JP2009167467A - High-strength cold-rolled steel sheet excellent in bending property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet having ≥780 MPa tensile strength with ductility and bending property. <P>SOLUTION: The steel sheet includes a chemical composition composed of 0.05-2.0% C, 0.2-2.0% Si, 0.5-2.8% Mn, 0.005-0.15% P, ≤0.02% S, 0.005-1.5% Al and the balance Fe with impurities, and has 60-80 vol% volume fraction of ferrite phase and further, the ratio of a nano-hardness (Hnf) of the ferrite phase and the nano-hardness (Hnm) of low-temperature transforming phase: Hnm/Hnf is made to be ≥3.0. In another embodiment, the volume fraction of the ferrite phase is 20-50 vol%, and further, the ratio of the nano-hardness (Hnf)of the ferrite phase and the nano-hardness (Hnm) of the low-temperature transforming phase: Hnm/Hnf can be made to be ≤2.0. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-strength cold-rolled steel sheet that is mainly used for press-molded automobile parts and the like and has a tensile strength of 780 MPa or more and excellent workability such as bendability.

High-tensile cold-rolled steel sheets with a tensile strength of 780 MPa used for automobile seat parts such as seat rails are required to have characteristics that balance strength, elongation, and bendability.
Today, it is possible to secure material strength, that is, steel plate strength by various strengthening methods, but the fact is that the workability decreases with increasing strength. As a high-strength thin steel sheet having excellent workability, a composite structure steel sheet having a low-temperature transformation phase such as ferrite and martensite or bainite as a second phase has been proposed. In particular, when emphasis is placed on elongation, it is known that a duplex phase steel (DP steel) may be used to increase the ferrite volume fraction, but it is known that the bendability deteriorates.

On the other hand, it has been clarified that when the bendability is emphasized and martensite or bainite single phase structure steel is used, the elongation deteriorates conversely.
Thus, despite the high strength steel sheet that combines both are strongly demanded, in the conventional high-strength steel sheet, the elongation, i.e. ductility and bendability of compatibility was difficult.

  Here, as a technique of high-strength steel sheet, for example, in Patent Document 1, by performing an overaging treatment at a relatively high temperature, the hardness of the low-temperature transformation phase is reduced, and the hardness difference with ferrite is reduced, thereby reducing the local ductility. A method of improving is disclosed. Moreover, this steel plate is ensuring the intensity | strength by raising the volume ratio of a low temperature transformation phase. However, there is no mention of the distribution of the low-temperature transformation phase. For example, there is no description about the possibility that local deformation becomes extremely non-uniform due to the non-uniform distribution of the low-temperature transformation phase due to segregation or the like, and the bendability deteriorates. there were.

  In Patent Document 2, excellent workability is ensured by using bainite as the low-temperature transformation phase and a single-phase structure including only the low-temperature transformation phase containing no ferrite. However, in order to secure the strength and to obtain a fine structure, it is necessary to add Mo, V, Ti, Mb, etc. as carbide forming elements, and there is a problem that the cost increases.

Furthermore, in Patent Document 3, as in Patent Document 1, the nano-hardness of each structure unit is specified together with the volume fraction of ferrite, bainite, retained austenite, and martensite, and the tensile strength is 980 MPa or more and the workability is improved. Excellent steel sheets are disclosed. However, in order to improve the hardenability, this steel sheet is excessively added with Mn in an amount of 2 to 4%. It is well known that Mn is prone to microsegregation during solidification, and when it is made into a steel plate, it segregates in a streak shape in the rolling direction. As a result, the low temperature transformation phase and the residual γ phase (austenite phase) are distributed in a streak pattern on the steel sheet, which adversely affects the bendability, but there is no mention of the danger in this steel sheet. Assuming that the effect of segregation could be avoided by some method, Patent Document 3 needs to control the volume fraction of the structure of four phases and the nano hardness of each phase in a complex manner such as bending and stretch flangeability. Therefore, there is a problem that manufacturing conditions are limited.
JP-A-63-293121 JP-A-3-277742 Japanese Patent Laid-Open No. 2005-2404

  The present invention solves the deterioration of local deformation characteristics of high-strength steel due to segregation, which is unavoidable when mass-producing steel materials, and achieves a tensile strength of 780 MPa or more and high-strength cooling excellent in workability such as bendability. It aims at providing about a rolled steel plate and its manufacturing method.

  The present inventors add Mn in order to easily obtain a low temperature transformation phase, but even if segregation of added Mn or the like cannot be avoided, it is a high strength steel having a high tensile strength of 780 MPa or more. Therefore, research was conducted to develop a thin steel sheet capable of achieving both elongation and bendability.

  Here, in addition to securing the strength and increasing the elongation due to the two-phase transformation in which the low-temperature transformation phase is dispersed, the present inventors are able to obtain local deformation characteristics, particularly bendability, which have not been obtained so far, based on the following knowledge. It was found that it can be given to.

  1) Striped surface irregularities (hereinafter referred to as streak defects) and surface flaws (hereinafter referred to as crack defects) accompanying bending deformation are caused by the difference in deformability between soft ferrite and hard phase. It is known that voids are generated at the interface between the two during bending deformation, and are caused by the propagation and propagation of cracks starting from these voids.

  2) The ferrite volume ratio (hereinafter referred to as ferrite ratio) is set to about 60% by volume or more, and at the same time, the ratio of each nano hardness of ferrite and low-temperature transformation phase (hereinafter simply referred to as “hardness ratio” or “hardness difference”) is 3 If the local strain due to bending deformation is absorbed mainly by ferrite by making it about twice or more, the strain propagation to the low-temperature transformation phase, which is hard and has low strain absorption capacity, becomes small, and the strain is uneven due to the local distribution of the low-temperature transformation phase. It becomes possible to avoid the occurrence of crystallization, and it is possible to prevent the occurrence of streak defects and crack defects accompanying bending deformation.

  3) When reducing the carbon content in consideration of weldability, if the ferrite ratio is reduced to about 50% by volume or less to ensure the strength, and at the same time, the hardness difference between the ferrite and the low-temperature transformation phase is about twice or less, bending deformation Can be absorbed not only in ferrite but also in a low-temperature transformation phase. In this case, since the fraction of the low temperature transformation phase is large, the local distribution of the low temperature transformation phase does not become apparent, and the occurrence of streak defects can be suppressed.

Here, the present invention is as follows.
(1) By mass%, C: 0.05 to 0.2%, Si: 0.2 to 2.0%, Mn: 0.5 to 2.8%, P: 0.005 to 0.15%, S: 0.02% or less, Al: 0.005 to 1.5%, balance Fe And a steel composition having a chemical composition comprising impurities, having a ferrite phase and a low-temperature transformation phase, wherein the volume fraction of the ferrite phase is 60% by volume or more and 80% by volume or less, and the nano hardness of the ferrite phase The ratio of Hnf to the nano-hardness Hnm of the low temperature transformation phase: A cold-rolled steel sheet having a tensile strength of 780 MPa or more, characterized by having a structure with Hnm / Hnf of 3.0 or more.

(2) The cold-rolled steel sheet according to (1) above, wherein the chemical composition has an Si content of 0.8% by mass or less, an Al content of 0.7-1.5% by mass, and a plated layer on the steel sheet surface.

(3) In mass%, C: 0.05-0.2%, Si: 0.2-2.0%, Mn: 0.5-2.8%, P: 0.005-0.15%, S: 0.02% or less, Al: 0.005-1.5%, balance Fe And a steel composition having a chemical composition comprising impurities, having a ferrite phase and a low-temperature transformation phase, wherein the volume fraction of the ferrite phase is 20% by volume or more and 50% by volume or less, and the nano hardness of the ferrite phase A ratio between Hnf and the nano-hardness Hnm of the low temperature transformation phase: a cold-rolled steel sheet having a tensile strength of 780 MPa or more, characterized by having a structure with Hnm / Hnf of 2.0 or less.

(4) The cold-rolled steel sheet according to (3), wherein the chemical composition has an Si content of 0.2 to 0.8 mass%, an Al content of 0.7 to 1.5 mass%, and a plated layer on the steel sheet surface.

(5) Any of the above (1) to (4), wherein the chemical composition further includes at least one of Cr: 0.01 to 1.0% and Mo: 0.01 to 1.0% by mass% instead of a part of Fe Cold-rolled steel sheet according to crab.

  By adjusting the ferrite rate in the DP structure and the nano hardness of the ferrite phase and the low-temperature transformation phase, even if the distribution of ferrite and the low-temperature transformation phase is uneven due to segregation such as Mn, elongation (ductility) and It is possible to provide a high-strength thin steel sheet that can achieve both bending characteristics. As a result, when manufacturing automobile seat rails, side sills (rockers), etc., it is possible to cope with thinner steel plates than before, which can greatly contribute to weight reduction and collision safety of automobile body parts.

 The reason why the steel composition and the manufacturing method are defined as described above in the present invention will be described more specifically.

(1) Two-phase structure and Mn segregation of ultra-high-strength steel Ultra-high-strength steel has a two-phase structure (hereinafter, DP structure) in which hard low-temperature transformation phase is dispersed in ductile-rich ferrite. Ensure strength and ductility at the same time. Further, in order to ensure hardenability, a large amount of Mn or the like is added, but segregation of Mn is unavoidable in ordinary mass-produced steel materials. Therefore, a low temperature transformation phase such as martensite constituting the DP structure is dispersed corresponding to such Mn segregation. Normally, Mn segregates in a streak shape parallel to the rolling direction, so that the low-temperature transformation phase is similarly distributed in a streak shape at intervals of about 100 μm.

(2) Streaky distribution of low-temperature transformation phase In DP steel, it can be seen that the region where the low-temperature transformation phase is dense due to the segregation and the rough region extend in the rolling direction at intervals of about 100 μm. Many. For example, in ultra-high-strength steels commonly used for automobiles, the ratio of ferrite phase and low-temperature transformation phase in DP steel is often about 50% to 50% by volume. Depending on the degree of Mn segregation and the cooling conditions, the proportion occupied is 60% by volume in the dense region and about 40% by volume in the rough region.

(3) Bending properties of steel plates High-tensile steel used for automobile seat rails and the like are required to have a predetermined tensile strength and at the same time have bending properties that do not cause streak defects or crack defects even at a bending radius of about R1 ^t . The ultra-high strength steel in which the low-temperature transformation phase is distributed in a streak shape described above exhibits a small elongation in a region where the low-temperature phase is dense, and a large elongation in a coarse region mainly composed of ferrite. For this reason, when bending deformation is applied perpendicularly to the streak direction, a dense region with a small extension protrudes from the surface, while a rough region with a large extension sinks into the surface, and as a result, as shown in FIG. As described above, uneven streak defects corresponding to the distribution of the low temperature transformation phase are generated on the steel sheet surface.

 Here, in FIG. 1, as shown in FIG. 1 (a), the low temperature transformation phase accumulates in the region where Mn segregation is observed. Therefore, when bending deformation is applied to this region, it is shown in FIG. 1 (b). As described above, since the low-temperature transformation phase has a small amount of strain, a decrease in thickness is observed in the peripheral region where the amount of strain is large, and this appears as uneven stripe defects.

(4) Improvement of strain absorption ability by increasing ferrite ratio The elongation of ferrite alone is 30% or more, which is more than 3 times that of as-quenched martensite alone. Therefore, if there is a certain amount of ferrite even at a low temperature transformation phase is dense, it is possible to absorb the distortion only in the ferrite, Ru is inevitable that the propagation of the distortion of the low-temperature transformation phase. Furthermore, if the hardness difference between the ferrite and the low temperature transformation phase is 3 times or more, this effect becomes more remarkable. As a result, the non-uniform distortion caused by bending is alleviated, and the occurrence of streak defects can be suppressed.

(5) Structure homogenization by reducing ferrite ratio On the other hand, by reducing the ferrite ratio and increasing the fraction of the low-temperature transformation phase, it is possible to avoid non-uniform distribution of the low-temperature transformation phase. As a result, non-uniform distortion due to bending is less likely to occur, and the occurrence of streak defects can be avoided. In this case, a certain amount of ferrite is required to ensure a constant elongation, but the hardness difference between the ferrite and the low temperature transformation phase should be about twice or less to facilitate strain propagation to the low temperature transformation phase. is there.

(6) Improvement of elongation and bendability by optimizing the ferrite ratio and the hardness ratio of ferrite and low-temperature transformation phase From the above, by adjusting the ferrite ratio as appropriate and optimizing the hardness ratio of ferrite and low-temperature transformation phase In addition, it is possible to obtain a steel sheet having excellent elongation and bending properties while ensuring a tensile strength of 780 MPa or more as an ultra high strength steel.

Although not particularly limited in the present invention, a so-called thin steel plate having a thickness of about 0.8 to 2.3 mm is usually suitable for bending forming.
“Elongation” refers to elongation in a so-called tensile test. In the present invention, it is usually about 13 to 18%, and “bending characteristics”
This refers to the bendability according to the V-block method in accordance with JIS Z 2248.

The steel plate according to the present invention exhibits an excellent effect in bending when forming a seat rail for an automobile, a side sill (rocker) and the like.
Here, as schematically shown in FIG. 2, in the present invention, when the ferrite ratio is sufficiently high, the hardness ratio of the two phases constituting the DP structure is set to 3 times or more, and the distortion caused by the bending deformation is the main. The generation of streak defects can be avoided by absorbing with ferrite (see the region (a) in FIG. 2).

  On the other hand, according to another embodiment, in the present invention, the ferrite ratio is reduced, the distribution of the low-temperature transformation phase is avoided, and the hardness ratio of the two phases is reduced to 2 or less, so that the low-temperature transformation phase is achieved. You may make it propagate distortion and suppress generation | occurrence | production of a stripe defect (refer the area | region (b) of FIG. 2).

As can be seen from these results, according to the present invention, good bendability can be ensured in any case.
(7) tissues by heat treatment, the optimization above ferrite ratio of hardness, A ₃ transformation point, by changing the holding time in the temperature range above the A ₁ transformation point, by controlling the pro-eutectoid ferrite amount Adjustment is possible. Further, as described later, by setting the cooling rate during quenching to a certain value or more, an arbitrary amount of ferrite can be obtained even at room temperature.

  Furthermore, the hardness difference between the ferrite and the low temperature transformation phase can be adjusted by setting the cooling rate to a certain value or more, the amount of carbon of the steel material, and further by an overaging treatment as will be described later.

<Steel component>
With respect to the elements added to the thin steel sheet shown in the present invention, the range and reasons for limitation will be described. In this specification, “%” indicating the steel composition is “% by mass” unless otherwise specified.

C: 0.05-0.20%
C is necessary for producing a low temperature transformation phase, and at least 0.05% needs to be added in order to ensure strength. However, if the added amount exceeds 0.20%, spot weldability deteriorates, so the upper limit of the added amount is 0.20%. Preferably, it is 0.06 to 0.18%.

Mn: 0.5-2.8%
Mn has the effect of increasing strength and toughness. Furthermore, since it also has the effect | action which improves hardenability, it is indispensable for the production | generation of low temperature transformation phases, such as a martensite. These effects are obtained when the Mn content is 0.5% or more. On the other hand, excessive addition of Mn not only makes it difficult to generate ferrite, but also tends to cause Mn segregation as described above. For this reason, the amount of Mn was limited to the range of 0.5 to 2.8%. Preferably, it is 1.2 to 2.7%.

P: 0.005-0.15%
P, as a strengthening element, preferably needs to be added in an amount of at least 0.005%, but if added in a large amount, the weldability may be deteriorated, so the content is made 0.15% or less. Preferably it is 0.02% or less.

Si: 0.2-2.0%
Si is an effective element for promoting the formation of ferrite and forming a DP structure. In order to exhibit this effect, the content of 0.2% or more is necessary. If the content exceeds 2.0%, the weldability is significantly reduced, so the upper limit is made 2.0%. In addition, a large amount of Si tends to deteriorate the adhesion and chemical conversion treatment of the plating. Therefore, when the content needs to be kept low as 0.8% or less, the generation of ferrite is insufficient. Therefore, the addition amount of Al is increased, and the addition amount of Si and Al may be adjusted to be 1.5% or less in total. A preferable range of Si is 0.3 to 1.5%.

Al: 0.005-1.5%
Al, like Si, is a ferrite-forming element. When the amount of Si added is small, it is indispensable for DP structure formation, and is added at 1.5% or less. Preferably, it is 0.7 to 1.2%.
As described above, the Al addition amount depends on the Si addition amount, but when the Si addition amount is limited in consideration of plating work, the Al addition amount is 0.5% or more, preferably 0.7% or more.
On the other hand, when there is no limitation on the amount of Si added, Al is mainly added for deoxidation. When Al is added for deoxidation, deoxidation is not sufficient if Al is less than 0.005%, and even if added over 0.5%, the effect is saturated and the effect is saturated, so 0.005-0.5% To do. Preferably, it is 0.01 to 0.1%.

S: 0.02% or less S is present as an impurity in the steel, but if it exceeds 0.02%, flaws are likely to occur during hot rolling and the surface properties deteriorate, so the content is made 0.02% or less.

Cr and / or Mo: 0.01 to 1.0% each
In this invention, the said element can be contained at least 1 type as needed. Cr and Mo are effective for strengthening ferrite. In order to ensure the effect, Cr and Mo are contained 0.01% or more. However, if Cr and Mo are contained in excess of 1%, the ductility is lowered, and these elements are expensive, so the upper limit is made 1.0%.

<Steel structure>
The cold-rolled steel sheet according to the present invention contains a ferrite phase and a low-temperature transformation phase such as martensite and bainite, and allows the presence of a phase such as residual austenite. And the balance, a cold-rolled steel sheet having a steel structure consisting of a low-temperature transformation phase. A typical example of the low-temperature transformation phase is martensite, but it may be a bainite phase or a mixed phase thereof.
(i) Ferrite ratio 60 volume% or more and 80 volume% or less and hardness ratio 3 times or less According to one aspect of the present invention, the ferrite ratio contained in the steel sheet is 60 volume% or more, and the ferrite and the low-temperature transformation phase The hardness ratio of the nano hardness is set to 3 times or more.

When the ferrite ratio is less than 60% by volume and the hardness ratio is less than 3 times, strain at the time of bending deformation is also introduced into the low temperature transformation phase. As described above, the low-temperature transformation phase is unevenly distributed in a streak shape due to segregation of Mn or the like, and therefore, there is a high possibility that a streak defect is generated with bending.
Furthermore, when the ferrite ratio exceeds 80% by volume, the volume fraction of the low-temperature transformation phase, which is a hard phase, decreases to less than 20% by volume, and it is difficult to increase the steel material strength to 780 MPa or more. Volume% or less.

(ii) Ferrite ratio of 20 volume% or more and 50 volume% or less and a hardness ratio of 2 times or less In another embodiment of the present invention, the ferrite ratio contained in the steel sheet is 50 volume% or less, and at the same time ferrite and a low-temperature transformation phase The hardness ratio of the nano hardness is set to 2 times or less. In this embodiment, when the ferrite ratio is more than 50% by volume and the hardness ratio is more than twice, or within the range where the ferrite ratio is less than 20% by volume and the hardness ratio is more than 2, non-uniform distortion occurs in the thin steel sheet during bending deformation. As a result, streak defects are generated. Furthermore, when the ferrite ratio is less than 20% by volume, although the bendability that is local deformation does not decrease, the total elongation that is a basic characteristic of the steel material decreases. Therefore, the lower limit of the ferrite ratio is set to 20% by volume.

That is, as shown as regions (a) and (b) in FIG. 2, if the ferrite volume ratio is more than 50% and less than 60%, the intended purpose cannot be achieved no matter how the hardness ratio is adjusted.
Here, “nano hardness” is calculated when the sharp diamond probe is pushed into the sample surface, the contact area between the probe and the sample is calculated from the depth of the push, and the indentation load is reduced by the contact area. The hardness determined by the deformation resistance. In this specification, as shown in the examples, data obtained by attaching a berkovich probe to a Triboscope manufactured by Hysitron, Inc. is shown.

As already mentioned, as a method of controlling the ferrite rate, after heating from the Ar ₃ point to the temperature range of the Ar ₁ point, and holding for a certain time, a composite structure consisting of a ferrite phase and a low temperature transformation phase To do. At this time, the ferrite rate can be arbitrarily controlled by selecting the holding temperature and holding time. Furthermore, after reaching this annealing temperature range, by immediately cooling to 50 ° C. or less, the hardness of the low-temperature transformation phase, which is a hard phase, can be secured, and by changing the cooling rate, the controlled ferrite rate can be increased even at room temperature. Can be obtained.

In addition, once the steel sheet structure is annealed at a temperature of Ac ₃ point or higher, and then cooled to a temperature range from the Ar ₃ point to the Ar ₁ point range, the ferrite phase and the low temperature transformation phase are maintained by holding for a certain period of time. It is good also as the composite structure which consists of.

In the present invention, the low-temperature transformation phase, occurs at the cooling stage after the cold-rolled steel sheet was heated and annealed to a temperature range of Ar ₁ to Ar _3, a low-temperature transformation phase at that time, martensite, bainite, etc. In addition, when the heat treatment at Ac _{3 to} 900 ° C. is performed prior to the annealing time, the ferrite ratio can be adjusted depending on the treatment time, and accordingly, the volume ratio of the low-temperature transformation phase changes accordingly.

  In the present invention, when the ferrite ratio is set to 60% by volume or more, the hardness ratio of the ferrite and the nano-hardness of the low temperature transformation phase is specified to be 3 times or more. The ferrite nano-hardness is about 3 to 4 GPa when the precipitation strengthening is not intentionally performed. On the other hand, it is known that the hardness of the low-temperature transformation phase is almost proportional to the amount of carbon. If 0.4% or more of carbon is contained, the hardness of the low-temperature transformation phase is 9 GPa, which is about three times that of ferrite.

  Considering weldability, the maximum carbon content of steel sheets for automobiles is about 0.2%. On the other hand, it is known that carbon in the ferrite phase is concentrated in the retained austenite by performing heat treatment in a two-phase region of ferrite and austenite.

    Therefore, if the ferrite ratio is 60% by volume or more, it is easy to solidly dissolve about 0.4% carbon in the retained austenite, and then a low-temperature transformation phase containing 0.4% or more carbon is obtained by cooling. be able to. As a result, it is possible to obtain a DP structure having a ferrite ratio of 60% by volume or more and having a low temperature transformation phase hardness of 3 times or more that of ferrite.

  On the other hand, when the ferrite ratio is 50% by volume or less, the concentration of carbon in the retained austenite is limited, the carbon content in the low temperature transformation phase does not increase so much, and the carbon content of the thin steel plate If the amount is 0.15%, the carbon content in the retained austenite is about 0.20%. In this case, the nano hardness of the low-temperature transformation phase obtained by subsequent cooling is about 5 GPa, and the hardness difference from the ferrite phase is a little less than 1.7 times, so that the ferrite ratio and hardness within the scope of the present invention can be obtained.

  Moreover, when the hardness ratio after cooling exceeds 2 times, the hardness difference with ferrite hardness should just be adjusted by performing an overaging process in the temperature range of 300-600 degreeC, and softening a low temperature transformation phase.

<Manufacturing method>
It is desirable that a steel having a desired component is melted by a known ordinary method such as a converter or an electric furnace, and a steel material such as a slab is formed by a continuous casting method. In place of the continuous casting method, an ingot casting method, a thin slab casting method, or the like may be used. This steel material is hot rolled to obtain a hot rolled steel sheet. In hot rolling, the cast steel material is not cooled to room temperature, but is charged directly into a heating furnace while being heated in a heating furnace, and then rolled directly, or directly heated after heat retention. Rolling may be performed, or rolling may be performed after the steel material is once cooled and then reheated.

  The hot-rolled steel sheet is pickled by a normal method and then cold-rolled to obtain a cold-rolled steel sheet. In order to make the austenite grain size during annealing finer and further improve the material stability, it is preferable that the rolling reduction of cold rolling is 30% or more.

The obtained cold-rolled steel sheet is annealed for 10 to 300 seconds in a temperature range of Ac _{3 to} 900 ° C. The annealing of the cold-rolled steel sheet is preferably continuous annealing, heated until the temperature of the cold-rolled steel sheet becomes an austenite single phase structure or higher (Ac ₃ or higher), and is 10 to 300 seconds in a temperature range of Ac _{3 to} 900 ° C. Annealing. Once the cold-rolled steel sheet has an austenite single-phase structure, it becomes a cold-rolled annealed sheet having a uniform microstructure.

If the annealing temperature is less than Ac ₃ , a band-like processed structure remains, the workability deteriorates, the mechanical properties fluctuate greatly, and the steel sheet lacks material stability. On the other hand, if the annealing time exceeds 300 seconds, the carbides become coarse, and the ferrite after annealing becomes coarse and the workability deteriorates.

From the above, the annealing conditions of the cold-rolled steel sheet were annealed for 10 to 300 seconds in a temperature range of Ac _{3 to} 900 ° C.
In this way, a cold-rolled steel sheet can be formed into a composite structure. At this time, as a method for controlling the ferrite ratio, as described above, the thin steel sheet is heated to a temperature range of Ar ₁ to Ar ₃ and is fixed for a certain period of time. By holding, a composite structure composed of a ferrite phase and an austenite phase is obtained. By selecting the annealing temperature and holding time at this time, the ferrite rate can be arbitrarily controlled. Specifically, since the austenite phase increases as the heating temperature increases and the ferrite phase increases as time elapses, the ferrite ratio can be adjusted by adjusting them. The ferrite phase is also preserved by subsequent cooling processes.

Further, after annealing at a temperature of Ac ₃ point or higher and 900 ° C. or lower, it is cooled to a temperature range of Ar _{1 to} Ar ₃ to form a composite structure composed of a ferrite phase and an austenite phase. If the cooling stop temperature is high, the austenite phase increases, and if the cooling rate is low, the ferrite phase increases. Therefore, the ferrite rate can be arbitrarily controlled by selecting the cooling stop temperature and the cooling rate at this time.

The cold-rolled steel sheet is cooled at an average cooling rate from Ar ₃ to Ar ₁ point to 200 ° C. at an average cooling rate of 100 ° C./second or more, preferably 800 ° C./second or more, thereby generating a low-temperature transformation phase. Ar ₃ to the average cooling rate from the temperature range of ₁ point Ar to 200 ° C. 100 ° C. / sec or more, preferably cooled at 800 ° C. / sec or more. When the average cooling rate is 100 ° C./second or less, a region where the quenching is locally incomplete occurs, and as a result, ferrite transformation and pearlite transformation occur, which causes deterioration of bendability. A predetermined ferrite ratio can be obtained by setting the average cooling rate to 100 ° C./second or more, desirably 800 ° C./second or more. The higher the average cooling rate, the better the bendability, but considering the cost, when water quenching is performed, the limit is about 1000 ° C./second. The final cooling temperature may be around 200 ° C. in consideration of practical use.

Furthermore, in the present invention, in order to obtain a hardness difference between an arbitrary ferrite phase and a low temperature transformation phase, an overaging treatment may be performed by holding for 30 to 600 seconds in a temperature range of 200 to 500 ° C. after cooling.

  By adjusting the steel material components, optimizing the hot rolling and annealing conditions after cold rolling, the average grain size of ferrite is 1-10 μm, and cold rolling with any ferrite ratio and hardness difference between ferrite and low temperature transformation phase A steel sheet can be obtained, and a high strength cold-rolled steel sheet having a tensile strength of 780 MPa or more and having both elongation and bendability can be obtained.

  In order to further improve the corrosion resistance, the cold-rolled steel sheet according to the present invention may be subjected to metal plating, for example, galvanizing such as hot dip galvanizing, galvannealed alloying, etc. In order to increase the adhesion, it is preferable to limit the Si content of the base steel sheet to 0.8% or less and the Al content to 0.7 to 1.5%.

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

Test specimens No. 1 to No. 4 containing the chemical components shown in Table 1 were made as trial products.
A steel having a desired component is melted by a known ordinary method such as a converter or an electric furnace, and a slab having a slab thickness of 270 mm is obtained by continuous casting by a continuous casting method, and after rough rolling by hot rolling after slab heating. The thickness was 40 mm after finishing rolling, and the thickness was 2.6 mm, and then cooled and wound up. Further, it was cold-rolled to a thickness of 1.5 mm, annealed at Ac ₃ points or more, and then subjected to continuous annealing.

  The thus obtained 1.5 mm thick cold rolled steel sheet was subjected to microstructure observation, tensile test, bending test, and nano hardness measurement. The ferrite ratio was calculated as an area ratio from a microstructure photograph. The test results are summarized in Table 2. The test method is as follows.

(Tissue observation)
Test specimens were collected from the cold-rolled annealed steel sheet, the cross section in the rolling direction and the structure of the cross section perpendicular to the rolling direction were observed with an electron microscope, and the ferrite ratio was determined by image analysis.

(Tensile test)
A JIS No. 5 tensile specimen was taken in a direction perpendicular to the rolling direction of the cold-rolled annealed steel sheet to obtain tensile properties (tensile strength TS, elongation El).

(Bending test)
A JIS No. 3 bending test piece with the direction perpendicular to the rolling direction as the longitudinal direction was taken from the cold-rolled annealed steel sheet, and the bendability was investigated by a V-block method in accordance with the provisions of JIS Z2248. The metal fitting with 90 ° apex was pushed in so that the burr was inside. The quality of bendability was judged visually, and the minimum bending radius was calculated by subtracting the minimum radius of the metal fitting that does not cause cracks or streaks after the test from the plate thickness and normalizing it. In addition, the pressing metal fittings having radii of 2, 1, 0.5, and 0 mm were used.

(Nano hardness)
The nano hardness of each of the ferrite and the low-temperature transformation phase was determined as follows. The measurement position was a cross section in a direction perpendicular to the rolling direction of the steel sheet, and the hardness of each phase at a position of 25 to 30 μm from the steel sheet surface to the sheet center direction was measured using a Hysitron Triboscope. Here, the load at the time of measurement was 500 μN, and the probe indentation depth was several tens of nm in any phase. The ten nano hardness values extracted for each phase were measured, and the average value was used.

  As is clear from Table 2, the minimum bending radius and the elongation during the tensile test vary depending on the steel structure, but assuming that it is used for press-molded automobile parts, the elongation is 10% or more and the bending radius R1t or less is satisfied. What to do was accepted.

It is explanatory drawing which showed typically the relationship between bendability and segregation. It is explanatory drawing which showed typically the range which can make favorable bendability and elongation compatible.

Claims (5)

In mass%, C: 0.05 to 0.2%, Si: 0.2 to 2.0%, Mn: 0.5 to 2.8%, P: 0.005 to 0.15%, S: 0.02% or less, Al: 0.005 to 1.5%, balance Fe and impurities Steel composition having a ferrite phase and a low temperature transformation phase, the volume fraction of the ferrite phase is not less than 60% by volume and not more than 80% by volume, and the nano hardness Hnf of the ferrite phase and the low temperature A ratio of the transformation phase to the nano hardness Hnm: a cold-rolled steel sheet having a tensile strength of 780 MPa or more, characterized by having a structure with Hnm / Hnf of 3.0 or more. The cold-rolled steel sheet according to claim 1, wherein the Si composition has a chemical composition of 0.2 to 0.8 mass%, an Al content of 0.7 to 1.5 mass%, and a plated layer on the steel sheet surface. In mass%, C: 0.05 to 0.2%, Si: 0.2 to 2.0%, Mn: 0.5 to 2.8%, P: 0.005 to 0.15%, S: 0.02% or less, Al: 0.005 to 1.5%, balance Fe and impurities Steel composition having a ferrite phase and a low-temperature transformation phase, the volume fraction of the ferrite phase being 20% by volume or more and 50% by volume or less, and the nano hardness Hnf of the ferrite phase and the low temperature A ratio of the transformation phase to the nano hardness Hnm: a cold-rolled steel sheet having a tensile strength of 780 MPa or more, characterized by having a structure with Hnm / Hnf of 2.0 or less. The cold rolled steel sheet according to claim 3, wherein the chemical composition has an Si content of 0.2 to 0.8 mass%, an Al content of 0.7 to 1.5 mass%, and a plated layer on the steel sheet surface. The cold rolling according to any one of claims 1 to 4, wherein the chemical composition further includes at least one of Cr: 0.01 to 1.0% and Mo: 0.01 to 1.0% by mass% instead of a part of Fe. steel sheet.

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