JP3377253B2 - High strength steel sheet excellent in perforation corrosion resistance and method for producing the same - Google Patents

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 excellent in perforation corrosion resistance and hole expandability and a method for producing the same, and more specifically, it is suitable for an automobile underbody member and a reinforcing member thereof. 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 a portion 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 a hot-dip galvanized steel sheet having excellent workability.

[0004]

Many of the undercarriage members and reinforcing members for automobiles are manufactured by arc-welding 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 have been 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
No. 6 publication proposes an alloyed galvanized steel sheet which improves the corrosion resistance of the plating base plate itself and exhibits excellent corrosion resistance even with a thin weight. However, even though it is a light weight, the presence of the plating layer inevitably causes defects during welding.

The inventors of the present invention have made various investigations with the aim of solving such problems, and as a hot-rolled steel sheet for underbody members for automobiles and reinforcing members thereof, have a resistance to bare use. Japanese Patent Application No. 4-2905 for a high-strength steel sheet excellent in perforation corrosion resistance and workability, especially in hole expansion
No. 0 proposed.

As a result of a more detailed examination of the invention of this 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 an improvement over the invention proposed in Japanese Patent Application No. 4-29050, which can stably obtain good corrosion resistance even in a more severe corrosive environment and is excellent in workability, in particular, hole expandability. It is an object of the present invention to provide a high-strength steel sheet and a manufacturing method thereof.

[0010]

[Means for Solving the Problems] The above-mentioned object is to provide a steel sheet which can obtain a certain degree of good corrosion resistance, with a composite structure of P and Cu as a basic component system and a metal structure of ferrite +
Bainite or ferrite + bainite + martensa
It was found that this can be achieved by using a mixed structure of Ito .

That is, according to the present invention, in% by weight, C : 0.018 to 0.061 % Mn: 0.1 to 2.0% P: 0.05 to 0.20 % S≤0.010 % Cu: 0.05 to 1.0% Al: 0.01 to 0.10% N≤ the 0.008% or as essential components, further contains Ti in accordance with the following formula (1)
Or (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ·· (1) or Si: 0.05 to 1.0%, Ni: 0.05 to 2.0%, Cr: 0.0
5 to 5.0%, Nb: 0.01 to 0.05%, containing one or more kinds, and the balance Fe and inevitable impurities,
Steel structure is ferrite + bainite or ferrite +
Provided is a high-strength steel sheet excellent in perforation corrosion resistance adjusted to a mixed structure composed of 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) which is more severe than the test described in Japanese Patent Application No. 4-29050.

A method for producing a hot rolled steel sheet according to the present invention is described in Japanese Patent Application No. 4-29.
The cold rolling speed after hot rolling is faster than that of No. 050, and the winding temperature is in the low temperature range. Because of this, ferrite + bay
It has a mixed structure of knight or ferrite + bainite + martensite and avoids pearlite transformation. It is this metallic structure that the steel of the present invention has further excellent resistance to pitting corrosion.

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 and has a lower limit.
Is 0.018% . However, if the content exceeds 0.1%, the workability deteriorates . In the present invention, the upper limit is 0.061 %.

Mn effectively acts to increase the strength, but 0.1
%, The effect is not observed, and if it exceeds 2.0%, the workability deteriorates.
It is contained in the range of.

P is a characteristic element in the steel of the present invention, which not only effectively acts to increase the strength, but also contains Cu in an amount of 0.05% or more in combination with Cu, so that a dense corrosion product having good adhesion can be obtained. To provide the effect of contributing to the improvement of pitting corrosion resistance. However, if P is contained in excess of 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, it should be 0.010% or less.

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 better. However, 0.008% is allowable, so 0.008% or less is set.

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, so it is contained within the range shown in the above formula (1). 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 increase the strength and improve the resistance to pitting corrosion. If less than 0.05%, the effect is not recognized, and if it exceeds 1.0%, the surface quality of the product is deteriorated and the workability is also deteriorated.
Include in the range of 1.0%. Si less than 0.03% is impure
It can be mixed as a thing.

Ni is an element which effectively acts to prevent hot brittleness due to Cu and to improve resistance to pitting corrosion, and is also effective for increasing strength and improving resistance to pitting corrosion. 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%. Ni of approximately 0.03% or less is an impurity.
Can be mixed.

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%. Cr of less than 0.04% is mixed as an impurity.
You can enter.

[0028] Nb effectively contributes to the increase in 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 then the structure of the steel is strictly adjusted, more specifically, ferrite + bainite or ferrite + bay.
It is characterized by adjusting the mixed structure of knight + martensite . 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.

The reason why the steel has good pitting corrosion resistance when it has this specific metal structure defined in the present invention is not necessarily clear.
_Effect on the corrosion resistance of distillate Fe ₃ C that has run out is estimated to have contributed.

That is, Fe ₃ C does not precipitate because pearlite does not exist in the steel whose structure is controlled. Even if the composition of the steel is such that it provides sufficient corrosion resistance in terms of composition, if Fe ₃ C is precipitated as pearlite, it will form a mixture of Fe ₃ C and base steel in a severe corrosive environment. It is presumed that a phenomenon in which a local battery is formed and corrosion progresses appears in between.

Organization specified in the present invention, that is, Ferei
To + bainite or ferrite + bainite + marte
In order to obtain a mixed microstructure of molybdenum , it can be advantageously performed by controlling the manufacturing conditions of the hot-rolled steel strip of the present invention steel. First, 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
Finish in the temperature range of 800 to 950 ℃, continue cooling at a cooling rate of 35 ℃ / s or more, and wind at a temperature of 530 ℃ or less.

If the extraction temperature of the steel slab from the heating furnace (steel slab heating temperature) is less than 1100 ° C., the carbonitride is not sufficiently melted and good workability cannot be obtained. In addition, heating above 1300 ° C only causes an increase in manufacturing cost, so it is advisable 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 950.
If it is finished in the temperature range of ° C, good workability can be obtained. If the hot rolling end temperature is less than the Ar ₃ transformation point or less than 800 ° C., equiaxed crystal grains cannot be obtained, and good workability cannot be obtained. Also, if the temperature exceeds 950 ° C, the crystal grain size becomes coarse, and the surface becomes rough after processing.

The cooling rate after the hot rolling is 35 ° C.
By cooling at a cooling rate of / s or more and winding at a temperature of 530 ° C or less, ferrite + bainite or ferrite
A mixed structure of g + bainite + 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, the cooling rate after hot rolling is 35
It is important to control the temperature above ℃ / s and the coiling temperature below 530 ℃.

[0036]

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: 30 ℃ / s, Winding temperature: 550 ℃ [Hot rolling condition B] Slab heating temperature: 1230 ℃ , Hot rolling end temperature: 900 ℃, subsequent cooling rate: 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, after pickling each of the obtained hot-rolled sheets, a test piece was sampled 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 erosion 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.

[0041]

[Table 1]

[0042]

[Table 2]

As is clear from the results in Table 2, P and Cu
Is added together, and the metal structure is ferrite + bainite
In addition, the steel of the present invention composed of ferrite, bainite, and martensite has excellent maximum pit depth and reduced corrosion weight loss even under CCT-I and severer CCT-II conditions, and is excellent in pitting corrosion resistance. You can see that

[0044]

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 Prefecture Steel Research Laboratory, Nisshin Steel Co., Ltd. (72) Toshihiro Kondo 11-Showa-cho, Kure-shi, Hiroshima No. 1 Nisshin Steel Co., Ltd. Steel Research Laboratory (56) References JP-A-3-82708 (JP, A) JP-A-5-112832 (JP, A) (58) Fields investigated (Int.Cl. ⁷⁾ , DB name) C22C 38/00-38/60 C21D 8/02 C21D 9/46

Claims (4)

(57) [Claims] 1. By weight%, C : 0.018 to 0.061 %, Mn: 0.1 to 2.0%, P: 0.05 to 0.20 %, S ≤ 0.010%, Cu: 0.05 to 1.0%, Al: 0.01 to 0.1%, N ≤0.008%, Si: 0.05 to 1.0%, Ni: 0.05 to 2.0%, Cr:
0.05 to 5% , Nb: 0.01 to 0.05% , at least one of which is Fe and inevitable impurities, and the steel structure is ferrite + bainite or ferrite + bainer.
A high-strength steel sheet with excellent resistance to perforation and corrosion that is adjusted to a mixed structure consisting of Ito and martensite . 2. In% by weight, C : 0.018 to 0.061 %, Mn: 0.1 to 2.0%, P: 0.05 to 0.20 %, S ≤ 0.010%, Cu: 0.05 to 1.0%, Al: 0.01 to 0.1%, N ≦ 0.008% 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 structure of steel is
Huelite + Bainite or Huelight + Bainite
+ High-strength steel sheet with excellent resistance to perforation and corrosion adjusted to a mixed structure consisting of martensite . 3. In% by weight, C : 0.018 to 0.061 %, Mn: 0.1 to 2.0%, P: 0.05 to 0.20 %, S ≤ 0.010%, Cu: 0.05 to 1.0%, Al: 0.01 to 0.1%, N ≤0.008%, Si: 0.05 to 1.0%, Ni: 0.05 to 2.0%, Cr:
0.05 to 5% , Nb: One or more of 0.01 to 0.05% , and Ti within the range according to the following formula (1), (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ・ ・ (1) The balance consists of Fe and unavoidable impurities, and the structure of steel is
Huelite + Bainite or Huelight + Bainite
+ High-strength steel sheet with excellent resistance to perforation and corrosion adjusted to a mixed structure consisting of martensite . 4. By weight%, C: 0.018 to 0.061%, Mn: 0.1 to 2.0%, P: 0.05 to 0.20 %, S ≤ 0.010%, Cu: 0.05 to 1.0%, Al: 0.01 to 0.1%, N ≦ 0.008%, further, Si: 0.05~1.0%, Ni: 0.05~2.0%, Cr:
0.05 to 5% , Nb: One or more of 0.01 to 0.05% , and / or Ti in the range according to the following formula (1) , (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ・ ・ (1) A steel slab with the balance Fe and inevitable impurities
It is heated in the temperature range of 100 to 1300 ℃ and hot rolled.
If the hot rolling is performed at an Ar ₃ transformation point or higher and 800 to 95
Finishes in the temperature range of 0 ℃, and continues to cool at 35 ℃ / s or more
By cooling with a coil and winding at a temperature of 530 ° C or less
Tissues are ferrite + bainite or ferrite + bay
Adjustable to a mixed structure consisting of knight + martensite
Preparative high-tensile hot rolled steel sheet manufacturing method having excellent pitting corrosion resistance corrosive you characterized.