JPS58221233A - Manufacture of high-strength cold-rolled steel plate with superior press formability - Google Patents

Abstract

PURPOSE:To manufacture easily a high-strength cold-rolled steel plate with supperior press formability by properly selecting conditions during the hot rolling of a steel, a draft during the cold rolling and conditions during the continuous annealing as well as the composition of the steel. CONSTITUTION:A steel contg. 0.02-0.06% C, 0.01-1.0% Si, 0.05-0.30% Mn, 0.01-0.15% P, <0.020% S, 0.020-0.07% sol.Al and <0.0030% N is hot rolled to a plate at a finishing temp. above the Ar3 point and coiled at >650 deg.C. The plate is cold rolled at >=70% draft and held at 750-900 deg.C for 30sec-5min in a continuous annealing furnace. The cold rolled steel plate is cooled from the A1 point to 800 deg.C at 5-30 deg.C/sec cooling rate, hardened in a jet of water, and tempered at 200-500 deg.C for 30sec-5min. A steel plate with 40-60kg/mm.<2> tensile strength and deep drawability is obtd. The plate has a composite structure consisting of ferrite and martensite.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-strength cold-rolled steel sheets with excellent press formability.
, 47 grade, and has good deep drawability.

Composite structure steel sheet is a high-strength steel sheet with a transformed structure reinforced with a mixed structure of ferrite and martensite, and its press formability is lower than that of conventional precipitation-strengthened or solid solution-strengthened high-strength steel sheets. It is seen as a very promising steel sheet for RC applications, which can help make automobiles lighter and more energy efficient.

However, the press formability of this composite structure steel sheet is particularly superior to conventional steel sheets in terms of elongation, ductility evaluated by n value,
Another advantage is that it has good shape fixability due to its overhang properties and low yield value, so even parts with special structures that could not be formed with conventional steel sheets can be formed by applying this composite structure steel sheet. It is possible to obtain high strength by hardening. However, even though composite texture steel sheets have such excellent performance, their range of applications has not been expanded to the extent that was expected at the beginning of their development. This is thought to be due to the disadvantages of this material, as well as poor deep drawing properties and poor elongation and lunge properties. However, composite structure steel sheets with a tensile strength of 40 to 60 Af/mI are usually rolled up at a low temperature during the hot rolling process to finely disperse martensite, so the T value is 1.
Although the elongation 7 lunge properties are influenced by martensite hardness, volume fraction, 5iil-, etc., the influence of this T value is also large, and is one of the reasons why the elongation 7 lunge properties of composite structure steel sheets are poor. Another example is the low T value.

The present invention was created after repeated studies in view of the above-mentioned circumstances, and has a tensile strength of 40 to 60 kff/fB.
- grade and has a high Lankford value with a T value of 1.4 or higher (3). , St:
0.0 1-1.0%, Mt+: 0.0
5-0.30%, P: 0.01-0.15%, S
: 0.020 or less, tral, Al: 0.
0 2 0 ~ 0.0 7 *, N2: 0.00
A steel containing 30% or less and the remainder consisting of h and unavoidable impurities is made into a slab by the usual method, then rolled in a hot rolling process at a finishing temperature of 3 points Ar or higher and a winding temperature of 650°C or higher to form a coil and then cooled. In the rolling process, it is rolled at a rolling rate of 70 inches or more, then held at 750-900°C for 30 seconds to 5 minutes in a continuous annealing furnace, and then slowly cooled to A1-800°C at a cooling rate of 5-30'C/see. After that, it is proposed to water-quench in jet water and temper at 200-500°C for 30 seconds to 5 minutes.

To further explain the present invention, the reason for limiting the steel components as described above is as follows.

It is preferable that C be low in order to increase the T value, but it is difficult to obtain a Vi composite structure (4) when C is less than 0.02-, and when it is more than 0.061, the T value is adversely affected by deep drawability. Since it is difficult to set the value to 1.4 or more, it was set to 0.02 to 0.06 inches.

St and P are effective elements for increasing strength without impairing ductility in a composite structure steel sheet, and the lower limit of each is required to be 0.01% to ensure this strength. In addition, the upper limit of these items should be 0.30 cm in order to obtain a good surface in the hot rolling process and annealing process.
P is 0.15% for rounding to ensure good spot weldability
It is necessary to do so.

- is an element necessary to obtain the above-mentioned composite structure, and the lower μn is, the higher the j-value can be obtained. However, if this blood is less than 0.05 mm, it will be difficult to obtain the above-mentioned composite structure, and hot brittleness will easily occur during the hot rolling process, causing surface defects. In the above case, 7 values 1, 4 due to the negative effect on the deep drawability of 1 itself.
Since it is difficult to obtain the above value, it is set to 0.05 to 0.30 inches.

Since S forms MnS and impairs ductility, it is preferable to have a small amount, and the content is set to 0.02% or less.

M is added for deoxidation and to fix N as AtN during high-temperature winding and suppress N aging, but so! If the amount is less than 0.020 inches, N cannot be fixed sufficiently during high-temperature winding, and if it is 0.07-6, it is sufficient to fix N, and adding more than this is not cost effective. Since it also has a disadvantage, it is set at 0.02 to 0.07%.

N is one of the impurity elements and is fixed as μN during high-temperature winding. However, if the N content is too large, the ductility deteriorates, so a small amount is preferable, and is limited to 0.003 or less.

Next, regarding the hot rolling conditions for the above-mentioned steel, it is essential to set the finishing temperature to Ar1 point or higher in order to obtain good deep drawability. In addition, high-temperature winding of 650°C or higher is necessary in order to obtain a high value of 7 by rapid heating during multiple continuous annealing by growing carbide, and at the same time, N is fixed as MN. and suppresses aging. These objectives cannot be achieved unless high temperature winding is performed at 650°C or higher.

After hot rolling as described above, pickling is performed before cold rolling, but the cold rolling rate in this case has a large effect on the T value.
In order to obtain a high 7 value in continuous annealing, it is preferable that the cold rolling rate is high, and it is necessary to set it to 70% or more.

The cold-rolled steel sheet is then annealed in a continuous annealing furnace equipped with water quenching equipment, and in the present invention, as described above, it has excellent deep drawability with a T value of 1.4 or more and a tensile strength of 40 to 40. 60
Special consideration has been taken to obtain a composite steel sheet with a strength of 14F grade. In other words, the reason why the heating temperature in this continuous annealing is set to 750 to 900°C is that in this temperature range, a mixed structure of ferrite and austenite is not formed, and this is a necessary condition for obtaining a mixed structure of ferrite and martensite in the subsequent water quenching. (7) If the temperature is outside the above range, it will be difficult to properly obtain the above-mentioned mixed structure of ferrite and martensite. Soaking time is limited to 30 seconds to 5 minutes □. If it is less than 30 seconds, it is difficult to obtain a stable 7 erite-austenite mixed structure, while if it is longer than 5 minutes, continuous annealing will reduce the work efficiency. There are also few special benefits due to K.

The steel plate after being heated and soaked under the above conditions is A, ~
It is quenched during 5-b up to a temperature range of 800°C. In this way, after heating to 750-900℃, soaking, and then slowly cooling to A1-800℃ and then water quenching, a higher heating temperature is preferable in order to increase the T value.
However, if water quenching is performed directly from a high temperature, the volume fraction of martensite will be high and the ductility will deteriorate, whereas if water quenching is performed after slow cooling to A, ~800°C as described above, martensitic The volume fraction of ferrite and martensite can be controlled within a preferable range for ductility, that is, 5 to 20 inches (8). ℃ or higher, the volume fraction of martensite is too high.Also, the cooling rate of 5 to 30 Vsee described above corresponds to the cooling rate of air cooling or gas jet cooling.The volume fraction of martensite after water quenching is 5
The tensile strength is controlled to be ~20, but the tensile strength is 40 to 6.
40 N when obtaining 0 #f/11111” class
For fld class, it is sufficient to add & or P alone, but 50
In the case of #f/*♂ or 60 kff/B♂ class, this alone is insufficient, and a large amount of a combination of St and P is added to ensure strength.

After water quenching, it is tempered at 200 to 500°C for 30 seconds to 5 minutes. If it is below 200°C, a large amount of solid solution C remains and aging becomes significant, which is undesirable. If it is above 500°C, martensite will soften significantly, so 500°C is the upper limit to ensure strength. Regarding the tempering time, if it is less than 30 seconds, a stable effect cannot be obtained, and if it is more than 5 m1n, the effect will be saturated, so it is meaningless.
A preferred product can be obtained by tempering for a few minutes.

Specific examples according to the present invention are shown below along with comparative examples thereof.

That is, the chemical composition of the steel sheets specifically used by the present inventors is shown in Table 1 below, together with comparative examples. N is above the range of the present invention, and A8 and A10 to A13 are within the scope of the present invention as chemical components, but the hot rolling conditions or continuous annealing conditions described below are outside the scope of the present invention. be.

Table 1 The above-mentioned rigid steels were produced in a converter, made into slabs by continuous casting, hot rolled, cold rolled, and then annealed in a continuous annealing furnace equipped with water quenching equipment. The hot rolling conditions, cold rolling rate and continuous annealing conditions for each steel are shown in Table 2 below.

(11) Table 2 (12) That is, the water quenching temperature of A8 is as high as 810°C, and
For items 9 to 13, the winding temperature after hot rolling is 560℃
This is lower than the scope of the present invention.

Furthermore, the results of testing and measuring the mechanical properties of each steel sheet obtained as described above are summarized in Table 3, line 9.

Table 3 (1) Punching diameter 10φ dO: Initial hole diameter wrinkle pressing force 50 ton d: Hole diameter punch at breakage 5
0φ-10R4 (15) That is, the 7 value of all of the items from ■ to ■ according to the present invention is 1.4.
It has high deep drawing strength and tensile strength of 40 to 60.
It can be obtained as a HfAl class 1 steel plate. It is clear that the elongation and run properties, which are evaluated using a large wave rating of 9, are considerably improved compared to those of the comparative example, and it is clear that the material has excellent reed and press formability.

The attached drawing No. 1 summarizes these results.
．． Figure 2 shows the relationship between tensile strength and 7 values, but it is clear that the inventive material and the comparative material are clearly different, and Figure 2 shows the relationship between tensile strength and 7 values. The relationship with the large wave rate is shown below, and it can be said that the material of the present invention has sufficient pressure superiority in this case as well.

According to the present invention as explained above, the tensile strength can be increased from 40 to 60 kgf/♂ by appropriately selecting the hot rolling conditions, cold rolling rate, and annealing conditions in continuous annealing as well as the composition of the steel.
This is a book that allows you to properly manufacture composite steel sheets with a good deep drawability and an industrial grade (16
) This is an invention with great effects.

[Brief explanation of drawings]

The drawings show the technical contents of the present invention, and Figure 1 is a diagram summarizing the tensile strength and T value of the inventive material and comparative material, and Figure 2 is a diagram showing the summary of the tensile strength and T value of the inventive material and the comparative material. This is a chart summarizing the relationship between tensile strength and large wave rate for comparative materials. Patent applicant Nippon Kokan Co., Ltd. Inventor Takashi Shimomura Good question
Hiro Osawa - Same
Masayuki Kinoshita /
En 406 θ 60 70 J Momme 1 Compensation (" - 4 Procedural Amendments (α - 2 Showa!N, 7.428 Japan Patent Office Commissioner Kazuo Sugi 1, Indication of the case Showa Kura 4 patent application No. 10/31E15 No. 3
, Person making the amendment Patent applicant related to the case: 8) Nippon Dokan Co., Ltd. 4, Agent Hakushu Patent Office 6, Specification subject to amendment 7, Contents of the amendment Amended as shown in the attached sheet Tsuburaya × To ask 1 Toyonaka 5th negative 10th number (ko 0
．． 30≠”, protect 1 and 1, A number 4, 1.0 reverses.”
I am corrected. □

Claims (1)

[Claims] c: o, 02-0.061sXSt: 0.01-1. . Chi, un: 0.05-0.30%, P: 0.01
~0.15%, S: 0.020% or less, sol, Al:
Steel containing 0.020 to 0.07%, Na: 0.0030 or less, and the balance consisting of h and unavoidable impurities, is heated at a finishing temperature of AQ point or higher and a winding temperature of 65% in a hot rolling process.
7 in the cold rolling process after rolling at 0℃ or higher to form a coil.
Rolled at a rolling rate of 0% or more, then heated to 750 in a continuous annealing furnace.
~900°C for 30 seconds to 5 minutes, then A1
A method for producing a high-strength cold-rolled steel sheet with excellent press formability, which comprises cooling from -800°C to 5-b, water quenching, and tempering to 200-500°C for 30 seconds to 5 minutes.