JP3350944B2 - High tensile cold rolled steel sheet with excellent ductility and corrosion resistance and manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] The present invention relates to a high-tensile cold-rolled thin steel sheet excellent in ductility and corrosion resistance, and suitable for a structural member formed into various shapes by press working, stretch flange working, and the like, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, various types of machinery and equipment have been strongly promoted in terms of both high performance and light weight. In response to this, many techniques for increasing the strength of steel sheets have been developed. In general, increasing the strength of a steel sheet involves deterioration of ductility, and it has been considered that it is extremely difficult to manufacture a steel sheet having both good workability and high strength.

[0003] Recently, however, it has recently been reported that 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 kept in that temperature region for a short time or stepwise cooling is performed. To partially transform austenite into bainite and finally [ferrite + bainite + residual austenite]
The structure consisting of
Stenite undergoes strain-induced transformation and shows large elongation. ''
Since the discovery of the phenomenon described above, attempts have been made to produce a high-ductility and high-strength steel sheet using this phenomenon.

For example, Japanese Unexamined Patent Publication (Kokai) No. 61-157625 discloses that in addition to 0.4 to 1.8% of Si (hereinafter,% representing the component ratio is referred to as% by weight) and 0.2 to 2.5% of Mn, A steel sheet containing one or more of P, Ni, Cu, Cr, Ti, Nb, V, and Mo [ferrite + austenite]
A method is disclosed in which after heating to a two-phase region, the mixed structure is realized by maintaining the mixture in a temperature range of 500 to 350 ° C. during cooling for 30 seconds to 30 minutes to obtain a high-tensile steel sheet exhibiting high ductility. .

Japanese Patent Publication No. 62-35461 discloses a method for producing a high-tensile steel sheet exhibiting high ductility, which comprises 0.7 to 0.7%.
A steel sheet containing 2.0% of Si and 0.5 to 2.0% of Mn is heated to a [ferrite + austenite] two-phase region in an annealing process, and then is cooled to 650 to 450 ° C. in a cooling process to obtain a total of 10 to 50%.
A method is disclosed in which a mixed structure steel sheet containing 10% or more by volume of ferrite and retained austenite in martensite or bainite is maintained at a constant temperature for 2 seconds.

However, in practice, even if a steel sheet having the above-mentioned mixed structure shows good ductility in a tensile test, it generally cannot always be said that the formability at the time of press working or the like is necessarily good. Was not satisfactory enough. For example, when the mixed-structure steel sheet is processed, most of the retained austenite is transformed into high-carbon martensite by strain-induced transformation in a later stage of deformation, so that local ductility is extremely poor. This phenomenon is remarkable in the case of stretch flange processing such as “hole expansion”, and as a result, the hole expandability of the mixed structure steel sheet is inferior to that of the conventional low carbon steel sheet. This is because, when punching is performed by punching, high-carbon martensite generated by strain-induced transformation is very hard, causing cracks, and the cracks propagate and propagate during subsequent hole expansion. It is considered.

[0007] Further, in the known mixed structure steel sheet manufacturing technology, it is necessary to change the C concentration in the steel in order to change the strength level. However, when the C concentration in the steel is reduced, the residual Au in the final product is reduced. As a result, the volume fraction of the austenite was reduced, and it was 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 slab heating in the hot rolling process, SiO ₂ and FeO undergo a eutectic reaction, resulting in a non-uniform low melting point scale and unevenness on the surface of the hot rolled sheet after pickling. Although these irregularities are slightly reduced by cold rolling, they still remain in the final product and cause deterioration of appearance.

In addition, a steel sheet reinforced with a transformation structure generally has a problem that its corrosion resistance is inferior to that of a steel sheet reinforced by solid solution. This problem is considered to be caused by the fact that a so-called "local cell" is easily formed in a composite structure steel sheet composed of a plurality of structures having different corrosion potentials, which leads to corrosion.

In view of the above, an object of the present invention is to provide excellent workability such as ductility, and good strength of corrosion resistance and appearance in which the strength level can be adjusted without a large change in the C content. The aim was to establish means that could provide stable high-strength steel sheets.

[0011]

The inventor of the present invention has made intensive studies to achieve the above object, and has obtained the following findings. (A) Investigation of the effect of Si and Al on the amount of austenite remaining in a continuously annealed steel sheet with the composition of 0.15% C-1.5% Mn as the standard composition. In both steel sheets to which Al and Al are added, approximately the same volume percentage of retained austenite is obtained. B) The total elongation of the Al-added steel sheet is slightly smaller than that of the Si-added steel sheet, but it is uniform from the total elongation. On the contrary, the local elongation excluding the elongation is large, indicating that the hole has good performance in terms of hole expandability. This is considered to be because in the steel sheet to which Al is added, since the retained austenite is stable, it hardly causes strain-induced transformation until it reaches a high strain region, and transforms after reaching a large deformation region. Although the cause of such a difference is unknown, it is presumed that the distribution form of the retained austenite changes.

(B) Further, the C concentration in the steel sheet and [Si + A
l), it is possible to arbitrarily change the tensile strength of the steel sheet while maintaining the same volume ratio of retained austenite by changing the balance between Si (%) and Al (%). is there.

(C) However, if the amount of Si is reduced by positively adding Al as a component for securing retained austenite, a smooth surface state is secured at the hot-rolled steel sheet stage, and the appearance deterioration of the final product is reduced. You will not be invited.

(D) Further, Mn and Cr are also elements known as stabilizing components of austenite. However, addition of Mn and Cr both to Al-added steel sheet or Si-added steel sheet is the same as that of residual austenite. Although it exerts a favorable effect on the securing and contributes to the improvement of the workability, when Cr is added, the effect that the corrosion resistance of the steel sheet is improved can also be secured.

(E) When manufacturing a cold-rolled annealed sheet from a steel in which Cr is contained in the Al- or Si-added steel to enhance corrosion resistance, devising the hot-rolling winding temperature and annealing conditions. This makes it easier to secure the amount of retained austenite which is favorable for ductility, and makes the productivity of corrosion-resistant high-strength steel sheets excellent in workability including ductility extremely stable.

The present invention has been completed on the basis of the above findings and the like, and is directed to " Cold-rolled thin steel sheet, C, Si, Mn, P,
Content of S, Cr, Al and N: 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, Cr: 0.5-5.0%, Al: 0.1 to 2.0%, N: 0.01% or less, and satisfy Si (%) + Al (%) ≥ 0.5, 7.0 ≥ Mn (%) + Cr (%) ≥ 1.0, with the balance being Fe and unavoidable impurities. A high strength and excellent ductility and corrosion resistance by having a composition having a composition containing at least 5% by volume of retained austenite in the composition. And "C, Si, Mn, P,
Content of S, Cr, Al and N: 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, Cr: 0.5-5.0%, Al: 0.1 to 2.0%, N: 0.01% or less, and satisfy Si (%) + Al (%) ≥ 0.5, 7.0 ≥ Mn (%) + Cr (%) ≥ 1.0, with the balance being Fe and unavoidable impurities. After hot rolling, a steel slab having the following composition is wound at 300 to 720 ° C., then, after descaling, cold rolled at a rolling reduction of 30 to 80%, and then subjected to continuous annealing or continuous hot-dip galvanizing. In the process, it is heated to a temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point, and 550 to 350 ° C. during the cooling.
Hold for at least 30 seconds in the temperature range of 400 ° C / m
By cooling slowly at a cooling rate of less than
% Of high-strength cold-rolled thin steel sheets containing high-strength, excellent ductility, and corrosion resistance by containing at least% of retained austenite ”.

Hereinafter, the reason why the composition of the steel sheet (slab) and the manufacturing conditions of the steel sheet are limited as described above in the present invention will be described together with the operation thereof.

A) Component composition C C is the most powerful austenite stabilizing element. To stabilize austenite at room temperature, it is necessary that at least 1% of C is contained in austenite. Although necessary, a sufficient austenite stabilizing effect can be secured at a content of 0.05% or more by selecting an annealing heat cycle. By adding a larger amount of C, a high-strength cold-rolled steel sheet having higher strength can be manufactured.
%, The steel sheet becomes too hard, and cannot be processed into a thin steel sheet in a normal sheet making process. Therefore, the C content is limited to 0.05 to 0.3%, but is preferably adjusted to 0.1 to 0.2%. Further, considering the weldability, 0.1 to 0.15% is most preferable.

Si Si is a ferrite stabilizing element, and increases the volume fraction of ferrite during annealing in the two-phase region to increase the equilibrium of the austenite phase.
Has the effect of increasing the concentration. In addition, Si also has a function of strengthening ferrite. However, if the content of Si exceeds 2.0%, the surface quality is significantly degraded due to the high Si scale peculiar to the Si-added steel sheet. Therefore, the Si content is set to 2.0% or less. In addition, the content of Si must be controlled in relation to the same ferrite stabilizing element, Al.
It is necessary to adjust the value of [Si (%) + Al (%)] to 0.5 or more.

Mn Mn is an austenite stabilizing element, and from this viewpoint, the Mn content is regulated by the sum of the Cr content having the same effect, and the content of [Mn (%) + Cr (%)] It needs to be adjusted so that the value is 1.0 or more. That is, the value of [Mn (%) + Cr (%)]
If it is less than 1.0, austenite is not stabilized. Just M
n content exceeds 4.0% or the value of [Mn (%) + Cr (%)] is 7.
If it exceeds 0, the steel sheet may be too hard and may not have sufficient ductility, so the upper limit of the Mn content is 4.0%.
And the value of [Mn (%) + Cr (%)] was in the range of 1.0 to 7.0. On the other hand, since Mn also has the effect of fixing S in steel as MnS and preventing hot brittleness, it is necessary to secure a content of at least 0.05% in order to ensure a desired effect on the effect. There is. FIG. 1 is a graph illustrating the content ranges of Mn and Cr relating to the steel sheet of the present invention.

[0020] P P is an element which is inevitably contained in steel as an impurity, it only low possible is preferred. In particular, if the content exceeds 0.1%, the ductility of the steel sheet deteriorates remarkably, so the P content is set to 0.1% or less.

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

Cr Cr is an element having an effect of stabilizing austenite like Mn, and thus Cr is added for this purpose. At the same time, Cr is an essential component for ensuring the desired corrosion resistance of the steel sheet. But also. If the Cr content is less than 0.5%, the desired corrosion resistance cannot be imparted to the steel sheet. On the other hand, if the Cr content exceeds 5.0%, it contributes to the stabilization of ferrite. In addition to making austenite unstable, pickling of the hot-rolled steel sheet becomes extremely difficult. Therefore, the Cr content is set to 0.5 to 5.0%, but is preferably adjusted to about 2 to 3%. As described above, the Cr content needs to be suppressed to 7.0% or less in total with the Mn content in order to ensure high ductility in the steel sheet.

Al As described above, Al is a ferrite stabilizing element like Si, and has an effect of increasing the C concentration of the austenite phase which is balanced by increasing the volume fraction of ferrite during the two-phase annealing. Have. However, the effect of stabilizing austenite is stronger than that of Si, and when the 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, if the Al content exceeds 2.0%, inclusions increase in the steel sheet, resulting in a decrease in ductility. Therefore, the Al content is 0.1 ~
2.0%, but to ensure the desired effect as a ferrite stabilizing element, the value of [Si (%) + Al (%)] must be
It is necessary to adjust it to be 0.5 or more. Note that FIG.
3 is a graph illustrating the content ranges of Si and Al according to the steel sheet of the present invention.

[0024] N N is also an element which is inevitably contained in steel as an impurity, the content thereof is preferably low. In particular, the N content
If it exceeds 0.01%, the amount of Al consumed as AlN is large.
Since the effect of the addition becomes small and the ductility deterioration due to AlN becomes noticeable, the upper limit of the N content is set to 0.01%.
It was decided.

B) Volume fraction of retained austenite The ductility of the steel sheet of the present invention as a final product depends on the volume fraction of retained austenite contained in the product.
%, The improvement of ductility due to the strain-induced transformation of austenite cannot be expected. In addition, since the ductility of a steel sheet improves with the increase of retained austenite, the volume fraction of retained austenite is preferably set to 10% or more.

C) Manufacturing Conditions Hot Rolling Winding Temperature In the case of the steel of the present invention, if it is rolled at a low temperature, it becomes hardened and hardened, so that it is difficult to remove the scale by subsequent pickling or cold rolling. Become. Conversely, when wound at a high temperature, cementite becomes coarse and soft, and pickling and cold rolling become easy. On the other hand, it takes too much time to re-dissolve cementite during soaking in annealing, and a sufficient amount of austenite is formed. Will not remain. Therefore, winding after hot rolling can avoid the above-mentioned disadvantages.
７720 ° C. However, since it is desired that pickling and cold rolling of the hot-rolled steel sheet are as easy as possible, it can be said that the winding temperature is preferably set at 550 to 650 ° C.

[0027] In the cold rolling reduction ratio reduction ratio of cold rolling is less than 30%, fully recrystallization ductility is deteriorated does not occur in the subsequent annealing step. on the other hand,
If the rolling reduction exceeds 80%, the load is excessively applied to the rolling mill. Therefore, the rolling reduction during cold rolling is set to 30 to 80%.

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

In the overaging treatment zone, the temperature is maintained between 550 and 350 ° C. for 30 seconds or more (preferably 2 minutes or more), or the temperature is slowly cooled between 550 and 350 ° C. at a cooling rate of 400 ° C./min or less. However, it is necessary to promote the concentration of C in austenite while transforming austenite to bainite. Here, if the temperature for promoting the concentration of C exceeds 550 ° C., bainite transformation does not occur. On the other hand, if the temperature is lower than 350 ° C., it becomes lower bainite and the concentration of C into austenite does not sufficiently occur. The cooling rate after the overaging treatment zone does not need to be particularly limited. Further, it goes without saying that the same annealing can be performed in a continuous hot-dip galvanizing line having a constant temperature holding zone having a length corresponding to 30 seconds or more. In the alloying process at the time of plating, if the maximum heating temperature is 600 ° C. or less, heating is performed after bainite transformation, so there is no particular adverse effect.

Next, the effects of the present invention will be described more specifically with reference to examples.

EXAMPLES First, steels having the respective component compositions shown in Table 1 were melted in an experimental vacuum furnace, and these were formed into a 25 mm thick experimental slab by hot forging. Next, the slab was placed in an electric furnace at 1250.
C. for 1 hour, followed by three-pass rolling in a temperature range of 1150 to 930 ° C. by a laboratory hot rolling mill to obtain a hot-rolled sheet having a thickness of 5 mm. Then, as a winding simulation, the steel sheet was subjected to forced air cooling or water spray cooling immediately after hot rolling.
The mixture was cooled to a temperature of 00 ° C., inserted into an electric furnace maintained at the temperature, maintained for 1 hour, and then cooled at a cooling rate of 20 ° C./hr.

Next, the obtained hot-rolled sheet was descaled by surface grinding to obtain a 3.2 mm-thick cold-rolled base material.
It was cold rolled to a thickness. The obtained cold-rolled sheet was subjected to continuous annealing simulation at 10 ° C./s in an infrared heating furnace.
Heat to 0 ° C., hold at that temperature for 40 seconds, then
Slowly cool to 0 ° C at a cooling rate of 3 ° C / s, then 50 ° C / s
At 400 ° C., and kept at that temperature for 3 minutes.

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 was also performed. In the hole expansion test, after cutting the annealed plate into 70 mm square and punching out a hole of 10 mm diameter with 0.1 mm clearance, a punch of 33 mmφ was pressed with 3 tons of wrinkle pressing force using a 36.5 mmφ inner diameter die. And the hole diameter at the limit of crack initiation was measured.

Further, the amount of retained austenite was measured for each annealed plate by X-ray reflection intensity measurement. Further, the corrosion resistance of each of the obtained annealed sheets was also investigated. Here, the corrosion resistance is measured by applying the polyester resin coating to the steel sheet and then performing cross-cutting and exposing it to the atmosphere for three years. At this time, the maximum width of the part where red rust is generated at the cross-cut part and the coating film is peeled off evaluated. Table 2 shows the results.

[0034]

[Table 1]

[0035]

[Table 2]

The total content of Si and Al in the steel was 1.6%
Test number 1 with front and back aligned and Al varied from 0.09 to 1.53%
Fig. 3 shows a part of the results of strength and workability investigations on Nos. 4 to 4, and also shows the test numbers 2 in which the total amount of Mn and Cr in the steel was adjusted to about 2.5% and Cr was changed to 0.2 to 2.5%. And 6 ~
FIG. 4 shows a part of the results of the corrosion resistance survey on No. 8.

The following can be seen from the results shown in Table 2 and FIG. That is, the steel sheet according to Test No. 1 in which only an amount of Al less than the specified value of the present invention is added has a smaller critical hole expansion rate than the other three types of steel, and the addition amount of Al increases. “Tensile strength × critical hole expansion rate” is improved. However, the steel sheet according to Test No. 5 in which the amount of Al added exceeded the prescribed value of the present invention had a small elongation.

On the other hand, the following can be seen from the results shown in Table 2 and FIG. That is, as the amount of Cr added increases, the maximum width of the red rust-generating coating peeled off at the cross cut portion after the air exposure test decreases, and thus the corrosion resistance of the steel sheet improves, but the mechanical properties hardly change.

Further, the result of Test No. 9 indicates that the elongation becomes small because the sum of Mn and Cr exceeds the specified value of the present invention. And the results of Test Nos. 10 and 13 are
It shows that the elongation is reduced when the C content in the steel is out of the range specified in the present invention.

[0040]

[Summary of Effects] As described above, according to the present invention, a high-tensile cold-rolled thin steel sheet having excellent ductility, good workability such as good hole expandability, and also excellent corrosion resistance can be obtained stably. Industrially extremely useful effects are obtained.

[Brief description of the drawings]

FIG. 1 is a graph illustrating a content range of Mn and Cr according to the steel sheet of the present invention.

FIG. 2 is a graph illustrating the content ranges of Si and Al according to the steel sheet of the present invention.

FIG. 3 shows the results of [Si +
5 is a graph showing the effect of Al content on “tensile strength × critical hole expansion rate” for a steel sheet having a content of [Al] of about 1.6%.

[FIG. 4] [Mn +
5 is a graph showing the effect of Cr content on “corrosion resistance” for a steel sheet having a content of [Cr] of about 2.5%.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/38 C22C 38/38

Claims (2)

(57) [Claims] 1. The contents of C, Si, Mn, P, S, Cr, Al and N are expressed by weight as follows: C: 0.05 to 0.3%, Si: 2.0% or less, Mn: 0.05 to 4.0%, P: 0.1% or less, S: 0.1% or less, Cr: 0.5 to 5.0%, Al: 0.1 to 2.0%, N: 0.01% or less, and Si (%) + Al (%) ≧ 0.5, 7.0 ≧ Mn (%) + Cr (%) ≧ 1.0, the balance being a component composition composed of Fe and unavoidable impurities, and having a structure containing 5% or more of retained austenite by volume. High-tensile cold-rolled thin steel sheet with excellent ductility and corrosion resistance. 2. The content of C, Si, Mn, P, S, Cr, Al and N in terms of weight percentage is as follows: C: 0.05 to 0.3%, Si: 2.0% or less, Mn: 0.05 to 4.0%, P: 0.1% or less, S: 0.1% or less, Cr: 0.5 to 5.0%, Al: 0.1 to 2.0%, N: 0.01% or less, and Si (%) + Al (%) ≧ 0.5, 7.0 ≧ Mn (%) + Cr (%) A steel slab satisfying ≧ 1.0 and having a balance of Fe and unavoidable impurities is rolled at 300 to 720 ° C. after hot rolling, and then, after descaling, is reduced to 30 to 80%. , And in a subsequent continuous annealing or continuous hot-dip galvanizing step, it is heated to a temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point, and 550 to 350 ° C. during the cooling.
Hold for at least 30 seconds in the temperature range of 400 ° C / m
A method for producing a high-tensile cold-rolled thin steel sheet comprising 5% or more by volume of retained austenite and having excellent ductility and corrosion resistance, characterized by gradually cooling at a cooling rate of not more than in.