JP2008231579A - Cold rolled steel sheet with ultrafine grain structure - Google Patents

Abstract

A high-tensile cold-rolled steel sheet having an ultrafine grain structure and excellent in strength-elongation balance and toughness among mechanical properties is provided.
SOLUTION: Steel components, particularly C, Si, Mn, Ni, Ti and Nb, are contained within the ranges satisfying the following formulas (1), (2) and (3), respectively, the balance being Fe and inevitable impurities The composition is such that the ferrite phase fraction is 65 vol% or more, the average crystal grain size of ferrite is 3.5 μm or less, and the martensite phase is 3 vol% or more of the whole structure.
637.5 + 4930 {Ti ^* + (48/93) ・ [% Nb]} ≧ A ₁ --- (1)
A ₃ ≦ 860 --- (2)
[% Mn] + [% Ni] ≧ 1.3 --- (3)
However, Ti ^* = [% Ti]-(48/32) · [% S]-(48/14) · [% N], A ₁ : Predicted value of A ₁ transformation point (° C) obtained by the formula , A ₃ : Predicted value of A ₃ transformation point (° C) obtained by the calculation formula
[Selection] Figure 1

Description

  The present invention is a cold-rolled steel sheet suitable for use as an automobile, household appliance, and further steel for machine structural use, particularly has an ultrafine grain structure, and has excellent strength, ductility, ductility, strength-ductility balance, etc. Further, the present invention relates to a high-tensile cold-rolled steel sheet having excellent stretch flangeability, or a high-tensile cold-rolled steel sheet having a low yield ratio, a small spring back during press formability, and excellent shape freezing property, and a method for producing the same. .

  Steel materials used as steel plates for automobiles, home appliances and mechanical structures are required to have excellent mechanical properties such as strength, workability and toughness. As a means for comprehensively improving these mechanical properties, it is effective to refine the structure. Therefore, many production methods for obtaining the microstructure have been proposed so far.

  Further, in recent years, with respect to high-strength steel, the goal is shifting to the development of high-strength steel sheets that can achieve both low cost and high-functional properties. Furthermore, steel plates for automobiles are required to have excellent impact resistance in addition to high strength from the viewpoint of occupant protection in the event of a collision.

  Furthermore, since many automotive parts made of steel sheets are formed by press working, excellent press forming is required as a steel sheet for automobile parts. In addition, parts that make up members, reinforcements, etc., which are skeleton members for securing the strength of automobile bodies, are often used for such applications because parts are often molded using stretch flange deformation. A steel sheet for automobile parts is also strongly required to have high stretch flangeability as well as high strength.

On the other hand, there is a problem that the springback after press forming is large as a problem when the strength of the steel sheet to be used is increased. If the amount of spring back is large, problems may occur when joining in combination with other parts, which may be a problem.
That is, when the material is molded in the mold and restrained, the bending portion receives a bending moment, but when the molded product is removed from the mold, the moment is reduced. The material deforms. This is a springback. Therefore, the springback increases as the strength of the material increases. In addition, when the strength level is the same, the higher the yield ratio (yield point / tensile strength), the greater the amount of springback. Therefore, in parts where springback is a problem, a low yield ratio is also strongly required. Yes.

  Under such circumstances, miniaturization of the structure of high-tensile steel is an important issue for the purpose of suppressing deterioration of ductility, toughness, durability ratio, and stretch flangeability associated with high tension.

As a means for refining the structure, a large reduction rolling method is conventionally known. The main point of the structure refining mechanism in this large rolling reduction method is to apply a large rolling to the austenite grains to promote γ-α strain-induced transformation (see, for example, Patent Document 1).
Moreover, the case where the control rolling method and the control cooling method are applied is also known (for example, refer patent document 2).

In addition, with regard to the raw steel, at least a part of the steel structure is made of ferrite, and the temperature is raised to a temperature range higher than the transformation point (Ac ₁ point) while plastic working is added thereto, or following this temperature rise, Ac Hold at a temperature range of _one or more points for a certain period of time, once part or all of the structure is reversely transformed into austenite, then ultrafine austenite grains appear, and then cooled to have an average crystal grain size of 5 μm or less, etc. There has been proposed a technique for forming a structure mainly composed of isotropic ferrite crystal grains (Patent Document 3).

All the techniques as described above are techniques aiming to refine crystal grains in the hot rolling process, that is, techniques aiming to refine the hot rolled sheet.
In this regard, for cold-rolled steel sheets that are thinner than hot-rolled steel sheets and are used in applications where the thickness accuracy and surface properties are severe, or where zinc or tin is plated on the surface, There are few techniques for refining crystal grains in the cold rolling-annealing process.

As a high-strength steel sheet having excellent workability, a dual-phase steel sheet (dual-phase steel sheet) made of a composite structure of ferrite and martensite is known. Since such a structure-strengthened steel sheet has hard martensite as the main strengthening factor, voids are likely to occur during processing due to a large hardness difference from the parent phase ferrite, and local elongation is low. Although it is slightly inferior to stretch flangeability, high stretch can be secured and the yield ratio can be lowered. Therefore, it is used when elongation and shape freezing are more important than stretch flangeability.
However, it is extremely difficult to obtain a material having both strength and elongation, with a product of tensile strength and elongation (TS × EL) exceeding 18000 MPa ·%, using conventional dual phase steel plates (dual phase steel plates). It was.

Japanese Patent Publication No. 5-65564 (Claims) JP 63-128117 A (Claims) JP-A-2-301540 (Claims)

The present invention was developed in view of the above-mentioned present situation, and by making it possible to make ultrafine grained cold-rolled steel sheets used as steel sheets for automobiles, home appliances and mechanical structures, strength, ductility, toughness It is another object of the present invention to propose a cold-rolled steel sheet having an ultrafine grain structure and an advantageous manufacturing method thereof, in which the balance between strength and ductility or stretch flangeability is improved.
In addition, the present invention achieves the ultrafine graining of a cold-rolled steel sheet, and in addition, by forming a dual-phase steel sheet composed of a composite structure of ferrite and martensite, strength, ductility, and shape freezing properties can be obtained. It is an object of the present invention to propose a low yield ratio cold-rolled steel sheet having an advantageously improved ultrafine grain structure together with its advantageous production method.

Now, as a result of intensive studies to achieve ultrafine graining on the cold-rolled steel sheet,
(1) By properly adjusting the alloying elements to control the recrystallization temperature of the steel sheet and the A ₁ and A ₃ transformation temperatures, by optimizing the recrystallization annealing temperature after cold rolling and the subsequent cooling rate, An ultrafine grain structure mainly composed of a ferrite phase with an average crystal grain size of 3.5 μm or less is obtained, and as a result, an excellent strength-elongation balance of TS × EL ≧ 17000 MPa ·% is obtained.
(2) Further, by controlling the cooling rate and adjusting the second phase, specifically, the remaining structure other than the ferrite phase is limited in the tensile strength (TS) by limiting the structure fraction other than the bainite phase. The product of hole expansion ratio (λ), which is an index of stretch flangeability, is an excellent strength-stretch flangeability balance of TS × λ ≧ 50000 MPa ·%.
(3) Furthermore, by controlling the cooling rate separately from the above (2) and adjusting the second phase, specifically, by setting the second phase to martensite, the yield ratio is TS x EL ≥18000 MPa ·%. It was found that an excellent strength-elongation balance (YR) ≦ 70% and a low yield ratio were obtained.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.03-0.16%,
Si: 2.0% or less,
Mn: 3.0% or less and / or Ni: 3.0% or less,
Ti: 0.2% or less and / or Nb: 0.2% or less,
Al: 0.01 to 0.1%,
P: 0.1% or less,
S: 0.02% or less and N: 0.005% or less, and C, Si, Mn, Ni, Ti and Nb are contained within the ranges satisfying the following formulas (1), (2) and (3) respectively, and the balance is It has an ultrafine grain structure characterized by having a steel structure with a composition of Fe and inevitable impurities, a ferrite phase fraction of 65 vol% or more, and an average crystal grain size of ferrite of 3.5 μm or less Cold rolled steel sheet.
Record
637.5 + 4930 {Ti ^* + (48/93) ・ [% Nb]} ≧ A ₁ --- (1)
A ₃ ≦ 860 --- (2)
[% Mn] + [% Ni] ≧ 1.3 --- (3)
However,
Ti ^* = [% Ti] − (48/32) ・ [% S] − (48/14) ・ [% N] --- (4)
A ₁ = 727 + 14 [% Si] -28.4 [% Mn] -21.6 [% Ni] --- (5)
A ₃ = 920 + 612.8 [% C] ² − 507.7 [% C] + 9.8 [% Si] ³
−9.5 [% Si] ² +68.5 [% Si] +2 [% Mn] ² −38 [% Mn]
+ 2.8 [% Ni] ² − 38.6 [% Ni] +102 [% Ti] +51.7 [% Nb] --- (6)
[% M] is the content of M element (mass%)

2. In the above 1, in the steel structure, the fraction of ferrite phase is 65 vol% or more, the average grain size of ferrite is 3.5 μm or less, and the remaining fraction other than ferrite phase is the fraction other than bainite phase. A cold-rolled steel sheet having an ultrafine-grained structure characterized by being limited to less than 3 vol% as a fraction of the entire structure.

3. In 1 above, the steel structure has a ferrite phase fraction of 65 vol% or more, an average crystal grain size of ferrite of 3.5 μm or less, and a martensite phase fraction of 3 vol% or more of the whole structure. A cold-rolled steel sheet having an ultrafine grain structure characterized by

4). In the above 1, 2 or 3, the steel plate is further in mass%,
Mo: 1.0% or less and
Cr: Cold-rolled steel sheet having an ultrafine grain structure characterized by having a composition containing one or two selected from 1.0% or less.

5. In any one of the above 1-4, the steel sheet is further in mass%,
A cold-rolled steel sheet having an ultrafine-grained structure characterized by having a composition containing 0.005% or less in total of one or more selected from Ca, REM and B.

6). % By mass
C: 0.03-0.16%,
Si: 2.0% or less,
Mn: 3.0% or less and / or Ni: 3.0% or less,
Ti: 0.2% or less and / or Nb: 0.2% or less,
Al: 0.01 to 0.1%,
P: 0.1% or less,
S: 0.02% or less and N: 0.005% or less, and C, Si, Mn, Ni, Ti and Nb are contained within the ranges satisfying the following formulas (1), (2) and (3) respectively, and the balance is A steel material having a composition of Fe and inevitable impurities is heated to 1200 ° C. or higher, then hot-rolled, and then cold-rolled, and then the temperature A ₃ (° C.) or higher obtained by the following formula (6) (A ₃ +30) A method for producing a cold-rolled steel sheet having an ultrafine-grained structure, characterized by performing recrystallization annealing at a temperature not higher than (° C.) and then cooling to at least 600 ° C. at a rate of 5 ° C./s or higher.
Record
637.5 + 4930 {Ti ^* + (48/93) ・ [% Nb]} ≧ A ₁ --- (1)
A ₃ ≦ 860 --- (2)
[% Mn] + [% Ni] ≧ 1.3 --- (3)
However,
Ti ^* = [% Ti] − (48/32) ・ [% S] − (48/14) ・ [% N] --- (4)
A ₁ = 727 + 14 [% Si] -28.4 [% Mn] -21.6 [% Ni] --- (5)
A ₃ = 920 + 612.8 [% C] ² − 507.7 [% C] + 9.8 [% Si] ³
−9.5 [% Si] ² +68.5 [% Si] +2 [% Mn] ² −38 [% Mn]
+ 2.8 [% Ni] ² − 38.6 [% Ni] +102 [% Ti] +51.7 [% Nb] --- (6)
[% M] is the content of M element (mass%)

7. In the above item 6, after the recrystallization annealing, after cooling to at least 600 ° C. at a rate of 5 ° C./s or more, the cooling time from 500 ° C. to 350 ° C. is further set to 30 seconds to 400 seconds. A method for producing a cold-rolled steel sheet having a fine grain structure.

8). 6. Ultrafine grain structure characterized in that, after recrystallization annealing, after cooling at least to 600 ° C. at a rate of 5 ° C./s or more, the cooling time from 500 ° C. to 350 ° C. is further set to less than 30 seconds. The manufacturing method of the cold-rolled steel plate which has this.

9. In the above 6, 7 or 8, the steel material is further in mass%,
Mo: 1.0% or less and
Cr: A method for producing a cold-rolled steel sheet having an ultrafine grain structure characterized by having a composition containing one or two selected from 1.0% or less.

10. In any one of the above 6 to 9, the steel material is further in mass%,
A method for producing a cold-rolled steel sheet having an ultrafine-grained structure, wherein the composition contains one or more selected from Ca, REM and B in a total content of 0.005% or less.

Thus, according to the present invention, a high-tensile cold-rolled steel sheet having an ultra-fine grain structure and excellent in mechanical properties, such as strength-elongation balance and toughness, as well as stretch flangeability, can be remodeled significantly. It can be manufactured stably without being accompanied.
In addition, according to the present invention, a high-tensile cold-rolled steel sheet that is particularly excellent in strength-elongation balance and excellent in shape freezing property can be stably obtained, both of which are extremely useful industrially.

The present invention will be specifically described below.
First, the reason why the composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.03-0.16%
C is not only an inexpensive strengthening component but also an element useful for generating a low-temperature transformation phase such as pearlite, bainite, or martensite. However, if the content is less than 0.03%, the effect of addition is poor. On the other hand, if the content exceeds 0.16%, ductility and weldability deteriorate, so C is limited to a range of 0.03% to 0.16%.

Si: 2.0% or less
Si, as a solid solution strengthening component, not only effectively contributes to improving the strength while improving the strength-elongation balance, but is also an effective element when martensiteizing the second phase. Addition deteriorates ductility, surface properties, and weldability, so Si was added at 2.0% or less. In addition, Preferably it is 0.01 to 0.6% of range.

Mn: 3.0% or less and / or Ni: 3.0% or less
Both Mn and Ni are austenite stabilizing elements, contributing to the refinement of crystal grains through the action of lowering the A ₁ and A ₃ transformation points, and the strength-ductility balance through the action of promoting the formation of the second phase. Has the effect of increasing However, addition of a large amount hardens the steel and, on the other hand, deteriorates the strength-ductility balance.
Mn also has an effect of detoxifying harmful solid solution S as MnS, so it is preferably contained in an amount of 0.1% or more. Moreover, it is preferable to contain Ni 0.01% or more.

Ti: 0.2% or less and / or Nb: 0.2% or less
By adding Ti and Nb, TiC, NbC and the like are precipitated, and the recrystallization temperature of the steel sheet is increased. For that purpose, it is preferable to contain each 0.01% or more. These may be added individually or in combination, but if they are added in excess of 0.2%, not only will the effect be saturated, but the amount of precipitates will increase and the ductility of the ferrite will increase. In any case, the content is 0.2% or less.

Al: 0.01 to 0.1%
Al acts as a deoxidizer and is an element effective for the cleanliness of steel, and it is desirable to add it in the deoxidation process. Here, if the amount of Al is less than 0.01%, the effect of addition is poor. On the other hand, if it exceeds 0.1%, the effect is saturated, and rather the production cost is increased, so Al is limited to the range of 0.01 to 0.1%.

P: 0.1% or less P is an element effective for achieving high strength at a low cost without causing a significant decrease in ductility. On the other hand, a large amount of P causes a decrease in workability and toughness. The content of was 0.1% or less. In addition, when the request | requirement with respect to workability and toughness is severe, since it is preferable to reduce P rather, it is desirable to set it as 0.02% or less in this case.

S: 0.02% or less S is not only a cause of hot cracking during hot rolling, but also exists as an inclusion such as MnS in the steel sheet and causes deterioration of ductility and stretch flangeability, so it is desirable to reduce it as much as possible. However, since up to 0.02% is acceptable, in the present invention, the S content is set to 0.02% or less. More preferably it is 0.005% or less

N: 0.005% or less Nitrogen causes aging deterioration and yield elongation, so the N content is limited to 0.005% or less.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
One or two selected from Mo: 1.0% or less and Cr: 1.0% or less
Both Mo and Cr can be included as a reinforcing component as required. However, addition of a large amount deteriorates the strength-ductility balance on the contrary, so that it is desirable to contain each at 1.0% or less. In order to sufficiently exhibit the above-described action, it is preferable to contain 0.01% or more of Mo and Cr.

0.005% or less total of one or more selected from Ca, REM and B
Ca, REM, and B all have the effect of improving workability and stretch flangeability by controlling the morphology of sulfides and increasing the grain boundary strength, and can be contained as required. However, excessive content may adversely affect cleanliness, so the total content is preferably 0.005% or less. In addition, in order to fully exhibit the above-described action, it is preferable to contain one or more selected from Ca, REM, and B in a total amount of 0.0005% or more.

As described above, the appropriate component composition range has been described. However, in the present invention, it is not sufficient that each component simply satisfies the above composition range. For C, Si, Mn, Ni, Ti and Nb, the following ( It is necessary to contain the formulas (1), (2), and (3) in ranges that satisfy each.
Record
637.5 + 4930 {Ti ^* + (48/93) ・ [% Nb]} ≧ A ₁ --- (1)
A ₃ ≦ 860 --- (2)
[% Mn] + [% Ni] ≧ 1.3 --- (3)
However,
Ti ^* = [% Ti] − (48/32) ・ [% S] − (48/14) ・ [% N] --- (4)
A ₁ = 727 + 14 [% Si] -28.4 [% Mn] -21.6 [% Ni] --- (5)
A ₃ = 920 + 612.8 [% C] ² − 507.7 [% C] + 9.8 [% Si] ³
−9.5 [% Si] ² +68.5 [% Si] +2 [% Mn] ² −38 [% Mn]
+ 2.8 [% Ni] ² − 38.6 [% Ni] +102 [% Ti] +51.7 [% Nb] --- (6)
[% M] is the content of M element (mass%)

The above A ₁ and A ₃ are predicted values of the Ac ₁ transformation point temperature (° C.) and Ac ₃ transformation point temperature (° C.) of the steel, respectively, and are components derived from the detailed basic experiments of the inventors. It is a regression equation. This predicted temperature (° C.) is particularly suitable when applied at a heating rate of 2 ° C./s or more and 20 ° C./s or less.
In the following, the reasons for limiting the above equations (1), (2), and (3) will be described in order.

Equation (1) is a condition that defines the amount of Ti and Nb added, and is based on the following findings.
In general, it is known that when Ti and Nb are added, TiC, NbC and the like are precipitated, and the recrystallization temperature of the steel sheet is increased. Therefore, a detailed investigation was made on the relationship between the addition amount of Ti and Nb and the recrystallization temperature Tre. When a certain amount or more of Ti and Nb was added, the recrystallization temperature became equivalent to A ₃ calculated by the above equation (6). It has been found.

Fig. 1 shows the relationship between the amount of Ti and Nb added and the recrystallization temperature Tre when various amounts of Ti and Nb were added in the steel composition adjusted to A ₁ = 700 ° C and A ₃ = 855 ° C. Results are shown. Here, the recrystallization temperature Tre was determined by performing continuous annealing in a laboratory with various heating temperatures, measuring hardness, and observing the structure. The Ti addition amount uses Ti ^* as the effective Ti amount for precipitating TiC, and the Nb addition amount uses 48/93 · [% Nb] to convert it into Ti. The relationship with the crystal temperature is shown.
According to the figure, when 637.5 + 4930 {Ti ^* + (48/93) · [% Nb]} reaches 700 ° C, that is, A ₁ or more, the recrystallization temperature Tre rapidly rises to near 855 ° C, that is, near A ₃ and becomes saturated. I understand that.

Next, in FIG. 2, A ₃ (varied by changing C, Si, Mn, Ni, etc.) under the condition of 637.5 + 4930 {Ti ^* + (48/93) · [% Nb]} ≧ A ₁ We are shown the results of examining the relationship between the recrystallization temperature Tre and a ₃ in the case of changing variously.
As shown in the figure, the recrystallization temperature Tre is equivalent to A ₃ under the condition of 637.5 + 4930 {Ti ^* + (48/93) · [% Nb]} ≧ A ₁ .

Although this reason is not necessarily clear, it can be considered as follows.
That is, when Ti and Nb are added and the recrystallization temperature rises due to the pinning force of these fine carbonitrides, and recrystallization cannot be performed in the ferrite (α) region below A ₁ , unrecrystallized processing α (Ferrite + austenite (γ)) at two-phase region temperature, recrystallized α nucleation from processed α and α → γ transformation nucleation at preferential nucleation sites such as high dislocation density and non-uniform deformation Conflicts occur. At this time, since the driving force of the α → γ transformation is larger than the driving force of recrystallization, γ nuclei are generated one after another in preference to the recrystallization α nucleation and occupy the preferential nucleation site.
Distortion (dislocation) is consumed by the atomic rearrangement in the α → γ transformation, and only the processed α having a low dislocation density remains, and recrystallization of the processed α becomes more difficult. The strain is completely eliminated only when the temperature rises, exceeds A _3, and enters the γ single phase region, and apparently recrystallization is completed. This is considered to be a mechanism in which the recrystallization temperature coincides with A ₃ and saturates.
Note that the α → γ transformation at this time nucleates from the processed α (many preferential nucleation sites), so that the γ grains at a high temperature at which recrystallization is completed are refined. Therefore, since it is effective to adjust the recrystallization temperature to A ₃ in order to refine the high-temperature γ grains during annealing, in the present invention, the formula (1), that is, 637.5 + 4930 {Ti ^* + (48/93) · [ % Nb]} ≧ Ti and Nb satisfying A ₁ were decided to be added. Preferably, Ti and Nb satisfying 637.5 + 4930 {Ti ^* + (48/93) · [% Nb]}> A ₁ are added.

Next, equation (2) is a condition for defining A ₃ .
As described above, when the expression (1) is satisfied, A ₃ substantially reaches the recrystallization temperature. Therefore, it is necessary to perform recrystallization annealing at a temperature equal to or higher than A ₃ . Here, when A ₃ exceeds 860 ° C., the recrystallization annealing temperature needs to be higher, γ grain growth is intense, and ferrite can be made into fine grains with an average crystal grain size of 3.5 μm or less. There wasn't. Therefore, it is necessary to satisfy A ₃ ≦ 860 ° C. Preferably, A ₃ <860 ° C., more preferably A ₃ ≦ 830 ° C.

Next, equation (3) is a condition that defines the amount of Mn or Ni, that is, the amount of austenite stabilizing element added.
As the austenite stabilizing element increases, the ferrite start line in the CCT diagram shifts to the low temperature side, which increases the degree of supercooling during the γ → α transformation in the cooling process after annealing and causes α to nucleate finely. The α crystal grains become finer. Here, in order to obtain fine grains with an average crystal grain size of 3.5 μm or less, [% Mn] + [% Ni] ≧ 1.3 (%) in addition to the above formulas (1) and (2) There was a need.
In addition, as long as [% Mn] + [% Ni] ≧ 1.3 (%) is satisfied, Mn and Ni may be added alone or in combination. [% Mn] + [% Ni]> 1.3 (%) is preferable, and [% Mn] + [% Ni] ≧ 2.0 (%) is more preferable.

Next, the steel structure will be described.
In the present invention, the steel structure has a ferrite phase with a volume fraction of 65% or more and an average crystal grain size of ferrite of 3.5 μm or less.
This is because, in order to obtain a cold-rolled steel sheet excellent in strength, ductility, toughness, and strength-elongation balance as expected in the present invention, it is necessary to have a steel structure mainly composed of fine ferrites, particularly a structure of a ferrite phase. This is because it is important that the fraction is 65 vol% or more and the average grain size of the ferrite phase is 3.5 μm or less.
Here, if the average crystal grain size of ferrite exceeds 3.5 μm, the strength-elongation balance deteriorates and the toughness decreases, and the structure fraction of ferrite, which is softer than other structures, reaches 65 vol%. Otherwise, the ductility is remarkably lowered and the workability becomes poor. A more preferable structural fraction of the ferrite phase is 75 vol% or more.

Further, as the second phase structure other than ferrite, martensite, bainite, pearlite and the like can be taken.
Here, when stretch flangeability is required, the steel structure may be a ferrite single-phase structure, but when a second phase other than ferrite exists, the remaining structure is a matrix phase ferrite. If the hardness difference is large, void forming sites are likely to be formed during processing. Therefore, a bainite phase structure having a small hardness difference is preferable.
If a large amount of phases such as martensite and pearlite other than ferrite and bainite are present, the hardness difference from the ferrite phase increases, or the phase itself adversely affects stretch flangeability, resulting in good stretch flangeability. However, it is acceptable if these phases are less than 3% in volume fraction.
Therefore, in particular, when the stretch flangeability is good, the steel structure has a ferrite phase fraction of 65 vol% or more, an average crystal grain size of ferrite of 3.5 μm or less, and a remaining structure other than the ferrite phase. Is a structure in which the structure fraction other than the bainite phase is limited to less than 3 vol% as a fraction of the entire structure.

Further, when a higher strength-elongation balance and shape freezing property are required, the second phase structure other than the ferrite phase should be a martensite phase, and the fraction of the whole structure should be 3 vol% or more. preferable.
This is because a low yield ratio can be effectively achieved by allowing martensite to be present in an amount of 3 vol% or more of the entire structure. In this respect, when bainite or pearlite is used, the yield point increases and the yield ratio increases, so that the shape freezing property decreases. Moreover, in pearlite, elongation tends to decrease, and it becomes difficult to obtain a good strength-elongation balance.

In order to obtain the martensite effect as described above, it is necessary to have a martensite phase of 3 vol% or more in terms of the fraction of the entire structure. More preferably, it is 5 vol% or more.
Further, the average crystal grain size of the martensite is preferably 2 μm or less. This is because when the average grain size of martensite exceeds 2 μm, the strength-elongation balance tends to be lower than when the average grain size of martensite is 2 μm or less, and TS × EL <18000 MPa ·% It is because it may become.
When the main phase is fine ferrite having an average crystal grain size of 3.5 μm or less as in the present invention, the average crystal grain size of martensite can generally be 2 μm or less.

Next, manufacturing conditions will be described.
The steel adjusted to the above-mentioned preferred component composition is melted in a converter or the like, and is made into a slab by a continuous casting method or the like. The slab, which is a steel material, is kept in a high temperature state or cooled and then heated to 1200 ° C. or higher, and then hot-rolled, and after cold rolling, the temperature A ₃ (° C.) or higher (A ₃ +30) Recrystallization annealing is performed at (° C) or lower, and then cooled to at least 600 ° C at a rate of 5 ° C / s or higher.

In the above process, when the heating temperature of the slab is less than 1200 ° C., TiC and the like are not sufficiently dissolved and coarsened, and the effect of increasing the recrystallization temperature and the effect of suppressing grain growth in the subsequent recrystallization annealing process are insufficient. Therefore, the heating temperature of the slab was set to 1200 ° C. or higher.
In the present invention, the hot finish rolling outlet temperature is not particularly limited, but if it is less than the Ar ₃ transformation point, α and γ are generated during rolling, and a band-like structure is likely to be formed in the steel sheet. Such a band-like structure remains even after cold rolling or annealing, and may cause anisotropy in material properties. Therefore, it is preferable that the finish rolling end temperature is equal to or higher than the Ar ₃ transformation point.

  The coiling temperature after the hot rolling is not particularly limited, but if it is less than 500 ° C or more than 650 ° C, precipitation of AlN for suppressing aging deterioration due to nitrogen tends to be insufficient, and the material properties tend to be inferior. It is in. Further, in order to make the structure of the steel sheet uniform and make the crystal grain size as fine and uniform as possible, the coiling temperature is preferably 500 ° C. or higher and 650 ° C. or lower.

Next, preferably after removing the oxidized scale on the surface of the hot-rolled steel sheet by pickling, it is subjected to cold rolling to obtain a cold-rolled steel sheet having a predetermined thickness. Here, pickling conditions and cold rolling conditions are not particularly limited, and may be according to a conventional method.
The rolling reduction during cold rolling is desirably 40% or more from the viewpoint of increasing the number of nucleation sites during recrystallization annealing and promoting the refinement of crystal grains. On the other hand, if the rolling reduction is increased too much, Since operation becomes difficult due to work hardening, the upper limit of the rolling reduction is preferably about 90% or less.

Next, the obtained cold-rolled steel sheet is heated to a temperature A ₃ (° C.) or more and (A ₃ +30) (° C.) or less shown in the above equation (6), and recrystallization annealing is performed.
In the steel material of the present invention whose components are adjusted as described above, since A ₃ is equivalent to the recrystallization temperature, recrystallization becomes insufficient at a temperature lower than A ₃ . On the other hand, at temperatures exceeding (A ₃ +30) (° C.), the growth of γ grains during annealing is severe and inappropriate for miniaturization, so A ₃ (° C.) or more and (A ₃ +30) (° C.) or less. Recrystallization annealing is performed. This recrystallization annealing is preferably performed in a continuous annealing line, and the annealing time for continuous annealing is preferably about 10 seconds to 120 seconds at which recrystallization occurs. This is because recrystallization is insufficient in a time shorter than 10 seconds, and a structure that remains stretched in the rolling direction remains, so that sufficient ductility may not be ensured. This is because the crystal grains are coarsened and a desired strength may not be obtained.

Subsequently, cooling is performed from the annealing temperature to at least 600 ° C. under a cooling rate of 5 ° C./s or more. Here, the cooling rate is an average cooling rate from the annealing temperature to 600 ° C. Here, when the cooling rate is less than 5 ° C./s, the degree of supercooling during the γ → α transformation during cooling is small, and the crystal grain size becomes coarse. Therefore, the cooling rate from the annealing temperature to 600 ° C. needs to be 5 ° C./s or more.
The reason why the end point temperature of the controlled cooling process is set to 600 ° C. is that the refinement of crystal grains is strongly influenced up to 600 ° C. at which the γ → α transformation starts. In the temperature range of less than 600 ° C., the second phase (martensite, bainite, pearlite, etc.) can be created by adjusting the cooling rate as appropriate.

  In particular, when stretch flangeability is required, the second phase is preferably bainite. For this purpose, following the above cooling, it is important that the cooling time in the temperature range from 500 ° C. to 350 ° C., that is, the residence time from 500 ° C. to 350 ° C. is 30 seconds or more and 400 seconds or less. If the cooling time is less than 30 seconds, the second phase tends to be martensite, the martensite structure fraction is 3 vol% or more, and the ductility / strength difference between the ferrite and the second phase becomes large. Degradation of sex. On the other hand, when the cooling time exceeds 400 seconds, the crystal grains tend to become coarse and the second phase tends to be brittle pearlite, the pearlite fraction becomes 3 vol% or more, and the stretch flangeability deteriorates. .

On the other hand, when particularly excellent strength-elongation balance and shape freezing property are required, the second phase is preferably martensite having a structure fraction of 3 vol% or more.
For this purpose, the cooling time in the temperature range from 500 ° C. to 350 ° C., that is, the residence time required for cooling from 500 ° C. to 350 ° C. is set to less than 30 seconds. When the cooling time is 30 seconds or more, the second phase tends to be bainite, the volume fraction of bainite is 3 vol% or more, and it becomes difficult to obtain a low yield ratio, and the shape freezing property is lowered.

  Thus, by using the above production method, a cold-rolled steel sheet having an ultrafine grain structure and having excellent strength-ductility balance and toughness, or even stretch flangeability, or particularly excellent strength-ductility balance and excellent shape. A cold-rolled steel sheet having freezing properties can be obtained.

Example 1
A slab having the component composition shown in Table 1 was heated under the conditions shown in Table 2 and hot-rolled according to a conventional method to obtain a 4.0 mm thick hot-rolled sheet. At this time, the hot finish rolling outlet temperature was not less than the Ar ₃ transformation point, and the coiling temperature was 600 ° C. This hot-rolled sheet is pickled and cold-rolled (rolling ratio: 60%) to form a 1.6 mm-thick cold-rolled sheet, and then recrystallized and annealed under the conditions shown in Table 2 in a continuous annealing line. To make a product plate.
The results of investigations on the structure, tensile properties, stretch flangeability and toughness of the product plate thus obtained are also shown in Table 3.

In addition, the structure was observed using an optical microscope or an electron microscope for the cross section in the rolling direction of the steel sheet, and the average crystal grain size of ferrite was determined, and the area ratio of each organization was determined and used as the volume ratio. Here, the average crystal grain size of ferrite was determined in accordance with the cutting method specified in JIS G 0552.
Tensile properties (tensile strength TS, elongation EL) were measured by a tensile test using JIS No. 5 test pieces collected with the rolling direction of the steel sheet as the longitudinal direction.
The stretch flangeability was evaluated by the following hole expansion test. That is, after punching a 10mmφ (D ₀ ) punched hole into a test piece collected in accordance with the Japan Iron and Steel Federation Standard JFST1001, the crack is penetrated through the plate thickness. The hole diameter D (mm) immediately after the measurement is obtained, and the following formula λ = {(D−D ₀ ) / D ₀ } × 100%
The hole expansion rate λ obtained in (1) was evaluated.
As for toughness, a ductile-brittle transition temperature vTrs (° C.) was measured by a method specified in JIS Z 2242 using a 2 mm V notch Charpy test piece.

As shown in Table 3, all of the inventive examples are fine, satisfying the ferrite phase structure fraction of 65 vol% or more and the average grain size of ferrite being 3.4 μm or less and 3.5 μm or less. In particular, steel plates Nos. 15 and 16 using G steel, in which the amounts of Ni and Mn are increased and A ₃ is greatly reduced, have an average crystal grain size of 0.9 μm and become ultrafine grains.
In addition, it can be seen that all the inventive examples have excellent strength-ductility balance with TS × EL of 17000 MPa ·% or more, and also have excellent ductility, with a ductile-brittle transition temperature of less than −140 ° C.
Furthermore, with respect to the remaining structure other than the ferrite phase, by restricting the structure fraction other than the bainite phase to less than 3 vol% as a fraction of the entire structure, the hole expansion workability is improved, and the strength-hole expansion balance (TS × λ) was 50000 MPa ·% or more, and it was markedly improved.

  In addition, when the second phase is martensite, the hole expansion rate of the remaining structure other than the ferrite phase is less than the case where the fraction of the structure other than the bainite phase is limited to less than 3 vol% in terms of the fraction of the entire structure. Although it is reduced, a particularly excellent strength-ductility balance of TS × EL ≧ 18000 MPa ·% is obtained.

On the other hand, in No. 10, since the heating temperature of the slab was low, TiC was coarsened, the effect of increasing the recrystallization temperature was suppressed, the effect of refining the crystal grain size of the steel sheet was not obtained, and the crystal grain size was increased. It was. TS x EL value is also small.
In No. 11, since the annealing temperature greatly exceeded the appropriate temperature (846 ° C.) of the present invention, the crystal grain growth was intense and the TS × EL value was inferior.
In No. 12, the annealing temperature was less than the lower limit of the present invention (816 ° C.), so recrystallization was not completed and the processed structure remained, resulting in inferior TS × EL value and increased ductile-brittle transition temperature. is doing.
In No. 13, since the cooling rate after annealing was low, the crystal grains became coarse and the strength decreased, leading to deterioration of the TS × EL value.
In No. 23, since the recrystallization temperature was less than A ₁ , the effect of refining γ grains by recrystallization annealing was not obtained, and coarse grains were formed, so that sufficient strength was not obtained.
In No. 24, since A ₃ exceeds 860 ° C., high temperature annealing is required. As a result, crystal grains grow and TS × EL value is inferior.
In No. 25, since the amount of (Ni + Mn) is small, the degree of supercooling during the γ-α transformation in the cooling process after annealing was small, and α could not produce fine nuclei, so the crystal grains became coarse .

Example 2
The slab having the component composition shown in Table 1 was heated under the conditions shown in Table 4 and hot-rolled according to a conventional method to obtain a 4.0 mm thick hot-rolled sheet. At this time, the hot finish rolling outlet temperature was not less than the Ar ₃ transformation point, and the coiling temperature was 600 ° C. This hot-rolled sheet is pickled and cold-rolled (rolling ratio: 60%) to form a 1.6 mm-thick cold-rolled sheet, and then recrystallized and annealed under the conditions shown in Table 4 in the continuous annealing line. To make a product plate.
Table 5 shows the results of investigation on the structure, tensile properties, and shape freezing property of the product plate thus obtained.

In addition, the structure is observed with an optical microscope or an electron microscope for the cross section in the rolling direction of the steel sheet, and the average crystal grain size of ferrite and martensite is obtained, and the area ratio of each organization is obtained and this is referred to as the volume ratio. did. Here, the average crystal grain sizes of ferrite and martensite were determined in accordance with the cutting method specified in JIS G 0552.
Tensile properties (yield point YP, tensile strength TS, elongation EL) were measured by a tensile test using a JIS No. 5 specimen taken with the rolling direction of the steel sheet as the longitudinal direction. The yield ratio YR was determined as YR = (YP / TS) × 100 (%).
The shape freezing property is obtained by performing hat bending with an opening length W ₀ = 100 mm, measuring the opening length W ₁ after processing, and determining the amount of change ΔW = W ₁ −W ₀ in the opening length. The tensile strength ratio ΔW / TS (mm / MPa) was used as an index. If this ΔW / TS is 0.040 or less, it can be said that the shape freezing property is excellent.

As shown in Table 5, invention examples both had an average crystal grain size of the ferrite occupying main phase i.e. 65% or more fraction is less and fine 3.1 .mu.m, and increase in particular the (Ni + Mn) content, A ₃ No. 14, which uses G steel with a reduced H, has an ultrafine grain with an average grain size of 0.9 μm. In all of the invention examples except No. 4, TS × EL is 18000 MPa ·% or more, and the strength-ductility balance is excellent, and the yield ratio YR is 70% or less, and the ratio ΔW / tensile strength ΔW / TS has a good shape freezing property of 0.040 or less.
In No. 4, although ferrite refinement could be achieved, the cooling time from 500 ° C to 350 ° C after annealing was long, so a bainite phase was formed in the second phase, and TS × EL was 17000 MPa · Although the yield ratio is over 70%, the yield ratio is over 70% and the shape freezing property is poor.
Further, as in Example 1, the toughness was also investigated, and as a result, it was confirmed that all of the inventive examples had a Charpy transition temperature of less than -140 ° C.

On the other hand, in No. 9, since the heating temperature of the slab was low, TiC was coarsened, the effect of increasing the recrystallization temperature was suppressed, the effect of refining the crystal grain size of the steel sheet was not obtained, and the crystal grain size was increased. Therefore, TS × EL value is inferior.
In No. 10, since the annealing temperature greatly exceeded the proper upper limit temperature (846 ° C.) of the present invention, the crystal grain growth was remarkable and the TS × EL value was inferior.
In No. 11, since the annealing temperature was less than the lower limit (816 ° C.) of the present invention, the recrystallization was not completed and the processed structure remained, so the TS × EL value was extremely inferior.
In No. 12, since the cooling rate after annealing was low, the crystal grains became coarse and the strength decreased, leading to deterioration of the TS × EL value.
In No. 21, since the recrystallization temperature T _X is less than A ₁ , the γ grain refinement effect due to recrystallization annealing cannot be obtained, and coarse grains are formed, so that sufficient strength and TS × EL value cannot be obtained. It was.
In No. 22, since A ₃ is higher than 860 ° C., high temperature annealing is required. As a result, crystal grains grew and TS × EL value decreased.
In No. 23, since the amount of (Ni + Mn) is small, the degree of supercooling during the γ-α transformation in the cooling process after annealing was small, and α could not produce fine nuclei. As a result, the strength and thus the TS × EL value decreased.

A ₁ = 700 ° C., in a steel composition which is adjusted to A ₃ = 855 ° C., which is a diagram showing the relationship between the Ti, Ti of changing variously the Nb addition amount, and the amount of Nb added and the recrystallization temperature. 637.5 + 4930 {Ti ^* + (48/93) · [% Nb]} ≧ A ₁ shows the relationship between A ₃ and recrystallization temperature Tre when A ₃ is variously changed under the condition of A ₁ is there.

Claims (3)

% By mass
C: 0.03-0.16%,
Si: 2.0% or less,
Mn: 3.0% or less and / or Ni: 3.0% or less,
Ti: 0.2% or less and / or Nb: 0.2% or less,
Al: 0.01 to 0.1%,
P: 0.1% or less,
S: 0.02% or less and N: 0.005% or less, and C, Si, Mn, Ni, Ti and Nb are contained within the ranges satisfying the following formulas (1), (2) and (3) respectively, and the balance is Fe and inevitable impurities composition, the ferrite phase fraction is 65 vol% or more, the average grain size of ferrite is 3.5 μm or less, and the martensite phase is 3 vol% or more of the whole structure A cold-rolled steel sheet having an ultrafine-grained structure, characterized in that the structure has a microstructure.
Record
637.5 + 4930 {Ti ^* + (48/93) ・ [% Nb]} ≧ A ₁ --- (1)
A ₃ ≦ 860 --- (2)
[% Mn] + [% Ni] ≧ 1.3 --- (3)
However,
Ti ^* = [% Ti] − (48/32) ・ [% S] − (48/14) ・ [% N] --- (4)
A ₁ = 727 + 14 [% Si] -28.4 [% Mn] -21.6 [% Ni] --- (5)
A ₃ = 920 + 612.8 [% C] ² − 507.7 [% C] + 9.8 [% Si] ³
−9.5 [% Si] ² +68.5 [% Si] +2 [% Mn] ² −38 [% Mn]
+ 2.8 [% Ni] ² − 38.6 [% Ni] +102 [% Ti] +51.7 [% Nb] --- (6)
[% M] is the content of M element (mass%) In claim 1, the steel sheet is further in mass%,
Mo: 1.0% or less and
Cr: Cold-rolled steel sheet having an ultrafine grain structure characterized by having a composition containing one or two selected from 1.0% or less. In Claim 1 or 2, a steel plate is further mass%,
A cold-rolled steel sheet having an ultrafine-grained structure characterized by having a composition containing 0.005% or less in total of one or more selected from Ca, REM and B.

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