JPH0673494A - Steel sheet for working excellent in dent resistance and its production - Google Patents

Abstract

PURPOSE:To produce a steel sheet for working excellent in dent resistance and face strain resistance by subjecting a steel slab constituted of a surface layer part having a specified compsn. contg. Ti or Nb and an inner part having a specified compsn. to specified hot rolling and cold rolling and thereafter executing specified box annealing. CONSTITUTION:The surface layer of the steel contains, by weight, 0.03 to 0.20% C, <=1.0% Si, 0.5 to 3% Mn, <=0.06% P, <=0.02% S, <=0.1% Al, <=0.008% N and 0.02 to 0.1% Ti or 0.01 to 0.08% Nb as well as 20C+10Si+5Mn+80P+300 (Ti+Nb)>=19. The other inner side contains 0.01 to 0.08% C, <=0.5% Si and Mn, <=0.03% P and S, 0.02 to 0.1% Al and 0.002 to 0.008% N. After being heated to >=1200 deg.C or directly, the steel slab having the three-layered structure is subjected to hot rolling at >=1110 deg.C starting temp., and it is finished at the Ar3 point or above. This hot rolled sheet is coiled at 500 to 650 deg.C. After that, at the time of subjecting the steel sheet to cold rolling and box annealing, it is heated to >=650 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mainly applied to automobile parts to be pressed, and has a tensile strength of 30 kgf / mm ² or more and is excellent in dent resistance, surface strain resistance and workability. And its manufacturing method.

[0002]

2. Description of the Related Art In recent years, steel sheets have been required to have high strength from the viewpoint of weight reduction and safety improvement of automobiles. In particular, since automobile outer panel panels are required to have dent resistance, there is a strong demand for higher strength steel sheets for outer panel. However, the increase in strength is accompanied by a decrease in formability, and particularly in the case of increasing the strength of the outer plate, there arises a problem that surface strain occurs due to an increase in yield strength.

In order to overcome such problems, so-called BH steel sheets have been developed, which have a relatively low yield strength during working, but the yield strength increases after baking for coating.

For example, Ti, Nb, etc. may be added to ultra-low carbon steel to limit the amount of solid solution C (Japanese Patent Laid-Open No. 59-38337).
JP-A-59-31827), a method of controlling the formation of precipitates by adjusting the amounts of N and S (JP-A-61-267).
57, JP-A-62-7822) and the like are known. These steel sheets have excellent surface strain resistance but insufficient dent resistance.

High-strength BH steel sheets have been developed in order to secure dent resistance, but many rear fenders and quarters have particularly severe formability due to their shapes, and use of high-strength BH steel sheets for these parts. It is very limited, and the actual situation is that high strength has not been solved.

It is assumed that an attempt is made to secure both dent resistance and surface distortion resistance / workability with a clad steel plate.
Although disclosed in Japanese Patent No. 133630, this method uses only solid solution strengthening to secure the strength of the surface layer portion.
Addition of a large amount of n and P is essential, and measures against deterioration of spot weldability have not been taken sufficiently.

[0007]

The present invention has a surface layer (both sides).
Is a high-strength, soft inner layer, so-called clad steel plate that solves the problems of securing dent resistance and surface strain resistance / workability. It is to provide a steel sheet that achieves both dent resistance and surface strain resistance / workability, which are difficult to achieve with a normal steel sheet, and a method for manufacturing the same by using the three-layer structure.

[0008]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the problems by hot and cold rolling a clad slab as a component of a specific surface layer and an inner layer and subjecting it to a specific annealing. Is as follows.

(1) The components of the surface layer are C: 0.
03-0.20%, Si: 1.0% or less, Mn: 0.5
~ 3%, P: 0.06% or less, S: 0.02% or less, A
1: 0.1% or less, N: 0.008% or less, and Ti:
0.02 to 0.1%, Nb: 0.01 to 0.08%, containing 1 or more, and 20 × C (%) + 10 × Si.
(%) + 5 × Mn (%) + 80 × P (%) + (Ti
(%) + Nb (%)) × 300 ≧ 19, balance F
e and unavoidable impurities, and the other interiors are in mass ratio, C: 0.01 to 0.08%, Si: 0.5% or less, Mn: 0.5% or less, P: 0.03% or less. , S:
0.03% or less, Al: 0.02 to 0.1%, N: 0.
A steel sheet containing 002 to 0.008% and having a three-layer structure composed of the balance Fe and unavoidable impurities and having excellent dent resistance, surface strain resistance, and workability.

(2) A plated steel sheet which is electroplated on both sides or one side of the steel sheet and which is excellent in dent resistance, surface strain resistance and workability.

(3) The components of the surface layer are C: 0.
03-0.20%, Si: 1.0% or less, Mn: 0.5
~ 3%, P: 0.06% or less, S: 0.02% or less, A
1: 0.1% or less, N: 0.008% or less, and Ti:
0.02 to 0.1%, Nb: 0.01 to 0.08%, at least one kind is contained, and 20 × C (%) + 10 × Si.
(%) + 5 × Mn (%) + 80 × P (%) + (Ti
(%) + Nb (%)) × 300 ≧ 19, balance F
e and unavoidable impurities, and the other inside is C: 0.01 to 0.08%, Si: 0.5% or less in mass ratio,
Mn: 0.5% or less, P: 0.03% or less, S: 0.0
3% or less, Al: 0.02-0.1%, N: 0.002
After making into a slab a steel having a three-layered structure containing 0.008% and the balance Fe and unavoidable impurities, when hot-rolling, when reheating, after heating to 1200 ° C or higher, rolling and reheating When directly hot rolling without
Rolling starts at 1100 ° C or higher, and hot rolling finish temperature is set to A
r ₃ points or more, coiled at 500 to 650 ° C., then cold rolled to perform box annealing, and heating to 650 ° C. or higher is excellent in dent resistance, surface strain resistance, and workability. Steel plate manufacturing method.

(4) A method for producing a plated steel sheet which is excellent in dent resistance, surface strain resistance and workability, which comprises electroplating both sides or one side of the steel sheet.

[0013]

The reason for limiting each constituent element of the present invention will be described in detail. First, the reasons for limiting the chemical composition of the surface layer will be described in detail. C: C is one of the strengthening elements, but if it exceeds 0.2%, the spot weldability deteriorates, so the upper limit of C is made 0.2%. Further, in the present invention, precipitation strengthening due to the formation of TiC, Nb (C, N) and the like and grain refinement of the structure are used as strengthening means. From this meaning, C must be at least 0.03%.

Si: Si is a strengthening element, so Si may be added in order to secure the strength of the surface layer portion, but an excessive addition causes deterioration of workability and weldability, so the upper limit is made 1%. The lower limit is not particularly specified and may be lowered to 0.005%.

Like Mn: Si, Mn is a strengthening element, and the lower limit is made 0.5% from the viewpoint of ensuring strength. The upper limit is 3% from the viewpoint of workability and weldability.

P: Since P is a strengthening element, it may be added to secure the strength of the surface layer portion, but it impairs secondary workability and weldability, so the upper limit is made 0.06%. The lower limit is not particularly specified and may be lowered to about 0.001%.

S: S deteriorates workability and weldability and promotes hot cracking, so the lower the better, the upper limit is 0.02%. It is preferably 0.015% or less.

Al: Al is used as a deoxidizing agent. Al
If the content is too large, alumina-based inclusions increase and the workability of the steel deteriorates, so the upper limit was made 0.1%. In the present steel, even if Si or Ti is used as a deoxidizing agent, it does not violate the gist of the present invention, so the lower limit of Al does not need to be specified. A desirable range is 0.002 to 0.08%.

N: N is 0.008% or less from the viewpoint of workability. In the present invention, a precipitation strengthening element is added to secure the strength of the surface layer. Therefore, 0.02 to 0.1% T
One or more kinds of Nb of i to 0.01 to 0.08% are added. If it is less than the lower limit, precipitation strengthening is insufficient. Addition exceeding the upper limit deteriorates workability and is not economical.

Further, in order to secure the strength of the surface layer, 20 × C
(%) + 10 × Si (%) + 5 × Mn (%) + 80 × P
(%) + (Ti (%) + Nb (%)) × 300 ≧ 19 must be satisfied. If it is less than the lower limit, the dent resistance is insufficient.

Although addition of Ca, REM, and Cr is not an essential condition of the present invention, Ca and REM are spheroidized inclusions,
That is, since it contributes to the reduction of adverse effects of inclusions on the workability and Cr contributes to the prevention of C contamination during box annealing, the selective addition is not against the gist of the present invention. However, the addition of these elements causes an increase in manufacturing cost, so the upper limit of the total addition amount of these elements is 0.5%.

Next, the reasons for limiting the chemical composition of the inner layer will be described in detail. The inner layer is basically soft to ensure workability and surface strain resistance. C: C is a strengthening element and deteriorates workability, so the upper limit is made 0.08%. The lower limit is 0.01% in consideration of deterioration of secondary workability and economy.

Si: Si is also a strengthening element and deteriorates workability, so the upper limit is made 0.5%. The lower limit need not be specified in particular and may be as low as about 0.005%.

Since Mn: C and Si deteriorate the workability similarly to Si, the upper limit is made 0.5%. In addition, since extreme reduction of Mn promotes hot cracking and is not economical, Mn is preferably set to 0.05% or more.

P: The upper limit is 0.03 because it promotes center segregation and deteriorates ductility and weldability.
%.

S: A-type inclusions are increased and workability is deteriorated. Also, since it promotes hot cracking, the upper limit is 0.0
3%. It is preferably 0.02% or less.

Al: AlN as a deoxidizer and during annealing
Required for precipitation. If it is less than 0.02%, its effect is not obtained, and if it exceeds 0.1%, alumina-based inclusions increase,
Deteriorates the ductility of steel.

N: N has an upper limit of 0.00 from the viewpoint of workability.
8% or less. On the other hand, in box annealing, it is necessary to precipitate AlN during annealing in order to improve the r value, so the lower limit of N is made 0.002%.

The present steel is thus cast into a clad slab having an inner layer and an outer layer and then hot rolled. The molten slab may be hot-rolled after being inserted into a heating furnace and heated, or may be directly hot-rolled without being inserted into the heating furnace. When inserting into a heating furnace and heating, TiC, Nb
It is necessary to heat to 1200 ° C. or higher for solutionizing (C, N) and AlN.

When hot rolling is carried out directly without inserting it into a heating furnace, a large amount of TiC, Nb until the finish rolling is performed.
In order to prevent precipitation of (C, N) etc., the rolling start temperature is set to 11
It is necessary to set it to 00 ° C or higher.

If the finish rolling finish temperature is less than Ar ₃ points, the workability of the product is deteriorated. Therefore, the finish rolling finish temperature is set to Ar ₃ point or higher. Cooling and winding after finishing rolling may be carried out by usual methods. Winding temperature is TiC or N
The temperature is set to 500 to 650 ° C. to precipitate b (C, N).

Below the lower limit, TiC or Nb (C,
The precipitation of N) is insufficient and it becomes difficult to secure the strength of the surface layer. On the other hand, when the upper limit is exceeded, precipitated TiC or Nb
Since (C, N) is coarsened, it becomes difficult to secure the strength of the surface layer, and the precipitation of AlN is increased, so that the r value is lowered.

The steel is hot-rolled, pickled and cold-rolled.
Cold rolling reduction is 60-85% to secure workability of inner layer
Is desirable.

Next, box annealing is performed. Here, workability is secured through recrystallization and grain growth. For this reason, the heating temperature for box annealing is set to 650 ° C. or higher. On the other hand, the heating temperature is 800 ℃
If it exceeds, the precipitation strengthening effect of the surface layer portion will decrease.

The box annealing operation may be carried out by a conventional method, for example, the temperature rising rate may be 5 to 100 ° C./hour and the heating and holding time may be about 1 to 20 hours. The temper rolling after annealing may be performed under normal conditions, for example, the pressure regulation rate of 0.5 to 2%.

In order to impart corrosion resistance to the steel sheet, zinc or the like may be electroplated. This plating may be applied to only one side or both sides.

When the clad slab of the present invention is produced by casting, for example, the two-line IN method (IN: immersion nozzle) disclosed in JP-B-44-27361 can be used. When using this method, divide the tundish into two rooms, one from each room.
A total of two dipping nozzles are inserted into the mold for casting.
Molten steels with different components are separately poured into each chamber in the tundish from a ladle.

In addition, it is possible to manufacture by arranging the molds in two stages, casting the inner layer part with the upper mold, and then casting the surface layer part with the lower mold. The casting may be either a continuous casting method or a ingot making method.

As for the cladding rate, it is preferable to control the thickness ratio of the inner layer / surface layer to 2 to 10. Here, the surface layer means a portion in which the front and back sides are combined. When it is less than 2, the ratio of the inner layer is small and the workability as a whole is insufficient. On the other hand, if it exceeds 10, the surface layer is too thin and it becomes difficult to secure the dent resistance.

[0040]

EXAMPLE The surface layer and the inner layer were adjusted to the chemical components shown in Table 1 (A to F) and melted by the two-in method.

[0041]

[Table 1]

A to D are components of the present invention. E is Ti and T (= 20 × C (%) + 10 × Si (%) + 5 on the surface layer)
X Mn (%) + 80 x P (%) + (Ti (%) + Nb
(%)) × 300), but F is C and P of the inner layer are outside the scope of the present invention. Incidentally, G and H are comparative single-layer slabs manufactured by a normal continuous casting method.

After hot rolling these slabs, descaling was performed in the pickling line, then cold rolling and box annealing. Some parts were galvanized by passing through an electroplating line. Table 2 shows conditions such as hot rolling, annealing, and coating weight. The cold rolling reduction was 75 to 80%, and the skin pass elongation after annealing was 1 to 1.3%. Table 2 also shows the tensile properties, dent resistance, and surface strain resistance of the products.

[0044]

[Table 2]

For the tensile test, a No. 5 test piece according to JIS Z2201 was used. The average r value is 0 with respect to the rolling direction.
In-plane average of r values in each direction of °, 45 °, 90 ° (= (0
R value in ° direction + r value in 90 ° direction + r value in 45 ° direction x 2) /
It is represented by 4).

The dent resistance was measured by subjecting a steel sheet to a cylindrical flat-bottomed bulge having a diameter of 100 mm, and 2.
After applying a 5% pre-strain, a steel indenter with a radius of 25 mm is
A method of measuring the dent amount by a three-point method with a span of 40 mm by pressing with a load of kgf to give plastic deformation (dent). The surface strain resistance was evaluated by visually forming a 600 × 600 mm steel plate into a kamaboko shape (with embossing) with a wrinkle holding force of 60 tons and visually evaluating the periphery of the embossing.

In Table 2, No. Nos. 1, 2, 4, 6, and 8 are steel plates manufactured according to the method of the present invention, and are superior in workability (elongation / r value), dent resistance, and surface strain resistance to the comparative materials. FIG. 1 compares the dent resistance of the steel of the present invention in Table 2 with that of a normal single-layer plate (Nos. 12 and 13), with the horizontal axis representing tensile strength. The thicker the plate thickness, the smaller the dent amount as the strength is higher, but the steel of the present invention has a smaller dent amount than the single-layer plate having the same strength even if the plate thickness is thin.

This means that the present invention can reduce the plate thickness from the viewpoint of dent resistance. Regarding the spot weldability of the steel of the present invention, the proper current range was wider and better than that of the single-layer plates (No. 12 and 13) having the same strength.

Other steel plates will be described. No. Three
No. 5 has a low r value because the winding temperature is too high or the heating temperature is too low, and the dent resistance is inferior to the steel of the present invention because the surface layer strength is insufficient. No. In Nos. 7 and 9, the r value is low because the finishing temperature or the annealing temperature is low. No. 10 is the surface T
Since i and T are low, the surface strength is insufficient and the dent resistance is poor.

No. No. 11 has a low r value due to high C and P in the inner layer, and also has poor spot weldability. Furthermore, since the inner layer is too hard, the strength difference between the inner layer and the surface layer is small, and the effect of multi-layering is weakened. It has the same level of dent resistance as a plate,
The surface strain resistance is also poor.

[0051]

EFFECTS OF THE INVENTION According to the present invention, it becomes possible to achieve both high strength, dent resistance, surface strain resistance and workability, which are increasingly required for automobile parts and the like. This leads to a reduction in fuel consumption (preventing waste of natural fuel) by reducing the thickness of the steel plate, and an improvement in safety due to an increase in collision strength, which has great social significance.

[Brief description of drawings]

FIG. 1 is a chart showing dent resistance (dent amount) and tensile strength.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] December 14, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0041

[Correction method] Change

[Correction content]

[0041]

[Table 1]

Claims (4)

[Claims] 1. The mass ratio of the components of the surface layer portion is C: 0.03 to 0.20%, Si: 1.0% or less, Mn: 0.5 to 3%, P: 0.06% or less, S : 0.02% or less, Al: 0.1% or less, N: 0.008% or less, and Ti: 0.02 to 0.1%, Nb: 0.01 to 0.08.
One or more of%, and 20 x C (%) + 10 x Si (%) + 5 x Mn (%) + 80 x P (%) + (Ti (%)
+ Nb (%)) × 300 ≧ 19, consisting of balance Fe and unavoidable impurities, and the other inside by mass ratio C: 0.01 to 0.08%, Si: 0.5% or less, Mn: 0.5 % Or less, P: 0.03% or less, S: 0.03% or less, Al: 0.02-0.1%, N: 0.002-0.008%, balance Fe and unavoidable impurities 3 A processing steel sheet with a layered structure and excellent dent resistance. 2. A galvanized steel sheet for processing, which has excellent dent resistance, wherein both sides or one side of the steel sheet is electroplated according to claim 1. 3. The mass ratio of the components of the surface layer portion is C: 0.03 to 0.20%, Si: 1.0% or less, Mn: 0.5 to 3%, P: 0.06% or less, S : 0.02% or less, Al: 0.1% or less, N: 0.008% or less, and Ti: 0.02 to 0.1%, Nb: 0.01 to 0.08.
One or more of%, and 20 x C (%) + 10 x Si (%) + 5 x Mn (%) + 80 x P (%) + (Ti (%)
+ Nb (%)) × 300 ≧ 19, consisting of balance Fe and unavoidable impurities, and the other inside by mass ratio C: 0.01 to 0.08%, Si: 0.5% or less, Mn: 0.5 % Or less, P: 0.03% or less, S: 0.03% or less, Al: 0.02-0.1%, N: 0.002-0.008%, balance Fe and unavoidable impurities 3 When the steel having a layered structure is made into a slab and hot-rolled, when it is reheated, it is heated to 1200 ° C or more and then rolled, and when it is directly hot-rolled without reheating, it is rolled at 1100 ° C or more. Start and set the hot rolling end temperature to Ar ₃ point or higher, and 500
A method for producing a steel sheet for working having excellent dent resistance, which comprises heating at 650 ° C. or higher in winding at −650 ° C., then cold rolling and box annealing. 4. A method for producing a plated steel sheet for processing which is excellent in dent resistance, which comprises electroplating both sides or one side of the steel sheet according to claim 3.