JP2002030381A - Thin steel sheet and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin steel sheet free from the generation of cracking at the time of press working or the like and having good workability and to provide a continuous casting method capable of obtaining the same thin steel sheet. SOLUTION: This thin steel sheet with a thickness of <=8 mm has a chemical composition on the average of the total sheet thickness of, by mass, 0.03 to 0.16% C, <=0.2% Si, <=1.4% Mn, <=0.03% P, <=0.02% S and <=0.10% sol.Al, and the balance Fe with impurity elements, and in which the ratios of the C content(C) t on the average and the Mn content(Mn) ton the average respectively in the region within 10% of the total thickness on both sides in the thickness direction from the center of the thickness to the C content (C) o on the average of the total sheet thickness and the Mn content(Mn) o on the average, i.e., (C) t/(C) o and (Mn) t/(Mn) o are respectively <=1.5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet or a cold rolled steel sheet after hot rolling, and further relates to a method for producing the thin steel sheet.

[0002]

2. Description of the Related Art Undercarriage parts of automobiles, structural parts of boilers, and the like are manufactured by using a continuously cast slab as a raw material for a hot-rolled thin steel plate or a hot-rolled steel material for a further cold rolling. It is manufactured by pressing a rolled thin steel plate.

In the vicinity of the center of the thickness of a continuously cast slab having a rectangular cross section, internal defects called center segregation or V segregation may occur. The center segregation is a segregation component such as C, S, P, and Mn that appears in the final solidified portion of the slab, and the V segregation appears near the final solidified portion of the slab.
These segregated components are concentrated in a V-shape. These segregations may cause a decrease in toughness in thick steel plates with a small reduction ratio from the slab, and may cause hydrogen-induced cracking in large diameter steel pipes manufactured by bending and welding from thick plates. Are known.

[0004] On the other hand, in the case of thin steel sheets, these segregations have not been a serious problem. This is because in a thin steel sheet, the reduction ratio from the slab is large, and these segregations are reduced. However, in recent years, there has been an increasing demand for improvement in productivity in continuous casting, and it has been aimed to increase the casting speed, and the tendency of occurrence of these segregations has become remarkable.
Further, in the case of undercarriage parts of automobiles, structural parts of boilers, and the like, more complicated press working has been performed, and a thin steel sheet having more excellent workability has been demanded. Since the press working becomes complicated under the situation where the segregation of the cast slabs is worsened, the occurrence of cracks during the press working is a problem in the thin steel sheet used for such an application.

[0005] The mechanism of generation of these segregations is considered as follows. As the solidification progresses, segregated components are concentrated between dendrite trees, which are solidified structures. The molten steel in which the segregated components are concentrated flows out from between the dendrite trees due to shrinkage of the slab during solidification or swelling of the slab called bulging. The flowed-out molten steel in which the segregation component is concentrated flows toward the solidification completion point of the final solidification part, and solidifies as it is to form a concentrated zone of the segregation component. These concentrated zones are center segregation or V-shaped segregation. As a countermeasure for preventing such segregation, for example, JP-A-7-210382 discloses that a slab including an unsolidified portion is bulged by 20 mm or more, and a thickness corresponding to the bulging amount is reduced by a pair of reduction rolls before solidification is completed. A way to do that has been proposed. This is a method for preventing central segregation and V-shaped segregation by forcibly discharging molten steel, in which segregated components remaining between dendrite trees are concentrated, to the upstream side in the casting direction. However, in this method, a dedicated roll pair is required to reduce the cast slab including the unsolidified portion, and the equipment becomes excessively large.

Japanese Patent Application Laid-Open No. 9-67619 discloses that a steel sheet containing 0.05 to 0.1% by mass of P and having improved corrosion resistance is provided with a center segregation in order to further improve the workability. When hot rolling is performed using the slab subjected to the reduction treatment, the finishing temperature is set to Ar ₃ to (Ar ₃
+ 100 ° C.), a subsequent cooling rate of 50 ° C./s, and a winding temperature of 300 to 500 ° C. have been proposed. As a specific method for reducing the center segregation, it has been proposed to reduce the cast slab including the unsolidified portion by light pressure, electromagnetically stir the unsolidified molten steel, or perform low-temperature casting. However, the light reduction method has an effect when the P content is large, but has little effect with a normal P content. Further, the equipment becomes excessively large in order to provide the electromagnetic stirring device. Furthermore, although there is no specific proposal for the degree of superheat of molten steel in a tundish at the time of low-temperature casting, clogging of the immersion nozzle occurs at low-temperature casting, and continuation of operation tends to be difficult.

[0007] As described above, the problem of cracking during the press working of a thin steel sheet due to the center segregation and the V-shaped segregation of the cast slab is described below. It is desired to obtain a thin steel sheet having good properties and a slab free of center segregation and V-shaped segregation, which is a material for hot rolling thereof, stably at low equipment cost.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a hot-rolled thin steel sheet or a cold-rolled thin steel sheet which does not crack during press working and has good workability. Further, it is another object of the present invention to provide a method for producing these thin steel sheets.

[0009]

SUMMARY OF THE INVENTION The gist of the present invention is a method of manufacturing a thin steel sheet shown in the following (1) and (2) and a method of manufacturing a thin steel sheet shown in the following (3) and (4). (1) The content in the average chemical composition of the entire plate thickness is 0.03 to 0.16% in mass%, Si: 0.2% or less, Mn: 1.4% or less, P: 0. 0.03% or less, S:
0.02% or less, sol. Al: 0.10% or less, the balance being 8 mm in thickness composed of Fe and impurity elements
The following thin steel sheets, which satisfy the following equations (a) and (b).

(C) t / (C) o ≦ 1.5 (B) (Mn) t / (Mn) o ≦ 1.5 (B) where (C) t: thickness Average C content (% by mass) in a region within 10% of the total thickness on both sides in the thickness direction from the center in the thickness direction (C) o: average C content (% by mass) of the entire plate thickness (Mn) ) T: Average Mn content (% by mass) in a region within 10% of the total thickness on both sides in the thickness direction from the center of the thickness (Mn) o: Average Mn content (% by mass) of the entire plate thickness (2) The steel sheet according to the above (1), further comprising, by mass%, one of Nb: 0.2% or less, Ti: 0.2% or less, and V: 0.2% or less. A thin steel sheet containing one or more kinds. (3) In mass%, C: 0.03 to 0.16%, Si:
0.2% or less, Mn: 1.4% or less, P: 0.03% or less, S: 0.02% or less, sol. Al: Steel containing 0.10% or less is melted, the superheat degree of the molten steel in the tundish is 80 ° C or less, the slab cross-section is rectangular, and the thickness is 300.
mm or less, the casting speed is 1.2 m / min to 3 m / min, and the distance between the axes of the slab supporting guide rolls adjacent to each other in the region of the slab until the center of the thickness is completely solidified is 450 mm.
A method for producing a thin steel sheet that is cast under the condition of not more than mm and then rolled into a thin steel sheet. (4) In mass%, C: 0.03 to 0.16%, Si:
0.2% or less, Mn: 1.4% or less, P: 0.03% or less, S: 0.02% or less, sol. Al: 0.10% or less, Nb: 0.2% or less, Ti: 0.
1% or less of 2% or less and V: 0.2% or less
A steel containing at least one seed is melted, the degree of superheat of the molten steel in the tundish is 80 ° C. or less, the cross section of the slab is rectangular and the thickness is 3
00 mm or less, the casting speed is 1.2 m / min to 3 m / min, and the distance between the axes of the guide rolls for supporting slabs adjacent to each other in the slab region until the center of the thickness is completely solidified is 4 mm.
A method for producing a thin steel sheet which is cast under conditions of 50 mm or less and then rolled to obtain a thin steel sheet.

The present inventors have solved the above-mentioned problem by obtaining the following findings and taking measures. The center segregation and the V-shaped segregation occurring near the center of the thickness of the slab are affected by the chemical composition of the steel, the thickness of the slab, the casting speed, and the like. For example, C, S, P, Mn
When the content rate is high, when the slab is thick, or when the casting speed is high, the degree of segregation is high, and these segregation zones spread in the thickness direction of the slab are generated. These segregation zones of the slab remain as segregation zones in the thin steel sheet. The thickness of the segregation zone of a thin steel sheet is affected by the hot rolling conditions such as heating temperature and time during hot rolling, and the heat treatment conditions of the thin steel sheet. It is likely to remain in proportion.

[0012] The ease with which cracks occur when a thin steel sheet obtained by hot rolling a cast slab or a thin steel sheet cold-rolled after hot rolling is caused by the segregation zone with respect to the total thickness of these thin steel sheets. Depends on the ratio of the thicknesses. Furthermore, the ease of occurrence of these cracks is low carbon steel to medium carbon steel thin steel sheets,
It depends on the segregation state of C and Mn existing in the segregation line.

Accordingly, in the thin steel sheet of the present invention, the content in the average chemical composition of the entire sheet thickness is expressed by mass% and C: 0.
03-0.16%, Si: 0.2% or less, Mn: 1.4
%, P: 0.03% or less, S: 0.02% or less, s
ol. Al: 0.10% or less, the balance being a thin steel sheet having a thickness of 8 mm or less composed of Fe and an impurity element,
10% of the total thickness from the center of the thickness to both sides in the thickness direction with respect to the average C content (C) o and the average Mn content (Mn) o of the total thickness of the thin steel sheet. The ratio of the average C content (C) t and the average Mn content (Mn) t in the region within (C) t / (C) o and (M
n) t / (Mn) o is 1.5 or less, respectively. Thereby, when press-working the thin steel plate which hot rolled the slab or the cold rolled thin steel plate after hot rolling, generation | occurrence | production of a crack can be prevented.

In the method for manufacturing a thin steel sheet according to the present invention, first,
In continuous casting, slabs with a thickness of 300 mm or less
Casting at a speed of up to 3 m / min. At this time, the superheat degree of the molten steel in the tundish is set to 80 ° C. or less, and the slab supports adjacent to each other in the slab region until the solidification of the thickness center is completed. Casting is performed under the condition that the distance between the axes of the guide rolls is 450 mm or less.

[0015] The thickness of the slab is 300 mm or less, 1.2
When casting at a speed of up to 3 m / min, the solidified structure near the center of the thickness of the slab has many fine equiaxed crystals, and the molten steel in which the segregation component is concentrated is between the equiaxed crystals. Since they are dispersed and held, center segregation and the like hardly occur. Further, since the degree of superheat of the molten steel in the tundish is appropriately set to 80 ° C. or less, the solidified structure near the center of the thickness of the slab is formed with many fine equiaxed crystals.

Further, when the distance between the axes of the guide rolls for supporting slabs adjacent to each other is set to 450 mm or less in the slab area until the thickness center portion is completely solidified, bulging of the slab can be suppressed. .

FIG. 2 is a schematic diagram showing the distance between the center axes of the guide rolls. The distance 7 between the roll axes is the casting direction 8
Means the distance between the axes of the guide rolls 6 adjacent to each other. In the area of the slab until the center of the thickness is completely solidified, in particular, there are many molten steels with concentrated segregation components, so the effect of preventing the occurrence of center segregation etc. by suppressing bulging of the slab is large.

Since the conditions of the slab thickness, casting speed, degree of superheat of molten steel, and distance between the shaft cores of the guide roll pair are combined to make the conditions within an appropriate range, the slab without generation of center segregation or the like is obtained. Can be obtained. Hot rolled thin steel sheet using such a slab without center segregation, or hot rolled thin steel sheet, and further cold rolled thin steel sheet, cracks occur during press working etc. Hard to do.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION A thin steel sheet according to the present invention and a method for manufacturing the thin steel sheet will be described below. The following% indication means% by mass. First, the chemical composition will be described. C: 0.03 to 0.16% C is an inexpensive and useful element for securing the strength of steel, and its lower limit is 0.03% in order to exhibit its effect.
And If it is contained too much, it is an element that deteriorates the workability of steel, and it is an element that has a small equilibrium distribution coefficient and is easy to segregate. The upper limit is 0.16%. Therefore,
C is set to 0.03 to 0.16%.

Si: 0.2% or less Si is usually added for deoxidation and strengthening of steel.
It is an element that can increase the strength without deteriorating the workability. In order to exhibit the effect, 0.01%
It is desirable to contain the above. However, if it is contained too much, the chemical conversion property of the steel material deteriorates.
2%.

Mn: 1.4% or less Mn is an inexpensive element as an element for increasing the strength, and in order to exert its effect, it is desirable to contain 0.2% or more. However, it is an element that is easily segregated, and the upper limit is 1.4%.

P: not more than 0.03% P is an impurity element. If P is contained in a large amount, it is an element that deteriorates the workability of steel, and is an element that tends to segregate, and the upper limit is made 0.03%. .

S: 0.02% or less S is an impurity element, which is an element that deteriorates the hot brittleness of steel, and its upper limit is made 0.02%.

Sol. Al: 0.10% or less Al is an element effective for deoxidation, and in order to exhibit its effect, it is desirable to contain 0.005% or more. However, when a large amount is contained, surface flaws are liable to be generated on the slab surface and the immersion nozzle is easily clogged during continuous casting, so the upper limit is set to 0.10%.

Ti, Nb and V described below are elements to be added as necessary, and one or two
It is better to add more than one species.

Ti: ≦ 0.2% Ti is an element effective for deoxidizing and fixing C and N. However, if the content exceeds 0.2%, not only does the effect become saturated, but also the workability of the steel deteriorates, so the upper limit is made 0.2%.

Nb: ≦ 0.2% Nb is an element effective for fixing C and N, and improves the aging resistance of steel. If it exceeds 0.2%, the effect is saturated and the workability of the steel deteriorates, so the upper limit is made 0.2%.

V: ≦ 0.2% V is an element effective for deoxidizing and fixing C and N. When the content exceeds 0.2%, the effect is saturated, and the effect of steel is reduced. Since the workability deteriorates, the upper limit is set to 0.2%.

As other elements, elements inevitably contained such as N are permissible as long as they do not substantially affect the properties of the steel.

Next, the thickness of the thin steel sheet and the segregation at the center of the thickness will be described. The upper limit of the thickness of the thin steel plate is 8mm
And Usually, the thickness of the used slab is 200-30.
If the thickness is 0 mm and the thickness of the thin steel sheet exceeds 8 mm, the reduction ratio from the normal slab becomes small, and the toughness of the thin steel sheet is likely to be reduced. Furthermore, the upper limit of the thickness of the thin steel plate is 8 m
The value of m is for product applications that target thin steel plates, such as undercarriage parts of automobiles and structural parts of boilers. Although the lower limit of the thickness of the thin steel sheet is not particularly limited, 0.7 mm is desirable from the viewpoint of product use.

Average C content (C) o of the total thickness of the thin steel sheet
And the average C content (C) t and the average Mn content (Mn) t in a region within 10% of the total thickness on both sides in the thickness direction from the thickness center with respect to the average Mn content (Mn) o. (C) t / (C) o and (Mn) t /
(Mn) o is 1.5 or less.

FIG. 3 shows (C) o, (Mn) o, (C) t
FIG. 3 is a schematic diagram showing (Mn) t. For example,
(C) t is a region 11 within 10% of the total thickness 12 on both sides in the thickness direction from the thickness center 10, that is, a segregation zone 9 which is a region within 20% of the total thickness at the center of the thickness.
Means the average content of C in the above.

C and Mn are elements that are useful for improving the strength of steel, but are elements that have a small equilibrium distribution coefficient and tend to segregate. Therefore, center segregation and V-shaped segregation occur near the center of the slab thickness, and when these segregation remain as segregation zones in the thin steel sheet, C and Mn segregate in the segregation zones. When a thin steel sheet having such a segregation zone is subjected to press working or the like, cracking may occur or stretch flangeability may deteriorate. The ease of occurrence of these cracks, etc. depends on the ratio of the thickness of the segregation zone to the total thickness of these thin steel sheets,
And the segregation state of C and Mn present in the segregation zone.

Specifically, if up to 20% of the total thickness of the thin steel sheet is defined as the segregation zone, the degree of segregation of C and Mn in the segregation zone, that is, the average C or Mn of the total thickness of the thin steel sheet. If the content of C or Mn in the segregation zone with respect to the content exceeds 1.5, cracks and the like are likely to occur.

When C and Mn segregate in the segregation zone as described above, the segregation zone is hardened, and when the thin steel sheet is subjected to shearing by press working or the like, cracks are generated starting from the hardened segregation zone. Or stretch flangeability deteriorates.
In particular, since C forms a carbide, not only the carbide serves as a starting point of the crack, but also the crack easily propagates along the carbide, and the stretch flangeability is remarkably deteriorated.

When a thin steel sheet is obtained by hot rolling a slab to be described later as a raw material, heating conditions such as a heating temperature and a heating time of the slab before rolling, and rolling conditions such as a rolling temperature and a winding temperature are as follows. Heating and rolling can be performed under normal conditions according to the steel. Furthermore, when a steel sheet obtained by hot rolling is used as a material and cold rolling is performed to obtain a thin steel sheet, ordinary methods such as pickling conditions, cold rolling conditions, and heat treatment conditions can be used.

Next, a manufacturing method for obtaining the above-described thin steel sheet will be described. First, when continuously casting a slab, in order to obtain a thin steel sheet having a segregation line as described above, a thickness of 3 mm is required.
A slab of 00 mm or less is cast at a speed of 1.2 to 3 m / min. At this time, the degree of superheat of the molten steel in the tundish is set to 80 ° C. or less, and the thickness of the slab until the solidification of the center of the thickness is completed. In the region, casting is performed under the condition that the distance between the axes of the slab supporting guide rolls adjacent to each other is 450 mm or less.

If the thickness of the slab exceeds 300 mm, center segregation or V-shaped segregation is likely to occur near the center of the thickness. In addition, the size of the continuous casting machine becomes large. Therefore, the thickness of the slab is 300 mm or less.

When the casting speed is less than 1.2 m / min, the occurrence of center segregation or V-shaped segregation in the center of the slab thickness can be suppressed, but the cleanliness of the slab surface layer deteriorates. When the cleanliness of the slab surface layer deteriorates, the surface quality such as the gloss of the surface of the thin steel plate deteriorates. If it exceeds 3 m / min, cracks are likely to occur on the slab surface, and furthermore, center segregation or V-shaped segregation tends to occur.

If the degree of superheat of the molten steel in the tundish exceeds 80 ° C., the solidified structure of the slab tends to be columnar, and center segregation or V-shaped segregation tends to occur.

When the degree of superheat of the molten steel is 80 ° C. or less, the solidified structure of the slab is formed with many fine equiaxed crystals. When the vicinity of the center of the slab is filled with fine equiaxed crystals, the molten steel in which the segregation component is concentrated is dispersed and held between the equiaxed crystals, so that center segregation and the like hardly occur. However, the degree of superheat of the molten steel in the tundish is preferably 10 ° C. or more. Below 10 ° C,
The immersion nozzle is easily clogged, the surface of the molten steel in the mold is easily solidified, and the operation tends to be unstable.

In the area of the slab until the center of the thickness is completely solidified, the distance between the axes of the guide rolls for supporting the slab adjacent to each other is set to 450 mm or less because the thickness center solid phase ratio is set to zero. This is for suppressing bulging of the slab between adjacent guide rolls in the region of the slab that exceeds.

If the bulging amount of the slab increases in the slab area until the thickness center portion is completely solidified, remarkable center segregation or V-shaped segregation is likely to occur. This is because there is a lot of molten steel in which the segregation component is concentrated in the area of these slabs. In addition, the distance between the center axes of the guide rolls is 450 m.
If m is exceeded, the bulging amount of the slab increases in the slab area until the thickness center portion is completely solidified.

If the distance between the center axes of the guide rolls is too small, the roll diameter of the guide rolls becomes too small, and the guide rolls are easily bent by the molten steel static pressure or the like, and the slabs are more likely to bulge. Therefore, the distance between the axes of the guide rolls is desirably 300 mm or more.

Heating and rolling conditions of the slab when hot-rolling a slab without center segregation or the like obtained as described above to obtain a thin steel sheet may be ordinary conditions. Further, pickling conditions, rolling conditions, heat treatment conditions, and the like in the cold rolling using the hot-rolled thin steel sheet as a material may be normal conditions.

[0046]

EXAMPLE A vertical bending type continuous casting machine having a vertical part length of 3 m, an arc radius of 10 m, a five-point bending four-point straightening machine, and a machine length of 45 m was used.
A test for casting a cast piece having a thickness of 230 mm and a width of 1800 mm was performed.

The steel used had a C content of 0.04 to 0.1.
6% by mass of low carbon steel and medium carbon steel. The casting speed was varied and tested in the range of 1.2 to 1.5 m / min. The distance between the axes of the guide rolls of the continuous casting machine is usually 250 mm in the vertical portion, 250 to 400 mm in the curved portion, and 4 mm in the horizontal portion.
In the test, the distance between the axes of the horizontal portion was changed in the range of 350 to 500 mm. This horizontal portion is in a range of 22 to 45 m from the meniscus. The degree of superheat of the molten steel in the tundish is 20 to
The temperature was varied within the range of 95 ° C. The specific water volume for secondary cooling is 1 to
2 liters / kg-steel.

Within the range of the above test conditions, it was confirmed by a one-dimensional unsteady heat transfer analysis in the slab thickness direction that the center solid fraction of the slab exceeds zero in the horizontal portion.
In addition, in some tests, the slab surface temperature was continuously measured, and a rivet test was performed on the slab at the horizontal position to confirm that the center solid fraction of the slab exceeded zero. did.

The cast slab obtained in each test was subjected to hot rolling to produce a steel strip having a thickness of 6 mm. At that time, the heating temperature of the slab is in the range of 1180 to 1270 ° C.
Rough rolling was completed at about 0 ° C., followed by finish rolling at a temperature of 930 to 870 ° C., and wound into a coil at a temperature of 500 to 600 ° C. In some tests, a steel strip having a thickness of 2 mm was obtained by using the steel strip as a raw material and cold rolling after pickling. This steel strip was continuously annealed at 800 ° C.

Although not performed in the present test, the rough bar may be heated using an induction heater or the like after the rough rolling of the continuous casting slab and before the finish rolling. After continuous annealing, galvanizing may be performed in-line.

For each steel strip obtained by hot rolling and then cold rolling, a sample was taken from the center in the width direction, and a hole expansion test was performed to evaluate the mechanical properties, chemical composition, and stretch flangeability. Was carried out. Mechanical properties are normal J
The tensile strength and elongation were investigated based on the method specified by IS.

The chemical composition of each steel strip was investigated by the following method. First, the average chemical composition of a steel strip was obtained by cutting a steel strip sample of full thickness with a 5 mm diameter drill blade,
Each element was analyzed by a chemical analysis method, and the average content of each element was determined. Next, on both sides in the thickness direction from the thickness center, the average C
And the Mn content, the steel strip is stepped in the thickness direction,
Cuttings of a predetermined thickness portion were collected, and the contents of C and Mn were determined by a chemical analysis method. EPM cross section in thickness direction
C and Mn may be analyzed by an instrumental analysis method using A or the like.

The hole expansion test was conducted according to the Japan Iron and Steel Federation Standard J.
The test was performed based on FST1001-1996. FIG.
It is a schematic diagram which shows the outline of a hole expansion test. FIG. 1A shows a state in which an initial hole is formed in a thin steel plate, and FIG. 1B shows a state in which the initial hole is expanded. In the hole expansion test, first, a punched hole 3 made by a punch 2 having an outer diameter of 10 mm is formed in a thin steel plate 1 as a test piece, and then the hole 3 is pushed and expanded by a conical punch 2. A crack is formed at the edge of the hole, and then the hole is expanded until the crack penetrates in the thickness direction. The diameter of the hole 4 when the crack penetrated in the plate thickness direction was determined, and the value obtained by dividing the diameter by the initial hole diameter of 10 mm was defined as the hole expansion ratio λ represented by the following formula (A), and the stretch flangeability of the thin steel plate was obtained. That is, the processability is evaluated. In this test, conditions such as the tip angle of the punch and the gap between the die 5 and the punch 2 were set in accordance with the above standards. λ = (D _h −D ₀ ) / D ₀ × 100 (A) where λ: hole expansion ratio (%) D ₀ : initial hole diameter (10 mm) D _h : crack has a plate thickness Table 1 and Table 2 show each test condition and each test result.

[0054]

[Table 1]

[Table 2] Test No. of the present invention example. 1 to No. In No. 23, the slab thickness during continuous casting was 230 mm, the casting speed was 1.2 m / min,
1.5 m / min or 2.5 m / min, the degree of superheat of the molten steel in the tundish is 22 to 75 ° C., and the distance between the axial centers of the guide rolls in the horizontal portion is 350 to 450 mm. Continuous casting was performed within the range of conditions specified by the casting method.

Test No. 1 to No. In 5, the steel used
Is medium carbon steel with a C content of 0.11% by mass.
The range of chemical composition conditions specified in the present invention, including
The steel in the enclosure. Test No. 1 to No. At 4
For example, a slab was hot-rolled into a steel strip having a thickness of 6 mm. Steel strip
In the thickness to the average C and Mn content of the total thickness of
Within 10% each of both sides in the thickness direction from the center
Of the average C and Mn contents (hereinafter referred to as the segregation zone
Is expressed in the range of 1.2 to 1.5.
It was within the range defined by the invention. The tensile strength is 44
~ 47kgf / mm ^Two , Elongation within 35-38%
And good mechanical properties. The hole expansion rate is 9
It was a steel strip having good workability at 2 to 95%. Ma
Test No. In 5, the cold-rolled steel strip with a thickness of 2 mm
did. The segregation degree of the segregation line is 1.5 for C and 1.2 for Mn.
Therefore, it was within the range of the conditions specified in the present invention. Tensile strength
Is 46kgf / mm^Two Good machine with elongation of 38%
Characteristic. The hole expansion rate is 111%, which is good.
The steel strip had workability.

Test No. 6-No. In Test No. 9, medium carbon steel having a C content of 0.16% by mass. 10 to N
o. In Test No. 13, each of the above-described Test Nos. 1 to No. According to the test contents of No. 5, continuous casting was performed within the range of conditions specified by the continuous casting method of the present invention. In each case, the segregation degree of the segregation zone of C and Mn is 1.1 to 1.5, so that the tensile strength is 46 to 52 kgf / mm ² and the elongation is 3
Good mechanical properties were found at 2-36%. Further, good results were obtained with a hole expansion ratio of 75 to 117%.

Test No. 14-No. In No. 19, the carbon content is 0.11 mass%, which is a medium carbon steel within the range of the chemical composition defined by the present invention, and Ti, Nb, and V are preferable elements for further improving the strength of the steel strip. One or two of these were tested as steels with a chemical composition. Each of the test Nos. 1 to No. According to the test contents of No. 4, continuous casting was performed within the range of conditions specified by the continuous casting method of the present invention. Test No. 2 using steel having the same C content. 1 to No. Approximately 20kg tensile strength compared to 4.
The f / mm ^{2 was} improved by about 64 to 66 kgf / mm ² , and the elongation was 26 to 27%, indicating good mechanical properties. Good results were obtained with a hole expansion ratio of 71 to 75%.

Test No. 20 and no. At 21, C
A medium-carbon steel having a content of 0.16% by mass within the range of the chemical composition specified in the present invention was tested as a steel having a chemical composition to which two types of Ti, Nb and V were added. Each of the test Nos. 6-No. According to the test contents of No. 8, continuous casting was performed within the range defined by the continuous casting method of the present invention. Test No. 2 using steel having the same C content. 6-No. 8 has a tensile strength of about 20 to 2
5kgf / mm ² about to improve, 71~75kgf / mm
² , and the elongation was 23 to 25%, indicating good mechanical properties. Good results were obtained with a hole expansion ratio of 68 to 69%.

Test No. 22 and No. In 23, C
A low-carbon steel having a content of 0.04% by mass within the range of the chemical composition defined by the present invention was tested as a steel having a chemical composition to which two kinds of Ti, Nb and V were added. Each of the test Nos. 10-No. According to the content of the test No. 12, continuous casting was performed within the range of conditions specified by the continuous casting method of the present invention. Test No. 2 using steel having the same C content. 10-No. 12 has a tensile strength of about 1
5 to 20 kgf / mm ² improved, 61 to 65 kgf
/ Mm ² , and the elongation was 27 to 29%, indicating good mechanical properties. The hole expansion ratio was 74 to 77%, which was a good result.

Test No. of Comparative Example 24-No. In No. 28, the slab thickness at the time of continuous casting was 230 mm, the casting speed was 1.5 m / min, and within the range defined by the continuous casting method of the present invention. 24 and No. In the case of No. 25, the test was performed with the distance between the axes of the guide rolls in the horizontal portion set to 480 mm and out of the range defined by the continuous casting method of the present invention. 26-No. In No. 28, the superheat of the molten steel in the tundish was set to 85 ° C. or 90 ° C., and the test was performed outside the range defined by the continuous casting method of the present invention. The steels used in these tests were the test Nos. 1 to
No. Medium carbon steel having a C content of 0.11% by mass within the range of the chemical composition conditions defined in the present invention as in No. 5.

Among them, Test No. 24-No. At 27, the slab was hot rolled to form a 6 mm thick steel strip. The segregation degree of the segregation zone of C and Mn of the steel strip is a value within the range of 1.5 to 1.7, and at least the segregation degree of the segregation zone of C or Mn is out of the range specified in the present invention. Met. Therefore, the tensile strength is not particularly problematic, but the elongation is 24-2.
It was as low as 9%, and the hole expansion rate was 60-68%, which was a bad result.

The test No. In No. 28, a cold-rolled steel strip having a thickness of 2 mm was formed. The segregation degree of the segregation zone is C = 1.
6, Mn was 1.7, which was a value outside the conditions defined in the present invention. Therefore, the tensile strength was not particularly problematic, but the elongation was as low as 29%, and the hole expansion ratio was 58%, which was a bad result.

Test No. 29-No. No. 32, medium carbon steel having a C content of 0.16% by mass. 33 ~
No. In the case of Test No. 36, a low carbon steel having a C content of 0.04 mass% was used, and 24-No. 28
The test was carried out according to the test contents of the above. That is, the test was performed with at least the distance between the center axes of the guide rolls in the horizontal portion or the degree of superheat of the molten steel in the tundish being outside the range defined by the continuous casting method of the present invention. Both are steel strip C
And the segregation degree of the segregation zone of Mn was in the range of 1.5 to 1.7, and at least the segregation degree of the segregation zone of C or Mn was a value outside the conditions defined in the present invention. Therefore, the tensile strength is not particularly problematic, but the elongation is 23 to 29%.
And the hole expansion rate is 52-7 in the hot rolled steel strip.
As low as 4% and 66% or 79 for cold rolled steel strip
%, Which was a bad result.

Test No. 37-No. In No. 42, a carbon content of 0.11 mass% is defined as a medium carbon steel within the range of the chemical composition defined by the present invention, and Ti, Nb, and V are preferable elements for improving the strength of the steel strip. One or two
The steel was tested as a seeded chemical composition. Each of the test Nos. 24-No. The test was carried out according to 27 test contents. That is, at least the distance between the axial centers of the guide rolls in the horizontal portion or the degree of superheat of the molten steel in the tundish was tested outside the range defined by the continuous casting method of the present invention. In any case, the segregation degree of the segregation zone of C and Mn in the steel strip is a value in the range of 1.4 to 1.6, and at least the segregation degree of the segregation band of C or Mn is out of the condition specified in the present invention. Value. Therefore, the tensile strength was not particularly problematic, but the elongation was as low as 20 to 22%, and the hole expansion ratio was as low as 49 to 72%, which was a bad result.

Test No. 43 and No. In 44, C
A medium-carbon steel having a content of 0.16% by mass within the range of the chemical composition specified in the present invention was tested as a steel having a chemical composition to which two types of Ti, Nb and V were added. Each of the test Nos. 29-No. The test was performed according to 31 test contents. That is, the test was performed with the distance between the axes of the guide rolls in the horizontal portion or the degree of superheat of the molten steel in the tundish being outside the range defined by the continuous casting method of the present invention. In each case, the degree of segregation of the C or Mn segregation zone of the steel strip was 1.6 or 1.7, which was out of the conditions specified in the present invention. Therefore, the tensile strength was not particularly problematic, but the elongation was as low as 16 to 17%, and the hole expansion ratio was as low as 48 to 49%.

Test No. 45 and no. In 46, C
A medium-carbon steel having a content of 0.04% by mass within the range of the chemical composition defined by the present invention was tested as a steel having a chemical composition to which two types of Ti, Nb and V were added. Each of the test Nos. 33-No. The test was performed according to 35 test contents. That is, the test was performed with the distance between the axes of the guide rolls in the horizontal portion or the degree of superheat of the molten steel in the tundish being outside the range defined by the continuous casting method of the present invention. At least the degree of segregation of the C or Mn segregation zone of the steel strip was 1.6 or 1.7, a value out of the conditions specified in the present invention. Therefore, the tensile strength is not particularly problematic, but the elongation is as low as 19 to 21%, and the hole expansion ratio is as low as 50 to 57%, which is a bad result.

[0067]

According to the thin steel sheet and the continuous casting method of the present invention, a hot-rolled thin steel sheet or a cold-rolled thin steel sheet which does not crack during press working and has good workability. Steel sheet can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an outline of a hole expanding test.

FIG. 2 is a schematic diagram showing the distance between the center axes of guide rolls.

FIG. 3. (C) o, (Mn) o, (C) t and (M)
n) It is a schematic diagram which shows t.

[Explanation of symbols]

1: thin steel plate 2: punch 3: hole 4: hole when a crack penetrates in the thickness direction 5: die 6: roll of guide roll 7: distance between roll axes 8: casting direction 9: segregation zone 10: Center of thickness 11: Area within 10% of total thickness 12: Total thickness _D0 : Diameter of initial hole (10 mm) _Dh : Diameter of hole when crack penetrates the plate thickness (mm)

Claims (4)

[Claims] (1) The content in the average chemical composition of the entire plate thickness is 0.03 to 0.16% in mass% and 0.03 to 0.1% in Si.
2% or less, Mn: 1.4% or less, P: 0.03% or less,
S: 0.02% or less, sol. Al: 0.10% or less, the balance being 8 m in thickness composed of Fe and impurity elements
m or less, and the following formula (a) and (b)
A thin steel sheet that satisfies the formula. (C) t / (C) o ≦ 1.5 (b) (Mn) t / (Mn) o ≦ 1.5 (b) where (C) t: from the center of thickness Average C content (% by mass) in a region within 10% of the total thickness on both sides in the thickness direction (C) o: Average C content (% by mass) of all sheet thicknesses (Mn) t: Average Mn content (% by mass) in regions within 10% of the total thickness on both sides in the thickness direction from the thickness center (Mn) o: Average Mn content (% by mass) of the entire plate thickness 2. The steel sheet according to claim 1, further comprising at least one of Nb: 0.2% or less, Ti: 0.2% or less, and V: 0.2% or less. Or a thin steel sheet containing two or more kinds. 3. C: 0.03 to 0.16% by mass%, S
i: 0.2% or less, Mn: 1.4% or less, P: 0.03
%, S: 0.02% or less, sol. Al: 0.10
% Of molten steel in the tundish, the degree of superheat of the molten steel in the tundish is 80 ° C. or less, the cross section of the slab is rectangular and the thickness is 3
00 mm or less, the casting speed is 1.2 m / min to 3 m / min, and the distance between the axes of the guide rolls for supporting slabs adjacent to each other in the slab region until the center of the thickness is completely solidified is 4 mm.
A method for producing a thin steel sheet, comprising casting under conditions of not more than 50 mm and then rolling to obtain a thin steel sheet. 4. C: 0.03 to 0.16% by mass%, S:
i: 0.2% or less, Mn: 1.4% or less, P: 0.03
%, S: 0.02% or less, sol. Al: 0.10
% Nb: 0.2% or less, Ti:
A steel containing one or more of 0.2% or less and V: 0.2% or less is smelted, the degree of superheat of the molten steel in the tundish is 80 ° C or less, and the slab cross-sectional shape is rectangular. The thickness of the slab supporting guide rolls adjacent to each other in the region of the slab until the solidification of the center of the thickness is completed is set to 450 mm or less at a thickness of 300 mm or less at a casting speed of 1.2 m / min to 3 m / min. A method for producing a thin steel sheet, which is cast under the following conditions, and then rolled into a thin steel sheet.