JPH04323345A - Cold rolled steel sheet excellent in press formability and baking hardenability - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet excellent in press formability and baking hardenability by preparing a steel having a specific composition where the contents of Nb, Ti, and Pb are controlled. CONSTITUTION:A cold rolled sheet of steel having a composition consisting of, by weight, 0.002-0.008% C, 0.1-2.0% Mn, 0.001-0.8% Si, 0.002-0.03% P, 0.004-0.020% S, 0.01-0.07% acid-soluble Al, 0.0005-0.004% N, 0.01-0.4% Pb, 0.01-0.04%, in total, of Nb and/or Ti, and the balance Fe with inevitable impurities is prepared. Further, it is preferable that annealing after cold rolling is done under the conditions of >= about 750 deg.C temp. and <= about 5 deg.C/sec cooling rate. By this method, the steel sheet excellent in press formability and having high baking hardenability can be obtained.

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 having excellent press formability and bake hardenability.

0002

[Prior Art] Manufacturers of thin steel sheets often manufacture so-called IF steel, which is made by adding carbonitride-forming elements such as Nb and Ti, singly or in combination, to ultra-low carbon steel in order to obtain high press formability. ing. In order to reduce the weight of automobile bodies, high-strength steel plates are manufactured by adding appropriate amounts of Si, Mn, P, etc. to IF steel, in line with the needs of automobile manufacturers for high-strength steel plates. In addition, as the strength of the steel sheet increases, wrinkles and surface distortion are more likely to occur during press forming.
Steel plates having so-called bake-hardenability, which are soft during press forming and then harden during paint baking, are also manufactured using IF steel of various strength levels.

IF steel with bake hardenability is produced by: 1) heating IF steel to a high temperature to dissolve some of the carbides;
(Japanese Unexamined Patent Publication No. 63-4899), 2) Controlling carbonitride-forming elements whose atomic ratio is equal to or less than that of C and N, according to the concentration of C and N. It is manufactured by two methods: adding solid solution C to the steel and leaving solid solution C in the steel (Japanese Unexamined Patent Publication No. 59-38337). However, these methods have the disadvantage that steel plates with high bake hardenability cannot be produced. If the amount of solid solution C in the steel is increased using methods 1) and 2) above, the bake hardenability will increase, but at the same time, the steel will become hard during press forming and will cause surface irregularities called stretcher strain. This has the disadvantage that the steel sheet becomes unsuitable for press forming.

0004

[Problems to be Solved by the Invention] The present invention provides a cold-rolled steel sheet with high bake hardenability based on IF steel which has excellent press formability. This advantageously solves the conventional problem that it is difficult to achieve both suppression of the occurrence of and high bake hardenability.

[0005]

[Means for Solving the Problems] The gist of the present invention is as follows. (1) In weight%, C: 0.002-0.008%, Mn: 0.1-2.0%, Si: 0.001-0.8%, P: 0.002-0.03% , S: 0.004-0.020%, Acid-soluble Al: 0.01-0.07%, N: 0.000
5 to 0.004%, Pb: 0.01 to 0.4%, and one or more types of Nb and Ti are added, the total amount of which is 0.01 to 0.04%, and the balance is Fe and unavoidable Cold-rolled steel sheet with excellent press formability and bake hardenability due to impurities.

(2) In weight%, C: 0.002-0.008%, Mn: 0.1-2.0%, Si: 0.001-0.8%, P: 0.002-0 .10%, S: 0.004-0.020%, Acid-soluble Al: 0.01-0.07%, N: 0.000
5 to 0.004%, Pb: 0.01 to 0.4%, B: 0.0002 to 0.002%, and one or more types of Nb and Ti are added, and the total amount added is 0.01 to 0.004%. A cold rolled steel sheet with excellent press formability and bake hardenability, with the balance being Fe and unavoidable impurities.

That is, the present invention applies P to so-called IF steel, which is made by adding appropriate amounts of Nb and Ti to ultra-low carbon steel that has excellent press formability.
This was done based on the new finding that adding an appropriate amount of B exhibits high bake hardenability and makes stretcher strain less likely to occur during press forming.The characteristics are high bake hardenability and high pressability. It is an IF steel containing Pb that exhibits formability.

[0008]

[Operation] The present invention will be explained in detail below. Since it is better to have less C in order to obtain high press formability, the upper limit was set to 0.008%. On the other hand, 0.002% or more of C is required to impart bake hardenability.

[0009]Si is added when manufacturing high-strength steel sheets, but if it is too much, it becomes difficult to remove scale from the steel surface during hot rolling, and surface flaws are likely to occur.
It was set to 8% or less. 0.001% or more is contained as an impurity element. Similarly to Si, the amount of Mn added is adjusted depending on the required tensile strength, but if it is too large, the cost of adding it increases, so the upper limit was set at 2.0%. Further, the lower the Mn content, the better the press formability is, but the removal of Mn increases the cost, so the lower limit was set at 0.1%.

[0010] Since P can significantly increase the strength with a small addition amount, it is added, like Mn and Si, when producing high-strength steel sheets. When manufacturing high-strength steel plates, P
If the amount added is too large, brittle cracking at low temperatures tends to occur, so the upper limit is set at 0.10%. When manufacturing mild steel, P is an impurity element, and the smaller the better, but generally 0.03% may be included as an impurity, so the upper limit of P is set to 0.03%.

[0011] Even when manufacturing high-strength steel sheets, the strength may be increased with Mn or Si, regardless of P. At that time, elongation is improved by significantly reducing P. The effect of improving elongation by reducing P is the same for mild steel. However, if P is significantly lowered, the cost of removing P in steelmaking will increase, so the lower limit is set at 0.002%. S exists in steel as an impurity element. You can also get rid of S, but 0.00
If it is less than 4%, scratches are likely to occur during continuous casting. More than 0.02% is usually not included in steel.

Acid-soluble Al is provided by Al added for deoxidation before casting. Acid soluble Al is 0.01
% or more, continuous casting cannot be performed successfully. T
When i is not added, N is added to precipitate it as AlN in the steel. In this case, the press formability improves as the amount of Al increases, but if it increases too much, it deteriorates. The maximum amount was set at 0.07%.

[0013] N is an impurity element, and since press formability is improved when the amount is small, the upper limit is set to 0.004%. Less than 0.0005% is impossible with current de-N technology in steelmaking. Pb is added in order to (1) increase bake hardenability and (2) make stretcher strain less likely to occur during press molding. Pb hardly forms a solid solution in the steel, but is dispersed in the steel in the form of particles, exists as a solid at room temperature and as a liquid at the annealing temperature, and is characterized by a larger coefficient of thermal expansion than steel. For this reason, it is thought that a tensile stress field is formed in the steel around the Pb particles during cooling after annealing, and the C atoms in a solid solution state in the steel gather here. When this is heated to the paint baking temperature of automobiles, the Pb particles expand, and because there are dislocations introduced during press forming, C atoms in a solid solution state are released from around the Pb particles, and these C atoms is thought to exhibit high bake hardenability because it fixes dislocations. On the other hand, Pb is significantly softer than steel at room temperature, so
If Pb particles exist in steel, Pb will be removed during press forming.
Since deformation begins around the particles, it is thought that this makes it difficult for yield point elongation to occur. The effects of (1) and (2) described above do not appear unless 0.01% or more of Pb is added. The effects of (1) and (2) become greater as the amount increases, but it has been found that adding more than 0.4% rapidly deteriorates the elongation properties necessary for press molding, so the upper limit was set at 0.4%.

One or more types of Nb and Ti are added to form carbonitrides. If it is not added, press formability will be low. The amount added is 0.01% in total.
The above is necessary. When the addition amount increases, the solid solution C decreases, and the amount of bake hardening decreases. The upper limit is 0.
04%. B is added when it is necessary to improve low temperature brittleness. In particular, when adding a large amount of P in order to obtain a high-strength steel sheet, P must be added. At that time, there is no effect unless it is added in an amount of 0.0002% or more. If the amount added exceeds 0.002%, press formability will be significantly deteriorated.

[0015] The steel of the present invention is preferably produced by cold rolling and subsequent annealing in view of its intended use. Any conventional method may be used, but N
b. When a large amount of Ti is added, the annealing temperature is preferably 750°C or higher. In this case, annealing beyond the Ac3 point increases the amount of bake hardening, but reduces press formability. The cooling rate after annealing is not particularly limited, but a faster cooling rate is preferable because the effects of (1) and (2) become greater. The cooling rate is preferably 5° C./sec or more, and it is particularly good to cool at a cooling rate of more than 100° C./sec. In order to obtain a hot-dip galvanized steel sheet, plating may be performed during annealing and cooling. Further, heat treatment for alloying the zinc plating layer after plating may be performed according to a conventional method. Of course, alloy plating with a zinc-iron composition may be used.

[0016] Although not particularly limited, it is preferable to produce a slab by continuous casting and hot rolling before cold rolling. Any conventional method for manufacturing IF steel may be used. The continuously cast slab may be charged into a hot rolling heating furnace without once being cooled to room temperature. Alternatively, the continuously cast slab may be hot rolled as it is without being charged into a heating furnace.

As mentioned above, alloyed hot-dip galvanized steel sheets, hot-dip galvanized steel sheets having a plating layer with an unalloyed iron-zinc composition, and electrically deposited plating layers containing zinc, iron, or chromium as the main components. It may also be an electroplated steel sheet. Further, temper rolling for shape correction after manufacturing may be carried out by a conventional method. Rust preventive oil, press lubricating oil may be applied, or a solid lubricating film may be applied. So-called surface treatment such as chromate treatment may also be performed.

[0018]

EXAMPLES Steels having various chemical compositions were manufactured using vacuum degassing equipment and continuously cast to a thickness of 245 mm. The slab was reheated to a temperature of 1050°C, and hot rolling was completed above the Ar3 transformation temperature to give a thickness of 3.2 mm.
．． Cooling was performed at 150° C./sec for 8 sec to 1 sec, and the film was wound up at 730° C. Thereafter, scale was removed from the steel surface by pickling with hydrochloric acid. After that, it is cold rolled to a thickness of 0.75m.
It was set as m. Thereafter, continuous annealing was performed for 40 seconds at the temperature shown in Table 1. The heating rate for annealing was 10°C/sec, and the cooling rate was 150°C/sec to 40°C.

Some of the materials were subjected to Sendzimer type continuous hot-dip plating to obtain alloyed hot-dip coated steel sheets with a coating weight of 60 g/m2. The bath Al at that time was 0.15%, and 5
Alloying treatment was performed at 00° C. until the liquid layer disappeared (about 10 to 20 seconds, depending on the chemical composition). The heating rate for annealing was 10°C/sec, and the maximum temperature of the steel plate was 840°C. Cooling was performed at 5°C/sec before plating and at 20°C/sec until 40°C after alloying.

[0020] These steel plates were subjected to 0.4% temper rolling and subsequently coated with oil whose main purpose was to prevent rust. Table 1 shows the chemical composition of the steel sheet analyzed after oil application. A JIS No. 5 tensile test piece was cut out from the steel plate with the rolling direction as the longitudinal direction, and subjected to a tensile test. Tensile test is 10mm/min
I went there. Tensile strength was measured as the strength level of the steel plate, and elongation at yield point and elongation were measured as measures of press formability. In particular, the elongation at yield point is closely related to the occurrence of stretcher strain during press forming, and the smaller the elongation at yield point, the less likely it is that stretcher strain will occur. yield point elongation,
The elongation is based on an inter-point distance of 50 mm. In practice, it takes time from the time it is shipped until it is used.
The tensile test was also conducted after being left at room temperature for 30 days after temper rolling.

[0021] First, the bake hardenability was determined at 2% with a distance of 50 mm between the scores.
A tensile prestrain of 2% was applied, and flow stress was measured at a tensile prestrain of 2%. Thereafter, the tensile test pieces were placed in a silicone oil bath heated to 170°C, and after 20 minutes, they were taken out and cooled with water, and the tensile test was performed again, and the upper yield point was measured. The amount of bake hardening was determined by subtracting the flow stress in (2) from the upper yield point in (2).

The steel of the present invention has a large elongation, and has a large amount of bake hardening even though the elongation at yield point is small compared to the comparative steel.

[0023]

[Table 1]

[0024]

[Effects of the Invention] According to the present invention, a steel plate having excellent press formability and high bake hardenability can be obtained, which will greatly contribute to the industrial world including automobiles.

Claims (2)

[Claims] Claim 1: In weight %, C: 0.002-0.008%, Mn: 0.1-2.0%, Si: 0.001-0.8%, P: 0.002-0. 03%, S: 0.004-0.020%, Acid-soluble Al: 0.01-0.07%, N: 0.000
5 to 0.004%, Pb: 0.01 to 0.4%, and one or more types of Nb and Ti are added, the total amount of which is 0.01 to 0.04%, and the balance is Fe and unavoidable Cold-rolled steel sheet with excellent press formability and bake hardenability due to impurities. 2. In weight %, C: 0.002-0.008%, Mn: 0.1-2.0%, Si: 0.001-0.8%, P: 0.002-0.008%. 10%, S: 0.004-0.020%, acid-soluble Al: 0.01-0.07%, N: 0.000
5 to 0.004%, Pb: 0.01 to 0.4%, B: 0.0002 to 0.002%, and one or more types of Nb and Ti are added, and the total amount added is 0.01 to 0.004%. A cold rolled steel sheet with excellent press formability and bake hardenability, with the balance being Fe and unavoidable impurities.