JP2528387B2 - Manufacturing method of ultra high strength cold rolled steel sheet with good formability and strip shape - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has a tensile strength of 150 to 2
00 kgf / mm ² and a martensite volume ratio of 80
Provided is a method for producing an ultra-high strength cold-rolled steel sheet having a fine two-phase structure in which the balance is up to 97% and the balance being ferrite, and having a good formability and strip shape.

[0002]

^2. Description of the Related Art In recent years, there has been an increasing demand for automobile safety, and a super-high strength cold-rolled steel sheet having a tensile strength of over 150 kgf / mm ² is used to make it lightweight and have a sufficient impact absorbing capacity. There is a movement to actively install reinforcement members such as hosments and door impact bars. In the ultra-high-strength cold-rolled steel sheet used for such applications, bending workability is particularly important in terms of workability,
In addition, it is necessary that the yield ratio is low from the viewpoint of shape fixability at the time of molding and that the strip shape is good from the viewpoint of workability on the molding line.

In the production of ultra-high strength cold rolled steel sheet, structural strengthening utilizing a hard low temperature transformation structure represented by martensite is used. In order to efficiently and inexpensively manufacture such an ultra-high strength cold rolled steel sheet, it is advantageous to use a water quenching type continuous annealing equipment, and usually, it is heated to a recrystallization heating temperature of Ac ₁ point or more for a short time. After holding, it is cooled to a predetermined temperature by forced air cooling, water quenching is performed from this temperature, and then overaging treatment is performed.

Regarding the method for producing an ultra-high strength cold-rolled steel sheet using such a water-quenching type continuous annealing equipment, conventional methods have been used.
For example, JP-A-53-28515 and JP-A-58-2
A number of methods are disclosed as seen in Japanese Patent No. 2327 and Japanese Patent Laid-Open No. 61-3843.

[0005]

[SUMMARY OF THE INVENTION] However, these inventions are all intended to at most 150 kgf / mm ² in tensile strength, the ultra-high strength cold rolled steel sheet of more than 150 kgf / mm ^2, for example JP It is only slightly seen in the examples of JP-A-53-28515 and JP-A-61-3843. Also, regarding these, formability or deterioration of the strip shape due to transformation strain observed in an ultra-high-strength cold-rolled steel sheet using such a martensite structure has not been sufficiently studied.

[0006]

The inventors of the present invention have repeatedly studied the structure, formability, and strip shape of an ultrahigh-strength cold-rolled steel sheet having a tensile strength of more than 150 kgf / mm ^2, and as a result, a steel having a specific composition has been obtained. In the hot-rolled sheet, the structure is uniformly refined, and the volume ratio of martensite is 80 to 9 in the subsequent continuous annealing.
It was found that an ultrahigh strength cold rolled steel sheet having good formability and strip shape can be produced by forming a fine two-phase structure in which the balance is 7% and the balance is ferrite.

That is, in the present invention, in% by weight, C: 0.1
8 to 0.3%, Si: 1.2% or less, Mn: 1 to 2.5
%, P: 0.020% or less, S: 0.003% or less, so
l. Al: 0.01 to 0.1%, and further N
b: 0.005-0.030%, V: 0.01-0.1
Steel containing 0%, Ti: 0.01 to 0.10%, and one or more of them in a total amount of 0.005 to 0.10%, with the balance being Fe and inevitable impurities, After hot rolling at a finishing temperature of Ar ₃ or more, winding at 500 to 650 ° C., pickling, cold rolling, and continuous annealing, Ac ₃ to [A
c ₃ + 70 ° C.] and soaking for 30 seconds or more, ferrite is precipitated in a volume ratio of 3 to 20% by primary cooling, then rapidly cooled to room temperature in jet water, and at a temperature of 120 to 300 ° C. After aged for 15 minutes, the martensite volume ratio is 80-97% and the balance is ferrite.
A method for producing an ultra high strength cold rolled steel sheet having a tensile strength of 150 to 200 kgf / mm ² and a good strippability, which is characterized in that the steel sheet has a phase structure.

[0008]

The reason for limiting the chemical composition of the steel used in the present invention is as follows.

C: 0.18 to 0.30%. C is an effective component for obtaining the strength of martensite, and the lower limit of 0.18% was determined as the minimum amount at which the desired strength can be obtained. In addition, the upper limit of 0.30% was determined because the impact characteristics remarkably deteriorated if added more than this.

Si: 1.2% or less. Si contributes most to the ductility among the mechanical properties of the steel sheet, and an increase in this addition amount improves the ductility. However, 1.2%
Since the above additions significantly deteriorate the impact properties, the upper limit was made this.

Mn: 1-2. 5 %. Mn is an element that lowers the transformation point and improves the hardenability of austenite, and plays an important role in controlling the volume ratio of martensite and obtaining a predetermined strength. That is, the lower limit is a limit for stably obtaining martensite, and the upper limit uses a water quenching type continuous annealing facility with a very fast cooling rate, so that the effect is saturated even if more is added, Also, it was determined from the fact that the band structure is significantly developed due to segregation during casting, which adversely affects bending workability.

P: 0.020% or less, S: 0.003%
Less than. Considering the workability of the steel sheet, P and S are preferably low, and therefore P is 0.020% or less, and particularly S
If its content is high, inclusions (MnS) increase, and not only the workability of the steel sheet but also the impact characteristics are significantly adversely affected, so the content is made 0.0030% or less.

Sol. Al: 0.01 to 0.10%. Al is used for deoxidation of steel, but sol. If Al is less than 0.01%, silicate inclusions remain and the workability of steel deteriorates. Therefore, it is necessary to set sol.Al to 0.01% or more. Further, sol. The residual Al causes an increase in surface defects and is not preferable, so the upper limit is set to 0. 10 %
And

Nb: 0.005 to 0.030%, V:
0.01-0.10%, Ti: 0.01-0.10%. In the present invention, further, any one of Nb, V and Ti or 2
In the above range, 0.005 to 0.10% is contained in total, but the addition of these elements is intended for the refinement of the structure, and the lower limit is the minimum amount necessary for the refinement of the structure. The upper limit was determined because if it is added more than this, precipitates increase and workability deteriorates significantly.

The reasons for limitation in manufacturing in the present invention are as follows. First, the steel of the above composition is hot-rolled at a finishing temperature of Ar ₃ points or higher, which was determined because the hot-rolled sheet structure becomes non-uniform below this temperature. The winding temperature is 50
The temperature is limited to 0 to 650 ° C. This is because when the upper limit is exceeded, the hot rolled sheet exhibits a band structure and deteriorates the workability of the final steel sheet. It was decided because. The hot rolled sheet is pickled and then cold rolled.

[0016] Next will be described about the continuous annealing heat cycle performed, initially heated to Ac ₃ ~ [Ac ₃ + 70 ° C.] After soaking for 30 seconds or more, the ferrite in the primary cooling
3 to 20% by volume is deposited, but the heating temperature is reduced
To + austenite 2 phase region, not austenite single phase
After setting the range AC ₃ to [AC ₃ + 70 ° C], add ferrite
It is ferrite + austenite two-phase region that precipitates
The resulting ferrite has a non-uniform shape and distribution,
Not only does it adversely affect the mechanical properties of the steel sheet, but also during quenching.
Absorption of strained ferrite due to martensitic transformation that occurs
Is not uniform, which causes deterioration of the strip shape.
It The upper limit of the heating temperature, [Ac ₃ + 70 ° C.], was determined because if the heating temperature is further increased, the austenite grains become coarse and the workability and impact properties are adversely affected. Further, the lower limit of 30 seconds of the soaking time was determined because a predetermined effect cannot be obtained in the entire plate thickness direction below this.
The soaking time here is defined as the time during which the maximum plate temperature during soaking is -20 ° C or higher.

Subsequent to the soaking, it is rapidly cooled to room temperature in jet water, but if it is left as it is, it is thermally unstable due to a large amount of solid solution C. Then, the temperature is kept at 120 to 300 ° C. for 1 to 15 minutes. Perform aging treatment. If the temperature is 120 ° C or lower, the desired effect cannot be obtained, and if it exceeds 300 ° C, martensite softens,
Since the strength sharply decreases, this 300 ° C. becomes the upper limit of the overaging treatment temperature. Also, the overaging time of 1 to 15 minutes,
If it is less than this, the predetermined effect cannot be obtained, and if it is more than this, not only the effect is deteriorated but also the operability is deteriorated.

In the present invention, the final structure is defined as a fine two-phase structure in which the volume ratio of martensite is 80 to 97% and the balance is ferrite. This is because, although the tensile strength is ultra high strength exceeding 150 kgf / mm ² , a low yield ratio can be realized, and the soft ferrite absorbs the strain during martensitic transformation, so that a good strip shape can be obtained. . In addition, the martensite volume ratio is 80
The content of martensite is set to 97% because the martensite volume ratio is 80% or less, that is, when the ferrite volume ratio is 20% or more, the bending workability is deteriorated due to an increase in hardness difference between ferrite and martensite. Is 97% or more, that is, when the ferrite volume ratio is 3% or less, the ferrite cannot absorb the strain due to the martensitic transformation, and the strip shape is significantly deteriorated.

When the steel having the above-described composition is limited to each of such ranges, the tensile strength becomes 1
With 50 to 200 kgf / mm ² , an ultra-high strength cold rolled steel sheet having good formability and strip shape is produced.

[0020]

EXAMPLES Some of the concrete examples of the present invention as described above will be described below.

Example 1 Composition steels A to J of the present invention having the composition as shown in Table 1.
After the steels of Comparative Steels K to T were tapped in a converter, they were continuously cast into slabs, which were hot-rolled under the conditions of the present invention as shown in Table 2 into hot-rolled sheets having a thickness of 2.8 mm. After washing
Cold rolled to a thickness of 1.2 mm. Next, in a water quenching type continuous annealing equipment, continuous annealing and overaging treatment were performed under various conditions within the scope of the present invention shown in Table 2. The mechanical properties and the martensite volume ratio of the steel sheet thus obtained are also shown in Table 2. In Table 2, the steels A to T are the component steels of the present invention, A, though both the hot rolling conditions and the continuous annealing conditions are within the scope of the present invention.
In comparison steels K to T whose components deviate from the scope of the present invention in comparison with the mechanical properties obtained in J to J, deterioration of bending workability and ductility and increase of yield ratio are observed.

[0022]

[Table 1]

[0023]

[Table 2]

As a representative example, FIG. 1 shows a summary of the results of studying the effects of martensite volume fraction, each mechanical property, and strip shape on Steel C in Table 1 above. Martensite volume ratio of 80 ~
By setting it to 97%, each characteristic is obtained stably and effectively.

Example 2 The composition steels A, C, and G of the present invention shown in Table 1 above were tapped in a converter and formed into a slab by continuous casting, which was formed into various slabs as shown in Table 3 below. A hot-rolled sheet having a thickness of 2.8 mm under hot-rolling conditions, pickled, cold-rolled to a thickness of 1.2 mm, and then water-quenched continuous annealing equipment under various conditions shown in Table 3 Continuous annealing and overaging treatment were performed. Table 3 also shows the mechanical properties, strip shape, and martensite volume ratio of the steel sheet thus obtained.

[0026]

[Table 3]

In Table 3, steels A, C and G are steel No. 1 in which both hot rolling conditions and continuous annealing conditions are within the scope of the present invention, although the components are within the scope of the present invention. In comparison with the mechanical properties and strip shapes obtained in Nos. 4, 4, 8, 9, 10, and 12, steel Nos. 6 and 7 in which the heating temperature in continuous annealing deviates from the range of the present invention in the ferrite + austenite two-phase region Steel Nos. 3, 5, 11, 13, 1 whose bending workability and strip shape are deteriorated and whose martensite volume ratio is out of the range of the present invention
In No. 4, bending workability, ductility, deterioration of strip shape and increase of yield ratio are observed.

As described above, according to the present invention, it is understood that an ultra-high strength cold-rolled steel sheet having a tensile strength of 150 to 200 kgf / mm ² and good strip form can be appropriately manufactured.

[0029]

According to the present invention as described above, it is possible to accurately manufacture an ultra-high strength cold-rolled steel sheet having a tensile strength of 150 to 200 kgf / mm ² and good formability and strip shape. Since it is possible to provide a member that is lightweight and has excellent formability in a traveling vehicle or the like, it is an invention that is industrially highly effective.

[Brief description of drawings]

FIG. 1 is a graph summarizing the effects of martensite volume fraction and soaking temperature on mechanical properties and strip shape in steel C in an example of the present invention.

Claims (1)

(57) [Claims] 1. C: 0.18 to 0.3% by weight, S
i: 1.2% or less, Mn: 1 to 2.5%, P: 0.02
0% or less, S: 0.003% or less, sol. Al: 0.01-
0.1%, and further, Nb: 0.005
0.030%, V: 0.01 to 0.10%, Ti: 0.
Steel containing 0.1 to 0.10% of any one kind or two or more kinds in a total amount of 0.005 to 0.10% and the balance of Fe and unavoidable impurities at a finishing temperature Ar of ₃ points or more. Hot rolling, winding at 500 to 650 ° C, pickling, cold rolling, continuous annealing followed by heating to Ac ₃ to [Ac ₃ + 70 ° C] and soaking for 30 seconds or more, and then ferrite at primary cooling At a volume ratio of 3 to 20%, then rapidly cooled to room temperature in jet water, subjected to overaging treatment at a temperature of 120 to 300 ° C. for 1 to 15 minutes, and a martensite volume ratio of 80 to 97%.
The tensile strength is 150 to 200 kg, which is characterized in that the steel sheet has a fine two-phase structure with the balance being ferrite.
Manufacturing method of ultra-high strength cold rolled steel sheet with good f / mm ² formability and strip shape.