JP5240037B2 - Steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel plate and a method for producing the same, and more specifically to a steel plate having a tensile strength of 590 MPa or more mainly used in the industrial field such as automobiles and a method for producing the same.

  In recent years, in the field of steel sheets for automobiles, the application of high-strength steel sheets having a high tensile strength of 590 MPa or more is being expanded in order to improve fuel efficiency and impact resistance. For this reason, these high-strength steel sheets have been used for applications that are more difficult to form than in the past.

  In general, formability decreases with increasing strength of a steel sheet. Thus, many proposals have been made so far in order to provide a high-strength steel sheet with high formability. For example, a so-called DP steel composed of two phases of ferrite and martensite with a reduced yield ratio and improved elongation has been proposed. Further, a so-called TRIP steel made of ferrite, bainite, and austenite with improved elongation characteristics has been proposed.

However, the steel plates proposed in these prior arts have insufficient stretch flangeability, which is important in the press forming of high strength steel plates in recent years.
On the other hand, Patent Document 1 discloses a hot-dip galvanized cold-rolled steel sheet that optimizes the second phase while making ferrite an ultrafine structure. According to this, stretch flangeability is improved.

Japanese Patent Laid-Open No. 2004-211126

  However, the manufacturing method disclosed in Patent Document 1 requires two heat treatment steps after cold rolling, which not only makes the manufacturing process complicated but also disadvantageous in cost. Furthermore, since the volume fraction of martensite and retained austenite, which contributes to the improvement of steel sheet elongation, is extremely limited to less than 3%, in the actual press forming in which various forming modes are combined, stretch flangeability is Even if sufficient, necking and cracking may occur due to insufficient elongation.

  For this reason, the present invention provides a high-strength steel sheet having a tensile strength of 590 MPa or more that achieves both good stretch flangeability and elongation, and a production method capable of producing it without going through complicated steps. For the purpose.

  The present inventors have intensively studied to solve the above problems. As a result, after strengthening the steel sheet with fine precipitates mainly composed of Ti carbonitride, by combining bainite advantageous for stretch flangeability, martensite and residual austenite contributing to improvement of elongation, It has been newly found that it is possible to provide both good stretch flangeability and elongation while having a high tensile strength of 590 MPa or more.

The present invention is based on these new findings, and the gist thereof is as follows.
In the present invention, C: 0.03% or more and 0.12% or less (in this specification, “%” relating to chemical composition means mass% unless otherwise specified), Si: 0.005% Or more, less than 0.5%, Mn: 2.0% to 3.0%, P: 0.05% or less, S: 0.005% or less, sol. Al: 0.001% to 0.2%, N: 0.0050% or less, Ti: 0.025% to 0.15% and Nb: 0% to 0.1%, the balance being with Fe and impurities, formula ^{(1): C * = C} + (12/14) × N- (12/48) × Ti- (12/93) C * is 0.010 or more defined by × Nb The total volume ratio of the martensite and retained austenite has a chemical composition of 0.074 or less, the ferrite volume fraction is 0.45 or more and 0.85 or less, the bainite volume fraction is 0.10 or more and 0.49 or less Is 0.01 or more and 0.05 or less, and C defined by Formula (2): C ^** = {C ^* / (1-Vf)} + {(Mn + Ni) / 6} + Cr / 5 + Mo / 2 When ^** has a steel structure is 0.45 or more 0.84 or less Moni, tensile strength more than 590 MPa, a total elongation of 25% or more, a steel sheet characterized by having mechanical properties are hole expansion ratio is 80% or more.

In the formulas (1) and (2), C, N, Ti, Nb, Mn, Ni, Cr and Mo represent the content (% by mass) of each element, and Vf represents the volume fraction of ferrite.
In the steel sheet according to the present invention, the chemical composition is replaced by a part of Fe, Cr: 1% or less, Mo: 0.5% or less, V: 0.1% or less, Cu: 1% or less, Ni: You may contain 1 type or 2 types or more selected from the group which consists of 1% or less and B: 0.005% or less.

In these steel plates according to the present invention, the chemical composition may contain one or two of Ca and Mg in total in an amount of 0.005% or less in place of part of Fe.
You may equip the surface of these steel plates concerning this invention with the plating layer.

From another point of view, the present invention is a method for producing a steel sheet, characterized by subjecting a steel sheet having the above-described chemical composition to a heat treatment that sequentially includes the following steps (A1) to (A5). In this specification, it is also referred to as a first manufacturing method.
(A1) A first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(A2) a first cooling step of cooling to 650 ° C. at an average cooling rate of 1 ° C./second or more and 10 ° C./second or less;
(A3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(A4) a second holding step of holding in a temperature range of 380 ° C. to 500 ° C. for 80 seconds to 1000 seconds; and (A5) a third cooling step of cooling to room temperature.

In the method for manufacturing a steel sheet according to the present invention, in the step (A4), the steel sheet may be subjected to a hot dipping process.
From another point of view, the present invention relates to a heat treatment and molten zinc in which the following steps (B1) to (B7) are sequentially applied to a steel sheet having the above-described chemical composition of Si: 0.005% or more and less than 0.1%. It is a manufacturing method of the steel plate characterized by performing a plating process. In this specification, it is also referred to as a second manufacturing method.
(B1) a first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(B2) a first cooling step of cooling to 650 ° C. at an average cooling rate of 1 ° C./second or more and 10 ° C./second or less;
(B3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(B4) a second holding step of holding in a temperature range of 380 ° C. to 550 ° C. for 40 seconds to 500 seconds;
(B5) Hot dip galvanizing step for hot dip galvanizing;
(B6) an alloying treatment step in which a temperature range of 480 ° C. to 550 ° C. is maintained for 10 seconds to 40 seconds; and (B7) a third cooling step of cooling to room temperature.

From still another aspect, the present invention provides a heat treatment and melting stepwise including the following steps (C1) to (C7) in the steel sheet having the above-described chemical composition of Si: 0.1% or more and less than 0.5%. A method for producing a steel sheet, characterized by performing a galvanizing treatment. In this specification, it is also referred to as a third manufacturing method.
(C1) a first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(C2) a first cooling step of cooling to 650 ° C. at an average cooling rate of 1 ° C./second or more and 10 ° C./second or less;
(C3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(C4) a second holding step of holding in a temperature range of 380 ° C. to 550 ° C. for 20 seconds to 250 seconds;
(C5) Hot dip galvanizing step for hot dip galvanizing;
(C6) an alloying treatment step of maintaining at a temperature range of 500 ° C. to 600 ° C. for 10 seconds to 40 seconds; and (C7) a third cooling step of cooling to room temperature.

  In addition, each said holding | maintenance process may hold | maintain at fixed temperature, or may be accompanied by a temperature fluctuation within a predetermined temperature range. For example, it may be accompanied by a temperature history such as a case where slow cooling is performed within a predetermined temperature range, a temperature increase caused by immersion in a hot dipping bath, or subsequent cooling.

  According to the present invention, a steel sheet having good stretch flangeability and elongation while having a high tensile strength of 590 MPa or more is obtained, which is extremely useful industrially.

Hereinafter, the reasons for limiting each condition in the present invention will be described.
(Chemical composition)
[C: 0.03% to 0.12%]
C is an important element for obtaining high tensile strength. If the C content is less than 0.03%, it is difficult to obtain a tensile strength of 590 MPa or more. Therefore, the C content is 0.03% or more. Preferably it is 0.04% or more. On the other hand, when the C content exceeds 0.12%, martensite, retained austenite, cementite or pearlite is excessively generated, and stretch flangeability is deteriorated. Therefore, the C content is 0.12% or less. Preferably it is 0.09% or less, More preferably, it is 0.07% or less.

[Si: 0.005% or more and less than 0.5%]
Si is an element effective for increasing the strength of a steel sheet, and is an element effective for securing retained austenite that contributes to an improvement in elongation. Furthermore, when producing an alloyed hot-dip galvanized steel sheet, the effect of moderately suppressing the alloying reaction is exhibited. From such a viewpoint, the Si content is set to 0.005% or more. For example, in the case of an alloyed hot-dip galvanized steel sheet, if the Si content is less than 0.005%, the plating adhesion may deteriorate.

  On the other hand, when the Si content is 0.5% or more, the chemical conversion processability may deteriorate in hot-rolled steel sheets and cold-rolled steel sheets, and the adhesion of plating may deteriorate in electroplated steel sheets. In a plated steel sheet, wettability with plating may deteriorate. Therefore, the Si content is less than 0.5%. Preferably it is 0.3% or less.

  As will be described later, in the alloyed hot-dip galvanized steel sheet, the steel structure can be more easily controlled by using the heat treatment for alloying, and thereby the stretch flangeability can be improved. In this case, when the alloying treatment temperature is relatively low, it is preferable that the Si content is less than 0.1% so that the alloying proceeds sufficiently, and the alloying treatment temperature is relatively high. In order to further improve the stretch flangeability, it is preferable to prevent the alloying from proceeding excessively by setting the Si content to 0.1% or more.

[Mn: 2.0% to 3.0%]
Mn has an effect of improving hardenability and is an extremely effective element for increasing the strength of a steel sheet. If the Mn content is less than 2.0%, the total elongation of 25% or more may not be obtained due to insufficient volume ratio of martensite or retained austenite. Therefore, the Mn content is 2.0% or more. Preferably it is 2.1% or more. On the other hand, if the Mn content exceeds 3.0%, the hardenability becomes too high and the volume ratio of martensite becomes excessive, which may significantly deteriorate the stretch flangeability. Therefore, the Mn content is 3.0% or less. Preferably it is 2.7% or less.

[P: 0.05% or less]
P is an element that is generally contained as an impurity, but may be positively contained because it has the effect of increasing the strength of the steel sheet by solid solution strengthening. However, when the P content is excessive, the deterioration of toughness becomes significant. Therefore, the P content is 0.05% or less. The lower limit of the P content does not need to be specified, but if the P content is less than 0.001%, a significant cost increase is caused, which is economically disadvantageous. For this reason, it is preferable to set it as 0.001% or more of P content.

[S: 0.005% or less]
S is an element contained as an impurity, and forms MnS to deteriorate stretch flangeability. Therefore, the S content is set to 0.005% or less as a range where the stretch flangeability deterioration is not remarkable. Preferably it is 0.003% or less, More preferably, it is 0.002% or less. Since the S content is preferably as low as possible, the lower limit of the S content need not be specified. However, when the S content is less than 0.0003%, a significant cost increase is caused, which is economically disadvantageous. For this reason, it is preferable that S content shall be 0.0003% or more.

[Sol. Al: 0.001% or more and 0.2% or less]
Al has the effect | action which deoxidizes molten steel and makes steel healthy. sol. When the Al content is less than 0.001%, deoxidation is not sufficient. Therefore, sol. The Al content is 0.001% or more. On the other hand, sol. Even if Al is added so that the Al content exceeds 0.2%, the effect due to the above action is saturated and the cost increases. Therefore, sol. The Al content is 0.2% or less.

[N: 0.0050% or less]
N is an element contained as an impurity. If the content exceeds 0.0050%, coarse nitrides are formed in the steel, and the stretch flangeability is remarkably deteriorated. Therefore, the N content is 0.0050% or less. Since the N content is preferably as low as possible, the lower limit of the N content need not be specified. However, when the N content is less than 0.0005%, a significant cost increase is caused, which is economically disadvantageous. For this reason, it is preferable that N content shall be 0.0005% or more.

[Ti: 0.025% or more and 0.15% or less]
Ti is one of the important elements in the present invention, which has the effect of strengthening the ferrite phase by forming fine precipitates by bonding or further complexing with C, N or the like. When the Ti content is less than 0.025%, the effect of strengthening ferrite cannot be obtained sufficiently. Therefore, the Ti content is 0.025% or more. On the other hand, even if Ti is contained in excess of 0.15%, the effect due to the above action is saturated and the cost increases. Therefore, the Ti content is 0.15% or less.

[Nb: 0% to 0.1%]
Nb is an optional element and is an element having an action of strengthening a ferrite phase by forming fine precipitates by bonding or further complexing with C or the like in the same manner as Ti. Even if Nb is contained in excess of 0.1%, the effect due to the above action is saturated and the cost increases. Therefore, the Nb content is 0% or more and 0.1% or less.

  By containing Ti or Nb that satisfies the above range, fine precipitates of Ti or Nb are formed, thereby strengthening the ferrite phase, thereby achieving the high elongation targeted by the present invention. Flangeability is obtained. In order to obtain the above-described effects more reliably, the total content of Ti and Nb is preferably 0.050% or more.

  On the other hand, if the contents of Ti and Nb are excessive, the hot-rolled steel sheet obtained by hot rolling tends to have a very fine and highly anisotropic structure. The steel sheet obtained by performing heat treatment after the intermediate rolling has a large anisotropy. Therefore, the total content of Ti and Nb is preferably 0.15% or less. In particular, since this effect becomes remarkable when Nb is large, the Nb content is more preferably 0.05% or less.

[Cr: 1% or less, Mo: 0.5% or less, V: 0.1% or less, Cu: 1% or less, Ni: 1% or less, and B: 0.005% or less Species or 2 or more types]
These elements are arbitrary elements and may be contained because they have an effect of increasing the strength of the steel sheet. If the content of each element exceeds the above range, the effect of increasing the strength is saturated and the cost increases. For this reason, content of each element shall be the said range. In order to obtain the effect of increasing the strength more reliably, Cr is 0.1% or more, Mo is 0.02% or more, V is 0.005% or more, Cu is 0.1% or more, and Ni is 0.1%. % Or more and B is preferably contained by 0.0002% or more.

[One or two of Ca and Mg in total 0.005% or less]
These elements are optional elements, and have the effect of improving stretch flangeability by controlling the form of sulfide. When the total content exceeds 0.005%, the effect by the above action is saturated. For this reason, the total content is set to 0.005% or less. In order to more reliably obtain the effect of the above action, the total content is preferably set to 0.0005% or more.

[C ^* : 0.010 to 0.074]
C content + (12/14) × N content - (12/48) × Ti content - (12/93) in the ^{C *} which is defined as × Nb content is less than 0.010, the heat treatment cooling process The amount of bainite, martensite and retained austenite produced in the above is insufficient, and the strength is insufficient, or the elongation is inferior even if the strength is present. On the other hand, when C ^* exceeds 0.074, the volume fraction of bainite, martensite, and retained austenite becomes excessive, and elongation deterioration and stretch flangeability deterioration may become remarkable. Therefore, C ^* is set to 0.010 or more and 0.074 or less.

The balance other than the above is Fe and impurities.
[Steel structure]
In the steel structure of the present invention, the volume fraction Vf of ferrite is 0.45 or more and 0.85 or less, the volume fraction of bainite is 0.10 or more and 0.49 or less, and the total volume fraction of martensite and retained austenite is 0.01. The C ^** is defined as {C ^* / (1-Vf)} + {(Mn content + Ni content) / 6} + Cr content / 5 + Mo content / 2. It is important that it is 0.45 or more and 0.84 or less.

  When the ferrite volume fraction Vf is less than 0.45, the bainite volume fraction exceeds 0.49, or the total volume fraction of martensite and retained austenite is less than 0.01, the balance between strength and elongation is to degrade. On the other hand, when the ferrite volume fraction Vf exceeds 0.85, the required strength cannot be obtained, and when the total volume fraction of martensite and retained austenite exceeds 0.05, stretch flangeability deteriorates, and bainite. When the volume ratio is less than 0.10, the strength is insufficient, or a large amount of martensite must be included to secure the strength, and the stretch flangeability deteriorates.

When C ^** is less than 0.45, the hardenability is insufficient, and pearlite and coarse cementite are generated in the cooling process of the heat treatment, so that both elongation and stretch flangeability deteriorate. On the other hand, when C ^** exceeds 0.84, martensite is excessively generated and stretch flangeability is remarkably deteriorated.

[Mechanical properties]
As for the mechanical properties of the steel sheet according to the present invention, the tensile strength is 590 MPa or more, the total elongation is 25% or more, and the hole expansion ratio is 80% or more.

The present invention provides a high-strength steel sheet having a tensile strength of 590 MPa or more that achieves both good stretch flangeability and elongation, and is specifically defined as described above.
[Plating layer]
The surface of the steel plate described above may be provided with a plating layer for the purpose of improving corrosion resistance. The plating may be electroplating or hot dipping. Examples of electroplating include electrogalvanizing and electro-Zn-Ni alloy plating. Examples of the hot dip plating include hot dip galvanizing, alloying hot dip galvanizing, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, and the like.

[Production method]
As the manufacturing conditions for the steel sheet, it is preferable to subject the steel sheet having a predetermined chemical composition to a heat treatment described later or further to a hot dip galvanizing process. Here, the production history of the steel sheet to be subjected to the heat treatment does not need to be specified in particular, and may be produced by a conventional method. However, in order to provide even better characteristics, the production history is preferably as follows.

  That is, in order to make precipitation of Ti or Nb fine precipitates easier in the hot rolling process or heat treatment process, it is necessary to keep Ti and Nb carbonitrides in solid solution in the stage of hot rolling. preferable.

From such a viewpoint, it is preferable that the temperature of the slab used for hot rolling is 1220 ° C. or higher. Since Ti and Nb tend to segregate in the center of the slab, the temperature is more preferably 1250 ° C. or higher in order to more effectively promote the solid solution of Ti and Nb carbonitrides. However, even if heating to a temperature exceeding 1380 ° C., the effect of promoting the solid solution of Ti and Nb carbonitrides is saturated and disadvantageous in terms of cost.
The finishing temperature of the hot rolling is preferably (Ar ₃ points + 80 ° C.) or more and 950 ° C. or less. In the present invention, since a large amount of Ti or further Nb is contained, when hot rolling is completed at a temperature lower than (Ar ₃ points + 80 ° C.), a very fine and highly anisotropic structure tends to be formed. Steel sheet obtained by heat treatment or steel sheet obtained by heat treatment after cold rolling has increased anisotropy, and cracking occurs due to the occurrence of earrings or insufficient elongation in a specific direction during processing such as press forming. Because there is a fear. Further, at a finishing temperature exceeding 950 ° C., scale wrinkles may occur.

Moreover, it is preferable that the coiling temperature of hot rolling shall be 400 degreeC or more and 700 degrees C or less from a viewpoint of suppressing the fluctuation | variation of the steel structure in a coil.
After hot rolling, if necessary, after performing skin pass rolling for flattening, pickling for removing the scale is performed, or further cold rolling is performed for heat treatment. When performing cold rolling, it is preferable that the rolling reduction of cold rolling shall be 30% or more and 70% or less from a viewpoint of the burden to an installation, or operativity.

The following three types of suitable manufacturing conditions can be mentioned for the heat treatment or the hot dip galvanizing treatment depending on the type of product.
[First manufacturing method]
A 1st manufacturing method is a manufacturing condition suitable in the case of manufacturing a hot-rolled steel plate, a cold-rolled steel plate, an electroplated steel plate, and a hot dipped steel plate, The following process (A1)-(A5) is carried out to the steel plate which has the said chemical composition. The heat treatment is sequentially performed.

(A1) A first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(A2) a first cooling step of cooling to 650 ° C. at an average cooling rate CR (° C./second) satisfying the following formula (3);
(A3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(A4) a second holding step for holding in a temperature range of 380 ° C. to 500 ° C. for 80 seconds to 1000 seconds; and (A5) a third cooling step for cooling to room temperature.
0.10 ≦ logCR-C ^* ≦ 1.00 (3)
In addition, when manufacturing a hot dipped steel plate, it is preferable to perform a hot dipping process in a process (A4).

In the step (A1), if the holding temperature is less than (Ac ₃ points-30 ° C.), it becomes difficult to secure a predetermined bainite volume fraction and the structure becomes non-uniform, so that elongation and stretch flangeability deteriorate. There is a case. On the other hand, when the holding temperature exceeds (Ac ₃ points + 50 ° C.), many of Ti and Nb carbides finely precipitated in the heating stage of the hot rolling process or heat treatment process are re-dissolved, and these fine precipitations The ferrite strengthening ability by the object is impaired, and martensite or retained austenite is hardly generated in the subsequent cooling process, and it becomes difficult to secure the intended tensile strength or elongation. In order to prevent excessive re-solidification of Ti and Nb carbides while adjusting the size of austenite in the above temperature range, the residence time in the above temperature range is preferably 20 seconds or more and 240 seconds or less.

The first cooling and the second cooling in the step (A2) and the step (A3) are for setting the volume ratios of ferrite, bainite, retained austenite, and martensite to predetermined ranges, respectively. If the value of logCR-C ^* when the average cooling rate in the first cooling in the step (A2) is CR (° C./second) is less than 0.10, it is difficult to ensure the target tensile strength. Or, the value of C ^** exceeds 0.84, excessive martensite is generated, and stretch flangeability is significantly deteriorated. On the other hand, if the value of logCR-C ^* exceeds 1.00, it will be difficult to ensure the required ferrite volume fraction, or the value of C ^** will be less than 0.45 and the hardenability will be insufficient. Since pearlite and coarse cementite are formed during the cooling process of heat treatment, both elongation and stretch flangeability deteriorate.

  In addition, if the average cooling rate in the second cooling in the step (A3) is less than 5 ° C./second, the volume ratio of pearlite or coarse cementite increases, and the target tensile strength cannot be obtained, or elongation and elongation flange Inferior. The upper limit of the second cooling in the step (A3), that is, the average cooling rate from 650 ° C. to 550 ° C. is not particularly limited, and is 1000 using known gas cooling, water cooling, roll cooling, air-water cooling and the like. It may be in the range of ° C / second or less.

  If the holding temperature in the step (A4) is less than 380 ° C., more than 500 ° C., or if the holding time is less than 80 seconds, the bainite transformation does not proceed sufficiently and finally martensite or residual Since austenite is generated excessively, stretch flangeability deteriorates. On the other hand, if the holding time exceeds 1000 seconds, the bainite transformation proceeds excessively, and it becomes difficult to ensure the intended tensile strength. Note that the effect of the present invention is not lost even if hot-dip plating is performed in the holding step of step (A4).

In step (A5), the steel structure formed in steps (A1) to (A4) is determined by cooling to room temperature.
[Second manufacturing method]
The second manufacturing method is a preferable manufacturing condition when manufacturing an alloyed hot-dip galvanized steel sheet. Si: 0.005% or more and less than 0.1% is applied to a steel sheet having the above chemical composition by the following step (B1). ) To (B7) are sequentially performed by heat treatment and hot dip galvanizing treatment.

(B1) a first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(B2) a first cooling step of cooling to 650 ° C. at an average cooling rate CR (° C./second) satisfying the following formula (3);
(B3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(B4) a second holding step of holding in a temperature range of 380 ° C. to 550 ° C. for 40 seconds to 500 seconds;
(B5) Hot dip galvanizing step for hot dip galvanizing;
(B6) an alloying treatment step in which a temperature range of 480 ° C. to 550 ° C. is maintained for 10 seconds to 40 seconds; and (B7) a third cooling step of cooling to room temperature.
0.10 ≦ logCR-C ^* ≦ 1.00 (3)
[Third production method]
The third production method is a suitable production condition in the case of producing an alloyed hot-dip galvanized steel sheet in the same manner as the second production method, but the stretch flangeability of the steel sheet is further improved by the second production method. Manufacturing conditions. That is, heat treatment and hot dip galvanizing treatment sequentially including the following steps (C1) to (C7) are performed on a steel sheet having the above chemical composition that is Si: 0.1% or more and less than 0.5%.

(C1) a first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(C2) a first cooling step of cooling to 650 ° C. at an average cooling rate CR (° C./second) satisfying the following formula (3);
(C3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(C4) a second holding step of holding in a temperature range of 380 ° C. to 550 ° C. for 20 seconds to 250 seconds;
(C5) Hot dip galvanizing step for hot dip galvanizing;
(C6) an alloying treatment step of maintaining in a temperature range of 500 ° C. to 600 ° C. for 10 seconds to 40 seconds; and (C7) a third cooling step of cooling to room temperature.
0.10 ≦ logCR-C ^* ≦ 1.00 (3)
In the second manufacturing method and the third manufacturing method, the combination of the Si content, the holding conditions before dipping in the hot dip galvanizing bath, and the alloying treatment conditions is extremely important, and these should be combined appropriately. Thus, it is possible to obtain a steel sheet having excellent stretch flangeability as compared with a steel sheet obtained by applying the first manufacturing method, and having good plating quality required for an alloyed hot-dip galvanized steel sheet. It can also be secured.

  The reasons for limitation in the steps (B1) to (B3) and (B7) of the second manufacturing condition and the steps (C1) to (C3) and (C7) of the third manufacturing condition are as described above. This is the same as the reason for limitation in steps (A1) to (A3) and (A5) in the production method.

  When the holding temperature and holding time in the step (B4) and the step (C4) are shorter than the respective lower limits, or when the alloying treatment temperature in the step (B6) and the step (C6) is lower than the respective lower limits, the bainite Insufficient production may result in poor stretch flangeability.

  Further, when the holding temperature and holding time in the step (B4) and the step (C4) are longer than the respective upper limits, or when the alloying treatment temperature in the step (B6) and the step (C6) is higher than the respective upper limits. In some cases, the decomposition of austenite proceeds excessively and the strength-elongation balance deteriorates.

  Further, if the alloying treatment time in the step (B6) or the step (C6) is less than 10 seconds or less than 40 seconds, the alloying treatment unevenness may occur or the plating adhesion may be deteriorated.

  In the second production method, since the alloying temperature in the step (B6) is relatively low, the Si content is preferably less than 0.1% so that alloying proceeds sufficiently. In the third production method, since the alloying temperature in the step (C6) is relatively high, the Si content is preferably 0.1% or more so that the alloying does not proceed excessively.

  The steel plates obtained by the first to third manufacturing methods may be subjected to skin pass rolling for flattening as necessary. In this case, in order to avoid deterioration of ductility, it is preferable that the elongation is 1.0% or less. Moreover, you may electroplate to the steel plate manufactured without performing the hot dipping by the said 1st manufacturing conditions.

  Thus, according to the present invention, a steel sheet having good stretch flangeability and elongation while having a high tensile strength of 590 MPa or more, specifically, a tensile strength of 590 MPa or more and a total strength of 25% or more. A steel sheet having an elongation and a hole expansion rate of 80% or more is provided.

The present invention will be described more specifically with reference to examples.
The steel plates of test numbers 1 to 26 shown in Table 1, Table 2, Table 4, and Table 5 were made on a trial basis. That is, molten steel melted to a predetermined chemical composition is made into a slab by continuous casting, and after heating this slab to 1270 ° C, it is hot-rolled at a finishing temperature of 910 ° C to a plate thickness of 2.6 mm, and then cooled. Winded up. After pickling, a part was further cold rolled to 1.6 mm and subjected to continuous annealing or continuous hot dip galvanization.

At this time, the holding time at the soaking temperature was set to 20 to 80 seconds. Some were further subjected to electrogalvanization or hot dip galvanization with an adhesion amount of 45 g / m ² per side. The temperature of the hot dip galvanizing bath was 460 ° C., and some hot dip galvanized steel sheets were subjected to alloying treatment for 10 to 30 seconds. The cold rolled steel sheet and the electrogalvanized steel sheet were subjected to temper rolling with an elongation of 0.2% after continuous annealing and the hot dip galvanized steel sheet after plating.

About the obtained steel plate of the test numbers 1-26, the JIS5 test piece was extract | collected in the orthogonal | vertical direction with respect to the rolling direction, and the tension test was implemented according to JIS.
Further, the hole expansion rate was measured according to the “JFS T 1001 hole expansion test method” of the Japan Iron and Steel Federation standard.

  Moreover, in the structure photograph of the number of fields of view 10 observed with a scanning electron microscope, the rolling direction cross section was subjected to nital corrosion, and the area% of ferrite, bainite, (total of martensite and residual austenite) was obtained by image analysis, The volume ratio was taken as.

  Also, the volume fraction of retained austenite at the (1/4) depth position of the plate thickness was measured by X-ray diffraction method, and the difference from the volume fraction obtained previously (total of martensite and retained austenite) was determined to be martensite. The volume ratio was considered.

  Furthermore, about the galvannealed steel plate, the plating external appearance was visually observed and the plating adhesion was investigated by the following method. That is, a blank diameter of 90 mm, a punch diameter of 50 mm, a punch shoulder radius of 5 mm, a die shoulder radius of 18 mm, a blank hold pressure of 14 kN, and a rust-preventive oil application were performed, and the tape was peeled from the outer wall of the test piece. The peel weight was measured, and those having a peel weight of 20 mg or less were considered good plating adhesion.

Tables 2 and 4 show the results of investigating the manufacturing conditions and mechanical properties.
Test numbers 1-6, 12-17, and 26 are examples of the present invention that satisfy all the conditions defined in the present invention, and test numbers 7-11, 18-25 are comparative examples that do not satisfy the conditions defined in the present invention. is there.

  The steel sheets of the inventive examples of test numbers 1 to 6, 12 to 17 and 26 all have a tensile strength of 590 MPa or more, a total elongation of 25% or more, and a hole expansion ratio of 80% or more. It is optimal as a high-strength steel sheet for press forming applications such as automobiles.

On the other hand, in the steel plate of test number 7, the S content exceeds the upper limit of the range of the present invention, and the Ti content falls below the lower limit of the range of the present invention and C ^* exceeds the upper limit of the range of the present invention. The total volume fraction of martensite and retained austenite exceeded the upper limit of the range of the present invention, and the stretch flangeability was unsatisfactory.

  Since the holding temperature in the first holding step in the present invention is lower than the lower limit of the range of the present invention, the steel plate of test number 8 has a volume fraction of bainite lower than the lower limit of the range of the present invention and martensite and residual austenite. The total volume ratio exceeded the upper limit of the range of the present invention, and the elongation and stretch flangeability were unsatisfactory.

The steel of test number 9 has a volume of ferrite because the value of logCR-C ^* is less than the lower limit of the range of the present invention when the cooling rate in the first cooling step in the present invention is CR (° C./second). The rate exceeded the upper limit of the range of the present invention, and the tensile strength was unsatisfactory.

  The steel plate of test number 10 has a holding time in the second holding step in the present invention below the lower limit of the present invention, so that the total volume ratio of martensite and retained austenite exceeds the upper limit of the range of the present invention, and the elongation and Stretch flangeability was unsatisfactory.

  Since the steel plate of test number 11 has a cooling rate in the second cooling step of the present invention below the lower limit of the range of the present invention, the volume fraction of ferrite exceeds the upper limit of the range of the present invention, The total volume fraction of martensite and retained austenite was both below the lower limit of the present invention, and the tensile strength was unsatisfactory.

In the steel sheet of test number 18, since the C content and C ^* are below the lower limit of the range of the present invention, the volume fraction of ferrite exceeds the upper limit of the range of the present invention, and the volume fraction of bainite is below the lower limit of the present invention. The total volume fraction of martensite and retained austenite was below the lower limit of the present invention, and the tensile strength was unsatisfactory.

In the steel sheet of test number 19, since the C content and C ^* exceed the upper limit of the range of the present invention, the volume ratio of bainite is lower than the lower limit of the present invention, and the total volume ratio of martensite and retained austenite is within the range of the present invention. Exceeding the upper limit, the stretch flangeability was unsatisfactory.

  In the steel sheet of test number 20, the Mn content exceeds the upper limit of the range of the present invention, so the total volume ratio of martensite and retained austenite exceeds the upper limit of the range of the present invention, and the N content is the upper limit of the range of the present invention. Therefore, elongation and stretch flangeability were unsatisfactory.

Steel with test Nos. 21, since the logCR-C ^* values when the cooling rate was set to CR (° C. / sec) in the first cooling step in the present invention is below the lower limit of the present invention, C ^** is the invention Exceeding the upper limit of the range, hardenability becomes excessive, the volume fraction of bainite is below the lower limit of the present invention, and the total volume ratio of martensite and retained austenite exceeds the upper limit of the range of the present invention, stretch flangeability Was unsatisfactory.

The steel sheet of test number 22 had poor tensile strength because the soaking temperature in the first holding step in the present invention exceeded the upper limit of the range of the present invention.
The steel plate of test number 23 has a value of logCR-C ^* when the cooling rate in the first cooling step in the present invention is CR (° C./second) and exceeds the upper limit of the present invention, and is held in the second holding step. Since the time is below the lower limit of the present invention, the volume fraction of ferrite falls below the lower limit of the range of the present invention, the total volume ratio of martensite and retained austenite falls below the upper limit of the present invention, and C ^** is the lower limit of the present invention. The elongation and stretch flangeability were unsatisfactory.

  In the steel plate of test number 24, the cooling rate in the second cooling step in the present invention is lower than the lower limit of the present invention, so the volume ratio of bainite and the total volume ratio of martensite and retained austenite are both lower limits of the present invention. The elongation and stretch flangeability were unsatisfactory.

  Furthermore, the steel plate of test number 25 has a processing temperature in the alloying treatment step of the present invention that exceeds the upper limit of the present invention, so that the total volume ratio of martensite and retained austenite falls below the lower limit of the present invention, and the elongation is poor. there were.

Claims (8)

In mass%, C: 0.03% to 0.12%, Si: 0.005% to less than 0.5%, Mn: 2.0% to 3.0%, P: 0.05% or less , S: 0.005% or less, sol. Al: 0.001% to 0.2%, N: 0.0050% or less, Ti: 0.025% to 0.15% and Nb: 0% to 0.1%, the balance being It consists of Fe and impurities, and has a chemical composition in which C ^* defined by the following formula (1) is 0.010 or more and 0.074 or less,
The volume ratio of ferrite is 0.45 or more and 0.85 or less, the volume ratio of bainite is 0.10 or more and 0.49 or less, and the total volume ratio of martensite and retained austenite is 0.01 or more and 0.05 or less, Furthermore, while having a steel structure in which C ^** defined by the following formula (2) is 0.45 or more and 0.84 or less,
A steel sheet having mechanical properties of a tensile strength of 590 MPa or more, a total elongation of 25% or more, and a hole expansion ratio of 80% or more.
C ^* = C + (12/14) × N− (12/48) × Ti− (12/93) × Nb
・ ・ ・ ・ ・ ・ ・ (1)
C ^** = {C ^* / (1-Vf)} + {(Mn + Ni) / 6} + Cr / 5 + Mo / 2
(2)
Here, C, N, Ti, Nb, Mn, Ni, Cr and Mo in the formula represent the content (% by mass) of each element, and Vf represents the volume fraction of ferrite.   Instead of part of Fe, the chemical composition is in mass%, Cr: 1% or less, Mo: 0.5% or less, V: 0.1% or less, Cu: 1% or less, Ni: 1% or less And B: 1 type, or 2 or more types selected from the group consisting of 0.005% or less.   The steel sheet according to claim 1 or 2, wherein the chemical composition contains 0.005% by mass or less of one or two of Ca and Mg instead of a part of Fe.   The steel plate provided with a plating layer on the surface of the steel plate in any one of Claim 1- Claim 3. A method for producing a steel sheet, comprising subjecting the steel sheet having the chemical composition according to any one of claims 1 to 3 to a heat treatment sequentially including the following steps (A1) to (A5):
(A1) A first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(A2) a first cooling step of cooling to 650 ° C. at an average cooling rate CR (° C./second) satisfying the following formula (3);
(A3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(A4) a second holding step for holding in a temperature range of 380 ° C. to 500 ° C. for 80 seconds to 1000 seconds; and (A5) a third cooling step for cooling to room temperature.
0.10 ≦ logCR-C ^* ≦ 1.00 (3)   In the said process (A4), the hot-dip plating process is performed to the said steel plate, The manufacturing method of the steel plate of Claim 5 characterized by the above-mentioned. Si: 0.005% by mass or more and less than 0.1% by mass The following steps (B1) to (B7) are sequentially applied to the steel plate having the chemical composition according to any one of claims 1 to 3. A method for producing a steel sheet, characterized by subjecting to heat treatment and hot dip galvanizing treatment:
(B1) a first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(B2) a first cooling step of cooling to 650 ° C. at an average cooling rate CR (° C./second) satisfying the following formula (3);
(B3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(B4) a second holding step of holding in a temperature range of 380 ° C. to 550 ° C. for 40 seconds to 500 seconds;
(B5) Hot dip galvanizing step for hot dip galvanizing;
(B6) an alloying treatment step in which a temperature range of 480 ° C. to 550 ° C. is maintained for 10 seconds to 40 seconds; and (B7) a third cooling step of cooling to room temperature.
0.10 ≦ logCR-C ^* ≦ 1.00 (3) Si: 0.1% by mass or more and less than 0.5% by mass Heat treatment having the following steps (C1) to (C7) sequentially on the steel sheet having the chemical composition according to any one of claims 1 to 3. And a method for producing a steel sheet, characterized by performing hot dip galvanizing treatment:
(C1) a first holding step of holding in a temperature range of (Ac ₃ points−30 ° C.) or more and (Ac ₃ points + 50 ° C.);
(C2) a first cooling step of cooling to 650 ° C. at an average cooling rate CR (° C./second) satisfying the following formula (3);
(C3) a second cooling step of cooling to 550 ° C. at an average cooling rate of 5 ° C./second or more;
(C4) a second holding step of holding in a temperature range of 380 ° C. to 550 ° C. for 20 seconds to 250 seconds;
(C5) Hot dip galvanizing step for hot dip galvanizing;
(C6) an alloying treatment step of maintaining at a temperature range of 500 ° C. to 600 ° C. for 10 seconds to 40 seconds; and (C7) a third cooling step of cooling to room temperature.
0.10 ≦ logCR-C ^* ≦ 1.00 (3)