JPS6176620A - Manufacture of composite structure type high strength cold rolled steel sheet - Google Patents

Abstract

PURPOSE:To obtain the titled sheet having improved yield behaviour, by heating and holding at a specified temp. a cold rolled steel sheet having specified quantities of C, Si, Mn, Al, P and S, next, applying slow and rapid coolings thereto. CONSTITUTION:The cold rolled steel sheet composed of 0.04-0.12wt% C, 0.1-1.0% Si, 1.0-2.0% Mn, 0.02-0.06% Al, 0.005-0.7% P, <=0.015% S and the balance Fe with inevitable impurities is heated to 750-850 deg.C, held at the attained temp. for >=5min, to vary a part of alpha phase to gamma phase and to form austenite. Next, said sheet is cooled slowly to >=500 deg.C temp. at 5-20 deg.C/sec average rate to transfer solid soln. C in alpha phase into gamma phase. Further, it is cooled rapidly to 350-200 deg.C range at 50-500 deg.C/sec average rate, and held at the end temp. of cooling for 1-5min. In this way, bainite or bainite + martensite is allowed to appear effectively.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet having a composite structure of ferrite + bainite (or even martensite), and in particular, it relates to a method for manufacturing a high-strength cold-rolled steel sheet having a composite structure of ferrite + bainite (or even martensite). This is a method for manufacturing high-strength cold-rolled steel sheets with a composite structure with good yield behavior by modifying the cooling conditions during annealing.

[Conventional technology]

Dual Phase@
The appearance of the dog caused the dog to move forward. In particular, dual 7A steel consisting of ferrite and martenite has high expectations because it has a low yield ratio and good formability, and also has the manufacturing advantage of not exhibiting yield elongation even after quenching. was received. However, the steel elongates by 7
It has the disadvantage of poor lunge properties and deep drawing properties, making it a bottleneck for practical application.

For these reasons, the inventors of the present invention have been unable to decide whether to stick to the two-phase structure of ferrite and martensite, and have therefore conducted research.

As a result, in the case of a composite structure in which the second phase is bainite (or may further contain martensite), the deep drawability (value 7) is greatly improved, and the elongation and lunge properties are significantly improved. Knowing that the bake hardenability (B, H, properties) would also be improved, he filed a patent application (Japanese Patent Application Laid-Open No. 1983-1999).
123956. 57-123957).

[Problem that the invention seeks to solve]

The present invention is a further development of the above-mentioned invention, and particularly aims to provide a method for manufacturing a composite structure type high-strength cold-rolled steel sheet with further improved yield behavior.

[Means for solving problems]

The main points of the method of the present invention are: C: 0.04-0.12 weight Lf6 (hereinafter simply referred to as "chi") Si: 0.1-1.0 mass Mn: 1.0-2.0% A1: 0.02-0.02% A cold-rolled steel sheet containing 0.06% P: 0.005-0.07% S: 0.015% or less, with the remainder consisting of Fc and unavoidable impurities, is heated to 750-850°C and then reaches a temperature of 5%. When cooling, cool at an average cooling rate of 5 to 200C/sec until the temperature exceeds 500℃, and
A temperature range of 50 to 200° C. is achieved by cooling at a temperature of 50 to 200° C. and maintaining the temperature at the end of cooling for 1 to 5 minutes.

[Effect]

The structure and effects of the present invention will be explained by clarifying the reason why the chemical composition and continuous annealing conditions of the steel plate in the present invention are determined as described above.

C is known as a functional element that strengthens and improves hardenability, but it also causes the appearance of a bainite phase and is an essential element for making the structure of the spot weld healthy. In order to satisfy the purpose of the present invention, it is necessary to add 0.04% or more. However, if the amount of C is excessive, the 7 value becomes small, resulting in a significant decrease in cold workability and significant hardening of spot welds, so the upper limit was set at 0.12%. In addition, when particularly excellent cold workability is required, it is desirable that the thickness be 0.07 inch or less.

Si and P are elements that promote the concentration of C in austenite, stabilize the austenite, and facilitate the appearance of a bainite phase, thereby imparting high strength and high ductility to a cold rolled steel sheet. However, in order to exhibit these effects, it is necessary to add 0.1% or more of Sl and 0.005% or more of P, and the above effects will be enhanced as the amount added increases, but regarding St. Not only does the above effect reach saturation at around 1.0% for PK and 0.07% for PK, but also a large amount of Si deteriorates the surface quality of the steel sheet, and a large amount of P makes the steel brittle. :1.0% and P:0.
The upper limit was set at 0.7%.

Mn is an element useful for preventing red heat embrittlement due to the coexistence of S, and is also necessary to enhance hardenability and form a desired structure. In the past, the present inventors have confirmed that the above effect is achieved by a blend of o, osq 6 or more, but in order to increase the strength of υ cold rolled steel sheet by using high Mn steel, it is necessary to It has been found that it is necessary to add more than .0 g. However, if the amount of b was increased, the 7 value would decrease, which would be contrary to the original purpose, so it was concluded that the upper limit should be 2.0%.

AI is not only necessary as a deoxidizing element, but also prevents aging by capturing N in the steel (forming AIN). However, if it is less than 0.02%, the effect is insufficient and it is necessary to add 0.02% or more, but if it exceeds 0.06%, AIN inclusions will increase and ductility and toughness will decrease. There is a problem of inviting

S not only deteriorates stretch flangeability but also inhibits grain growth during continuous annealing and lowers the T value, so it is considered an impurity element and needs to be suppressed to 0.015% or less.

Although the basic constituent elements of the present invention have been clarified through the above description, supplementary explanations will be added regarding other elements that can play a significant role in achieving the objects of the present invention.

Cr is an austenite stabilizing element, and can greatly contribute to increasing the strength of the composite structure and cold-rolled pan plate of the present invention. There is no particular lower limit set because such an effect can be achieved even with the presence of a very small amount, but adding more than 1 chlorine will not produce any greater effect and will only increase the cost. is the upper limit.

Next, like AI, B traps N in the steel to prevent aging, and like Cr, it also exerts an austenite stabilizing effect and contributes to high strength. And these effects are 0.
It is exhibited by adding 0.002% or more, and 0.005%
The recommended addition range is 0 because the effect is saturated when
．． It was set at 0002 to 0.005.

Next, continuous annealing conditions will be explained.

First, regarding heating, the cold rolled steel sheet manufactured in the previous step is introduced into a continuous annealing furnace, heated to 750 to 850° C. while running, and then held at the reached temperature for 5 minutes or less. The reason for heating to the above temperature is to change a part of the α phase to the γ phase and make austenite appear by heating to between the Ac1 transformation point and the Ac3 transformation point. That is, the heating described above causes the hepterythrite phase and austenite phase to coexist, thereby preparing for the formation of a composite structure. The reason why the period was set at 5 minutes or less is that sufficient γ phase can be extracted by holding for up to 5 minutes, and holding for more than 5 minutes will result in an unnecessarily long total reactor length, resulting in equipment economical problems. This is because it makes sex worse.

The beginning of the cooling process is a gradual cooling step from the above-mentioned holding temperature to 500°C. This step is a means for transferring the solid solution C in the average cooling rate 5~b into the γ phase. ), the decrease in solid solution C in the α phase contributes to the improvement of the α phase and thus the ductility of the cold-rolled steel sheet, and the increase in C in the γ phase improves the hardenability of the γ phase, making it even more important. Bainite (or bainite +
This is done to effectively bring out martensite.

Therefore, in order to exhibit the slow cooling effect, the solid solution C must be removed from the α phase to the γ phase.
It is necessary to diffuse it sufficiently into the phase, and the cooling rate is 20
It must be slowed down by more than ℃/second.

However, if a slower speed (specifically, slower than 5° C./second) is desired, the furnace length will need to be increased, which will reduce the economic efficiency of the equipment. 500℃ during slow cooling process with Nao
If the temperature is lowered to below , the γ phase that was painstakingly formed during the initial heating disappears and returns to the α phase, making it impossible to form a composite structure. That is, in order to obtain the desired composite structure, the rapid cooling described below must be started at a temperature of 500° C. or higher, at which the α phase and the γ phase are appropriately dispersed and coexist. Now, the quenching stage is performed until the temperature of the steel plate falls within the range of 450 to 200°C, and the average cooling rate must be 50 to 200°C. This rapid cooling changes the γ phase to bainite (
or bainite and martensite). Therefore, if the rapid cooling ends at a high temperature (450°C) or if it proceeds at a rate slower than 50°C/sec, the formation of the bainite etc. is insufficient, and in the former case, the γ phase remains. α due to subsequent air cooling (air cooling after leaving the continuous annealing furnace)
In the latter case, the γ phase changes to α during the slow cooling process.
Return to phase. In any case, the product after annealing will have a ferrite-only structure derived from the α phase, and the object of the present invention will not be achieved. On the other hand, if the rapid cooling progresses too much to less than 200°C, or if the cooling rate is too fast and exceeds 500°C/sec, martensite will be produced excessively, leading to deterioration of ductility.

The steel plate that has been rapidly cooled in this way has a temperature of 1
Hold for ~5 minutes. This step is essential because the solid solution C in the ferrite is diffused to the grain boundaries and precipitated as Fe and C, thereby preventing a decrease in ductility due to a large amount of solid solution C in the ferrite.

If the holding time is less than 1 minute, the diffusion of solid solution C becomes insufficient and the above effect cannot be obtained, so 1 minute was set as the lower limit. Therefore, the longer the holding time is, the more the grain boundary precipitation effect of FC is enhanced, but this effect reaches saturation in about 5 minutes, so 5
The upper limit was set at 10 minutes. Furthermore, if you try to secure a holding time exceeding 5 minutes, there will be an economic disadvantage in that the length of the furnace will become longer.
come. In addition, in this holding step, there is no problem if the temperature changes slightly within a range of, for example, ±30°C, but if the temperature decreases significantly, the diffusion rate of solid solution C becomes too slow and the above effect cannot be obtained.
On the other hand, if the temperature rises, a problem arises in that solid solution C in the α phase precipitates as fine Fe5C within the crystal grains, reducing ductility.

After completing the heating, slow cooling, rapid cooling, and heat retention steps as described above, the q-plate exits the continuous annealing furnace and undergoes air cooling. After baking, the product steel sheet is a composite I! consisting of ferrite + bainite (or even martensite). 'l (indicates a weave, but the area ratio of bainite is 5 to 50% (particularly preferably 8 to 2
0%), the resulting steel plate has stable quality, resulting in a high-strength product with excellent yield behavior. In addition, when martensite is produced in addition to bainite, it is advantageous in terms of lowering the yield ratio and improving elongation. However, if the amount is too large, the deep drawability will deteriorate, so it is recommended that the area ratio be kept at 15 inches or less, preferably 8 inches or less.

There are no particular restrictions on the means to ensure the heat treatment pattern as described above, and it may be carried out according to a general continuous annealing operation, but the rapid cooling method in particular will be described.
Any method can be selected from among a water-cooled roll method, a boiling water injection method, a boiling water immersion method, a heat vibration method, and the like.

〔Example〕

Continuously cast 220 m of ingots consisting of the chemical components shown in Table 1.
I made mt slug. Rough rolling (30 mmj) + hot rolling (3.2 mmt) and cold rolling (0.8 mmt) were performed, and continuous annealing was performed. Comparative example (A
, W) and Example (J+Rt*Ra), and the heat pattern was changed for each example (J+Rt*Ra), and the example of R1 is shown in FIG. That is, heating is carried out at a rate of 30°C/sec from room temperature to 800°C, and after reaching 800°C, the temperature is maintained for 2 minutes. Then, it was slowly cooled at a cooling rate of 10° C./second to obtain Example R0.
After the temperature reached 700°C, rapid cooling at 100°C/sec (water-cooled roll method) was started. Rapid cooling was stopped at 300'C, and the same temperature was maintained for 3 minutes (during which time the temperature was increased to 280°C, other conditions were as in Example R1. Comparative Example A was heated to 800°C).
After being held for 2 minutes, it was air cooled, and once it had cooled down to room temperature, it was reheated to 300°C and then kept in the same manner as in Example R1. Comparative Example W is obtained by slow cooling to 600°C as in Example R, and then cooled to room temperature by water cooling.
It was reheated and kept warm in the same manner.

Regarding the obtained example steel sheets and comparative example steel sheets, yield stress (YS: kgf/mm, tensile strength TS: kg
f〆d), elongation (El: reward), yield ratio (YR: dimensionless)
, tensile strength x elongation (TsXEI: kgf%/mn?) and elongation 7 lunge property (hole expansion rate 2%), the results shown in Table 2 were obtained.

Table 1 (H: Remaining Fe and unavoidable impurities) Table 2 (A) Table 2 (B) Table 2 (C) As shown in Table 1, all of the pot seeds ~ ■ satisfy the component conditions of the present invention. However, comparative examples that do not satisfy the cooling conditions (cooling pattern in annealing) of the present invention generally have high yield ratios and low TSXEI and elongation 7 lung properties. On the other hand, it can be seen that in the examples of the present invention, the yield ratio is low and the strength-ductility balance is also good even when comparing the same strength.

〔Effect of the invention〕

In the present invention, a cold rolled steel sheet having a specific composition is subjected to annealing using a heat pattern under specific conditions (particularly including a quenching process). As a result, a cold-rolled steel sheet was obtained that exhibits a yield behavior that is even better than that of conventional steel sheets.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a heat pattern in an example.

Claims (1)

[Claims] C: 0.04 to 0.12% by weight (hereinafter simply referred to as %) S
Contains i: 0.1-1.0% Mn: 1.0-2.0% Al: 0.02-0.06% P: 0.005-0.07% S: 0.015% or less A cold-rolled steel sheet, the balance of which is Fe and unavoidable impurities, is heated to 750-850°C and then held at the temperature reached for 5 minutes or less, and during cooling, the heating temperature is 5-20°C/20°C until the temperature reaches 500°C or higher. Cooled at an average cooling rate of 45 seconds.
Cooling is performed at an average cooling rate of 50 to 500°C/sec to a temperature range of 0 to 200°C, and the cooling end temperature is 1
A method for producing a composite structure type high-strength cold-rolled steel sheet, characterized by holding the steel sheet for ~5 minutes.