US20110162762A1

US20110162762A1 - High strength steel sheet and method for manufacturing the same

Info

Publication number: US20110162762A1
Application number: US13/062,574
Authority: US
Inventors: Hiroshi Matsuda; Yoshimasa Funakawa; Yasushi Tanaka
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2008-09-10
Filing date: 2009-09-08
Publication date: 2011-07-07
Also published as: KR101341731B1; JP5365112B2; CN102149841A; JP2010065273A; CA2734978A1; KR20110042369A; CA2734978C; CN102149841B; TW201020329A; EP2327810A1; EP2327810A4; EP2327810B1; WO2010030021A1; TWI412609B

Abstract

A high strength steel sheet having composition includes, on a percent by mass basis, C: 0.17% to 0.73%; Si: 3.0% or less; Mn: 0.5% to 3.0%; P: 0.1% or less; S: 0.07% or less; Al: 3.0% or less; and N: 0.010% or less, satisfies Si+Al≧0.7%, and the remainder includes Fe and incidental impurities, with a microstructure that has an area percentage of a total amount of lower bainite and whole martensite 10% to 90% relative to the whole steel sheet microstructure, an amount of retained austenite is 5% to 50%, an area percentage of bainitic ferrite in upper bainite is 5% or more relative to the whole steel sheet microstructure, as-quenched martensite is 75% or less of the total amount of lower bainite and whole martensite, and an area percentage of polygonal ferrite is 10% or less relative to the whole steel sheet microstructure, an average amount of C in retained austenite is 0.70% or more, and tensile strength is 980 MPa or more.

Description

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2009/065981, with an interational filing date of Sep. 8, 2009 (WO 2010/030021 A1, published Mar. 18, 2010), which is based on Japanese Patent Application No. 2008-232437, filed Sep. 10, 2008, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a high strength steel sheet used in industrial fields of automobile, electric apparatus, and the like and which has excellent workability, especially elongation and stretch-flangeability, and a tensile strength (TS) of 980 MPa or more, and a method for manufacturing the same.

BACKGROUND

In recent years, enhancement of fuel economy of the automobile has become an important issue from the viewpoint of global environmental conservation. Consequently, there is an active movement afoot to reduce the thickness of car components through increases in strength of car body materials to reduce the weight of a car body itself.
In general, to increase the strength of a steel sheet, it is necessary to increase the proportion of a hard phase, e.g., martensite or bainite, relative to the whole microstructure of the steel sheet. However, the increase in strength of the steel sheet through the increase in proportion of the hard phase causes a reduction in workability. Therefore, development of a steel sheet having high strength and excellent workability in combination has been desired. Various complex microstructure steel sheets, e.g., a ferrite-martensite double phase steel (DP steel) and a TRIP steel taking the advantage of the transformation induced plasticity of retained austenite, have been developed.
In the case where the proportion of the hard phase in the complex microstructure steel sheet increases, the workability of the steel sheet is affected by the workability of the hard phase significantly. This is because in the case where the proportion of the hard phase is small and that of soft polygonal ferrite is large, the deformability of polygonal ferrite is predominant over the workability of the steel sheet, and even in the case where the workability of the hard phase is inadequate, the workability, e.g., elongation, is ensured, whereas in the case where the proportion of the hard phase is large, deformability of the hard phase itself rather than deformation of polygonal ferrite exerts an influence directly on formability of the steel sheet. Therefore, if the workability of the hard phase itself is inadequate, deterioration of the workability of the steel sheet becomes significant.
Consequently, as for a cold rolled steel sheet, after conducting a heat treatment to adjust the amount of polygonal ferrite generated during annealing and cooling thereafter, the steel sheet is water-quenched to generate martensite, the temperature is raised again, and the steel sheet is kept at high temperatures so that martensite is tempered, carbides are generated in martensite, which is a hard phase, and thereby, workability of martensite is improved. However, such quenching•tempering of martensite needs a specific production facility, for example, a continuous annealing facility having a water quenching function. Therefore, in the case where a common facility is used, in which after the steel sheet is water-quenched, it is not possible to raise the temperature again and maintain high temperatures, the strength of the steel sheet can be increased but the workability of martensite, which is a hard phase, cannot be improved.
Furthermore, as for a steel sheet in which the hard phase is other than martensite, there is a steel sheet in which a primary phase is polygonal ferrite, a hard phase is bainite and pearlite, and carbides are generated in such bainite and pearlite serving as the hard phase. This steel sheet exhibits improved workability not only by polygonal ferrite, but also by generating carbides in the hard phase to improve the workability of the hard phase in itself and, in particular, an improvement of the stretch-flangeability is intended. However, since the primary phase is polygonal ferrite, it is difficult to allow an increase in strength to 980 MPa or more in terms of tensile strength (TS) and the workability to become mutually compatible. In this connection, even when the workability of the hard phase itself is improved by generating carbides in the hard phase, the level of workability is inferior to that of polygonal ferrite. Therefore, if the amount of polygonal ferrite is reduced to increase the strength to 980 MPa or more in terms of tensile strength (TS), adequate workability cannot be obtained.
Japanese Unexamined Patent Application Publication No. 4-235253 proposes a high strength steel sheet having excellent bendability and impact characteristic, wherein alloy components are specified and the steel microstructure is fine uniform bainite including retained austenite.
Japanese Unexamined Patent Application Publication No. 2004-76114 proposes a complex microstructure steel sheet having excellent bake hardenability, wherein predetermined alloy components are specified, the steel microstructure is bainite including retained austenite, and the amount of retained austenite in the bainite is specified.
Japanese Unexamined Patent Application Publication No. 11-256273 proposes a complex microstructure steel sheet having excellent impact resistance, wherein predetermined alloy components are specified, the steel microstructure is specified in such a way that bainite including retained austenite is 90% or more in terms of area percentage and the amount of austenite in the bainite is 1% or more, and 15% or less, and the hardness (HV) of the bainite is specified.
However, the above-described steel sheets have problems as described below.
Regarding the component composition described in JP '253, it is difficult to ensure the amount of stable retained austenite which exerts a TRIP effect in a high strain region in the case where a strain is applied to a steel sheet. Therefore, although bendability is obtained, elongation is low when plasticity becomes unstable, and punch stretchability is poor.
Regarding the steel sheet described in JP '114, bake hardenability is obtained. However, in the case where an increase in strength is intended in such a way that the tensile strength (TS) becomes 980 MPa or more, or furthermore, 1,050 MPa or more, it is difficult to ensure the strength or workability, e.g., elongation and stretch-flangeability, when the strength increases because the microstructure primarily contains bainite and, furthermore, ferrite while martensite is minimized.
The steel sheet described in JP '273 is for the purpose of improving the impact resistance and the microstructure contains bainite having a hardness of HV 250 or less as a primary phase, specifically at a content exceeding 90%. Therefore, it is difficult to make the tensile strength (TS) 980 MPa or more.
It could therefore be helpful to provide a high strength steel sheet having excellent workability, especially the elongation and the stretch-flangeability, and a tensile strength (TS) of 980 MPa or more, as well as an advantageous method for manufacturing the same.

SUMMARY

Our high strength steel sheets include a steel sheet in which galvanizing or galvannealing is applied to a surface of the steel sheet.
Excellent workability refers to a value of TS×T.EL satisfying 20,000 MPa·% or more and a value of TS×λ satisfying 25,000 MPa·% or more. In this regard, TS represents tensile strength (MPa), T.EL represents total elongation (%), and λ represents hole-expansion limit (%).
We conducted intensive research on the component composition and microstructure of steel sheets. We found that strength was increased through the use of a lower bainite microstructure and/or a martensite microstructure, stable retained austenite which was advantageous to obtain a TRIP effect, was ensured through the use of upper bainite transformation while the C content was increased in such a way that the amount of C in the steel sheet became 0.17% or more, a portion of the martensite was converted to tempered martensite and, thereby, a high strength steel sheet having excellent workability, especially a balance between the strength and the elongation and a balance between the strength and the stretch-flangeability in combination, and a tensile strength of 980 MPa or more was obtained.
We thus provide:

- 1. A high strength steel sheet characterized by having a composition containing, on a percent by mass basis,
  - C: 0.17% or more, and 0.73% or less,
  - Si: 3.0% or less,
  - Mn: 0.5% or more, and 3.0% or less,
  - P: 0.1% or less,
  - S: 0.07% or less,
  - Al: 3.0% or less, and
  - N: 0.010% or less,
- while it is satisfied that Si+Al is 0.7% or more, and the remainder includes Fe and incidental impurities,
  - wherein regarding the steel sheet microstructure, it is satisfied that the area percentage of a total amount of lower bainite and whole martensite is 10% or more, and 90% or less relative to the whole steel sheet microstructure, the amount of retained austenite is 5% or more, and 50% or less, the area percentage of bainitic ferrite in upper bainite is 5% or more relative to the whole steel sheet microstructure, as-quenched martensite is 75% or less of the above-described total amount of lower bainite and whole martensite, and the area percentage of polygonal ferrite is 10% or less (including 0%) relative to the whole steel sheet microstructure, the average amount of C in the above-described retained austenite is 0.70% or more, and the tensile strength is 980 MPa or more.
- 2. The high strength steel sheet according to the above-described item 1, characterized in that the above-described steel sheet further contains at least one type of element selected from, on a percent by mass basis,
  - Cr: 0.05% or more, and 5.0% or less,
  - V: 0.005% or more, and 1.0% or less, and
  - Mo: 0.005% or more, and 0.5% or less.
- 3. The high strength steel sheet according to the above-described item 1 or item 2, characterized in that the above-described steel sheet further contains at least one type of element selected from, on a percent by mass basis,
  - Ti: 0.01% or more, and 0.1% or less and
  - Nb: 0.01% or more, and 0.1% or less.
- 4. The high strength steel sheet according to any one of the above-described items 1 to 3, characterized in that the above-described steel sheet further contains, on a percent by mass basis,
  - B: 0.0003% or more, and 0.0050% or less.
- 5. The high strength steel sheet according to any one of the above-described items 1 to 4, characterized in that the above-described steel sheet further contains at least one type of element selected from, on a percent by mass basis,
  - Ni: 0.05% or more, and 2.0% or less, and
  - Cu: 0.05% or more, and 2.0% or less.
- 6. The high strength steel sheet according to any one of the above-described items 1 to 5, characterized in that the above-described steel sheet further contains at least one type of element selected from, on a percent by mass basis,
  - Ca: 0.001% or more, and 0.005% or less, and
  - REM: 0.001% or more, and 0.005% or less.
- 7. A high strength steel sheet characterized by including a galvanized layer or a galvannealed layer on a surface of the steel sheet according to any one of the above-described items 1 to 6.
- 8. A method for manufacturing a high strength steel sheet, characterized by including the steps of hot-rolling a billet having a component composition according to any one of the above-described items 1 to 6, conducting cold-rolling to produce a cold-rolled steel sheet, annealing the resulting cold-rolled steel sheet for 15 seconds or more, and 600 seconds or less in an austenite single phase region and, thereafter, conducting cooling to a cooling termination temperature: T° C. determined in a first temperature range of 350° C. or higher, and 490° C. or lower, wherein cooling to at least 550° C. is conducted while the average cooling rate is controlled at 5° C./s or more, subsequently, maintenance is conducted in the first temperature range for 15 seconds or more, and 1,000 seconds or less and, then, maintenance is conducted in a second temperature range of 200° C. or higher, and 350° C. or lower for 15 seconds or more, and 1,000 seconds or less.
- 9. The method for manufacturing a high strength steel sheet according to the above-described item 8, characterized in that a galvanizing treatment or a galvannealing treatment is applied during cooling to the above-described cooling termination temperature: T° C. or in the above-described first temperature range.

A high strength steel sheet having excellent workability, especially the elongation and the stretch-flangeability, and a tensile strength (TS) of 980 MPa or more, as well as an advantageous method for manufacturing the same can be provided. Therefore, the utility value in the industrial fields of automobiles, electric, and the like is very large, and in particular, the usefulness in weight reduction of an automobile body is significant.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram showing a temperature pattern of a heat treatment in our manufacturing method.

DETAILED DESCRIPTION

Initially, the reason for limitation of the steel sheet microstructure in such a way that described above will be described. Hereafter, “area percentage” refers to an area percentage relative to the whole steel sheet microstructure.
Area Percentage of Total Amount of Lower Bainite and Whole Martensite: 10% or more, and 90% or Less
Lower bainite and martensite are microstructures necessary to increase the strength of the steel sheet. If the area percentage of a total amount of lower bainite and whole martensite is less than 10%, the steel sheet does not satisfy the tensile strength (TS) of 980 MPa or more. On the other hand, if the total amount of lower bainite and whole martensite exceeds 90%, the upper bainite is reduced and, as a result, stable retained austenite, in which C is concentrated, cannot be ensured. Consequently, a problem occurs in that the workability, e.g., elongation, deteriorates. Therefore, the area percentage of the total amount of lower bainite and whole martensite is 10% or more, and 90% or less. A preferable range is 20% or more, and 80% or less. A more preferable range is 30% or more, and 70% or less.

Proportion of As-Quenched Martensite in Total Amount of Lower Bainite and Whole Martensite: 75% or Less

If the proportion of as-quenched martensite in the martensite exceeds 75% of the total amount of lower bainite and whole martensite present in the steel sheet, the tensile strength becomes 980 MPa or more, but the stretch-flangeability is poor. The as-quenched martensite is very hard, and deformability of the as-quenched martensite in itself is very low. Therefore, workability, especially stretch-flangeability, of the steel sheet deteriorates significantly. Furthermore, since the difference in hardness between the as-quenched martensite and the upper bainite is significantly large, if the amount of as-quenched martensite is large, the interface between the as-quenched martensite and upper bainite increases. Consequently, fine voids are generated at the interface between the as-quenched martensite and the upper bainite during punching or the like, and in stretch-flange forming conducted after the punching, voids are coupled to each other so that cracking develops easily and, thereby, stretch-flangeability deteriorates. Therefore, the proportion of as-quenched martensite in the martensite is 75% or less relative to the total amount of lower bainite and whole martensite present in the steel sheet. Preferably, the proportion is 50% or less. In this regard, the as-quenched martensite is a microstructure in which no carbide is detected in the martensite and can be observed with SEM. Amount of retained austenite: 5% or more, and 50% or less
The retained austenite undergoes martensitic transformation through a TRIP effect during working and, thereby, strain dispersive power is enhanced to improve elongation.
Retained austenite in which the amount of concentrated C is increased, is formed in the upper bainite through the use of upper bainite transformation. As a result, retained austenite capable of making the TRIP effect apparent even in a high strain region during working can be obtained. In the case where such retained austenite and martensite are present in combination and used, good workability is obtained even in a high strength region in which the tensile strength (TS) is 980 MPa or more. Specifically, the value of TS×T.El can be 20,000 MPa·% or more, and a steel sheet having an excellent balance between strength and elongation can be obtained.
Since the retained austenite in the upper bainite is formed between laths of bainitic ferrite in the upper bainite and distributes finely, large amounts of measurement at high magnification is necessary for determination of the amount (area percentage) thereof through microstructure observation, and it is difficult to quantify accurately. However, the amount of retained austenite formed between laths of the bainitic ferrite is an amount corresponding to the amount of formed bainitic ferrite to some extent. We found that an adequate TRIP effect was able to be obtained and the tensile strength (TS) of 980 MPa or more and TS×T.El of 20,000 MPa·% or more were able to be achieved if the area percentage of bainitic ferrite in the upper bainite was 5% or more, and the amount of retained austenite determined by an intensity measurement with X-ray diffraction (XRD), which was a previously employed technique to measure the amount of retained austenite, specifically, an X-ray diffraction intensity ratio of ferrite to austenite, was 5% or more. In this regard, we ascertained that the amount of retained austenite determined by the previously employed technique to measure the amount of retained austenite is equivalent to the area percentage of retained austenite relative to the whole steel sheet microstructure.
In the case where the amount of retained austenite is less than 5%, an adequate TRIP effect is not obtained. On the other hand, if the amount exceeds 50%, hard martensite generated after the TRIP effect is made apparent becomes excessive, deterioration of tenacity and the like become problems. Therefore, the amount of retained austenite is within the range of 5% or more, and 50% or less. The range is preferably more than 5%, and more preferably within the range of 10% or more, and 45% or less. The range is further preferably within the range of 15% or more, and 40% or less.

Average Amount of C in Retained Austenite: 0.70% or More

Regarding a high strength steel sheet having a tensile strength (TS) of 980 MPa to 2.5 GPa class, to obtain excellent workability through the use of the TRIP effect, the amount of C in the retained austenite is important. C is concentrated into the retained austenite formed between laths of bainitic ferrite in the upper bainite. It is difficult to accurately evaluate the amount of C concentrated into the retained austenite between the laths. However, we found that excellent workability was obtained when the average amount of C in the retained austenite determined from the amount of shift of a diffraction peak in the X-ray diffraction (XRD), which was a previously employed method for measuring the average amount of C in the retained austenite (an average of the amount of C in the retained austenite), was 0.70% or more.
In the case where the average amount of C in the retained austenite is less than 0.70%, martensitic transformation occurs in a low strain region during working so that the TRIP effect in a high strain region to improve the workability is not obtained. Therefore, the average amount of C in the retained austenite is 0.70% or more. The amount is preferably 0.90% or more. On the other hand, if the average amount of C in the retained austenite exceeds 2.00%, the retained austenite becomes excessively stable, martensitic transformation does not occur during working, and the TRIP effect is not apparent so that elongation deteriorates. Therefore, it is preferable that the average amount of C in the retained austenite is 2.00% or less. More preferably, the average amount is 1.50% or less.

Area Percentage of Bainitic Ferrite in Upper Bainite: 5% or More

Generation of bainitic ferrite due to upper bainite transformation is necessary to concentrate C in untransformed austenite to obtain retained austenite, which makes the TRIP effect apparent in a high strain region during working and which enhances strain resolution. The transformation from austenite to bainite occurs over a wide temperature range of about 150° C. to 550° C., and bainite generated in this temperature range include various types. In many cases in the previous technology, such various types of bainite has been specified as bainite simply. However, to obtain the desired workability, it is necessary that the bainite microstructure is specified clearly. Therefore, the upper bainite and the lower bainite are defined as described below.
Upper bainite is characterized in that lath-shaped bainitic ferrite and retained austenite and/or carbides present between bainitic ferrite are included and fine carbides regularly arranged in the lath-shaped bainitic ferrite are not present. On the other hand, lower bainite is characterized in that lath-shaped bainitic ferrite and retained austenite and/or carbides present between bainitic ferrite are included, as is common to upper bainite, and in the lower bainite, fine carbides regularly arranged in the lath-shaped bainitic ferrite are present.
That is, the upper bainite and the lower bainite are distinguished on the basis of presence or absence of fine carbides regularly arranged in the bainitic ferrite. The above-described difference in the generation state of carbides in the bainitic ferrite exerts a significant influence on concentration of C into the retained austenite. That is, in the case where the area percentage of bainitic ferrite in the upper bainite is less than 5%, even when bainite transformation proceeds, the amount of C formed into carbides in the bainitic ferrite increases. As a result, the amount of concentration of C into the retained austenite present between laths decreases, and a problem occurs in that the amount of retained austenite, which exerts the TRIP effect in a high strain region during working, decreases. Therefore, it is necessary that the area percentage of bainitic ferrite in the upper bainite is 5% or more in terms of area percentage relative to the whole steel sheet microstructure. On the other hand, if the area percentage of bainitic ferrite in the upper bainite exceeds 85% relative to the whole steel sheet microstructure, it may become difficult to ensure the strength. Consequently, it is preferable that the area percentage is 85% or less.

Area Percentage of Polygonal Ferrite: 10% or Less (Including 0%)

If the area percentage of polygonal ferrite exceeds 10%, it becomes difficult to satisfy the tensile strength (TS): 980 MPa or more and, at the same time, strain is concentrated on soft polygonal ferrite present together in the hard microstructure during working so that cracking occurs easily during working. As a result, the desired workability is not obtained. If the area percentage of the polygonal ferrite is 10% or less, even when the polygonal ferrite is present, a state in which a small amount of polygonal ferrite is discretely dispersed in a hard phase is brought about, concentration of strain can be suppressed, and deterioration of workability can be avoided. Therefore, the area percentage of the polygonal ferrite is 10% or less. The area percentage is preferably 5% or less, further preferably 3% or less, and may be 0%.
The hardness of the hardest microstructure in the steel sheet microstructure is HV ≦800. That is, in the case where as-quenched martensite is not present in the steel sheet, any one of tempered martensite, lower bainite, and upper bainite becomes the hardest phase. All of these microstructures are phases which become HV ≦800. Alternatively, in the case where as-quenched martensite is present, the as-quenched martensite becomes the hardest microstructure. Regarding the as-quenched martensite, the hardness becomes HV ≦800, a significantly hard martensite exhibiting HV >800 is not present, and good stretch-flangeability can be ensured.
The steel sheet may include pearlite, Widmanstaetten ferrite, and lower bainite as the remainder microstructure. In that case, it is preferable that the allowable content of the remainder microstructure is 20% or less in terms of area percentage. More preferably, the allowable content is 10% or less.
The basic configuration of the steel sheet microstructure of the high strength steel sheet is as described above, and the following configuration may be added as necessary.
Next, the reason for limitation of component composition of the steel sheet in such a way that described above will be described. In this connection, % hereafter representing the following component composition refers to percent by mass.

C: 0.17% or More, and 0.73% or Less

The element C is an indispensable element to increase the strength of the steel sheet and ensure the amount of stable retained austenite, and an element necessary to ensure the amount of martensite and retain austenite at room temperature. If the amount of C is less than 0.17%, it is difficult to ensure the strength and workability of the steel sheet. On the other hand, if the amount of C exceeds 0.73%, hardening of welded and heat-affected zones is significant so that weldability deteriorates. Therefore, the amount of C is within the range of 0.17% or more, and 0.73% or less. The range is preferably more than 0.20% and 0.48% or less, and further preferably 0.25% or more.

Si: 3.0% or Less (Including 0%)

The element Si contributes to an improvement in the strength of steel by strengthening through solid solution. However, if the amount of Si exceeds 3.0%, an increase in the amount of solid solution into the polygonal ferrite and the bainitic ferrite causes deterioration of workability and tenacity, and causes deterioration of surface characteristics due to occurrence of red scale and the like and deterioration of wettability and adhesion of the coating in the case where hot dipping is applied. Therefore, the amount of Si is 3.0% or less. The amount is preferably 2.6% or less. The amount is further preferably 2.2% or less.
Moreover, Si is an element useful for suppressing generation of carbides and facilitating generation of retained austenite. Therefore, it is preferable that the amount of Si is 0.5% or more. However, in the case where generation of carbides is suppressed by merely Al, Si is not necessarily added, and amount of Si may be 0%.

Mn: 0.5% or More, and 3.0% or Less

The element Mn is useful for strengthening steel. If the amount of Mn is less than 0.5%, carbides are deposited in a temperature range higher than the temperature, at which bainite and martensite are generated, during cooling after annealing. Consequently, it is not possible to ensure the amount of hard phase, which contributes to strengthening of steel. On the other hand, the amount of Mn exceeding 3.0% causes deterioration of castability and the like. Therefore, the amount of Mn is 0.5% or more and 3.0% or less. The range is preferably 1.5% or more and 2.5% or less.

P: 0.1% or Less

The element P is useful for strengthening steel. If the amount of P exceeds 0.1%, the impact resistance deteriorates due to embrittlement based on grain boundary segregation, and in the case where galvannealing is applied to a steel sheet, the alloying rate is reduced significantly. Therefore, the amount of P is 0.1% or less. The amount is preferably 0.05% or less. In this connection, it is preferable that the amount of P is reduced. However, reduction to less than 0.005% causes a significant increase in cost. Therefore, it is preferable that the lower limit thereof is about 0.005%.

S: 0.07% or Less

The element S generates MnS to become an inclusion and causes deterioration of the impact resistance and cracking along a metal flow of welded zones. Therefore, it is preferable that the amount of S is minimized. However, since excessive reduction in the amount of S causes an increase in production cost, the amount of S is 0.07% or less. Preferably, the amount is 0.05% or less, and more preferably 0.01% or less. In this connection, reduction of S to less than 0.0005% is attended with a significant increase in production cost. Therefore, the lower limit thereof is about 0.0005% from the viewpoint of the production cost.

Al: 3.0% or Less

The element Al is useful for strengthening steel and, in addition, is a useful element which is added as a deoxidizing agent in a steel making process. If the amount of Al exceeds 3.0%, inclusion in a steel sheet increases and elongation deteriorates. Therefore, the amount of Al is 3.0% or less. The amount is preferably 2.0% or less.
Moreover, Al is an element useful for suppressing generation of carbides and facilitating generation of retained austenite. Furthermore, it is preferable that the amount of Al is 0.001% or more to obtain a deoxidation effect, and more preferably 0.005% or more. In this regard, the amount of Al is the amount of Al contained in the steel sheet after deoxidation.

N: 0.010% or Less

The element N causes maximum deterioration of the aging resistance of steel and is preferably minimized. If the amount of N exceeds 0.010%, deterioration of the aging resistance becomes significant and, therefore, the amount of N is 0.010% or less. In this connection, reduction of N to less than 0.001% causes a significant increase in production cost so that the lower limit thereof is about 0.001% from the viewpoint of production cost.
Up to this point, the basic components have been described. However, only satisfaction of the above-described component ranges is not adequate, and it is necessary that the following formula is satisfied.
Si+Al≧0.7%
As described above, both Si and Al are elements useful for suppressing generation of carbides and facilitating generation of retained austenite. Regarding suppression of generation of carbides, an effect is exerted by containing Si or Al alone, but it is necessary to satisfy that a total of the amount of Si and Al is 0.7% or more. In this connection, the amount of Al in the above-described formula is the amount of Al contained in the steel sheet after deoxidation.
In addition, the components described below can be contained appropriately besides the above-described basic components.
At Least One Type Selected from Cr: 0.05% or More, and 5.0% or Less, V: 0.005% or More, and 1.0% or Less, and Mo: 0.005% or More, and 0.5% or Less
The elements Cr, V, and Mo function to suppress generation of pearlite during cooling from an annealing temperature. The effect thereof is obtained at Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. On the other hand, if Cr: 5.0%, V: 1.0%, and Mo: 0.5% are exceeded, the amount of hard martensite becomes too large, and the strength becomes high more than necessary. Therefore, in the case where Cr, V, and Mo are contained, the ranges are Cr: 0.05% or more and 5.0% or less, V: 0.005% or more and 1.0% or less, and Mo: 0.005% or more and 0.5% or less.
At Least One Type Selected from Ti: 0.01% or More, and 0.1% or Less and Nb: 0.01% or More, and 0.1% or Less
The elements Ti and Nb are useful for strengthening steel through deposition, and the effect thereof is obtained when the individual contents are 0.01% or more. On the other hand, if the individual contents exceed 0.1%, workability and shape fixability deteriorate. Therefore, in the case where Ti and Nb are contained, the ranges are Ti: 0.01% or more and 0.1% or less and Nb: 0.01% or more and 0.1% or less.

B: 0.0003% or More, and 0.0050% or Less

The element B is useful for suppressing generation•growth of ferrite from austenite grain boundaries. The effect thereof is obtained when the content is 0.0003% or more. On the other hand, if the content exceeds 0.0050%, workability deteriorates. Therefore, in the case where B is contained, the range is B: 0.0003% or more and 0.0050% or less.
At Least One Type Selected from Ni: 0.05% or More, and 2.0% or Less and Cu: 0.05% or More, and 2.0% or Less
The elements Ni and Cu are useful for strengthening steel. Furthermore, in the case where galvanizing or galvannealing is applied to a steel sheet, internal oxidation of a steel sheet surface layer portion is facilitated and, thereby, adhesion of the coating is improved. These effects are obtained when individual contents are 0.05% or more. On the other hand, if the individual contents exceed 2.0%, the workability of the steel sheet deteriorates. Therefore, in the case where Ni and Cu are contained, the ranges are Ni: 0.05% or more and 2.0% or less and Cu: 0.05% or more and 2.0% or less.
At Least One Type Selected from Ca: 0.001% or More, and 0.005% or Less and REM: 0.001% or More, and 0.005% or Less
The elements Ca and REM are useful for spheroidizing the shape of sulfides and improve the adverse effect of sulfides on stretch-flangeability. The effects thereof are obtained when individual contents are 0.001% or more. On the other hand, if the individual contents exceed 0.005%, increases of inclusion and the like are invited to cause surface defects, internal defects, and the like. Therefore, in the case where Ca and REM are contained, the ranges are Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.
The components other than those described above are Fe and incidental impurities. However, components other than those described above may be contained within the bounds of not impairing the effects of our steel sheets.
Next, a method for manufacturing a high strength steel sheet will be described.
After a billet adjusted to have the above-described favorable component composition is produced, hot-rolling is conducted and, then, cold-rolling is conducted to produce a cold-rolled steel sheet. These treatments are not specifically limited and may be conducted following usual methods.
Favorable production conditions are as described below. After the billet is heated to a temperature within the range of 1,000° C. or higher and 1,300° C. or lower, the hot rolling is terminated in a temperature range of 870° C. or higher and 950° C. or lower. The resulting hot-rolled steel sheet is taken up in a temperature range of 350° C. or higher and 720° C. or lower. Subsequently, the hot-rolled steel sheet is pickled and, thereafter, cold-rolling is conducted at a reduction ratio within the range of 40% or more and 90% or less to produce a cold-rolled steel sheet.
In this connection, it is assumed that the steel sheet is produced through usual individual steps of steel making, casting, hot rolling, pickling, and cold rolling. However, for example, production may be conducted through thin slab casting or strip casting while a part of or an entire hot rolling step is omitted.
A heat treatment shown in FIG. 1 is applied to the resulting cold-rolled steel sheet. The explanation will be conducted below with reference to FIG. 1.
Annealing is conducted for 15 seconds or more and 600 seconds or less in an austenite single phase region. The steel sheet contains upper bainite, lower bainite, and martensite, which are transformed from untransformed austenite in a relatively low temperature range of 350° C. or higher and 490° C. or lower, as primary phases. Therefore, it is preferable that polygonal ferrite is minimized and annealing in an austenite single phase region is required. The annealing temperature is not specifically limited insofar as it is in the austenite single phase region. If the annealing temperature exceeds 1,000° C., growth of austenite grains is significant, coarser configuration phases are generated by downstream cooling, and tenacity and the like deteriorate. On the other hand, in the case where the annealing temperature is lower than A₃point (austenite transformation point), polygonal ferrite has already been generated in an annealing stage, and it becomes necessary that a temperature range of 500° C. or more is cooled very rapidly to suppress growth of polygonal ferrite during cooling. Therefore, it is necessary that the annealing temperature is the A₃point or higher and, preferably, 1,000° C. or lower.
Furthermore, if the annealing time is less than 15 seconds, in some cases, reverse transformation to austenite does not proceed adequately or carbides in the steel sheet are not dissolved adequately. On the other hand, if the annealing time exceeds 600 seconds, an increase in cost is invited along with high energy consumption. Therefore, the annealing time is within the range of 15 seconds or more, and 600 seconds or less. Preferably, the annealing time is within the range of 60 seconds or more, and 500 seconds or less. The A₃point can be calculated on the basis of
A₃point (° C.)=910−203×[C %]½+44.7×[Si %]−30×[Mn %]+700×[P %]+130×[Al %]−15.2×[Ni %]−11×[Cr %]−20×[Cu %]+31.5×[Mo %]+104×[V %]+400×[Ti %].
In this connection, [X %] represents percent by mass of component element X of the steel sheet.
The cold-rolled steel sheet after annealing is cooled to a cooling termination temperature: T° C. determined in a first temperature range of 350° C. or higher and 490° C. or lower, wherein cooling to at least 550° C. is conducted while the average cooling rate is controlled at 5° C./s or more. In the case where the average cooling rate is less than 5° C./s, excessive generation and growth of polygonal ferrite, deposition of pearlite and the like occur so that a desired steel sheet microstructure is not obtained. Therefore, the average cooling rate from the annealing temperature to the first temperature range is 5° C./s or more. Preferably, the average cooling rate is 10° C./s or more. The upper limit of the average cooling rate is not specifically limited insofar as variations do not occur in the cooling termination temperature. If the average cooling rate exceeds 100° C./s, variations in microstructure in a longitudinal direction and a sheet width direction of a steel sheet becomes large significantly. Therefore, 100° C./s or less is preferable.
The steel sheet cooled to 550° C. is cooled succeedingly to the cooling termination temperature: T° C. The rate of cooling of the steel sheet in the temperature range of T° C. or higher and 550° C. or lower is not specifically limited except that a maintenance time in the first maintenance temperature range is 15 seconds or more and 1,000 seconds or less. However, in the case where the steel sheet is cooled at a too low rate, carbides are generated from untransformed austenite and, thereby, there is a high probability that a desired microstructure is not obtained. Therefore, it is preferable that the steel sheet is cooled at an average rate of 1° C./s or more in a temperature range of T° C. or higher and 550° C. or lower.
The steel sheet cooled to the cooling termination temperature: T° C. is kept in the first temperature range of 350° C. or higher and 490° C. or lower for a period of 15 seconds or more, and 1,000 seconds or less. If the upper limit of the first temperature range exceeds 490° C., carbides are deposited from the untransformed austenite and, thereby, a desired microstructure is not obtained. On the other hand, in the case where the lower limit of the first temperature range is lower than 350° C., a problem occurs in that lower bainite is generated rather than upper bainite and the amount of C concentrated into austenite is reduced. Therefore, the first temperature range is 350° C. or higher and 490° C. or lower. Preferably, the range is 370° C. or higher and 460° C. or lower.
Moreover, in the case where the maintenance time in the first temperature range is less than 15 seconds, a problem occurs in that the amount of upper bainite transformation is reduced and the amount of C concentrated into untransformed austenite is reduced. On the other hand, in the case where the maintenance time in the first temperature range exceeds 1,000 seconds, carbides are deposited from untransformed austenite which serves as retained austenite in the final microstructure of the steel sheet, stable retained austenite, into which C has been concentrated, is not obtained and, as a result, a desired workability is not obtained. Therefore, the maintenance time is 15 seconds or more and 1,000 seconds or less. Preferably, the range is 30 seconds or more and 600 seconds or less.
After maintaining the first temperature range is completed, the resulting steel sheet is cooled to a second temperature range of 200° C. or higher and 350° C. or lower at any rate and is kept in the second temperature range for a period of 15 seconds or more and 1,000 seconds or less. If the upper limit of the second temperature range exceeds 350° C., a problem occurs in that lower bainite transformation does not proceed and, as a result, the amount of as-quenched martensite increases. On the other hand, in the case where the lower limit of the second temperature range is lower than 200° C. as well, a problem occurs in that lower bainite transformation does not proceed and the amount of as-quenched martensite increases. Therefore, the second temperature range is 200° C. or higher and 350° C. or lower. Preferably, the range is 250° C. or higher and 340° C. or lower.
Moreover, in the case where the maintenance time is less than 15 seconds, an adequate amount of lower bainite is not obtained, and desired workability is not obtained. On the other hand, in the case where the maintenance time exceeds 1,000 seconds, carbides are deposited from the stable retained austenite in the upper bainite generated in the first temperature range and, as a result, desired workability is not obtained. Therefore, the maintenance time is 15 seconds or more and 1,000 seconds or less. Preferably, the range is 30 seconds or more and 600 seconds or less.
In this regard, in a series of heat treatments, the maintenance temperature is not necessarily a constant insofar as the maintenance temperature is within the above-described predetermined temperature range, and fluctuation within a predetermined temperature range does not impair the steel sheets. The same goes for the cooling rate. Furthermore, the steel sheet may be heat-treated with any facility insofar as only the thermal history is satisfied. In addition, temper rolling may be applied to the surface of the steel sheet or a surface treatment, e.g., electroplating, may be applied after the heat treatment to correct the shape.
The method for manufacturing a high strength steel sheet can further include a galvanizing treatment or a galvannealing treatment in which an alloying treatment is further added to the galvanizing treatment. The galvanizing treatment or, furthermore, the galvannealing treatment may be conducted during the above-described cooling to the first temperature range or in the first temperature range. In this case, the maintenance time in the first temperature range is 15 seconds or more and 1,000 seconds or less, in which a treatment time of the galvanizing treatment or the galvannealing treatment in the first temperature range is included. In this connection, it is preferable that the galvanizing treatment or the galvannealing treatment is conducted with a continuous galvanizing and galvannealing line.
Furthermore, the method for manufacturing a high strength steel sheet can include that the high strength steel sheet is produced following the above-described manufacturing method where steps up to the heat treatment have been completed and, thereafter, the galvanizing treatment or, furthermore, the galvannealing treatment is conducted.
Alternatively, after maintaining the second temperature range following the manufacturing method, the galvanizing treatment or the galvannealing treatment can be conducted succeedingly.
A method for applying a galvanizing treatment or a galvannealing treatment to a steel sheet is as described below.
The steel sheet is immersed into a plating bath, and the amount of adhesion is adjusted through gas wiping or the like. It is preferable that the amount of Al dissolved in the plating bath is 0.12% or more and 0.22% or less in the case of the galvanizing treatment and 0.08% or more and 0.18% or less in the case of the galvannealing treatment.
Regarding the treatment temperature, as for the galvanizing treatment, the temperature of the plating bath may be 450° C. or higher and 500° C. or lower and, furthermore, in the case where the galvannealing treatment is applied, it is preferable that the temperature during alloying is 550° C. or lower. In the case where the alloying temperature exceeds 550° C., carbides are deposited from untransformed austenite and in some cases, pearlite is generated. Consequently, the strength or the workability, or the two are not obtained. In addition, the powdering property of the coating layer deteriorates. On the other hand, if the temperature during alloying is lower than 450° C., in some cases, alloying does not proceed. Therefore, it is preferable that the alloying temperature is 450° C. or higher.
It is preferable that the coating mass is 20 g/m²or more and 150 g/m²or less per surface. If the coating mass is less than 20 g/m², the corrosion resistance becomes inadequate. On the other hand, even when 150 g/m²is exceeded, the corrosion-resisting effect is saturated and an increase in the cost is likely.
It is preferable that the degree of alloying of the coating layer (Fe percent by mass (Fe content)) is 7 percent by mass or more and 15 percent by mass or less. If the degree of alloying of the coating layer is less than 7 percent by mass, alloying variations occur so that the quality of outward appearance deteriorates, or a so-called a ζ phase is generated in the coating layer so that the sliding property of the steel sheet deteriorates. On the other hand, if the degree of alloying of the coating layer exceeds 15 percent by mass, large amounts of hard brittle Γ phase is formed so that the adhesion of the coating deteriorates.

EXAMPLES

Our steel sheets and methods will be described below in further detail with reference to the examples. However, the following examples do not limit the scope of this disclosure. In this connection, modification of the configurations and range is included in the scope of this disclosure.
An ingot obtained by melting a steel having a component composition shown in Table 1 was heated to 1,200° C. and subjected to finish hot rolling at 870° C. The resulting hot-rolled steel sheet was taken up at 650° C. and, subsequently, the hot-rolled steel sheet was pickled. Thereafter, cold rolling was conducted at a reduction ratio of 65% to produce a cold-rolled steel sheet having a sheet thickness: 1.2 mm. The resulting cold-rolled steel sheet was subjected to a heat treatment under the conditions shown in Table 2. In this connection, the cooling termination temperature: T in Table 2 refers to a temperature at which cooling of a steel sheet is terminated in cooling of the steel sheet from the annealing temperature.
Furthermore, a part of cold-rolled steel sheets were subjected to a galvanizing treatment or a galvannealing treatment. As for the galvanizing treatment, plating was conducted on both surfaces at a plating bath temperature: 463° C. in such a way that a mass per unit area (per surface): 50 g/m²was ensured. Moreover, as for the galvannealing treatment, plating was conducted on both surfaces while the alloying condition was adjusted in such a way that a mass per unit area (per surface): 50 g/m²was ensured and the degree of alloying (Fe percent by mass (Fe content)) became 9 percent by mass. The galvanizing treatment and the galvannealing treatment were conducted after cooling was once conducted to T° C. shown in Table 2.
The resulting steel sheet was subjected to temper rolling at a reduction ratio (elongation percentage): 0.3 after a heat treatment in the case where a plating treatment is not conducted, or after a galvanizing treatment or a galvannealing treatment in the case where these treatments were conducted.

TABLE 1

Steel
type	C	Si	Mn	Al	P	S	N	Cr	V	Mo	Ti	Nb	B

A	0.311	1.96	1.54	0.041	0.009	0.0024	0.0025	—	—	—	—	—	—
B	0.299	1.98	1.99	0.042	0.013	0.0019	0.0034	—	—	—	—	—	—
C	0.305	2.52	2.03	0.043	0.010	0.0037	0.0042	—	—	—	—	—	—
D	0.413	2.03	1.51	0.038	0.012	0.0017	0.0025	—	—	—	—	—	—
E	0.417	1.99	2.02	0.044	0.010	0.0020	0.0029	—	—	—	—	—	—
F	0.330	1.45	2.82	0.040	0.012	0.0031	0.0043	—	—	—	—	—	—
G	0.185	1.52	2.32	0.048	0.020	0.0050	0.0044	—	—	—	—	—	—
H	0.522	1.85	1.48	0.040	0.011	0.0028	0.0043	—	—	—	—	—	—
I	0.320	0.99	2.25	0.041	0.014	0.0018	0.0042	—	—	—	—	—	—
J	0.263	1.50	2.29	0.039	0.011	0.0010	0.0036	0.9	—	—	—	—	—
K	0.270	1.35	2.27	0.043	0.004	0.0020	0.0035	—	0.21	—	—	—	—
L	0.221	1.22	1.99	0.040	0.040	0.0030	0.0043	—	—	0.19	—	—	—
M	0.202	1.75	2.52	0.045	0.044	0.0020	0.0044	—	—	—	0.035	—	—
N	0.175	1.51	2.18	0.042	0.022	0.0020	0.0044	—	—	—	—	0.07	—
O	0.212	1.51	2.37	0.043	0.030	0.0010	0.0029	—	—	—	0.020	—	0.0011
P	0.480	1.52	1.33	0.044	0.015	0.0020	0.0038	—	—	—	—	—	—
Q	0.310	1.42	2.02	0.043	0.015	0.0030	0.0023	—	—	—	—	—	—
R	0.335	2.01	2.22	0.043	0.004	0.0028	0.0041	—	—	—	—	—	—
S	0.329	1.88	1.65	0.040	0.021	0.0020	0.0031	—	—	—	—	—	—
T	0.330	0.01	2.33	1.010	0.025	0.0020	0.0033	—	—	—	—	—	—
U	0.291	—	2.75	0.042	0.012	0.0040	0.0024	—	—	—	—	—	—
V	0.290	0.48	2.22	0.130	0.006	0.0020	0.0035	—	—	—	—	—	—
W	0.145	0.50	1.42	0.320	0.007	0.0018	0.0041	—	—	—	—	—	—
X	0.190	1.00	0.41	0.036	0.013	0.0020	0.0038	—	—	—	—	—	—

Steel						A₃point
type	Ni	Cu	Ca	REM	Si + Al	(° C.)	Remarks

A	—	—	—	—	2.00	850	Steel
B	—	—	—	—	2.02	842	Steel
C	—	—	—	—	2.56	862	Steel
D	—	—	—	—	2.07	838	Steel
E	—	—	—	—	2.03	820	Steel
F	—	—	—	—	1.49	787	Steel
G	—	—	—	—	1.57	841	Steel
H	—	—	—	—	1.89	815	Steel
I	—	—	—	—	1.03	787	Steel
J	—	—	—	—	1.54	807	Steel
K	—	—	—	—	1.39	827	Steel
L	—	—	—	—	1.26	849	Steel
M	—	—	—	—	1.80	872	Steel
N	—	—	—	—	1.55	848	Steel
O	—	—	—	—	1.55	848	Steel
P	0.52	—	—	—	1.56	806	Steel
Q	—	0.55	—	—	1.46	805	Steel
R	—	—	0.003	—	2.05	824	Steel
S	—	—	—	0.002	1.92	848	Steel
T	—	—	—	—	1.02	873	Steel
U	—	—	—	—	0.04	732	Comparative
							Steel
V	—	—	—	—	0.61	111	Comparative
							Steel
W	—	—	—	—	0.82	859	Comparative
							Steel
X	—	—	—	—	1.04	868	Comparative
							Steel

Note)
Underline indicates that the value is out of the appropriate range.

TABLE 2

					Average cooling	Cooling rate
			Annealing	Annealing	rate to	550° C. to
Sample	Steel	Coating	temperature	time	550° C.	T° C.
No.	type	*2	(° C.)	(s)	(° C./s)	(° C./s)

1	A	CR	880	180	4	15
2	A	CR	900	180	20	20
3	A	CR	900	200	50	50
4	A	CR	900	200	50	50
5	B	CR	800	200	20	20
6	B	CR	880	200	20	20
7	B	CR	880	350	35	35
8	C	CR	890	150	25	25
9	C	CR	900	200	20	20
10	D	CR	900	200	20	20
11	D	CR	900	200	50	50
12	E	CR	880	250	15	15
13	F	CR	870	300	20	20
14	F	GI	870	300	12	12
15	G	CR	890	200	20	20
16	H	CR	880	200	25	25
17	I	CR	900	250	30	30
18	I	GA	900	250	20	20
19	J	CR	900	200	20	20
20	K	CR	900	200	40	40
21	L	CR	900	200	30	30
22	M	CR	900	200	20	20
23	N	CR	900	200	20	20
24	O	CR	900	200	20	20
25	P	CR	900	200	20	20
26	Q	CR	900	200	30	30
27	R	CR	900	200	30	30
28	S	CR	900	200	30	30
29	T	CR	900	200	30	30
30	U	CR	900	200	13	13
31	V	CR	900	200	20	20
32	W	CR	900	200	40	40
33	X	CR	900	200	15	15

		Maintaining	Second temperature
	Cooling	time in	range

	termination	first	Maintaining	Maintaining
Sample	temperature	temperature	temperature	times
No.	(° C.)	range (s)	(° C.)	(s)	Remarks

1	430	100	300	100	Comparative
					Example
2	400	5	320	90	Comparative
					Example
3	420	100	330	180	Example
4	400	100	330	300	Example
5	400	120	300	100	Comparative
					Example
6	520	200	330	300	Comparative
					Example
7	400	100	330	350	Example
8	400	80	110	120	Comparative
					Example
9	380	120	310	300	Example
10	400	100	330	300	Example
11	400	300	250	10	Comparative
					Example
12	400	200	340	550	Example
13	450	100	330	250	Example
14	450	100	330	200	Example
15	400	90	240	420	Example
16	370	400	200	500	Example
17	400	150	250	300	Example
18	450	100	280	100	Example
19	370	90	300	300	Example
20	420	90	300	300	Example
21	420	200	300	300	Example
22	420	180	300	300	Example
23	420	100	300	300	Example
24	420	100	300	300	Example
25	420	300	300	300	Example
26	420	120	300	300	Example
27	420	100	300	300	Example
28	420	100	300	300	Example
29	420	120	300	300	Example
30	420	100	300	300	Comparative
					Example
31	420	100	300	300	Comparative
					Example
32	420	60	300	300	Comparative
					Example
33	420	60	300	300	Comparative
					Example

*1 Underline indicates that the value is out of the appropriate range.
*2 CR: No coating (cold-rolled steel sheet) GI: Galvanized steel sheet GA :Galvannealed steel sheet

Various characteristics of the thus obtained steel sheet were evaluated by the following methods.
A sample was cut from each steel sheet and was polished. Microstructures of ten fields of view of a surface parallel to the rolling direction were observed with a scanning electron microscope (SEM) at 3,000-fold magnification, the area percentage of each phase was measured, and a phase structure of each crystal grain was identified.
The steel sheet was ground•polished up to one-quarter of a sheet thickness in the sheet thickness direction and the amount of retained austenite was determined by X-ray diffractometry. As for an incident X-ray, Co—Kα was used and the amount of retained austenite were calculated from the average value of the intensity ratio of each of (200), (220), and (311) faces of austenite to the diffraction intensity of each of (200), (211), and (220) faces of ferrite.
As for the average amount of C in the retained austenite, a lattice constant was determined from the intensity peak of each of (200), (220), and (311) faces of austenite based on the X-ray diffractometry, and the average amount of C (percent by mass) in the retained austenite was determined from the following calculation formula:
a0=0.3580+0.0033×[C %]+0.00095×[Mn %]+0.0056×[Al %]+0.022×[N %]
where, a0 represents a lattice constant (nm) and [X %] represents percent by mass of an element X. The percent by mass of an element other than C was percent by mass relative to whole steel sheet.
The tensile test was conducted based on JIS Z2241 by using a test piece of JIS No. 5 size taken in a direction perpendicular to the rolling direction of the steel sheet. The TS (tensile strength) and the T.E (total elongation) were measured, a product of the strength and the total elongation (TS×T.El) was calculated and, thereby, the balance between the strength and the workability (elongation) was evaluated. Cases where TS×T.El≧20,000 MPa·% were evaluated as “good.”
The stretch-flangeability was evaluated on the basis of the Japan Iron and Steel Federation Standard JFST 1001. Each of the resulting steel sheets was cut into 100 mm×100 mm, a hole having a diameter: 10 mm was punched with a clearance of 12% of sheet thickness. Thereafter, a dice having an inside diameter: 75 mm was used, a 60° circular cone punch was pushed into the hole while holding was conducted with a holddown force: 88.2 kN, a hole diameter at crack occurrence limit was measured, and a hole-expansion limit λ (%) was determined from the formula (1):
hole-expansion limit λ(%)={(Df−D0)/D0}×100 (1)
where Df represents a hole diameter (mm) at occurrence of crack and D0 represents an initial hole diameter (mm).
The thus measured λ was used, the product of the strength and the hole-expansion limit (TS×2) was calculated and, thereby, the balance between the strength and the stretch-flangeability was evaluated.
Stretch-flangeability was evaluated as “good” in the case where TS×λ≧25,000 MPa·%.
Furthermore, the hardness of the hardest microstructure in the steel sheet micro-structure was determined by a method described below. That is, as a result of microstructure observation, in the case where as-quenched martensite was observed, 10 points of the as-quenched martensite were measured with an ultramicro-Vickers at a load: 0.02 N, and an average value thereof was assumed to be the hardness of the hardest microstructure in the steel sheet microstructure. In this connection, in the case where as-quenched martensite is not observed, as described above, the microstructure of any one of the tempered martensite, the upper bainite, and the lower bainite becomes the hardest phase in our steel sheets. In the case of our steel sheets, the hardest phase was a phase showing HV ≦800.
The above-described evaluation results are shown in Table 3.

	TABLE 3

	(As-

Area percentage relative to whole steel sheet micro structure (%)

quenched

Sample	Steel		LB*²+	As-				αb + LB +	M)/(M +
No.	type	αb*²	M*²	quenched M	α*²	γ²³	Remainder	M + γ	LB) (%)

1	A	3	6	0	58	1	32	10	0
2	A	4	89	10	3	4	0	97	11
3	A	54	31	10	2	13	0	98	32
4	A	56	30	7	2	12	0	98	23
5	B	21	49	10	21	6	3	76	20
6	B	37	49	10	3	8	3	94	20
7	B	50	38	10	0	12	0	100	26
8	C	50	35	28	0	15	0	100	80
9	C	52	34	11	0	14	0	100	32
10	D	45	39	9	0	16	0	100	23
11	D	58	21	18	0	21	0	100	86
12	E	25	63	30	0	12	0	100	48
13	F	15	78	30	0	7	0	100	38
14	F	14	76	30	2	8	0	98	39
15	G	70	14	2	7	9	0	93	14
16	H	16	78	17	0	6	0	100	22
17	I	37	52	20	0	11	0	100	38
18	I	36	55	38	0	9	0	100	69
19	J	16	75	14	1	9	0	100	19
20	K	22	69	21	0	9	0	100	30
21	L	20	69	22	0	11	0	100	32
22	M	36	56	13	0	8	0	100	23
23	N	33	58	35	0	9	0	100	60
24	O	35	55	15	0	10	0	100	27
25	P	30	57	25	0	13	0	100	44
26	Q	40	44	18	0	16	0	100	41
27	R	22	68	17	0	10	0	100	25
28	S	60	29	12	0	11	0	100	41
29	T	42	47	25	0	11	0	100	53
30	U	40	41	20	9	2	8	83	49
31	V	39	54	24	4	3	0	96	44
32	W	78	8	3	0	3	11	89	38
33	X	8	1	1	70	0	21	9	100

	Average amount
	of C in
Sample	retained	TS	T.EL	λ	TS × T.EL	TS × λ
No.	γ (%)	(MPa)	(%)	(%)	(MPa · %)	(MPa · %)	Remarks

1	—	841	21	38	17661	31958	Comparative
							Example
2	0.91	1492	12	20	17904	29840	Comparative
							Example
3	1.14	1166	19	34	22154	39644	Example
4	1.23	1156	21	34	24276	39304	Example
5	0.68	1296	13	20	16848	25920	Comparative
							Example
6	0.57	1467	11	22	16137	32274	Comparative
							Example
7	1.22	1302	18	23	23436	29946	Example
8	0.94	1482	20	5	29640	7410	Comparative
							Example
9	1.16	1371	19	20	26049	27420	Example
10	1.36	1335	24	21	32040	28035	Example
11	1.20	1203	29	8	34887	9624	Comparative
							Example
12	1.15	1695	15	18	25425	30510	Example
13	0.81	1710	14	19	23940	32490	Example
14	0.75	1632	13	19	21216	31008	Example
15	0.74	1098	21	45	23058	49410	Example
16	1.09	1820	14	18	25480	32760	Example
17	0.85	1395	16	21	22320	29295	Example
18	0.82	1314	17	20	22338	26280	Example
19	0.81	1783	12	17	21396	30311	Example
20	0.83	1612	13	19	20956	30628	Example
21	0.73	1870	11	14	20570	26180	Example
22	0.82	1285	21	20	26985	25700	Example
23	0.79	1045	25	38	26125	39710	Example
24	0.86	1230	19	28	23370	34440	Example
25	0.92	1771	14	19	24794	33649	Example
26	0.95	1596	13	20	20748	31920	Example
27	0.96	1482	14	37	20748	54834	Example
28	1.05	1465	17	28	24905	41020	Example
29	1.07	1355	19	33	25745	44715	Example
30	—	1183	13	23	15379	27209	Comparative
							Example
31	—	1288	12	23	15456	29624	Comparative
							Example
32	—	901	14	32	12614	28832	Comparative
							Example
33	—	735	14	30	10290	22050	Comparative
							Example

*¹Underline indicates that the value is out of the appropriate range.
*²αb: Bainitic ferrite in upper bainite LB: Lower bainite M: Martensite α: Polygonal ferrite γ: Retained austenite
*³Amount of retained austenite determined by X-ray diffractometry was assumed to be area percentage relative to whole steel sheet microstructure.

As is clear from Table 3, our steel sheets satisfy that the tensile strength is 980 MPa or more, the value of TS×T.El is 20,000 MPa·% or more, and TS×λ≧25,000 MPa·%. Therefore, it was able to be ascertained that high strength and excellent workability, especially excellent stretch-flangeability, were provided in combination.
On the other hand, regarding Sample No. 1, the average cooling rate to 550° C. was out of the appropriate range. Therefore, a desired steel sheet microstructure was not obtained. Although TS×λ≧25,000 MPa·% was satisfied, the tensile strength (TS) ≧980 MPa and TS×T.El≧20,000 MPa·% were not satisfied. Regarding Sample No. 2, the maintenance time in the first temperature range was out of the appropriate range. Regarding Sample No. 5, the annealing temperature was lower than A₃point. Regarding Sample No. 6, the cooling termination temperature: T was out of the first temperature range. Regarding Sample No. 8, the maintenance temperature in the second temperature range was out of the appropriate range. Regarding Sample No. 11, the maintenance time in the second temperature range was out of the appropriate range. Therefore, a desired steel sheet microstructure was not obtained. Although the tensile strength (TS) ≧980 MPa was satisfied, any one of TS×T.El≧20,000 MPa·% and TS×λ≧25,000 MPa·% was not satisfied. Regarding Sample Nos. 30 to 34, the component compositions were out of the appropriate range. Therefore, a desired steel sheet microstructure was not obtained, and at least one of the tensile strength (TS) ≧980 MPa, TS×T.El≧20,000 MPa·%, and TS×λ≧25,000 MPa·% was not satisfied.

Claims

1. A high strength steel sheet having a composition comprising, on a percent by mass basis,

C: 0.17% to 0.73%;

Si: 3.0% or less;

Mn: 0.5% to 3.0%;

P: 0.1% or less;

S: 0.07% or less;

Al: 3.0% or less; and

N: 0.010% or less,

satisfies Si+Al≧0.7%, and the remainder includes Fe and incidental impurities,

with a microstructure, it is satisfied that the that has an area percentage of a total amount of lower bainite and whole martensite 10% to 90% relative to the whole steel sheet microstructure, an amount of retained austenite is 5% to 50%, an area percentage of bainitic ferrite in upper bainite is 5% or more relative to the whole steel sheet microstructure, as-quenched martensite is 75% or less of the total amount of lower bainite and whole martensite, and an area percentage of polygonal ferrite is 10% or less relative to the whole steel sheet microstructure, an average amount of C in retained austenite is 0.70% or more, and tensile strength is 980 MPa or more.

2. The high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Cr: 0.05% to 5.0%;

V: 0.005% to 1.0%; and

Mo: 0.005% to 0.5%.

3. The high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ti: 0.01% to 0.1%; and

Nb: 0.01% to 0.1%.

4. The high strength steel sheet according to claim 1, further comprising, on a percent by mass basis,

B: 0.0003% to 0.0050%.

5. The high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ni: 0.05% to 2.0%; and

Cu: 0.05% to 2.0%.

6. The high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ca: 0.001% to 0.005%; and

REM: 0.001% to 0.005%.

7. A high strength steel sheet comprising a galvanized layer or a galvannealed layer on a surface of the steel sheet according to claim 1.

8. A method for manufacturing a high strength steel sheet comprising:

hot-rolling a billet having a component composition according to claim 1;

conducting cold-rolling to produce a cold-rolled steel sheet;

annealing the resulting cold-rolled steel sheet for 15 to 600 seconds in an austenite single phase region; and

conducting cooling to a cooling termination temperature: T° C. determined in a first temperature range of 350° C. to 490° C., wherein cooling to at least 550° C. is conducted while an average cooling rate is controlled at 5° C./s or more, subsequently, temperature is maintained in the first temperature range for 15 to 1,000 seconds and, then, temperature is maintained in a second temperature range of 200° C. to 350° C. for 15 to 1,000 seconds or less.

9. The method according to claim 8, further comprising applying galvanizing treatment or a galvannealing treatment during cooling to a cooling termination temperature: 1° C. or in the first temperature range.

10. The high strength steel sheet according to claim 2, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ti: 0.01% to 0.1%; and

Nb: 0.01% to 0.1%.

11. The high strength steel sheet according to claim 2, further comprising, on a percent by mass basis,

B: 0.0003% to 0.0050%.

12. The high strength steel sheet according to claim 3, further comprising, on a percent by mass basis,

B: 0.0003% to 0.0050%.

13. The high strength steel sheet according to claim 2, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ni: 0.05% to 2.0%; and

Cu: 0.05% to 2.0%.

14. The high strength steel sheet according to claim 3, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ni: 0.05% to 2.0%; and

Cu: 0.05% to 2.0%.

15. The high strength steel sheet according to claim 4, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ni: 0.05% to 2.0%; and

Cu: 0.05% to 2.0%.

16. The high strength steel sheet according to claim 2, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ca: 0.001% to 0.005%; and

REM: 0.001% to 0.005%.

17. The high strength steel sheet according to claim 3, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ca: 0.001% to 0.005%; and

REM: 0.001% to 0.005%.

18. The high strength steel sheet according to claim 4, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ca: 0.001% to 0.005%; and

REM: 0.001% to 0.005%.

19. The high strength steel sheet according to claim 5, further comprising at least one element selected from the group consisting of, on a percent by mass basis,

Ca: 0.001% to 0.005%; and

REM: 0.001% to 0.005%.