JPH05195145A - High tension steel sheet excellent in pitting corrosion resistance and its production - Google Patents

Abstract

PURPOSE:To obtain a high strength steel sheet excellent in pitting corrosion resistance and capable of withstanding use as a chassis member for an automobile in the naked state. CONSTITUTION:This high tension steel sheet excellent in pitting corrosion resistance consists of, by weight, <=0.1% C, 0.10-2.0% Mn, 0.05-0.20% P, <=0.010% S, 0.05-1.0% Cu, 0.05-2.0% Mo, 0.01-0.1% Al, <=0.008% N and the balance Fe with inevitable impurities or further contains one or more among 0.05-1.0% Si, 0.05-2.0% Ni, 0.05-5.0% Cr and 0.01-0.05% Nb and/or Ti satisfying an inequality (48/32)XS<=Ti<={(48/32)XS+(48/14)XN}X2.

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 resistance to perforation corrosion and a method for producing the same, and more particularly to a hot-rolled steel sheet suitable for undercarriage members for automobiles and its reinforcing member applications. Regarding

[0002]

2. Description of the Related Art In recent automobiles, it is necessary to guarantee no perforation and corrosion for 10 years. For this reason,
The materials used are required to have improved corrosion resistance, but there is a strong demand for improved corrosion resistance, especially for underbody materials. For example, in cold regions such as North America and Canada, road surface is prevented from freezing and corrosion caused by salts 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 hot-dip galvanized steel sheets having excellent workability.

However, many of the underbody members and reinforcing members for automobiles are manufactured by arc-welding individual parts in many cases, and when a plated steel sheet is used as a material, defects such as blowholes occur during welding. However, there was the problem that a sound weld could not be obtained.

On the other hand, automobiles are required to have improved fuel efficiency in view of the preservation of the global environment, etc., and the steel materials used have been promoted to be lighter due to higher strength. The reduction in plate thickness due to this increase in strength is disadvantageous from the viewpoint of perforation corrosion resistance.

From this point of view, for example, Japanese Patent Laid-Open No. 2-2241
Japanese Unexamined Patent Publication No. 6 discloses an alloyed galvanized steel sheet which improves the corrosion resistance of the plating base plate itself and exhibits excellent corrosion resistance even in a thin weight. However, even though it is a light-weight coating, the presence of the plating layer inevitably causes defects during welding. Moreover, the original plate is not sufficiently corrosion-resistant so that it can be used naked.

[0007]

SUMMARY OF THE INVENTION The present invention has been made for the purpose of solving the above problems, and can be used as a hot-rolled steel sheet for an automobile underbody member or a reinforcing member thereof in a bare state. It is intended to provide a high-strength steel sheet having excellent resistance to perforation and corrosion and a method for producing the same.

[0008]

According to the present invention, the weight percent is
, C ≦ 0.1%, Mn: 0.10 to 2.0%, P: 0.05 to 0.20%,
S ≦ 0.010%, Cu: 0.05 to 1.0%, Mo: 0.05 to 2.0%,
In addition to containing Al: 0.01 to 0.1%, N: 0.008% or less,
In some cases, Si: 0.05 to 1.0%, Ni: 0.05 to 2.
1% or more of 0%, Cr: 0.05 to 5.0%, Nb: 0.01 to 0.05% and / or Ti in the range according to the following formula (1), the balance being Fe and unavoidable impurities Provide a high-strength steel sheet excellent in piercing corrosion. (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 (1).

Then, a steel billet having this chemical composition is
Since the material is heated in the temperature range of 1100 to 1300 ℃ and hot-rolled, and the hot-rolling is completed at the Ar ₃ transformation point or higher and in the temperature range of 800 to 950 ℃, and continuously wound at the temperature of 700 ℃ or lower. Provided is a method for producing a high-strength hot-rolled steel sheet having excellent pitting corrosion resistance.

[0010]

[Function] As a result of various test studies to solve the above-mentioned problems, the present inventors have found that among the basic components of the steel sheet,
Strictly adding C, Mn, P, Cu, Mo within the above range, and further adding at least one of Si, Ni, Cr and Nb within the above range,
It was also found that by adding a small amount of Ti that satisfies Eq. (1), high strength and excellent resistance to pitting corrosion can be achieved. From the viewpoint of manufacturing conditions, it was found that by appropriately controlling the conditions of hot rolling, it is possible to provide workability that satisfies the characteristics required for automotive underbody parts.

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

C is an element effective for increasing the strength. 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, but 0.1
%, The effect is not observed, and if the content exceeds 2.0%, the workability deteriorates, so 0.10-2.0%
The range is.

P is an indispensable additional 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 provide a dense corrosion product with good adhesion. Which serves to improve the resistance to pitting corrosion. However, if P is contained in excess of 0.2%, the workability will deteriorate, so 0.05 to 0.
Add in the range of 2%.

S deteriorates the workability. Particularly, when MnS is combined with Mn to form MnS, the hole expandability is deteriorated. Therefore, it is preferable that S is as small as possible, but 0.010
% Is acceptable, so 0.010% or less.

Cu is an element that effectively acts to improve the resistance to pitting corrosion when added in combination with P as described above. If the addition amount is less than 0.05%, the effect is not observed, and if the addition amount exceeds 1.0%, not only the effect is saturated, but also the manufacturing cost increases, so the addition range is set to 0.05 to 1.0%.

Mo is also an essential element in the steel of the present invention,
It is effective in increasing the strength and improving the resistance to pitting corrosion. 0.05
If less than%, those effects are not recognized. If it is added in excess of 2.0%, the effect of improving the pitting corrosion resistance is saturated and the workability deteriorates, which is also a cost disadvantage. For this reason, the addition range of Mo is set to 0.05 to 2.0%.

N adversely affects corrosion resistance and workability.
Therefore, it is desirable that the amount is as small as possible, but 0.008% is acceptable in the present invention, so the content is made 0.008% or less.

Al is added as a deoxidizing agent, but if it is less than 0.01%, the effect as a deoxidizing agent is not recognized, and if it exceeds 0.1%, the effect is saturated, and Al is produced. Since the cost will be high, the upper limit was set to 0.1%.

Si is an element which is effective in increasing the strength and improving the corrosion resistance. If it is less than 0.05%, its effect is not observed, and if it exceeds 1.0%, the surface properties of the product are deteriorated and the workability is also deteriorated, so it is contained in the range of 0.05 to 1.0%.

Ni is an element that effectively acts to suppress the occurrence of hot cracking during hot rolling due to the addition of Cu, and is also effective in increasing the strength and improving the resistance to pitting corrosion. Addition amount is 0.05
If less than%, those effects are not recognized. Even if the content exceeds 2.0%, the effect is saturated and the cost is increased. For this reason, the Ni content range is 0.05 to 2.0%.

Cr is also an element effective for increasing the strength and improving the resistance to pitting corrosion. If less than 0.05%, the effect is not recognized, and if added over 5.0%, not only the effect is saturated, but also the manufacturing cost is significantly increased compared to the effect, so it is contained in the range of 0.05 to 5.0%. Let

Nb effectively reduces the crystal grain size of the steel sheet to enhance the strength. However, if the addition amount is less than 0.01%, the effect is not recognized, and if the addition amount exceeds 0.05%, precipitation of Nb carbonitrides increases and the strength increases but the workability deteriorates significantly. It is contained in the range of%.

Ti is an element that fixes S and N and effectively acts to improve workability, especially hole expandability. This effect is not observed when the Ti amount is less than (48/32) × S as shown in the equation (1). Also [(48/32) × S + (48/14) × N] × 2
Even if the content exceeds the above range, the effect is saturated and the cost increases, so it is necessary to contain within the range shown in the above formula (1).

By containing the above chemical components in the above range, a high-strength steel sheet excellent in perforation corrosion resistance can be obtained. Further, various hot rolling conditions are examined when manufacturing the hot rolled steel sheet. As a result, they have found that by appropriately controlling the extraction temperature, the hot rolling end temperature and the coiling temperature, excellent workability can be imparted to this steel sheet.

If the extraction temperature of the steel slab from the heating furnace during hot rolling (steel slab heating temperature) is less than 1100 ° C., the dissolution of carbonitrides will be insufficient and good workability cannot be obtained. Further, heating above 1300 ° C does not show any further effect on the dissolution of carbonitrides, but rather leads to an increase in manufacturing costs.
Control to the range of.

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 less than the Ar ₃ transformation point or less 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 after the processing, rough skin called orange peel occurs. Also, when the winding temperature exceeds 700 ° C, the scale layer on the surface becomes thick and the pickling property deteriorates.
It is necessary to wind at a temperature below 700 ° C.

[0028]

【Example】

[Example 1] Steel having the chemical composition shown in Table 1 was melted to obtain a continuously cast slab. This slab was heated to 1230 ° C and hot-rolled at a hot rolling finish temperature of 900 ° C and a coiling temperature of 550 ° C to obtain a hot-rolled sheet with a thickness of 2.6 mm. The obtained hot-rolled sheet was pickled, and then test pieces were collected and subjected to a combined cycle corrosion test of salt spray, drying, and wet (3 cycles / day) under the conditions shown in FIG. Perform this corrosion test for 30 cycles, 60 cycles, 12 cycles
After 0 cycles and up to 240 cycles were performed, corrosion products were removed and the maximum erosion depth and corrosion weight loss were measured to evaluate the pitting corrosion resistance. The test results are shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

As is clear from Table 2, P of the example of the present invention,
The steel containing Cu and Mo added together has a smaller corrosion weight loss and a shallower maximum erosion depth than those of the steels of No. 17 to 21 in which P, Cu and Mo are not added or the composite addition is not performed at the same time. It can be seen that it has excellent resistance to perforation and corrosion. Comparative example
No. 21 is a steel in which P and Cu are added but no Mo is added, and it can be seen that both the corrosion weight loss and the maximum erosion depth are improved by the combined addition of P and Cu. It cannot be said that it is a sufficient value compared with steel.

[Example 2] Of the steels shown in Table 1, No.
2, 8 and 15 were hot-rolled under the hot-rolling conditions shown in Table 3 to a sheet thickness of 2.6 mm, and the hot-rolled sheet was subjected to a tensile test and a hole expansion test. Tensile tests were performed on JIS No. 5 test pieces, and hole expandability tests were performed by punching to a diameter of 10 mm with a clearance of 20% and using a ball head punch with a diameter of 50 mm. The results of these tests are shown in Table 4.

[0033]

[Table 3]

[0034]

[Table 4] As is clear from Table 4, in the case of A, B and E which adopted the hot rolling conditions specified in the present invention, a hot rolled sheet showing good values in both tensile properties and hole expandability was obtained. No rough surface is observed. When the extraction temperature is lower than that specified in the present invention, and when the hot rolling end temperature is F, the rough surface after processing is not recognized, but the ductility and hole expandability are poor, and the underbody parts for automobiles It is not possible to satisfy the workability required as. On the other hand, under the condition D in which the hot rolling finish temperature is high, the tensile properties and the hole expansibility are almost the same values as those of the examples of the present invention, but roughening of the surface occurs after processing.

[0035]

As described above, according to the present invention, a high-strength steel sheet having excellent resistance to perforation and corrosion can be obtained. This steel sheet has an effect not seen in conventional materials as an underbody member or a reinforcing member for automobiles even without plating treatment. Exert. That is, not only is it effective in reducing the weight of automobiles by increasing the strength, but it has also been possible to greatly improve the perforation corrosion resistance, especially as a material for the underbody parts where plated steel sheets could not be used until now.

[Brief description of drawings]

FIG. 1 is a test cycle diagram showing conditions of a corrosion test.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mikio Soge 11-11 Showa-cho, Kure City, Hiroshima Prefecture Nisshin Steel Co., Ltd.

Claims (3)

[Claims] 1. By weight%, C ≦ 0.1%, Mn: 0.10 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 following are high-strength steel sheets with the balance being Fe and unavoidable impurities with excellent perforation and corrosion resistance. 2. By weight%, C ≦ 0.1%, Mn: 0.10 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%
In addition to containing the following, 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 kinds and / or Ti in the range according to the following formula (1), and the balance Fe and inevitable impurities. High-strength steel plate (48/32) x S ≤ Ti ≤ {(48/32) x S + (48/14) x N} x 2 ··· (1) with excellent perforation and corrosion resistance. 3. By weight%, C ≦ 0.1%, Mn: 0.10 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%
In addition to the following, depending on the case, Si: 0.05
~ 1.0%, Ni: 0.05 ~ 2.0%, Cr: 0.05 ~ 5.0%, Nb:
0.01 to 0.05% of one or more and / or the following formula
A steel slab containing Ti in the range according to (1) and the balance Fe and unavoidable impurities is heated in a temperature range of 1100 to 1300 ° C. to hot-roll, and the hot-rolling is performed at an Ar ₃ transformation point. Above, and finished in the temperature range of 800 ~ 950 ℃, continue to 700 ℃
Method for producing high-strength hot-rolled steel sheet excellent in perforation corrosion resistance consisting of winding at the following temperature, (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ··· (1).