JP2016050337A - High strength high ductility steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength high ductility steel sheet used for an automobile thin steel sheet excellent in strength-ductility balance, capable of securing elongation of 32% or more with tensile strength of 1180 MPa or more.SOLUTION: There is provided a high strength high ductility steel sheet having a component composition containing, by mass%, C:0.10 to 0.30%, Si:1.0 to 3.0%, Mn:4.0 to 7.0% and the balance iron with inevitable impurities, a steel structure consisting of retained austenite of 40% or more by area percentage, ferrite of 5% or less by area percentage and the balance bainite, martensite, tempered bainite and tempered martensite, a zone having IQ value by EBSD of 5000 or less of 25% or more, the zone having IQ value of 6000 or more of 30% or more by area percentage, and Mn concentration in the retained austenite is 1.5 times or more as Mn content of the whole steel sheet.SELECTED DRAWING: None

Description

  The present invention relates to a high-strength and highly ductile steel sheet useful as an automatic thin steel sheet and the like, and more particularly to a technique for improving the strength / ductility balance of a steel sheet made of medium Mn steel containing about 5% by mass of Mn.

  For example, steel sheets used in automobile frame parts and the like are required to have higher strength for the purpose of collision safety and fuel efficiency reduction by reducing the weight of the car body, and excellent molding for processing into complex frame parts Workability is also required. For this reason, there is a demand for the development of a steel sheet capable of securing a tensile strength (TS) of 1180 MPa or more and an elongation (EL) of 32% or more as specific mechanical characteristics (hereinafter also simply referred to as “characteristics”). ing.

  Various materials have been proposed as steel plate materials to meet the above-mentioned demands. In recent years, medium Mn steel having an excellent balance between strength and ductility and having a Mn content of about 4 to 10% by mass has attracted attention.

  For example, in Patent Document 1, a steel slab having a component composition containing Mn: 4.0 to 7.0% and Al: 0.5 to 2.0% by weight% is hot-rolled to 550 to 650 ° C. After being rolled up, the steel is cold-rolled and annealed at a two-phase region temperature. The martensite is 40 to 50%, the retained austenite is 20 to 40%, and has a steel structure composed of the remaining ferrite. A cold-rolled steel sheet is disclosed. This high-strength cold-rolled steel sheet can prevent delayed fracture by containing a predetermined amount of Al, but only a property of about 30% elongation at a tensile strength of 1200 MPa has been obtained. Has not yet been satisfied.

  Moreover, in patent document 2, after hot-rolling the steel slab of the component composition containing Mn: 3.5-10.0 mass%, coiling in the low temperature range of two phase region temperature, it cold-rolling, A high-strength steel sheet having a steel structure containing 30% or more of ferrite and 10% or more of retained austenite obtained by annealing at a two-phase region temperature is disclosed. This high-strength steel sheet is intended to ensure the ductility and obtain a predetermined strength-ductility balance by coexisting ferrite and retained austenite for each required amount. It has not reached.

  In Patent Document 3, a steel slab having a composition containing Mn: 3.5 to 10% by mass is hot-rolled and wound at 500 to 600 ° C., and then cold-rolled and 300 to 600 ° C. The primary heating up to is rapidly heated to an average temperature rising rate of 1 to 50 ° C./s, and the secondary heating from 600 ° C. to the annealing temperature is set to an average heating rate of 0.1 to 10 ° C./s, and the annealing temperature: 650 By heating up to 750 ° C., high strength having a structure containing 30 to 70% ferrite phase and 10% or more retained austenite phase by volume ratio, and the average interval between the retained austenite phases is 1.5 μm or less A thin steel sheet is disclosed. Although this high-strength thin steel sheet is excellent in the impact energy absorption capability, it is assumed that the strength-ductility balance is insufficient because a considerable amount of ferrite is contained.

JP 2011-523442 A JP 2013-0761162 A JP 2012-251239 A

  Accordingly, an object of the present invention is to provide a high-strength and high-ductility steel sheet having an excellent strength-ductility balance that can ensure a tensile strength of 1180 MPa or more and an elongation of 32% or more.

The high strength and high ductility steel sheet according to the first invention of the present invention is
Ingredient composition is mass%,
C: 0.10 to 0.30%,
Si: 1.0-3.0%,
Mn: 4.0 to 7.0%
The balance consists of iron and inevitable impurities,
Steel structure
Residual austenite is 40% or more in area ratio,
Ferrite is 5% or less in area ratio, and the balance consists of bainite, martensite, tempered bainite, and tempered martensite,
The area where IQ value by EBSD is 5000 or less is 25% or more in area ratio,
The area where the IQ value is 6000 or more is 30% or more in area ratio,
Furthermore, the Mn concentration in the retained austenite is not less than 1.5 times the Mn content of the entire steel sheet.

  According to the present invention, in a steel plate made of medium Mn steel, bainite (including tempered bainite) and martensite (tempered martensite) are contained in the steel structure by controlling the proportions of the low and high IQ value regions by EBSD. It is excellent in the strength-ductility balance that can ensure the tensile strength is 1180 MPa or more and the elongation is 32% or more by increasing the concentration degree of Mn in the retained austenite High strength and high ductility steel sheets can be provided.

  In order to solve the above-mentioned problems, the present inventors have made various studies on measures that can secure a tensile strength of 1180 MPa or more and an elongation of 32% or more as a mechanical property of a steel plate made of medium Mn steel. I came. As a result, the inventors have thought that the desired characteristics can be secured by the following thought research.

  That is, it is a well-known technical matter that a large amount of retained austenite can be secured in the structure by increasing the Mn content of steel and performing two-phase region heating during annealing.

  On the other hand, as the control of the matrix (matrix) structure, the ferrite phase is mainly introduced as in the prior art mentioned in the background art above. Strength decreases and it is difficult to improve the strength-ductility balance. In addition, even if the structure before annealing is a martensite single phase and this is heated in a two-phase region, since the annealing temperature must be lowered in the medium Mn steel, the effect of improving the ductility by tempering the martensite in the parent phase Cannot be expected, and sufficient ductility of the steel sheet cannot be secured.

  Therefore, in order to improve the ductility while ensuring sufficient strength, it is considered effective to sufficiently increase the ductility of the retained austenite while increasing the ductility of the parent phase. And, as a means to increase the ductility while ensuring the strength, while introducing a lath-like hard structure in the matrix phase, mixing martensite with high strength and bainite with high strength but slightly lower strength than martensite It is effective to introduce.

  However, bainite and martensite, including those after tempering, have a lath structure, and are difficult to distinguish by microscopic observation.

  Therefore, since bainite and tempered bainite have a lower dislocation content than martensite and tempered martensite, the EBSD-IQ method is used as a means for discriminating them. The low IQ value region is regarded as martensite or tempered martensite, while the high IQ value region is regarded as bainite or tempered bainite, and the respective ratios of the low IQ value region and the high IQ value region are constant values. I needed more.

  Moreover, in order to exert the effect of improving the ductility of retained austenite, it is effective to improve the stability of retained austenite. Therefore, by increasing the concentration of Mn in retained austenite, the retained austenite is stabilized. It was decided to increase the effect of improving ductility.

  As a result of further investigation based on the above findings, the present inventors have completed the present invention.

  Hereinafter, the component composition which comprises the high intensity | strength highly ductile steel plate (henceforth "the steel plate of this invention") based on this invention is demonstrated first. Hereinafter, all the units of chemical components are mass%. In addition, “content” of each component may be simply referred to as “amount”.

[Component composition of the steel sheet of the present invention]
C: 0.10 to 0.30%
C contributes to the amount of retained austenite and is an essential element for ensuring strength and ductility. C contributes to bainite transformation during holding at the coiling temperature after hot rolling, and contributes to ductility by increasing the area ratio of the low IQ region and the area ratio of the high IQ region. In order to effectively exhibit such an action, it is necessary to contain C by 0.10% or more, preferably 0.12% or more, more preferably 0.14% or more. However, if the amount of C is excessive, the rate of bainite transformation is significantly reduced, and the amount of bainite cannot be sufficiently secured before annealing, and the ductility is deteriorated. Therefore, the amount of C is 0.30% or less, preferably 0.8. 28% or less, more preferably 0.26% or less.

Si: 1.0-3.0%
Si contributes to an increase in strength by solid solution strengthening and is an essential element that contributes to improving ductility by ensuring the amount of retained austenite by suppressing the decomposition of retained austenite. In order to effectively exhibit these functions, it is necessary to contain Si by 1.0% or more, preferably 1.2% or more, and more preferably 1.4% or more. However, if the amount of Si is excessive, the ductility of the parent phase is deteriorated and the ductility of the steel sheet is deteriorated. Therefore, the amount of Si is 3.0% or less, preferably 2.8% or less, more preferably 2.6%. The following.

Mn: 4.0 to 7.0%
Mn is very effective for securing a large amount of retained austenite and can be concentrated in the retained austenite to increase the stability of the retained austenite. As a result, Mn is an essential element that contributes to improving ductility. Mn also contributes to bainite transformation during holding at the coiling temperature after hot rolling, and contributes to ductility by increasing the area ratio of the low IQ region and the area ratio of the high IQ region. In order to effectively exhibit these actions, it is necessary to contain Mn at 4.0% or more, preferably 4.3% or more, and more preferably 4.6% or more. However, if the amount of Mn is excessive, the bainite transformation is significantly delayed, and bainite cannot be sufficiently formed in the stage before annealing, and the characteristics cannot be secured. Therefore, the amount of Mn is 7.0% or less, preferably Is 6.7% or less, more preferably 6.4% or less.

  The steel of the present invention contains the above-described elements as essential components, and the balance is iron and inevitable impurities (P, S, Al, N, O, etc.), but in the range not impairing the action of the present invention, Allowable components (Cr, Mo, B, Cu, Ni, Nb, Ti, V, Ca, Mg, etc.) can be contained.

  Next, the structure characterizing the steel sheet of 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 does not contain ferrite in principle, and is controlled in a structure composed of a predetermined amount of retained austenite and the balance substantially consisting of bainite and martensite (including those tempered). It is different from the prior art.

<Residual austenite: 40% or more in area ratio>
Residual austenite is a useful structure that contributes to higher strength and higher ductility by work-hardening the material by deformation-induced transformation during deformation. In order to effectively exhibit such an action, the retained austenite is required to be 40% or more, preferably 41% or more, and more preferably 42% or more in terms of area ratio. From the above viewpoint, the more retained austenite is preferable, but the upper limit is about 50% in terms of area ratio in this component steel.

<Ferrite: 5% or less in area ratio>
In order to ensure the strength of the steel sheet, it is necessary to make the matrix phase a high-strength structure. Therefore, it is preferable not to include a soft structure such as ferrite as much as possible, but mixing up to 5% in area ratio is allowed. .

<Balance: bainite, martensite, tempered bainite, and tempered martensite>
As described above, in order to ensure the strength of the steel sheet, it is necessary to make the parent phase a high-strength structure, so the remaining structure is a hard structure, bainite and martensite, and a mixed structure of these tempered structures. To do.

<Region where IQ value by EBSD is 5000 or less: Area ratio is 25% or more, and IQ value is 6000 or more: Area ratio is 30% or more>
The IQ value obtained by EBSD is introduced as a parameter indicating the content ratio of a structure having a high strength and a structure having a high ductility. Here, EBSD (Electron Back Scatter Diffraction) is a technique for analyzing a Kikuchi pattern obtained from reflected electrons generated when an electron beam is incident on the surface of a test piece. The IQ value is a value related to the strength of the Kikuchi pattern obtained by EBSD, and is a numerical value obtained by parameterizing the completeness of the crystal at the measurement site. The IQ value is high when the integrity of the crystal is high, and the IQ value is low when the integrity is low. A region having an IQ value of 5000 or less is regarded as a region corresponding to a martensite phase (including a tempered martensite phase) having low crystal integrity, in which many lattice defects such as dislocations are introduced during transformation. A region having an IQ value of 6000 or more was regarded as a region corresponding to a bainite phase (including a tempered bainite phase) having fewer defects than the martensite phase and high crystal integrity. And each ratio of the area | region whose IQ value is 5000 or less corresponding to the martensite phase which is a high intensity | strength structure | tissue, and the area | region whose IQ value is 6000 or more corresponding to the bainite phase which is a highly ductile structure | tissue is controlled. Thus, high strength and high ductility can be realized. In order to effectively exhibit such an action, the area where the IQ value is 5000 or less is 25% or more in area ratio, preferably 28% or more, more preferably 30% or more, and the area where the IQ value is 6000 or more is The area ratio is 30% or more, preferably 32% or more, and more preferably 35% or more.

<Mn concentration in the retained austenite: 1.5 times or more the Mn content of the entire steel sheet>
By concentrating Mn in the retained austenite, the retained austenite is stabilized, work-induced transformation can be performed in a higher strain region, and the work hardening degree in the high strain region is increased, so that ductility is improved. In order to effectively exhibit such an action, the Mn concentration in the retained austenite: 1.5 times or more, preferably 1.6 times or more, more preferably 1.7 times or more of the Mn content of the whole steel sheet. And

[Each measurement method of area ratio of each phase and Mn concentration in residual γ]
Here, each measuring method of the area ratio of each phase and the Mn concentration in the retained austenite will be described.

  First, the steel plate was subjected to nital corrosion, observed with an optical microscope (magnification 400 times) to identify each phase other than retained austenite, and the area ratio of each phase was measured by image analysis.

  Next, the area ratio of retained austenite was measured by X-ray diffraction after grinding to a thickness of 1/4 of the steel sheet and then chemical polishing (ISIJ Int. Vol. 33, (1933), No. 7, p. . 776).

  Regarding the Mn concentration in the retained austenite, a thin film sample was prepared using FIB (Nova 200 manufactured by FEI), and this was observed with a transmission electron microscope with a spherical aberration correction function (JEM-ARM200F manufactured by JEOL Ltd.). However, the Mn concentration was measured using the attached EDS analyzer (JED-2300T manufactured by JEOL Ltd.).

[Measurement method of IQ value by EBSD]
The IQ value is measured by EBSD, using a “field emission scanning electron microscope JSM-6500F manufactured by JEOL Ltd.”, observing an image of a 50 μm × 50 μm region with a 0.125 μm measurement step, and analyzing software “EDAX” Analysis was performed using “TSM OIM”. Then, from the distribution of the IQ value, the area ratios of the region having the IQ value of 5000 or less and the region having the IQ value of 6000 or more were calculated.

  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, steel having the above component composition is melted and hot rolled after being made into a slab (steel material) by ingot forming or continuous casting, the coiling temperature is kept below 400 ° C., and held for 30 to 120 minutes, and then to room temperature Cool to hot rolled material. Next, after removing the scale by pickling or the like after this hot-rolled material, after performing cold rolling, or even if performing cold rolling, the hot rolling material is kept at a low pressure reduction rate of about 20% or less, and then the annealing temperature: A high-strength and highly ductile steel sheet can be obtained by annealing at (Ae1 + Ae3) / 2 ± 20 ° C. under the condition of holding time: 3000 s or more.

<Winding temperature: less than 400 ° C., holding time: 30 to 120 min>
This is because part or all of the tissue is bainite. In this component steel, bainite is difficult to be formed when the coiling temperature is set to 400 ° C. or higher. Therefore, the amount of bainite cannot be secured even if the coil is held for a long time. Further, when the coiling temperature is set to 500 ° C. or higher, ferrite is formed. Start to be. Although the minimum of coiling temperature is not specifically limited, Since it will become difficult to wind up if coiling temperature is made too low, it is about 300 degreeC.
Moreover, in order to make the bainite transformation proceed appropriately, it is necessary to appropriately control the holding time. If the holding time is too short, the bainite transformation becomes insufficient and the area ratio of the low IQ region decreases, so the holding time is 30 min or longer, preferably 40 min or longer, more preferably 50 min or longer. On the other hand, if the holding time is too long, the bainite transformation proceeds too much and the area ratio of the high IQ region decreases, so the holding time is 120 min or less, preferably 100 min or less, more preferably 80 min or less.

<Cold rolling is performed or, even if cold rolling is performed, a low pressure reduction rate of about 20% or less is maintained>
This is because the lath-like structure such as martensite and bainite formed during the hot rolling is ensured as much as possible.

<Annealing temperature: (Ae1 + Ae3) / 2 ± 20 ° C., holding time: 50 min or more>
By heating and holding at a two-phase temperature for a predetermined time, Mn is diffused and distributed between ferrite and austenite to ensure the concentration of Mn in austenite and to be introduced during winding during hot rolling This is because, in the bainite, carbides are formed during heating during annealing, but the carbides are sufficiently dissolved. If the holding time is insufficient, carbides remain in the bainite and easily become a starting point of fracture, which causes deterioration of strength and ductility. Therefore, the holding time is 50 min or more, preferably 70 min or more, more preferably 90 min or more. . The upper limit of the holding time is not particularly limited, but if the holding time is too long, the productivity is lowered. Therefore, the holding time is preferably 300 min or less, more preferably 280 min or less.
Ae1 (° C.) and Ae3 (° C.) are obtained from thermodynamic calculation software (Thermo-Calc Software, Thermo-Calc manufactured by AB) using TCFE7 as a thermodynamic database, and the contents of C, Mn, Si, and Al (Mass%), the phase fraction of each phase of FCC, BCC, and cementite at each temperature is obtained, and the temperature at which transition from the two-phase state of BCC-cementite to the three-phase state of FCC-BCC-cementite is performed is Ae1 (° C) -The temperature at which the transition from the two-phase state of BCC to the single phase of FCC was defined as Ae3 (° C).

  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.

  The steel materials having the components shown in Table 1 below were hot-rolled and then wound up under the conditions of temperature and holding time shown in Table 2 below to produce hot rolled sheets having a thickness of 3.0 mm. And after pickling this hot-rolled sheet, it annealed on the conditions of the temperature and holding time which are shown in the same table 2, and produced the hot-rolled steel sheet.

For each hot-rolled steel sheet, the area ratio of each phase and its Mn concentration, as well as the region where the IQ value by EBSD is 5000 or less, and 6000 or more are measured by the measurement method described in the above-mentioned section [Mode for Carrying Out the Invention]. Each area ratio of the region was measured.
In addition, since all the experimental samples used in this example consisted of a bainite phase, a martensite phase, a tempered bainite phase, and a tempered martensite other than the ferrite phase and the retained austenite phase, the following Table 3 As for the area ratio of each phase, only the area ratios of the retained austenite phase and the ferrite phase are described.

  Moreover, in order to evaluate a strength-ductility balance about each said hot-rolled steel plate, tensile strength TS and elongation (total elongation) EL were measured by the tension test. The tensile test was carried out in accordance with JIS Z 2241 using a JIS 14B test piece (t = 2 mm, w = 7 mm, GL = 20 mm) so as to be parallel to the rolling direction.

  The measurement results are shown in Table 3 below. In the same table, the properties of the hot-rolled steel sheet were determined to be acceptable (◯) when the tensile strength TS was 1180 MPa or higher and the elongation EL was 32% or higher, and was rejected (X).

  As shown in Table 3 above, the steel No., which is an inventive steel (evaluation is ○). 1 is an invention steel that satisfies the requirements of the structure provision of the present invention as a result of being manufactured under the recommended production conditions using a steel type that satisfies the requirements of the composition provision of the present invention, the characteristics satisfy the acceptance criteria, It was confirmed that a high-strength and ductile steel plate excellent in strength-ductility balance was obtained.

  On the other hand, steel No. which is a comparative steel (evaluation of x). Nos. 2 to 12 do not satisfy at least one of the requirements of the component specification and the organization specification of the present invention, and the characteristics do not satisfy the acceptance criteria.

  That is, Steel No. Nos. 2 to 7 use steel types that satisfy the requirements of the component provisions of the present invention, but are manufactured under conditions that partially deviate from the recommended production conditions. ing.

  For example, steel no. 2 is a case where the temperature is not maintained at the time of winding, but the retained austenite is decreased, the area ratio of the low IQ region is decreased, and TS is inferior.

  Steel No. 3 is the case where the temperature is not maintained during winding, but the area ratio of the low IQ region is lowered even if the retention time during the subsequent annealing is sufficiently lengthened to secure the amount of retained austenite. Is inferior.

  Steel No. In No. 4, since the winding temperature is too high, the bainite transformation does not proceed, the area ratio of the low IQ region is low, and the EL is inferior.

  Steel No. In No. 5, since the holding time during annealing is too short, both the amount of retained austenite and the degree of Mn concentration are insufficient, and both TS and EL are inferior.

  Steel No. In No. 6, since the annealing temperature is too low, the amount of retained austenite formed by reverse transformation during annealing cannot be secured, and both TS and EL are inferior.

  Steel No. On the other hand, since the annealing temperature of No. 7 is too high, the amount of retained austenite formed by reverse transformation at the time of annealing is too large, and the subsequent austenite cannot be ensured by martensite during cooling, resulting in poor EL.

  On the other hand, Steel No. Although Nos. 8 to 12 are manufactured under recommended manufacturing conditions, steel types that partially deviate from the requirements of the component provisions of the present invention are used, so the requirements of the structure provisions are not satisfied and the characteristics are inferior.

  For example, steel no. Since 8 (steel type b) has too little Si, residual austenite is decomposed and the amount thereof is insufficient, and EL is inferior.

  Steel No. 9 (steel type c), on the contrary, has too much Si, so that the area ratio of the high IQ region is insufficient, the structure becomes brittle, and the EL is inferior.

  Steel No. 10 (steel type d) has an excessively small amount of Mn, so that the area ratio of the high IQ region is lowered and the amount of retained austenite is also insufficient, and both TS and EL are inferior.

  Steel No. No. 11 (steel type e) has too little C, so a large amount of ferrite is formed, the IQ value decreases, the area ratio of the high IQ region decreases, the amount of retained austenite is also insufficient, and both TS and EL Inferior.

  Steel No. 12 (steel type f), on the contrary, has too much C content, so that the bainite transformation during hot rolling is suppressed, the IQ value increases as a whole, the area ratio of the low IQ region decreases, and the EL is inferior. Yes.

Claims (1)

Ingredient composition is mass%,
C: 0.10 to 0.30%,
Si: 1.0-3.0%,
Mn: 4.0 to 7.0%
The balance consists of iron and inevitable impurities,
Steel structure
Residual austenite is 40% or more in area ratio,
Ferrite is 5% or less in area ratio, and the balance consists of bainite, martensite, tempered bainite, and tempered martensite,
The area where IQ value by EBSD is 5000 or less is 25% or more in area ratio,
The area where the IQ value is 6000 or more is 30% or more in area ratio,
Furthermore, the Mn density | concentration in the said retained austenite is 1.5 times or more of Mn content of the whole steel plate. The high intensity | strength high ductility steel plate characterized by the above-mentioned.