JPH0499227A - Production of ultrahigh-strength cold rolled steel sheet excellent in workability - Google Patents

Abstract

PURPOSE:To produce an ultrahigh-strength cold rolled steel sheet having high ductility and good bendability by uniformly refining the structure of the hot rolled steel sheet of specified composition, eliminating the banded structure and applying specified continuous annealing to the sheet. CONSTITUTION:A steel contg., by weight, 0.1-0.25% C, 0.5-2% Mn, 0.3-1.5% Si <=0.020% P, <=0.003% S, 0.01-0.10% sol. Al, 0.015-0.10% of one or >=2 kinds among 0.005-0.030% Nb, 0.01-0.10% V and 0.01-0.10% Ti in total, the balance Fe and inevitable impurities is hot-rolled at the finishing temp. of Ar3 to [Ar3+40 deg.C], wound at 500-650 deg.C, pickled and cold-rolled. The steel sheet is held between points A1 and A3 in the succeeding continuous annealing, soaked, cooled under specified conditions and overaged to form a composite structure consisting of 5-50% ferrite and the balance martensite. An ultrahigh-strength cold rolled steel sheet having >= about 100kgf/mm<2> tensile strength and excellent in workability is obtained in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention provides an ultra-high strength cold-rolled steel sheet with excellent workability, more specifically, a tensile strength of 100 kgf/am'' or more.

The present invention relates to a method for manufacturing an ultra-high strength cold-rolled steel sheet with excellent workability and having a two-phase structure consisting of ferrite with a volume fraction of 5 to 50% and martensite as the remainder.

[Conventional technology]

In recent years, from the perspective of automobile safety and weight reduction, reinforcement members such as bumpers, bumper reinforcements, and door impact bars have been given a tensile strength of 100 kg f/m.
Ultra-high strength cold-rolled steel sheets with a strength of n2 or higher are now being used.
Requirements regarding the workability of these ultra-high strength cold-rolled steel sheets are becoming increasingly strict.

In the production of ultra-high-strength cold-rolled steel sheets, structural strengthening using hard, low-temperature transformed phases such as martensite and bainite is used. In order to efficiently and inexpensively manufacture such ultra-high-strength cold-rolled steel sheets, it is advantageous to use water-quenching type continuous annealing equipment. After that, it is cooled to a predetermined temperature by forced air cooling, water quenching is performed from this temperature, and then an overaging treatment is performed.

For example, JP-A-61-3843 discloses that after controlling the recrystallization heating temperature and setting the austenite phase volume fraction during recrystallization heating within a predetermined range, water quenching and overaging treatment are performed to reduce yield. A method for manufacturing an ultra-high strength cold rolled steel sheet with a good strength-ductility balance is disclosed.

[Problem to be solved by the invention]

However, in terms of bendability, which is the main formability of ultra-high-strength materials with a tensile strength of 100 kgf/aa" or higher, such materials aiming for high ductility are inferior to low-ductility martensite and bainite single-phase steel sheets. Due to its inferiority, its use is very limited.

[Means to solve the problem]

As a result of repeated studies on the microstructure, ductility, and bendability of ultra-high-strength cold-rolled steel sheets with such composite structures, the inventors of the present invention have made the microstructure uniform and fine in hot-rolled steel sheets with specific components.
And the band organization was eliminated.

It has been discovered that by creating a structure in which ferrite and martensite are uniformly and finely distributed through subsequent continuous annealing, it is possible to produce an ultra-high strength cold-rolled steel sheet that has high ductility and good bendability.

That is, in the present invention, C: 0.1 to 0°25% in weight%
, Mn: 0, 5-2%, Si: 0.3-1.5
%, P: 0.020% or less, S: 0.003% or less.

Contains Sol, Al: 0.01 to 0.10%,
In addition, Nb: 0.005 to 0.030%,
V: 0°01~0.10%, Ti: 0.01~0
．． 10% of one or more types in total 0.015 to 0
．． The steel containing Fe in the range of 10% and the balance consisting of Fe and unavoidable impurities was heated to a finishing temperature of Ar, ~ CA r, + 40
After hot rolling at 500-650'C], pickling and cold rolling, followed by continuous annealing, heating between A1 and A3 points and soaking, 3-30'C/see. 8o cooling rate
It is cooled to a temperature range of 0 to 500°C, then rapidly cooled to room temperature in jet water, and overaged at a temperature of 100 to 400°C, and has a composite structure consisting of 5 to 50% ferrite and the remainder martensite. A method for producing an ultra-high-strength cold-rolled steel plate having a tensile strength of 100 kgf/mn'' or more and excellent workability, which is characterized by obtaining a cold-rolled steel plate.6 Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the steel used in the present invention are as follows.

C: 0.1 to 0.25% C is an essential element for obtaining martensite, and if the amount of C is low, the specified amount of martensite cannot be obtained, resulting in insufficient strength, so the lower limit is set to 0. It was set at 1%. Furthermore, in order to obtain good weldability, the upper limit was set at 0.25%.

M, n: 0.5-2% Mn is an element that lowers the transformation point and further improves the hardenability of austenite, and plays an important role in controlling the volume fraction of martensite and obtaining the desired strength. do. If Mn is less than 0.5%, martensite cannot be stably obtained.On the other hand, in the present invention, water quenching type continuous annealing equipment with a very fast cooling rate is used.
Even if it is added in excess of %, the effect will be saturated. For the above reasons, Mn is specified in the range of 0.5% to 2%.

Si: 0.3 to 1.5% Si can reduce the volume ratio to martensite 1 by solid solution strengthening, and also promotes uniform precipitation of ferrite during cooling after soaking. It has the effect of
This can provide a good strength-ductility balance. If Si is less than 0.3%, it is not possible to sufficiently reduce the volume fraction of martensite due to solid solution strengthening and promote the precipitation of ferrite, so the lower limit is set to 0.3%. On the other hand, unlike Mn, "S" is an element that increases the transformation point, and if it is added in excess of 1.5%, it becomes necessary to raise the heating temperature considerably to obtain martensite, which causes operational problems. Since it significantly deteriorates toughness, the upper limit was set at 1.5%.

P: 0.020% or less, S: 0.003% or less P and S are preferably lower when considering the workability of the steel plate. Therefore, P is set to 0.020% or less, and in particular, S: If the content is high, inclusions (MnS) will increase and have a significant negative effect on the workability of the steel plate, so it is set at 0.003% or less.

Sol, A ], : 0.01-0.10% A1 is
It is used for deoxidizing steel, but S01°A1 is 0.0
If the Sol. The Al content must be 0°01% or more. Further, since residual Sol and Al in excess of 0.10% is undesirable as it increases surface flaws, the upper limit was set at 0.10%.

Honsha aJ ['lic' Rani, Nb: 0.005-0
．． 030%, V: 0.01-0.10%, Ti: 0
．． One or more of 01 to 02 10% in total o,
O is contained in a range of 15 to 0.10%. These elements are added for the purpose of refining the structure, and the lower limit was defined as the minimum amount necessary for refining the structure. Further, the upper limit was specified because if the amount added exceeds this amount, the amount of precipitates increases and the ductility deteriorates significantly.

Next, the reason for manufacturing limitations will be explained.

First, the steel with the above composition has a finishing temperature Ar, ~(A r).

+40'C]. This regulation of finishing temperature is aimed at refining the structure of the hot-rolled sheet; if the upper temperature limit is exceeded, the structure of the hot-rolled sheet becomes coarse, and if the temperature is below the lower limit, the structure of the hot-rolled sheet becomes coarse. It becomes uniform. Further, the winding temperature is limited to 500 to 650°C. When the coiling temperature exceeds the upper limit, the hot rolled sheet exhibits a band structure, which deteriorates the workability of the final product, while when it falls below the lower limit, the hot rolled sheet becomes hard, causing operational problems. After pickling, this hot-rolled sheet is
Cold rolled.

Next, a thermal cycle in continuous annealing will be explained.

First, by heating and soaking in the range between A1 and A1 points, a mixed structure of ferrite and austenite is formed.
Furthermore, by cooling to a temperature range of 800 to 500° C. at 3 to 5 degrees, a mixed structure in which ferrite, ferrite, and austenite are uniformly and finely distributed can be obtained. The lower limit of the above cooling rate is
It was stipulated with productivity in mind. Further, at a cooling rate exceeding the above upper limit, precipitation of ferrite is suppressed and a mixed structure in which ferrite and austenite are uniformly and finely distributed cannot be obtained, so the upper limit of the cooling rate was specified as above.

Subsequently, by rapidly cooling to room temperature in jet water, austenite transforms into martensite, and a mixed structure in which ferrite and martensite are uniformly and finely distributed is obtained.

However, as it is, there is a large amount of solid solution C and it is thermally unstable, so an overaging treatment is subsequently performed at a temperature of 100 to 400°C. This over-aging treatment cannot achieve the desired effect at temperatures below 100°C, while at temperatures above 400°C martensite softens and the strength rapidly decreases, so 400°C is the upper limit of the over-aging temperature. .

Furthermore, in the present invention, the volume fraction of ferrite in the final structure is defined as 5 to 50%, but this is because if the volume fraction of ferrite is less than 5%, the ductility will be significantly deteriorated, while if it exceeds 50%, the specified strength will not be achieved. Not only is this not possible, but the difference in hardness between ferrite and martensite increases, resulting in significant deterioration of bendability.

By performing the treatments limited to each range as described above, an ultra-high strength cold rolled steel sheet with a tensile strength of 100 kgf/mn or more and excellent workability can be manufactured.

〔Example〕

Invention steel A- having the composition shown in Table 1
After steels H and comparative steel weights ~Q were tapped in a converter, they were made into slabs by continuous casting, and this was made into hot-rolled sheets with a thickness of 2.8 cm under various hot-rolling conditions shown in Table 2. , after pickling, thickness 1.
It was cold rolled to 2m.

Next, continuous annealing and overaging treatment were performed using a water quenching type continuous annealing equipment under various conditions shown in Table 2. The mechanical properties and ferrite volume fraction of the steel sheet thus obtained are also shown in Table 2.

In the same table, 1. For example, Steels 1 and 2 are both steels with the composition of the present invention and have the same annealing heat cycle, but Steel 2, whose hot rolling conditions are outside the range of the present invention, has a lower temperature than Steel 1. Steels 10 and 11 have poor bendability, and the hot rolling conditions for Steels 10 and 11 satisfy the scope of the present invention, but the cooling rate from heating and soaking in the annealing cycle to quenching falls within the scope of the present invention. It can be seen that Steel 10, which is out of the range, has inferior ductility compared to Steel 11.

In addition, comparative steel weight whose component composition is outside the range of the present invention ~
It can be seen that in Q, the ductility or bendability is not good even though the hot rolling conditions and annealing heat cycle meet the range of the present invention. On the other hand, steels 1.3.4.6.7.8.11.12.13.15 to 1 whose composition, hot rolling conditions, and annealing heat cycle satisfy the range of the present invention
It can be seen that in No. 9, not only a good balance of strength and ductility but also good bendability was obtained.

As described above, it can be seen that according to the present invention, an ultra-high strength cold rolled steel sheet having a tensile strength of 100 kgf101!1" or more and excellent in strength/ductility balance and bendability can be produced.

/ /

Claims (1)

[Claims] In weight%, C: 0.1 to 0.25%, Mn: 0.5 to 2
%, Si: 0.3 to 1.5%, P: 0.020% or less,
S: 0.003% or less, Sol. Al: 0.01-0.
Contains 10%, and further contains Nb: 0.005 to 0
．． 030%, V: 0.01-0.10%, Ti: 0.0
1 to 0.10% of one or more types in total of 0.01
The steel containing Fe in the range of 5 to 0.10% with the balance consisting of Fe and unavoidable impurities was heated at a finishing temperature of Ar_3 to [Ar_3+
After hot rolling at 40℃] and winding at 500 to 650℃,
Pickling and cold rolling, followed by continuous annealing, heating and soaking between A_1 and A_3 points, and then cooling at a cooling rate of 3 to 30°C/sec to 80°C.
It is cooled to a temperature range of 0 to 500°C, then rapidly cooled to room temperature in jet water, and overaged at a temperature of 100 to 400°C, and has a composite structure consisting of 5 to 50% ferrite and the remainder martensite. A method for producing an ultra-high-strength cold-rolled steel plate with a tensile strength of 100 kgf/mm^2 or more and excellent workability, the method comprising obtaining a cold-rolled steel plate.