JPH05163533A - Manufacture of baking hardening type steel sheet with composite structure suitable for deep drawing - Google Patents

Abstract

PURPOSE:To economically manufacture cold rolled steel sheet having desired strength characteristic as well as an excellent deep drawing characteristic without adding Ti, Nb, B, etc. CONSTITUTION:The steel, containing 0.002-0.010% C, <=1.0% Si, 0.2-2.5% Mn, <=0.20% P and 0.005-0.10% Al are contained, and of which the remainder is composed of Fe and inevitable impurities, is hot rolled and cold rolled; successively in continuous annealing, heated not less than transformation point but not more than Ac3 point: cooled to 500-750 deg.C at an average cooling rate of 20 deg.C/sec; and cooled to the transportation point Ms or less at the average cooling rate of 20 deg.C or above. Consequently, the structure composed of ferrite and martensite of not less than 0.1% and not more than 5% in volume percentage is obtained. This steel sheet is provided with a necessary material characteristic as a cold rolled sheet steel for which deep drawing characteristic is required, and it is excellent especially in strength characteristic.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet, and more particularly to a method for producing a composite microstructure bake hardenable steel sheet having excellent deep drawability.

[0002]

2. Description of the Related Art In recent years,
There is a strong demand for cold-rolled steel sheets having high press formability and high r-value as automobile parts and outer panels for electric devices. In particular, regarding automobile parts, from the viewpoint of ensuring safety and reducing the weight for improving fuel efficiency, in addition to a high r value, high strength steel sheets are also required.

Conventionally, as a steel sheet having an improved r value, a so-called IF (Interstitial Free) obtained by adding Ti and / or Nb, which is sufficient to firmly bond C and N, to an ultra-low carbon steel.
Steel is known. In order to increase the strength of this steel type, solid solution strengthening elements such as Si, Mn, and P are added thereto. However, these solid solution strengthening elements are added during the annealing after cold rolling. It is difficult to obtain a high r value and strength together because it has a tendency to suppress nucleation and crystal grain growth of the (222) crystal orientation, which is advantageous for the value. Further, the yield ratio (yield strength / tensile strength) is high, and the press formability is poor.

On the other hand, for example, as shown in Japanese Patent Publication No. 3-2224 and Japanese Patent Publication No. 3-21611, Ti, Nb and B are added to low carbon steel to form a composite structure after cold rolling and annealing. Therefore, it has been proposed to obtain a high r value, non-aging at room temperature and high bake hardenability (BH property), but the annealing temperature is as high as around 900 ° C. and the economy is poor. Further, since the annealing temperature is high, the volume fraction of ferrite during annealing is small, the texture tends to be random, and the safety of the material is also poor.

The present invention solves the above-mentioned drawbacks of the prior art and economically provides a cold-rolled steel sheet having desired strength characteristics and excellent deep drawability without adding Ti, Nb, and B. The object is to provide a method that can be obtained.

[0006]

Means for Solving the Problems As a result of intensive studies for solving the above-mentioned problems, the present inventors have added an appropriate alloying element such as Mn to Al-killed steel and added it to a usual high r-value Al.
Hot rolling and cold rolling according to the manufacturing method of steel, when performing the continuous annealing method, by controlling the heating temperature and cooling process of continuous annealing, it is possible to produce a composite structure bake hardenable steel sheet having excellent deep drawability. The present invention has been found out here, and the present invention has been completed.

That is, according to the present invention, C: 0.002 to 0.02.
010%, Si ≦ 1.0%, Mn: 0.2-2.5%, P ≦
Steel containing 0.20% and Al: 0.005 to 0.10%, the balance of which is Fe and unavoidable impurities, is hot-rolled and cold-rolled, and is continuously annealed at a temperature of Ac ₁ By heating to an Ac of ₃ points or less, cooling from 500 to 750 ° C. at an average cooling rate of 20 ° C./second or less, and then cooling to an Ms transformation point or less at an average cooling rate of 20 ° C./second or more, A gist of the present invention is to provide a method for producing a composite microstructure bake hardenable steel sheet having excellent deep drawability, which is characterized by obtaining a microstructure consisting of ferrite and martensite of 0.1% to 5% in volume ratio.

The present invention will be described in more detail below.

[0009]

[Action]

First, the reasons for limiting the chemical composition of steel in the present invention will be explained.

C: As the amount of C increases, the amount of martensite increases and hinders the improvement of the r value, so the upper limit of the amount of C is made 0.01%. However, if it is less than 0.002%, it is difficult to obtain martensite.
The C content is in the range of 0.002-0.01%.

Si: Si is an element effective for solid solution strengthening,
Reduces the C concentration in ferrite and reduces C in austenite
Since the concentration is increased, the hardenability is improved. But 1.
If it exceeds 0%, the deterioration of the r value becomes large, so the upper limit is made 1.0%.

Mn: Mn is an element that improves the hardenability of steel and is effective for solid solution strengthening, but if it exceeds 2.5%, the deterioration of the r value becomes large, so its upper limit is 2. 5%.
However, if it is less than 0.2%, not only is it difficult to obtain martensite, but also hot cracking occurs, so the lower limit is made 0.2%.

P: P is an element effective for solid solution strengthening and an element effective for forming a composite structure as a ferrite forming element. However, if it exceeds 0.20%, the r value is deteriorated and the secondary processing resistance is improved. Since the brittleness deteriorates, the upper limit is set to 0.20%.

Al: Al is a deoxidizing agent and is at least 0.
It is necessary to add 005%. But 0.1
If it is added in excess of 0%, not only the deoxidizing effect is saturated, but also alumina-based inclusions are generated and formability is deteriorated, so the Al amount is made 0.005 to 0.10%.

Next, manufacturing conditions will be described.

The steel having the above chemical composition is hot-rolled and cold-rolled according to a conventional method for producing low carbon Al killed steel, and then subjected to continuous annealing.

However, in the continuous annealing, first, the heating temperature is set to the Ac ₁ transformation point or more and the Ac ₃ transformation point or less, and the ferrite.
Two phases of austenite. If the heating temperature is lower than the Ac ₁ transformation point, austenite cannot be obtained, and therefore martensite cannot be obtained because transformation is not accompanied in the cooling process. Further, when the heating temperature exceeds the Ac ₃ transformation point, all the {111} textures that improve the r value obtained in the cold rolling annealing process are transformed into austenite, and the texture obtained in the subsequent cooling process has a random orientation. Therefore, a high r value cannot be obtained.

Then, 20 to 500 to 750 ° C.
C in the austenite is concentrated by slow cooling at an average cooling rate of not more than ° C / sec. If the average cooling rate up to this temperature range exceeds 20 ° C./sec, the concentration of C in austenite is small, bainite transformation occurs, and it is difficult to obtain martensite. In addition, the amount of low-temperature transformation products also increases, and a high r value cannot be obtained.

Thereafter, by cooling to below the Ms transformation point at an average cooling rate of 20 ° C./sec or more, ferrite and martensite in a volume ratio of 0.1% to 5% can be obtained, and deep drawing is performed. A composite structure bake hardenable steel sheet having excellent properties can be obtained. If the average cooling rate is lower than 20 ° C./sec, bainite or pearlite is formed, martensite cannot be obtained, and predetermined strength and bake hardenability cannot be obtained.

The composite structure steel sheet obtained by the above method has the following characteristics. (1) As-annealed, there is no yield point elongation and the yield ratio is low. (2) Work hardening coefficient (n value) is high, and uniform elongation and total elongation are large. (3) At room temperature, the recovery of elongation at yield is extremely slow and has a slow aging property and BH property. (4) It has a high r value. All of these are properties that are strongly required for press forming steel sheets.

The reason for having such excellent characteristics is as follows. That is, the heating temperature of the continuous annealing is set to the Ac ₁ transformation point or more and the Ac ₃ transformation point or less, and the ferrite
By making the two phases of austenite, C moves from ferrite to austenite due to the difference in solid solubility, and by concentrating, C concentration in ferrite becomes small. By cooling this under the above conditions to convert the austenite enriched with C into martensite, a composite structure steel sheet of soft ferrite and hard martensite can be obtained. As a result, a {111} texture develops in the annealing heating process, and a clean ferrite can produce a steel sheet having a high r value and good ductility. Further, due to the formation of mobile dislocations in the surrounding ferrite base material and the internal stress associated with the martensitic transformation, dislocations easily occur around the martensitic phase when stress is applied from the outside, and the plastic deformation is propagated uniformly. For this reason, there is no yield point elongation while annealing,
Low yield ratio. Further, it has a good BH property due to the presence of an appropriate solid solution C in the ferrite. Therefore, a composite microstructure bake hardenable steel sheet having excellent deep drawability can be manufactured.

Next, an embodiment of the present invention will be shown.

[0024]

【Example】

[Table 1] 1250 is an ultra-low carbon steel having the chemical composition shown in
After the solution heat treatment at ℃, finish rolling at 900 ℃ to finish the hot rolling, then at 700 ℃ for 1 hour furnace cooling, after the winding treatment, 80% cold rolling, 0.8mm of Into steel plate,

[Table 2] Continuous annealing was performed under the heating temperature and cooling conditions shown in. The material properties of the obtained steel sheet are also shown in Table 2.

JIS 13B was used as the tensile test piece, and the room temperature aging (AI amount) was 10% from 10% tensile strain.
The amount of increase in YP after treatment at 0 ° C. for 1 hour was evaluated. Bake hardenability (BH amount) is 170 ° C x 2 from 2% tensile strain
The amount of increase in YP after 0 minute treatment was evaluated.

In Table 2, No. 1 to No. 2 and No. 6 to No. 7
Are examples of the present invention, all of which have delayed aging when the AI amount is 5 N / mm ² or less and have a high BH amount of 60 N / mm ² or more. In addition, the r value is 1.6 or more, indicating excellent deep drawability.

On the other hand, in Comparative Example No. 3, since the cooling rate from the intermediate temperature to room temperature is slow, a martensite structure cannot be obtained and it is aged at room temperature. Also, the heating temperature is high,
Since it became an austenite single phase, the texture became random and the r value was low. In No. 4, since the heating temperature is lower than the Ac ₁ transformation point, a martensite structure cannot be obtained,
Aging at room temperature. Since No. 5 has a high cooling rate up to an intermediate temperature, it has a large bainite structure and a low r value. No.8
Has a high heating temperature in order to obtain solid solution C, the texture becomes random, and the deep drawability is low. No. 9 is IF steel,
Since there is no solid solution C, there is no BH amount. No. 10 has a large amount of C and a high volume ratio of martensite, and thus has a low r value.

[0028]

As described in detail above, according to the present invention,
Has the material properties required for cold-rolled steel sheets that require deep drawability, such as panel materials for automobiles, and in particular has excellent strength properties,
Since an increase in yield strength can be obtained after forming at 340 to 500 N / mm ² , it is possible to manufacture a steel sheet that is easy to form and has excellent dent resistance.

Claims (1)

[Claims] 1. In weight% (hereinafter the same), C: 0.002-
0.010%, Si ≦ 1.0%, Mn: 0.2-2.5%, P
≦ 0.20%, Al: 0.005 to 0.10%, the balance Fe and inevitable impurities steel is hot-rolled and cold-rolled, the subsequent continuous annealing, Ac ₁ transformation point By heating to less than Ac ₃ points and cooling from 500 to 750 ° C. at an average cooling rate of 20 ° C./second or less, and then cooling to an Ms transformation point or less at an average cooling rate of 20 ° C./second or more. ,
A method for producing a composite microstructure bake hardenable steel sheet having excellent deep drawability, which comprises obtaining a microstructure consisting of ferrite and a martensite content of 0.1% to 5% by volume.