JP2019002078A - Ultra high strength steel sheet excellent in yield ratio and workability - Google Patents

Abstract

To provide an ultra high strength steel sheet excellent in yield ratio and workability capable of satisfying a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more and a total elongation of 10% or more.SOLUTION: The ultra high strength steel sheet is provided that has a component composition containing, 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% and the balance iron with inevitable impurities, P, S and N in the inevitable impurities being limited to P:0.1% or less, S:0.01% or less and N;0.01% or less respectively. The steel sheet has structure composed of, by a mass ratio based on all structure, martensite of 90% or more and retained austenite of 0.5% or more. In the steel sheet, a region where local Mn concentration is 1.2 times or more the Mn content of total steel sheet is present in an amount of 1% or more by area percentage. The steel sheet has a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more and a total elongation of 10% or more.SELECTED DRAWING: None

Description

  The present invention relates to an ultra-high strength steel sheet excellent in yield ratio and workability. As a steel plate type of the ultra high strength steel plate according to the present invention, various plated steel plates such as a hot dip galvanized steel plate and an alloyed hot dip galvanized steel plate are included in addition to the cold rolled steel plate.

  Steel sheets used for automobile frame members are required to have high strength for the purpose of improving fuel efficiency by reducing the weight of the vehicle body, and also to have a high yield ratio in order to ensure collision safety. On the other hand, excellent workability is also required in order to form a complex shaped part.

  For this reason, provision of the ultra high strength steel plate which has high yield ratio and high elongation (total elongation; EL) is desired. Specifically, 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.

  In addition, steel sheets for automobiles are welded at the time of assembling the vehicle body or attaching parts, but the weldability depends greatly on the composition of the steel sheet, and in particular, if a large amount of C and Mn is added, the weldability may be deteriorated. Are known. Therefore, the steel sheet for automobiles is required to satisfy the above mechanical characteristics with a component composition satisfying C of 0.35% by mass or less and Mn of 1.5% by mass or less.

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

(1) Increase the amount of retained austenite and use its TRIP action.
(2) Increase the amount of soft ferrite (including bainitic ferrite).

  However, in the above means (1), in order to leave a large amount of austenite, it is necessary to increase the addition amount of C and Mn, and C ≦ 0.35 mass% and Mn ≦ 1.5 mass% are satisfied. There was a problem that sufficient weldability could not be secured.

  On the other hand, the above means (2) requires a certain amount of the soft phase in order to ensure the elongation, cannot satisfy the yield ratio of 0.75 or more, and cannot secure sufficient collision safety. It was.

  For example, in Patent Document 1, by increasing the Mn content in a steel sheet, a large amount of austenite is retained, thereby improving hydrogen embrittlement resistance in an ultra-high strength region having a tensile strength of 1180 MPa or more and punching. There has been proposed a steel plate that is also excellent in delayed fracture resistance in a hole-drilled part.

  However, as shown in the Examples, the steel sheets of the present invention all have a room for improvement in terms of weldability because the Mn content in the steel sheet exceeds 1.5 mass%. .

  Patent Document 2 discloses a component composition satisfying C of 0.35% by mass or less and Mn of 1.5% by mass or less by increasing the fraction of the soft ferrite phase, and a tensile strength of 1470 MPa or more. And the steel plate which can implement | achieve 10% or more of total elongation is proposed.

  However, as shown in the examples, the steel sheet does not achieve a yield ratio of 0.75 or more, and has a problem in that sufficient collision safety cannot be ensured.

JP 2008-81788 A JP 2010-90432 A

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

The ultra-high strength steel sheet excellent in yield ratio and workability according to the first invention of the present invention,
% By mass
C: 0.15-0.35%,
Si: 0.5-3.0%
Mn: 0.5 to 1.5%
Al: 0.001 to 0.10%
Each
The balance consists of iron and inevitable impurities,
Among the inevitable impurities, P, S, and N are
P: 0.1% or less,
S: 0.01% or less,
N: each having a component composition limited to 0.01% or less,
The area ratio for all tissues
Martensite: 90% or more,
Retained austenite: having a structure of 0.5% or more,
A region where the local Mn concentration is 1.2 times or more of the Mn content of the entire steel sheet is 1% or more in terms of area ratio,
The tensile strength is 1470 MPa or more, the yield ratio is 0.75 or more, and the total elongation is 10% or more.

The ultra-high-strength steel sheet excellent in the yield ratio and workability according to the second invention of the present invention is the first invention,
Ingredient composition is further mass%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
B: 0.0002 to 0.0050%
1 type or 2 types or more are included.

The ultra-high strength steel sheet excellent in yield ratio and workability according to the third invention of the present invention is the above first or second invention,
Ingredient composition is further mass%,
Mo: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Nb: 0.01-0.3%
Ti: 0.01 to 0.3%,
V: 0.01 to 0.3%
1 type or 2 types or more are included.

The ultra-high-strength steel sheet excellent in the yield ratio and workability according to the fourth invention of the present invention is any one of the first to third inventions described above.
Ingredient composition is further mass%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%
1 type or 2 types are included.

  According to the present invention, without increasing the average concentration of C and Mn in the entire steel sheet, the steel structure has martensite as the main structure, and Mn is concentrated in the retained austenite to ensure weldability. However, it has become possible to provide an ultra-high strength high plate having high strength, high yield ratio and excellent workability.

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

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

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention contains a predetermined amount of retained austenite (hereinafter, austenite may be expressed as γ) in which Mn is further concentrated after the matrix phase is martensite. It is a feature.

<Martensite: 90% or more>
Martensite needs an area ratio of 90% or more, preferably 92% or more, more preferably 94% or more in order to achieve a high yield ratio of 0.75 or more while realizing a tensile strength of 1470 MPa or more. In this specification, martensite is used to include both untempered fresh martensite and tempered martensite.

<Residual austenite: 0.5% or more>
Residual austenite needs an area ratio of 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more in order to improve the total elongation using its TRIP action.

<A region where the local Mn concentration is 1.2 times or more of the Mn content of the entire steel sheet: 1% or more in terms of area ratio>
In order to further improve the total elongation by securing the residual austenite to the high strain region by concentrating Mn in the retained austenite and enhancing the stability of the retained austenite, and to ensure the total elongation of 10% or more It is. On the other hand, from the viewpoint of ensuring weldability, the average Mn concentration in the steel sheet needs to satisfy 1.5% by mass or less, so that the Mn concentration region is formed in the steel sheet of the present invention. That is, the retained austenite formed in the Mn concentration region is stabilized while keeping the Mn concentration of the matrix phase low. Thereby, a part of area | region where local Mn density | concentration becomes 1.2 times or more of Mn content of the whole steel plate will exist as a retained austenite, and will contribute to the further improvement of total elongation.
The retained austenite formed in the steel sheet of the present invention is very fine and the Mn concentration cannot be measured directly. Therefore, a region where the local Mn concentration is 1.2 times or more of the Mn content of the entire steel sheet is 1% or more (preferably 1.1% or more, more preferably 1.2% or more) in terms of area ratio. Thus, it is ensured that Mn is sufficiently concentrated in the retained austenite.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.15-0.35%
C is an important element that greatly affects the strength of the steel sheet. In order to ensure 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, weldability deteriorates, so 0.35% or less, preferably 0.3% or less, and more preferably 0.25% or less.

Si: 0.5 to 3.0%
Si is an element useful for suppressing the formation of carbides and promoting the formation of retained austenite. In order to effectively exhibit such an action, Si is contained in an amount of 0.5% or more, preferably 0.8% or more, more preferably 1.1% or more. However, if Si is excessively contained, the weldability is remarkably deteriorated, so that it is 3.0% or less, preferably 2.5% or less, more preferably 2.0% or less.

Mn: 0.5 to 1.5%
Mn is also a useful element that contributes to increasing the strength of the steel sheet as a solid solution strengthening element. Moreover, it has the effect of suppressing the ferrite transformation at the time of cooling by improving hardenability. Furthermore, since it has the effect of stabilizing austenite, highly stable retained austenite can be formed. In order to effectively exhibit such an action, 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 securing weldability, it is desirable that the amount of Mn is as low as possible, 1.5% or less, preferably 1.3% or less, and more preferably 1.15% or less.

Al: 0.001 to 0.10%
Al is a useful element added as a deoxidizer, and in order to obtain such an action, 0.001% or more, preferably 0.01% or more, more preferably 0.03% or more is contained. Let However, if Al is contained excessively, the cleanliness of the steel is deteriorated, so that it 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 elements as essential components, and the balance is iron and unavoidable impurities (P, S, N, O, etc.). Among the unavoidable impurities, P, S, and N are as follows: It can be contained up to each allowable range.

P: 0.1% or less P is unavoidably present as an impurity element and contributes to an increase in strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and embrittles the grain boundaries to improve workability. Since it deteriorates, the amount of P is limited to 0.1% or less, preferably 0.05% or less, and more preferably 0.03% or less.

S: 0.01% or less S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of a crack at the time of deformation, thereby reducing workability. Hereinafter, it is preferably limited to 0.005% or less, more preferably 0.003% or less.

N: 0.01% or less N is also unavoidably present as an impurity element and lowers the workability of the steel sheet by strain aging. Therefore, the N content is 0.01% or less, preferably 0.005% or less, and more preferably. Is limited to 0.003% or less.

  In addition, the following permissible components can 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%
These elements are useful elements that have the effect of enhancing the hardenability and suppressing the transformation from austenite. In order to obtain such an action, it is preferable that each element is contained in the above lower limit value or more. The above elements may be contained alone or in combination of two or more. However, even if these elements are contained excessively, the effect is saturated and it is economically wasteful, so that each element is set to the above upper limit value or less.

Mo: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Nb: 0.01-0.3%
Ti: 0.01 to 0.3%,
V: 0.01 to 0.3%
These elements are useful elements for improving strength without degrading workability. In order to obtain such an action, it is preferable that each element is contained in the above lower limit value or more. The above elements may be contained alone or in combination of two or more. However, if these elements are contained excessively, coarse carbides are formed and the workability deteriorates, so that each element is set to the above upper limit value or less.

Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%
These elements are useful elements for improving the workability by refining inclusions and reducing the starting point of fracture. In order to obtain such an action, it is preferable to contain 0.0005% or more of any element. The above elements may be used alone or in combination of two. However, if it is excessively contained, inclusions become coarse and workability deteriorates. Therefore, both elements are made 0.01% or less.

  Next, preferable production conditions for obtaining the steel sheet of the present invention will be described below.

[Preferred production method of the steel sheet of the present invention]
First, a steel having the above composition is melted and made into a slab (steel material) by ingot forming or continuous casting, and a soaking temperature of 1200 ° C. or less (more preferably 1150 ° C. or less) and a finishing temperature of 900 ° C. or less (more preferably). 880 ° C. or lower) is hot-rolled (hot rolled) and cooled from the finishing temperature to the Ac1 point or lower to obtain a bainite or pearlite single-phase structure or a two-phase structure containing ferrite.

  After the hot rolling, an annealing treatment is performed under the condition of maintaining at 680 ° C to Ac1 point (more preferably 690 ° C to [Ac1-10 ° C]) for 0.8 h or more (more preferably 1 h or more). By this annealing treatment, the carbide is spheroidized and coarsened, and Mn in the carbide is concentrated to 1.2 times or more of the amount of Mn added to the steel sheet. In this annealing treatment, after cooling to Ac1 point or less, it may be kept in the above temperature range, or may be gradually cooled in this temperature range, or after hot rolling, it is once cooled to less than 680 ° C. You may go after.

Ac1 points are based on the chemical composition of the steel sheet, by Lesley, "Steel Materials Science", translated by Kouta Shigeaki, Maruzen Co., 1985, p. The following equation (1) described in H.273 can be used.
Ac1 (° C.) = 723-10.7 × Mn−16.9 × Ni + 29.1 × Si + 16.9 × Cr (1)
Here, the element symbol in the above formula represents the content (% by mass) of each element.

  After the annealed sheet is cold-rolled (cold-rolled), the carbide is austenitized by heat-treating (gamma heat-treating) the cold-rolled sheet at austenite single-phase temperature (Ac3 point or higher) for 52 s or more. To do. Since Mn is concentrated in the carbide by the preceding annealing treatment, austenite having a high Mn concentration is formed. From this austenite single-phase region temperature, the residual austenite in which Mn is concentrated to 1.2 times or more of the amount of Mn added to the steel sheet by quenching to room temperature at a cooling rate of 100 ° C./s or more is a matrix It can be formed in martensite.

Ac 3 points are based on the chemical composition of the steel sheet, by Lesley, “Iron & Steel Materials Science”, translated by Sadako Koda, Maruzen Co., Ltd., 1985, p. The following equation (2) described in H.273 can be used.
Ac3 (° C.) = 910−203 × √C−30 × Mn + 44.7 × Si + 700 × P + 400 × Al-15.2 × Ni-11 × Cr-20 × Cu + 400 × Ti + 31.5 × Mo + 104 × ∨ (2 )
Here, the element symbol in the above formula represents the content (% by mass) of each element.

  And by tempering the said heat processing board on the conditions hold | maintained at 150-300 degreeC for 30-1200 s, a tempered martensite can be formed and a strength-elongation balance can be improved, this invention steel plate (yield ratio and processing) An ultra-high strength steel sheet having excellent properties can be obtained.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

〔Test method〕
After melting steel having each component composition of A to K shown in Table 1 below, an ingot having a thickness of 120 mm was produced, and hot rolling was performed using the ingot to obtain a thickness of 2.8 mm. Annealing was performed under the annealing conditions shown in Table 2. After this annealed plate was pickled, it was cold-rolled to a thickness of 1.0 mm to obtain a cold-rolled plate, and the cold-rolled plate was subjected to gamma heat treatment and tempering under the conditions shown in Table 2 below.

〔Measuring method〕
Using each obtained steel sheet, the area ratio of martensite and retained austenite, and the local Mn concentration were measured. Moreover, in order to evaluate the mechanical characteristic of a steel plate, it measured also about the yield strength (YS), the tensile strength (TS), and total elongation (EL). These measurement methods are shown below.

(Martensite area ratio)
As for the area ratio of martensite, each steel plate was mirror-polished, its surface was corroded with 3% nital solution to reveal the metal structure, and then the plate was formed using SEM (Scanning Electron Microscope). The structure of 1/4 part in thickness is observed at a magnification of 2000 times for 5 fields of view of approximately 40 μm × 30 μm, and the areas that appear gray are defined as martensite, and the area ratio obtained for each field is arithmetically averaged to martensite. Area ratio.

(Area ratio of retained austenite)
The area ratio of retained austenite was obtained by grinding and polishing each steel plate to ¼ of the plate thickness in the plate thickness direction and measuring the X-ray diffraction intensity.

(Local Mn concentration)
The local Mn concentration is approximately 20 μm × 20 mm, 3 fields, quantitative analysis using field emission electron beam microanalyzer (FE-EPMA), and in each field, the measurement area is 1 μm × 1 mm small area. And the Mn concentration in each small region was averaged. The ratio of the small region where the average Mn concentration is 1.1 times or more the Mn content of the steel sheet is calculated by defining the area ratio of the Mn concentrated region in each field of view, and the area of the Mn concentrated region in three fields of view. Evaluation was performed by arithmetically averaging the rates.

(Yield strength, tensile strength and total elongation)
Yield strength (YS) and tensile strength are obtained by using each steel plate to be evaluated and preparing a test piece No. 5 described in JIS Z 2201 with the long axis in the direction perpendicular to the rolling direction and measuring according to JIS Z 2241. (TS) and total elongation (EL) were determined, and the yield ratio (YR) was determined from YS / TS.

〔Measurement result〕
The measurement results are shown in Table 3 below. In this example, when the tensile strength (TS) is 1470 MPa or more, the yield ratio (YR) is 0.75 or more, and the total elongation (EL) is 10% or more, the result is “good”, and the yield ratio and workability are improved. It was determined to be an excellent ultra high strength steel sheet. On the other hand, those having a tensile strength (TS) of less than 1470 MPa, a yield ratio (YR) of less than 0.75, or a total elongation (EL) of less than 10% were judged as rejected with x. In addition, what attached | subjected the shading to each item of Tables 1-3 shows that it does not satisfy the requirements of this invention, the recommended manufacturing conditions, and a mechanical characteristic.

  As shown in Table 3, the invention steels (steel Nos. 3, 8, 11, 12, 15 to 20) satisfying the requirements of the present invention (the above-described component requirements and the above-mentioned structural requirements) all have a tensile strength TS. An ultra-high strength steel plate having a yield ratio and workability of 1470 MPa or more, a yield ratio YR of 0.75 or more and a total elongation EL of 10% or more was obtained.

  On the other hand, comparative steels (steel Nos. 1, 2, 4 to 7, 9, 10, 13, 14) lacking at least one of the requirements of the present invention (the component requirements and the structure requirements) are tensile. At least one of the properties of strength TS, yield ratio YR, and total elongation EL is inferior.

  For example, steel no. 1 and 6 are the production numbers in Table 2. As shown in FIGS. 1 and 6, since the annealing temperature after hot rolling is too low outside the recommended range, as shown in Table 3, Mn does not concentrate sufficiently in the retained austenite and the total elongation EL is inferior. ing.

  On the other hand, Steel No. Nos. 5 and 10 show the production numbers in Table 2. As shown in FIGS. 5 and 10, since the annealing temperature after hot rolling is too high outside the recommended range, Mn is homogenized by diffusion, and as shown in Table 3, Mn is concentrated in the retained austenite. The total elongation EL is inferior.

  Steel No. 2 and 7 show the production numbers in Table 2. As shown in Tables 2 and 7, since the annealing holding time after hot rolling is too short outside the recommended range, as shown in Table 3, Mn does not concentrate sufficiently in the retained austenite, and the total elongation EL is Inferior.

  Steel No. 4 and 9 show the production numbers in Table 2. As shown in FIGS. 4 and 9, since the heat treatment temperature is too low outside the recommended range, it does not sufficiently austenite, and as shown in Table 3, the martensite is insufficient, the tensile strength TS and the yield ratio YR. Is inferior.

  Steel No. No. 13 has a C content that is too low as shown in steel type E in Table 1. As shown in Table 3, both martensite and residual ferrite are insufficient, and Mn is sufficiently concentrated in the residual austenite. The tensile strength TS and the yield ratio YR are inferior.

  Steel No. No. 14 has a Mn content that is too low as shown in steel type F in Table 1. Therefore, as shown in Table 3, both martensite and residual ferrite are insufficient, and tensile strength TS and yield ratio YR are inferior.

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

Claims (4)

% By mass
C: 0.15-0.35%,
Si: 0.5-3.0%
Mn: 0.5 to 1.5%
Al: 0.001 to 0.10%
Each
The balance consists of iron and inevitable impurities,
Among the inevitable impurities, the content of a predetermined composition is limited,
The area ratio for all tissues
Martensite: 90% or more,
Retained austenite: having a structure of 0.5% or more,
A region where the local Mn concentration is 1.2 times or more of the Mn content of the entire steel sheet is 1% or more in terms of area ratio,
An ultra-high strength steel sheet excellent in yield ratio and workability, characterized by a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more. Ingredient composition is further mass%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
B: 0.0002 to 0.0050%
Including one or more of
The ultra-high strength steel sheet excellent in yield ratio and workability according to claim 1. Ingredient composition is further mass%,
Mo: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Nb: 0.01-0.3%
Ti: 0.01 to 0.3%,
V: 0.01 to 0.3%
Including one or more of
The ultra-high strength steel sheet excellent in yield ratio and workability according to claim 1 or 2. Ingredient composition is further mass%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%
Including one or two of the following:
The ultra-high-strength steel plate excellent in the yield ratio and workability of any one of Claims 1-3.