WO2004106571A1

WO2004106571A1 - High strength thin steel sheet excellent in resistance to delayed fracture after forming and method for preparation thereof, and automobile parts requiring strength manufactured from high strength thin steel sheet

Info

Publication number: WO2004106571A1
Application number: PCT/JP2003/006617
Authority: WO
Inventors: Toshiki Nonaka; Nobuhiro Fujita; Hirokazu Taniguchi
Original assignee: Nippon Steel Corporation
Priority date: 2003-05-27
Filing date: 2003-05-27
Publication date: 2004-12-09
Also published as: EP1637618A1; US20070006948A1; AU2003235443A1; DE60333400D1; EP1637618A4; EP1637618B1; US20110120598A1

Abstract

A thin steel sheet which contains 7 vol % or less of retained austenite and 100 to 100,000 pieces per 1 square millimeter of crystallized products and/or precipitates having a particle diameter of 0.01 to 5.0 μm, and has a chemical composition in mass %: C: 0.05 to 0.3 %, Si: 3.0 % or less, Mn: 0.01 to 3.0 %, P: 0.02 % or less, S: 0.02 % or less, Al: 0.01 to 3.0 %, N: 0.01 % or less, Mg: 0.0002 to 0.01 %, and the balance: Fe and inevitable impurities.

Description

Description High-strength thin steel sheet with excellent delayed fracture resistance after forming and its manufacturing method, and high-strength automotive parts made from high-strength thin steel sheet

〔Technical field〕

The present invention relates to a high-strength thin steel sheet in which placing cracks and delayed fracture, which are particularly problematic in a high-strength thin steel sheet, are suppressed, a method of manufacturing the same, and a strength part for automobiles manufactured by using the same.

(Background technology)

Conventionally, high-strength steel is often used for applications such as ports, PC steel wires, and line pipes.However, when the strength exceeds 78 OMPa, delayed blasting occurs due to the penetration of hydrogen into the steel. It is known to occur.

On the other hand, in the case of thin steel sheets, (i) hydrogen is released in a short time even if hydrogen enters because of the small thickness, and (ii) steel sheets with a workability of more than 780 MPa Since there was almost no use, it can be said that the awareness of problems with delayed blasting was low.

However, recently, due to the necessity of reducing the weight of automobiles and improving collision safety, ultrahigh-strength steel sheets of 780 MPa or more are press-formed into reinforcing materials such as pumper-packed beam beams and sheet trails. The number of cases where pipe forming, bending, end face processing, or hole expansion processing is provided is increasing rapidly. Therefore, there is an urgent need to develop ultra-high strength thin steel sheets with delayed fracture resistance.

To date, most techniques for improving delayed fracture resistance have been developed for steels that are often used as-is and below yield strength or yield stress, such as port, bar, and plate. For example, in the development of strip steel and bolt steel, the development of tempered martensite has been carried out, and a new development of elucidation of delayed fracture has been developed. (Japan) Iron and Steel Institute of Japan Study group Heisei

(January 1999) reported that an additive element exhibiting temper softening resistance, such as Cr, Mo or V, was effective in improving delayed fracture resistance. This technology shifts the delayed fracture mode from grain boundaries to intragranular fracture by precipitating and utilizing this as a hydrogen trap site. ,

However, since these steels have a C content of 0.4% or more and contain many alloying elements, the workability and weldability required for thin steel sheets are poor, and the precipitation of alloy carbides takes several hours or more. Therefore, there is a problem in manufacturability.

In addition, Japanese Patent Application Laid-Open No. 11-293383 discloses that an oxide mainly composed of Ti and Mg is effective in preventing generation of hydrogen defects. ,

However, this is intended for thick steel plates.Despite the consideration of delayed fracture after welding with high heat input, it is subject to the high degree of forming required for thin steel plates and the paring accompanying edge processing. The effect on delayed fracture phenomena such as occurrence is not considered.

Furthermore, no consideration is given to workability, which is a basic property of thin steel sheets.

On the other hand, regarding delayed rupture of thin steel sheets, it has been reported that delayed rupture was promoted due to the addition-induced transformation of the amount of retained austenite (for example, Yamazaki et al., CAMP—ISIJ vol. 5 p1839) ~ 1842 (1992), see).

This takes into account the forming of thin steel sheets, It reports on regulations on the amount of residual austenite that does not degrade the performance.

In other words, the above report relates to a high-strength thin steel sheet with a specific structure, and cannot be said to be a fundamental measure for improving delayed rupture resistance.

[Disclosure of the Invention]

As mentioned above, a development example in which measures were taken to prevent delayed fracture in forming before use, while ensuring the formability, which is a basic characteristic, while taking into account the working environment of thin steel sheets and the manufacturability of existing equipment, in particular. Is almost.

In view of the above background, the present inventors have come to find a method for fundamentally improving delayed fracture resistance by sufficiently considering the use environment of thin steel sheets and the production method using existing facilities.

In other words, by forming a compound or composite compound of Mg and controlling the morphology of these compounds, the delayed fracture resistance after forming can be improved without deteriorating the formability of the high-strength steel sheet. I found that it could be improved.

In addition, we have found an effective method for producing high-strength thin steel sheets using the current production facilities (hot rolling, continuous annealing, hot-dip galvanizing, electric plating, etc.). Details are as follows.

(1) In mass%,

C: 0.05-0.3%,

S i: 3.0% or less,

Mn: 0.01 to 3.0%,

P: 0.02% or less,

S: 0.02% or less, A1: 0.01 to 3.0%,

N: 0.01% or less,

M g: 0.0 00 0 2 to 0.0 1%

And the balance consists of iron and unavoidable impurities.The volume fraction of retained austenite in the structure of the steel sheet is 7% or less.

At least one of Mg oxides, sulfides, complex precipitates and complex precipitates,

Average particle size d: 0.01 to 5.0 // m

Density (0: 100 to 100,000 per square mm, and distribution: the ratio of standard deviation σ from average particle diameter to average particle diameter d ≤ 1.0 is satisfied, and

A high-strength thin steel sheet with excellent delayed fracture resistance after forming, characterized in that the volume fraction V V (%) of residual austenite and the tensile strength T S (MP a) satisfy formula (A).

Formula (A): 1 0 0 0 (V 7 - 0. 1) "5 - 5 + α (M g - 4 0) 2

-5 0 (d— 0.2) ² + l. Llnp + 7 0 0 (TS-6 8 0)-° ⁹ ≥ 10

Where α = — 0.05 (Mg ≤ 4 0), a =-0.02 (

(Mg> 40)

V y: Volume ratio of residual austenite (%)

M g: M g amount (mass p p m)

d: diameter (μm)

P density (number Roh mm ²⁾

T S: Tensile strength (MPa)

And, furthermore,

(i) 1 0 0 0 (V-0.1) — When ⁵ ' ⁵ ≥ 10, 1 0 0 0 ( ^{V - 0. 1) -. 5} 5 = 1 0

(ii) 2≤M g≤1 0 0 p p m

(iii) When 0.01 ≤ d ≤ 5.0 / zm and (d-0.2) ² ≤ 0.2, then (d-0.2) ² = 0.2

(iv) 1 0 0 ≤ p ≤ 1 0 0 0 0 0 pcs / mm ²

(v) 7 8 0 MP a ≤ T S

It is.

(2) In addition, mass. /. so,

V: 0.05 to 1%,

T i: 0.02 to 1%,

Nb: 0.02 to 1%,

Zr: 0.02 to 1%

A high-strength thin steel sheet excellent in delayed fracture resistance after forming as described in (1), characterized by containing one or more of the following.

(3) Further, in mass%,

Cr: 0.05-5%,

Mo: 0.05-5%,.

W: 0.05 to 5%

A high-strength thin steel sheet having excellent delayed fracture resistance after forming according to (1) or (2), characterized by containing one or more of the following:

(4) Further, in mass%,

Cu: 0.05 to 2.0%

The high-strength thin steel sheet excellent in delayed fracture resistance after forming according to any one of the above (1) to (3), characterized by containing:

(5) Furthermore, in mass%,

N i: 0.005 to 2.0%, C o: 0.05 to 2.0%

High-strength ^ steel excellent in delayed crushing resistance after forming according to any one of the above (1) to (4), characterized by containing one or more of the following:

(6) Further, in mass%,

Β: 0.0 0 0 0 2 to 0.1%

The high-strength thin steel sheet excellent in delayed fracture resistance after forming according to any one of the above (1) to (5), characterized by containing:

(7) Further, in mass%,

R EM: 0.0 0 0 0 5 to 0.0 1%,

C a: 0.0 0 0 5 to 0.0 1%,

Y: 0.0 0 0 0 5 to 0.0 1%

The high-strength thin steel sheet having excellent delayed crushing resistance after forming according to any one of the above (1) to (6), characterized by containing one or more of the following.

(8) The high-strength thin steel sheet having excellent delayed fracture resistance according to the above (1) to (7) is a hot-rolled steel sheet or a cold-rolled steel sheet. High strength thin steel sheet with excellent delayed fracture.

(9) A high-strength thin steel sheet having excellent delayed fracture resistance according to the above (1) to (7), wherein the steel sheet has been subjected to a surface treatment with zinc plating. High strength thin steel sheet with excellent delayed fracture resistance.

(10) A high-strength thin steel sheet having excellent delayed fracture resistance after the forming process described in the above (8) or (9), further comprising a film laminating process. High strength thin steel sheet with excellent delayed fracture resistance.

(11) A composition comprising the composition according to any one of (1) to (7). A piece is manufactured, hot-rolled at a finishing temperature of ₃ points or more of Ar, wound up at 500 to 800 ° C, then pickled, and then cooled at a rolling reduction of 30 to 8 °%. Rolling, then soaking at a temperature of 600 ° C. or more to 950 ° C. or less, recrystallization annealing, and then temper rolling are performed. Method for producing high-strength thin steel sheet with excellent quality.

(12) In the method for producing a high-strength thin steel sheet having excellent delayed rupture resistance after forming described in (11) above, the steel sheet may have a temperature of 200 to 700 ° C after annealing. A method for producing a high-strength thin steel sheet having excellent delayed fracture resistance after forming, characterized in that the steel sheet is held for 10 to 10 hours.

(13) A high-strength component for automobiles, which is made of a high-strength thin steel sheet having excellent delayed fracture resistance after the forming process according to (1) to (7).

[Brief description of drawings]

Figure 1 is a diagram showing the relationship between equation (A) and delayed rupture time.

FIG. 2 is a diagram showing the relationship between equation (A) and residual austenite. FIG. 3 is a diagram showing the relationship between the formula (A) and the amount of Mg.

FIG. 4 is a diagram showing the relationship between the equation (A) and the density.

[Best mode for carrying out the invention]

In tempered martensite steel, delayed blasting is thought to be caused by the accumulation of hydrogen at the former austenite grain boundaries and the like, and voids and the like originate from that portion.

Therefore, if the hydrogen trap site is evenly and finely dispersed and hydrogen is trapped in that part, the diffusible hydrogen concentration decreases, and the sensitivity to delayed fracture decreases.

As disclosed in the aforementioned Japanese Patent Application Laid-Open No. It has been found that in thick steel sheets with a combined addition of Mg, Mg and Ti, controlling the oxide morphology improves the resistance to delayed fracture caused by hydrogen.

However, if high residual stress is generated by the forming process as in the case of a thin steel sheet, or if paris etc. remains on the machined end face, the delayed rupture resistance is inevitably deteriorated. As described above, there are few studies on delayed fracture characteristics taking into account the usage of thin steel sheets, and the problem of delayed fracture characteristics in thin steel sheets is the problem of controlling the oxide form of Mg and Ti. It cannot be solved only by. There is also concern that the fine dispersion of the trap sites may degrade the ductility, a basic property of thin steel sheets.

In view of the above-described background, the present inventors have developed various types of crystallization and precipitation to ensure or improve the use environment of thin steel sheets, that is, delayed rupture resistance even after forming. In addition to the effects of the materials, the effects of the strength and structure of the steel sheet were examined.

As a result, we found a technique to improve or ensure delayed fracture resistance even under high residual stress and occurrence of edge crevices in the use environment of thin steel sheets. That is,

(i) Control of the dispersion morphology of the oxide or sulfide containing Mg and the compound crystallized or precipitated with them.

(ii) the amount of retained austenite in the microstructure of the steel sheet, and

(i i i) strength of steel sheet,

Respectively, to effectively disperse the SM g compound or compound crystallization / precipitate at the hydrogen trap site, and to achieve both ductility and delayed fracture resistance after forming. Can be.

Then, as a condition for satisfying this, the formula (A) is defined as ( Equation (A) will be described in detail later. ).

In addition, if the oxide or sulfide containing Mg in (i) and the compound crystallized or precipitated with them are present in the grains (excluding the phase interface of the microstructure such as the former austenite grain boundary). This is more effective for improving delayed blasting properties.

In order to control the above (i), (ii) and (iii), the production conditions are specified, and oxides, nitrides, sulfides, and other crystallized or precipitated substances of various elements are trapped in hydrogen. It is also effective to control the form that can be a site.

In the present invention, by satisfying the expression (A), the delayed fracture resistance of the high-strength thin steel sheet can be sufficiently ensured even after the forming.

This is because the interaction between the dislocations and residual stress fields introduced by the forming process of the thin steel sheet and the particles that become the trap sites is caused by the dislocations and residual stresses introduced during hot rolling and cooling after welding of the thick steel sheet. This is thought to be due to the difference between the interaction between the stress field and the particles that serve as trap sites, and the difference in the heat treatment methods for thin and thick steel plates.

The above (i) and (ii) are limited as follows. Residual austenite amount: Since the residual austenite increases the delayed fracture susceptibility when it becomes martensite due to work-induced transformation, the upper limit is set to 7% by volume.

Average particle size: The average particle size was limited to 0.5Ομπι to 0.5ΐμπι. A certain size is required as a hydrogen trap site, and the presence of a large amount of fine particles is not preferable in terms of ensuring the ductility of a thin steel sheet, and makes production difficult.

Therefore, the lower limit of the average particle diameter was set to 0.1Ομππι. In addition, coarse particles do not act as a trap site and can be a starting point for destruction. Therefore, 5.0 μππ was set as the upper limit of the average particle diameter. Density: The particle density was set to 100 to: L 0000 particles mm ² . The low particle density means that the number of trap sites is small, and the delayed fracture resistance after processing cannot be ensured. Therefore, the lower limit was set to 100 particles / mm ² .

Further, when the particle density is high density, Rukoto to deteriorate the ductility and formability, and, since the resistance to delayed fracture improving effect saturates, and the 1 0 0 0 0 0 ZMM ² as the upper limit .

Distribution: Regarding the distribution of particles, the ratio of the standard deviation σ from the average particle diameter to the average particle diameter d satisfies the equation σ / d ≤ 1.0. σ Ζ (1> 1.0) means that the particle distribution is wide-ranging, and the effect of improving delayed blasting is smaller than that of the same average particle size. Therefore, the upper limit of a Z d is set to 1.0.

Here, measurement of particles containing the Mg compound will be described. For particle measurement, use a thin film or extracted replica sample and observe it with a scanning or transmission electron microscope at a magnification of 50,000 to 100,000, and a minimum of 30 fields of view. Is the value obtained by measuring.

The particle diameter is evaluated by the circle equivalent diameter by image analysis. When determining the density, multiple precipitates or crystals are counted as one.

The composition analysis was performed using EDX and EELS, and the structural analysis was performed by analyzing the diffraction pattern.

Each composite compound is composed of a compound (Carbide, Nitride, Oxide or Sulfide) containing an alloying element (for example, Ti, Nb, V, Cr, Mo, REM, Ca, etc.) in addition to Mg. Things).

Hereinafter, the present invention will be described in more detail.

The present invention relates to a high-strength thin steel plate, and mainly relates to a steel plate having a tensile strength of 780 MPa or more and a thickness of 0.5 mm to 4.0 mm. Things.

Next, equation (A) will be described. Based on the assumption that the volume fraction of residual austenite, the average particle size, the density, the amount of Mg, and the tensile strength can be cited as factors of delayed rupture resistance, FIG. ) It was set.

Formula (A): 1 0 0 0 (V 7-0.1) — ⁵ ' ⁵ + H (M g-40) ²

-5 0 (d — 0.2) ² + 1.1 .1 np + 7 0 0 (TS-680)-° ⁹ ≥ 10

Where, o; = — 0.05 (Μ g ≤ 4 0), a =-0.02 (

(Mg> 40)

V y: Volume ratio of residual austenite (%)

M g: M g amount (mass p p m)

d: Particle size (μm)

p density (pcs Zmm ² )

T S tensile strength (M P a)

And, furthermore,

(I) 1 0 0 0 ( V- 0 1.) - When the ^⁵ · ⁵ ≥ 1 0 is, 0 0 0 (V- 0 1 .) "5 - 5 = 1 0

(ii) 2 ≤ Mg ≤ 1 0 0 p p m

(iii) When O. 0 1 ≤ d ≤ 5.0 / m and (d-0 2) ≤ 0.2, (d _ 0.2) ² = 0.2

(iv) 1 0 0 ≤ p ≤ 1 0 0 0 0 0 pieces mm ²

(v) 7 8 0 M P a ≤ T S

It is.

Left function f of formula _{(A) (ν γ, M} g, d, p TS) putting a, f (V, M g, d, p, TS) significantly delayed fracture resistance value is 1 0 or more Is improved. Figures 2 to 4 show the effect of each variable on delayed fracture resistance. In the figure, 示し indicates that the delayed fracture resistance is good, and X indicates that it is poor.

FIG. 2 is a diagram showing a correlation between f (V y) and the volume ratio V of retained austenite. The preconditions are as follows: Mg: 30 ppm, average particle diameter: 0.4 μm, density: 1500 pieces Zmm ² , tensile strength: 1480 MPa.

If Vγ is high, the delayed fracture resistance deteriorates, but the invention steel with a high f (Vy) value shows good delayed fracture resistance when V0 is 7% or less.

In addition, even when is 7% or less, the comparative steel of X has f (V7) <10 because Mg, particle size, and density are out of the ranges defined by the present invention, and the delayed fracture resistance is deteriorated. It is.

FIG. 3 is a diagram showing a correlation between f (M g) and the amount of Mg added. The prerequisites are as follows: volume ratio of residual austenite: 3.0%, average particle diameter: 0.4 μm, density: 1500 / mm ² , tensile strength: 148 MP .

There is a region where the Mg is in the range of 20 to 70 ppm, particularly, where the delayed blasting resistance is good. In the case of X with Mg of less than 100 ppm, the residual austenite, particle size, and density are out of the range defined by the present invention. Is worse.

FIG. 4 is a diagram showing the correlation between f ( _P ) and the density of crystallized substances and precipitates. The preconditions are as follows: volume fraction of residual austenite: 3.0%, Mg: 30 ppm, tensile strength: 138 MPa. If the density is low, it can be said that delayed crushing resistance is poor.

In addition, although the density p is within the range defined by the present invention, that of X is the range where the residual austenite, Mg, and particle size are limited by the present invention. Since it was out of the surrounding area, it was f (p)-10 and the delayed rupture resistance was poor.

Based on the above, it was concluded that if the above requirements satisfied expression (II), the delayed blasting resistance was excellent.

Next, the reasons for limiting the chemical components of steel in the present invention will be described.

C is an element that can increase the strength of a steel sheet. In particular, it produces a hard phase such as martensite-austenite and is an essential element for high strength. To obtain a strength of 7 8 OMPa or more, 0.055% or more is necessary, but conversely, if it is contained too much, the cementite, which is the starting point of brittle fracture, will increase, and hydrogen embrittlement will occur. Is likely to occur. Therefore, the upper limit was set to 0.3%.

Si is a substitutional solid solution strengthening element that greatly hardens the material, is effective in increasing the strength of the steel sheet, and is an element that suppresses cementite precipitation. However, removing scale in hot rolling is costly and disadvantageous economically, so the upper limit is 3.0%.

In addition, if the addition amount is large, the plating property is deteriorated. Therefore, in order to improve the plating property, 0.6% or less is desirable.

Mn is an element effective for increasing the strength of a steel sheet. However, since this effect cannot be obtained at less than 0.01%, the lower limit was set to 0.01%. Conversely, if the addition amount is large, not only co-segregation with P and S is promoted, but also the workability may be deteriorated, so the upper limit is 3.0%.

P is an element that promotes grain boundary destruction due to grain boundary segregation, and it is preferable that P be low. However, extreme reduction is not preferable in terms of manufacturing cost. In addition, since P is an element that deteriorates corrosion resistance, the upper limit is set to 0.02%. S is an element that promotes the absorption of hydrogen in a corrosive environment. It is desirable that S be low, but extremely reducing it is not preferable in terms of manufacturing cost. Particularly, in order to enhance the workability, the lower the better, the upper limit is set to 0.02%.

A1 is added in an amount of 0.01% or more for deoxidation.However, if the addition amount increases, inclusions such as alumina increase, thereby deteriorating the workability and the weldability. The upper limit is 0%. The addition of 0.2% or more has an effect of suppressing the generation of residual austenite, and is therefore preferable.

Since N contributes to the deterioration of workability and the occurrence of blowholes during welding, a smaller N is better. If the content exceeds 0.01%, the workability deteriorates. Therefore, the upper limit is set to 0.01%.

Mg is not only effective in improving the delayed fracture resistance of the compound itself, but also forms complex precipitates or crystallized substances with other elements, and changes their morphology to delayed fracture resistance. Since it is an element necessary for controlling to contribute to improvement, it is added in an amount of 0.0002% or more.

However, if the content exceeds 0.01%, coarse oxides and sulfides are generated, which is not effective for shape control, and also lowers the formability, which is a basic characteristic required for thin steel sheets. 0.01%.

Next, V, T i, N b, and Z r are strong carbide forming elements, which are elements necessary for forming precipitates and inclusions and improving strength and delayed fracture resistance.

Further, V is an element effective for increasing the strength of the steel sheet and reducing the grain size.

However, since this effect cannot be obtained if the content is less than 0.005%, the lower limit is set to 0.005%. Conversely, when the content exceeds 1%, precipitation of carbonitride becomes remarkable, and ductility decreases remarkably. Therefore, the upper limit was set to 1%. Further, Ti is an element effective for increasing the strength of the steel sheet and reducing the grain size. However, if the content is less than 0.02%, the number of precipitates decreases, so the lower limit is set to 0.02%. Conversely, if it exceeds 1%, coarse precipitates or crystallized substances are formed, and workability and delayed crushing resistance are reduced. For this reason, the upper limit was set at 1%.

Further, Nb is an element effective for increasing the strength and refining the steel sheet. However, these effects cannot be obtained at less than 0.002%, so the lower limit was made 0.002%. Conversely, if the content exceeds 1%, the precipitation of carbonitrides increases and the workability and delayed fracture resistance deteriorate, so the upper limit was made 1%.

Further, Zr is an element effective for increasing the strength and reducing the grain size of the steel sheet. However, if the content is less than 0.02%, the number of precipitates decreases, so the lower limit was made 0.02%. Conversely, if it exceeds 1%, coarse precipitates or crystallized substances are formed, and workability and delayed crushing resistance are reduced. Therefore, the upper limit was set to 1%.

Next, Cr, Mo, and W are carbide forming elements and temper softening resistance elements, and are elements necessary for improving strength and delayed rupture resistance.

Further, Cr is an element effective for increasing the strength of the steel sheet. However, these effects cannot be obtained at less than 0.005%, so the lower limit was made 0.05%. Conversely, if the content exceeds 5%, the workability decreases, so the upper limit was set to 5%.

In addition, Mo is not only an effective element for improving the hardenability of steel sheets and stably obtaining martensite in continuous annealing equipment, but also has the effect of strengthening grain boundaries and suppressing the occurrence of hydrogen embrittlement. It is an element that plays. However, these effects cannot be obtained at less than 0.005%, so the lower limit was made 0.05%. Also, if it exceeds 5%, these effects will saturate, The upper limit is set at 5%.

Further, W is an element effective for increasing the strength of the steel sheet. However, this effect cannot be obtained at less than 0.05%, so the lower limit was made 0.05%. Conversely, if the content exceeds 5%, the workability decreases, so the upper limit was set to 5%.

Next, Cu is effective for strengthening, and the fine precipitation of Cu itself delays and contributes to the improvement of fracture. Therefore, the addition of 0.05% or more was made. On the other hand, excessive addition causes deterioration of workability, so the upper limit was set to 2.0%.

Next, Ni and Co are strengthening elements that enhance hardenability.

In addition, Ni is an element that forms Ni sulfide to suppress hydrogen intrusion and improve delayed blasting properties, and to enhance the hardenability of the steel sheet to ensure the strength of the steel sheet.

However, these effects cannot be obtained if the content is less than 0.05%, so the lower limit is set to 0.005%. Conversely, if the content exceeds 2%, workability deteriorates, so the upper limit was set to 2%. '

Further, since Co is effective for strengthening, it was added in an amount of 0.05% or more. In addition, since excessive addition causes deterioration of workability, the upper limit is set to 2.0%.

Next, B is an element effective in increasing the strength of the steel sheet. However, if the content is less than 0.002%, this effect cannot be obtained, so the lower limit is set to 0.0002%. Conversely, if the content exceeds 0.1%, the hot workability deteriorates, so the upper limit was made 0.1%.

Next, REM, Ca, and Y are effective for controlling the morphology of inclusions and contribute to delayed rupture resistance, so they were added in an amount of 0.0005% or more. On the other hand, 'excessive addition degrades hot workability, so the addition was made 0.01% or less. Next, the manufacturing method will be described.

First, a piece having the specified components is hot-rolled. At this time, in order to prevent the strain from being excessively applied to the ferrite grains and to reduce the workability,

Finish rolling is performed at Ar ₃ or more.

If the finish rolling temperature is too high, the composite precipitate or crystal having a recrystallized grain size and Mg after annealing becomes coarser than necessary, so the finish rolling temperature is desirably 940 ° C or lower.

Regarding the coiling temperature, the higher the temperature, the more recrystallization and grain growth are promoted, and workability can be expected to be improved. However, scale formation that occurs during hot rolling is promoted, and pickling properties are reduced. It should be below 0 ° C.

On the other hand, if the coiling temperature is too low, the steel sheet hardens and the load during cold rolling increases. For this reason, the winding temperature is set to 500 ° C. or higher.

In cold rolling after pickling, if the draft is low, it is difficult to correct the shape of the steel sheet, so the lower limit of the draft is set to 30%. Further, if rolling is performed at a rolling reduction exceeding 80%, cracks will occur at the edges of the steel sheet and shape irregularities will occur, so the upper limit is set to 80%.

If the continuous annealing temperature is too low, it will be in an unrecrystallized state and the steel structure will be hardened.On the other hand, if it is too high, the crystal grains may become coarse and the surface may be roughened during pressing. Not less than 950 ° C. Annealing is performed using continuous annealing equipment or box annealing equipment.

If necessary, after annealing, the temperature may be maintained in a temperature range of 200 to 700 ° C. for 1 minute to 10 hours, and then cooled. By this heat treatment, alloy carbides or nitrides (for example, carbonitrides containing V, Cr, Mo, W) are precipitated.

These precipitates act as new hydrogen trap sites, and the delayed fracture resistance is further improved. If the temperature and time conditions are low and short, The above temperature range was used because precipitation did not occur, and the precipitates became coarse and did not function as a trap site at high temperatures for long periods of time. In the above-mentioned piece, when the production speed is high, the Mg compound becomes excessively fine, and when the production speed is low, the Mg compound becomes coarse, and the number of particles is reduced. In some cases, the effect cannot be fully exhibited.

The production speed of the piece is preferably 0.05 to 20.OmZ. Furthermore, in order to stably utilize the delayed rupture property improving effect of the Mg compound, the content of 1.0 to 3.0 OmZ is preferable.

The steel sheet of the present invention may be any of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet. Further, the plating may be any of normal zinc plating and aluminum plating. The plating may be hot dip plating or electroplating, and may be subjected to an alloying heat treatment after plating, or may be multi-layer plating.

Also, a steel sheet that is not plated or a steel sheet that has been subjected to a film lamination treatment on a plated steel sheet does not depart from the scope of the present invention.

Furthermore, in the case of a high-strength component for an automobile (for example, a reinforcing member such as a bumper or a door impact beam) formed from the high-strength thin steel sheet (for example, a steel sheet of 780 MPa or more) of the present invention, excellent material properties (strength, Rigidity etc.) were maintained, and shock absorption and delayed fracture resistance were also good.

Next, the present invention will be described based on examples.

Steels with the components shown in Table 1 were melted and made into slabs by continuous production in accordance with a conventional method. Symbols A to J are steels of the components according to the present invention, and symbols K to M are those whose components deviate from the scope of the present invention. These steels are heated in a heating furnace at a temperature of 116 to 125 ° C and hot rolled at a finishing temperature of 87 to 900 ° C, Winded at 50 ° C.

Subsequently, except for the symbol H, cold rolling was performed after pickling, followed by recrystallization annealing, and then temper rolling of 0.4% to obtain a cold-rolled steel sheet.

Further, reference numeral I, J are basis weight and alloyed molten zinc plated steel sheet of single-sided 5 0 gZrn ^2, for J, was further subjected to film lamination. Table 2 shows the steel sheet manufacturing methods and material properties.

Table 3 shows the evaluation of the delayed fracture resistance of the steel sheet. The evaluation method was to bend a strip test of 80 mm x 30 mm, attach a water-resistant strain gauge to the surface, immerse it in 0.5 mo 1 Z1 sulfuric acid, and electrolyze by electric current. This is a method for invading hydrogen.

Thereafter, the occurrence of cracks was evaluated. The bending radii were 5 mm, 10 mm, and 15 mm, and the applied stress was 60 MPa and 90 MPa, respectively.

As shown in Tables 2 and 3, reference numerals 1, 2, 3, 5, and 7 to 12, which are examples of the present invention, show sufficient tensile strength and ductility to be applied to reinforcing parts of automobiles. Also, the time until crack generation is long, and it is excellent in delayed rupture resistance.

On the other hand, in Comparative Examples 4, 6, and 13 to 15, any one of the components and the annealing temperature is out of the range limited by the present invention.

Symbols 4 and 6 indicate that the value of the formula (A) deviates from the range of the present invention, and that the time until crack generation is short. Reference numerals 13 to 15 deviate from the component range of the present invention, and the number of crystallized substances and precipitates serving as hydrogen trap sites is small, or conversely, cracks occur because hydrogen is trapped too much. Occur And the difference from the delayed fracture resistance obtained in the present invention is apparent.

Table 1 Classification C S i Mn P S Al N Mg T i Nb V

A Invented steel 0.15 0. 50 2.50 0. 016 0. 006 0, 035 0. 006 0. 0042

Β Invented steel 0.12 0.62 2.60 0.017 0.006 0. 032 0.005 0.0038 0. 050

C Inventive steel 0.15 0.50 2.90 0.015 0.004 0.035 0.004 0.0039 0.050

D Invented steel 0.14 0.44 2.60 0.015 0.005 0.034 0.006 0.0052 0.100 0.042

E Invented steel 0.15 0. 50 2.60 0, 007 0. 002 0. 030 0. 003 0. 0028 0, 050 0. 012

C

F Inventive steel 0.16 丄. 03 2.30 0. 011 0. 001 0. 054 0. 004 0, 0055 0. 054

G Invention steel 0.16 1.52 2.33 0.012 0.003 0.325 0.005 0.0033 0.131

B "

H Invented steel 0.21 0, 52 丄. 51 0. 011 0. 002 0. 312, 0.004 0.0032 0. 011

1

免 Immunity ^ Steel U. Lb Z o. 101 U. UUo ϋ. 003 0. 7 l 0. UU1 ϋ. UU48 0.

J Invented steel 0.15 0.01 2.55 0.0.09 0.003 l.2l 0.003 0.0054 0.088 0.041

K Comparative steel 0.15 0.50 2.50 0.016 0.006 0.035 0.006 0.051

L Comparative steel 0.12 0.48 2.33 0.015 0.005 0.035 0.005 0.0012 1.311

M Comparative steel 0.18 0.52 2.10 0. 011 0.003 0. 035 0. 002 0.008

Table 1 (continued)

Board thickness

Cr Mo W Cu Ni Co B REM Ca Y Lolo f

(. mm; \

A, Nopachi

1.2 Cold rolled steel sheet

B 1.4 Cold rolled steel sheet c

0.300 1.2 Cold rolled steel sheet

D, / eight

0. 01 0. 01 1.0 Cold rolled steel sheet, (J. ΌΖ U. Ul U. U 0.8 Cold rolled steel sheet

CO

r Ul U. 01 U. 0005

I. 6 Cold rolled steel sheet U. Ubl U. 11 u. Uuy U. UUU5 0, 0016 1.4 Cold rolled steel sheet

H 3.4 Hot rolled steel sheet

I ϋ. Zoo 1. ffifn mesas steel plate

J 0.02 0.286 0.013 0.02 0.0012 0.0022.1.8 Galvanized steel sheet

K 1.2 Cold rolled steel sheet

L 2.12 0. 012 1.4 Cold rolled steel sheet

M 0.0011 0.0015 1.6 Cold rolled steel sheet

Table 2

3 Steel making conditions Tensile properties No. No. Classification Cylindrical structure t degree. Caro temperature Finishing temperature Winding degree iff degree τ T F T

(rn Me min (° C) (° C) (° C) (° C) (MPa) (o / ₀ )

1 A Invention example 1.5 1180 880 650 850 1410 8

2 B Invention example 1.4 1190 870 700 820 1160 12

3 c Inventive example 2.1 1240 880 650 820 1380 8

4 Comparative example 1.7 1190 880 550 550 1610 2

5 D Invention example 1.5 1230 900 600 840 1360 9 t 0

CO 6 Comparative example 1.6 1210 870 550 97 1310 10

7 E Inventive example 1.3 1200 880 600 820 1410 8

8 F Invention example 1 5 1150 700 830 1480 sg G Explanation example 1.8 1160 880 600 800 1420 7

10 H Invention example 1.7 1230 900 550 1400 8

11 I Inventive example 1.6 1200 900 650 810 1390 8

12 J Inventive example 1.5 5 1220 880 600 820 1530 8

13 K Comparative example 1.7 1180 890 600 840 1410 8

14 L Comparative example 1.8 1190 890 600 850 1390 4

15 M Comparative example 1.2 2 1220 890 600 830 1470 8

Table 3

Time to crack generation (min)

One

Classification Residual 0 / Ratio Diameter (A) Bending radius 15mm Bending radius 10mm Bending radius 5mm No. (%) (Μm) (pcs.Zmm ² ) Stress 60 Stress 90 Stress 60 Stress 90 Stress 60 Stress 90 kgf / mm ^ kgf / mm kgf / mm ² kgf / mm ² kgf / mm ² kgf / mm ² Example 2.b 0.2 1000 15.92 〇〇〇〇〇〇

2 Examples 3.7 0.18 1550 11.62 〇〇〇〇〇〇

3 Example 4.2 0.12 2500 10.63 〇〇〇〇〇〇

4 Comparative car father 3.0 0.45 1000 8.82 〇 X 〇 X X X

"

5 Invention 2.7 1 0U0 17.4 〇〇〇〇〇〇

0 Comparative example 3.3 0.12 300 9.45 X X X X X X

7 Example 3.1 0.2 1600 11.63 〇 63 〇〇〇〇

8 Example 2.7 0.18 3400 15.61 ο ο ri u U

y destination ^ ί! m, (J. Ib o4UL) 丄. L6 〇〇〇〇〇〇

10 Invention example 2.8 0.14 2600 14.46 〇〇〇〇〇〇

11 Invention Example 3.1 0.13 2200 12.11 〇〇〇〇〇〇

12 Invention example 2.4 0.12 1200 19.02 〇〇〇〇〇〇

13 Comparative Example 2.5 0.19 1000 9.55 〇 X X X X X

14 Comparative Example 4.2 0.2 1200 6.21 X X X X X X

15 Comparative Example 3.2 0.2 1000 3.31 XXXXXX

[Industrial applicability]

As described above, in the high-strength thin steel sheet according to the present invention, the Mg compound or the composite crystallization / precipitate, which is a hydrogen trap site, is effectively dispersed, and the ductility and the delayed fracture resistance after the forming process are reduced. It is possible to balance soil properties.

Even in the automotive strength parts (for example, reinforcement members such as pampers and door impact beams) formed and processed from the high-strength thin steel sheet of the present invention, excellent material properties are maintained, and shock absorption and delayed fracture resistance are maintained. Was also good.

Claims

Range of request

1. In mass%,

C: 0.05-0.3%,

S i: 3.0% or less,

Mn: 0.01 to 3.0%,

P: 0.02% or less,

S: 0.02% or less,

A1: 0.01 to 3.0%,

N: 0.01% or less,

Mg: 0.0000 to 0.001%

With the balance being iron and unavoidable impurities, with a volume fraction of residual austenite in the steel sheet structure of 7% or less, Mg oxides, sulfides, complex precipitates and composites. At least one of the precipitates is

Average particle diameter d: 0.01 ~ 5.0 jum

Density J: 100 to 100,000 pieces per square mm and distribution: Ratio of standard deviation σ from average particle diameter to average particle diameter d ≤

Ten

Satisfy and

The volume fraction V γ (%) of residual austenite and the tensile strength T S (M P a) satisfy equation (A).

High strength thin steel sheet type which is excellent in delayed fracture resistance after molding, characterized in that (A): 1 0 0 0 (. V y - 0 1) "5 · 5 + α (M g - 4 0) ^Two

-5 0 (d — 0.2) ² + l. Llnp + 7 0 0 (TS — 6 8 0) "° ⁹ ≥ 10 And 0; = —0.05 (Mg≤40), a: —0.02 (Mg> 40)

V y: Volume ratio of residual austenite (%)

M g M g amount (mass p P m)

d Particle size (μm)

P density (pcs / mm ² )

T S Tensile strength (MP a)

And, furthermore,

(I) 1 0 0 0 ( V - 0. 1) - 5. time of 5 0, 1 0 0 0 (V - 0. 1) "5 · 5 = 1 0

(ii) 2≤M g≤1 0 0 p p m

(iii) When O. 0 1 ≤ d ≤ 5.0 μm, (d-0 2) ≤ 0.2, then (d _ 0.2) ² = 0.2

(iv) 1 0 0 ≤ p ≤ 1 0 0 0 0 0 pcs / mm ²

(v) 7 8 0 MP a≤ T S

It is.

2. Further mass%

V: 0 0 0 5 to 1%,

T i: 0 0 0 2 to 1%,

Nb: 0 0 0 2 to 1%,

Zr: 0 0 0 2 to: 1%

2. The high-strength thin steel sheet having excellent delayed fracture resistance after forming according to claim 1, characterized by containing one or more of the following.

3. Furthermore, in mass%,

C r: 0.05 to 5%,

M0: 0.05-5%,

W: 0.05 to 5% 3. The high-strength thin steel sheet having excellent delayed fracture resistance after forming according to claim 1 or 2, characterized by containing one or more of the following.

4. Furthermore, in mass%,

Cu: 0.05 to 2.0%

A high-strength thin steel sheet having excellent delayed fracture resistance after forming according to any one of claims 1 to 3, wherein the steel sheet comprises:

5. Furthermore, in mass%,

N i: 0.005 to 2.0%,

C o: 0.05 to 2.0%

The high-strength thin film having excellent delayed crushing resistance after molding according to any one of claims 1 to 4, characterized by containing one or more of the following.

6. Furthermore, in mass%,

B: 0, 0 0 0 2 to 0.1%

A high-strength thin steel sheet having excellent delayed rupture resistance after forming according to any one of claims 1 to 5, characterized by comprising:

7. Furthermore, in mass%,

R EM: 0.0 0 0 0 5 to 0.0 1%,

C a: 0.0 0 0 0 5 to 0.0 1%,

Y: 0.0 0 0 0 5 to 0.0 1%

The high-strength thin steel sheet having excellent delayed fracture resistance after forming according to any one of claims 1 to 6, characterized by containing one or more of the following.

8. The high-strength thin steel sheet excellent in delayed fracture resistance after forming according to claims 1 to 7 is a hot-rolled steel sheet or a cold-rolled steel sheet, characterized in that it has a delayed fracture resistance after forming processing. High-strength steel sheet with excellent loamy properties.

9. Excellent resistance to delayed fracture after forming as described in claims 1 to 7 A high-strength thin steel sheet having excellent delayed rupture resistance after forming, characterized in that the steel sheet is subjected to a surface treatment with zinc plating.

10. The high-strength thin steel sheet having excellent delayed fracture resistance after forming according to claim 8 or 9, further comprising a film laminating treatment, wherein the delayed fracture resistance after forming is provided. High-strength thin steel sheet with excellent heat resistance.

1 1. A piece having the composition according to any one of claims 1 to 7 is manufactured, hot-rolled at a finishing temperature of Ar ₃ or more, and rolled at 500 to 800 ° C. Then, after pickling, cold rolling is performed at a rolling reduction of 30 to 80%, and then, recrystallization annealing is performed by soaking at a temperature of 600 to 950 ° C. Next, a method for producing a high-strength thin steel sheet having excellent delayed fracture resistance after forming, characterized by subjecting to temper rolling.

12. The method for producing a high-strength thin steel sheet having excellent delayed fracture resistance after forming according to claim 11, wherein after annealing, a temperature range of 1 minute to 1 minute in a temperature range of 200 to 700 ° C. A method for producing a high-strength thin steel sheet having excellent delayed fracture resistance after forming, characterized by holding for 0 hours.

13. A strength part for automobiles characterized by being made of a high-strength thin steel sheet having excellent delayed fracture resistance after forming according to claims 1 to 7.