JPH0585620B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a cold rolled steel sheet with excellent bake hardenability and workability. (Prior Art) In recent years, high-strength steel plates have been increasingly used in the automobile industry due to the need to reduce the weight of vehicle bodies to improve fuel efficiency and to ensure the safety of passengers in the event of a collision. Additionally, due to the style of car bodies, there are many parts that are difficult to process, and the use of super-processable steel sheets is increasing. In particular, efforts are being made to reduce the thickness and weight of automobile doors and other exterior panels. but,
As the thickness of the plate decreases, it tends to lose its elasticity, and even when pressed with a finger, it may dent. Therefore, there is a desire for a steel plate that is easy to press and harden when used, and steel plates that utilize bake-hardening properties are being used to harden the steel plate by utilizing the heat from the baking of the paint after pressing. Bake-hardenable steel sheets have a mechanism in which solute carbon is left in the steel sheet manufacturing process, and the steel sheets are hardened through strain aging during paint baking.Because solute carbon remains, the steel sheets can be aged and pressed even at room temperature. Sometimes it has already hardened, causing press cracks. There is a need for measures to prevent aging deterioration at room temperature, which is a drawback of bake-hardenable steel sheets. Conventional manufacturing of bake-hardenable steel sheets involves box-type annealing as shown in Japanese Patent Publication No. 57-17064.
P in the area where about 0.015% of solid solution carbon remains:
Add about 0.06% to make TS = 35 kg class, and
For continuous annealing, C: 0.03%, P: about 0.03%
TS = 35 kg class. Furthermore, as a super-formable steel sheet, JP-A-59-
As shown in Publication No. 74232, ultra-low carbon steel, C:
It is a bake-hardenable steel sheet containing 0.0030%, Nb: 0.0018%, and Ti: 0.0010%. (Problems to be Solved by the Invention) However, the amount of solid solution carbon to remain varies depending on conditions such as components, annealing temperature, time, cooling rate, etc., and the amount of bake hardening changes. Even if this change in the amount of bake hardening is still acceptable, deterioration due to room temperature aging due to changes in solute carbon must be prevented. As the amount of solute carbon increases, the amount of bake hardening increases and the room temperature aging tends to deteriorate, so a steel sheet with excellent room temperature aging properties is desired even though the amount of bake hardening increases. The present invention aims to solve the problem that, although the amount of bake hardening increases due to an increase in solute carbon, the aging property at room temperature also deteriorates. (Means for Solving Problems) The gist of the present invention is C: 0.005% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.1% or less, Al: 0.1%.
Contains N: 0.005% or less, and Ti, Nb
The essential condition is the composite addition of Ti, and Ti is 48/14
[N (%) - 0.003%] ≦ Ti (%), Ti (%) < [48/12
C
(%) +48/14N (%)], 0.003%≦Ti＜0.025% 3
Contains within the range that satisfies the conditions, and Nb is 93/12 [C
(%)-0.005%〕≦Nb(%)≦93/12[C(%)-0.00
1
%] and is contained within the range satisfying 0.003%≦Nb(%)<0.025%, Nb(%) + Ti(%)<0.04%, and further Cr: 0.03 to 0.35%, V:
0.01~0.15%, Mo: 0.02~0.16%, W: 0.02~
A steel containing one or more of 0.30% as an essential element, or further containing 0.0002 to 0.0030% of B, with the remainder consisting of Fe and unavoidable elements, is hot rolled by a conventional hot rolling method. After forming the steel strip, after cold rolling in a conventional manner, annealing is performed at a temperature above the recrystallization temperature and below 750℃ using box type annealing, or annealing is performed at a temperature above the recrystallization temperature and below 900℃ using continuous annealing. This is a method for producing a cold rolled steel sheet with excellent bake hardenability and workability. Bake-hardenable steel sheets utilize strain aging by leaving solute carbon behind, so aging at room temperature can cause deterioration of the material before pressing, which may cause press cracks. Bake-hardenable steel sheets are soft and have excellent workability before being pressed, and are expected to harden by baking the paint and become hard when used. However, conventional manufacturing methods have had the disadvantage that it is difficult to adjust the amount of solid solute carbon, and that the relationship between softness before pressing and hardness after baking is incompatible. Therefore, as a result of intensive study, the present inventors found that ultra-low carbon steel sheets with Nb and Ti additions as ultra-formable steel sheets.
By containing one or more of Cr, V, Mo, and W as essential elements, there is no aging at room temperature and the original properties of a bake-hardenable steel sheet that become hard after painting is baked. We found that it is a property of By adding Cr etc., solid solution carbon decreases,
Room temperature aging disappears. However, carbon does not precipitate as a complete carbide, but exists in a weakly bonded state or cluster form before precipitation of Cr, etc., and becomes solid solution carbon by the heat during paint baking, which contributes to bake hardenability. It was discovered that the steel sheet does not age at room temperature and can be given the properties that a bake-hardenable steel sheet should have, which becomes hard after painting and baking. The present inventor was constructed based on this knowledge, and the present invention will be described in detail below. Figure 1 shows C: 0.0025%, Si: 0.009%, Mn:
0.21%, P: 0.01%, Ti: 0.015%, Nb: 0.005%
The basic component is steel containing Cr, and the amount of Cr is 0.01~
Steel with a change in the range of 0.4% was melted, hot-rolled according to a conventional method at a finishing temperature of 890℃ and a coiling temperature of 720℃, and then cold-rolled according to a conventional method to obtain a steel plate with a thickness of 0.8mm. After that, it was continuously annealed at 800℃ for 1 minute, then cooled at 100℃/sec, and then heated to 400℃.
FIG. 3 is a diagram examining the relationship between Cr content, bake hardening amount, and yield elongation when overaging treatment was performed for 3 minutes and a skin pass of 1% was applied. From the figure,
Regardless of the presence or absence of Cr content, the amount of bake hardening is large, but the yield elongation is reduced. This means that
Due to the addition of Cr, there is no aging at room temperature, which is an inherent property of a bake-hardenable steel sheet that becomes hard after painting is baked. The reason for limiting the chemical components is as follows. When C is contained in an amount exceeding 0.005%, the amount of solid solution carbon increases, and room temperature aging properties deteriorate. In order to fix C, the amount of Nb added increases, which is economically disadvantageous. For this reason, the upper limit of C was set at 0.005%. Si contains 0.8% or less, and has a positive effect on bake hardenability due to the effect of dissolving C. Furthermore, when C is lowered, the strength of the steel sheet decreases, so Ni is included to compensate for the decrease in strength. However, if the content exceeds 0.8%, an oxide film will increase on the surface of the steel sheet and workability will deteriorate, so the upper limit was set at 0.8%. If Mn is contained in an amount exceeding 1.0%, processability will decrease, so 1.0% is set as the upper limit. A small amount of P improves workability, but it is added as a solid solution element. However, if the amount exceeds 0.1%, processability deteriorates, so 0.1% was set as the upper limit. Al contains 0.1 or less, N is fixed as AlN,
This is to prevent deterioration due to aging at room temperature, and the upper limit is
The reason why it is set at 0.1% is because even if it is contained more than that, it exceeds the amount of N that is fixed, and the remainder after fixing N deteriorates workability. N content is preferably 0.005% or less, and it is desirable to keep it as low as possible, so that the amount of Ti added can be reduced.
In order to improve room temperature aging properties, the upper limit has been set to 0.005.
%. B is included as necessary, but it is an element that strengthens hardening by rapid cooling in the annealing process, and is an element that improves bake hardenability, and if its content is less than 0.0002%, the effect will be lost. The lower limit is 0.0002%, and 0.00030
If the content exceeds 0.0030%, scratches are likely to occur during the hot rolling process and the surface quality of the steel sheet will be significantly impaired, so the upper limit is set at 0.0030%. Ti and Nb are added as a combined addition. First, Ti fixes N and improves room temperature aging properties, reduces bonding with C and improves bake hardenability, and remains in the steel as solid solution Ti so that it does not impair workability. 48/1, subject to
4 [N (%) - 0.003%] ≦ Ti (%), Ti (%) < [48/
12
C (%) + 48/14N (%)], 0.003%≦Ti<0.025%, and competes with Nb to produce a bake-hardenable steel sheet with excellent workability. Nb combines with C to improve room-temperature aging properties, and also improves bake hardenability by taking advantage of the incomplete bonding, and remains in the steel as solid solution Nb, impairing workability. provided that there is no
93/12 [C (%) - 0.005%] ≦Nb (%) ≦ 93/12 [
C
(%) −0.001%] and 0.003%≦Nb(%)＜0.025
%, Nb (%) + Ti (%) <
The content satisfies 0.04%, and the competition with Ti creates a bake-hardenable steel sheet with excellent workability. Containing one or more of Cr, V, Mo, and W as essential elements are all carbide-forming elements, and their combination with C reduces solid solution carbon and improves room temperature aging. In addition, during paint baking, the heat decomposes and increases solid solute carbon, which has the effect of improving bake hardenability.
The reason for setting the content to 0.02% and W: 0.02% is to obtain the above effect. Also, the upper limit of each is Cr: 0.35
%, V: 0.15%, Mo: 0.16%, and W: 0.30% because even if it is contained more than that, it exceeds the content required to obtain the above effect, and furthermore, if it is contained more than that, it will cause carbide-forming elements. This is because C is fixed as a complete carbide and bake hardenability cannot be obtained. Next, the manufacturing process will be described. The hot rolling process produces a hot rolled steel strip using a conventional method, but it is desirable to set the finishing temperature to the _A3 transformation point or higher; if it is lower than that, rolling distortion will remain and the structure will not be uniform. The reason why it is desirable to set the winding temperature to below the _A1 transformation point is because if the winding temperature is higher than this, the cooling after winding will be slow and the structure will become non-uniform. Another reason is that the amount of scale produced is large and the pickling properties are poor. The cold rolling process is a conventional method, and the rolling ratio is preferably 40 to 90%. The range of annealing temperature is set at a lower limit of the recrystallization temperature or higher and an upper limit of 750° C. or lower for box type annealing because annealing at an annealing temperature exceeding that temperature increases carbides and deteriorates workability. Further, in continuous annealing, the lower limit is set to be higher than the recrystallization temperature and the upper limit is set to 900°C or lower because annealing at an annealing temperature exceeding that increases carbide size and deteriorates workability. Furthermore, in continuous annealing, after annealing, it is possible to perform an overaging treatment by cooling and adjust the solute carbon to adjust the bake hardenability. As described above, according to the present invention, it is possible to economically produce a cold rolled steel sheet with excellent bake hardenability and workability. (Example) Next, the present invention will be explained in detail by giving examples. Example 1 The steel shown in Table 1 manufactured by the ingot method or continuous casting method was continuously hot rolled, pickled, cold rolled, and annealed under the manufacturing conditions shown in Table 2. Afterwards, various properties of the obtained steel plate were investigated. As can be seen from Table 3, the desired characteristic values could not be obtained by the comparative methods other than the method of the present invention, whereas the desired characteristic values could be obtained by the method of the present invention.

【table】

【table】

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【table】

[Table] (Effects of the Invention) As explained above, according to the present invention, it is possible to economically produce a cold rolled steel sheet with excellent bake hardenability and workability.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between Cr content, bake hardening amount, and yield elongation.

Claims (1)

[Claims] 1 Contains C: 0.005% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.1% or less, Al: 0.1% or less, N: 0.005% or less, and Ti, Nb. The condition is that Ti should be added in combination, and Ti should be 48/14 [N (%) - 0.003%] ≦Ti
(%), Ti (%) <[48/12C (%) + 48/14N (%)]
,
Contains within the range that satisfies the three conditions of 0.003%≦Ti<0.025%, and Nb is 93/12 [C (%) - 0.005%]≦
Nb (%) ≦93/12 [C (%) - 0.001%] and
Contains within the range of 0.003% ≦ Nb (%) < 0.025%, Nb (%) + Ti (%) < 0.04%, and furthermore, Cr: 0.03 to 0.35%, V: 0.01 to 0.15
%, Mo: 0.02 to 0.16%, W: 0.02 to 0.30%, and the steel contains one or more of the following as essential elements, with the remainder consisting of Fe and unavoidable elements.
After making a hot rolled steel strip by conventional hot rolling, after conventional cold rolling, box annealing is performed at a temperature above the recrystallization temperature and 750°C or below, or continuous annealing is performed at a temperature above the recrystallization temperature, A method for producing a cold-rolled steel sheet with excellent bake hardenability and workability, characterized by annealing at a temperature of 900°C or less. 2 Contains C: 0.005% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.1% or less, Al: 0.1% or less, N: 0.005% or less, and Ti and Nb are added in combination. is the essential condition, Ti is 48/14 [N (%) - 0.003%] ≦ Ti (%),
Ti (%) <[48/12C (%) + 48/14N (%)], 0.003
%
Contains within the range that satisfies the three conditions of ≦Ti<0.025%,
Nb is 93/12 [C (%) - 0.005%] ≦Nb (%) ≦93/12
[C (%) - 0.001%] and 0.003%≦Nb (%)<
Contains within the range satisfying 0.025%, Nb (%) + Ti
(%) <0.04%, and further Cr: 0.03 to 0.35%, V: 0.01 to 0.15%,
Steel containing one or more of Mo: 0.02 to 0.16% and W: 0.02 to 0.30% as essential elements, further containing B: 0.002 to 0.030%, and the balance consisting of Fe and inevitable elements. After making it into a hot-rolled steel strip by conventional hot rolling, after conventional cold rolling, box annealing is performed at a temperature above the recrystallization temperature and 750°C or below, or continuous annealing is performed at a temperature above the recrystallization temperature. , a method for producing cold-rolled steel sheets with excellent bake hardenability and workability, characterized by annealing at a temperature of 900°C or less.