JPH06336641A - High tensile strength steel plate excellent in pitting corrosion resistance and its production - Google Patents

Abstract

PURPOSE:To produce a high strength steel plate excellent in pitting corrosion resistance and capable of withstanding use in a bare state as an automotive under carriage member even under a severe corrosive environment. CONSTITUTION:This steel plate has a chemical compsn. contg., by weight, <=0.1% C, 0.1 to 2.0% Mn, 0.05 to 0.20% P, <=0.010% S, 0.05 to 1.0% Cu, 0.05 to 0.20% Mo, 0.01 to 0.10% Al and <=0.008% N and contg., at need, prescribed ranges of one or >= two kinds among 0.05 to 1.0% Ti and/or Si, 0.05 to 2.0% Ni, 0.05 to 5.0% Cr and 0.01 to 0.05% Nb, and the balance Fe with inevitable impurities, and its metallic structure is formed of a single phase of ferrite or bainite or a mixed one of >= two kinds among ferrite, bainite and martensite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet having excellent perforation corrosion resistance and hole expandability, and a method for producing the same, and more specifically, it is suitable for a vehicle underbody member and its reinforcing member. Regarding rolled steel sheets.

[0002]

2. Description of the Related Art In recent automobiles, it is necessary to guarantee no perforation corrosion for 10 years. For this reason, the steel materials used are required to have improved corrosion resistance, but especially for suspension materials, there is a strong demand for improved corrosion resistance. For example, in cold regions such as North America and Canada, road surfaces are prevented from freezing and corrosion caused by salt sprayed for snow melting is promoted. Therefore, further improvement in corrosion resistance is strongly desired centering around suspension members.

Conventionally, as one of the methods for improving the corrosion resistance, it has been generally practiced to replace the part where a bare steel plate is used with a plated steel plate mainly for hot dip galvanizing. Against this background, many plated steel sheets having excellent corrosion resistance have been developed. For example, JP-A-2-310354 describes a method for producing hot-dip galvanized steel sheets having excellent workability.

[0004]

Many of the undercarriage members and reinforcing members for automobiles are manufactured by arc welding of individual parts in many cases. However, there was a problem that a sound weld could not be obtained.

On the other hand, automobiles are desired to have improved fuel efficiency in order to save energy and preserve the global environment, and the steel materials used are being promoted to be lighter in weight (reduced in plate thickness) due to higher strength.
The reduction in plate thickness due to higher strength is disadvantageous from the viewpoint of perforated corrosion.

From this point of view, for example, Japanese Patent Laid-Open No. 2-2241
Japanese Patent Publication No. 6 proposes an alloyed galvanized steel sheet which improves the corrosion resistance of the plated original plate itself and exhibits excellent corrosion resistance even with a thin weight. However, even though it is a light weight, it is inevitable that defects will occur during welding due to the presence of the plating layer.

The inventors of the present invention have made various investigations with the objective of solving such problems, and as a hot-rolled steel sheet for underbody members for automobiles and reinforcement members thereof, have a resistance to bare use. A high-strength steel sheet having excellent perforation corrosion resistance and workability, in particular, hole expandability, and a manufacturing method thereof are disclosed in Japanese Patent Application No. 4-2907
Proposed to No. 9.

As a result of a more detailed examination of the invention of the prior application, although a good corrosion resistance can be obtained to some extent,
It was found that the corrosion resistance may deteriorate slightly in more severe corrosive environments.

Therefore, the present invention is a further improvement of the invention proposed in Japanese Patent Application No. 4-29079, and in the more severe corrosive environment while maintaining the workability of the steel of the prior invention, particularly the hole expandability. The purpose of the present invention is to provide a high-strength steel sheet that can stably obtain good corrosion resistance.

[0010]

Means for Solving the Problems The above-mentioned object is, as in the steel sheet of the invention of the prior application, that the composite structure of P and Cu is used as a basic component system and that the metallographic structure is a single phase of ferrite or bainite, or is a ferrite. , Bainite and martensite have been found to be achieved.

That is, according to the present invention, in% by weight, C≤0.1% Mn: 0.1-2.0% P: 0.05-0.20% S≤0.010% Cu: 0.05-1.0% Mo: 0.05-2.0%, Al: 0.01 ~ 0.10% N ≦ 0.008% as an essential component, and in some cases contains Ti according to the following formula (1), (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ・ ・ (1) Further, if necessary, Si: 0.05 to 1.0%, Ni: 0.05 to
2.0%, Cr: 0.05 to 5.0%, Nb: 0.01 to 0.05%, one or more, and the balance Fe and inevitable impurities. The steel structure is a single-phase structure of ferrite or bainite, or Provided is a high-strength steel sheet excellent in pitting corrosion resistance adjusted to a mixed structure consisting of two or more phases of ferrite, bainite and martensite.

As a method for producing a high-strength steel sheet having such a chemical composition and a metal structure and having excellent perforation corrosion resistance, a slab of steel having the chemical composition is produced.
This slab is heated to a temperature range of 1100 to 1300 ℃ and the hot rolling finish temperature is above the Ar ₃ transformation point and 800 to 95
A method for producing hot-rolled steel sheet is carried out, which comprises controlled hot rolling in a temperature range of 0 ° C, cooling from the hot rolling finishing temperature at a cooling rate of 35 ° C / s or more, and winding at a temperature of 530 ° C or less. provide.

[0013]

The steel sheet of the present invention, as shown in the examples below,
It has excellent perforation corrosion resistance in a corrosive environment test (CCT-II) that is more severe than the test described in Japanese Patent Application No. 4-29079.

A method for manufacturing a hot rolled steel sheet according to the present invention is described in Japanese Patent Application No. 4-29
Compared with No. 079, the cold rolling rate after hot rolling is faster and the winding temperature is in the low temperature range. Therefore, it has a single-phase structure of ferrite or bainite or a mixed structure of two or more phases of ferrite, bainite and martensite, and pearlite transformation does not occur. It can be considered that the steel of the present invention has more excellent resistance to perforation and corrosion due to this metallic structure.

The action of each component of the steel sheet according to the present invention and the reason for limiting the content range thereof are as follows.

C is an element effective for increasing the strength of steel.
However, if the content exceeds 0.1%, the workability deteriorates, so the upper limit was made 0.1%.

Mn effectively acts to increase the strength of steel,
If less than 0.1%, the effect is not observed, and if more than 2.0% is contained, the workability deteriorates.
It is contained in the range of 2.0%.

P is a characteristic element in the steel of the present invention, which not only effectively acts to strengthen the steel, but also contains Cu in an amount of 0.05% or more in combination with Cu to form a dense corrosion product with good adhesion. Which contributes to the improvement of pitting corrosion resistance. However, if the P content exceeds 0.20%, the workability deteriorates, so 0.05 to 0.
Include in the range of 2%.

S deteriorates workability. In particular, when MnS is combined with Mn to form MnS, the hole expandability is deteriorated. Therefore, it is desirable that S is as small as possible, but 0.01
Since 0% is acceptable, 0.010% or less is set.

Cu is an element that effectively acts to improve the corrosion resistance when it is contained in combination with P as described above. If the content is less than 0.05%, the effect is not observed, and if the content exceeds 1.0%, not only the effect is saturated, but also the manufacturing cost increases, so 0.05-1.0%
It is contained in the range of.

Al is contained as a deoxidizing agent, and 0.01% or more is necessary to fulfill its role.
However, if the content exceeds 0.10%, inclusions such as Al ₂ O ₃ increase and the workability and surface quality deteriorate. Therefore, the content is set to 0.01 to 0.10%.

N is an element detrimental to corrosion resistance and deep drawability, and the smaller the amount, the more preferable. However, 0.008% is acceptable, so N is set to 0.008% or less.

Mo is an element that effectively acts to increase the strength and improve the resistance to pitting corrosion. In particular, it is very effective in improving the corrosion resistance, and can further improve the corrosion resistance of the P- and Cu-added steel. If the Mo content is less than 0.05%, the effect is not observed, and if the Mo content exceeds 2.0%, the effect saturates and the workability deteriorates, which further increases the cost. 0.05 ~
It is contained in the range of 2.0%.

Ti is an element that fixes S and N and effectively acts to improve the workability, especially the hole expandability.
This effect is not observed below (48/32) × S. On the other hand, if the content exceeds {(48/32) × S + (48/14) × N} × 2, the effect will be saturated and the cost will increase. It is necessary to let

Further, in the present invention, in order to increase the strength of the steel sheet or improve the corrosion resistance, Si: 0.05 to 1.0%, N
i: 0.05 to 2.0%, Cr: 0.05 to 5.0%, Nb: 0.01 to 0.05
%, One kind or two or more kinds may be contained.

Si is an element that effectively acts to enhance the strength and improve the resistance to pitting corrosion. If the content is less than 0.05%, the effect is not recognized, and if the content exceeds 1.0%, the surface quality of the product is deteriorated and the workability is also deteriorated.
Include in the range of 1.0%.

Ni is an element which effectively acts to prevent hot brittleness due to Cu and to improve perforation corrosion resistance, and is also effective for increasing strength and improving perforation corrosion resistance. Ni content is
If less than 0.05%, those effects are not recognized. Also, 2.
Even if the content exceeds 0%, the effect is saturated and the cost is increased. Therefore, Ni is contained in the range of 0.05 to 2.0%.

Cr is an element that effectively acts to enhance the strength and improve the resistance to pitting corrosion. If the content is less than 0.05%, those effects are not recognized. Further, if the content exceeds 5.0%, not only the effect is saturated but also the manufacturing cost is remarkably increased as compared with the effect, so the content is made 0.05 to 5.0%.

Nb effectively contributes to high strength by refining the metal structure of the steel sheet. However, if the content is less than 0.01%, the effect is not recognized, and if the content exceeds 0.05%, the strength is increased but the workability is significantly deteriorated, so the content is set to 0.01 to 0.05%.

In the present invention, the steel having such a composition is used, and the structure of the steel is strictly adjusted, more specifically, a single-phase structure of ferrite or bainite, or among ferrite, bainite and martensite. It is characterized by adjusting to a mixed tissue of two or more kinds. In the case of this particular metal structure, the steel can stably obtain good corrosion resistance even in a severe corrosive environment, as will be shown in Examples described later, and remarkably has good perforation corrosion resistance. I understood.

It is speculated that the disappearance of Fe ₃ C contributes to the fact that the steel has good pitting corrosion resistance when it has this specific metallographic structure defined in the present invention. To be done.

[0032] That _is, in the steel, which was controlled at the organization does not occur pearlite transformation. Therefore, Fe ₃ C does not precipitate. Even if the steel has the action of having sufficient corrosion resistance in terms of composition, it can be treated as pearlite with F
e ₃ If the C is precipitated, Fe ₃ C is in a severe corrosive environment
It is presumed that a local battery may be formed between the base metal and the ground iron, which may cause corrosion to proceed.

The structure specified in the present invention, that is, a single-phase structure of ferrite or bainite, or ferrite,
A mixed structure of two or more of bainite and martensite can be advantageously formed by controlling the manufacturing conditions of the hot-rolled steel strip of the steel of the present invention. That is, a slab of the steel is heated in a temperature range of 1100 to 1300 ° C and hot-rolled, and the hot-rolling is performed at an Ar ₃ transformation point or higher and 800-95 ° C.
It is possible to finish in the temperature range of 0 ° C, continue cooling at a cooling rate of 35 ° C / s or more, and wind at a temperature of 530 ° C or less.

If the extraction temperature of the steel slab (heating temperature of the slab) from the heating furnace is less than 1100 ° C., the melting of carbonitrides is insufficient and good workability cannot be obtained. In addition, since heating above 1300 ° C only causes an increase in manufacturing cost, it is preferable to heat the steel slab to the range of 1100 to 1300 ° C and hot-roll it.

Hot rolling is performed at an Ar ₃ transformation point or higher and 800 to 95.
Good workability can be obtained when finishing in the temperature range of 0 ° C.
If the hot rolling end temperature is lower than the Ar ₃ transformation point or lower than 800 ° C., equiaxed crystal grains cannot be obtained, and good workability cannot be obtained. Further, when the temperature exceeds 950 ° C, the crystal grain size becomes coarse and the surface becomes rough after processing.

After the hot rolling, the cooling rate was set to 35 ° C.
By cooling at a cooling rate of / s or more and winding at a temperature of 530 ° C or less, a single-phase structure of ferrite or bainite or a mixed structure of two or more types of ferrite, bainite and martensite can be obtained. If the cooling rate after hot rolling is less than 35 ° C / s, pearlite transformation occurs during cooling, and even if the coiling temperature exceeds 530 ° C, pearlite transformation occurs in the subsequent cooling process. In this case, good corrosion resistance cannot be stably obtained. Therefore, it is important to control the cooling rate after hot rolling to 35 ℃ / s or higher and the coiling temperature to 530 ℃ or lower.

[0037]

EXAMPLE Steels having the chemical composition values shown in Table 1 were melted to produce a plurality of continuously cast slabs for each steel. Each steel slab was hot-rolled under the following two conditions A and B to obtain a hot-rolled sheet having a plate thickness of 2.6 mm. [Hot rolling condition A] Slab heating temperature: 1230 ℃, Hot rolling end temperature: 900 ℃, Cooling rate after that: 30 ℃ / s, Winding temperature: 550
℃ [Hot rolling condition B] Slab heating temperature: 1230 ℃, Hot rolling end temperature: 900 ℃, Cooling rate after that: 80 ℃ / s, Winding temperature: 450
℃

A sample was taken from the obtained hot-rolled sheet and observed under an optical microscope to examine the metal structure. The results are also shown in Table 2. In the table, F indicates ferrite, P indicates pearlite, B indicates bainite, and M indicates martensite.

Further, each hot-rolled sheet thus obtained was pickled, and then test pieces were taken and subjected to a corrosion test. For the corrosion test, a 70 × 150 mm test piece was cut out, the end face and the back face were sealed, and the compound corrosion test under the following two conditions was performed up to 240 cycles.
After removing the corrosion products, the maximum corrosion depth and the corrosion weight loss were measured to evaluate the pitting corrosion resistance. The test results are shown in Table 2.

Composite corrosion test (CCT-I): As shown in FIG. 1 , a salt spray test at 35 ° C. according to JIS Z2371 for 2 hours, a drying test at 60 ° C. for 4 hours, a humidity of 95% or more at 50 ° C. Wet test is a total of 8 hours of 2 hours and 240 cycles
Do a cycle.

Complex corrosion test (CCT-II): As shown in FIG. 2 , salt spray test at 50 ° C. according to JIS Z2371 for 2 hours, drying test at 70 ° C. for 4 hours, humidity at 60 ° C. of 95% or more. Wet test is a total of 8 hours of 2 hours and 240 cycles
Do a cycle.

[0042]

[Table 1]

[0043]

[Table 2]

As is clear from the results of Table 2, P and Cu
The steels according to the present invention in which the alloys are added in combination and the metallographic structure is the ferrite single phase or the ferrite + bainite and the ferrite + martensite are CCT-I and the more severe CCT-
Even under the condition II, the maximum erosion depth is shallow and the corrosion weight loss is small, indicating that the pitting corrosion resistance is excellent.

[0045]

As described above, according to the present invention, it is possible to obtain a high-strength steel sheet having excellent perforation corrosion resistance even in a severe corrosive environment. When this steel plate is used as an underbody member or a reinforcing member for automobiles, it exhibits durability that conventional materials do not have, and it can contribute to weight reduction of automobiles due to higher strength.

[Brief description of drawings]

FIG. 1 is a test cycle diagram showing conditions (CCT-I) of a corrosion test.

FIG. 2 is a test cycle diagram showing conditions (CCT-II) of a corrosion test.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mikio Soge 11-1 Showa-cho, Kure-shi, Hiroshima Inside Steel Research Laboratory, Nisshin Steel Co., Ltd. (72) Toshihiro Kondo 11-11 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd.

Claims (6)

[Claims] 1. By weight%, C ≦ 0.1%, Mn: 0.1 to 2.0
%, P: 0.05 to 0.20%, S ≦ 0.010%, Cu: 0.05 to 1.0
%, Mo: 0.05 to 2.0%, Al: 0.01 to 0.1%, N ≦ 0.008
%, The balance Fe and unavoidable impurities, and the microstructure of the steel is adjusted to a single-phase structure of ferrite or bainite or a mixed structure of two or more phases of ferrite, bainite and martensite. A high-strength steel sheet with excellent corrosion resistance. 2. In weight%, C ≦ 0.1%, Mn: 0.1 to 2.0
%, P: 0.05 to 0.20%, S ≦ 0.010%, Cu: 0.05 to 1.0
%, Mo: 0.05 to 2.0%, Al: 0.01 to 0.1%, N ≦ 0.008
%, Furthermore, Si: 0.05 to 1.0%, Ni: 0.05 to 2.0%, C
r: 0.05 to 5% or Nb: 0.01 to 0.05%, the balance consisting of Fe and unavoidable impurities, and the steel structure is a single phase structure of ferrite or bainite, or ferrite, bainite and A high-strength steel sheet excellent in perforation corrosion resistance adjusted to a mixed structure composed of two or more phases of martensite. 3. By weight%, C ≦ 0.1%, Mn: 0.1 to 2.0
%, P: 0.05 to 0.20%, S ≦ 0.010%, Cu: 0.05 to 1.0
%, Mo: 0.05 to 2.0%, Al: 0.01 to 0.1%, N ≦ 0.008
%, And Ti within the range according to the following formula (1), (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ・ ・ (1) Remainder Of Fe and unavoidable impurities, and the structure of the steel is adjusted to the single-phase structure of ferrite or bainite, or the mixed structure of two or more phases of ferrite, bainite and martensite. Excellent high-tensile steel plate. 4. In weight%, C ≦ 0.1%, Mn: 0.1 to 2.0
%, P: 0.05 to 0.20%, S ≦ 0.010%, Cu: 0.05 to 1.0
%, Mo: 0.05 to 2.0%, Al: 0.01 to 0.1%, N ≦ 0.008
%, Furthermore, Si: 0.05 to 1.0%, Ni: 0.05 to 2.0%, C
Contains at least one of r: 0.05 to 5% or Nb: 0.01 to 0.05% and Ti in the range according to the following formula (1), and (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ・ ・ (1) The balance consists of Fe and inevitable impurities, and the steel structure is a single-phase structure of ferrite or bainite, or 2 of ferrite, bainite and martensite. A high-strength steel sheet with excellent resistance to perforation and corrosion that is adjusted to a mixed structure consisting of more than one phase. 5. In weight%, C ≦ 0.1%, Mn: 0.1 to 2.0
%, P: 0.05 to 0.20%, S ≦ 0.010%, Cu: 0.05 to 1.0
%, Mo: 0.05 to 2.0%, Al: 0.01 to 0.1%, N ≦ 0.008
%, The balance being Fe and unavoidable impurities, a steel slab is heated to a temperature range of 1100 to 1300 ° C. and hot-rolled. At that time, the hot-rolling is performed at an Ar ₃ transformation point or higher and 800 to
The structure of steel is single-phase microstructure of ferrite or bainite, or of ferrite, bainite and A method for producing a high-strength hot-rolled steel sheet excellent in perforation corrosion resistance, which comprises adjusting to a mixed structure composed of two or more phases of martensite. 6. The steel slab has a composition of Si: 0.0
5 to 1.0%, Ni: 0.05 to 2.0%, Cr: 0.05 to 5% or N
b: containing at least one of 0.01 to 0.05% and / or Ti in a range according to the following formula (1):
(48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ··· (1).