JP6121292B2 - High-strength steel sheet having high yield ratio and formability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel sheet having a high yield ratio and formability used for automobile parts and the like.

  As is well known, steel sheets used for automobile parts are required to be thin in order to improve fuel efficiency, and high strength steel sheets are required to achieve both thinning and securing of component strength. . Therefore, specifically, it is required to increase the tensile strength (TS) of the steel plate to 590 MPa or more. Furthermore, from the viewpoint of ensuring collision safety, it is also required to increase the yield strength (YS) of the steel sheet. Specifically, a steel sheet having a yield ratio (YR = YS / TS) of 0.70 or more is required. ing. Steel sheets are also required to have excellent formability in order to process into complicated parts. For this reason, when the strength of TS is 590 MPa or more, the product of TS and total elongation (EL) (TS × EL) is required to be 25000 MPa% or more. In addition, from the viewpoint of the part forming process, not only EL but also a steel sheet excellent in not only the hole expansion ratio (λ) but also λ60% or more, preferably λ70% or more is desired. Is anxious.

Conventionally, Patent Document 1 and the like can be cited as a response to these demands, and this document discloses a high-strength cold-rolled steel sheet having a high yield ratio and excellent formability of TS of 590 MPa or more. Has been.
However, the steel sheet disclosed in this document satisfies the above-mentioned levels of individual properties such as TS, YR and λ, but the product of TS and total elongation (EL) (TS × EL) is insufficient, and the strength At the same time, it has a problem that it cannot sufficiently meet the demand for a steel sheet whose formability is maintained at a high level.

On the other hand, a TRIP steel that uses a work-induced martensitic transformation of residual γ is conventionally known as a steel plate excellent in TS × EL. In general, TRIP steel is manufactured in a continuous annealing line (CAL), and after austenite is generated in the CAL soaking zone, it is cooled to a predetermined temperature and held at the predetermined temperature in an overaging zone (aus) Temper), a part of austenite is transformed into bainite, C is concentrated in the remaining austenite, and after final cooling, residual γ is obtained.
However, in the steel sheet obtained by this CAL, a part of austenite undergoes martensite transformation at the time of final cooling in the production process, and martensite (hereinafter simply referred to as martensite) that has not been tempered to the final structure. ) Is included. Due to the expansion during the martensitic transformation, movable dislocations are introduced into the matrix phase, so that YR is lowered, and martensite is very hard and thus becomes the starting point of fracture and lowers λ. Therefore, even in YR and λ, there is a problem that the demand for a high level steel sheet cannot be fully met.

JP 2008-156680 A

  Therefore, the object (problem) of the present invention is to solve the above-mentioned problems of the prior art, yield strength (YS), tensile strength (TS), yield ratio (YR), total elongation (EL), hole expansion rate ( A high-strength cold-rolled steel sheet having a high yield ratio and excellent formability with a high level of (λ) and the product (TS × EL) of tensile strength (TS) and total elongation (EL), and a method for producing the same. There is.

The invention described in claim 1
C: 0.10 to 0.30% (meaning mass%, the same shall apply hereinafter)
Si: 0.50 to 2.50%,
Mn: less than 1.00% (including 0%)
P: 0.100% or less (including 0%),
S: 0.010% or less (including 0%),
Al: 0.50 to 3.00%,
N: 0.0020 to 0.0100%,
The balance consists of iron and inevitable impurities,
Steel structure Ferrite fraction: 60-90%
Bainitic ferrite fraction: 10% or less (including 0%)
Residual γ fraction: 5% or more Tempered martensite fraction 5% to 30%,
Martensite fraction 5% or less (including 0%)
The average particle diameter of the equivalent circle diameter of cementite present in the tempered martensite is 200 nm or less, and the number density is 10 / μm ² or more,
It is a high-strength steel sheet having a high yield ratio and formability characterized by being.

The invention according to claim 2 has a high yield ratio and formability according to claim 1, wherein the number density of cementite particles having an equivalent circle diameter of 200 nm or more present in the tempered martensite is 1 piece / μm ² or less. High strength steel plate.

The invention according to claim 3
Furthermore,
V: 0.01-1.00%,
Ti: 0.01-0.30%,
Nb: 0.01-0.30%
A high-strength steel sheet having a high yield ratio and formability according to claim 1 or 2, comprising one or more of the above.

The invention according to claim 4
Furthermore,
Cr: 0.01 to 3.00%,
Mo: 0.01 to 1.00%,
Cu: 0.01-2.00%,
Ni: 0.01 to 2.00%,
B: 0.0001% to 0.005%,
A high-strength steel sheet having a high yield ratio and formability according to any one of claims 1 to 3, comprising one or more of the following.

The invention described in claim 5
Furthermore,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 0.0001 to 0.01%,
A high-strength steel sheet having a high yield ratio and formability according to any one of claims 1 to 4, comprising one or more of the above.

The invention described in claim 6
A steel sheet obtained by subjecting a steel material having the composition shown in any one of claims 1 to 5 to hot rolling and cold rolling,
After heating to a temperature of [0.6Ac1 + 0.4Ac3] or higher and soaking for 10 to 1000 s at that temperature, it is cooled to 750 to 550 ° C at a cooling rate of 15 ° C / s or less, and then 100 ° C / cooling to room temperature at a cooling rate of s or more,
Furthermore, the steel sheet is reheated to 300 to 480 ° C., tempered to hold at the temperature for 10 to 1000 s, and then cooled to room temperature. A high strength steel sheet having a high yield ratio and formability, It is a manufacturing method.

The invention described in claim 7
A steel sheet obtained by subjecting a steel material having the composition shown in any one of claims 1 to 5 to hot rolling and cold rolling,
After heating to a temperature of [0.6Ac1 + 0.4Ac3] or higher and soaking for 10 to 1000 s at that temperature, it is cooled to 750 to 550 ° C at a cooling rate of 15 ° C / s or less, and then 100 ° C / cooling to room temperature at a cooling rate of s or more,
Further, the steel sheet was reheated to 480 to 580 ° C. at a heating rate of 8 ° C./s or more, and tempered to hold it for 20 s or less at the temperature or within a temperature drop range of 10 ° C. or less from the temperature. Then, it is the manufacturing method of the high strength steel plate which has a high yield ratio and a formability characterized by cooling to normal temperature with the cooling rate over 5 degrees C / s.

According to the present invention, the yield strength (YS), tensile strength (TS), yield ratio (YR), total elongation (EL), hole expansion ratio (λ), and tensile strength (TS) and total elongation (EL) A high-strength cold-rolled steel sheet having a high yield ratio and excellent formability can be obtained with a high product (TS × EL). Specifically, to provide a high-strength cold-rolled steel sheet having the characteristics that YS is 420 MPa or more, TS is 590 MPa or more, YR is 0.70 or more, λ is 60% or more, and (TS × EL) is 25000 MPa% or more. Can do.
In addition, according to the present invention, the high-strength cold-rolled steel sheet can be manufactured relatively easily by an existing continuous annealing line (CAL) without adding a new manufacturing process. It is advantageous.

First, the specific technical idea of the present invention will be described in comparison with the conventional TRIP steel.
The inventors of the present invention have made extensive studies with a view to improving YR and λ and further improving TS × EL based on the conventional TRIP steel utilizing residual γ. As described above, in this steel, martensite is generated in the final cooling after austempering in the overaging zone, so that movable dislocations are introduced and YR is lowered, and hard martensite is the starting point of fracture. Since λ decreased, it was thought that YR could be improved by preventing the movable dislocations from moving easily, and λ could be improved by softening martensite. Furthermore, it was thought that if the bainitic ferrite contained in the steel was refined to a size of about martensite, the TS × EL balance could be improved by the refinement effect.

  This TRIP steel has a structure mainly composed of ferrite + bainitic ferrite + martensite + residual γ. As a result of investigation based on the above idea, this structure is mainly composed of ferrite + tempered martensite + residual γ. By contriving to change the organization, YR and λ can be improved, and at the same time, TS x EL balance can be improved.

  And as a basic means for obtaining this steel structure (structural control by heat treatment), the improvement of YR and λ may be tempered after the introduction of martensite. That is, when the dislocations are fixed by tempering, the movement of dislocations is suppressed until high stress is applied and YR is improved, and martensite is tempered and solid solution C is appropriately precipitated as cementite, thereby martensite. Since the deformability of the site is improved and it becomes difficult to become a starting point of destruction, λ is improved.

  However, in order to improve YR and λ by the above-described structure control in such conventional TRIP steel, it is necessary to add a tempering step after final cooling after austempering in existing CAL in the production of a steel sheet. Cost increases and practical application is difficult. Also, in the idea of concentrating C in austenite to create residual γ by utilizing the bainite transformation in austemper as in conventional TRIP steel, a large amount of bainitic ferrite is contained in the final structure. The TS × EL balance is inferior to a fine structure having a size of about martensite.

  Thus, based on the conventional TRIP steel, this steel structure is converted into the structure mainly composed of ferrite + tempered martensite + residual γ, and the idea of adding tempering for this purpose is the We faced new problems in the stage of actually making steel sheets that satisfy the characteristics.

  After that, the inventors continued to study in order to advantageously solve the above problems, and as a result, they found that it was possible to concentrate a large amount of C in austenite by means other than bainite transformation. That is, by adjusting the steel component composition appropriately, especially by adjusting the amount of Mn and Al, a sufficient amount of C can be concentrated in austenite in α-γ two-phase heating. I found it. By rapidly cooling this structure to room temperature, some austenite undergoes martensite transformation, and a structure mainly composed of ferrite, martensite and residual γ is obtained. By tempering this structure, mainly ferrite and tempered martensite are obtained. It was confirmed that a structure composed of residual γ was obtained and desired YR, TS × EL, and λ were obtained.

  Moreover, tempering, which is indispensable for the present structure control, is not performed after the final cooling after CAL austempering, and can be performed in an overaged zone, so there is no need for additional steps for tempering. Therefore, it can be relatively easily built in the existing CAL process, and the problem of the manufacturing cost is also solved.

  Furthermore, the inventors based on this new idea, knowledge, and basic structure control means, a specific steel sheet for obtaining a high strength steel sheet having a high yield ratio and formability, all of which have a high level of the above-mentioned characteristics. The present inventors have completed the present invention by repeating experiments and verifications on the component composition, structure conditions, and production conditions of the above. Hereinafter, the present invention will be described in detail.

[Component composition of this steel sheet]
First, the component composition of the steel sheet will be described from the basic elements. These elements are the objectives (problems) of the present invention: yield strength (YS), tensile strength (TS), yield ratio (YR), total elongation (EL), hole expansion ratio (λ), and tensile strength. It is positioned as an essential component composition for achieving the product (TS × EL) of (TS) and total elongation (EL).
In addition, unit% display of a component composition means the mass% altogether.

C: 0.10 to 0.30%
C is an essential element for obtaining a desired structure (tempered martensite + residual γ) and ensuring high (TS × EL), and 0.10% for effectively exhibiting such an action. It is necessary to add more. However, more than 0.30% is not suitable for welding. Preferably it is 0.13% or more, More preferably, it is 0.15% or more. Moreover, Preferably it is 0.25% or less.

Si: 0.50 to 2.50% or less Si is useful as a solid solution strengthening element and contributes to high strength. It is also effective in securing residual γ and increases TS × EL. In order to exhibit such an action effectively, it is necessary to add 0.50% or more. However, if it exceeds 2.50%, TS × EL decreases due to excessive solid solution strengthening. Preferably it is 0.80 to 2.20%, More preferably, it is 1.00 to 2.00%.

Mn: Less than 1.00% (including 0%)
Mn is an important element that requires special attention in the present invention, and acts as a solid solution strengthening element. However, when the amount of Mn solid solution is high, the amount of C that can be concentrated in austenite in the ferrite-austenite two-phase region decreases, The amount of residual γ obtained by rapid cooling from there decreases, and TS × EL decreases. In order to sufficiently increase the C concentration in austenite in the ferrite-austenite two-phase region, it is necessary to limit the amount of Mn to less than 1.00%. It is preferably less than 0.50% (including 0%), more preferably less than 0.10% (including 0%).

P: 0.1% or less (including 0%)
If it exceeds 0.1%, EL and λ deteriorate. Preferably it is 0.03% or less.

S: 0.01% or less (including 0%)
S is inevitably present as an impurity element, forms sulfide inclusions such as MnS, and is an element that lowers λ as a starting point of cracking. Preferably it is 0.005% or less.

Al: 0.50 to 3.00%
Al is a particularly important element in the present invention. The amount of C that can be concentrated in austenite in the ferrite-austenite two-phase region is increased, and the amount of residual γ obtained by quenching from there is increased, thereby improving TS × EL. Let In order to exhibit such an action effectively, it is necessary to add 0.50% or more of Al. However, if it exceeds 3.00%, the soaking austenitizing temperature becomes too high, and casting itself becomes difficult. Preferably it is 0.60% or more, more preferably 0.70% or more. Moreover, Preferably it is 2.00% or less, More preferably, it is 1.00% or less.

N: 0.0020 to 0.0100%
N is an element that is inevitably present, but forms a precipitate when combined with a carbonitride-forming element such as Al, and contributes to improvement in strength and refinement of the structure. In order to effectively exhibit such an action, N needs to be contained in an amount of 0.0020% or more. On the other hand, if the N content is too large, the amount of dissolved Al decreases and the amount of residual γ decreases, so the upper limit is limited to 0.0100%.

  In addition to the above basic component composition, in order to further improve the above-described various characteristics as the object (problem) of the present invention, desirable selective elements that can contain one or more of these element groups are as follows: Explained.

V: 0.01-1.00%,
Ti: 0.01-0.30%,
Nb: 0.01-0.30%
These elements (groups) are effective selection elements for securing high strength (TS, YS) by precipitation strengthening and refinement of the structure. If each is less than the lower limit, sufficient strength cannot be obtained, and if it exceeds the upper limit, the moldability is lowered and EL and λ are insufficient. If the addition amount is out of each range, desired YR and (TS × EL) cannot be achieved. Preferably, V is 0.02 to 0.50%, Ti is 0.02 to 0.20%, and Nb is 0.02 to 0.20%.

Cr: 0.01 to 3.00%,
Mo: 0.01 to 1.00%,
Cu: 0.01-2.00%,
Ni: 0.01 to 2.00%,
B: 0.0001 to 0.005%,
These elements (groups) are also reinforcing elements and are useful selection elements for securing desired (TS, YS). If each is less than the lower limit, sufficient strength cannot be obtained, and if it exceeds the upper limit, the moldability is lowered and EL and λ are insufficient. If the addition amount is out of each range, desired YR and (TS × EL) cannot be achieved. Preferably, Cr is 0.02 to 1.00%, Mo is 0.02 to 0.50%, Cu is 0.02 to 0.50%, Ni is 0.02 to 0.50%, and B is 0. 0.003 to 0.001%.

Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 0.0001 to 0.01%,
These elements (groups) are effective elements for controlling the form of inclusions such as sulfides and improving moldability (El, λ), and sufficient moldability cannot be obtained below the lower limit. If it exceeds, the effect is saturated and wasted. Preferably, Ca is 0.001 to 0.005%, Mg is 0.001 to 0.005%, and REM is 0.0005 to 0.005%.

[Structure of this steel sheet]
Next, a novel and characteristic steel sheet structure (steel structure) in the present invention will be described. In addition, the fraction of each structure | tissue shows the area fraction which observed the structure | tissue of the direction (plate width direction) perpendicular | vertical to the rolling direction of a steel plate in the (plate thickness) / 4 position. The method for measuring and evaluating the tissue will be described later.

Ferrite fraction: 60-90%
Since ferrite (α) is a soft phase, it is effective to increase EL and λ, and it is necessary to increase C to concentrate C in austenite to obtain residual γ in the ferrite-austenite two-phase region. . If it is less than 60%, TS × EL cannot be secured. If it exceeds 90%, TS590 MPa or more cannot be secured. Preferably it is 63 to 87%, more preferably 65 to 85%.

Bainitic ferrite fraction: 10% or less (including 0%)
When bainitic ferrite (BF) is generated, tempered martensite, which is a finer lath structure, is reduced, and the TS × EL balance is lowered. Therefore, to improve TS × EL, it is necessary to limit the amount to 10% or less. Preferably it is 8% or less, More preferably, it is 7% or less.

Residual γ fraction: 5% or more Residual γ (γR) enhances TS × EL by transformation-induced martensite transformation during deformation. Preferably it is 6% or more, More preferably, it is 8% or more. Since the residual γ is transformed into hard martensite, λ decreases when introduced excessively. Therefore, it is 25% or less, preferably 20% or less.

Tempered martensite fraction: 5-30%
Tempered martensite (TM) contributes to the improvement of TS × EL because it has a fine lath structure and is appropriately softened and has ductility. If it is less than 5%, TS590 MPa cannot be secured. On the other hand, if it exceeds 30%, the strength becomes too high and TS × EL is lowered. Preferably it is 8-25%, More preferably, it is 10-20%.

Martensite fraction: 5% or less (including 0%)
Since martensite (M) that has not been tempered becomes a starting point of fracture at the time of deformation, it is necessary to significantly reduce λ. Preferably it is 4% or less, More preferably, it is 3% or less.

The average equivalent circle diameter of cementite particles present in tempered martensite is 200 nm or less, and the number density thereof is 10 particles / μm ² or more. The particle size of cementite particles in tempered martensite is reduced and the number thereof is increased. Therefore, it is possible to increase the deformability of the tempered martensite to suppress the starting point of fracture and effectively increase λ. If the circle equivalent average particle diameter exceeds 200 nm or the number density is less than 10 / μm ² , the cementite becomes coarse and becomes the starting point of fracture, or the solid solution C is high and the deformability of tempered martensite is insufficient. Since λ decreases, each is defined as above.

The number density of the cementite particles having an equivalent circle diameter of 200 nm or more present in the tempered martensite is 1 / μm ² or less. Although not essential in the present invention, the cementite present in the tempered martensite is coarser than 200 nm. Since cementite may cause rupture to lower λ, it is preferable to limit as described above. More preferably, those having an equivalent circle diameter of 200 nm or more are 0.8 pieces / μm ² or less, and more preferably 0.6 pieces / μm ² or less.

  Although the steel structure in the present invention is as described above, it is desirable not to include other structures (such as pearlite) other than the essential structure, but a very small amount of 5% or less of the entire structure. If so, it is allowed to contain it.

[Production method of this steel sheet]
Next, the manufacturing method of this steel plate is demonstrated. However, from the melting of steel, which is the material of the steel plate, to casting, hot rolling and cold rolling of the steel material (slab) are based on the conditions normally adopted, so this is omitted here, and the features of the present invention The following description will focus on various conditions relating to heat treatment of the steel sheet after cold rolling, that is, soaking, tempering, heating before and after, and cooling.

(Heating and soaking conditions)
“Heating, soaking temperature: [0.6Ac1 + 0.4Ac3] or more”
First, the steel sheet after cold rolling is heated and soaked (continuous annealing is preferable, but batch annealing may be used), but it is necessary to heat to [0.6Ac1 + 0.4Ac3] or more and perform soaking at this temperature. There is. That is, the soaking temperature (T1) is set to [0.6Ac1 + 0.4Ac3] or higher. If the temperature is lower than this temperature, the cementite cannot be dissolved to increase the C concentration in the austenite, and a large amount of undissolved cementite remains, so that sufficient residual γ cannot be obtained and TS × EL characteristics cannot be secured. .

“Soaking time: 10 to 1000 s”
The soaking time (t1) maintained at the soaking temperature is 10 to 1000 s. If it is less than 10 s, a large amount of undissolved cementite remains and a sufficient residual γ cannot be obtained (TS × EL), so that sufficient characteristics cannot be secured. Further, if it exceeds 1000 s, the productivity is hindered.

(Cooling conditions after soaking)
“First cooling to 750 to 550 ° C. at a cooling rate of 15 ° C./s or less after soaking”
In the present invention, after the soaking, two-stage cooling of the first cooling and the second cooling is performed to room temperature. The first cooling is performed in a temperature range from 750 to 550 ° C. after soaking, that is, a temperature range from the soaking temperature to 750 to 550 ° C. at a cooling rate of 15 ° C./s or less (first cooling rate: CR1)). Cool down. After cementite is dissolved by high-temperature soaking at the time of soaking, the ferrite fraction is increased by this slow cooling, and the austenite is made to have a high C until it becomes residual γ even at room temperature. When it is gradually cooled to a temperature range of less than 550 ° C., austenite is transformed into pearlite, so the amount of residual γ decreases. If it exceeds 15 ° C./s, ferrite transformation does not occur sufficiently, and C concentration to austenite becomes insufficient, and the amount of residual γ decreases. Preferably, the temperature range from the soaking temperature to 660 ° C. is 13 ° C./s or less, more preferably the temperature range from the soaking temperature to 670 ° C. is cooled at a cooling rate of 10 ° C./s or less. In order not to impede productivity, the lower limit of the cooling rate is 0.5 ° C./s.

“Second cooling to room temperature at a cooling rate of 100 ° C./s or higher after the first cooling”
In the second cooling, the temperature range from the first cooling to room temperature, that is, the temperature range from 750 to 550 ° C. rapid start temperature (T2) to room temperature is 100 ° C./s or more (second cooling rate: CR2 ) By this rapid cooling, a part of austenite is converted into residual gamma while undergoing martensitic transformation while avoiding pearlite transformation and bainite transformation. The cooling is preferably performed at a cooling rate of 250 ° C./s or more, more preferably 500 ° C./s or more.

(Reheating, tempering and cooling conditions)
There are two methods for this reheating, tempering and cooling conditions. Therefore, since the heating, soaking, and cooling conditions after soaking described above are common, one of the two reheating, tempering and cooling conditions described below is selected for these conditions. And combine them. Condition A is a condition set for a method of tempering at a low temperature over time, and condition B is a condition set for a method of tempering at a high temperature in a short time.

<Reheating, tempering and cooling conditions A>
“Tempering temperature: 300-480 ° C.”
The steel sheet brought to room temperature by the second cooling is further reheated and tempered. At that time, it is necessary to reheat to 300 to 480 ° C. and to temper in this temperature range. That is, the tempering temperature is 300 to 480 ° C. This is because the martensite up to room temperature is tempered and the movable dislocations introduced during the martensite transformation are fixed. At low temperatures where the tempering temperature is less than 300 ° C., YR, TS × EL, and λ all decrease. If the temperature exceeds 480 ° C., TS cannot be secured at 590 MPa or more, and the residual γ is decomposed, thereby making it impossible to increase TS × EL. In addition, in this condition A, although it does not specifically limit about the heating rate (temperature rising rate rate) at the time of reheating from normal temperature to the said tempering temperature 300-480 degreeC, In the case of tempering of this seed steel plate Usually, a heating rate of, for example, 1 ° C./s or more and less than 8 ° C./s is sufficient, and it is not necessary to heat at high speed as in Condition B described later.

"Tempering time: 100-300s"
The tempering time maintained at the above tempering temperature needs to be 100 to 300 s. This is also because the martensite up to room temperature is tempered and the movable dislocations introduced during the martensite transformation are fixed, for the same reason as the tempering temperature. Both YR, (TS × EL), and λ decrease in a short time when the tempering time is less than 100 s. If the same time exceeds 300 s, it becomes difficult to maintain TS at 590 MPa or more, and the residual γ is decomposed, thereby making it impossible to improve (TS × EL).

"Cooling conditions after tempering"
The cooling condition after the tempering (third cooling) is not particularly limited, but the ordinary cooling (relative cooling) may be used.

<Reheating, tempering and cooling conditions B>
“Heating rate of reheating: 8 ° C / s or more”
The steel sheet brought to room temperature by the second cooling is tempered by reheating to 480 to 580 ° C., which is higher than the condition A, and heating at that time is performed at a high speed of 8 ° C./s or more. . By reducing the residence time at 300 to 480 ° C. at which coarse cementite particles are easily formed, the number density of cementite particles having a circle-equivalent diameter of 200 nm or more present in tempered martensite is 1 / μm ² or less to further increase λ. . A heating rate (temperature increase rate) of less than 8 ° C./s is not preferable because λ decreases.

“Tempering temperature: 480-580 ° C.”
Under these conditions, the temperature is tempered at 480 to 580 ° C. This is because the tempering of martensite is insufficient in the tempering temperature range of Condition A and EL and λ are lowered by the rapid temperature rise to avoid the formation of coarse cementite particles.

“Tempering time: 20 s or less at tempering temperature or within a temperature drop range of 10 ° C. or less from tempering temperature”
The tempering time maintained at the above tempering temperature is a short time of 20 s or less. However, if the tempering temperature is within a range of 10 ° C. or less even if the tempering temperature is lowered in consideration of the case where the tempering temperature is not maintained at a constant temperature from the start to the end and there is a temperature drop, the above 480 to 480 It is assumed that the material has been tempered at a temperature equivalent to the tempering temperature selected at 580 ° C., and the tempering time including this case is 20 s or less. That is, it means that the tempering is terminated by holding for 20 seconds or less at a tempering start temperature to a tempering start temperature of −10 ° C.
When this tempering time exceeds 20 s, coarse cementite particles are generated and λ decreases. Moreover, TS590MPa cannot be ensured because martensite is excessively tempered.

“Cooling conditions after tempering: Cooling to room temperature at a cooling rate exceeding 5 ° C./s”
When the tempering is completed, it is cooled to room temperature (third cooling), but it is necessary to perform the cooling rate (third cooling rate) at this time under a condition exceeding 5 ° C./s.
This is because when the cooling rate is 5 ° C./s or less, the residence time of 300 to 480 ° C. is prolonged, so that coarse cementite is formed and λ decreases.

[Measurement and evaluation method of steel sheet structure]
(Ferrite, bainitic ferrite fraction)
In SEM observation, the polygonal black part is defined as ferrite, and the lath-shaped part having a lath width of 0.3 μm or more is defined as bainitic ferrite and the area ratio is measured. After corroding with a 3% nital solution to reveal the metal structure, a scanning electron microscope (SEM) image at a magnification of 2000 times was observed for approximately 5 fields of 40 μm × 30 μm region, and 100 points per field of view were calculated by a dot calculation method. Measurements were made to determine the area.

(Tempered martensite, martensite fraction)
An extracted replica sample was prepared, and a transmission electron microscope (TEM) image at a magnification of 50000 times was observed for three fields of view of 2.4 μm × 1.6 μm. The presence or absence of cementite in the lath structure having a lath width of less than 0.3 μm was confirmed. The structure in which cementite was present was identified as tempered martensite, and the structure in which no cementite was present was identified as martensite.

(Particle size and number density of cementite in tempered martensite)
From the contrast of the TEM image, the black portion is discriminated as a cementite particle and marked, and by image analysis software, the equivalent circle diameter D (D = 2 × (A / π) 1 is calculated from the area A of each marked cementite particle. / 2) was calculated, and the number of cementite particles having a predetermined particle size per unit area was determined.

(Example)
Hereinafter, the Example of this invention is given and the outstanding effect is demonstrated.
The experiment was performed in the lab. After melting and casting the steel shown in Table 1, it was hot rolled into a slab with a plate thickness of 30 mm, and then heated to 1150 ° C. and hot rolled to a plate thickness of 2.9 mm at a rolling end temperature of 900 ° C. Thereafter, air cooling was performed at 650 ° C. for 10 s, and the furnace was placed in a holding furnace at 500 ° C. to perform furnace cooling (simulating winding of a hot-rolled steel sheet). The hot-rolled steel sheet was then cold-rolled at a cold rolling rate of 52% to obtain a 1.4 mm cold-rolled steel sheet.

  These steel plates are subjected to heat treatment (soaking, cooling, tempering) under various conditions as shown in Table 2, and the mechanical properties after treatment (YS, TS, YR, EL, λ, TS × EL) are measured and investigated. did. The result is the heat treatment No. Table 3 shows the steel grade (component composition) and structure. In the other columns of the structure (area ratio) in Table 3, P represents pearlite and Θ represents cementite.

  In the evaluation column of Table 3, TS ≧ 590 MPa, YR ≧ 0.70, TS × EL ≧ 25000 MPa%, and λ ≧ 60% are all satisfied with ○, TS ≧ 590 MPa, YR Those satisfying all of ≧ 0.70, TS × EL ≧ 25000 MPa%, and λ ≧ 70% were marked with “◎”, and those other than the above were marked with “x”.

  As can be seen from the various properties in Table 3, the cold-rolled steel sheet according to the present invention (invention example) has excellent characteristics as compared with other cold-rolled steel sheets (comparative example), and strict requirements for automobile parts. It can be seen that this is a steel plate with high quality that fully meets the requirements.

Claims (7)

C: 0.10 to 0.30% (meaning mass%, the same shall apply hereinafter)
Si: 0.50 to 2.50%,
Mn: less than 1.00% (including 0%)
P: 0.100% or less (including 0%),
S: 0.010% or less (including 0%),
Al: 0.50 to 3.00%,
N: 0.0020 to 0.0100%,
The balance consists of iron and inevitable impurities,
Steel structure has a ferrite fraction of 60-90%,
Bainitic ferrite fraction: 10% or less (including 0%)
Residual γ fraction: 5% or more,
Tempered martensite fraction 5% -30%,
Martensite fraction 5% or less (including 0%)
The average particle diameter of the equivalent circle diameter of cementite present in the tempered martensite is 200 nm or less, and the number density is 10 / μm ² or more,
A high-strength steel sheet having a high yield ratio and formability. The high-strength steel sheet having a high yield ratio and formability according to claim 1, wherein the number density of cementite particles having an equivalent circle diameter of 200 nm or more present in the tempered martensite is 1 piece / µm ² or less. Furthermore,
V: 0.01-1.00%,
Ti: 0.01-0.30%,
Nb: 0.01-0.30%
A high-strength steel sheet having a high yield ratio and formability according to claim 1 or 2, comprising one or more of the following. Furthermore,
Cr: 0.01 to 3.00%,
Mo: 0.01 to 1.00%,
Cu: 0.01-2.00%,
Ni: 0.01 to 2.00%,
B: 0.0001 to 0.005%,
A high-strength steel sheet having a high yield ratio and formability according to any one of claims 1 to 3, comprising one or more of the following. Furthermore,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 0.0001 to 0.01%,
A high-strength steel sheet having a high yield ratio and formability according to any one of claims 1 to 4, comprising one or more of the following. It is a manufacturing method of the high strength steel plate according to any one of claims 1 to 5 ,
A steel plate obtained by subjecting steel material to hot rolling and cold rolling,
Heat to a temperature of [0.6Ac1 + 0.4Ac3] or higher,
After soaking for 10 to 1000 seconds at the temperature,
Cool to 750-550 ° C. at a cooling rate of 15 ° C./s or less,
Next, it is cooled to room temperature at a cooling rate of 100 ° C./s or more,
Furthermore, the steel sheet is reheated to 300 to 480 ° C.,
After tempering for 100-300 s at this temperature,
A method for producing a high-strength steel sheet having a high yield ratio and formability, characterized by cooling to room temperature.
It is a manufacturing method of the high strength steel plate according to any one of claims 1 to 5 ,
A steel plate obtained by subjecting steel material to hot rolling and cold rolling,
Heat to a temperature of [0.6Ac1 + 0.4Ac3] or higher,
After soaking for 10 to 1000 seconds at the temperature,
Cool to 750-550 ° C. at a cooling rate of 15 ° C./s or less,
Next, it is cooled to room temperature at a cooling rate of 100 ° C./s or more,
Furthermore, the steel sheet is reheated to 480 to 580 ° C. at a heating rate of 8 ° C./s or more,
After tempering for 20 seconds or less at the temperature or within a temperature drop range of 10 ° C. or less from the temperature,
A method for producing a high-strength steel sheet having a high yield ratio and formability, characterized by cooling to room temperature at a cooling rate exceeding 5 ° C / s.