected. Among them, the absolute intensity was obtained for a q value of 1 nm ⁻¹ .

(7) Other steel structures:
In the present specification, steel structures other than the above-described ferrite, tempered martensite, tempered bainite retained austenite, and cementite are not particularly defined. However, pearlite, tempered bainite, untempered martensite, and the like may exist in addition to the steel structure such as ferrite. If the steel structure such as ferrite satisfies the above-described structure condition, the effect of the present invention is exhibited even if pearlite or the like is present in the steel.

2. Composition The composition of the high-strength steel sheet according to the present invention will be described below. First, basic elements C, Si, Al, Mn, P and S will be described, and further elements that may be selectively added will be described.
In addition, unit% display of a component composition means the mass% altogether.

(1) C: 0.15 to 0.35%
C is an essential element to ensure high strength-ductility balance (TS x EL balance), etc. by increasing the amount of desired structure, especially residual γ. Therefore, it is necessary to add 0.15% or more. However, more than 0.35% is not suitable for welding. Preferably it is 0.18% or more, More preferably, it is 0.20% or more. Further, it is preferably 0.30% or less. When the C content is 0.25% or less, welding can be performed more easily.

(2) Total of Si and Al: 0.5 to 3.0%
Si and Al have a function of suppressing precipitation of cementite and leaving residual austenite, respectively. In order to exhibit such an action effectively, it is necessary to add Si and Al in total of 0.5% or more. However, if the total of Si and Al exceeds 3.0%, the deformability of the steel decreases, and TS × EL decreases. Preferably it is 0.7% or more, More preferably, it is 1.0% or more. Moreover, it is preferably 2.5% or less.
Al may be added in an amount that functions as a deoxidizing element, that is, less than 0.10% by mass, and is 0 for the purpose of, for example, suppressing the formation of cementite and increasing the amount of retained austenite. A larger amount such as 7% by mass or more may be added.

(3) Mn: 1.0 to 4.0%
Manganese suppresses the formation of ferrite. In order to exhibit such an action effectively, it is necessary to add 1.0% or more. However, if it exceeds 4.0%, the MA becomes coarse and the hole expansibility deteriorates. Preferably it is 1.5% or more, More preferably, it is 2.0% or more. Moreover, preferably 3.5. % Or less.

(4) P: 0.05% or less P is unavoidably present as an impurity element. If P exceeds 0.05%, EL and λ deteriorate. Therefore, the P content is 0.05% or less (including 0%). Preferably, it is 0.03% (including 0%) or less.

(5) S: 0.01% or less S is unavoidably present as an impurity element. If S exceeding 0.01% is present, sulfide inclusions such as MnS are formed, which becomes a starting point of cracking and lowers λ. Therefore, the S content is 0.01% or less (including 0%). Preferably, it is 0.005% (including 0%) or less.

(6) Balance In one preferred embodiment, the balance is iron and inevitable impurities. As inevitable impurities, mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. In addition, for example, like P and S, it is usually preferable that the content is small. Therefore, although it is an unavoidable impurity, there is an element that separately defines the composition range as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition ranges are separately defined.

  However, it is not limited to this embodiment. Any other element may be further included as long as the characteristics of the high-strength steel sheet of the present invention can be maintained. Other elements that can be selectively contained as described above are exemplified below.

3. Characteristics As described above, the high-strength steel sheet of the present invention has high levels of TS, YR, TS × EL, LDR, λ, impact resistance characteristics, and SW cross tension. These characteristics of the high-strength steel sheet of the present invention will be described in detail below.

(1) Tensile strength (TS)
It has a TS of 980 MPa or more. Preferably, TS is 1180 MPa or more. If TS is less than 980 MPa, excellent fracture characteristics can be obtained more reliably, but this is not preferable because the load resistance at the time of collision becomes low.

(2) Yield ratio (YR)
It has a yield ratio of 0.75 or more. Thereby, combined with the above-described high tensile strength, high yield strength can be realized, and the final product obtained by processing such as deep drawing can be used under high stress. Preferably, it has a yield ratio of 0.80 or more.

(3) Product of TS and total elongation (EL) (TS x EL)
TS × EL is 20000 MPa or more. By having TS × EL of 20000 MPa or more, a high level of strength-ductility balance having both high strength and high ductility can be obtained. Preferably, TS × EL is 23000 MPa or more.

(4) Deep drawability (LDR)
LDR is an index used for evaluation of deep drawability. In cylindrical drawing, the diameter of the obtained cylinder is d, and the maximum diameter of a disk-shaped steel plate (blank) that can be obtained without breaking by one deep drawing process is D, and d / D is It is called LDR (Limiting Drawing Ratio). More specifically, a cylindrical sample having a plate thickness of 1.4 mm and various diameters is subjected to cylindrical deep drawing with a die having a punch diameter of 50 mm, a punch angle radius of 6 mm, a die diameter of 55.2 mm, and a die angle radius of 8 mm. The LDR can be obtained by obtaining the sample diameter (maximum diameter D) that has been drawn without breaking.

  The high-strength steel sheet of the present invention has an LDR of 2.05 or more, preferably 2.10 or more, and has excellent deep drawability.

(5) Hole expansion rate (λ)
The hole expansion ratio λ is obtained in accordance with Japan Iron and Steel Federation standard JFS T1001. A punched hole having a diameter d ₀ (d ₀ = 10 mm) is formed in the test piece, a punch having a tip angle of 60 ° is pushed into the punched hole, and the diameter of the punched hole when the generated crack penetrates the plate thickness of the test piece. d is measured and obtained from the following equation.
λ (%) = {(d−d ₀ ) / d ₀ } × 100

  The high-strength steel sheet of the present invention has a hole expansion ratio λ of 20% or more, preferably 30% or more. Thereby, excellent workability such as press formability can be obtained.

(6) Plate thickness reduction rate in the tensile test (R5 tensile plate thickness reduction rate)
Using a test piece in which an arc-shaped notch with a radius of 5 mm was provided on a No. 5 test piece, the test was performed at a deformation rate of 10 mm / min in the tensile test, and the sample was broken. Thereafter, the fracture surface was observed, and a value (t ₁ / t ₀ ) obtained by dividing the thickness t ₁ of the fracture surface in the thickness direction by the original thickness t ₀ was defined as the thickness reduction rate.
The plate thickness reduction rate in this test is 50% or more, preferably 52% or more, more preferably 55% or more. Thereby, even if it deform | transforms greatly at the time of a collision, since it becomes difficult to fracture | rupture, the steel plate which has the outstanding impact resistance characteristic can be obtained.

(7) Cross tensile strength of spot welding The cross tensile strength of spot welding was evaluated according to JIS Z 3137. The spot welding was performed by stacking two 1.4 mm steel plates. Spot welding was performed with a dome radius type electrode with a pressurizing force of 4 kN and a current increased by 0.5 kA in the range from 6 kA to 12 kA, and the current value (minimum current value) at which dust was generated during welding was examined. A cross joint spot-welded at a current 0.5 kA lower than the minimum current value was used as a sample for measuring the cross tensile strength. A cross tensile strength of 6 kN or more was defined as “good”. The cross tensile strength is preferably 8 kN or more, more preferably 10 kN or more.
When the cross tensile strength is 6 kN or more, when automobile parts and the like are manufactured from a steel plate, a part having high joint strength during welding can be obtained.

4). Manufacturing Method Next, a manufacturing method of the high strength steel plate according to the present invention will be described.
The inventors of the present invention have the above-mentioned desired steel structure by performing heat treatment (multi-step austemper treatment), which will be described in detail later, on the rolled material having a predetermined composition, and as a result, the above-mentioned desired characteristics. It was found that a high-strength steel sheet having
Details will be described below.

FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to the present invention, particularly heat treatment.
The rolled material to be heat-treated is usually produced by hot rolling followed by cold rolling. However, the present invention is not limited to this, and either one of hot rolling and cold rolling may be performed. The conditions for hot rolling and cold rolling are not particularly limited.

(1) Austenitizing treatment As shown in [1] and [2] in FIG. 1, the rolled material is austenitized by heating to a temperature of Ac ₃ point or higher and heating for a predetermined heating time. The heating time at this heating temperature is, for example, 1 to 1800 seconds. The upper limit of the heating temperature is preferably Ac ₃ points or more and Ac ₃ points + 100 ° C. or less. This is because coarsening of crystal grains can be suppressed by setting the temperature to Ac ₃ points + 100 ° C. or lower. The heating temperature is more preferably Ac ₃ points + 10 ° C. or higher, Ac ₃ points + 90 ° C. or lower, and further preferably Ac ₃ points + 20 ° C. or higher, Ac ₃ points + 80 ° C. or lower. This is because the austenite can be more completely suppressed and the formation of ferrite can be suppressed, and the coarsening of crystal grains can be more reliably suppressed.
Although heating at the time of austenitization shown by [1] in FIG. 1 may be performed at an arbitrary heating rate, a preferable average heating rate is 1 ° C./second or more, more preferably 20 ° C./second.
(2) Cooling and residence in a temperature range of 300 ° C. to 500 ° C. After the above austenite formation, cooling is performed, as shown in [5] in FIG. Within a temperature range of 300 to 500 ° C., a cooling rate of 10 ° C./second or less is used for 10 seconds or more and less than 300 seconds.
Cooling is performed at an average cooling rate of 15 ° C./second or more and less than 200 ° C./second at least between 650 ° C. and 500 ° C. This is because the formation of ferrite during cooling is suppressed by setting the average cooling rate to 15 ° C./second or more. Moreover, generation | occurrence | production of the excessive thermal distortion by rapid cooling can be prevented by making a cooling rate into less than 200 degrees C / sec. As a preferable example of such cooling, as shown in [3] in FIG. 1, a relatively low average cooling of 0.1 ° C./second or more and 10 ° C./second or less is performed up to a rapid cooling start temperature of 650 ° C. or more. Cooling at a rate, and as shown in [4] of FIG. 1, cooling is performed at an average cooling rate of 20 ° C./second or more and less than 200 ° C./second from a rapid cooling start temperature to a residence start temperature of 500 ° C. or less. be able to.

It is allowed to stay for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 to 500 ° C. That is, in a temperature range of 300 to 500 ° C., the cooling rate is set to 10 ° C./second or less for 10 seconds or more. The state where the cooling rate is 10 ° C./second or less includes the case where the cooling rate is maintained at a substantially constant temperature (that is, the cooling rate is 0 ° C./second) as shown in [5] in FIG.
Due to this residence, bainite is partially formed. And since bainite has a lower carbon solid solubility limit than austenite, it expels carbon beyond the solid solubility limit. As a result, an austenite region enriched with carbon is formed around bainite.
This region becomes slightly coarse retained austenite after cooling and reheating described later. By forming this slightly coarse retained austenite, the deep drawability can be enhanced as described above.

If the retention temperature is higher than 500 ° C., the carbon concentration region becomes too large, and not only retained austenite but also MA becomes coarse, so that the hole expansion rate decreases. On the other hand, if the retention temperature is lower than 300 ° C., the carbon concentration region is small, the amount of coarse retained austenite is insufficient, and the deep drawability deteriorates.
On the other hand, if the residence time is shorter than 10 seconds, the area of the carbon-enriched region is reduced, the amount of coarse retained austenite is insufficient, and the deep drawability is deteriorated. On the other hand, when the residence time is 300 seconds or more, the carbon enrichment region becomes too large, and not only retained austenite but also MA becomes coarse, so the hole expansion rate decreases.
Further, if the cooling rate during the residence is higher than 10 ° C./second, sufficient bainite transformation does not occur, and therefore a sufficient carbon enriched region is not formed, and the amount of coarse retained austenite is insufficient.

Therefore, it is retained for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 to 500 ° C. It is preferable to retain for 10 seconds or more at a cooling rate of 8 ° C./second or less in a temperature range of 320 to 480 ° C., and hold at a constant temperature for 3 to 80 seconds.
More preferably, it is retained for 10 seconds or more at a cooling rate of 3 ° C./second or less within a temperature range of 340 to 460 ° C., and during that time, it is held at a constant temperature for 5 to 60 seconds.

(3) Cooling to a cooling stop temperature between 100 ° C. and less than 300 ° C. After the above-mentioned residence, as shown in [6] of FIG. Cooling is performed at an average cooling rate of 10 ° C./second or more until a cooling stop temperature of 10 ° C. In one of the preferred embodiments, as shown in [6] of FIG. 1, the above-mentioned end temperature of residence (for example, the holding temperature shown in [5] of FIG. 1) is set as the second cooling start temperature.
By this cooling, the martensitic transformation is caused while leaving the above-described carbon-enriched region as austenite. By controlling the cooling stop temperature within a temperature range of 100 ° C. or more and less than 300 ° C., the amount of austenite remaining without transformation to martensite is adjusted, and the final amount of retained austenite is controlled.

When the cooling rate is slower than 10 ° C./second, the carbon concentration region spreads more than necessary during cooling, and the MA becomes coarse, so the hole expansion rate decreases. When the cooling stop temperature is lower than 100 ° C., the amount of retained austenite is insufficient. As a result, although TS increases, EL decreases and TS × EL balance is insufficient.
When the cooling stop temperature is 300 ° C. or higher, coarse untransformed austenite increases and remains even after the subsequent cooling, resulting in coarse MA size and a decrease in the hole expansion ratio λ.
In addition, a preferable cooling rate is 15 degreeC / degrees C or more, and a preferable cooling stop temperature is 120 degreeC or more and 280 degrees C or less. A more preferable cooling rate is 20 ° C./s or more, and a more preferable cooling stop temperature is 140 ° C. or less and 260 ° C. or less.

  As shown in [7] in FIG. 1, the cooling stop temperature may be maintained. A preferable holding time in the case of holding can be 1 to 600 seconds. Even if the holding time is increased, there is almost no influence on the characteristics, but if the holding time exceeds 600 seconds, the productivity is lowered.

(4) Reheating to a temperature range of 300 to 500 ° C. As shown in FIG. 1 [8], reheating at 30 ° C./second or more from the above-described cooling stop temperature to a reheating temperature in the range of 300 to 500 ° C. Heat at speed. By rapidly heating, the residence time in the temperature range where precipitation and growth of carbides are promoted can be shortened, and formation of fine carbides can be suppressed. A preferable reheating rate is 60 ° C./s or more, more preferably 70 ° C./s.
Such rapid heating can be achieved by a method such as high-frequency heating or electric heating.

After reaching the reheating temperature, the temperature is maintained as shown in [9] of FIG. At that time, it is preferable that the tempering parameter P represented by the following formula (1) is 10000 or more and 14500 or less and the holding time is 1 to 150 seconds. The tempering parameter P of the steel plate of this embodiment is represented by the following formula (1).
P = T (K) × (20 + log (t / 3600) (1)
Here, T is a tempering temperature (K), and t is a holding time (seconds).

  At the time of reheating, redistribution of carbon that is supersaturated in martensite occurs. Specifically, two phenomena occur: carbon diffusion from martensite to austenite and precipitation of carbide (cementite) in the martensite lath. Of these two phenomena, carbide precipitation is likely to occur when holding at low temperature for a long time. Moreover, even if it is a case where it hold | maintains at high temperature, when a heating rate is slow or holding time is too long, a carbide | carbonized_material will precipitate. On the other hand, carbon diffusion from martensite to austenite strongly depends on the diffusion rate, and can be sufficiently performed at a high temperature in a short time.

  The cementite particles present in the martensite are likely to become the starting point of collision fracture, which causes a decrease in collision resistance. Therefore, at the time of reheating, it is desirable to perform a reheating treatment that promotes carbon diffusion from martensite to austenite while suppressing the precipitation of carbide (cementite) in the martensite lath. Therefore, it is effective to perform rapid heating and heat treatment at a high temperature in a short time.

However, in order to obtain sufficient tensile strength by causing sufficient carbon diffusion, it is necessary to control the tempering parameter P as a factor of the combination of temperature and time within a certain range.
If the tempering parameter P is less than 10,000, carbon diffusion from martensite to austenite does not occur sufficiently, the austenite becomes unstable, and the amount of retained austenite cannot be ensured, resulting in insufficient TS × EL balance. On the other hand, if the tempering parameter P is larger than 14500, the formation of carbides cannot be prevented even in a short time treatment, the amount of retained austenite cannot be secured, and the TS × EL balance deteriorates. Even if the tempering parameters are appropriate, if the heating rate is too low and the time is too long, carbides are formed in the martensite lath, and crack propagation is likely to occur at the time of impact deformation, and the impact resistance characteristics deteriorate. The amount of carbide in the martensite lath can be determined from the scattering intensity of X-ray small angle scattering.

If the reheating temperature is lower than 300 ° C., the diffusion of carbon is insufficient and a sufficient amount of retained austenite cannot be obtained, resulting in a decrease in TS × EL. When the reheating temperature is higher than 500 ° C., the retained austenite is decomposed into cementite and ferrite and the retained austenite is insufficient, and the characteristics cannot be ensured.
If the holding is not performed or the holding time is shorter than 1 second, there is a possibility that the carbon diffusion is insufficient. For this reason, it is preferable to hold for 1 second or more at the reheating temperature. If the holding time is longer than 150 seconds, similarly, carbon may be precipitated as cementite. For this reason, the holding time is preferably 150 seconds or less.
A preferable reheating temperature is 320 to 480 ° C, and a more preferable reheating temperature is 340 to 460 ° C.
Preferably, the tempering parameter P is 10500 to 14500, and a preferable holding time at this time is 1 to 150 seconds. A more preferable tempering parameter P is 11000 to 14000, and a preferable holding time at this time is 1 to 100 seconds, and more preferably 1 to 60 seconds.

After the reheating, as shown in [10] in FIG. 1, it may be cooled to a temperature of 200 ° C. or lower such as room temperature. A preferable average cooling rate up to 200 ° C. or less can be 10 ° C./second.
The high-strength steel sheet of the present invention can be obtained by the above heat treatment.

  If it is those skilled in the art who contacted the manufacturing method of the high-strength steel plate which concerns on embodiment of this invention demonstrated above, the high-strength steel plate which concerns on this invention by the manufacturing method different from the manufacturing method mentioned above by trial and error can be obtained. There is a possibility.

1. Sample Production After a cast material having the chemical composition shown in Table 1 was manufactured by vacuum melting, this cast material was hot forged into a steel plate having a thickness of 30 mm, and then subjected to hot rolling. Table 1 also shows the Ac ₃ points calculated from the composition. The hot rolling conditions do not substantially affect the final structure and properties of this patent, but after heating to 1200 ° C., the plate is subjected to multi-stage rolling. The thickness was 2.5 mm. At this time, the end temperature of the hot rolling was 880 ° C. Then, it cooled to 600 degreeC at 30 degree-C / sec, stopped cooling, and after inserting into the furnace heated at 600 degreeC, it hold | maintained for 30 minutes, and then cooled in the furnace, and was set as the hot-rolled steel plate.
The hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled to 1.4 mm. The cold-rolled plate was heat-treated to obtain a sample. The heat treatment conditions are shown in Table 2. In Table 2, for example, a number indicated in [] as in [2] corresponds to a process having the same number indicated in [] in FIG. In Table 2, sample no. 1, 4, 7, and 26 are samples that were not retained for 10 seconds or more at a cooling rate of 10 ° C./second or less within a temperature range of 300 to 500 ° C. in the process corresponding to [5] in FIG. In particular, sample no. Reference numerals 1 and 26 are samples that are rapidly cooled to 200 ° C. after starting rapid cooling at 700 ° C. (samples in which steps corresponding to [5] and [6] in FIG. 1 are skipped). Sample No. 9 is a sample (steps corresponding to [6] to [8] in FIG. 1) that is held at that temperature after cooling to the reheating temperature instead of cooling to a cooling stop temperature between 100 ° C. and less than 300 ° C. Sample skipped).
The reheating corresponding to [8] was performed by an electric heating method.
In Tables 1 to 4, numerical values marked with an asterisk (*) indicate that they are out of the scope of the present invention.

2. Steel structure By the method described above for each sample, the ferrite fraction, the total fraction of tempered martensite and tempered bainite (described as “tempered M / B” in Table 3), the amount of retained austenite (residual γ amount), MA Average size, average size of residual austenite (residual γ average size), ratio of residual austenite of size 1.5 μm or more to total austenite (in Table 3, described as “residual γ ratio of 1.5 μm or more”), The scattering intensity when the q-value of X-ray small angle scattering was 1 nm ⁻¹ was determined. For the measurement of the amount of retained austenite, a two-dimensional micro part X-ray diffractometer (RINT-RAPIDII) manufactured by Rigaku Corporation was used. The obtained results are shown in Table 3.
In this example, the steel structures (remaining structures) other than the steel structures described in Table 3 are sample Nos. Samples other than 9 are martensite that has not been tempered. 9 is a bainite that has not been tempered.

3. Mechanical properties About the obtained sample, YS, TS, and EL were measured using the tensile tester, and YR and TSxEL were computed. Further, the hole expansion ratio λ, LDR, the cross tensile strength (SW cross tension) of the spot welded portion, and the R5 tensile plate thickness reduction rate were determined by the above-described methods. Table 4 shows the obtained results.

  Consider the results in Table 4. Sample No. 13, 15, 18, 21 and 28 to 36 are examples which satisfy all the requirements (composition, production conditions and steel structure) defined in the present invention. All of these samples have a tensile strength (TS) of 980 MPa or more, a yield ratio (YR) of 0.75 or more, TS × EL of 20000 MPa or more, an LDR of 2.05 or more, a hole expansion ratio of 20% or more (λ ), SW cross tension of 6 kN or more and R5 tensile sheet thickness reduction rate (RA) of 50% or more.

  In contrast, sample no. No. 1 was not retained in the temperature range of 300 to 500 ° C. after austenitization, so the amount of retained austenite having a size of 1.5 μm or more was not sufficient, and as a result, sufficient deep drawability was not obtained. . [7] Since the holding time was as long as 300 seconds, carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.

  Sample No. In [2], since the [5] holding temperature was as long as 300 seconds, the MA average size was excessive, and as a result, a sufficient hole expansion rate was not obtained.

  Sample No. No. 3 [6] Since the cooling rate was as low as 1 ° C./second, the average MA size was excessive, and as a result, a sufficient hole expansion rate was not obtained. [7] Since the holding time was as long as 300 seconds, carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.

  Sample No. No. 4 [5] Since the holding time was as short as 3 seconds, the amount of retained austenite having a size of 1.5 μm or more was not sufficient, and sufficient deep drawability could not be obtained.

  Sample No. No. 5 had a high [5] holding temperature of 550 ° C., and therefore the average MA size was excessive. As a result, a sufficient hole expansion ratio and a sufficient deep drawability could not be obtained.

  Sample No. No. 6 [5] Since the holding temperature was as low as 250 ° C., the amount of retained austenite having a size of 1.5 μm or more was not sufficient.

  Sample No. No. 7 [6] Since the cooling stop temperature was as high as 350 ° C., the total amount of tempered martensite and tempered bainite was insufficient, the MA average size was excessive, and the average size of retained austenite was excessive. As a result, a sufficient hole expansion rate and deep drawability could not be obtained.

  Sample No. No. 8 [1] Since the heating temperature was as low as 780 ° C., the amount of ferrite was excessive, and the total amount of tempered martensite and tempered bainite was insufficient. As a result, sufficient tensile strength and yield ratio were not obtained. .

  Sample No. No. 9 [6] Since the cooling stop temperature was as high as 400 ° C., martensite and bainite were not formed, the average size of MA was excessive, and the average size of retained austenite was also excessive. As a result, sufficient tensile strength and yield ratio were not obtained. Furthermore, since it is held at that temperature for 300 seconds ([9] holding time), the formation of carbides is small. As a result, λ decreased.

  Sample No. No. 10 [5] Since the cooling stop temperature was as low as 20 ° C., the amount of residual γ decreased, and the amount of residual austenite having a size of 1.5 μm or more was not sufficient. As a result, sufficient TS × EL value and sufficient deep drawability could not be obtained.

  Sample No. No. 11 [8] Since the reheating rate was as low as 30 ° C./second, carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.

  Sample No. No. 12 [4] Since the quenching start temperature is as low as 580 ° C., the amount of ferrite becomes excessive, and the total amount of tempered martensite and tempered bainite is insufficient. As a result, sufficient tensile strength and yield ratio cannot be obtained. It was.

  Sample No. No. 14 [4] Since the cooling rate was as low as 8 ° C./second, the amount of ferrite was excessive, the total amount of tempered martensite and tempered bainite was insufficient, and the MA average size was excessive. As a result, sufficient tensile strength and yield ratio were not obtained.

  Sample No. No. 16 had a long [9] holding time of 300 seconds, so carbide (cementite) was precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.

  Sample No. No. 17 [8] Since the reheating rate was as low as 15 ° C./second, carbide (cementite) precipitated. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.

  Sample No. No. 19 [7] Since the reheating temperature was 550 ° C. higher, the parameter became 14604 higher. Therefore, the amount of residual γ is reduced and the amount of retained austenite having a size of 1.5 μm or more is not sufficient. As a result, TS × EL and deep drawability decreased. Further, since the scattering intensity of X-ray small angle scattering is large, it can be said that the volume fraction of about 1 nm cementite is large. As a result, the impact resistance characteristics (thickness reduction rate) decreased.

  Sample No. No. 20, [8] The reheating temperature was as low as 250 ° C., so the parameter was as low as 9280. Therefore, the diffusion of carbon is insufficient, the amount of residual γ is reduced, and the amount of residual austenite having a size of 1.5 μm or more is not sufficient. As a result, TS × EL and deep drawability decreased.

Sample No. No. 22 had a small amount of C, an insufficient amount of retained austenite, and an insufficient amount of retained austenite having a size of 1.5 μm or more. As a result, sufficient TS × EL and deep drawability could not be obtained.
Sample No. No. 23 had a large amount of Mn and a short amount of retained austenite, and as a result, sufficient TS × EL was not obtained.

Sample No. No. 24 has a small amount of Mn, an excessive amount of ferrite, and a total amount of tempered martensite and tempered bainite is insufficient. As a result, sufficient tensile strength and yield ratio were not obtained.
Sample No. In No. 25, the amount of Si + Al was small, the total amount of tempered martensite and tempered bainite was insufficient, the amount of retained austenite was small, the MA average size was excessive, and the average size of residual austenite was excessive. As a result, sufficient TS × EL, hole expansion ratio and deep drawability were not obtained.

  Sample No. No. 26 had an excessive amount of C and was not retained in the temperature range of 300 to 500 ° C. after austenitization, so that sufficient SW cross tensile strength could not be obtained.

  Sample No. In No. 27, the amount of Si + Al was excessive, and sufficient TS × EL was not obtained.

4). Summary As described above, a steel sheet satisfying the composition and steel structure defined in the present invention has a tensile strength (TS), a yield ratio (YR), a product of (TS) and total elongation (EL) (TS × EL), LDR. It was confirmed that the hole expansion rate (λ), the thickness reduction rate (RA) of the fractured portion during the tensile test, and the cross tensile strength of the spot welded portion were all at high levels.
Moreover, according to the manufacturing method of this invention, it has confirmed that the steel plate which satisfy | fills the composition and steel structure prescribed | regulated to this invention can be manufactured.

Claims (6)

C: 0.15% by mass to 0.35% by mass,
Total of Si and Al: 0.5% by mass to 3.0% by mass,
Mn: 1.0% by mass to 4.0% by mass,
P: 0.05 mass% or less,
S: 0.01% by mass or less,
And the balance consists of Fe and inevitable impurities,
Steel structure
The ferrite fraction is 5% or less,
The total fraction of tempered martensite and tempered bainite is 60% or more,
The amount of retained austenite is 10% or more,
The average size of MA is 1.0 μm or less,
The average size of retained austenite is 1.0 μm or less,
Residual austenite having a size of 1.5 μm or more is 2% or more of the total retained austenite amount,
A high-strength steel sheet having a scattering intensity of 1.0 cm ⁻¹ or less when the q value in X-ray small angle scattering is 1 nm ⁻¹ .   The high-strength steel sheet according to claim 1, wherein the C content is 0.30 mass% or less.   The high-strength steel sheet according to claim 1 or 2, wherein the amount of Al is less than 0.10% by mass. C: 0.15% by mass to 0.35% by mass, Si and Al in total: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% by mass, P: 0.0. Preparing a rolled material comprising 05% by mass or less, S: 0.01% by mass or less, and the balance being Fe and inevitable impurities;
Heating the rolled material to a temperature of Ac ₃ point or higher to austenite;
After the austenitization, cooling is performed between 650 ° C. and 500 ° C. at an average cooling rate of 15 ° C./second or more and less than 200 ° C./second, and at a cooling rate of 10 ° C./second or less within a range of 300 to 500 ° C. for 10 seconds. As mentioned above, making it hold for less than 300 seconds,
After the residence, cooling at an average cooling rate of 10 ° C / second or more from a temperature of 300 ° C or more to a cooling stop temperature between 100 ° C and 300 ° C;
Heating from the cooling stop temperature to a reheating temperature in the range of 300 to 500 ° C. at an average heating rate of 30 ° C./second or more;
Holding at the reheating temperature T so that the tempering parameter P defined by the formula (1) is 10,000 to 14500 and the holding time t is 1 to 150 seconds;
After the holding, cooling from the reheating temperature to 200 ° C. at an average cooling rate of 10 ° C./second or more,
A method for producing a high-strength steel sheet including
P = T × (20 + log (t / 3600)) (1)
Here, T: reheating temperature (K), t: holding time (second).   The manufacturing method according to claim 4, wherein the retention includes holding at a constant temperature within a range of 300 to 500 ° C. 5.   The manufacturing method according to claim 4 or 5, wherein the tempering parameter is 11000 to 14000 and the holding time is 1 to 150 seconds.

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