JPH0673498A - Cold rolled steel sheet excellent in baking hardenability in coating/baking at low temperature and its production - Google Patents

Abstract

PURPOSE:To provide a cold rolled steel sheet having baking hardenability in coating and baking treatment at a low temp. without adding expensive alloy elements. CONSTITUTION:In a steel sheet obtd. by subjecting steel contg. 0.001 to 0.03% C, <=0.3% Si, 0.05 to 1.0% Mn, <=0.10% P, 0.01 to 0.1% acid soluble Al and <=0.005% N, and furthermore contg., at need, one or two kinds of 0.01 to 0.1% Ti and 0.01 to 0.05% Nb, and the balance Fe with inevitable impurities to hot rolling, cold rolling and continuous annealing, the cementite density in the grains after annealing is regulated to 4X10<4> pieces/mm<2> and the content of solid solution C is regulated to <=10ppm. The steel sheet can be obtd. by subjecting the steel having the same chemical components to hot rolling at the Ar3 transformation point or above, coiling it at 600 to 700 deg.C, subjecting it to cold rolling and thereafter executing optimum overaging treatment by continuous annealing at the recrystallization temp. to 850 deg.C for 30sec to 3min.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention has press moldability and 150 ° C.
The present invention relates to a cold-rolled steel sheet excellent in coating bake hardenability at low temperatures and also excellent in normal temperature aging and a method for producing the same.

[0002]

2. Description of the Related Art Generally, as a material for processing such as cold rolled steel sheet, a soft material is required so that it can be easily processed at the time of processing, and a material having high strength is required as a product after processing. Required. For example, in the case of automobile outer panels, it is common to use low-yield stress steel plates so that pressing of complicated shapes can be performed easily and that the dimensional accuracy after processing does not deteriorate due to springback or strain. is there.

However, large parts such as doors and hoods using such low-yield stress steel plates cannot be expected to be work hardened, so that there is a problem that they are extremely deformed by an external force.

In order to solve such a problem, so-called paint bake hardenable steel sheets have been used in which the outer plate strength is increased by the precipitation hardening of carbon during paint baking. Conventionally, ultra-low carbon steels such as Ti and Nb have been used for automobile parts that require high workability.
While use the added IF (I nterstitial-atoms F ree ) the steel were common, not obtained paint bake hardenability due to the absence of solid solution carbon in the IF steel. For this reason, in recent years, a quasi-IF steel has been proposed in which the components are prepared such that a small amount of solid solution carbon remains so as to obtain paint bake hardenability. However, in these steel sheets, it is often difficult to obtain a desired coating bake hardening amount because the compositional variations at the steel making stage are directly reflected in the final mechanical properties.

From the above viewpoints, a low carbon Al-killed cold-rolled steel sheet is generally used for a member requiring paint bake hardenability, but a continuously annealed low-carbon Al-killed cold rolled steel sheet is The press formability such as tensile properties is not good,
Moreover, it is usually inferior in normal temperature aging property. Therefore,
In the low carbon Al-killed cold rolled steel sheet, which is expected to have high bake hardenability, if the tensile properties and the extreme temperature aging similar to those of the ultra low carbon IF steel sheet can be obtained, the merit of reducing the weight of the automobile is very large.

The bake-hardening amount depends almost on the amount of solute carbon remaining after annealing. In continuous annealing, heating-soaking is followed by rapid cooling, so steels that do not contain strong carbonitride-forming elements (for example, Ti) such as low carbon Al-killed cold-rolled steel sheets are more likely than box-annealed materials. Although it is easy to leave a large amount of solid solution carbon in the final product and is advantageous in obtaining bake hardenability, there is a fatal defect that the room temperature aging resistance is poor. For this reason,
Even if the material has the same material level as the box annealed material immediately after manufacturing, material deterioration due to aging and recovery of yield point elongation occur during pressing after several months, causing problems such as cracks and stretcher strain on the press. It often occurs. This is evaluated by the room temperature aging index and generally needs to be 3 kgf / mm ² or less.

[0007] Regarding the aging effect, many studies have been vigorously made in the past. In order to improve the non-aging effect under a time constraint like continuous annealing, in principle, after recrystallization annealing, rapid cooling is performed from a state in which solute carbon is abundant at a cooling rate of 1000 ° C./sec or more, and then It is only necessary to temper, but in this case, the introduction and freezing of various defects such as quenching strain, and in some cases a quenched structure is formed. Hardened,
As a result, the workability is greatly deteriorated.

On the other hand, after the recrystallization annealing, the gas or a mixture of this and water is cooled at a relatively slow cooling rate of about 1 to 300 ° C./sec and overageed at about 400 ° C. Although it softens, the aging property is considerably inferior to the above example. In order to avoid this, if the overaging temperature is lowered, the time required for the overaging treatment is prolonged, resulting in a decrease in production efficiency and an increase in cost. Further, since the non-aging as described above is to reduce the amount of solute carbon in the final product stage, the bake hardenability is inevitably deteriorated.

Further, from the viewpoint of improving the room temperature aging, a supercooling type overaging treatment method is disclosed in JP-A-51-20715.
No. 6, JP-A-60-258428, JP-B-60-443
No. 77 and Japanese Examined Patent Publication No. 2-93023, all of them are mainly aimed at obtaining non-aging property, and are completely free from bake hardenability at coating, let alone harden bake at low temperature. Not considered.

Bake hardenability is the property of precipitation hardening by iron carbides that precipitate during the heating process when the paint is baked after press molding, but the equivalent strain amount during press molding is 2%, and under heating conditions. About 170 ℃ × 20
It is believed to correspond to minutes. However, in recent years
From the viewpoint of energy saving, the development of paints that can be baked at low temperatures has been promoted, and along with this, there has been a demand for steel plates that can obtain the same amount of hardening as conventional paints even at low temperatures.

As described above, the bake hardenability is the precipitation hardening of iron carbide, and since the strain during press forming is at most several percent, the dislocation density, which becomes the nucleus of precipitation of iron carbide, may not be sufficiently multiplied. However, when the baking temperature is lowered, the amount of hardening sharply decreases. It is presumed that this is because the low temperature baking reduces the nucleation rate and growth rate of iron carbide.

As described above, in the case of a general low carbon Al-killed cold-rolled steel sheet, there is no steel sheet having a paint bake hardenability at a low temperature.

The present invention was developed in order to cope with such a current situation, and an object thereof is 3 kgf / mm ² in a coating baking treatment at 150 ° C or less without adding an expensive alloying element. Low carbon Al having the above bake hardenability
It is intended to provide a killed cold rolled steel sheet and a method for manufacturing the same.

[0014]

In general, bake hardenability rapidly decreases with a decrease in temperature, and is generally smaller than the room temperature aging index value at 150 ° C. or lower. This tendency is remarkable in the steel sheet having an intra-grain cementite density of 4 × 10 ⁴ pieces / mm ² or less after continuous annealing. In the case of a steel sheet having a room temperature aging index of 3 kgf / mm ² or less, bake hardenability at low temperature cannot be expected.

The inventors of the present invention have studied the reason for this. As a result, in order to reduce the room temperature aging index to 3 kgf / mm ² or less, the amount of solid solution carbon remaining after continuous annealing is 10 ppm.
In order to obtain the curability of 3 kgf / mm ² or more in coating baking at a low temperature of 150 ° C. or lower, for example, 130 ° C., the amount of solid solution carbon is regulated as follows, and the annealing is required Later cementite density is 4 × 10 ⁴ pieces / mm ²
We have found that the above restrictions need to be regulated.

That is, the intra-grain cementite density is 4 × 1.
If it is less than 0 ⁴ pcs / mm ² , the bake hardening amount will decrease sharply when the baking temperature is lower than the current 170 ° C, whereas if it is 4 × 10 ⁴ pcs / mm ² or more, the decrease in the bake hardening amount will result in lower temperature baking. The temperature does not occur. These relationships are schematically shown in FIG. This is because the amount of pre-strain introduced by press molding is at most about 2%. That is, the distribution state of cementite generated by the overaging treatment influences the degree of growth of dislocations generated by prestrain. Therefore, in the case of deformation of at most about 2% due to pre-strain equivalent to press molding, not only the dislocation density itself introduced is not large, but also when the intra-grain cementite density is less than 4 × 10 ⁴ pieces / mm ^2. Found out that dislocations (which are a factor for developing bake hardenability) as precipitation sites of iron carbide are not grown at all in the crystal grains, so that a high hardening amount cannot be obtained by low temperature baking. Note that this is because the precipitation of iron carbide becomes the nucleation law in low temperature bake hardening, and the diffusion law becomes the higher the baking temperature, so bake hardenability is obtained without depending on the intragranular cementite density. It is estimated to be.

As described above, in order to obtain a cold-rolled steel sheet excellent in coating bake hardenability at a low temperature, the present inventors have earnestly studied the chemical composition of the steel and the continuous annealing conditions, and as a result, the coating bake at a low temperature. In order to accelerate the precipitation rate of iron carbide, it was concluded that it is effective to disperse cementite generated during overaging treatment finely and uniformly inside the ferrite grains.

In order to realize these in the manufacturing process, the composition of the steel is such that the C content is 0.001 to 0.03%, the Si content is 0.3% or less, and the Mn content is 0.05 to 1.0. %, P amount of 0.
It is necessary to regulate the amount of Al to 10% or less, the amount of acid-soluble Al to 0.01 to 0.1%, and the amount of N to 0.005% or less. Overaging treatment conditions during continuous annealing are also a major factor in achieving these, but specific limiting conditions are not necessarily required other than setting the quenching end temperature after soaking to be equal to or lower than the overaging treatment start temperature. . When the overaging treatment condition during continuous annealing is a constant temperature type, coarse cementite precipitates and forms at grain boundaries even if the composition of the steel is regulated as described above, whereas once undercooling-reheating It was found that cementite can be finely and uniformly precipitated inside the ferrite grains by performing such overaging treatment.

As described above, this steel sheet is essentially subjected to a supercooling-type overaging treatment to increase the intragrain cementite density and extremely reduce the amount of solute carbon. However, since these cementites are finely and evenly dispersed inside the ferrite grains, even dislocations due to the strain of at most about 2% introduced during press molding are uniformly dispersed. Therefore, the precipitation of iron carbide at the time of baking the paint is promoted, and the precipitate effectively acts on the hardening, so that 150% after applying a strain of 2% corresponding to the molding strain amount of the door or the hood. The amount of curing at the time of coating baking below is 3 kgf / mm ² or more. That is, it is an ideal steel sheet that is easy to work during working and has high strength as a finished product even after coating baking at low temperature.

In the present invention, C: 0.001 to 0.03%, S
i: 0.3% or less, Mn: 0.05 to 1.0%, P: 0.10%
Below, acid-soluble Al: 0.01-0.1%, N: 0.005%
It contains the following and, if necessary, Ti: 0.01 to 0.1.
%, Nb: 0.01 to 0.05% of 1 type or 2 types,
Hot-rolling steel with the balance Fe and inevitable impurities
A steel sheet obtained by cold rolling and continuous annealing is characterized by having an intragranular cementite density after annealing of 4 × 10 ⁴ pieces / mm ² or more and a solute C amount of 10 ppm or less.
The gist is a cold-rolled steel sheet that is excellent in paint bake hardenability at low temperatures below ℃.

According to another aspect of the present invention, C: 0.001 to 0.
03%, Si: 0.3% or less, Mn: 0.05 to 1.0%, P:
0.10% or less, acid-soluble Al: 0.01 to 0.1%, N:
If less than 0.005% is contained, Ti: 0.0
1-0.1%, Nb: 0.01-0.05% 1 type or 2 types are contained, and the balance consists of Fe and unavoidable impurities. The steel is hot-rolled at the Ar ₃ transformation point or higher, After winding at 600 to 700 ° C. and cold rolling, when performing continuous annealing at a recrystallization temperature or higher and 850 ° C. or lower for 30 seconds to 3 minutes, the intragranular cementite density after annealing is 4 × 10 ⁴ pieces / mm ² The gist is a method for producing a cold-rolled steel sheet excellent in paint bake hardenability at a low temperature of 150 ° C. or lower, which is characterized by performing the overaging treatment as described above.

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

C: When the C content is in the range of 0.001 to 0.03%, stretcher strain does not occur during press molding, and further, when the coating is baked at a low temperature of 150 ° C. or less, the curing is 3 kgf / mm ² or more. You can get the quantity. But,
If it is less than 0.001%, 4 × 10 ⁴ particles / mm ² or more of intragranular cementite cannot be obtained after continuous annealing, and carbon cannot be sufficiently solid-dissolved, so that the coating bake hardenability cannot be exhibited. Further, if the C content exceeds 0.03%, good deep drawability cannot be secured even if the intragrain cementite density is restricted to 4 × 10 ⁴ pieces / mm ² or more.

Si: Si is an effective element for ensuring the necessary strengthening without deteriorating the workability, but
If the Si content exceeds 3%, the effect of the present invention cannot be obtained. That is, Si is one of the important elements constituting the present invention, and when it exceeds 0.3%, it is 4 × during overaging treatment.
Cementite of 10 ⁴ pieces / mm ² or more is finely and uniformly precipitated inside the ferrite grains, and dislocations introduced at the time of press forming cannot be uniformly and dispersed, and precipitation of iron carbide in the baking process at low temperature is impossible. You cannot get the effect of promotion.

Mn: Mn is required to be 0.05% or more in order to prevent hot embrittlement of steel, but addition of a large amount of Mn causes deterioration of press workability, especially deep drawability, so its upper limit is 1. 0.0%.

P: P, like Si, is one of the important elements constituting the present invention. When it exceeds 0.10%, cementite is finely and uniformly precipitated inside the ferrite grains during the overaging treatment. Dislocations introduced at the time of press forming cannot be uniformly dispersed, and the effect of promoting the precipitation of iron carbide in the baking process at low temperature cannot be obtained.
Therefore, the amount of P is set to 0.10% or less.

Acid-soluble Al: Acid-soluble Al is important for fixing N in steel, preventing excessive normal temperature aging due to N, and obtaining bake hardenability by only solute carbon. For that purpose, 0.01% or more is required. The amount of the acid-soluble Al is 0.01 to 0.1 because the content of 0.1% or more exceeds the amount for fixing N and the rest after fixing N deteriorates the workability.
The range is%.

N: As the amount of N increases, the material deteriorates. That is, even if it is attempted to coarsen the precipitated AlN by the winding treatment at a high temperature after hot rolling, since the size is insufficient to obtain high crystal grain growth during annealing, the amount of N increases with An increase in the amount of AlN deposited deteriorates the material. From this point of view, the N content is 0.005% or less.

Ti, Nb: Ti, Nb is a solid solution carbon other than a few ppm that contributes to baking and hardening of the coating, fixed as a carbide, and r
It is an element effective in improving the deep drawability represented by a value, and one or two of Ti and Nb can be added if necessary. From the viewpoint, when Ti is added, the Ti content is set to 0.01 to 0.1%, and the Nb content is set to 0.01 to 0.05%.

Intra-grain cementite density: The intra-grain cementite density is an important factor constituting the present invention and is 4 × 10.
^{When the} number is ⁴ pieces / mm ² or more, a curing amount of 3 kgf / mm ² or more can be obtained when baking the coating at a low temperature of 150 ° C. or less. However, if it is less than 4 × 10 ⁴ pieces / mm ² , dislocations introduced during press molding cannot be uniformly dispersed, and a curing amount cannot be obtained in the coating baking process at low temperature. On the other hand, even if excessively present, there is no problem in obtaining a curing amount in the coating baking process at a low temperature, but if it is 30 × 10 ⁴ pieces / mm ² or more, the tensile property is deteriorated, so it is preferably made less than this. The method for measuring the intra-grain cementite density was determined by a scanning electron micrograph of 1000 times that obtained by etching with a cementite developing solution such as a picric acid solution.

Next, the manufacturing conditions of the present invention will be described.

The steel having the above chemical composition is hot-rolled into a steel ingot obtained by a conventional method, wound and then cold-rolled. The hot-rolling is performed at a temperature not lower than the Ar ₃ transformation point. , 600
Wind up at ~ 700 ° C. The hot rolling temperature is set to the Ar ₃ transformation point or higher in order to prevent generation of coarse grains and work structure in the surface layer of the hot rolled steel sheet which adversely affects the texture formation after cold rolling-recrystallization annealing. , The winding temperature is 600 ~ 700 ℃
Is defined as the precipitation form of carbides necessary for obtaining paint bake hardenability at low temperature and good press formability, and Al
This is to cause precipitation of N. If the temperature is lower than 600 ° C., these effects cannot be obtained, and if the temperature exceeds 700 ° C., the workability is hindered and the cost is increased due to the high temperature winding.

After cold rolling, continuous annealing is performed. As for the continuous annealing condition, the soaking during annealing is set to 850 ° C. or higher, the recrystallization temperature or higher.
It is carried out under the condition that the temperature is maintained at 30 ° C. or lower for 30 seconds to 3 minutes, and the overageing treatment is performed so that the intra-grain cementite density after annealing becomes 4 × 10 ⁴ pieces / mm ² or more.

The soaking during annealing is performed at a temperature not lower than the recrystallization temperature and 850 ° C.
The reason for carrying out under the condition of holding for 30 seconds to 3 minutes below is to obtain a recrystallized structure capable of exhibiting good press formability,
If it exceeds 850 ° C, the amount of austenite during soaking increases,
When this austenite transforms into ferrite again, the texture becomes random and the r value is lowered. Further, the upper limit of the soaking time is set to about 3 minutes which is adopted in ordinary continuous annealing.

The condition of the overaging treatment may be any condition as long as the intragranular cementite density after annealing is 4 × 10 ⁴ pieces / mm ² or more. In order to disperse the cementite that precipitates during the overaging treatment finely and uniformly inside the ferrite grains, the quenching end point temperature after soaking is below the overaging start temperature (including room temperature).
Is preferable (that is, supercooling-reheating gradient overaging). At that time, it is desirable to perform subcooling at a secondary cooling (rapid cooling) rate of 50 ° C./sec or more after primary cooling (slow cooling) to 300 ° C. or less.

According to the supercooling-reheating gradient overaging, the cementite has a precipitation form in which it is precipitated in the ferrite grains.
The fine cementite density greatly increases, and the amount of solute C decreases accordingly. The larger the number of cementite nuclei precipitated in the grains during supercooling, the higher the cementite precipitation rate during overaging treatment, and the solid solution C decreases in a shorter time. In addition,
The intra-grain cementite density increases with a decrease in the supercooling temperature before the gradient overaging, and hardly depends on the reheating temperature of the overaging treatment. Further, the final amount of solid solution C mainly depends on the reheating gradient overaging end temperature if the temperature necessary and sufficient for diffusion of C can be secured for reheating, and the amount of solid solution C decreases as the end temperature decreases. .

However, in the case of isothermal overaging, the cementite precipitates at the grain boundaries and the precipitation within the grains is small, so that the above-mentioned intragrain cementite density cannot be obtained.

It goes without saying that the cold-rolled steel sheet according to the present invention can be easily applied to surface-treated steel sheets such as hot-dip galvanized steel sheets and alloyed steel sheets.

Next, examples of the present invention will be shown.

[0040]

【Example】

Continuous cast slabs having the chemical composition shown in Table 1 were produced. These steel ingots are hot-rolled at a heating temperature of 1230 ° C. and a finishing temperature of 900 ° C., and after finishing, 620 to 7
The hot rolled steel strip was wound in the temperature range of 00 ° C. Further, the steel strip was cold-rolled to a thickness of 0.8 mm with a rolling reduction of about 73%, and continuously annealed under the conditions shown in Table 1 to manufacture a cold-rolled steel sheet. The continuous annealing was soaking, primary cooling (gradual cooling), secondary cooling, and heat treatment by reheating from the supercooling temperature.

Steels No. 1 to No. 13 are obtained by changing the cementite density by variously changing the continuous annealing conditions using the same steel type. Steels No. 3 to No. 8 are examples of the present invention. , And others are comparative examples. Steel No. 14 to No. 25 are the results of the investigation of the influence of chemical components. No. 15 and No. 17
No. 18, No. 21, No. 23 to No. 25 are examples in which the chemical components are within the scope of the present invention, and others are comparative examples deviating from the scope of the present invention.

As shown in the characteristics of the steel sheet obtained in Table 2,
In the examples of the present invention, not only a high 130 ° C. coating bake hardening amount of 3 kgf / mm ² or more is obtained, but also tensile properties such as yield stress, normal temperature aging property, and r value are good.

[0044]

As described in detail above, according to the present invention,
A low-carbon Al-killed type that has a bake hardenability of 3 kgf / mm ² or more in a paint baking treatment at 150 ° C or less without adding an expensive alloy element, and also has good room temperature aging properties, tensile properties, and press formability. A rolled steel sheet can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the baking temperature and the amount of bake hardening at various intra-grain cementite densities. When the solid line shows an intra-grain cementite density of 4 × 10 ⁴ pieces / mm ² or more, the dotted line shows the intra-grain cementite density of 4 × This is the case of less than 10 ⁴ pieces / mm ² .

[Table 1]

[Table 2]

Claims (2)

[Claims] 1. In weight% (hereinafter the same), C: 0.001 to
0.03%, Si: 0.3% or less, Mn: 0.05 to 1.0%,
P: 0.10% or less, acid-soluble Al: 0.01 to 0.1%,
N: 0.005% or less is contained, and if necessary, Ti:
Steel containing 0.01 to 0.1% and Nb: 0.01 to 0.05% of 1 or 2 and the balance Fe and inevitable impurities is hot-rolled, cold-rolled and continuously annealed. In the steel sheet obtained by carrying out the annealing, the intragrain cementite density after annealing is 4 × 10
⁴ / mm ² or more with excellent cold-rolled steel sheet in bake hardenability properties at low temperatures below 0.99 ° C., wherein the amount of solute C is 10ppm or less. 2. C: 0.001 to 0.03%, Si: 0.3%
Hereinafter, Mn: 0.05 to 1.0%, P: 0.10% or less, acid-soluble Al: 0.01 to 0.1%, N: 0.005% or less, and if necessary, further Ti: 0.01 to 0.1%, Nb: 0.
Steel containing 01 to 0.05% of 1 type or 2 types and the balance of Fe and unavoidable impurities is hot-rolled at an Ar ₃ transformation point or higher, wound at 600 to 700 ° C., and cold-rolled. After that, when performing continuous annealing for 30 seconds to 3 minutes at a recrystallization temperature or higher and 850 ° C. or lower, the intragrain cementite density after annealing was 4 ×.
A method for producing a cold-rolled steel sheet excellent in coating bake hardenability at a low temperature of 150 ° C. or lower, which is characterized by performing an overaging treatment such that 10 ⁴ pieces / mm ² or more.