JPS582248B2 - Manufacturing method for hot-dip galvanized steel sheet with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing hot-dip plated steel sheets having extremely excellent ductility and drawability.

Conventionally, plated steel sheets have been mainly used in fields where workability is hardly required, such as roofing materials, and low carbon rimmed steel has been used as the material, and this has been sufficient to accomplish its mission.

However, in recent years, the application of plated steel sheets to new fields has been proposed, and the use of so-called pre-coated materials and parts that require high workability related to exhaust gas regulations has increased, and conventional rimmed steel has not been able to meet these demands. I can't do it anymore.

In other words, when rimmed steel is used as the base material, C and N contained in the steel are rapidly cooled after being annealed and plated on a continuous plating line (Sendzimer type). Because it remains in a supersaturated solid solution state, the tensile strength and yield strength are high, and the elongation and
The stretchability, deep drawability, etc. deteriorate, resulting in poor workability, and plated steel sheets made from rimmed cold-rolled steel sheets have poor workability and cannot withstand high-level processing.

Therefore, when it is desired to impart a certain degree of workability to a conventional plated steel sheet, it is necessary to
A so-called over-aging treatment has been adopted in which annealing is performed at a temperature of 0 to 400° C. to precipitate supersaturated solid solution C and N and soften the material.

However, although the materials of products subjected to this over-aging treatment are slightly improved, they are still insufficient to withstand high-level processing, and the effect of improving processability is limited. It has not yet been applied for any purpose.

On the other hand, it is known that the deep drawability of steel sheets is improved by the addition of Ti, for example, in Japanese Patent Publications Nos. 44-18066 and 46-27.
No. 38, No. 42-12348, No. 49-31844, and No. 50-31531.

Further, it is stated in Japanese Patent Publication No. 46-20563 that when Ti is contained in an amount of 0.05 to 0.5%, the processing adhesion of the plating layer is improved and a galvanealed steel sheet with good workability can be obtained.

However, when containing Ti, especially the Ti/C ratio is 4.
When Ti is contained in the above amount, the effect of improving the workability of the steel becomes remarkable, but when this Ti-containing steel is processed on a continuous hot-dip plating line such as the Sendzimer type, so-called Since it is reduced after being weakly oxidized in the gas cleaning process, in steels that contain a large amount of easily oxidized alloying elements such as Ti, the reduction cannot be carried out sufficiently, resulting in insufficient reduction, which causes the problem of frequent non-plating. .

Figure 1 shows the results of investigating this relationship.In the Sendzimer type hot-dip plating line, as the Ti content increases, the defective rate due to non-plating increases, and when Ti is 0.2
It was found that at 0%, the defective rate reached as high as 3%.

Therefore, even if we expect the effect of improving deep drawability by adding Ti,
When performing continuous hot-dip plating, there is a major problem in that the rate of non-plating defects increases with the addition of Ti, and true results cannot be obtained.

The present invention has been made to overcome this problem, and its gist is to suppress the amount of Ti added to 0.15% or less, and to reduce the amount of Ti, C, and N and related amounts to By adding Nb, it was possible to reduce the defective rate due to non-plating and to obtain a hot-dip coated steel sheet with good workability.In addition to consideration of such component composition, manufacturing conditions It has been found that by controlling the above, it is possible to obtain a hot-dip plated steel sheet with even better workability.

That is, the present invention is an ultra-low carbon material containing at least one of V or Nb in an amount that satisfies the relationship of V; 0.07% or less, and the remainder being iron and inevitable impurities. The gist of this is a method for producing hot-dip galvanized steel sheets with excellent workability using hot-rolled steel sheets as the raw material, and furthermore, steel with the above composition is hot-rolled at a temperature of Ar 3 or higher, and Processing characterized by top annealing at a temperature of 850°C, then cold rolling at a reduction rate of 40% or more, and then continuous annealing at temperatures of 750°C to Ac 3 points in a Sendzimer type continuous hot-dip plating apparatus to perform hot-dip plating. Provided is a method for manufacturing hot-dip plated steel sheets with excellent properties.

The reasons for limiting each requirement of the present invention as described above will be described below.

C: The lower the carbon (C) content, the more the quench age hardening caused by C is reduced, so for the purpose of the present invention, the lower the carbon (C) content, the more desirable it is.

However, it is not easy to reduce Ti, V, and N to less than 0.001% using current steel manufacturing methods.
By adding b, the content of C in an amount of 0.001% or more to 0.02% is allowed, and it also has a favorable effect on various mechanical properties.

However, if the C content exceeds 0.02%, the amount of Ti, V, and Nb added to fix C and prevent adverse effects on quench age hardening increases, which is economically disadvantageous. However, the yield strength increases, the elongation decreases, and the workability deteriorates, which is not preferable for the purpose of the present invention.

Therefore, the C content of the material is 0.001~0.020%
is limited to.

Si: Si is an element necessary for manufacturing the steel material of the present invention,
If it exceeds 0.05%, it hardens the steel and impairs workability.

For this reason, the Si content was limited to 0.05% or less.

Mn; Mn is useful for suppressing hot embrittlement caused by S, and is usually added in an amount such that Mn/S≧15, but in the steel of the present invention, since Ti is added, the form of TiS is Since S is fixed by
When the Mn content exceeds 0.25%, the average plastic strain ratio (
The upper limit of Mn was set at 0.25% because the Mn content decreased and the drawability deteriorated.

Ti: Ti is known as an element that has a strong affinity with oxygen, carbon, nitrogen, sulfur, etc., and deep-drawing steel with Ti added in an amount that gives an effective Ti/C ratio of 4 or more is also produced by the aforementioned Tokko Showa. It is stated in each issue of the publication.

However, when steel which has been made to have good deep drawability by containing Ti in this way is subjected to Sendzimer type continuous hot-dip plating, problems arise in the plating properties as described above.

In other words, steel containing a large amount of Ti, which has a strong affinity for oxygen, tends to be under-reduced in the weak oxidation-reduction treatment when gas cleaning the surface of a cold-rolled sheet before hot-dip plating, as shown in Figure 1. As a result, the defective rate due to non-plating increases.

As is clear from FIG. 1, when Ti% exceeds 0.15%, the defective rate due to non-plating becomes about 2% or more.

Normally, if the defective rate exceeds 2%, it will cause a problem when the product is shipped in the form of a coil, so from this point of view it is necessary to suppress the Ti content to 0.15% or less.

V, Nb; Although V and Nb are strong carbonitride-forming elements, they have a common effect in that they have a relatively weak affinity with oxygen.

Therefore, there is no problem of poor plating properties due to non-plating such as Ti, and the above-mentioned problem of having to limit the Ti content from the viewpoint of plating properties can be compensated for by adding these elements. These elements fix C and N that could not be fixed, improving ductility and deep drawability, resulting in a hot-dip plated product of high quality and good workability.

Note that these V and Nb provide supplementary effects to Ti, so they are not used alone, but are used in the form of a composite addition with Ti, and their content can be adjusted to reduce the deficiency of Ti. From the stoichiometric relationship for compensation, Ti-
In the case of a V-based material, it is necessary to satisfy the following conditions in the case of a Ti--Nb based material, and to set C and N in amounts that can be fixed.

However, as the content of V and Nb increases, as shown in the examples below, the steel hardens and the test value deteriorates, so the upper limit for each is 0.07% for V and 0.0% for Nb.
7% and 0.07% for Nb.

N: N is an element that causes aging in steel together with C,
Furthermore, since it also deteriorates workability, the lower the value, the more desirable it is.

However, with current steel manufacturing methods, it is unavoidable that some amount of N remains.

However, if N exceeds 0.01%, the amounts of Ti, V, and Nb required to eliminate the adverse effects of N on workability and aging properties will increase, so it is limited to 0.01% or less.

Al; Although the content of Al is not necessarily necessary for the material of the steel of the present invention, it is an element necessary for manufacturing the steel of the present invention.

That is, it is an element necessary as a deoxidizing agent to improve the addition yield of Ti, V, and Nb.

However, excessive addition will harden the steel, so sol. Al
The amount was limited to 0.10% or less.

By adjusting the additive elements as described above, it is possible to obtain hot-dip plated steel sheets of high quality and excellent workability on a continuous hot-dip plating line for zinc, aluminum, etc., but by further controlling the manufacturing conditions, This makes it possible to provide excellent workability.

The manufacturing conditions will be described below.

For melting, for example, undeoxidized converter molten steel is decarburized to 0.02% or less of C in the steel using a vacuum degassing device, deoxidized with Al, and then an alloying amount is added in the form of ferroalloy. Then, the composition is adjusted to be within the range of the steel of the present invention to obtain molten steel.

This molten steel is ingot-formed and bloomed using a conventional method, or a slab is manufactured by continuous casting.

The obtained slab was subjected to normal hot rolling conditions, that is, finishing temperature A
A hot-rolled steel sheet is obtained by hot rolling at r3 points or higher and a coiling temperature of 500 to 650°C.

Next, after pickling this hot rolled steel sheet,
Top annealing at a temperature of ℃.

This top annealing is a treatment for softening the steel by coagulating carbides (TiC, VC, NbC).

One of the features of the present invention is that the top annealing temperature is set in the temperature range of 750 to 850°C.

That is, as is clear from the experimental results shown in FIG. 2, the top annealing temperature for the steel of the present invention is 750 to 85
The hardness reaches its minimum in the 0°C range, and the material becomes soft.

This is thought to be because at a top annealing temperature of less than 750°C, the rate of aggregation of carbides becomes slow, and at a temperature exceeding 850°C, solid solution of carbides occurs again.

Therefore, for the purpose of the present invention, this top annealing temperature is 7.
A temperature between 50 and 850°C is very beneficial.

Next, this top annealed hot rolled steel sheet is cold rolled, at a cold rolling reduction rate of 40% or more, preferably 60%.
~85%.

Good workability can be obtained by setting this cold rolling reduction to 40% or more.

Next, when performing recrystallization annealing and hot-dip plating on this cold-rolled steel sheet in a Sendzimer-type continuous hot-dip plating line, the annealing temperature is set to 750°C or more at AC
The range shall be below point B.

Controlling this annealing temperature range is also very beneficial for the purpose of the present invention and can improve workability.

That is, as an example is shown in Fig. 3, the steel of the present invention has a high recrystallization temperature because it contains Ti, V, and Nb, and it takes a long time to soften at an annealing temperature of less than 750°C. However, continuous annealing is difficult.

Continuous annealing can be performed at a temperature of 750°C or higher, softening in a short time and providing good workability.The higher the temperature, the more advantageous it is, but it is economically disadvantageous to raise the strip temperature to a temperature higher than the Ac3 point. Not only that, but the material deteriorates due to α←→γ transformation.

Therefore, in a continuous hot-dip plating line after cold rolling, 750
Recrystallization annealing treatment at a temperature range of 3 degrees Celsius to Ac3 is suitable.

Examples according to the method of the present invention will be described below in comparison with comparative examples.

Example 1 Molten steel produced in a 90-ton LD converter was decarburized in a vacuum degassing device, deoxidized with Al, and then Fe-Ti and Fe-V or Fe-Ti and Fe-Nb were added to the surface. Molten steel having the chemical composition shown in 1 was obtained.

Each of these molten steels was cast according to the ordinary ingot making method, and six 15-ton ingots each were manufactured.

All ingots were smelted in the usual way and 180 x 93
After making the slab into a 5mm slab, it was removed and hot-rolled at a hot rolling finishing temperature of 870-900°C and a winding temperature of 600-630°C to obtain a plate with a thickness of 2.5mm. It was made into a 7 mm hot rolled coil.

After pickling these, top annealing was performed at 800°C for 4 hours, and then the plate thickness was 0. The steel sheet was cold rolled to a thickness of 8 mm, annealed at a temperature of 800° C. on a Sendzimer type continuous aluminum plating line (threading speed 50 m/min), and hot-dip aluminum plated to produce an aluminum-plated steel sheet.

Next, each coil is subjected to 1% temper rolling to produce a product.
A sample was taken from the middle part of the coil corresponding to the third melting number, and its mechanical properties were examined.

The test results are shown in Table 2.

Note that the test piece sampling direction was the L direction, and the plate thickness was 0.8 mm.

From the results in Table 2, it is clear that even if the manufacturing conditions are the same, the steels with chemical composition values within the range of the present invention are superior in ductility and deep drawability compared to comparative steels.

In addition, the product of the present invention had an extremely low rate of non-plating, and a good plated steel sheet was obtained.

Example 2 Molten steel produced in a 90-ton LD converter was decarburized by a vacuum degassing device, and after Al decarburization, Fe-Ti and Fe-Nb
C; 0.006%, Sl; 0.02%,
Mn; 0.2 0%, P; 0.014%, S; 0.01
2%, N; 0.003%, sol. Al; 0.023%
, Ti: 0.0-7%, Nb: 0.0-5%. Molten steel having chemical components within the range of the present invention was produced, and an aluminum hot-dip plated steel plate was produced by changing the top annealing conditions and continuous annealing conditions as follows. did.

That is, six 15-ton ingots were produced by casting the molten steel according to the ordinary ingot making method, and these were then bloomed using the ordinary method to obtain 18 ingots.
After forming a slab of 0 x 935 mm, defects were removed and hot rolled at a finishing temperature of 870 to 905°C and a coiling temperature of 605 to 635°C to obtain a hot rolled coil with a plate thickness of 2.7 mm.

After pickling these, top annealing was performed under the conditions shown in Table 3, and then the plate thickness was 0. It was cold rolled to 8 mm.

Subsequently, continuous annealing was performed on a Sendzimer type continuous aluminum plating line at the annealing temperatures shown in Table 3 (threading speed 5
0 m/min), and hot-dip aluminum plating was applied to produce an aluminum-plated steel sheet.

Each coil was subjected to 1% temper rolling to produce a product, and a sample was taken from the middle part of each coil to examine its mechanical properties.

The test results are shown in Table 3.

The specimen collection direction is the L direction, and the plate thickness is 0. 8m
It is m.

Lump no. 1 and 2 are manufacturing conditions within the scope of the present invention, and N
o. 3 to 6 are outside the condition range regulated by the present invention.

From the results in Table 3, it is clear that under the top annealing conditions and continuous annealing conditions within the scope of the present invention, plated steel sheets with superior ductility and deep drawability and good workability can be obtained compared to those outside the range regulated by the present invention. It is clear that a plated steel sheet that can withstand advanced processing and can be applied to new applications has been provided.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between Ti content and defective rate due to non-plating, FIG. 2 is a diagram showing the relationship between top annealing temperature and hardness, and FIG. 3 is a diagram showing the relationship between continuous annealing conditions and hardness.

Claims (1)

[Claims] 1 C; 0.001 to 0.02% Si: 0.05% or less Mn: 0.25% or less N; 0.01% or less sol, Al; 0.10% or less Ti; 0 .15% or less, and further contains at least one of V or Nb.
．． 0.7% or less and the remainder is iron and unavoidable impurities, hot rolled at a temperature of Ar3 or higher, top annealed at a temperature of 750 to 850°C, Production of a hot-dip plated steel sheet with excellent workability, characterized in that it is then cold-rolled at a rolling reduction of 40% or more, and then continuously annealed at a temperature of 750°C to 3 AC points in a Sendzimer-type continuous hot-dip plating apparatus to perform hot-dip plating. Law.