KR20170103905A - Ultra-high strength steel sheet with excellent yield ratio and workability - Google Patents

Abstract

C, Si, Mn, and Al, the balance being iron and inevitable impurities, and P, S, and N among the inevitable impurities are each limited to a specific amount, and martensite has a composition of 90 Area percent or more, residual austenite of 0.5% or more by area, a region where the concentration of Mn in the local region is 1.2 times or more of the Mn content of the entire steel sheet is 1% or more by area, a tensile strength is 1470 MPa or more, Ultra high strength steel sheet with a total elongation of 10% or more.

Description

Ultra-high strength steel sheet with excellent yield ratio and workability

The present invention relates to an ultra high strength steel sheet excellent in yield ratio and workability. The steel sheet of the super high strength steel sheet according to the present invention includes various coated steel sheets such as cold rolled steel sheets, hot-dip galvanized steel sheets, and galvannealed galvanized steel sheets.

A steel plate used for a skeleton member of an automobile is required to have a high strength for the purpose of improving the fuel efficiency by reducing the weight of the vehicle body and a high yield ratio is also required in order to secure collision safety. On the other hand, excellent workability is also required for molding into a component having a complicated shape.

Therefore, it is urgently desired to provide an ultra-high strength steel sheet having a high elongation (elongation: EL) while having a high yield ratio. Concretely, a steel sheet having a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more is required.

It is known that automotive steel sheets are welded at the time of assembling a vehicle body or at the time of installation of components. However, it is known that the weldability depends greatly on the composition of the steel sheet, and particularly when C and Mn are added in large amounts, the weldability is deteriorated. Therefore, steel sheets for automobiles are required to satisfy the above-mentioned mechanical properties with a component composition satisfying C of 0.35 mass% or less and Mn of 1.5 mass% or less.

Conventionally, in order to increase the elongation of the high-strength steel sheet, mainly the following two means have been employed.

(1) The amount of retained austenite is increased, and its TRIP action is utilized.

(2) Increase the amount of soft ferrite (including bainitic ferrite).

However, in the means of (1) above, it is necessary to increase the addition amount of C and Mn in order to retain a large amount of austenite, and C? 0.35 mass% and Mn? 1.5 mass% There was a problem that it could not be done.

On the other hand, in the means of (2) above, a certain amount of soft phase is required to secure elongation, so that the yield ratio can not be satisfied at 0.75 or more, and sufficient collision safety can not be ensured.

For example, Patent Document 1 discloses that by increasing the Mn content in a steel sheet and retaining a large amount of austenite, it is possible to improve the water-proofing property at an ultra-high strength region having a tensile strength of 1180 MPa or more, A steel sheet having excellent resistance to delayed fracture in a drilled portion has been proposed.

However, as shown in the examples of the steel sheet, the inventive steel had a Mn content exceeding 1.5% by mass in the steel sheet, and there was room for improvement in terms of weldability.

Patent Document 2 discloses a steel sheet having a tensile strength of 1470 MPa or more and a total elongation of 10% or more in terms of a composition satisfying C of 0.35 mass% or less and Mn of 1.5 mass% or less by increasing the fraction of soft ferrite phase, A steel sheet capable of realizing the above is proposed.

However, as shown in the embodiment, the steel sheet can not realize a yield ratio of 0.75 or more, so that sufficient collision safety can not be ensured.

Japanese Patent Application Laid-Open No. 2008-81788 Japanese Patent Application Laid-Open No. 2010-90432

Therefore, an object of the present invention is to provide an ultra-high strength steel sheet excellent in yield ratio and workability, having a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more.

The ultra-high strength steel sheet excellent in yield ratio and workability according to the first invention of the present invention,

In terms of% by mass,

C: 0.15 to 0.35%

Si: 0.5 to 3.0%

Mn: 0.5 to 1.5%

Al: 0.001 to 0.10%

Respectively,

The balance being iron and inevitable impurities,

Of the inevitable impurities, P, S, and N,

P: not more than 0.1%

S: 0.01% or less,

N: not more than 0.01%

Respectively,

As an area ratio for the entire tissue,

Martensite: 90% or more,

Residual austenite: 0.5% or more

≪ / RTI >

The area where the local Mn concentration is 1.2 times or more the Mn content of the entire steel sheet is 1% or more in area ratio,

A tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more

.

The ultra-high strength steel sheet excellent in yield ratio and workability according to the second invention of the present invention is the steel sheet according to the first invention,

The composition of matter, in% by mass,

Cu: 0.05 to 1.0%

Ni: 0.05 to 1.0%

B: 0.0002 to 0.0050%

Or a combination of two or more of them.

The ultra-high strength steel sheet excellent in yield ratio and workability according to the third invention of the present invention is the steel sheet according to the first or second invention,

The composition of matter, in% by mass,

Mo: 0.01 to 1.0%

Cr: 0.01 to 1.0%

Nb: 0.01 to 0.3%

Ti: 0.01 to 0.3%

V: 0.01 to 0.3%

Or a combination of two or more of them.

The ultra-high strength steel sheet excellent in yield ratio and workability according to the fourth invention of the present invention is the steel sheet according to any one of the first to third inventions,

The composition of matter, in% by mass,

Ca: 0.0005 to 0.01%

Mg: 0.0005 to 0.01%

Or one or two of the above.

INDUSTRIAL APPLICABILITY According to the present invention, martensite is a main structure and the Mn is concentrated in the retained austenite without increasing the average concentration of C and Mn in the whole steel sheet, It is possible to provide an ultra-high strength steel sheet having a high strength and excellent workability.

Hereinafter, the present invention will be described in more detail.

First, a structure characterizing an ultra-high strength steel sheet (hereinafter also referred to as " steel sheet steel of the present invention ") excellent in yield ratio and workability according to the present invention will be described.

[Tissue of Inventive Steel Sheet]

As described above, the steel sheet of the present invention is characterized by further containing a predetermined amount of retained austenite (hereinafter sometimes referred to as " a ") obtained by concentrating Mn to martensite .

<Martensite: 90% or more>

The martensite is required to have an area ratio of not less than 90%, preferably not less than 92%, more preferably not less than 94%, in order to achieve a tensile strength of not less than 0.75 while realizing a tensile strength of the steel sheet of 1470 MPa or more. In the present specification, on the other hand, martensite is used to mean both untempered fresh martensite and tempered tempering martensite.

On the other hand, since all of the residual austenite other than the residual austenite may be martensitic, the upper limit of the martensite area ratio is 99.5%, preferably 99% or less, when the residual austenite falls within the lower limit (0.5%).

&Lt; Residual austenite: 0.5% or more >

The retained austenite is required to have an area ratio of not less than 0.5%, preferably not less than 0.6%, more preferably not less than 0.7% in order to improve the overall elongation by utilizing its TRIP action.

On the other hand, since all but the martensite may be retained austenite, the upper limit of the retained austenite area percentage is 10%, preferably 5% or less, more preferably 5% or less when the martensite is subjected to the lower limit (90% 3% or less, particularly preferably 2% or less.

As described above, the steel sheet of the present invention may be composed of only two phases of martensite and retained austenite (the total area ratio of the two phases is 100%), but inevitably, other phases (ferrite, bainite, Etc.) may occur. Even if such another phase exists, the sum of the area ratios may be 9.5% or less. The sum of area ratios of the other phases is preferably 7.5% or less, more preferably 5.5% or less.

&Lt; Area in which the Mn concentration of the local region is 1.2 times or more the Mn content of the entire steel sheet: 1% or more in area ratio>

The Mn is concentrated in the retained austenite to increase the stability of the retained austenite so that the retained austenite remains in the highly deformed region to further improve the overall elongation and to secure a total elongation of 10% or more. On the other hand, from the viewpoint of securing the weldability, it is necessary to satisfy the average Mn concentration of 1.5% by mass or less in the steel sheet. That is, the retained austenite formed in the Mn-enriched region is stabilized while keeping the Mn concentration of the mother phase low. As a result, a part of the area where the local Mn concentration becomes 1.2 times or more the Mn content of the entire steel sheet exists as the retained austenite, contributing to the improvement of the overall elongation.

On the other hand, the retained austenite formed in the inventive steel sheet is very fine, and the Mn concentration can not be directly measured. Therefore, it has been found that the Mn concentration in the local region is 1% or more (preferably 1.1% or more, more preferably 1.2% or more) in the area ratio of 1.2 times or more of the Mn content of the entire steel sheet, It is ensured that Mn is sufficiently concentrated in the kneading.

Next, the composition of components constituting the inventive steel sheet will be described. Hereinafter, the units of the chemical components are all% by mass.

[Composition of the steel sheet of the present invention]

C: 0.15 to 0.35%

C is an important element that greatly affects the strength of the steel sheet. In order to secure the strength of the steel sheet, C is contained in an amount of 0.15% or more, preferably 0.16% or more, and more preferably 0.17% or more. However, if C is contained excessively, the weldability deteriorates. Therefore, the content of C is 0.35% or less, preferably 0.3% or less, more preferably 0.25% or less.

Si: 0.5 to 3.0%

Si is an element useful for suppressing the generation of carbide and promoting the formation of retained austenite. To effectively exhibit such an effect, Si is contained at 0.5% or more, preferably at least 0.8%, more preferably at least 1.1%. However, when Si is excessively contained, the weldability is remarkably deteriorated. Therefore, it is 3.0% or less, preferably 2.5% or less, and more preferably 2.0% or less.

Mn: 0.5 to 1.5%

Mn is a solid solution strengthening element and is a useful element contributing to the increase in the strength of the steel sheet. In addition, by increasing the hardenability, there is also an effect of suppressing the ferrite transformation at the time of cooling. Further, since the effect of stabilizing austenite is obtained, residual austenite having high stability can be formed. In order to effectively exhibit such an effect, Mn is contained in an amount of 0.5% or more, preferably 0.7% or more, more preferably 0.9% or more. However, from the viewpoint of ensuring the weldability, the amount of Mn is preferably as low as possible, preferably 1.5% or less, preferably 1.3% or less, more preferably 1.15% or less.

Al: 0.001 to 0.10%

Al is a useful element to be added as a deoxidizing agent. In order to obtain such action, the content of Al is 0.001% or more, preferably 0.01% or more, more preferably 0.03% or more. However, if Al is contained excessively, the deterioration of the cleanliness of the steel is deteriorated. Therefore, the content of Al is 0.10% or less, preferably 0.08% or less, more preferably 0.06% or less.

The steel sheet of the present invention contains the above element as an essential component and the remainder is iron and unavoidable impurities (P, S, N, O, etc.), but P, S and N among the inevitable impurities .

P: not more than 0.1%

P is inevitably present as an impurity element and contributes to an increase in strength due to solid solution strengthening. However, P is segregated at the old austenite grain boundary to deteriorate workability by embrittling the grain boundary, so that the P content is 0.1% or less, preferably 0.05 %, More preferably 0.03% or less.

S: not more than 0.01%

S is inevitably present as an impurity element and forms an MnS inclusion and becomes a starting point of cracking at the time of deformation, thereby lowering the workability. Therefore, the amount of S is 0.01% or less, preferably 0.005% or less And not more than 0.003%.

N: not more than 0.01%

N is inevitably present as an impurity element and degrades the workability of the steel sheet due to strain aging. Therefore, the N content is limited to 0.01% or less, preferably 0.005% or less, more preferably 0.003% or less.

In addition, the following permissible components may be further contained within the range not impairing the action of the present invention.

Cu: 0.05 to 1.0%

Ni: 0.05 to 1.0%

B: 0.0002 to 0.0050%

One or more of

These elements are useful elements having an effect of increasing the hardenability and inhibiting the transformation from austenite. In order to obtain such an action, it is preferable that each of the elements is contained at least the lower limit value. These elements may be contained singly or in combination of two or more. However, even if these elements are contained in excess, the effect becomes saturated and is economically useless, so that the respective elements are made to be not more than the respective upper limit values.

Mo: 0.01 to 1.0%

Cr: 0.01 to 1.0%

Nb: 0.01 to 0.3%

Ti: 0.01 to 0.3%

V: 0.01 to 0.3%

One or more of

These elements are useful elements for improving the strength without deteriorating the processability. In order to obtain such an action, it is preferable that each of the elements is contained at least the lower limit value. These elements may be contained singly or in combination of two or more. However, if these elements are contained in excess, a coarse carbide is formed and the workability deteriorates. Therefore, each of the elements is made to be the upper limit value or less.

Ca: 0.0005 to 0.01%

Mg: 0.0005 to 0.01%

One or two of

These elements are elements useful for improving workability by making the inclusions finer and reducing the origin of fracture. In order to obtain such an action, it is preferable that any element is contained in an amount of 0.0005% or more. The above elements may be used singly or in combination. However, if it is contained excessively, on the contrary, the inclusions are coarsened and the workability deteriorates, so that the content of any element is 0.01% or less.

Next, preferable manufacturing conditions for obtaining the inventive steel sheet will be described below.

[Preferred Production Method of Steel Sheet of the Present Invention]

First, a steel having the above-mentioned composition is firstly melted and turned into a slab (steel material) by coarse ingot or continuous casting, and then the steel having a soaking temperature of 1200 캜 or lower (more preferably 1150 캜 or lower) and a finishing temperature of 900 캜 or lower More preferably 880 占 폚 or lower), and then cooled to a temperature below the Ac1 point from the finish temperature to obtain a bimetal or pearlite single phase structure or a two phase structure including ferrite.

After the hot rolling, the annealing treatment is carried out under the condition that the temperature is kept at 680 캜 to Ac 1 point (more preferably 690 캜 - [Ac-10 캜]) for 0.8 hours or longer (more preferably 1 hour or longer). By this annealing treatment, the carbide is spheroidized and coarsened, and Mn in the carbide is concentrated to 1.2 times or more the amount of Mn added to the steel sheet. On the other hand, this annealing treatment may be carried out after cooling to the Ac1 point or lower and then maintained in the above-mentioned temperature range, or it may be cooled in this temperature range or after cooling to 680 ° C or lower after hot rolling.

On the other hand, the Ac1 point can be determined from the chemical composition of the steel sheet by Leslie, &quot; Steel Materials Science &quot;, Kodanariasu, Maruzen Co., 1985, p. (1) described in 273 above.

Ac1 (占 폚) = 723-10.7 x Mn-16.9 x Ni + 29.1 x Si + 16.9 x Cr ... (One)

Here, the symbol of the element in the above formula represents the content (mass%) of each element.

The carbide is austenitized by performing a heat treatment (? Heat treatment) under the condition that the annealed sheet is cold-rolled (cold-rolled) and then the cold-rolled sheet is maintained at austenite single phase reverse temperature (Ac3 point or more) for 52s or more. Since Mn is concentrated in the carbide by annealing at the front stage, austenite having a high Mn concentration is formed. By rapidly cooling the austenite single-phase reverse temperature to room temperature at a cooling rate of 100 ° C / s or more, the residual austenite concentrated to 1.2 times or more of the amount of Mn added to the steel sheet can be formed in the martensite have.

On the other hand, the Ac3 point was determined from the chemical composition of the steel sheet by Leslie, &quot; Steel Materials Science &quot;, Kodanariasu, Maruzen Co., 1985, p. (2) described in 273 above.

Ac 3 (° C.) = 910-203 × √C-30 × Mn + 44.7 × Si + 700 × P + 400 × Al-15.2 × Ni-11 × Cr-20 × Cu + 400 × Ti + V ... (2)

Here, the symbol of the element in the above formula represents the content (mass%) of each element.

The tempering martensite can be formed by tempering the annealed sheet at a temperature of 150 to 300 占 폚 for 30 to 1200 seconds to improve the strength-elongation balance, and the steel sheet of the present invention (yield ratio and workability Excellent super high strength steel sheet) is obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is of course not limited by the following Examples, and it is of course possible to carry out the present invention by appropriately modifying it within the range suitable for the purposes of the preceding and latter parts All of which are included in the technical scope of the present invention.

Example

〔Test Methods〕

Ingots A to K shown in the following Table 1 were melted to prepare ingots each having a thickness of 120 mm. The ingot was hot-rolled to a thickness of 2.8 mm and then annealed under the annealing conditions shown in Table 2 below . The annealed sheets were pickled and cold rolled to a thickness of 1.0 mm to form cold-rolled sheets. The cold-rolled sheets were subjected to a γ-ray heat treatment and tempering under the conditions shown in Table 2 below.

〔How to measure〕

The area ratio of the martensite and the retained austenite and the local Mn concentration were measured using the obtained steel sheets. Further, in order to evaluate the mechanical properties of the steel sheet, the yield strength (YS), tensile strength (TS) and total elongation (EL) were also measured. These measurement methods are described below.

(Area ratio of martensite)

With respect to the area ratio of martensite, each steel sheet was subjected to mirror polishing, and the surface thereof was corroded with 3% of the remnant liquid to develop the metal structure, and then the plate thickness was measured using an SEM (Scanning Electron Microscope) A 1/4 part of the tissue was observed at a magnification of 2,000 times with respect to a field of view of approximately 40 mu m x 30 mu m in an outline, and the gray area was defined as martensite, and the area ratio obtained with respect to the field of view was arithmetically averaged to obtain martensite Area ratio.

(Area ratio of retained austenite)

The area ratio of the retained austenite was obtained by grinding and polishing each steel sheet to 1/4 of the plate thickness in the plate thickness direction and measuring the X-ray diffraction intensity.

(Local Mn concentration)

The local Mn concentration was quantitatively analyzed by using a field-of-view electron emission microanalyzer (FE-EPMA) at 3 fields in an area of approximately 20 mu m x 20 mm. In each field of view, the measurement area was divided into small areas of 1 mu m x 1 mm And the Mn concentration in each small region was averaged. The average Mn concentration is calculated as the area ratio of the Mn enriched region in each field of view by calculating the ratio of the small region having the Mn content of 1.2 times or more of the Mn content of the steel sheet and the area ratio of the Mn enriched region of the three fields is arithmetically averaged I did.

(Yield strength, tensile strength and total elongation)

(YS), tensile strength (TS) and tensile strength (TS) of the test specimen No. 5 described in JIS Z 2201 by taking the major axis in the direction perpendicular to the rolling direction and measuring it according to JIS Z 2241, And the total elongation (EL) were obtained. Further, the yield ratio (YR) was calculated from YS / TS.

[Measurement result]

The measurement results are shown in Table 3 below. In the present embodiment, it is assumed that a steel having a tensile strength (TS) of 1470 MPa or more, a yield ratio (YR) of 0.75 or more and a total elongation (EL) of 10% . On the other hand, when the tensile strength (TS) was less than 1470 MPa or the yield ratio (YR) was less than 0.75 or the total elongation (EL) was less than 10%

As shown in Table 3, the inventive steels (steels Nos. 3, 8, 11, 12 and 15 to 20) satisfying the requirements of the present invention (the above-mentioned component requirements and the above described structural requirements) all had a tensile strength TS of 1470 MPa or more, A yield ratio YR of 0.75 or more and a total elongation EL of 10% or more were satisfied. Thus, an ultrahigh strength steel sheet excellent in yield ratio and workability was obtained.

In contrast, the comparative steels (Steel Nos. 1, 2, 4 to 7, 9, 10, 13 and 14) lacking at least one of the requirements of the present invention The characteristics of at least one of the non-YR and the full elongation EL are inferior.

For example, 1 and 6 are the same as those in Production No. 2 in Table 2. As shown in Table 3, Mn is not sufficiently concentrated in the retained austenite because the annealing temperature after hot rolling is too low to exceed the recommended range, as shown in Figs. 1 and 6, respectively.

On the other hand, 5 and 10 are the same as those in Production Example 1 of Table 2. As shown in Table 5 and Table 10, since the annealing temperature after hot rolling is too high beyond the recommended range, Mn is homogenized by diffusion. As shown in Table 3, Mn is not concentrated in the retained austenite, EL is behind.

In addition, 2 and 7 are the same as those in Production Example 2 of Table 2. As shown in Table 2, since Mn is not sufficiently concentrated in the retained austenite, the total elongation EL is inferior because the annealing holding time after hot rolling is too short to exceed the recommended range as shown in Figs.

In addition, 4 and 9 show the results of the production of No. 2 in Table 2. 4 and 9, the austenitization was not sufficiently carried out because the γ-ray heat treatment temperature was too low beyond the recommended range. As shown in Table 3, martensite was insufficient and tensile strength TS and yield ratio YR This is behind.

In addition, 13, as shown in Table 1, since the C content is too low, martensite and retained austenite are both insufficient as shown in Table 3, and Mn in the retained austenite is not sufficiently concentrated , The tensile strength TS and the yield ratio YR are inferior.

In addition, 14, as shown in Table 1, the manganese content and retained austenite are insufficient as shown in Table 3 because the Mn content is too low, and the tensile strength TS and the yield ratio YR are inferior.

As described above, it was confirmed that an ultra-high strength steel sheet excellent in yield ratio and workability was obtained by satisfying the requirements of the present invention.

While the invention has been described in detail and with reference to specific embodiments thereof, it is evident to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

The present application is based on Japanese Patent Application (Patent Application No. 2015-026736) filed on February 13, 2015, the content of which is incorporated herein by reference.

The ultrahigh strength steel sheet of the present invention is excellent in yield ratio and workability and is useful as a cold rolled steel sheet or various coated steel sheets for a vehicle body.

Claims (2)

In terms of% by mass,
C: 0.15 to 0.35%
Si: 0.5 to 3.0%
Mn: 0.5 to 1.5%
Al: 0.001 to 0.10%
Respectively,
The balance being iron and inevitable impurities,
Of the inevitable impurities, P, S, and N,
P: not more than 0.1%
S: 0.01% or less,
N: not more than 0.01%
Respectively,
As an area ratio for the entire tissue,
Martensite: 90% or more,
Residual austenite: 0.5% or more
&Lt; / RTI &gt;
The area where the local Mn concentration is 1.2 times or more the Mn content of the entire steel sheet is 1% or more in area ratio,
A tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more
Which is excellent in yield ratio and workability. The method according to claim 1,
A super high strength steel sheet excellent in yield ratio and processability, wherein the composition further comprises at least one of the following (a) to (c) in mass%.
(a) at least one of Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, and B: 0.0002 to 0.0050%
(b) at least one or two or more of the following: 0.01 to 1.0% of Mo, 0.01 to 1.0% of Cr, 0.01 to 0.3% of Nb, 0.01 to 0.3% of Ti and 0.01 to 0.3% of V
(c) one or two of Ca: 0.0005 to 0.01% and Mg: 0.0005 to 0.01%