JP2616350B2 - Ultra-high tensile cold rolled steel sheet 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 an ultra-high-strength cold-rolled steel sheet having a tensile strength of 1180 N / mm ² or more and used for, for example, an automobile door guard bar and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, there has been an increasing demand for improving the safety of automobiles due to an increase in traffic accidents. In particular, in order to secure the collision strength of a vehicle body in a side collision, the amount of door guard bars used has been reduced. It is increasing. On the other hand, due to the necessity of weight reduction for the purpose of improving fuel efficiency and power performance of automobiles, thinning and strengthening of steel sheets constituting each part of the body of the automobile are being promoted in parallel.

[0003] For example, a door guard bar conventionally has 590-
A 780 N / mm ² grade cold rolled steel sheet was used. However, with these changes in the environment for safety and weight reduction,
In recent years, has become as high-strength cold-rolled steel sheet of 980 ~1470N / mm ² class is used, in recent years, especially TS ≧ 1180N /
Ultra-high tensile cold rolled steel sheets of mm ² have come to be used frequently.

However, the formability of this ultra-high tensile cold-rolled steel sheet tends to decrease as the tensile strength increases. Therefore, various techniques for ensuring moldability have been proposed. For example,
JP-A-63-14817 discloses C: 0.03 to 0.20% (hereinafter, unless otherwise specified, "%" means "% by weight"), Si: 0.3 to 1.5%, Mn:
0.5 to 2.6% as a basic component, and if necessary, Ti:
One or two kinds of 0.01 to 0.25% and Nb: 0.01 to 0.3% plus B: 0.0003 to 0.01%, steel containing a balance of Fe and unavoidable impurities, under specific conditions, hot rolling,
Japanese Patent Application Laid-Open No. Heisei 3 discloses a technology for producing a high-strength thin steel sheet having excellent bending properties with a tensile strength of 785 to 1470 N / mm ² by performing heat treatment, cold rolling, annealing, quenching, and overaging.
JP-A-277742 discloses that C: 0.08 to 0.20%, Mn: 1.5 to 3.
5%, Cr: 0.15 to 0.50%, Al: 0.01 to 0.10%, Mo: 0.02 to 0.20%, V: 0.015 to 0.150%, Ti: 0.
01-0.10% and Nb: 1 selected from 0.01-0.10%
An ultra-high tensile cold-rolled steel sheet containing TS or more, and having a steel composition of the balance Fe and unavoidable impurities and having a structure mainly composed of bainite, TS ≧ 1176 N / mm ² and excellent stretch flangeability Further, JP-A-3-277743 discloses that C: 0.08 to 0.20%, Si: 0.1 to 1.5%, Mn: 1.5 to 1.5%.
3.5%, Cr: 0.15 ~ 0.50%, Al: 0.01 ~ 0.10%,
And Mo: 0.02 to 0.20%, V: 0.015 to 0.150%, Ti:
Composite structure of martensite and ferrite containing one or two or more selected from 0.01 to 0.10% and Nb: 0.01 to 0.10%, the balance being steel composition of Fe and unavoidable impurities and containing residual austenite Having,
TS ≧ 1176N / mm ² , El: Excellent elongation / strength balance at 14% or more, YR ≦ 60%, BH ≧ 167N / mm ² with excellent workability and BH properties,
Ultra-high tensile strength cold rolled steel sheets having excellent weldability have been proposed.

However, these proposals are mainly aimed at improving formability such as bending characteristics and ductility, and do not consider suppression of delayed fracture at all.
Particularly, in the case of an ultra-high tensile cold-rolled steel sheet having TS ≧ 1180 N / mm ² , even with these proposals, sufficient formability cannot be ensured, and it is not easy to manufacture a door guard bar having a predetermined shape.

In view of the above, a door guard bar has been proposed in which a cold-rolled steel sheet is conventionally formed into a hat shape, but a cold-rolled steel sheet is formed into a pipe shape by bending and the ends are arc-welded. ing. Further, use of an electrogalvanized steel sheet has been studied from the viewpoint of improving rust prevention.

[0007]

However, according to the study results of the present inventor, when arc welding or electrogalvanizing is performed on a pipe-shaped member made of ultra-high-strength steel having TS ≧ 1180 N / mm ² ,
During arc welding or electrogalvanizing, it was found that hydrogen penetrates into the inside of the pipe-shaped member to cause delayed fracture, and it was found that the door guard bar as a reinforcing member needs to be improved promptly.

[0008] Here, an object of the present invention is to prevent delayed fracture that occurs in an ultra-high tensile strength cold-rolled steel sheet subjected to arc welding or electrogalvanizing, and more specifically, is excellent in delayed fracture resistance. Another object of the present invention is to provide an ultra-high tensile strength cold-rolled steel sheet having a tensile strength of 1180 N / mm ² or more used for, for example, an automobile door guard bar, and a method for producing the same.

[0009]

As is well known, delayed fracture is a phenomenon in which hydrogen is diffused into defects such as dislocations, vacancies, and grain boundaries in high-strength steel to embrittle the material and apply stress. It is a phenomenon that causes destruction in the state. For example, if residual stress exists in the material, stress concentrates on the hard second phase mainly composed of hard martensite and precipitates in steel, but the hydrogen in the steel concentrates on these, causing delayed fracture. Occur. In high-strength steels, hard secondary materials are used to increase the strength of the material.
It is said that since high-tensile steel contains a large amount of phases and precipitates, delayed fracture is more likely to occur.

As a result of intensive studies based on such basic matters, the present inventors have found that C: 0.10 to 0.20% and Cr: 0.50%.
Ultra-1.3%, Mo: 0.20-1.0%, Cr + Mo: 0.70-1.50%
Hot and cold rolling, cold rolling, continuous annealing, quenching, and overaging treatments under ultra-high tensile strength cold rolled steel containing specific conditions, bainite-based fine and uniform structure It is possible to suppress the concentration of hydrogen in the hard second phase and the precipitates in the steel as well as increase the energy absorption of the fracture by increasing the propagation path of the crack that causes the fracture phenomenon, The inventors have found that an ultra-high tensile strength cold rolled steel sheet excellent in delayed fracture resistance can be manufactured, and completed the present invention.

Here, the gist of the present invention is as follows:
C: 0.10 to 0.20%, Si: 0.50% or less, Mn: 1.50 to 3.00
%, P: 0.030% or less, S: 0.003% or less, Al: 0.01 to
0.10%, Cr: more than 0.50 to 1.3% or less, Mo: 0.20 to 1.0%, Cr + Mo: 0.70 to 1.50%, steel composition consisting of balance Fe and unavoidable impurities, characterized by having a bainite-based structure An ultra-high tensile cold-rolled steel sheet with excellent delayed fracture resistance.

This ultra-high tensile cold rolled steel sheet has a C: 0.10 to 0.20
%, Si: 0.50% or less, Mn: 1.50 to 3.00%, P: 0.030
%, S: 0.003% or less, Al: 0.01 to 0.10%, Cr: 0.
More than 50 to 1.3% or less, Mo: 0.20 to 1.0%, and Cr + Mo: 0.
A steel having a steel composition containing 70 to 1.50%, subjected to hot rolling at Ar ₃ transformation point or more of the finishing temperature, after winding in a temperature range of 600 to 700 ° C., subjected to pickling and cold rolling , 800-
Continuous annealing is performed at a temperature of 900 ° C for 20 to 200 seconds, and then quenched from a temperature of 600 to 700 ° C at a cooling rate of 60 to 250 ° C / second, and then a temperature of 250 to 350 ° C. To
It is provided by performing overaging treatment under the condition of holding for 60 to 600 seconds.

[0013]

Hereinafter, the present invention will be described in detail together with the functions and effects. First, the reason for limiting the composition of the ultra-high tensile strength cold-rolled steel sheet according to the present invention will be described.

C: C is an element effective for securing the strength, and is a target value of the strength when the C content is less than 0.10%.
The amount of martensite and the amount of bainite required to secure TS ≧ 1180 N / mm ² are insufficient. On the other hand, C content is 0.20%
If it is excessive, the strength of the welded portion when the pipe-shaped door guard bar is assembled by welding is insufficient. Therefore, the C content is limited to 0.10% or more and 0.20% or less.

Si: Si is an element for improving the hardenability of a material and is effective for producing an ultra-high tensile strength cold rolled steel sheet. However, if added over 0.50%, the formation of two phases of the structure is promoted to promote the formation of ferrite, and the concentration of hydrogen in the hard second phase mainly composed of martensite occurs, which promotes the occurrence of delayed fracture. I do. Therefore, the Si content is limited to 0.50% or less.

Mn: Mn is an element effective for securing strength.
If the Mn content is less than 1.50%, the desired strength cannot be obtained. On the other hand, if the Mn content exceeds 3.00%, wrought MnS is easily generated, which is one of the causes of delayed fracture.
Therefore, the Mn content is limited to 1.50% or more and 3.00% or less.

P: P is an element which may cause a decrease in toughness particularly in a high-strength material and cause brittle fracture during cold rolling. Therefore, P is desirably as small as possible. However, the reduction requires cost, and the content of about 0.030% is acceptable. Therefore, the P content is limited to 0.030% or less.

S: To prevent the occurrence of delayed fracture, use Mn
It is important to reduce S as much as possible.
If it exceeds 3%, MnS is generated. Therefore, the S content is 0.
Limited to 003% or less.

Al: Al is added in an amount of 0.01% or more for the purpose of deoxidation for the purpose of improving the cleanliness of steel. However, if the content of Al is about 0.10%, the deoxidizing effect is sufficiently ensured. It only increases costs. Therefore, the Al content is 0.01
% And 0.10% or less.

Cr, Mo, Cr + Mo: Cr and Mo are both very important elements for exhibiting the effects of the present invention, and these elements are particularly added in combination. As described above, delayed fracture is likely to occur when a hard second phase or a precipitate is present as compared with the base material. On the other hand, fractures (cracks) generally tend to progress along crystal grain boundaries. When Cr or Mo is added, a fine and uniform structure mainly composed of bainite can be obtained, so that the concentration source of hydrogen is eliminated and the grain boundary area which is the path of the fracture increases, so that the energy absorbed by the fracture increases. Thus, the occurrence of delayed fracture is suppressed. Further, although its mechanism is unknown, Mo prevents hydrogen from entering steel.

In order to suppress the occurrence of delayed fracture, it is necessary to add Cr + Mo in an amount of 0.70% or more.
%, The effect saturates and causes an increase in cost. Therefore, Cr + Mo is limited to 0.70% or more and 1.50% or less.

Further, Mo exhibits an effect of suppressing delayed fracture even in a small amount as compared with Cr. Even if the content of Mo is about 0.20%, the delayed fracture can be prevented if Cr + Mo is in the range of 0.70 to 1.50%. Is suppressed. Therefore, the lower limit of the Mo content is set to 0.20%. On the other hand, Mo is an expensive alloying element, and adding Mo more than 1.0% only increases the cost. Therefore, the Mo content is
Limited to 0.20% or more and 1.0% or less.

If the content of Cr is less than 0.50%, it is difficult to obtain a uniform structure, while if the content is more than 1.3%, the effect is saturated and the cost is merely increased. Therefore, the Cr content is limited to more than 0.50% and 1.3% or less. Compositions other than the above are Fe and unavoidable impurities.

The ultra-high tensile strength cold rolled steel sheet according to the present invention has a fine and uniform structure mainly composed of bainite. The bainite amount is 50% or more in area ratio. The term "fine" refers to the degree obtained by the present invention, which corresponds to a grain size number of 11 or more. Further, the term “uniform” means the degree of uniformity obtained by the present invention, which means that there is almost no difference in the structure (bainite ratio) in the thickness and width directions. Other than bainite, it has martensite and ferrite, and the respective amounts are 10 to 40% and 0 to 5% in area ratio. Next, a method for producing an ultra-high tensile strength cold-rolled steel sheet according to the present invention having the above-described steel composition and structure will be described.

C: 0.10 to 0.20%, Si: 0.50% or less, M
n: 1.50 to 3.00%, P: 0.030% or less, S: 0.003% or less, Al: 0.01 to 0.10%, Cr: more than 0.50 to 1.3%, Mo:
Steel having a steel composition containing 0.20% or more and 1.0% and Cr + Mo: 0.70 to 1.50%, for example, C: 0.10 to 0.20%, Si: 0.
50% or less, Mn: 1.50 to 3.00%, P: 0.030% or less,
S: 0.003% or less, Al: 0.01 to 0.10%, Cr: more than 0.50 to 1.
3% or less, Mo: 0.20% or more and 1.0%, and Cr + Mo: 0.70 ~
Hot rolling is performed on a steel having a steel composition consisting of 1.50%, the balance being Fe and unavoidable impurities, at a finishing temperature not lower than the Ar ₃ transformation point. The reason for limiting the hot rolling finish temperature to the Ar ₃ transformation point or higher is that ultra-high strength steel has large deformation resistance during hot rolling,
In particular, when the finishing temperature is lower than the Ar ₃ transformation point, the deformation resistance sharply increases, and it becomes difficult to perform hot rolling. Desirably, it is 870 ° C or more and 950 ° C or less.

After the completion of the hot rolling, winding is performed in a temperature range of 600 to 700 ° C. If the winding temperature is lower than 600 ° C., the hot-rolled steel sheet becomes hard and the subsequent cold rolling becomes difficult. Meanwhile, 70
At a winding temperature exceeding 0 ° C., a band structure is generated, and a non-uniform structure is formed in the thickness direction even after cold rolling and annealing, which promotes delayed fracture. Therefore, the winding temperature should be between
Limit to 0 ° C or less. After winding, pickling and cold rolling are performed in accordance with a normal process. These conditions are not particularly limited in the present invention.

After the cold rolling, the temperature is lowered to a temperature range of 800 to 900 ° C.
Perform continuous annealing under the condition of holding for 20 to 200 seconds. In the present invention, it is important to form a fine and uniform structure in order to improve delayed fracture resistance, and it is desirable that continuous annealing be performed in the austenite single phase region as much as possible. However, if the annealing temperature is too high or if the annealing time is too long, a part of the austenite crystal grains are coarsened, and a uniform structure cannot be obtained. Therefore, in order to obtain a fine and uniform structure, a continuous annealing temperature: 800 ° C to 900 ° C and a holding time: 20 seconds to 200 °
The continuous annealing is performed under the condition of less than second.

After continuous annealing under such conditions, 60
Rapid cooling is performed at a cooling rate of 60 to 250 ° C / sec from a temperature range of 0 to 700 ° C. For cooling after continuous annealing, rapid cooling is started from a temperature range of 600 ° C to 700 ° C. Cooling rate is 60-250 ° C /
Seconds. The reason for limiting the cooling conditions in this way is to avoid that Cr and Mo, which are the features of the present invention, form carbides during cooling and become carbides. Since these carbides are easily formed in the temperature range of 500 to 600 ° C., the temperature range of 500 to 600 ° C. is cooled in as short a time as possible in order to prevent the formation of carbides. For this purpose, it is necessary to cool from a temperature range of 600 ° C. or more at a cooling rate of 60 ° C./sec or more. On the other hand, if the temperature is rapidly cooled from a temperature range exceeding 700 ° C., the flatness of the coil decreases during cooling, and it becomes difficult to pass the sheet in the continuous annealing step. Therefore, in the present invention, cooling is performed at a cooling rate of 60 ° C./sec or more from a temperature range of 600 ° C. or more and 700 ° C. or less. The reason why the upper limit of the cooling rate is set to 250 ° C./sec or less is that if the cooling rate is more than 250 ° C./sec, water cooling is performed as a cooling means, but this makes it difficult to control the cooling end temperature. Because.

The quenching end temperature is set at 25 ° C. to obtain a fine baitnite structure in the subsequent overaging treatment.
It is desirable to set the temperature range from 0 ° C to 350 ° C. After quenching, over-aging treatment is performed under the condition that the temperature is kept in the temperature range of 250 to 350 ° C for 60 to 600 seconds. To improve the delayed fracture resistance, a very fine and uniform structure is obtained. For that purpose, it is necessary to continue the overaging treatment after quenching.

The reason why the treatment temperature is limited to not less than 250 ° C. and not more than 350 ° C. is that the structure becomes too coarse at a holding temperature exceeding 350 ° C., whereas the ratio of martensite increases in a temperature range below 250 ° C. Becomes uneven. In addition, a holding time of at least 60 seconds is required to stably perform the overaging treatment, and if the holding time exceeds 600 seconds, the continuous annealing speed decreases, and the efficiency of the entire process is reduced. Will decrease. Therefore, in the present invention, the overaging processing time is limited to 60 seconds or more and 600 seconds or less.

Thus, the ultra-high tensile strength cold rolled steel sheet according to the present invention is manufactured. Hereinafter, the present invention will be described in detail with reference to examples, but these are exemplifications of the present invention, and the present invention is not limited thereto.

[0032]

Example 1 Using a vacuum melting furnace, C: 0.12%, Si: 0.4
7%, Mn: 2.04%, P: 0.010%, S: 0.001%, Al: 0.03
% Of steel, Cr is in the range of 0.01 to 1.45%, and Mo is 0.1%.
The ingots varied in the range of 005 to 1.05% were cast, forged to a thickness of 50 mm, and the final rolling temperature (finishing temperature): 920 ° C, hot-rolled to a thickness of 4.0 mm, and then 630 ° C
Spray cooling until quenched and then 660 ℃ in a box furnace
A heat treatment equivalent to winding, in which the furnace was cooled after soaking under the condition of 4 h, was performed.

Grind the front and back surfaces of the obtained specimen to obtain a plate thickness of 2.
After reducing the thickness to 4 mm, a cold-rolled steel sheet having a thickness of 1.2 mm was further cold-rolled. The cold-rolled specimen was subjected to a heat cycle (continuous annealing: 850) shown in FIG. 1 by a continuous annealing simulator.
C. × 40 seconds, rapid cooling: 700 ° C. to 100 ° C./second, overaging treatment: 300 ° C. × 120 seconds) to form an ultra-high tensile cold-rolled steel sheet, and its delayed fracture resistance was investigated.

The delayed fracture resistance was evaluated by performing a four-point bending test shown in FIG. 2 and measuring the time until fracture occurred. That is, as shown in Fig. 2 (a), a test piece having a total length of 68 mm and a width of 6.0 mm was cut out, and 0.2 mm wide x depth from the side in the plate thickness direction (convex side during bending) at the center in the longitudinal direction.
A notch of 1.0 mm x tip R of 0.1 mm was created and held in tap water heated to 55 ° C. A four-point load of 250 N / mm ² with 10 mm intervals from the top and 60 mm intervals from the bottom with the notch at the center was used.
(See FIG. 2 (b)), and the bolts were fixed. At this time, in the vicinity of the notch of the test piece, an elongation strain having a strain rate of 0.24% is provided on the bending convex side.

FIG. 3 is a graph showing the test results. From FIG. 3, it can be seen that complex addition of specific amounts of Cr and Mo is effective for improving delayed fracture. If the delayed fracture occurrence time in this test method is 200 hours or more, it is considered that there is no practical problem even if it is used as an automobile door guard bar. To achieve this, it is necessary that Cr + Mo ≥ 0.7%. Recognize. In particular, it can be seen that Mo can improve delayed fracture with a small amount of addition as compared with Cr, and that even a small amount of 0.2% or more is effective.

[0036]

Example 2 Steel slabs 1 to 12 having the steel compositions shown in Table 1
Under the following manufacturing conditions, hot rolling, winding, pickling, cold rolling, continuous annealing, quenching, overaging treatment and temper rolling were performed to produce an ultra-high tensile strength cold rolled steel sheet.

[Steel making] Converter → Vacuum degassing (RH): Component adjustment → Continuous casting: Casting →
Slab production 240t (mm) [Hot rolling] Heating: 1200-1220 ° C, Finishing: 870-900 ° C, Winding:
630-660 ° C, sheet thickness: 2.4 mm [Pickling, cold rolling] Conventional process, sheet thickness: 1.2 mm [continuous annealing, quenching, overaging treatment] See Table 1 for conditions [temper rolling] rolling reduction : 0.3 to 0.5% JIS No. 5 specimen is cut out from the ultra-high tensile strength cold-rolled steel sheets 1 to 12 manufactured as described above, and a tensile test is performed.
And El were measured, and the delayed fracture resistance was measured in exactly the same manner as in Example 1 described above. The results are summarized in Table 2.

[0038]

[Table 1]

[0039]

[Table 2]

From Table 2, it can be seen that all of the samples produced according to the present invention have a delayed fracture onset time of more than 200 hours. On the other hand, Samples Nos. 7 to 12, which are comparative examples that do not satisfy the scope of the present invention, have a delayed fracture occurrence time of less than 200 hours. Therefore, according to the present invention, it can be seen that the delayed fracture resistance of the ultra-high tensile strength cold rolled steel sheet is significantly improved, and that the steel sheet can be used as an automobile door guard bar.

[0041]

As described above in detail, the present invention makes it possible to improve the delayed fracture resistance of an ultra-high tensile strength cold rolled steel sheet. Therefore, the occurrence of delayed breakage of the door guard bar joined to the door inner panel can be suppressed or eliminated.

[Brief description of the drawings]

FIG. 1 is a heat cycle showing heat treatment conditions in Example 1.

FIGS. 2A and 2B are explanatory views of a test method for measuring delayed fracture resistance in an example, where FIG. 2A shows a load application position, and FIG. 2B shows a load application condition.

FIG. 3 is a graph showing the results of Example 1.

Claims (2)

(57) [Claims] C .: 0.10 to 0.20% by weight, Si: 0.50% by weight
%, Mn: 1.50 to 3.00%, P: 0.030% or less, S:
0.003% or less, Al: 0.01 to 0.10%, Cr: more than 0.50 to 1.3%
Hereinafter, Mo: 0.20 to 1.0% and Cr + Mo: 0.70 to 1.50%,
An ultra-high tensile strength cold-rolled steel sheet having excellent delayed fracture resistance, having a steel composition comprising the balance of Fe and unavoidable impurities, and having a structure mainly composed of bainite. 2. C: 0.10 to 0.20% by weight, Si: 0.50% by weight
%, Mn: 1.50 to 3.00%, P: 0.030% or less, S:
0.003% or less, Al: 0.01 to 0.10%, Cr: more than 0.50 to 1.3%
Hereinafter, a steel having a steel composition containing Mo: 0.20 to 1.0% and Cr + Mo: 0.70 to 1.50% is hot-rolled at a finishing temperature not lower than the Ar ₃ transformation point, and rolled in a temperature range of 600 to 700 ° C. After pickling, pickling and cold rolling are performed, and continuous annealing is performed under the condition that the temperature is maintained in a temperature range of 800 to 900 ° C for 20 to 200 seconds, and 600
Rapid quenching at a cooling rate of 60-250 ° C / sec from a temperature range of ~ 700 ° C, and over-aging treatment at a temperature of 250-350 ° C for 60-600 seconds. Manufacturing method of ultra-high tensile cold rolled steel sheet with excellent delayed fracture.