JP2002363649A - Method for producing high strength cold rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a cold rolled steel sheet having a low yield ratio, excellent workability and weldability, and high tensile strength by which the problems of a high yield ratio and bad shape freezability on press forming which are the characteristics of a precipitation type and fine grain type high strength steel sheet are solved. SOLUTION: Steel containing, by mass, 0.016 to 0.2% C and one or more kinds selected from 0.025 to 1% Ti, 0.01 to 1.5% Nb, and 0.01 to 1% V is hot- rolled, and is thereafter coiled at <=650 deg.C. The steel sheet is cold-rolled at a draft of <=85%, and is subsequently annealed at a heating rate of >=30 deg.C/s in the temperature range from 600 deg.C to the completion of recrystallization. In this production method, within 1 second after the completion of final rolling in the hot rolling, the steel sheet is cooled at a cooling rate of >=100 deg.C/s in the temperature range of >=80 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a high-strength cold-rolled steel sheet having a low yield ratio and excellent workability and weldability.

[0002]

2. Description of the Related Art In recent years, high-strength cold-rolled steel sheets having high tensile strength have been attracting attention as steel sheets used for automobile parts and home electric appliances. Conventionally, as a steel strengthening mechanism, No. 74
・ As shown on p.41 of the 75th Nishiyama Memorial Technical Lecture (The Iron and Steel Institute of Japan, 1981), solid solution strengthening, precipitation strengthening,
Five mechanisms of transformation strengthening, grain refinement, and work hardening are known. Of these, the increase in strength due to work hardening significantly deteriorates the workability of steel, and therefore, in the production of high-strength thin steel sheets, the following strengthening mechanism is usually used.

Here, the solid solution strengthening is performed by adding a solid solution element such as Si, Mn, or P, and is an effective means for increasing the strength without significantly deteriorating the workability. But S
When i and Mn are added in large amounts, the surface properties of the steel sheet deteriorate. That is, Si generates red scale, and Mn causes surface defects accompanying slab cracking, thereby deteriorating the steel sheet surface properties. Further, since the solid solution amount of P is limited, it is difficult to produce a high-strength material having a tensile strength exceeding 500 MPa only by the solid solution strengthening mechanism.

[0004] Precipitation strengthening is performed by adding carbonitride forming elements such as Ti, Nb, and V to finely precipitate carbonitrides, and the fine precipitates act as trap sites for dislocations, resulting in tensile strength. Can be increased. However, in the case of precipitation strengthening, the yield strength is inevitably increased, so that there is a problem that the shape freezing property during press molding is inferior.

[0005] Further, in the case of producing a cold-rolled steel sheet which is thinner than a hot-rolled steel sheet and which is excellent in surface roughness and sheet thickness accuracy, the addition of a large amount of carbonitride forming element requires cold-rolling. During the subsequent annealing, the recrystallization temperature rises. Therefore, annealing in the austenite region is inevitable, and in this case, workability is significantly deteriorated due to coarsening of crystal grains. Therefore, especially in the case of a cold-rolled steel sheet, it is difficult to increase the strength by precipitation strengthening.

[0006] Transformation strengthening is performed by rapid cooling from a high temperature range, and by controlling the cooling, a mixed structure of a martensite phase or a bainite phase which is a hard transformation phase and a ferrite phase which is a soft phase is formed to have high strength and workability. It is possible to obtain a steel sheet having excellent properties. However, when it is manufactured as a hot-dip galvanized plate, it cannot be rapidly cooled due to its manufacturing process. Therefore, it is necessary to add a large amount of quenching promoting elements such as Cr and Mo, which causes a problem that the production cost is increased.

[0007] Further, in many cases, thin steel sheets are required to have weldability, and there is also a problem that the hard phase formed by rapid cooling is tempered and softened by the influence of heat during welding. Thus, the strengthening mechanism based on the formation of the low-temperature transformation phase is disadvantageous from the viewpoint of weldability.

[0008] The grain refining is carried out by strong working hot.
This is done by quenching. Therefore, thin objects are possible,
Moreover, when manufacturing as a cold rolled sheet excellent in surface roughness and sheet thickness accuracy, it is difficult to reduce the grain size. Furthermore, grain refinement can increase tensile strength by acting as a trap site for dislocations at the grain boundaries, but inevitably involves an increase in yield strength, resulting in poor shape freezing during press molding. is there.

As described above, according to the production method utilizing these strengthening mechanisms, a thin steel sheet having low yield strength, excellent shape freezing property during press molding, good workability, and HAZ softening during welding is produced. It was difficult. Among them, the following several technologies have been proposed.

Regarding a method for producing a low-yield-ratio high-strength cold-rolled steel sheet excellent in workability, for example, as disclosed in Japanese Patent Publication No. 60-54373, Ti / C is 2 to 20 by weight% of Ti. After the hot rolled steel and the cold rolled steel are added by ordinary methods, recrystallization annealing by rapid heating is performed, and the steel is cooled at an average cooling rate of 20 ° C./s or less.

This method aims at fixing C with Ti in particular, and focuses on not leaving solid solution C by slow cooling after recrystallization annealing. Regarding the rapid heating, it is a viewpoint that it is more preferable as quickly as possible from the viewpoint of productivity.Since the expression is limited to heating to 750 ° C. within one minute in the examples, substantially 15 to 20 ° C.
It seems that a heating rate of about / s is intended.

Regarding a method for producing a high-tensile cold-rolled steel sheet having excellent stretch flangeability, for example, as disclosed in Japanese Patent No. 2688384, a steel to which 0.005 to 0.045 wt% of Nb is added is subjected to hot rolling and cold rolling. After rolling, there is a method in which heating and annealing are performed at a heating rate of 5 ° C./s or more. In this method, it is intended to realize grain refinement particularly by adding Nb.

Further, it is considered that a higher heating rate is advantageous for miniaturization, so that the heating rate is 5 ° C./s or more, preferably 10 ° C./s.
s or more, and the example shows an example up to a maximum of 20 ° C./s. At this time, the grain size is reduced to an average of 11 μm as the ferrite grain size.

As a method for producing a high-strength cold-rolled steel sheet having excellent bake hardenability, for example, as disclosed in JP-A-4-365814, one or more of Ti and Nb and Cr and Mo are added. There is a method of performing hot rolling, cold rolling, and recrystallization annealing on the resulting steel. Here, the higher the heating rate during annealing, the higher the workability is improved by the development of the (111) plane. However, in the case of a high-strength cold-rolled steel sheet, this is not important, and although not particularly specified, the normal heating method is used. About 5 to 5000 ° C./s.

[0015]

However, the above-mentioned prior art has the following problems. Tokiko Showa 60-5
In the technique described in Japanese Patent No. 4373, similarly to solid solution C, solid solution N which greatly affects yield strength is fixed with Al. However, as shown in p.55 of the 74.75th Nishiyama Memorial Technical Lecture (The Iron and Steel Institute of Japan, 1981), it is clear that Ti preferentially forms nitride. Therefore, if C and N are to be fixed on top of that, a large amount of Ti needs to be added, which causes a problem of increasing the manufacturing cost.

As for the technique described in Japanese Patent No. 2688384, in such a method of realizing finer grains by adding Nb, an increase in yield strength is inevitable as conventionally pointed out. Therefore, there is a problem that the shape freezing property at the time of press molding is deteriorated.

With respect to the technique described in Japanese Patent Application Laid-Open No. 4-365814, the obtained material characteristic values also have a high yield ratio (= yield strength / tensile strength). Such a high yield ratio is considered to be due to the influence of Ti, Nb-based carbonitride precipitation as conventionally pointed out, and therefore, even in this method, the shape freezing property during press molding is deteriorated. Is not resolved. The heating rate at the time of annealing is not described in the examples, and it is reasonable to consider that rapid heating is not particularly intended and is not performed as an example.

The present invention solves the problem of high yield ratio, which is a feature of precipitation-type and fine-grained high-strength steel sheets, and poor shape freezing property during press forming. An object of the present invention is to provide a method for producing a cold-rolled steel sheet having excellent workability and weldability and high tensile strength.

[0019]

The above object is achieved by the following invention. The invention, in mass%, C: 0.016 ~
0.2%, Ti: 0.025-1%, Nb: 0.01-1.5%, V: 0.01
After hot rolling 65% of steel containing at least one of
Winding at 0 ° C or less, cold rolling at a rolling reduction of 85% or less, 6
A method for producing a high-strength cold-rolled steel sheet, comprising annealing a temperature range from 00 ° C. to the end of recrystallization at a heating rate of 30 ° C./s or more.

The present invention has been made as a result of intensive studies to solve the above-mentioned problems. In the course of the research, a strengthening mechanism based on the formation of a low-temperature transformation phase was disadvantageous from the viewpoint of weldability, and the present invention aimed at increasing the strength by precipitation. Regarding precipitation, the carbonitride is not precipitated as much as possible in the hot rolling stage, and rapid heating is performed in the recrystallization annealing process after the subsequent cold rolling, whereby the structure is drastically refined and precipitation strengthening is performed. It has been found that by the combined action, a great increase in strength can be obtained, the workability is good, and the yield ratio also drops significantly.

The cold-rolled steel sheet to which the present invention is applied includes a steel sheet subjected to a surface treatment such as a hot-dip galvanized material or an electro-galvanized material.

First, the chemical components of the present invention will be described.

C: 0.016-0.2% C is an inexpensive and effective element for increasing the strength of steel. Furthermore, the addition of Ti, Nb, and V causes fine precipitation of carbides, suppresses grain growth, and contributes to an increase in strength by precipitation strengthening. To obtain this effect, 0.016% is required as the C content. On the other hand, the addition of a large amount of C exceeding 0.2% causes an increase in the amount of pearlite, and not only deteriorates ductility and stretch flangeability, but also adversely affects weldability. Therefore, the amount of C is specified in the range of 0.016 to 0.2%.

Ti: 0.025-1%, Nb: 0.01-1.5%, V: 0.01
11% Ti, Nb and V are all carbonitride forming elements, and contribute to an increase in strength by finely depositing carbonitride. To obtain this effect, Ti ≧ 0.025%, Nb ≧ 0.01%, V ≧ 0.01%
It is necessary to contain at least one of the following.

In the case of a process in which the temperature of the slab is temporarily lowered from continuous casting and then reheated in a hot-rolling heating furnace, especially when a large amount of Ti and Nb are added, carbonitrides remain in the hot-rolling heating furnace. It does not completely re-dissolve and remains coarse and does not contribute to an increase in strength. Therefore, in this case, it is preferable to add Ti and Nb at about 0.2% or less.

In the case where hot rolling is directly started from the continuous casting without going through the reheating process, there is no upper limit of the addition amount of Ti and Nb. However, Ti, Nb, and V having a C equivalent or more do not contribute to the strength increase and are economically disadvantageous. Therefore, the upper limits of Ti, Nb, and V are 1%, 1.5%, and 1%, respectively.
%.

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

Winding temperature: 650 ° C. or less In the case of winding after hot rolling, when winding is performed at a high temperature exceeding 650 ° C., carbonitrides of Ti, Nb, and V precipitate during the cooling process after winding. I do. Therefore, no precipitate-free zone near the grain boundary, which is considered to be formed by precipitation after cold rolling and recrystallization annealing, is not formed, and as a result, the yield strength increases.

Furthermore, in the recrystallization annealing process after cold rolling, the carbonitride hinders recrystallization, so that the annealing temperature and time must be increased, and as a result, grain growth is promoted and strength is reduced. Will be invited. Therefore, 650 after hot rolling
It is specified that the film is to be wound at a temperature of not more than ° C. The lower limit of the winding temperature is not particularly specified, and the winding may be performed at room temperature from the viewpoint of material properties.

Cold Rolling Ratio: 85% or Less In cold rolling, excessive cold rolling at a cold rolling ratio exceeding 85% promotes the development of a recrystallized texture disadvantageous to workability. Further, since the load of the cold rolling mill increases, the cold rolling ratio is specified to be 85% or less.

Heating rate in annealing: 30 ° C./s or more The heating process in recrystallization annealing after cold rolling is the most important part of the present invention. If this heating rate is slow,
Strain recovery progresses during heating, and when the temperature reaches the annealing target temperature, the driving force for generating recrystallization nuclei decreases, and as a result, fine grains cannot be obtained.

The heating rate in the annealing should be sufficiently high,
/ s or more suppresses the recovery of distortion during heating,
At the annealing target temperature, recrystallization nuclei can be generated at once, and an ultrafine structure can be obtained. At the same time, the precipitation of carbonitride from the grain boundary after recrystallization is promoted, and a non-precipitated zone is formed in the vicinity of the grain boundary, whereby a low yield ratio steel can be obtained.

Further, at a temperature higher than or equal to the two-phase range, austenite grains are miniaturized with the progress of recrystallization of fine ferrite grains, and C concentration is promoted. The micronized martensite phase is easily formed), which also contributes to a low yield ratio. Therefore, the heating rate in recrystallization annealing after cold rolling is 30
Specified as ℃ / s or more.

The heating method is not particularly limited, but heating may be performed by induction heating, direct energization, or the like. here,
From the viewpoint of productivity, it is preferable to heat at a heating rate of 30 ° C./s or more from room temperature to the annealing temperature, but in the range from room temperature to 600 ° C., the amount of strain recovery itself is small even at low speed heating. Therefore, the temperature may be lower than 30 ° C./s. Therefore, the heating rate of 30 ° C./s or more in the present invention is,
This means that heating is performed at a heating rate of 30 ° C./s or more at least in the region from 600 ° C. to the annealing temperature, more specifically, from 600 ° C. to the recrystallization completion temperature.

In the above invention, further, within 1 second after the end of the final rolling of hot rolling, at a cooling rate of 100 ° C./s or more,
A method for producing a high-strength cold-rolled steel sheet characterized by cooling over a temperature range of not less than ° C.

According to the present invention, the precipitation conditions of carbonitrides are more reliably suppressed by defining the cooling conditions after finish rolling. Cooling is performed on the runout table from hot rolling to winding up for 1 s immediately after the end of hot final finishing rolling.
Within 80 ° C or more at a cooling rate of 100 ° C / s or more. As described above, the cooling is started before the precipitation of the carbonitride starts, and the temperature region where the precipitation is remarkable is rapidly cooled to suppress the precipitation of the carbonitride.

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In practicing the invention, first,
The steel of the above chemical composition is melted. The smelting method can be appropriately applied, such as a normal converter method or an electric furnace method. Other elements are not particularly specified in the invention, but the following components are preferable in the production of the high-strength cold-rolled steel sheet of the invention.

Si: preferably 2.0% or less Si has the effect of solid-solution strengthening ferrite without deteriorating workability, and has the effect of increasing the balance between strength and workability. Therefore, Si is added according to the required strength level. Is preferred. However, since the addition of a large amount of Si deteriorates toughness and weldability, the upper limit is preferably about 2.0%.

Further, the addition of a large amount of Si promotes the formation of firelite on the slab surface during hot rolling and heating, and promotes the generation of a so-called red scale surface pattern. In the case of steel sheets that require surface properties or hot-dip galvanized steel sheets, the upper limit should be about 0.5%, more preferably about 0.2%, because non-plating defects due to Si are also induced. Is preferred.

Mn: preferably 2.5% or less Mn is an element effective for increasing the strength as solid solution strengthening, and is preferably added according to the required strength level. However, a large amount of Mn addition causes deterioration of weldability.
The upper limit is preferably about 5%.

P: preferably 0.1% or less P is an element effective for increasing the strength as solid solution strengthening. In the case of Si-added steel, it suppresses the generation of red scale. It is preferable to add them. However, the addition of a large amount of P promotes segregation at grain boundaries and lowers ductility and toughness, so the upper limit is preferably about 0.1%.

S: preferably 0.01% or less S significantly reduces hot ductility, thereby inducing hot cracking and significantly deteriorating the surface properties. Furthermore, not only does it hardly contribute to strength, but also forms coarse MnS as an impurity element, and in the case of Ti-added steel,
By generating a large amount of coarse Ti-based sulfide, ductility and stretch flangeability are reduced. Therefore, the upper limit of S is preferably set to 0.01%.

Sol.Al: preferably 0.1% or less sol.Al acts as a deoxidizing element to reduce inclusions in steel, but when added in large amounts, alumina-based inclusions increase. However, since the ductility decreases, the upper limit is preferably about 0.1%.

N: preferably 0.01% or less When a large amount of N is added, slab cracking occurs during hot rolling,
It is preferable to set the upper limit to about 0.01% because there is a possibility that surface flaws may occur.

Cu, Ni, Cr, Mo, B: added as required. Further, depending on the required strength level, Cu, Ni, Cr,
Additional elements such as Mo and B may be added. However, addition of Cu exceeding 1% tends to cause surface flaws due to hot cracking. Also, the addition of Ni, Cr, and Mo exceeding 1% increases the alloy cost. Regarding B, the effect saturates even if it exceeds 0.01%. Therefore, when Cu, Ni, Cr, and Mo are added, the content is 1% or less, and when B is added, the content is 0.01% or less.

After the molten steel is cast into a slab, it is heated as it is or cooled and hot-rolled. The hot-rolled steel sheet after finish rolling is wound at the above-mentioned winding temperature and subjected to ordinary cold rolling.

For annealing, rapid heating is performed under the above-mentioned heating conditions. Although the annealing temperature (recrystallization temperature) is not particularly specified, annealing is performed for each chemical component to a temperature required for recrystallization annealing, and in that range, it is preferable to keep the temperature as low as possible. Although the annealing time is not specified, the time for staying in the temperature range higher than the recrystallization temperature is not particularly required.

The cooling after annealing may be either standing cooling or rapid cooling. In particular, when manufactured as a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, the heat history in the process may be used.

As described above, according to the present invention, it is possible to produce a cold-rolled steel sheet having a low yield ratio and excellent workability and weldability and high tensile strength. For this reason, the scope of the present invention is not limited, but is considered as follows.

That is, in the recrystallization annealing process after the cold rolling, when gradual heating is performed, the recovery of the strain progresses during the heating, and most of the strain accumulated in the cold rolling disappears. In addition, recrystallization requires long-term holding at a high temperature. Then, the recrystallization nucleus is also preferentially generated from the grain boundary at the stage of the hot-rolled steel sheet, and the nucleus grows, whereby the recrystallization proceeds.

On the other hand, when rapid heating is performed, the temperature can reach the recrystallization temperature range without recovery of strain during heating. And since the strain is immediately shifted to the high temperature range without recovery, the driving force for the recrystallization nucleation becomes enormous, and it is instantaneous not only from the grain boundaries at the hot rolling stage but also from within the grains. It is thought that nucleation proceeds. It is presumed that an ultrafine structure is formed due to the increase in the number of nucleation sites.

Since the recrystallization process by the rapid heating proceeds instantaneously, the carbonitride forming element which is in a solid solution state in the hot rolling step cannot be precipitated in the heating step, but precipitates after the recrystallization is completed. Become. As a result, the precipitation of carbonitrides occurs preferentially at the recrystallized grain boundaries, but most of the precipitation occurs at the grain boundaries due to the ultra-fine grains and the large grain boundary area of the rapidly heated material. become.

As a result, the carbonitride precipitated at the grain boundaries suppresses grain growth, and a non-precipitated zone (PFZ) is formed near the grain boundaries. In a composite strengthened steel with such an ultra-fine grain structure and precipitation, when a work strain such as that at the time of press forming is applied while realizing high strength, a strain is generated in a non-precipitated zone near a grain boundary. It is considered that the yield strength decreases due to the concentration of.

Further, in rapid heating in the two-phase region or higher, austenite grains are also miniaturized as recrystallization of fine ferrite grains progresses, and C enrichment is extremely accelerated. Therefore, it is presumed that, during cooling after annealing, quenching is likely to occur (a martensite phase is easily generated), and that residual strain in the steel sheet also contributes to lowering the yield ratio. As a result, according to the present invention, a low yield ratio can be realized while realizing high strength.

As described above, the structure of the cold rolled steel sheet of the present invention is as follows:
A fine martensite phase is formed at the grain boundaries of the fine ferrite grains. Therefore, in a strict sense, it can be said that it is a dual-phase structure steel, but its second phase volume fraction is very small (about several percent) and not large enough to secure the strength, so that the structure of the cold-rolled steel sheet of the present invention is substantially Can be said to be a fine ferrite structure.

Although the fine martensite at the ferrite grain boundary affects the yield stress, it is considered that the fine martensite is not so large as to be a starting point of a crack at the time of plastic deformation. As a result, it is possible to prevent a decrease in stretch flangeability, which is inevitable in dual-phase steel.

[0057]

An embodiment of the present invention will be described. Note that the present invention is not limited to only these examples.

Steel having the components shown in Table 1 was melted in a laboratory vacuum melting furnace, and once cooled to room temperature.

[0059]

[Table 1]

Thereafter, the steel ingot was reheated at 1250 ° C. and subjected to lab hot rolling. After the rolling, the steel sheet was once cooled under various water cooling conditions, then air-cooled to a temperature corresponding to winding, kept in a furnace at that temperature for 1 hour, and then cooled in a furnace to obtain a heat treatment equivalent to winding. After hot rolling, cold rolling was performed at various cold pressure ratios, and then recrystallization annealing was performed under various heating conditions, and air cooling was performed after the annealing. 1.2m after cold pressing
m. Table 2 shows the experimental conditions.

[0061]

[Table 2]

Here, steel sheets Nos. 4 to 7, 10 to 13, and 15 to 20 are invention steel sheets. Steel sheets Nos. 1 to 3 are comparative steel sheets whose heating rates during annealing, steel sheets Nos. 8 to 9 are coiling temperatures, and No. 14 is a cold-rolling rate outside the range of the present invention.

Using the annealed samples, tensile properties, hole expanding properties (stretch flange properties), and weldability were investigated. Here, the weldability was investigated by examining the hardness distribution of the welded part by TIG bead-on welding, and the hardness ratio of the HAZ softened part to the base metal hardness (HAZ
(Hardened part hardness / base material part hardness). Table 3 summarizes the characteristic values of these samples.

[0064]

[Table 3]

Hereinafter, the relationship between the manufacturing conditions and the characteristic values will be described with reference to the drawings. First, the influence of the heating rate on annealing is shown in FIGS. Here, regarding steel type A, the final finishing temperature is 850 ° C., the cooling start time is 3 s, the falling temperature is 100 ° C., the winding temperature is 600 ° C., the cold rolling rate is 60%, and the annealing is cold rolling. The effects of the heating rate during annealing on the material properties when the annealing temperature is 800 ° C and the annealing time is 1 s are shown.

FIG. 1 shows the effect of heating rate on yield and tensile strength. As shown in the figure, when the heating rate is below the lower limit of 30 ° C./s in the claims of the present invention, the yield strength is 490.
~ 500MPa, while the tensile strength is 550 ~ 570MPa, when the heating rate is 30 ° C / s or more which is the claim of the present invention,
The yield strength decreased to 400 MPa or less, and the tensile strength increased to 600 MPa or more.

FIG. 2 shows the effect of the heating rate on the yield ratio (= yield strength / tensile strength). As shown in the figure, when the heating rate is lower than the lower limit 30 ° C./s of the claims of the present invention,
The yield ratio was 86 to 88%, whereas the heating rate was as low as 63% or less when the heating rate was 30 ° C./s or more, which is claimed in the present invention.

FIG. 3 shows the effect of the heating rate on the total elongation. As shown in the figure, when the heating rate is lower than 30 ° C./s, which is outside the scope of the present invention, the total elongation is 29 to 31.
%, Whereas the heating rate is within the scope of the present invention.
When the temperature was 30 ° C./s or more, it was as high as 34% or more.

FIG. 4 shows the effect of the heating rate on the hole expansion rate. As shown in the figure, when the heating rate is lower than the lower limit of 30 ° C./s in the claims of the present invention, the hole expansion rate is 58 to 65.
%, Whereas the heating rate is within the scope of the present invention.
When the temperature was 30 ° C./s or more, the value was as high as 78% or more.

FIG. 5 shows the effect of the heating rate on the HAZ softening hardness ratio. As shown in the figure, when the heating rate is lower than the lower limit 30 ° C./s of the claim of the present invention, the HAZ softening hardness ratio is 0.79 to 0.82, whereas the heating rate is the claim 30 of the present invention. At ℃ / s or more, it was as high as 0.95 or more.

As described above, the heating rate during annealing was set to 30 ° C. /
By setting it to s or more, it is possible to produce a steel plate having low yield, high strength, excellent ductility and hole expandability, and which is difficult to be softened by HAZ during welding.

FIGS. 6 to 10 show that, for steel type A, the final finishing temperature was 850 ° C., the cooling start time was 3 s, the drop temperature was 100 ° C., the cold rolling rate was 60%, and the annealing was cold rolling. When the heating rate is 100 ° C./s, the annealing temperature is 800 ° C., and the annealing time is 1 s, the effect of the winding temperature during hot rolling on the material properties is shown.

FIG. 6 shows the effect of the winding temperature on the yield and tensile strength. As shown in the figure, when the winding temperature exceeds 650 ° C., which is outside the scope of the present invention, the yield strength is 470 to 480 MPa and the tensile strength is 570 to 580 MPa, whereas the winding temperature is When the temperature was 650 ° C. or lower, which is the scope of the invention, the yield strength was reduced to 380 MPa or less, and the tensile strength was increased to 605 MPa or more.

FIG. 7 shows the effect of the winding temperature on the yield ratio (= yield strength / tensile strength). As shown in the figure, the winding process temperature is out of the range of the present invention.
When the temperature exceeds ℃, the yield ratio is 82%, whereas when the winding temperature is 650 ° C or less, which is the claim of the present invention, the yield ratio is 63%.
% Or less.

FIG. 8 shows the effect of the winding temperature on the total elongation. As shown in the figure, when the winding temperature exceeds 650 ° C., which is outside the scope of the present invention, the total elongation is 3
On the other hand, when the winding treatment temperature is 650 ° C. or less, which is the claimed range of the present invention, it is as high as 34% or more.

FIG. 9 shows the effect of the winding temperature on the hole expansion rate. As shown in the figure, when the winding processing temperature exceeds 650 ° C., which is outside the scope of the present invention, the hole expansion rate is 66 to 67%, whereas the winding processing temperature is within the scope of the present invention. At a certain temperature of 650 ° C or lower, it increased to 79% or more.

FIG. 10 shows the effect of the winding temperature on the HAZ softening hardness ratio. As shown in the figure, when the winding temperature exceeds 650 ° C., which is outside the scope of the present invention, H
While the AZ softening hardness ratio was 0.83 to 0.84, when the winding treatment temperature was 650 ° C. or less, which is the claimed range of the present invention, it increased to 0.94 or more.

As described above, the winding temperature during annealing is set at 65 ° C.
By controlling the temperature to 0 ° C. or lower, it is possible to produce a steel sheet having low yield, high strength, excellent ductility and hole expandability, and which is difficult to soften HAZ during welding.

Further, among the steel sheets of the invention, as shown in steel sheet No. 15, under the hot rolling conditions, the cooling start time after the final rolling was 0.5 s, the cooling rate was 200 ° C./s, and the temperature drop was 1 s.
By setting the temperature to 50 ° C, the same steel type A is used for the steel sheet No. 5 which has the same conditions after the winding process and has higher strength, higher ductility, lower yield ratio, higher hole expansion ratio, and higher HAZ softening hardness ratio. I was able to.

Further, as shown in Comparative steel sheet No. 14, in the cold rolling after the hot rolling, the cold pressure ratio was 90%. The total elongation of the steel sheet No. 5 of the present invention was remarkably reduced.

[0081]

As described above, according to the present invention, by limiting the hot rolling and winding conditions and the annealing conditions, carbonitride is not precipitated as much as possible in the hot rolling stage, and the carbonitride is not reprecipitated after the subsequent cold rolling. Rapid heating in the crystal annealing process drastically refines the structure and combines with precipitation strengthening to achieve a significant increase in strength, good workability, and large yield stress. Can be reduced.
As a result, a method for producing a high-strength cold-rolled steel sheet having a low yield ratio and excellent workability and weldability is provided, and an industrially effective effect is brought.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of heating rate during annealing after cold pressure on yield and tensile strength.

FIG. 2 is a graph showing the effect of the heating rate during annealing after cold pressure on the yield ratio.

FIG. 3 is a diagram showing the effect of a heating rate during post-cold-pressure annealing on total elongation.

FIG. 4 is a diagram showing the effect of the heating rate during post-cold pressure annealing on the hole expansion rate.

FIG. 5 is a graph showing the effect of the heating rate during annealing after cold pressure on the HAZ softening hardness ratio.

FIG. 6 is a diagram showing the effect of the winding treatment temperature after hot rolling on yield and tensile strength.

FIG. 7 is a diagram showing the effect of the winding treatment temperature after hot rolling on the yield ratio.

FIG. 8 is a diagram showing the effect of the winding temperature after hot rolling on the total elongation.

FIG. 9 is a diagram showing the effect of the winding treatment temperature after hot rolling on the hole expansion ratio.

FIG. 10 is a diagram showing the effect of the winding treatment temperature after hot rolling on the HAZ softening hardness ratio.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yasushi Tanaka 1-2-1 Marunouchi, Chiyoda-ku, Tokyo F-term in Nihon Kokan Co., Ltd. 4K037 EA01 EA02 EA05 EA06 EA11 EA13 EA15 EA16 EA17 EA18 EA20 EA23 EA25 EA27 EA28 EA31 EA32 FD04 FE01 FE02 FG00 FH00 FJ01 FJ04 FJ05 HA00

Claims (2)

[Claims] [Claim 1] In mass%, C: 0.016 to 0.2%, and Ti:
0.025 to 1%, Nb: 0.01 to 1.5%, V: 0.01 to 1%
After hot rolling, the steel containing more than one kind is taken up at 650 ° C or less, cold rolled at a rolling reduction of 85% or less, and the temperature range from 600 ° C to the end of recrystallization is 30 ° C / s or more. A method for producing a high-strength cold-rolled steel sheet, comprising annealing at a low temperature. 2. Within 1 s after the end of the final rolling of hot rolling, 10
The method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein cooling is performed at a cooling rate of 0 ° C / s or more over a temperature range of 80 ° C or more.