JP2818675B2 - Manufacturing method of surface-conditioned cold-rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a cold-rolled and surface-treated steel sheet used for an automobile body and the like. The present invention relates to a method for producing a cold-rolled steel sheet in which the concentration of a predetermined component on the surface is adjusted so as to have good chemical conversion treatment (zinc phosphate treatment) as a base treatment for coating and also excellent spot weldability.

<Conventional technology> Cold-rolled steel sheet for press working is conventionally low C of 0.01% or more.
-Although rimmed steel and low C-Al killed steel were manufactured by box annealing, in consideration of recent demands for energy saving and shortening of production lead time, conversion to a continuous annealing method is being actively promoted.

In the continuous annealing method, the heating and soaking time is extremely short.
Therefore, in order to make the drawability comparable to that of the box annealing material, measures have been taken such as setting the hot-rolling coiling temperature of the low-C steel to be higher than before and the annealing temperature to be higher than that of the box annealing method. Furthermore, since the cooling time is extremely short, carbon dissolved during the annealing is precipitated by performing overaging treatment. However, even with such a special treatment, since the solute carbon still remains, it was difficult to obtain ordinary-temperature delayed aging regardless of workability.

In view of such a situation, as a means of obtaining the same aging resistance as that of the low C-Al killed steel annealed in a box and the higher workability, C ≦ 0.01 wt% and Al ≦ 0.20 wt% are used. A technique of forming a very low C steel to be contained and adding a carbonitride forming element such as Ti, Tb, B or the like as necessary is becoming common in conjunction with the progress of steelmaking technology.

In fact, the ultra-low C cold rolled steel sheet produced in this way has excellent grain growth during annealing, has a good overall elongation value (El), which can be evaluated for particularly good workability, especially in a tensile test, and also has a Rank Ford. It shows the value (value), and is currently widely used as a steel plate for press working.

<Problems to be Solved by the Invention> However, with the increase in the use of this type of steel sheet, it has been clarified that the steel sheet also has the following various problems. It was a stage.

First, the ultra-low C steel is originally close to dull iron, and therefore has extremely excellent surface cleanliness. Therefore, in the chemical conversion treatment (zinc phosphate treatment) as a base treatment of the coating after pressing, the reactivity is somewhat inferior to the conventional low C-Al killed steel, and the fineness of the generated zinc iron phosphate crystal and the chemical conversion It was slightly disadvantageous to the low C-Al killed steel in terms of stability when the processing conditions were changed.

Next, in terms of weldability, the composition of the heat-affected zone (HAZ) in the case of ultra-low C steel generally tends to be coarse, and the strength tends to be lower than that of the welded portion or the base metal. Therefore, it could not be said that it was more advantageous than the low C-Al killed steel in terms of strength and fatigue properties of the weld. For these reasons, at present, the application of ultra-low C steel to electric resistance welded steel pipes and the like, which require a relatively long time for welding, has not yet been achieved.

Further, since ultra-low C steel is rich in ductility, it is very tenacious, and when punching or shearing is performed under the same conditions as low C-Al-killed steel, the so-called burrs formed on the end face of the steel have low C. -Generates more than Al-killed steel. These burrs, when peeled off in a later pressing step, induce so-called star defects. Ultra-low C steel has such a danger, and it has been strongly desired to improve the punchability of ultra-low C steel in order to reduce the burr height.

Further, to further improve the workability, the amount of impurity elements is inevitably reduced, so that the surface concentration of the elements in the steel during annealing is suppressed. This results in a reduction in the surface hardness of the steel sheet. Therefore, when press forming is performed, if lubrication is not sufficient, the steel sheet surface and the press die mesh at the time of contact, not only the surface scratch defect of the steel sheet is induced, but in extreme cases even press cracking is accompanied . Such a decrease in so-called slidability is most remarkable when a combination of an extremely low carbon steel with a small amount of impurity elements and a continuous annealing method that cannot secure a sufficient time for enrichment on the surface is used.

In order to overcome the above situation, the present inventors have conducted various studies. In order to solve the above-mentioned problems while maintaining good mechanical properties (El, value, etc.), it is considered necessary to use an ultra-low C steel. On the other hand, the problems with the ultra-low C steel mentioned above are more or less the existence of elements near the surface,
It is clear that they have a close effect. Accordingly, the present inventors have conducted a number of investigations and laboratory confirmations, and as a result, carburized under appropriate conditions in a continuous annealing furnace so that a concentrated layer of carbon or nitrogen was present at an appropriate thickness and concentration on the surface. Alternatively, it has been found that if exposed to an immersion atmosphere and then cooled under appropriate conditions to prevent re-diffusion, the drawbacks of ultra-low C steel can be solved at once.

Therefore, by cooling the steel sheet exposed to the carburizing or nitriding atmosphere in a continuous annealing furnace under appropriate conditions, it is possible to improve properties such as mold resistance, chemical conversion treatment, and spot weldability. It is an object of the present invention to provide a method for producing a cold-rolled steel sheet in which the concentration of a predetermined component on the surface is adjusted.

<Means for Solving the Problems> That is, in continuously annealing a cold-rolled steel sheet as an original sheet using an annealing furnace having heating, soaking, and cooling zones, C: 0.01 wt% is used as the cold-rolled steel sheet of the original sheet. Using the following, in the late soaking or early cooling in the annealing furnace,
A layer in which the predetermined component is concentrated (hereinafter, referred to as a surface-concentrated layer) is formed on the surface of the cold-rolled steel sheet by using an atmosphere for concentrating the predetermined component, and the average cooling rate to 500 ° C. is 2 immediately.
An object of the present invention is to provide a method for producing a surface-conditioned cold-rolled steel sheet, which is cooled to a temperature exceeding 0 ° C./sec.

When a carburized layer is formed as a surface-concentrated layer, C is 0.01%.
When an original plate of not more than wt% is used and a nitriding layer is formed as a surface-concentrated layer, it is preferable to use an original plate having C and N of 0.01 wt% or less.

The steel sheet as the original sheet further contains Si: 1.0 wt% or less,
Mn: 1.0 wt% or less, P: 0.2 wt% or less, S: 0.05 wt% or less, Al:
0.01 to 0.1 wt% and N: 0.01 wt% or less, with the balance being F
e and unavoidable impurities are preferable, and the original plate is composed of 0.001 to 0.15 wt% of Ti and / or 0.001% of Nb.
More preferably, it contains 0.10 wt%.

 Hereinafter, the present invention will be described in more detail.

The present invention relates to a method for forming a carburized layer or a nitriding layer on a surface of a cold-rolled steel sheet (original sheet) having the following composition by a continuous annealing method in order to obtain required characteristics.

According to the present invention, an ultra-low carbon steel sheet containing C ≦ 0.01 wt% is used as a base plate, and a carburizing atmosphere and carburizing conditions are set as described below using a continuous annealing furnace having heating, soaking, and cooling zones. Accordingly, the present invention provides a method for producing a surface-conditioned cold-rolled steel sheet having excellent properties such as mold resistance, chemical conversion treatment, and spot weldability.

The steel sheet to which the present invention is applied basically adjusts the C concentration of the surface by carburization so as to satisfy the characteristics aimed at by the present invention. In order to obtain good mechanical properties such as
It is indispensable to set the extremely low C region of wt% or less. Above this, it is not possible to obtain a material that is as low or better than low C steel.

The present invention also provides a method for nitriding as described later using a very low carbon steel sheet containing C ≦ 0.01 wt% and N ≦ 0.01 wt% as a base plate and using a continuous annealing furnace having heating, soaking and cooling zones. Provided is a method for producing a surface-conditioned cold-rolled steel sheet having excellent characteristics such as mold galling resistance, chemical conversion treatment properties, and spot weldability by setting an atmosphere and nitriding conditions.

The steel sheet to which the present invention is applied basically adjusts the N concentration on the surface by nitriding so as to satisfy the characteristics intended by the present invention. In order to obtain good mechanical properties such as press formability such as the shape, it is essential that C is 0.01 wt% or less and N is an extremely low C and N range of 0.01 wt% or less. Above this, it is not possible to obtain materials of low C, N steel or better.

Furthermore, although the original steel sheet contains Si, Mn, P, S, and Al within the range described below, it is not preferable for the purpose of the present invention.

Si has the effect of strengthening the steel and is added in a required amount according to the desired strength. However, if the added amount exceeds 1.0 wt%, the deep drawability is adversely affected.

Like Mn, Mn also has the effect of strengthening the steel to avoid hot cracking due to the residual S, and is added in a required amount depending on the desired strength. However, if the added amount exceeds 1.0 wt%, the workability also deteriorates. 1.0 wt% or less is preferable because it has an adverse effect.

Both P and Si, like Mn, have the effect of strengthening steel and are added in the required amount according to the desired strength. However, if the added amount exceeds 0.2 wt%, the workability is adversely affected. Good to do.

The smaller the content of S, the better the deep drawability is. Therefore, it is preferable to reduce the content of S as much as possible.
In the following, it does not adversely affect so much, so it is preferable to set the content to 0.05 wt% or less.

Al is added as a deoxidizing agent and for improving the yield of carbonitride forming elements described later, that is, for improving the aging resistance by fixing N in steel, but if the content is less than 0.01 wt%, the addition of Al is added. The effect is poor. On the other hand, even if it is added in excess of 0.1% by weight, the effect reaches saturation, so it is preferable to be in the range of 0.01 to 0.1% by weight.

N is preferably set to 0.01% by weight or less for the same reason as described for C.

Further, the cold-rolled steel sheet used as the original sheet in the present invention includes Ti
And / or Nb may be added as described below.

The addition of these elements is effective for fixing C and N which form a solid solution in steel and deteriorate aging resistance. Moreover,
Since the size of the formed precipitate is appropriately coarse, the growth of grains during continuous annealing is promoted, which is advantageous for improving workability, particularly El and the value.

Ti is a carbonitride forming element and solid solution in steel (C, N)
In order to preferentially form a {111} orientation that is advantageous for deep drawability. However, the addition amount is 0.00
If it is less than 1 wt%, the effect of the addition is poor. On the other hand, if it exceeds 0.15 wt%, no further effect can be obtained, but rather the deterioration of the surface properties and ductility of the steel sheet.
Limited to the range of 0.15 wt%.

Nb is a carbide forming element, and is added to reduce the solid solution C in steel and to preferentially form the {111} orientation that is advantageous for deep drawability by minimizing the microstructure of the hot-rolled steel sheet. You. However, if the addition amount is less than 0.001 wt%, the effect of the addition is poor. On the other hand, if it exceeds 0.10 wt%, no further effect can be obtained, and rather it leads to deterioration of ductility. Limited to.

The steel sheet having the above components is subjected to continuous annealing.

CAL (Continuous An) that is preferably used in the present invention
nealing Line) has heating, soaking and cooling zones. Carburizing is performed in a condition suitable for carburizing in an atmosphere for carburizing from the latter stage of the soaking zone to the cooling zone.

As a carburizing atmosphere, in a gas mainly composed of N ₂ + H ₂
Use what added an appropriate amount of CO. And carburizing conditions are selected appropriately.

In addition, the nitriding is performed from the latter part of the soaking zone to the cooling zone under an atmosphere suitable for nitriding under conditions suitable for nitriding.

As a nitriding atmosphere, in a gas mainly composed of N ₂ + H ₂
An N source such as NH _{3 to} which an appropriate amount is added is used. And
Nitriding conditions should be appropriately selected.

In the present invention, in the latter stage of soaking in the annealing furnace or in the early stage of cooling, the original plate is exposed to a carburizing or nitriding atmosphere for a required time to diffuse C or N on the surface of the original plate, thereby forming a concentrated layer of C or N. To form

After forming a carburized layer or a carburized layer on the surface of the original sheet as described above, the carburized layer or the carbonitrided layer is formed at a temperature of 20 ° C./50° C. from the annealing temperature to 500 ° C. so as not to disappear by re-diffusion. Cool at an average cooling rate of more than a second.

The reason why the temperature is limited to 500 ° C. is that at a temperature higher than this, the re-diffusion rate of the concentrated C or N is high, and during this time, cooling is performed at an average cooling rate of more than 20 ° C./sec. 20
At a cooling rate of not more than ° C./sec, the condensed element in the once formed C or N surface concentrated layer diffuses toward the inside of the steel sheet, and the surface concentrated layer cannot be maintained.

As described above, the C or N concentration in the surface layer is appropriately adjusted by the carburizing treatment by the continuous annealing method, that is, the surface conditioning is excellent in various properties such as mold galling resistance, chemical conversion treatment properties, and spot weldability. Cold rolled steel sheet can be manufactured.

The surface-conditioned cold-rolled steel sheet manufactured as described above can be further subjected to a surface treatment. Examples of the surface treatment include zinc-based alloy plating such as Zn-Fe or Zn-Ni-plated steel sheet, and zinc-based composite plating such as lower resin film and upper Zn-Ni steel sheet, in addition to zinc plating.

<Example> Next, a specific description will be given based on an example of the present invention.

(Example 1) Five types of low C steels shown in Table 1 were produced by RH degassing after converter melting and continuous casting.

The slab was placed in a heating furnace, heated to 1230 ° C., finished at 880 ° C. by hot rolling, and wound at 535 ° C. to form a 3.2 mm thick hot rolled coil. Next, a cold-rolled steel sheet having a thickness of 0.8 mm was formed by cold rolling after pickling. The cold-rolled coil is connected to a continuous annealing line (CA
In L), rapid heating, soaking, and rapid cooling were performed under the following conditions. At this time, carburizing was performed in the soaking / cooling zone using the following atmosphere.

Table 2 shows the results.

(1) annealing conditions Atsushi Nobori rate 20 ° C. / sec soaking temperature 750 ° C. soaking time of 30 seconds cooling rate 2 ° C. / sec or more (2) a carburizing atmosphere _{H 2 3% Co 0.5% N} 2 remaining (Example 2) The steel sheets shown in Table 1 used in Example 1 were nitrided in the same manner as in Example 1. Table 3 shows the results.

(1) annealing conditions Atsushi Nobori rate 20 ° C. / sec soaking temperature 600 ° C. soaking time of 30 seconds cooling rate 2 ° C. / sec or more (2) nitriding atmosphere _{H 2 3% NH 3 5%} N 2 remaining <Effect of the Invention> According to the present invention, in the continuous annealing method, the carburizing and nitriding atmosphere and the carburizing or nitriding conditions are appropriately selected, and the subsequent cooling rate is appropriately selected, so that the mold resistance to galling is improved. It is possible to manufacture a surface-conditioned cold-rolled steel sheet having excellent properties such as chemical conversion property and spot weldability.

Continuing from the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corporation Research and Development Headquarters (72) Inventor Hideo Abe 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Technical Research (56) References JP-A-60-149729 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 8/20, 8/26

Claims (6)

(57) [Claims] (1) When continuously annealing a cold-rolled steel sheet as an original sheet using an annealing furnace having heating, soaking and cooling zones, the cold-rolled steel sheet of the original sheet is C: 0.01 wt% or less. A layer in which the predetermined component is concentrated (hereinafter referred to as a surface-concentrated layer) is formed on the surface of the cold-rolled steel sheet using an atmosphere for concentrating the predetermined component at a later stage of soaking in the annealing furnace or at an early stage of cooling. Immediately, the average cooling rate to 500 ℃ is 20 ℃
A method for producing a surface-conditioned cold-rolled steel sheet, wherein the steel sheet is cooled so as to be more than 1 / sec. 2. The method according to claim 1, wherein the surface-concentrated layer is a carburized layer. 3. The cold rolled steel sheet of the original sheet is C: 0.01 wt% or less,
The method for producing a surface-conditioned cold-rolled steel sheet according to claim 1, wherein N: 0.01 wt% or less is used, and the surface-concentrated layer is a nitriding layer. 4. The cold-rolled steel sheet as an original sheet further comprises: Si: 1.0% by weight or less, Mn: 1.0% by weight or less, P: 0.2% by weight or less, S: 0.05% by weight or less, Al: 0.01 to 0.1% by weight and N: contains 0.01wt% or less,
The method for producing a surface-conditioned cold-rolled steel sheet according to any one of claims 1 to 3, wherein the balance comprises Fe and unavoidable impurities. 5. The cold-rolled steel sheet as an original sheet further has a Ti: 0.001-0.
The method for producing a surface-conditioned cold-rolled steel sheet according to any one of claims 1 to 4, comprising 15 wt% and / or Nb: 0.01 to 0.10 wt%. 6. The method according to claim 1, wherein the surface-concentrated layer has an average cooling rate of 20 ° C. /
The surface-adjusted cold-rolled steel sheet according to any one of claims 1 to 5, wherein the ratio of the C or N concentration of the surface layer portion and the central portion after cooling to more than seconds is a surface concentrated layer of 2.9 times or more. Manufacturing method.