JP2001131695A - High strength thin steel sheet excellent in secondary working brittleness resistance and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength thin steel sheet having surface properties, non-aging properties and workability applicable to use for an automotive outer plate and also excellent in secondary working brittleness resistance and to provide a method for producing the same. SOLUTION: This high strength thin steel sheet excellent in secondary working brittleness resistance has a composition composed of, by weight, 0.0040 to 0.02% C, <=0.05% Si, 0.7 to 3.0% Mn, 0.04 to 0.15% P, <=0.02% S, 0.01 to 0.1% Al, <=0.004% N, <=0.2% Nb, and the balance substantially Fe with inevitable impurities and also satisfying the following relationship (1): (12/93)×Nb*/C>=1.0...(1), where Nb*=Nb-(93/14)×N.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a high-strength thin steel sheet used for automobiles, household electric appliances, building materials, etc., in particular, a high-strength thin steel sheet having excellent secondary work brittleness resistance, and a high-strength galvanized thin steel sheet. And their manufacturing methods.

[0002]

2. Description of the Related Art In general, high-strength galvanized steel sheets used for automobiles and pressed to suppress not only the high-level surface properties usable as an outer panel, but also the deep drawability and the generation of stretcher strains. Is required to be non-aging. Until now, in order to enhance deep drawability and non-aging properties, we have reduced the amount of C and N as much as possible, and at the same time, added IF such as Ti and Nb to fix IF steel with harmful dissolved C and N as carbonitride. High-strength thin steel sheets based on them have been developed.

However, in IF steel, since grain boundaries are clean and brittle, the part that has undergone large compression processing during shrinkage flange deformation during deep drawing causes brittle fracture in subsequent processing, so-called secondary processing. There is a problem of high sensitivity to brittleness. In addition, the higher the strength of the steel sheet, the lower the grain boundary strength, which tends to make secondary working brittle.

[0004] Therefore, in developing a high-strength thin steel sheet excellent in deep drawing workability, it is very important to improve the resistance to secondary working brittleness. The following various techniques have been proposed in order to increase the resistance to secondary working brittleness.

[0005] Japanese Patent Publication No. 61-32375: Lowering the r value, increasing the yield strength and elongation by adding Nb in an equivalent ratio to Ti and C in an equivalent ratio of N or less to N. Technology to prevent lowering of steel, and at the same time, to leave a part of solid solution C at the grain boundaries to increase the resistance to secondary working brittleness (hereinafter referred to as the conventional technology)
1).

[0006] Japanese Patent Application Laid-Open No. HEI 5-128845: A technique for restricting the lower limit of the amount of C, positively adding Mn and Cr, and increasing the amount of solute C to increase the resistance to secondary working embrittlement (hereinafter referred to as conventional technology). Technology
2).

Japanese Unexamined Patent Publication No. Hei 5-70836: The lower limit of the amount of C fixed by Ti and Nb is secured to secure the amount of carbides that suppress the growth of crystal grains, and the strength is enhanced by the refinement of crystal grains. -Technology to increase the brittleness resistance in secondary processing by controlling the upper limit of Si and P contents and adding B at the same time as obtaining ductility balance (hereinafter, conventional technology 3).

Japanese Patent Application Laid-Open No. Hei 2-75837: r value is increased by reducing the amount of C as much as possible and fixing with Ti, and fine NbC is precipitated by adding Nb to form a saw-like crystal grain boundary. Technology to improve secondary working brittleness (hereinafter, conventional technology 4).

[0009]

However, the prior arts 1 to 4 have the following problems.

Prior arts 1 and 2: In both cases, since solid solution C remains to enhance secondary work brittleness resistance, there is a concern about aging problems when the temperature is maintained for a long time in a relatively high temperature environment such as in summer. You. In addition, in the prior art 1, the temperature is 100 ° C for 1 hour.
The aging resistance is evaluated by the accelerated test of the above, but when evaluated by a long-term aging test over several months at room temperature, when the added amount of Ti or Nb is equal to or less than the equivalent ratio to C and N, the above Yield point elongation that causes stretcher strain even if it is determined that there is no problem in the acceleration test
(YPEl) is often observed.

Prior Art 3: The secondary work brittleness is enhanced by the addition of B. However, B segregates at the grain boundaries and has a favorable texture to suppress the crystal rotation during cold working and obtain a high r value. Inhibits development and deteriorates deep drawability.

Prior art 4: Addition of Nb causes the grain boundaries to be saw-shaped, increasing the resistance to secondary working embrittlement. However, the saw-shaped grain boundaries increase the binding force of the grain boundaries and concentrate the deformation in the crystal grains to increase ductility. descend. Furthermore, in order to increase the r value, C, N, the addition of Ti at an equivalent ratio or more to S (in the example, more than 0.03% is added), striped after hot-dip galvanizing. Ti
Surface irregularities called marks occur and cannot be used for applications requiring surface properties such as automobile outer panels.

The present invention has been made to solve such a problem, and has excellent surface properties, non-aging property, workability, and excellent secondary work embrittlement resistance applicable to automotive outer panel applications. It is an object of the present invention to provide a high-strength thin steel sheet, a high-strength galvanized thin steel sheet, and a method for producing them.

[0014]

Means for Solving the Problems The above-mentioned problems are expressed in wt% and C:
0.0040-0.02%, Si: 0.05% or less, Mn: 0.7-3.0%, P: 0.04
~ 0.15%, S: 0.02% or less, Al: 0.01 to 0.1%, N: 0.004% or less, Nb: 0.2% or less, the balance substantially consisting of Fe and unavoidable impurities, and the following formula (1) The problem is solved by a high-strength thin steel sheet with satisfactory secondary work brittleness resistance.

(12/93) × Nb * / C ≧ 1.0 (1) However, Nb * = Nb− (93/14) × N High-strength (for example, 440 MPa class) thin steel sheet is manufactured based on IF steel. For this purpose, a large amount of a solid solution strengthening element is usually added to secure the strength. When a large amount of Si or P is added as a solid solution strengthening element, the surface properties are remarkably deteriorated, so that it cannot be applied to automotive outer panels. Further, when a large amount of Mn is added, the α / γ transformation point is reduced, so that the annealing temperature range is restricted and the workability is deteriorated. Furthermore, as described above, when a grain boundary strengthening element such as B is added from the viewpoint of secondary working brittleness resistance, it is disadvantageous for workability such as non-aging property and deep drawability.

Accordingly, the present inventors have determined that there is basically a limit in simultaneously satisfying the surface properties, non-ageing properties, workability, and secondary work brittleness resistance of the conventional IF steel, and have determined that the conventional technology is not sufficient. As a result of examining the technology for improving the resistance to secondary working brittleness without using it, it was found that if C, Nb content and Nb / C are controlled within specific ranges, a high-strength thin steel sheet satisfying the above characteristics simultaneously can be obtained. I found it.

The details will be described below.

C: In order to ensure a tensile strength of 340 MPa or more, it is necessary to add 0.0040 wt% or more, but if it exceeds 0.02 wt%, the ductility is significantly reduced. Therefore, 0.0040-0.02
wt%. In order to properly control the form and dispersion state of the precipitate, and to bring out better moldability and more preferable overall characteristics, the amount of C added is specified in the range of 0.0050 to 0.0080 wt%, more preferably 0.0050 to 0.0074 wt%. Is preferred.

C is an important element in the present invention, and the above characteristics change depending on the ratio of Nb / C (atomic equivalent ratio). Therefore, it is necessary to manage Nb / C as described later.

Si: An element effective for securing strength,
If it is added in excess of 0.05 wt%, the adhesion of the galvanized coating will be degraded.

Mn: An element effective for precipitating S in steel as MnS to prevent hot cracking of a slab and to increase strength without deteriorating galvanized adhesion. In order to ensure the specified tensile strength, it is necessary to add 0.7 wt% or more.However, exceeding 3.0 wt% not only causes a significant rise in slab cost, but also lowers the α / γ transformation temperature, so the annealing temperature The range is limited, and the workability also deteriorates. Therefore, it is set to 0.7 to 3.0 wt%.

P: It is necessary to add 0.04 wt% or more to secure the strength. However, if added in excess of 0.15 wt%, the adhesion of zinc plating is deteriorated.

S: Reduces hot workability and increases slab susceptibility to hot cracking. Further, since workability is deteriorated due to precipitation of fine MnS, the content is set to 0.02 wt% or less.

Al: N in steel is precipitated as AlN, and is added so as not to leave solid solution N as much as possible. If the content is less than 0.01 wt%, such effects are not sufficient, and if the content exceeds 0.1 wt%, the effect corresponding to the added amount cannot be obtained.

N: Precipitates and renders harmless as AlN, but the content is made 0.004 wt% or less so that the lower limit amount of Al is rendered as harmless as possible.

Nb: An important element in the present invention together with C. As described below, it fixes solid solution C, refines crystal grains, and improves secondary work brittleness resistance, aging resistance and workability. Contribute greatly. From the economic point of view, its content is
0.2 wt% or less, desirably Nb ≦ 0.140 wt%. Also, in order to properly control the form and dispersion state of the precipitates and to further improve the secondary workability, it is preferable to set Nb> 0.035 wt%, and further improve the secondary workability and improve the overall performance. For further improvement, it is desirable that Nb ≧ 0.080 wt%.

Slabs of various component systems are manufactured, hot-rolled, pickled, cold-rolled, annealed at 830 ° C., and reduced at a rate of 0.5
% Temper rolling to measure the r value as an index of deep drawability, the recovery of YPEl after an accelerated test at 100 ° C for 1 hour, and the transition temperature for secondary work embrittlement to evaluate non-aging properties Was done. Here, the secondary processing embrittlement transition temperature is a blank having a diameter of 105 mm is punched from a steel sheet, deep-drawn in a cup shape, and the processed cup is immersed in a refrigerant such as ethyl alcohol at various temperatures and then conical. This is the temperature at which the cup breaks while expanding the end of the cup with a punch, and shifts from ductile fracture to brittle fracture determined from fracture surface observation.

FIG. 1 shows the relationship between (12/93) × Nb * / C and the r value. When (12/93) × Nb * / C ≧ 1.0, a high r value of 1.75 or more can be obtained, and it can be seen that excellent workability is exhibited.

FIG. 2 shows the relationship between (12/93) × Nb * / C and YPEl. If (12/93) × Nb * / C ≧ 1.0, no recovery of YPEl was observed, indicating that excellent non-aging properties are exhibited.

FIG. 3 shows the relationship between the tensile strength TS and the secondary working embrittlement transition temperature. As is clear from comparison at the same TS level, it is understood that steel satisfying the component system of the present invention exhibits superior secondary work brittleness resistance compared to the conventional steel.

From the above results, if the component amount is within the range of the present invention and the above-mentioned formula (1) is satisfied, it has non-aging property and workability applicable to automotive outer panels, and A high-strength thin steel sheet excellent in secondary work brittleness resistance is obtained. In order to control the morphology and dispersion state of the precipitates and to secure excellent secondary work brittleness resistance, it is desirable to regulate (12/93) × Nb * / C in the range of 1.3 to 2.2.

The following three points can be considered as reasons why excellent secondary work brittleness is obtained by the present invention. (1) Due to the precipitation of NbC, the grain size of the annealed sheet is refined, so that the toughness is improved. (2) According to the electron microscope observation, the size and dispersion form of precipitated NbC are optimized by setting (12/93) × Nb * / C ≧ 1.0, that is, fine NbC is uniform in the grains. A microstructure that is thought to be a so-called precipitate depletion zone (PFZ) is formed near the grain boundaries and has very few precipitates near the grain boundaries. Is suppressed. (3) By increasing the n value in the low strain region of 1% to 10%, the amount of distortion at the punch bottom contact part during drawing increases, and the amount of inflow during deep drawing decreases, resulting in shrinkage of the flange. The degree of compression processing in is reduced.

The effect of the present invention can be achieved by the above-described limitation of the amount of elements. Further, in order to improve the quality and the resistance to secondary working embrittlement, Ti and B are replaced by Ti ≦ 0.019 wt% and B: 0.000
It can be added in the range of 1 to 0.002 wt%.

Formability is improved by forming Ti: carbonitride and making the structure of the hot-rolled sheet finer. However, if added in excess of 0.05 wt%, the precipitates become coarse and sufficient effects cannot be obtained. More desirably, particularly from the viewpoint of the surface properties of the hot-dip galvanizing, the upper limit is set to less than 0.02 wt%, and the lower limit is set to 0.005 w
It is desirable to be t%.

B: added to strengthen crystal grain boundaries and improve secondary work brittleness resistance, but if added in excess of 0.002 wt%, formability is greatly reduced, so the upper limit is 0.002 wt%. And In the steel of the present invention, the crystal grains are refined and exhibit extremely excellent secondary work brittleness resistance.Therefore, in order to minimize the decrease in formability, the amount of B added is preferably 0.0001 to 0.001 wt%.
It is preferable to regulate to within the range.

The high-strength thin steel sheet of the present invention has the following formula:
The solid solution C and N are completely fixed by (1), so their BH (bake hardenability) is less than 2 kgf / mm2, even if they are held for a long time in a relatively high temperature environment such as summer. However, aging does not matter. Furthermore, it has excellent workability of the welded part, and can respond to new technologies such as tailored blanks.

The high-strength thin steel sheet of the present invention is a step of hot-rolling a steel slab having the above components at a finishing temperature not lower than the Ar3 transformation point, and winding the steel sheet after hot rolling at 500 to 700 ° C. It can be manufactured by a method for manufacturing a high-strength thin steel sheet having a step and a step of annealing the rolled steel sheet after cold rolling. Further, when a zinc-based plating treatment is applied to the annealed steel sheet, a high-strength galvanized thin steel sheet having excellent secondary work brittleness resistance can be obtained. As the zinc-based plating, hot-dip galvanizing, electro-galvanizing, or the like can be applied.

The reason for hot rolling at a finishing temperature higher than the Ar3 transformation point is that rolling at a temperature lower than the Ar3 transformation point deteriorates the workability of the final product. The reason for winding at 500 to 700 ° C is that the temperature must be 500 ° C or higher in order to sufficiently precipitate NbC, and 700 ° C or lower in order to prevent indentation flaws due to scale peeling of the steel sheet surface.

Here, the hot rolling of the slab can be carried out after heating in a reheating furnace or directly without heating. The conditions of the cold rolling, annealing, and galvanizing are not particularly limited, but the desired effects can be obtained by ordinary conditions.

[0040]

EXAMPLE After melting steel No. 1 to No. 20 shown in Table 1,
A slab was manufactured by continuous casting. 1200 ℃ this slab
After finishing, finishing temperature 890 ℃ ~ 940 ℃, winding temperature 600 ℃ ~ 660
After hot rolling at 50 ° C. to form a hot-rolled steel sheet, and after pickling, cold rolling of 50 to 85% was performed, followed by continuous annealing, continuous annealing and galvanizing (annealing temperature 800 ° C. to 840 ° C.). In continuous annealing and hot dip galvanizing, hot dip galvanizing was performed at 460 ° C. after annealing, and alloying of the plated layer was immediately performed at 500 ° C. in an in-line alloying furnace. Thereafter, temper rolling at a reduction of 0.7% was performed. Then, the mechanical properties and surface properties of these steel sheets were investigated. Further, a vertical cracking test was performed by the above-described method, and the secondary working embrittlement transition temperature Tc was evaluated.

Table 2 shows the results.

The steel sheets Nos. 1 to 10 of the present invention are all non-aged, have excellent surface properties, and have extremely excellent secondary working embrittlement transition temperature and temperature as compared with Comparative Examples having the same strength level. It shows very good mechanical test values.

Further, the steel sheet of the present invention, as originally intended,
It is a high-strength galvanized steel sheet with high surface properties applicable to automotive outer panel applications, non-aging, excellent workability, and excellent secondary work brittleness resistance. I have.

On the other hand, the comparative steel sheets Nos. 11 to 20 exhibited at least one of the mechanical test values, the non-aging property, the transition temperature for secondary working embrittlement, and the surface properties, which were higher than those of the steel sheets of the present invention. Inferior. For example, with respect to Nos. 14, 15, and 17 to 20, the amounts of Si and Ti added for securing the strength are high, so that the surface properties of the hot-dip galvanized are remarkably inferior. All comparative steel sheets except No. 12, 16, 19 have high secondary working embrittlement transition temperature,
When compared at the same strength level, the comparative steel sheet has a lower r-value and elongation and shows a higher YP than the steel sheet of the present invention.

[0045]

【table 1】

[0046]

[Table 2]

[0047]

Since the present invention is constructed as described above, it has a surface property, non-ageing property and workability applicable to automotive outer panel applications and a high secondary workability brittle resistance. A high-strength thin steel sheet and a method for producing the same can be provided.

The high-strength thin steel sheet of the present invention can be widely applied to household electric appliances, building materials and the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between (12/93) × Nb * / C and an r value.

FIG. 2 is a diagram showing a relationship between (12/93) × Nb * / C and YPEl.

FIG. 3 is a diagram illustrating a relationship between a tensile strength TS and a secondary working embrittlement transition temperature.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Seiji Nakamura 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Masahiko Sekiguchi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Kokan Co., Ltd. (72) Inventor Akira Miyamoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Nihon Kokan Co., Ltd. F term (reference) 4K037 EA01 EA02 EA04 EA05 EA15 EA16 EA18 EA19 EA23 EA25 EA27 EA31 FA03 FC04 FC07 FE01 FE02 FE03 FH01 FJ05 FM02 GA05 GA07 HA02 HA05 JA06

Claims (5)

[Claims] (1) In wt%, C: 0.0040 to 0.02%, Si: 0.05% or less, M
n: 0.7 to 3.0%, P: 0.04 to 0.15%, S: 0.02% or less, Al: 0.01 to
0.1%, N: 0.004% or less, Nb: 0.2% or less, balance substantially Fe
And a high-strength thin steel sheet comprising inevitable impurities and excellent in secondary work brittleness resistance satisfying the following formula (1). (12/93) × Nb * / C ≧ 1.0… (1) where Nb * = Nb− (93/14) × N 2. The high-strength thin steel sheet according to claim 1, further comprising 0.05 wt% or less of Ti. 3. The high-strength steel sheet according to claim 1, further comprising 0.002 wt% or less of B. A step of hot rolling a steel slab having the component according to any one of claims 1 to 3 at a finishing temperature equal to or higher than the Ar3 transformation point; and A method for producing a high-strength thin steel sheet having excellent secondary working brittleness resistance, comprising: a step of winding at 500 to 700 ° C; and a step of annealing the rolled steel sheet after cold rolling. A step of hot-rolling a steel slab having the composition according to any one of claims 1 to 3 at a finishing temperature equal to or higher than the Ar3 transformation point, A step of winding at 500 to 700 ° C., a step of annealing the rolled steel sheet after cold rolling, and a step of subjecting the annealed steel sheet to a zinc-based plating treatment, which has excellent secondary work brittleness resistance. A method for manufacturing high-strength galvanized thin steel sheets.