JPH07224353A - Hot rolled corrosion resistant steel sheet and its production - Google Patents

Abstract

PURPOSE:To produce a hot rolled corrosion resistant steel sheet excellent in fatigue resistance in the weld zone and to provide a method for producing the same. CONSTITUTION:(1) This hot rolled corrosion resistant steel sheet excellent in fatigue resistance in the weld zone has a compsn. contg. 0.006 to 0.08% C, 0.60 to 1.60% Si, 0.50 to 1.50% Mn, 0.03 to 0.10% P, 0.30 to 1.00% Mo, 0.20 to 1.00% Cu, 0.10 to 1.00% Ni, 0.01 to 0.10% Al and 0.0050 to 0.0150% N, and in impurities, the content of Cr is regulated to <=0.1% and S to <=0.010%. (2) This hot rolled corrosion resistant steel sheet is produced by reheating or directly feeding the stock steel slab having the above components to be subjected to hot rolling, and coiling at 350 to 550 deg.C. The finish rolling is completed at the temp. of Ar3 transformation point or above. Thus, the hot rolled high strength steel sheet excellent in pore expandability, fatigue resistance and poring resistance in the arc weld zone and suitable as the stock for automobile under carriage parts can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is mainly applied to the material for undercarriage parts of automobiles and is hot rolled with a tensile strength of 540 to 780 N / mm ² which is excellent in fatigue resistance and corrosion resistance of welded parts. Regarding steel plates.

[0002]

2. Description of the Related Art In recent years, automobiles have been required to improve fuel consumption in order to cope with environmental problems and the like. For this reason, high-strength steel sheets have been used to reduce the thickness of parts and reduce their weight. In particular, in the undercarriage parts of automobiles, the effect of improving fuel efficiency by reducing the weight is great, so use of up to 780 N / mm ² class high-strength steel sheets is being considered.

In order to cope with the weight reduction (thinning) of the underbody parts, it is necessary not only to increase the strength of the material but also to prevent the perforation resistance and the fatigue resistance of the welded portion from deteriorating as the thickness decreases. However, generally, when the wall thickness is reduced, these performances are deteriorated. Therefore, a high-strength steel sheet that has been subjected to galvannealing has been used.

However, when arc welding is performed on this plated steel sheet, blowholes are generated by the vapor of zinc and the perforation resistance and fatigue resistance are rather reduced. Therefore, the welding speed is slowed or the welded portion is not welded before welding. It is necessary to remove the zinc plating of the above, and there is a problem that workability is significantly impaired. Therefore, recently, a corrosion resistant steel plate in which the steel plate itself has corrosion resistance has been proposed.

As a steel sheet for automobiles having high corrosion resistance, for example, Japanese Patent Publication No. Sho 60-9584 discloses C: 0.001 to 0.05%, Si: 0.00
05-0.3%, Mn: 0.01-0.29%, Al: 0.001-0.07%,
Cu: 0.26-0.35%, P: 0.005-0.02% and Ni: 0.03
~ 0.09% as the basic component, and other Nb, V, Mo, B 1
Species or two or more species (0.01-0.3% for Nb, V, Mo, B = 0.
0001 to 0.05%) and, if necessary, Cr: 0.01 to 0.1% is disclosed.

As a highly corrosion-resistant steel sheet having excellent formability, for example, Japanese Patent Publication No. 1-53344 discloses C: 0.02% or less, Si:
0.3% or less, Mn: 0.5% or less, P: 0.025% or less, S:
0.025% or less, acid-soluble Al: 0.02 to 0.15%, Ti: C amount of 4
It is disclosed that the content is more than double and 0.02% to 0.3%, Cr: 5.5 to less than 10%, Cu: 0.01 to 0.50% and Ni: 0.01 to 0.50%.

The applicant of the present invention has disclosed that, in Japanese Patent Application Laid-Open Nos. 4-141525 and 4-141526, Cr: 0.5 to 5.0%,
A method for producing a hot-rolled high-strength steel sheet having excellent corrosion resistance, which is most characterized by containing P: 0.12% or less and Cu: 0.5 to 3.0%, is shown.

[0008] Corrosion resistance A steel sheet for automobiles is characterized by adding Cu to improve the corrosion resistance. However, none of the above inventions considers corrosion resistance and fatigue, which is an important factor. In Japanese Examined Patent Publication No. 1-53344, the evaluation of corrosion resistance is carried out without coating, and it cannot be said that this steel sheet is suitable as a material for undercarriage parts of automobiles on the premise of painting.

Further, Cr deteriorates the chemical conversion treatment property and also the coating adhesion. Therefore, there is a problem that the coating adhesion of a steel sheet containing an excessive amount of Cr deteriorates when electrodeposition coating is performed as in an automobile underbody part.

JP-A-4-329848, JP-A-4-337037, JP-A-5-51646 and JP-A-5-179346 disclose high strength in consideration of fatigue characteristics, notch fatigue strength and formability. A hot rolled steel sheet is shown.

In the above-mentioned conventional steel sheets, corrosion resistance, formability or general fatigue characteristics are taken into consideration, but none of them are simultaneously examining corrosion resistance and weld fatigue resistance.

Generally, when the strength of the material increases, the fatigue resistance of the arc welded portion relatively decreases.

This tendency is particularly remarkable in a material having a tensile strength of 540 N / mm ² or more. The main causes of this include stress concentration at the toe of the weld bead, which is the origin of the crack, and softening of the weld heat affected zone.

In the undercarriage parts of automobiles, the weld heat affected zone is particularly apt to corrode due to salt water accumulated near the weld bead, repeated stress during running, and residual stress during welding. Further, since the holes due to the corrosion serve as a concentrated source of stress, the fatigue performance after being exposed to the corrosive environment becomes important.

[0015]

SUMMARY OF THE INVENTION The object of the present invention is to provide high hole expandability and corrosion resistance of the arc welded portion, as well as excellent fatigue resistance, which is important in actual use as an undercarriage component of automobiles. It is to provide a high-strength hot-rolled steel sheet and a manufacturing method thereof.

[0016]

The gist of the present invention lies in the following hot-rolled corrosion-resistant steel sheet and its manufacturing method.

(1)% by weight, C: 0.006 to 0.08%, Si:
0.60 to 1.60%, Mn: 0.50 to 1.50%, P: 0.03 to 0.10%,
Mo: 0.30-1.00%, Cu: 0.20-1.00%, Ni: 0.10-1.00
%, Al: 0.01 to 0.10% and N: 0.0050 to 0.0150%, the balance consisting of Fe and unavoidable impurities, and Cr in the impurities is 0.1% or less and S is 0.010% or less. Hot-rolled corrosion-resistant steel sheet with excellent fatigue resistance in the weld zone.

(2) Welding, characterized in that the raw steel slab having the above-mentioned components is reheated or directly sent to be hot-rolled, finish rolling is completed at ₃ or more points of Ar, and wound at 350 to 550 ° C. Method for producing hot-rolled corrosion-resistant steel sheet having excellent fatigue resistance of the part.

In order to improve the corrosion resistance of the present invention, Cu,
P is added and the amount of S is suppressed, the amount of Cr is suppressed to prevent deterioration of corrosion resistance after coating, and Si, Mo and N are added to improve fatigue resistance of the welded part. It is a feature.

[0020]

First, the reasons why the chemical compositions of the steel sheet of the present invention and the raw material steel to be the subject of the method of the present invention are determined as described above will be described together with the operation effects. % Means% by weight.

C: 0.006 to 0.08% If the C content is less than 0.006%, the grain boundary strength decreases, and when P is segregated at the grain boundaries, an impact is applied after working, causing brittle fracture along the grain boundaries. Secondary processing brittleness resistance decreases. On the other hand, if it exceeds 0.08%, the second phase such as pearlite and bainite increases, and the workability decreases. Therefore, the range of the C content is 0.006 to 0.08%.

Si: 0.60 to 1.60% Si improves workability, particularly hole expansibility which is important for underbody parts. Further, the bead shape of the arc welded portion is smoothed to reduce the stress concentration at the toe portion of the bead and improve the fatigue resistance. This effect is not recognized unless the Si content is 0.60% or more. On the other hand, if it exceeds 1.60%, the chemical conversion treatment, which is the base treatment of the coating, deteriorates, and therefore the corrosion resistance after coating also deteriorates. Therefore, the Si content range is 0.60 to 1.60%
And

Mn: 0.50 to 1.50% Mn is an element necessary for obtaining the tensile strength of 540 to 780 N / mm ² grade required by the steel sheet of the present invention. Mn content is 0.50
If it is less than 10%, the hardenability becomes insufficient and the required strength cannot be obtained. On the other hand, if it exceeds 1.50%, this effect is saturated and the cost increases. Therefore, the range of Mn content is
It was set to 0.50 to 1.50%.

P: 0.03 to 0.10% P improves the corrosion resistance when added together with Cu. This effect appears when the P content is 0.03% or more. However, if it exceeds 0.10%, although it is preferable from the viewpoint of corrosion resistance, it causes a reduction in the secondary processing brittleness resistance. Therefore, the P content range is 0.03 to 0.
It was set to 10%.

Cr, Mo, N: Cr is 0.1% or less, Mo is 0.30 to
Since 1.00% and N is 0.0050 to 0.0150% Cr is an impurity that deteriorates chemical conversion treatability and fatigue resistance, it is desirable to make it as low as possible. Particularly, in the heat-affected zone of welding, the surface temperature of Cr is increased and the oxidation thereof occurs due to the temperature rise due to welding, so that the chemical conversion treatment property is remarkably lowered and the adhesion of the coating film is lowered. As a result, the corrosion resistance after coating deteriorates.

When the Cr content exceeds 0.1%, perforation, which is particularly important for undercarriage parts, is promoted. Therefore, not only the perforation resistance but also the holes due to corrosion serve as a stress concentration source to reduce fatigue resistance. Bring

Mo has its own effect of improving corrosion resistance,
It also improves hardenability. Furthermore, it is the most effective element for forming fine carbides such as MoC and preventing softening of the heat-affected zone, which is one of the factors that reduce the fatigue resistance of welds.

N is a low temperature (about 170 ° C.) that is equivalent to baking.
Even before and after), it diffuses and moves easily, sticks to the strain (dislocation) generated by processing and welding of the underbody parts, and combines with the iron atoms in the base material to form fine iron nitride. In the weld heat affected zone, the quenching effect of Mo and Mn cannot be exhibited in the region where the temperature does not rise above the A ₁ transformation point. N is an element effective in preventing softening of this region.

The results of experiments conducted by the present inventors to confirm the effects of Cr, Mo and N on fatigue resistance will be described.

Material steels having the chemical compositions shown in Table 1 were melted in a vacuum melting furnace.

[0031]

[Table 1]

After forging these, finishing temperature 900-92
It was hot-rolled at 0 ° C to a plate pressure of 2 mm and spray-cooled to 450 ° C. Then, after holding in a heat treatment furnace kept at 450 ° C. for 2 hours, it was cooled and taken out to obtain a test material. The obtained test piece was subjected to arc welding, subjected to chemical conversion treatment and electrodeposition coating to obtain a sample for corrosion test.

After the corrosion test for 3 months, the coating film was removed and a fatigue test piece was cut out to carry out a fatigue test. The result is shown in FIG. The test conditions are as follows.

Welding conditions: CO ₂ arc welding, wire Y
GW-12, welding speed 100cm / min, welding current 200A,
Welding voltage 25V Chemical conversion treatment: dip method, chemical conversion liquid PBL3020 (manufactured by Nippon Parker Co., Ltd.) Electrodeposition coating: PTU-80 (manufactured by Nippon Paint Co., Ltd.), film thickness 20
μm Corrosion test: Volvo cycle test (outdoor exposure, once / week
5% NaCl aqueous solution is sprayed on the test piece) Fatigue test: Schenk-type swing bending fatigue test (manufactured by Tokyo Koki Co., Ltd.) Fatigue resistance depends on the strength of the material, so the fatigue resistance is evaluated by the tensile strength of the material. The "durability ratio" excluding the fatigue limit strength was used. Generally, the durability ratio of the arc welded portion of the mild steel plate is 0.7 or more under this test condition. Therefore, even with the high strength steel plate, the same durability ratio is suitable for the weight reduction of the underbody parts.

FIG. 1 shows the effect of C on the durability ratio of arc welds.
It is a figure which shows the influence of the content of r, Mo, and N. As is clear from Fig. 1, an increase in the Cr content causes deterioration of the durability ratio, and in order to increase the durability ratio to 0.7 or more, the content should be 0.1
% Or less.

Further, by properly containing Mo and N, a durability ratio of 0.7 or more can be obtained. The content of these elements must be 0.30% or more for Mo and 0.0050% or more for N.

If the Mo content exceeds 1.00%, the effect of improving the durability ratio is saturated, and since Mo is an expensive element, the cost increases. Therefore, the Mo content range is 0.30 to 1.
It was set to 00%.

If the N content exceeds 0.0150%, bubble-like defects are likely to occur at the slab stage, and flaws called hegging are generated on the surface of the steel sheet, impairing the surface quality. Therefore, N
The content range was 0.0050 to 0.0150%.

Cu: 0.20 to 1.00% Like P, Cu is an essential element for improving corrosion resistance. However, a content of 0.20% or more is necessary to obtain the effect. On the other hand, when it exceeds 1.00%, this effect is saturated. Therefore, the Cu content range is 0.20 to 1.00%.

Ni: When 0.10 to 1.00% Cu is contained, Cu checking (Cu is concentrated in the balance due to the oxidation of iron on the slab surface to lower the melting point,
Cu easily corrodes the grain boundaries of the steel, causing cracks on the slab surface), and in order to prevent this, it is necessary to add Ni at 1/2 or more of the Cu content.

As described above, Ni is an essential element for preventing Cu checking. Since this effect is obtained when the Cu content is 1/2 or more, the lower limit was made 0.10%. On the other hand, the upper limit of 1.00% is sufficient to prevent Cu checking,
Further, since Ni is also an expensive element, if it exceeds 1.00%, it causes a cost problem. For these reasons, the Ni content range is
It was set to 0.10 to 1.00%.

Al: 0.01 to 0.10% Al is an element necessary as a deoxidizing agent. Al content 0.01%
If it is less than the above, the deoxidizing effect is small. On the other hand, if it exceeds 0.10%, the deoxidizing effect is saturated and the cost is increased.

S: 0.010% or less S is a desirable impurity if it is low unless there is an economic problem. If the S content exceeds 0.010%, it will bond with Mn.
It becomes MnS, which is the starting point of perforation in a corrosive environment. In addition, MnS also serves as the starting point for cracks during hole expansion,
Poor expandability is reduced. Therefore, the upper limit of S content is 0.01
It was set to 0%.

Next, the reasons for limiting the manufacturing method and manufacturing conditions of the present invention will be described.

The hot rolling is carried out by a usual method after the raw material steel slab of the above components produced by melting in a converter or the like and produced by continuous casting is directly fed or heated to 1100 ° C. or higher.

Finish rolling in hot rolling is completed in a temperature range of Ar ₃ or higher. This is because when rolling is performed in a temperature range of less than the Ar ₃ point, ferrite transformation occurs, and the ferrite undergoes processing to deteriorate ductility.

The winding must be carried out in the temperature range of 350 to 550 ° C. In the case of steel plates used for difficult-to-machine parts among the undercarriage parts, improvement of workability is also important. In the case of such an underbody part, the hole expansibility is a particularly important characteristic. In order to improve the hole expandability, it is effective to make the material structure a uniform ferrite-bainite structure.

If the winding temperature exceeds 550 ° C., it becomes difficult to obtain bainite. On the other hand, if the temperature is lower than 350 ° C, the martensite structure is mixed and the uniformity of the structure is impaired.

[0049]

Example Steels having the chemical compositions shown in Table 2 were melted in an actual converter and then continuously cast into a slab having a thickness of 250 mm. The obtained slab was heated to 1250 ° C., then hot-rolled to a thickness of 2.6 mm at a finishing temperature of 870 to 900 ° C. and wound at the winding temperature shown in Table 2.

[0050]

[Table 2]

After pickling these coils, a sample was taken and subjected to welding, chemical conversion treatment, under the same test conditions as above.
After applying the electrodeposition coating, a corrosion test was conducted. The period of this corrosion test was set to one year to eliminate seasonal factors.

The coating film on the test piece after the corrosion test was removed and the maximum corrosion depth of the heat-affected zone of the weld was measured, which was used as an index for evaluating the puncture resistance. Then, the fatigue test sample was cut out from the position showing the maximum corrosion depth, and the fatigue test was performed.
The fatigue resistance was evaluated by the durability ratio defined by the following formula.

Durability ratio (%) = (fatigue limit strength / material tensile strength) × 100 Further, JIS No. 5 test pieces were cut out from the pickled coil, and the tensile strength was measured and the hole expandability was also evaluated. It was

The hole expandability is determined by forming a punched hole having a diameter of 12 mm, expanding the hole with a conical punch having a tip angle of 60 degrees with the burr on the punch side, and measuring the hole diameter when a crack is generated. Was evaluated by the method of obtaining. The definition of the hole expansion ratio is as follows.

Pore expansion ratio (%) = [(hole diameter at the time of crack occurrence−punching hole diameter) / punching hole diameter] × 100 Table 3 shows the evaluation test results.

[0056]

[Table 3]

As is clear from Table 3, all of the examples of the present invention are high strength hot rolled steel sheets excellent in perforation resistance, fatigue resistance (durability ratio) and hole expansion resistance. Comparative examples J, Mo or N with low Si content Comparative examples M and N with low Si content
In each case, the fatigue resistance is lowered. In Comparative Example Q in which the Cr content is too high, both fatigue resistance and puncture resistance are reduced. Comparative example K in which P or Cu and Ni contents are insufficient
And L, the perforation resistance is poor. In Comparative Examples O and P, the winding temperature was not appropriate, and the hole expandability was lower than that of Example B of the present invention having the same composition.

[0058]

According to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet which is excellent in fatigue resistance and puncture resistance of the arc welded portion. According to the method of the present invention, it is possible to improve the hole expandability by controlling the winding temperature, and it is suitable as a material for undercarriage parts of automobiles, and it is also possible to manufacture a steel sheet for difficult forming applications.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of contents of Cr, Mo and N on a durability ratio of an arc welded portion.

Claims (2)

[Claims] 1. C: 0.006 to 0.08% by weight, Si: 0.60
~ 1.60%, Mn: 0.50-1.50%, P: 0.03-0.10%, Mo:
0.30-1.00%, Cu: 0.20-1.00%, Ni: 0.10-1.00%,
Welds containing Al: 0.01 to 0.10% and N: 0.0050 to 0.0150%, the balance consisting of Fe and unavoidable impurities, Cr in the impurities being 0.1% or less and S being 0.010% or less. Hot-rolling corrosion-resistant steel sheet with excellent fatigue resistance. 2. C: 0.006 to 0.08% by weight, Si: 0.60
~ 1.60%, Mn: 0.50-1.50%, P: 0.03-0.10%, Mo:
0.30-1.00%, Cu: 0.20-1.00%, Ni: 0.10-1.00%,
Reheat a raw steel slab containing Al: 0.01 to 0.10% and N: 0.0050 to 0.0150%, the balance consisting of Fe and unavoidable impurities, Cr in the impurities being 0.1% or less and S being 0.010% or less. Alternatively, a method for producing a hot-rolled corrosion-resistant steel sheet excellent in fatigue resistance of a welded portion, which comprises directly sending and hot-rolling, finishing rolling with Ar ₃ points or more, and winding at 350 to 550 ° C.