JPH0756056B2 - Method for producing high strength galvanized steel sheet having high r value - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a hot-dip galvanized steel sheet having high strength and an unprecedentedly high r-value by using a steel containing Cu. . Recent demands on the characteristic values of cold-rolled steel sheets for processing from users are high strength, high workability and high rust resistance. The present invention relates to a manufacturing method for providing a steel sheet that meets these requirements.

(Prior Art) The present inventors have found that a high-strength cold-rolled steel sheet with good workability (high r value) can be produced by using Cu-added steel and limiting the continuous annealing conditions after cold rolling. It is already known as proposed in JP-A-64-4429.

The steel sheet can be further provided with anticorrosion property by electrogalvanizing. However, it is economical and difficult to increase the basis weight of an electrogalvanized steel sheet in order to obtain higher rust prevention, and for that purpose, hot dip galvanizing is more suitable.

However, when the invention steel sheet described in JP-A-64-4429 is hot dip galvanized through an ordinary continuous annealing line, the plating adhesion is poor and the high rust preventive property is not satisfied in this component range. Further, the cold-rolled steel sheet of the invention described in JP-A-64-4429, which is passed through a hot dip coating line immediately after cold rolling, not only has poor plating adhesion, but also has a precipitation treatment step, resulting in high strength. It was not possible to satisfy the demand for new high corrosion-resistant steel sheet.

(Problems to be Solved by the Invention) Recently, along with the increase in the number of parts having complicated shapes that require high work deformation, the strength of parts has been increased and the weight of parts has been reduced by thinning steel plates. It is becoming increasingly necessary to improve the rust prevention properties at the same time.
That is, the demand from the user side of the high-strength hot-dip galvanized steel sheet having workability is only ever increasing, but the conventional steel sheet cannot satisfy such a demand from the user side.

So far, high-strength cold-rolled steel sheets having a high r value and a tensile strength of 45 to 75 kgf / mm ² have been obtained. However, this steel sheet is only for high r value and high strength, and no consideration is given to a method for obtaining hot dip galvanizing property. That is, a hot-dip galvanized high-strength steel sheet having a high r value has not been obtained so far.

In view of such an actual situation and in order to meet the above-mentioned demand, the present invention provides a novel high-strength hot-dip galvanized steel sheet having a tensile strength in the range of 45 to 75 kgf / mm ² and a high r value. It is intended to provide a manufacturing method.

(Means for Solving the Problem) The inventors of the present invention have used a continuous annealing method on an industrial scale that is usually adopted at present and a continuous hot-dip galvanizing method. On the premise of the industrial manufacturing method of hot-dip galvanized steel sheet, a study was conducted to add various elements to low carbon steel singly or in combination. As a result, the C content of the steel containing Cu, the Si content reduction, the Al content limitation, and
It was newly discovered that a steel sheet having high strength, a high r value, and at the same time good plating property can be produced by limiting the temperature condition of the hot dip coating line.

The present invention is based on the above findings. That is, the high-strength hot-dip galvanized steel sheet having a high r value of the present invention has, as weight%, C: 0.010% or less, Si: 0.03% or less,
Mn: 0.05-0.5%, Cu: 0.80-2.2%, Sol.Al:0.005-0.03
%, P: 0.10% or less, S: 0.001 to 0.030%, N: 0.0050% or less, and other inevitable elements as a basic component, and one or two of Ti and Nb, and further Ni. the is intended to contain each optionally, and the invention is a steel having the above component composition is hot rolled at Ar ₃ transformation point or higher, wound at 450 ° C. or less, or more 750 ° C. after cold rolling Has a high r-value, which is characterized by performing recrystallization annealing at a temperature of 500 ° C. to 700 ° C. and then reducing the steel sheet surface by heating for 0.5 minutes to 3 minutes, and then performing hot dip coating. The method for producing a high-strength hot-dip galvanized steel sheet constitutes the invention.

The present invention will be described in detail below.

First, the reason for limiting the component range in the present invention is as follows.

It is necessary to reduce the C content as much as possible in order to secure an extremely high r value and high ductility even at a high strength level. Therefore, the amount of C needs to be 0.010% or less. When the amount of C is more than this, the r value and the ductility decrease, and the object of the present invention cannot be achieved.

Si is effective as an element for obtaining high strength, but has a great adverse effect on plating adhesion. Therefore, from the viewpoint of plating adhesion, Si: 0.03% or less (do not add and mix as impurities as much as possible).

The Mn content is preferably low in order to enhance the r value and ductility of the steel sheet, and the upper limit is 0.5%. If the Mn content is too low, surface defects on the slab are likely to occur, so the lower limit is 0.05%.
And

When Cu is added to the ultra-low carbon steel, it has the effect of increasing the strength of the steel sheet by precipitating it after the development of the recrystallization texture having a high r value is completed. Fig. 1 shows C: 0.0025%, Mn: 0.1
5%, Si: 0.01%, S: 0.015%, P: 0.07%, N: 0.0025%, S
ol.Al: Steel containing 0.025% as the basic component, Cu content of 0.71 ~
The steel added in the range of 2.06% is melted, hot-rolled and cold-rolled according to the usual method to obtain a steel plate with a thickness of 0.8 mm, and then 850
Recrystallization annealing is performed at ℃ and it is cooled to room temperature. It is a figure which showed the influence of the reduction treatment temperature on the tensile strength of the steel sheet which carried out the continuous hot dip galvanizing line, and then performed the reduction treatment for 60 seconds at various temperatures.

In the figure, curve a is Cu: 2.06%, curve b is Cu: 1.68%, curve c
Indicates the tensile strength of a steel sheet with Cu: 0.80% and the curve d shows the tensile strength of a steel sheet with Cu: 0.71%. From the figure, it is understood that a Cu content of 0.8% or more is required to obtain high strength in the reduction treatment of hot dip galvanizing. On the other hand, if it exceeds 2.2%, the surface quality at the hot-rolled steel sheet stage deteriorates, so Cu needs to be in the range of 0.8 to 2.2%.

Sol.Al is an element necessary to obtain Al killed,
It is necessary to make it as low as possible because it lowers the plating adhesion, and it is necessary to set it in the range of 0.005 to 0.03%.

P is an element that increases the strength of the steel sheet and is added according to the required strength level. However, if 0.10% is exceeded, secondary work cracking will occur, so this is the upper limit.

The S content is preferably low in order to increase the r value and the ductility of the steel sheet, and the upper limit is 0.030%. If the amount of S is too low, it will be difficult to industrially melt, so the lower limit is set to 0.001%.

N is preferably as low as 0.0050% or less in order to increase the r value and obtain high ductility.

One or two of Ti and Nb 0.01-0.2%, 0.005
When added in the range of up to 0.2%, C and N are fixed by these and the steel sheet becomes a non-aging steel sheet. When it becomes a non-aging steel sheet, the deterioration of ductility due to aging disappears and a higher ductility steel sheet can be obtained. Also, the addition of one or two of Ti and Nb has the effect of increasing the r value of the steel sheet.

Ni keeps the surface quality of the hot-rolled steel sheet, which is the raw material, high quality,
It is effective in preventing hot embrittlement. 0.15 if needed
It may be added in the range of up to 0.45%.

Next, in the hot rolling step, the high temperature slab directly sent from the continuous casting machine or the high temperature slab obtained by heating is hot rolled at a temperature of Ar ₃ or higher.

Regarding the coiling temperature after hot rolling, when coiled over 450 ° C, Cu finely precipitates in the hot rolled sheet, which has a bad effect on the recrystallization texture during subsequent cold rolling and annealing. The coiling temperature after hot rolling shall be 450 ° C or lower.

Next, regarding cold rolling, in order to obtain a high r value, a higher cold rolling reduction is better. A cold rolling reduction of 50 to 85% is suitable for the purpose of the present invention.

The cold-rolled sheet is annealed by performing continuous annealing at a temperature of 750 ° C or higher to recrystallize and simultaneously form a solid solution of Cu. On this occasion,
At a temperature of less than 750 ° C, recrystallization is not completed and a high r value cannot be obtained, and further, solid solution of Cu is not sufficient and strength cannot be obtained. After passing the continuous annealing line, the continuous hot dip galvanizing line is passed to obtain high strength and hot dip galvanizing is performed at the same time. In that case, the surface of the steel sheet is usually reduced before it is immersed in the hot dip galvanizing bath, but the method is a conventional non-oxidizing heating-
Either a reduction method or a direct reduction method using a burner reduction area may be used. Alternatively, a method of performing appropriate pretreatment and then heating by a radiant tube or direct energization may be used. Here, Cu precipitation treatment is performed at the same time as the heat reduction treatment before hot dip galvanizing. At this time, the reduction treatment temperature
If the temperature is less than 450 ° C. and less than 0.1 minutes, the precipitation of Cu is insufficient and high strength cannot be obtained. On the other hand, if the temperature exceeds 700 ° C. for more than 3 minutes, the precipitates become coarse and sufficient strength cannot be obtained. Further, if the reduction treatment temperature is less than 500 ° C and the reduction treatment time is less than 0.5 minutes, a good reduced surface required for securing hot dip coating adhesion cannot be obtained. Therefore, the reduction treatment temperature is 500 ° C. or more and 700 ° C. or less, and 0.5 minutes or more and 3 minutes or less.

The previous studies have been conducted on the premise that the continuous annealing line and the continuous hot-dip galvanizing line are passed through different lines respectively.However, the temperature conditions required for recrystallization annealing and the precipitation and plating of Cu It is possible to use only one line as long as the temperature condition required for adhesion is satisfied.

In addition, in order to improve the spot weldability of the hot-dip galvanized steel sheet, alloying treatment may be performed at a temperature of about 530 ° C. after hot-dip galvanizing. In this case, the alloying temperature is
Since it is in the range of Cu precipitation strengthening, the strength is further improved as compared with the case where no alloying treatment is performed, and in this case as well, a steel sheet having a high r value and high strength can be obtained.

(Example) Steel pieces A to J shown in Table 1 were hot-rolled and wound under the conditions shown in the same table to obtain a hot-rolled steel sheet having a thickness of 3.2 mm. The steel sheet was cold-rolled to 0.8 mm, recrystallized by a continuous annealing line at the temperature shown in the table, and then hot-dip galvanized by a hot-dip galvanizing line. This line is a non-oxidative heating-reduction furnace system. The temperature of the plating bath was 460 ° C., and the immersion time was 4 seconds. The reduction treatment temperatures are shown in the table. Table 2 shows the mechanical properties and plating adhesion of this steel sheet.

The steels A to E of the present invention have a high strength exceeding 45 kgf / mm ² , but have an extremely high r value and have characteristics not found in conventional steels having good plating adhesion. On the other hand, Comparative Steel F has a large amount of C and thus has a low r value and a low elongation. Comparative steel G
Has a high r-value, but the strength does not increase because the amount of Cu is small, and the target strength is not achieved. Comparative Steel H has a low soaking temperature during continuous annealing, so that recrystallization has not been completed and r-value elongation is low. Comparative Steel I has a low reduction treatment temperature in the hot-dip galvanizing line, so that the intended strength is not obtained. Comparative Steel J has a high amount of Si in the steel and therefore has poor plating adhesion.

(Effect of the Invention) As described in detail above, according to the present invention, it is possible for the first time to manufacture a high-strength hot-dip galvanized steel sheet having a high r value and a tensile strength of 45 to 75 kgf / mm ² .

[Brief description of drawings]

FIG. 1 is a graph showing the effect of Cu content on the tensile strength of an ultra-low carbon cold-rolled steel sheet using the reduction treatment temperature of the hot dip coating line as a parameter.

Claims (4)

[Claims] 1. As weight%, C: 0.010% or less, Si: 0.03% or less, Mn: 0.05 to 0.5%, Cu: 0.8 to 2.2%, Sol.Al: 0.005 to 0.03%, P: 0.10% or less, Steel containing S: 0.001 to 0.030%, N: 0.0050% or less is hot-rolled at a temperature of ₃ points or more of Ar, and 450 ° C.
It is rolled up into a hot-rolled steel strip and then cold-rolled. A method for producing a high-strength hot-dip galvanized steel sheet having a high r value, which comprises heating for 3 minutes or more and 3 minutes or less to reduce the surface of the steel sheet, and then performing hot dip coating. 2. As weight%, C: 0.010% or less, Si: 0.03% or less, Mn: 0.05 to 0.5%, Cu: 0.8 to 2.2%, Sol.Al: 0.005 to 0.03%, P: 0.10% or less, S: 0.001 to 0.030%, N: 0.0050% or less, and one or two of Ti or Nb from 0.01 to 0.
Steel containing 2%, 0.005 to 0.2% is hot-rolled at a temperature of Ar ₃ points or higher, wound at 450 ° C or lower to form a hot-rolled steel strip, and then cold-rolled. Cold rolled steel strip 750
Recrystallization annealing at a temperature of ℃ or more, followed by heating at a temperature of 500 ℃ or more and 700 ℃ or less for 0.5 minutes or more and 3 minutes or less to reduce the surface of the steel sheet, and then perform hot dip coating Of manufacturing a high-strength galvanized steel sheet having: 3. As weight%, C: 0.010% or less, Si: 0.03% or less, Mn: 0.05 to 0.5%, Cu: 0.8 to 2.2%, Sol.Al: 0.005 to 0.03%, P: 0.10% or less, Steel containing S: 0.001 to 0.030%, N: 0.0050% or less, Ni: 0.15 to 0.70% is hot-rolled at a temperature of ₃ points or more of Ar, and 450 ° C.
It is rolled up to form a hot rolled steel strip, and then cold rolled, the obtained cold rolled steel strip is recrystallized and annealed at a temperature of 750 ° C or higher, and then 0.5 ° C at a temperature of 500 ° C or higher and 700 ° C or lower. A method for producing a high-strength hot-dip galvanized steel sheet having a high r value, which comprises heating for 3 minutes or more and 3 minutes or less to reduce the surface of the steel sheet, and then performing hot dip coating. 4. As weight%, C: 0.010% or less, Si: 0.03% or less, Mn: 0.05 to 0.5%, Cu: 0.8 to 2.2%, Sol.Al: 0.005 to 0.03%, P: 0.10% or less, S: 0.001 to 0.030%, N: 0.0050% or less, Ni: 0.15 to 0.70%, and 0.01 or 0 of one or two of Ti or Nb, respectively.
Steel containing 2%, 0.005 to 0.2% is hot-rolled at a temperature of Ar ₃ points or higher, wound at 450 ° C or lower to form a hot-rolled steel strip, and then cold-rolled. Cold rolled steel strip 750
Recrystallization annealing at a temperature of ℃ or more, followed by heating at a temperature of 500 ℃ or more and 700 ℃ or less for 0.5 minutes or more and 3 minutes or less to reduce the surface of the steel sheet, and then perform hot dip coating Of manufacturing a high-strength galvanized steel sheet having: