JPH05271857A - High tensile strength steel sheet and its production - Google Patents

Abstract

PURPOSE:To provide a high tensile strength steel sheet excellent in workability, corrosion resistance, and surface treatability. CONSTITUTION:The steel sheet has a composition which contains 0.05-0.3% C, <=2% Si, 0.05-4.0% Mn, <=0.1% P, <=0.1% S, 0.1-2% Cu in the range not lower than Si(%)/5, 0.1-2% Al, <=0.01% N, and Ni by the amount not lower than Cu(%)/3 (Ni is not needed when Cu is =0.5%) and satisfies [Si(%)+Al(%)>=0.5] and [Mn(%)+Ni(%)>=0.5]. Further, this steel sheet has a structure containing retained austenite by >=5% by volume ratio. This steel sheet can b< produced by subjecting a slab of a steel with the above composition to hot rolling, to coiling at 300-720 deg.C, to descaling treatment, and to cold rolling at 30-80% draft and then subjecting the resulting steel sheet to heating up to a temp. in the region between the Ac1 transformation point and the Ac3 transformation point in the subsequent continuous annealing or continuous hot-dip galvanizing stage and to holding, in the course of cooling, at a temp. in the range between 550 and 350 deg.C for >=30sec or to slow cooling through the temp. region at <=100 deg.C/min cooling rate.

Description

Detailed Description of the Invention

The present invention relates to a high-strength thin steel sheet having excellent ductility, corrosion resistance and surface treatment property, which is suitable as a structural member formed into various shapes by press working, stretch flange working, etc., and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, various machines and devices have been strongly promoted to have higher performance and lighter weight. In response to this, many technologies for strengthening steel plates have been developed. Generally, it is said that it is very difficult to manufacture a steel sheet having both good workability and high strength, since increasing the strength of the steel sheet causes deterioration of ductility.

However, recently, "a low-carbon steel sheet to which Si and Mn are added in combination is rapidly cooled to 350 to 550 ° C. after annealing in a two-phase region, and is held in that temperature region for a short time or is cooled stepwise. And partially transform austenite into bainite, and finally [ferrite + bainite + residual austenite]
The structure consisting of
Stenite undergoes strain-induced transformation and shows a large elongation. "
Since the discovery of this phenomenon, attempts have also been made to manufacture high ductility and high strength steel sheets by utilizing this phenomenon.

For example, in Japanese Patent Laid-Open No. 61-157625, there is 0.4 to 1.8% Si (hereinafter,% representing the component ratio is% by weight) and 0.2 to 2.5% Mn, and if necessary, an appropriate amount. A steel sheet containing one or more of P, Ni, Cu, Cr, Ti, Nb, V, and Mo [ferrite + austenite]
After heating to the two-phase region, a method for producing a high-tensile steel sheet exhibiting high ductility by realizing the mixed structure by maintaining the temperature region of 500 to 350 ° C during cooling for 30 seconds to 30 minutes is disclosed. ..

Further, Japanese Patent Publication No. 62-35461 discloses a method for producing a high-strength steel sheet exhibiting high ductility.
After heating a steel sheet containing 2.0% Si and 0.5 to 2.0% Mn to the [ferrite + austenite] two-phase region in the annealing process, a total of 10 to 50 was applied between 650 and 450 ° C in the cooling process.
A method is disclosed in which a steel sheet having a mixed structure containing 10% or more by volume of ferrite and retained austenite is contained in martensite or bainite by maintaining a constant temperature for 2 seconds.

However, in practice, a steel sheet having the above-described mixed structure generally cannot be said to have good formability at the time of press working even if it shows good ductility in a tensile test. I was not completely satisfied with it. For example, when the mixed structure steel sheet is processed, most of the retained austenite has undergone strain-induced transformation into high carbon martensite in the latter stage of deformation, resulting in extremely poor local ductility. This phenomenon remarkably appears in the case of stretch flanging such as "hole expansion", so that the hole expandability of the steel sheet having the mixed structure is inferior to that of the conventional low carbon steel sheet. This is because when performing punching by punching, cracks occur because the high carbon martensite generated by strain-induced transformation is extremely hard, and this crack expands and propagates during subsequent hole expansion. It is considered.

Further, in the known manufacturing technique for a steel sheet having a mixed structure, it is necessary to change the C concentration in the steel in order to change the strength level. Since the volume ratio of the austenite decreases, it has been difficult to produce "a cold-rolled steel sheet containing a large amount of retained austenite in a relatively low strength region and exhibiting high ductility".

Further, when the amount of Si added to the steel sheet becomes large,
During heating of the slab in the hot rolling process, SiO ₂ and FeO undergo a eutectic reaction to cause non-uniform scaling of the low melting point, resulting in unevenness on the surface of the hot rolled sheet after pickling. Although these irregularities were slightly reduced by cold rolling, they still remained in the final product, causing deterioration in appearance.

Further, in the case of a steel sheet having a transformation structure strengthened, there is also a problem that its corrosion resistance is generally inferior to that of a solid solution strengthened steel sheet. This problem is considered to occur because a so-called "local battery" is likely to be formed in a steel sheet having a composite structure composed of a plurality of structures having different corrosion potentials, and this is associated with corrosion. Moreover, recently, strict requirements have come to be made on the surface treatability (plating treatability, etc.) of these steel sheets.

From the above, the object of the present invention is that the workability including ductility is excellent, and the strength level can be adjusted without a large change in the C content.
The aim was to establish means for stably providing high-strength steel sheets with good corrosion resistance, appearance, and surface treatment.

[0011]

Therefore, the inventors of the present invention have made extensive studies in order to achieve the above object, and have been able to obtain the following findings. (A) The results of investigating the effects of Si and Al on the amount of retained austenite in a continuous annealed steel sheet having a standard composition of 0.15% C-1.5% Mn are shown below. a) Retained austenite of approximately the same volume ratio can be obtained in steel sheets to which both Si and Al are added if the addition amounts are the same.b) The total elongation of steel sheets to which Al is added is Si-added. Although it is slightly smaller than the steel sheet, the local elongation obtained by subtracting the uniform elongation from the total elongation is conversely large, and it has good performance in terms of hole expandability. It is considered that this is because, in the steel sheet to which Al is added, the retained austenite is stable, and thus strain-induced transformation is unlikely to occur until the high strain region is reached, and the transformation is performed after reaching the large deformation region. Although the cause of such a difference is unknown, it is presumed that the distribution form of retained austenite changes.

(B) In this case, the C concentration in the steel sheet and [Si
+ Al], the tensile strength of the steel sheet can be arbitrarily changed by changing the balance of Si (%) and Al (%) without changing the total content. Is.

(C) Furthermore, if Al is positively added as a component for securing retained austenite and the amount of Si is reduced, a smooth surface state is secured at the stage of hot-rolled steel sheet, and deterioration of appearance in the final product is prevented. No more inviting.

(D) When Cu is added to steel having a standard composition of 0.15% C-1.5% Si-1.5% Mn, the following effects are observed. a) Removability of high Si scale generated during slab heating during hot rolling process is improved (This is because Cu contained in “slab surface layer that is oxidized during heating to scale” is oxidized. It is thought that this is because the scale peels off at the interface between the scale and the base metal, improving the peelability of the scale.), B) Corrosion resistance when used as a cold-rolled steel sheet without surface treatment ( This is considered to be because the Cu concentrated layer contributes to improving the corrosion resistance of the steel sheet surface), c) when the steel sheet of the composition is plated by a continuous hot dip galvanizing line, the wettability and alloying of the plating The processability is improved (this is also considered to be the action of the Cu-enriched layer).

(E) When a cold-rolled annealed sheet is manufactured from a steel whose properties have been improved by adding Cu to the Al- and Si-added steel, by carefully devising the hot-rolling winding temperature and the annealing conditions, It is much easier to secure the amount of retained austenite favoring ductility, and the productivity of high-strength steel sheets with excellent ductility and other workability, corrosion resistance, and surface treatment is greatly stabilized.

The present invention has been completed on the basis of the above findings and the like. "A thin steel plate is made of C, Si, Mn, P, S,
Content of Cu, Al and N: C: 0.05-0.3%, Si: 2.0% or less, Mn: 0.5
~ 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2.0% and Si (%) / 5 or more, Al: 0.1 to 2.0%, N: 0.01% or less, and Si (%) + Al (%) ≥ 0.5 and the balance is a component composition consisting of Fe and unavoidable impurities, or when the range of Cu content is expanded by adding Ni: C: 0.05 to 0.3%, Si: 2.0% or less, Mn: 0.05
~ 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2.0% and Si (%) / 5 or more, Al: 0.1 to 2.0%, N: 0.01% or less, Ni: Cu
(%) / 3 or more, and the composition is Si (%) + Al (%) ≧ 0.5, Mn (%) + Ni (%) ≧ 0.5, and the balance is Fe and inevitable impurities. By having a structure having a residual austenite content of 5% or more in terms of volume ratio,
It features high strength and excellent ductility, corrosion resistance, and surface treatment. "Furthermore," C, Si, Mn, P,
Content of S, Cu, Ni, Al and N is C: 0.05-0.3%, Si: 2.0% or less, Mn: 0.05
~ 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2.0% and Si (%) / 5 or more, Al: 0.1 to 2.0%, N: 0.01% or less, Ni: Cu (%) / 3 or more (however, it does not have to be included when Cu is 0.5% or less), and Si (%) + Al (%) ≧ 0.5 and Mn (%) + Ni (%) ≧ 0.5 are satisfied. A steel slab having a composition with the balance being Fe and unavoidable impurities is hot-rolled, wound at 300 to 720 ° C., then descaled, cold-rolled at a rolling reduction of 30 to 80%, and thereafter. In the continuous annealing or continuous hot dip galvanizing process, heating to a temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point and below, and 550 to 350 ° C during cooling.
Hold for 30 seconds or more in the temperature range of 100 ℃ / m
By gradually cooling at a cooling rate of in or less, the volume ratio is 5
% Or more of retained austenite and having a high strength and excellent ductility, corrosion resistance, and surface treatment, it is possible to stably manufacture a high-strength thin steel sheet ”.

In the following, the reason why the composition of the steel sheet (steel slab) and the manufacturing conditions of the steel sheet are limited as described above in the present invention will be explained together with its action.

[Action]

A) Component composition C C is the most powerful austenite stabilizing element, and it is necessary that 1% or more of C is contained in austenite in order to stabilize austenite at room temperature. However, by selecting the heat cycle of annealing, it is possible to secure a sufficient austenite stabilizing effect with a content of 0.05% or more. Then, by adding a larger amount of C, a high-strength high-strength cold-rolled steel sheet can be manufactured.
If the content exceeds%, the steel plate becomes too hard, and it becomes impossible to process it into a thin steel plate in a normal plate-making process. Therefore, although the C content is limited to 0.05 to 0.3%, it is preferably adjusted to 0.1 to 0.2%.

Si Si is a ferrite stabilizing element, and increases the volume fraction of ferrite during annealing in the two-phase region to equilibrate with C in the austenite phase.
It has the effect of increasing the concentration. At the same time, Si also has a function of strengthening ferrite. However, if the Si content exceeds 2.0%, the high Si scale peculiar to the Si-added steel sheet significantly deteriorates the surface quality, so the Si content was set to 2.0% or less. The content of Si must be controlled in relation to Al, which is the same ferrite stabilizing element, and in order to obtain the desired effect on the above action.
It is necessary to adjust the value of [Si (%) + Al (%)] to be 0.5 or more.

Mn Mn is an austenite-stabilizing element, and from this viewpoint, the Mn content is regulated by the total amount with the content of Ni (added as necessary) having a similar action, [Mn (%) ＋
It is necessary to adjust the value of Ni (%)] to be 0.5 or more. That is, if the value of [Mn (%) + Ni (%)] is less than 0.5, austenite is not stabilized. However, if the Mn content exceeds 4.0%, the steel plate may become too hard and sufficient ductility may not be obtained, so the upper limit of the Mn content was limited to 4.0%.

P P is an element inevitably contained in steel as an impurity, and it is preferable that P P is as low as possible. In particular, if the content of P exceeds 0.1%, the ductility deterioration of the steel sheet becomes remarkable, so the P content was set to 0.1% or less.

S S is also an element that is unavoidably contained in steel as an impurity, and it is preferable that S S is also lower. In particular, if the content of MnS exceeds 0.1%, the precipitation amount of MnS becomes conspicuous, which not only hinders the ductility of the steel sheet, but also consumes Mn added as an austenite stabilizing element as the precipitate. , S content was set to 0.1% or less.

Al As described above, Al is a ferrite stabilizing element like Si, and has an effect of increasing the C concentration of the equilibrium austenite phase by increasing the volume fraction of ferrite during the two-phase region annealing. Have However, it has a stronger effect of stabilizing austenite than Si, and if a content of 0.1% or more is secured, the effect of improving the local ductility of the steel sheet can be obtained.
On the other hand, when the Al content exceeds 2.0%, inclusions increase in the steel sheet and ductility decreases. Therefore, the Al content is 0.1 ~
Although it was set to 2.0%, in order to secure the desired effect as a ferrite stabilizing element, the value of [Si (%) + Al (%)] along with Si should be
It is necessary to adjust it to be 0.5 or more. Note that FIG.
3 is a graph showing the Si and Al content ranges of the steel sheet of the present invention.

Cu Cu is used for “improving the removability of high Si scale formed on a slab in the hot rolling process”, “improving corrosion resistance when used as a cold rolled steel sheet without surface treatment”, and “continuous It is added for the purpose of "improving the wettability of the plating and the alloying processability when plating is performed on a hot-dip galvanizing line.
If less than 0.1% or less than Si (%) / 5, these effects are not sufficient, while if more than 2.0% is contained in a large amount,
Although the cause is unknown, the stacking fault energy of the retained austenite is too low to prevent the strain-induced transformation, resulting in a marked decrease in ductility. Therefore, the Cu content is determined to be in the range of 0.1 to 2.0% and Si (%) / 5 or more. In addition, FIG. 2 shows Si and Cu related to the steel sheet of the present invention.
5 is a graph illustrating a content range of.

When the Ni steel contains Cu in an amount of more than 0.5%, defects called crack cracks occur on the surface during heating of the slab.
The cause is that a low melting point alloy phase containing a large amount of Cu is generated in the austenite grain boundaries, but Ni has an action of increasing the melting point and suppressing the generation of this defect. In order to obtain the desired effect on the above action of Ni, it is necessary to secure a Ni content of Cu (%) / 3 or more, but when Cu is 0.5% or less, there is almost no possibility of crack cracking. Therefore, Ni addition is not always necessary. However, as described above, Ni is an austenite stabilizing element, and in order to stabilize austenite, it is necessary to pay attention so that the total amount of Mn (Mn + Ni) is 0.5% or more. That is, if the value of [Mn (%) + Ni (%)] is less than 0.5, austenite is not stabilized. Therefore, the Ni content is adjusted so as to secure the above value in consideration of the Mn content. It

N 2 N is also an element that is unavoidably contained in steel as an impurity, and it is preferable that the content thereof be low (current steelmaking technology makes it very difficult to reduce N in steel having a large C content). Yes 0.0
Although there is an economic limit of about 01%, the lower the better. In particular, when the N content exceeds 0.01%, the amount of Al consumed as AlN is large and the effect of Al addition becomes small, and the deterioration of ductility due to AlN becomes conspicuous. Therefore, the upper limit of the N content is 0.01%. Defined as%.

B) Volume Ratio of Retained Austenite The ductility of the steel sheet of the present invention as a final product depends on the volume ratio of retained austenite contained in the product, and the volume ratio is 5%.
If it is less than%, improvement in ductility due to strain-induced transformation of austenite cannot be expected. Since the ductility of the steel sheet improves as the amount of retained austenite increases, the volume ratio of retained austenite is preferably 10% or more.

C) Manufacturing conditions Hot rolling coiling temperature In the case of the steel of the composition of the present invention, when it is coiled at a low temperature, it is hardened by being hardened, so that it becomes difficult to remove the scale by subsequent pickling or to carry out cold rolling. Become. On the other hand, when coiled at a high temperature, the cementite becomes coarse and becomes soft, which facilitates pickling and cold rolling, but it takes too long to re-dissolve cementite during soaking during annealing, and sufficient austenite is formed. It will not remain. Therefore, winding after hot rolling can avoid the above-mentioned inconvenience.
It was decided to carry out at -720 degreeC. However, since it is desired that the hot-rolled steel sheet is as easy as possible to be pickled and cold-rolled, it can be said that the winding temperature is preferably 550 to 650 ° C.

Cold rolling reduction ratio If the reduction ratio of cold rolling is less than 30%, recrystallization does not occur completely in the subsequent annealing step and ductility deteriorates. on the other hand,
If the rolling reduction exceeds 80%, the rolling mill is overloaded, so the rolling reduction during cold rolling was set to 30 to 80%.

Continuous Annealing Conditions In continuous annealing of a cold-rolled steel sheet, first, in order to form a two-phase structure of [ferrite + austenite], heating is performed in a temperature range from Ac ₁ transformation point to Ac ₃ transformation point. However, if the heating temperature is too low, it takes too long to re-dissolve cementite, and if it is too high, the volume ratio of austenite becomes too large and the C concentration in austenite decreases.
It is desirable to soak at 800 to 850 ° C. And
After soaking, in order to gradually cool and grow ferrite to increase the C concentration in austenite, it is desirable that the cooling rate up to 700 ° C. be 10 ° C./s or less. Further, in the temperature range below 700 ° C. before entering the overaging treatment zone, it is desirable that the cooling rate is conversely 50 ° C./s or more in order to suppress the pearlite transformation of austenite.

In the overaging treatment zone, it is held for 30 seconds or longer (preferably 2 minutes or longer) between 550 and 350 ° C, or is gradually cooled between 550 and 350 ° C at a cooling rate of 100 ° C / min or less. However, it is necessary to promote the concentration of C in austenite while transforming austenite into bainite. Here, if the temperature for promoting the concentration of C exceeds 550 ° C., bainite transformation does not occur, while if it falls below 350 ° C., it becomes a lower bainite and C is not sufficiently concentrated in austenite. Preferably 400-450
℃. The cooling rate after the overaging treatment zone does not have to be particularly limited. Further, it goes without saying that similar annealing can be performed even in a continuous hot-dip galvanizing line having a constant temperature holding zone of a length corresponding to 30 seconds or more. In the alloying treatment during the plating treatment, if the maximum heating temperature is 600 ° C. or lower, heating is performed after the bainite transformation, so there is no particular adverse effect.

Next, the effects of the present invention will be described more specifically by way of examples.

EXAMPLE Steels having the respective compositional components shown in Table 1 were melted in an experimental vacuum furnace, and hot forged into 25 mm thick experimental slabs. Next, the slab is heated to 1250 ° C. in an electric furnace for 1 hour.
After soaking for an hour, it was rolled in a temperature range of 1150 to 930 ° C. for 3 passes by an experimental hot rolling mill to obtain a hot rolled sheet having a thickness of 5 mm. Then, as a coiling simulation, the steel sheet is subjected to forced air cooling or water spray cooling at 500 ° C. immediately after hot rolling.
It was cooled to the temperature of 1, and then inserted into an electric furnace maintained at the temperature and held for 1 hour, and then cooled at a cooling rate of 20 ° C./hr.

The hot-rolled sheet thus obtained was then descaled by surface grinding to obtain a cold-rolled base metal having a thickness of 3.2 mm, which was 1.4 mm.
Cold rolled to thickness. The cold-rolled sheet thus obtained was subjected to continuous annealing simulation in an infrared heating furnace at 10 ° C./s for 82 ° C.
Heat to 0 ° C, hold at that temperature for 40 seconds, then 70
Gradually cool to 0 ℃ at a cooling rate of 3 ℃ / s, then 50 ℃ / s
It was cooled to 400 ° C. at the cooling rate of, and held at that temperature for 3 minutes.

In these treatments, first, the surface roughness of pickled skin was measured at the hot rolled steel sheet stage, and the presence or absence of cracks on the surface and edges was checked. Next, a JIS No. 5 tensile test piece was sampled from the annealed steel sheet and subjected to a tensile test, and a hole expansion test and a wet box cycle corrosion test were also performed.

In the hole expansion test, the annealed plate was cut into 70 mm square, and a hole having a diameter of 10 mm was punched out with a clearance of 0.1 mm. A 33 mmφ punch was pushed in and the hole diameter at the crack initiation limit was measured.

The wet box cycle corrosion test was carried out by a method of exposing to the atmosphere for 3 months while spraying salt water twice a week and measuring the corrosion depth at that time. The results are shown in Table 2.

[0036]

[Table 1]

[0037]

[Table 2]

Incidentally, a part of the workability investigation results regarding test Nos. 3 to 6 in which the total addition amount of Si and Al in the steel was kept constant and Al was changed to 0.07 to 1.54% is shown in FIG. Fig. 3 shows a part of the workability investigation results regarding the test Nos. 8 to 11 in which the Mn content of No. 2 was changed, and the results of the wet box cycle test of Test Nos. 11 to 15 in which the Cu content was further changed. Each is shown in FIG.

The following can be seen from the results shown in Table 2 and FIG. That is, the steel sheet according to Test No. 3 in which only an amount of Al less than the specified value of the present invention was added had a smaller limiting hole expansion rate than the other three types of steel sheets, and the total elongation was increased when the amount of Al added was large. At the same time, the marginal hole expansion ratio is improved at the same time. However, in the steel sheet according to test number 7 in which the added amount of Al exceeds the specified value of the present invention, the total elongation is lower than that in the steel sheet according to test number 6 having substantially the same strength level.

Further, the steel sheets according to Test No. 8 contained only austenite stabilizing elements (Mn,
Since (Ni) is not added, it has been quenched and has a [ferrite + martensite] structure, so the marginal hole expansion ratio is extremely low. On the other hand, in the steel sheet according to Test No. 11, Mn was added in an amount exceeding the specified value of the present invention, and the amount of retained austenite was sufficient, but stress-induced stress was significantly high. Does not cause transformation and has a low total elongation.

On the other hand, the following can be seen from the results shown in Table 2 and FIG. In other words, as the amount of Cu added increases, the "corrosion depth measured by the wet box cycle test" increases with the increase.
However, when the Ni addition amount is less than the specified value of the present invention as in the steel sheet according to Test No. 15, cracks are observed in the hot rolled sheet.

The results of Test Nos. 16 and 19 show that the desired strength or ductility cannot be obtained when the C content in the steel is out of the specified range of the present invention.

As described above, the hole expandability, which was a drawback of the conventional "ferrite + bainite + retained austenitic steel" in which Si was added as a ferrite stabilizing element, was remarkably improved by improving the local ductility by adding Al. By adding Cu at the same time, the surface roughness and corrosion resistance of the hot-rolled steel sheet peculiar to Si-added steel can be significantly improved without impairing the ductility and cracks of the hot-rolled steel sheet.

Separately from these tests, a continuous hot-dip galvanizing test was performed on the cold-rolled steel sheet according to the present invention manufactured in this example. All of them showed good wettability of the plating, and the alloy It was confirmed that the chemical treatment was also satisfactory.

[0045]

[Summary of Effects] As described above, according to the present invention, a high-strength thin steel sheet having excellent ductility and good workability such as hole expandability, and at the same time excellent corrosion resistance and surface treatment property is obtained. The effect is extremely useful in industry, such as stable production.

[Brief description of drawings]

FIG. 1 is a graph illustrating the content ranges of Si and Al related to the steel sheet of the present invention.

FIG. 2 is a graph showing the Si and Cu content ranges of the steel sheet of the present invention.

FIG. 3 is a graph showing the effect of Al content and Mn content on workability, which was created based on the results of the examples.

FIG. 4 is a graph showing the effect of Cu content on the corrosion depth in a wet box cycle test, which was created based on the results of Examples.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/16

Claims (4)

[Claims] 1. The content of C, Si, Mn, P, S, Cu, Al and N is C: 0.05 to 0.3% by weight, Si: 2.0% or less, Mn: 0.5.
~ 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2.0% and Si (%) / 5 or more, Al: 0.1 to 2.0%, N: 0.01% or less, and Si (%) + Al (%) ≥ 0.5, the balance is a component composition consisting of Fe and unavoidable impurities, and has a structure containing residual austenite in a volume ratio of 5% or more. A high-strength thin steel sheet with excellent ductility, corrosion resistance, and surface treatment characteristics. 2. C, Si, Mn, P, S, Cu, Ni, Al and N
Content by weight C: 0.05-0.3%, Si: 2.0% or less, Mn: 0.05
~ 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2.0% and Si (%) / 5 or more, Al: 0.1 to 2.0%, N: 0.01% or less, Ni: Cu
(%) / 3 or more, satisfying Si (%) + Al (%) ≧ 0.5, Mn (%) + Ni (%) ≧ 0.5, and the balance being Fe and inevitable impurities, and A high-strength thin steel sheet excellent in ductility, corrosion resistance, and surface treatment, characterized by having a structure containing residual austenite in a volume ratio of 5% or more. 3. The content of C, Si, Mn, P, S, Cu, Al and N is C: 0.05 to 0.3% by weight, Si: 2.0% or less, Mn: 0.5.
~ 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2.0% and Si (%) / 5 or more, Al: 0.1 to 2.0%, N: 0.01% or less, and Si (%) + Al (%) ≧ 0.5 and the balance of the steel composition consisting of Fe and unavoidable impurities is hot rolled, rolled at 300 to 720 ° C., and then descaled. After cold rolling at 30 to 80%, in the subsequent continuous annealing or continuous hot dip galvanizing step, the material is heated to a temperature range of Ac ₁ transformation point or more and Ac ₃ transformation point or less, and 550 to 350 ° C. during the cooling.
Hold for 30 seconds or more in the temperature range of 100 ℃ / m
A method for producing a high-strength thin steel sheet excellent in ductility, corrosion resistance, and surface treatment including a retained austenite content of 5% or more in terms of volume ratio, characterized by slow cooling at a cooling rate of in or less. 4. C, Si, Mn, P, S, Cu, Ni, Al and N
Content by weight C: 0.05-0.3%, Si: 2.0% or less, Mn: 0.05
~ 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2.0% and Si (%) / 5 or more, Al: 0.1 to 2.0%, N: 0.01% or less, Ni: Cu
(%) / 3 or more, while satisfying the requirements of Si (%) + Al (%) ≧ 0.5, Mn (%) + Ni (%) ≧ 0.5 and the balance being Fe and unavoidable impurities After hot rolling, the roll is wound at 300 to 720 ° C., and after descaling, cold rolling is performed at a rolling reduction of 30 to 80%, and then in the subsequent continuous annealing or continuous hot dip galvanizing process, Ac ₁ transformation point or higher Ac Heating to a temperature range below ₃ transformation points, and 550 to 350 ° C during cooling
Hold for 30 seconds or more in the temperature range of 100 ℃ / m
A method for producing a high-strength thin steel sheet excellent in ductility, corrosion resistance, and surface treatment including a retained austenite content of 5% or more in terms of volume ratio, characterized by slow cooling at a cooling rate of in or less.