JPH09118929A - Manufacture of corrosion resistant hot rolled steel sheet excellent in resistant to secondary operation brittleness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for more inexpensively manufacturing a hot- rolled steel sheet excellent in resistance to secondary operation brittleness and corrosion resistance as a material for automotive skirt parts. SOLUTION: A slab consisting of 0.01-0.07% C, 0.8-2.0% Mn, <=0.04% P, <=0.002% S, <=0.05% Cu, <=0.005% Ti, <=0.005% Nb and the balance Fe with inevitable impurities is heated at 1,050-1,300 deg.C and, after completing hot rolling at 830-900 deg.C, the hot rolled steel sheet is cooled at cooling rate of >=35 deg.C/sec and coiled at <=300 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a corrosion-resistant hot-rolled steel sheet, and more particularly to a method for producing a corrosion-resistant steel sheet having excellent secondary work embrittlement resistance which is mainly used for undercarriage parts of automobiles.

[0002]

2. Description of the Related Art In recent years, there has been an increase in the use of plated steel sheets as steel sheets for undercarriage parts of automobiles due to the demand for further improvement in corrosion resistance. However, plated steel sheets are expensive and have drawbacks such as deterioration of hole expandability, reduction of workability due to suppression of blowholes in welds, and enormous cost for measures against blowholes.

For this reason, there is a demand for a relatively inexpensive steel sheet which is superior in corrosion resistance, in place of the plated steel sheet. As a hot rolled steel sheet for underbody parts that can meet these requirements, it is necessary to have both excellent perforation resistance and secondary work embrittlement resistance.

That is, as to the corrosion resistance, the underbody parts are one of the uses for the reinforcing member, and therefore, excellent perforation resistance in the bare state is required, and the underbody parts having a complicated shape are subjected to severe press forming. In some cases, it is necessary to have excellent secondary work embrittlement resistance.

As a conventional technique, for example, Japanese Patent Application Laid-Open No. 5-1178
No. 02 discloses a high-strength steel sheet for automobile underbody, which has excellent corrosion resistance, in a weight ratio of C: 0.005 to 0.20%, Si: 0.005 to
1.0%, Mn: 0.1 to 2.5%, P: 0.050 to 0.10%, S:
0.001 to 0.010%, Al: 0.005 to 0.1%, N: 0.0005 to
0.0100%, Cu: 0.10 to 0.50%, Nb: 0.01 to 0.05%, Mo:
A steel sheet having a steel composition of 0.10 to 0.20% has been proposed.

Further, Japanese Patent Laid-Open No. 5-171289 discloses a method for producing a corrosion-resistant hot rolled steel sheet having good stretch flangeability.
By weight ratio, C: 0.05% or less, P: 0.03 to 0.15%, Mn: 0.
5 to 2.5%, Si: 2.0% or less, Cu: 0.05 to 0.50%, and
Ti: 0.02-0.06%, Nb: 0.01-0.04% and Ni: 0.050
~ 0.50% of steel sheet containing one or more of them is hot-rolled at a finishing temperature of 880 ° C or more, an average cooling rate of 700-600 ° C is 60 ° C / s or less, and a winding temperature is 350-550. A method of manufacturing under the condition of ° C is disclosed.

However, in the prior arts disclosed in these publications, in order to improve the corrosion resistance of the steel sheet, P or
It was necessary to include Cu. However, when P is added, P segregates at the grain boundaries, resulting in a marked deterioration in secondary processing resistance. Further, when Cu is added, surface defects due to hot brittleness occur. Furthermore, in order to prevent the surface defects caused by Cu, it is necessary to add expensive Ni together, and eventually, an expensive material cannot be avoided.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is generally to provide a method for producing a corrosion-resistant hot-rolled steel sheet excellent in secondary work embrittlement resistance.

More specifically, the object of the present invention is, as a material for undercarriage parts for automobiles, for example, in a longitudinal cracking test, the generation temperature thereof is −60 ° C. or less, which is excellent in secondary working brittleness resistance and ordinary steel. It is an object of the present invention to provide a method for manufacturing a hot-rolled steel sheet having excellent corrosion resistance of 60% or less at a low cost at a low cost.

[0010]

Means for Solving the Problems Accordingly, the present inventors have:
The following findings were obtained through repeated studies on a method of manufacturing a hot-rolled steel sheet that is inexpensive and has the above-described characteristics in terms of materials and manufacturing process.

(1) The addition amount of P, which was thought to be essential for improving the perforation resistance, was suppressed to 0.04% or less,
C: 0.01 to 0.07% and an appropriate amount of C are added, and a hot rolled steel sheet excellent in secondary work embrittlement resistance can be manufactured by manufacturing under appropriate rolling conditions.

(2) In order to improve the puncture resistance even if the addition of Cu, which causes surface defects, is suppressed, Mn: 0.8-2.0
%, The amount of C added is controlled to 0.01 to 0.07%, and the perforation resistance can be improved by manufacturing under appropriate rolling conditions.

(3) After the hot rolling is completed, the material is rapidly cooled at a cooling rate of 30 ° C./sec or more and is wound at a low temperature of 300 ° C. or less, so that the crystal grains can be made finer and the above two characteristics can be achieved. A synergistic improvement of is realized.

Therefore, in the present invention, in% by weight, C: 0.01 to
0.07%, Mn: 0.8-2.0%, P: 0.04% or less,
S: 0.002% or less, Cu: 0.05% or less, Ti: 0.
1050 to 1300 ° C for a slab having a steel composition of 005% or less, Nb: 0.005% or less, and the balance of iron and unavoidable impurities.
After finishing the hot rolling at 830 ℃ or more and 900 ℃ or less, it is cooled at a cooling rate of 35 ℃ / sec or more and wound at a temperature of 300 ℃ or less. It is a method of manufacturing an excellent corrosion-resistant hot-rolled steel sheet.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the scope of the present invention is defined as described above will be described in detail below. First, the reason for defining the chemical composition of the raw material steel to be the subject of the present invention will be described together with its function and effect. In the present specification, “%” means% by weight unless otherwise specified.

C: 0.010 to 0.07% C is an element which has an important influence on secondary work embrittlement resistance and perforation resistance. The presence of C has the effect of increasing the bond strength of the grain boundaries. Therefore, if C <0.010%, it is difficult to prevent the deterioration of the secondary work embrittlement resistance due to P segregated at the grain boundaries. On the other hand, if it exceeds 0.07%, corrosion resistance decreases due to an increase in pearlite. Therefore, in the present invention, the C content is set to 0.010 to 0.070%. Preferably 0.010-0.
It is 030%.

Mn: 0.8-2.0% Mn is an important element for improving the puncture resistance, and its effect is particularly large in the low C region (C: 0.07% or less). The mechanism by which Mn improves perforation resistance is not clear, but Mn
The addition of Al has the effect of suppressing the formation of pearlite, which is the starting point of corrosion. In addition, since Mn forms a solid solution in ferrite, it is considered that the potential difference between ferrite and cementite in bainite becomes small and it becomes difficult to become a starting point of corrosion. The effect of improving the perforation resistance of Mn becomes large at 0.8% or more. Further, even if Mn is added in excess of 2.0%, not only the corrosion resistance is not improved but the formability is remarkably lowered.
Therefore, the upper limit was set to 2.0%. Preferably 1.0-1.5
%.

P: 0.040% or less P is an element which improves the puncture resistance by preferably adding 0.03% or more. However, P easily segregates at the grain boundaries and deteriorates the secondary work embrittlement resistance. Therefore, the content of P is 0.
040% or less. More preferably, it is limited to 0.02% or less.

S: 0.002% or less S is an element that deteriorates the perforation resistance. Particularly when the addition amount of Cu and P is small as in the present invention, the corrosion resistance is greatly deteriorated due to the presence of S. When S is 0.002% or less, MnS, which is the starting point of corrosion, becomes extremely small, and the corrosion resistance is improved.
On the other hand, when S exceeds 0.002%, deterioration of perforation resistance starts to occur. Therefore, the S content should be 0.002% or less. It is preferably 0.001% or less.

Cu: 0.05% or less Cu is an element that generally improves the puncture resistance by adding 0.2% or more, but due to hot brittleness, it causes surface defects and greatly reduces the yield. Further, since Ni added as a countermeasure against this surface flaw is very expensive, the addition of Ni causes a large increase in manufacturing cost. According to the present invention,
As for surface defects, when Cu is less than 0.05%, surface defects do not occur even if Ni is not added. Therefore, the addition amount of Cu is set to 0.05% or less.

Ti, Nb: 0.005% or less Originally, Ti and Nb were added as strengthening elements, and in a material in which Cu and P were positively added, they had little influence on the puncture resistance. However, as in the present invention, C
It was found that Ti and Nb rather deteriorate the puncture resistance when the amounts of u and P added are small. This is probably because the carbonitrides of Ti and Nb deteriorate the corrosion resistance. Therefore, the upper limits of Ti and Nb are each set to 0.005%. Preferably, each is limited to 0.003% or less. The lower limits are not limited, but preferably 0.001% or more.

Next, the reasons for limiting the manufacturing conditions will be described. First, the raw material steel slab may be, for example, a raw material steel slab that is melted in a converter or the like and that is manufactured by continuous casting or the like and that satisfies the above-mentioned range of components.

The heating temperature is 1050-1300 ° C. This is 10
If the temperature is lower than 50 ° C., the edge or the like may be partially lowered from the Ar ₃ point during hot rolling, which may cause deterioration of workability or defective shape. If it exceeds 1300 ° C, damage to the furnace and scale loss will increase. Therefore, the heating temperature is set to 1050-1300 ° C. It is preferably 1100 to 1230 ° C.

The finish rolling is completed in the temperature range of 830 ° C to 900 ° C. This is because rolling in a temperature range of less than 830 ° C. may cause ferrite transformation, and the ferrite may be processed and deteriorate ductility. Also, if the temperature exceeds 900 ° C., the crystal grains become coarse, which causes deterioration of the secondary work embrittlement resistance.

The average cooling rate and coiling temperature after hot rolling are important for secondary work embrittlement resistance and perforation resistance. Regarding the effect of improving the secondary processing brittleness resistance, C is 0.01% or more,
By limiting P to 0.04% or less, and setting an average cooling rate of 35 ° C / sec or more and a winding temperature of 300 ° C or less,
A very good improvement effect is shown. This is a result of the two effects of preventing grain boundary segregation of P and refining the crystal grains to act synergistically under such manufacturing conditions.

Regarding the effect of improving the perforation resistance, C: 0.
By limiting the amount to 07% or less and Mn: 0.8% or more, and setting the average cooling rate to 35 ° C / sec or more and the coiling temperature to 300 ° C or less, a very remarkable improvement effect is exhibited. This is also considered to be a result of synergistic effects such as difficulty in forming pearlite, which is a starting point of corrosion, and reduction in potential difference in bainite. Average cooling rate of 35 ° C / sec or more, winding temperature The reason why the temperature is limited to 300 ° C. or lower is to exert the above effects, but the upper limit and the lower limit of each are not particularly limited. However, in the case of the average cooling rate, the average cooling rate is about 40 ° C / sec when water is cooled from 850 ° C, so from the practical viewpoint, the upper limit is considered to be approximately 50 ° C / sec. More preferably, the average cooling rate is 40 to 45 ° C / sec. The lower limit of the coiling temperature is not particularly limited, but a temperature up to the point where martensite is not formed by cooling is allowed in relation to the average cooling rate. Therefore, the temperature is generally up to the Ms point, specifically about 100 ° C or higher.

Here, the reason why the winding temperature and the Mn content are limited as described above in the present invention will be explained from the viewpoint of their action and effect. Hot-rolled steel sheets were manufactured using slabs having the steel composition shown in Table 1 within the composition range of the present invention or slabs outside the composition range of the present invention. At this time, the heating temperature is 1050 to 1300 ℃, and the finishing temperature is 830 to 900.
℃ and within the scope of the present invention. The cooling rate is determined in relation to the coiling temperature. For coiling temperatures below 450 ° C, 35 ° C / sec.
As described above, for those exceeding 450 ° C, it was set to less than 35 ° C / sec.
A test piece was sampled from the steady portion of the hot-rolled steel sheet having a plate thickness of 2.6 mm thus obtained and evaluated.

[0028]

[Table 1]

FIG. 1 shows the coiling temperature, C, and T which affect the corrosion resistance.
It is the graph which showed the influence of each content of i and Nb.
As the test piece for evaluating the puncture resistance, a portion having no defect such as a flaw on the surface was used. For the corrosion test, a 150 × 70 (mm) size test piece was sealed on the back and side surfaces and placed on a plate that was placed outdoors at an angle of 30 ° from the horizon, and NaCl:
5% salt water was sprayed once a week for 6 months. After the test, the test pieces were evaluated for corrosion. In this case, the maximum perforation depth was obtained by dividing one test piece into 10 equal parts and determining the largest amount of corrosion among the respective maximum perforation depths.

From the results shown in FIG. 1, it can be seen that the corrosion resistance is improved due to the lowering of the winding temperature. The components within the scope of the present invention have a great effect, and particularly at a coiling temperature of 300 ° C. or less, they have corrosion resistance equivalent to that of Cu and P-added steel, which is said to have excellent perforation resistance. On the other hand, it is clear that the effect of lowering the coiling temperature is small for the Ti and Nb added steels.

FIG. 2 shows the influence of Mn on the corrosion resistance in the corrosion test conducted in the same manner as in FIG. 1 except that Mn is the manufacturing condition of the test piece. The method for evaluating perforation resistance is the same as in FIG.

From the results shown in FIG. 2, it is understood that Mn is effective for the perforation resistance under the components and manufacturing conditions within the scope of the present invention. The effect becomes large at 0.80% or more. But,
It can be seen that the effect is saturated even if added over 2.00%.

FIG. 3 shows the coiling temperature, C, which affects the secondary work embrittlement resistance of the hot-rolled steel sheet produced as described above except that the C and P contents are changed as shown in Table 2 below. ,
It shows the effect of P content.

[0034]

[Table 2]

After working a disk from a steel sheet taken to investigate the secondary working brittleness resistance, drawing was carried out at a drawing ratio of 1.8% [= circle diameter / (punch diameter + sheet thickness)], and then the The drawn product is cooled to the test temperature and then given an impact to evaluate the secondary work brittleness resistance by the difference between the brittle fracture surface and the ductile fracture surface of the fracture surface.
The brittle fracture surface generation temperature was determined.

From the results shown in FIG. 3, it can be seen that when the components within the scope of the present invention are contained, excellent secondary work embrittlement resistance is exhibited. Moreover, the effect is remarkable when the winding temperature is 300 ° C or lower. On the other hand, in the case of the comparative steels with C = 0.008% and P = 0.07%, deterioration of the secondary work embrittlement resistance is observed. It can also be seen that the winding temperature sensitivity is becoming stronger.

Therefore, in the present invention, in order to give the same corrosion resistance as that of the Cu and P-added steel and to have excellent secondary work embrittlement resistance in combination, the coiling temperature is 300 in the components within the scope of the present invention. It should be below ℃. Next, the working effects of the present invention will be described more specifically by way of examples.

[0038]

Example In this example, a slab having the composition components shown in Table 3 was treated at the heating temperature, rolling finishing temperature, winding temperature and cooling rate shown in Table 3 to obtain a hot rolled steel sheet having a thickness of 2.6 mm. Manufactured.

Using the hot-rolled steel sheet produced in this manner, the tensile properties, puncture resistance, secondary work embrittlement resistance, and the presence or absence of surface flaws on the steel sheet were investigated. The results are summarized in Table 4.

In the tensile test in this example, the JIS No. 5 test piece was used in the direction of 0 °. The evaluation of corrosion resistance was the same as in the cases of FIGS. 1 and 2 described above. The secondary work brittleness resistance was the same as in the case of FIG. 3 described above. As shown in Table 4, within the scope of the present invention, tensile properties, corrosion resistance, secondary work embrittlement resistance,
Although all the surface defects show excellent properties, it is understood that any of these properties are inferior outside the scope of the invention.

[0041]

[Table 3]

[0042]

[Table 4]

[0043]

As described above, according to the present invention,
A hot rolled steel sheet excellent in secondary work embrittlement resistance and corrosion resistance can be manufactured by suppressing the addition amount of P, adding an appropriate amount of C and Mn, and manufacturing under appropriate rolling conditions. As a hot-rolled steel sheet which is a comparatively inexpensive alternative to a plated steel sheet and which is excellent in secondary processing brittleness resistance and corrosion resistance, it exhibits a great effect as a hot-rolled steel sheet suitable for undercarriage parts for automobiles.

[Brief description of the drawings]

FIG. 1 is a graph showing the effects of winding temperature, C content, Ti, and Nb on perforation resistance.

FIG. 2 is a graph showing the effect of Mn on perforation resistance.

FIG. 3 is a graph showing the effects of winding temperature, C, and P on the secondary processing brittleness resistance.

Claims (1)

[Claims] 1. By weight%, C: 0.01 to 0.07%, Mn: 0.8 to 2.0%, P: 0.
04% or less, S: 0.002% or less, Cu: 0.05% or less, Ti: 0.
005% or less, Nb: 0.005% or less, a slab having a steel composition with the balance being iron and unavoidable impurities is heated at 1050-1300 ° C, and hot rolling is completed at 830 ° C or more and 900 ° C or less, then at 35 ° C. A method for producing a corrosion-resistant hot-rolled steel sheet excellent in secondary work embrittlement resistance, which comprises cooling at a cooling rate of not less than / sec and winding at a temperature of 300 ° C or less.