WO2018088421A1

WO2018088421A1 - High-strength cold-rolled thin steel sheet and method for producing high-strength cold-rolled thin steel sheet

Info

Publication number: WO2018088421A1
Application number: PCT/JP2017/040219
Authority: WO
Inventors: 美絵小幡; 植田　圭治; 金子　真次郎
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2016-11-10
Filing date: 2017-11-08
Publication date: 2018-05-17
Also published as: JP6597889B2; JPWO2018088421A1

Abstract

Provided are: a high-strength cold-rolled thin steel sheet which has a tensile strength of 980 MPa or more, while exhibiting high ductility and high stretch flangeability; and a method for producing this high-strength cold-rolled thin steel sheet. This high-strength cold-rolled thin steel sheet has a specific composition and a structure that comprises, in a volume percentage, from 10% to 70% (inclusive) of polygonal ferrite, from 5% to 40% (inclusive) of bainitic ferrite, more than 15% but 40% or less of residual austenite and more than 0% but 30% or less of martensite. The polygonal ferrite has an average crystal grain size of 10.0 μm or less; and the polygonal ferrite has an aspect ratio of 1.5 or more. The residual austenite has an average crystal grain size of 2.0 μm or less; and the residual austenite has an aspect ratio of 2.0 or more.

Description

High strength cold-rolled steel sheet and method for producing high-strength cold-rolled steel sheet

The present invention relates to a high-strength cold-rolled steel sheet and a method for producing a high-strength cold-rolled steel sheet. More specifically, the present invention relates to a high-strength cold-rolled thin steel sheet having a tensile strength TS of 980 MPa or more and suitable for automobile parts and a method for producing the same.

In recent years, from the viewpoint of preservation of the global environment, there has been a demand for improved fuel economy of automobiles, and it has been promoted to apply high-strength cold-rolled thin steel sheets having a tensile strength of 980 MPa or more to body parts and the like (for example, Patent Documents 1 to 3).
Furthermore, recently, there has been an increasing demand for improving the collision safety of automobiles, and from the viewpoint of ensuring the safety of passengers in the event of a collision, the tensile strength for structural members such as the skeleton part of a vehicle body is extremely high at 1180 MPa or more. Application of high-strength cold-rolled thin steel sheets having strength has also been studied (for example, Patent Documents 1 to 3).

JP 2011-157583 A JP 2007-32237 A JP 2008-174802 A

However, even if the conventional cold-rolled thin steel sheet has a high strength of 980 MPa or more, the ductility may be insufficient or the stretch flangeability may be insufficient.

Accordingly, an object of the present invention is to provide a high-strength cold-rolled thin steel sheet having a tensile strength of 980 MPa or more and having high ductility and high stretch flangeability, and a method for producing the same.
“Thin steel plate” refers to a steel plate having a thickness of 5 mm or less.

As a result of intensive studies to achieve the above object, the present inventors have found that a cold-rolled thin steel sheet having a specific composition and structure has a high tensile strength of 980 MPa or more, and has ductility and stretch flangeability. The present invention was completed.

That is, the present invention provides the following [1] to [5].
[1] By mass%, C: more than 0.15% to 0.45% or less, Si: 0.50% to 2.50%, Mn: 1.50% to 3.50%, P: 0.00. 001% or more and 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, and Al: 0.010% or more and 1.00% or less, and the balance is Fe and inevitable impurities Composition and volume ratio of 10% to 70% polygonal ferrite, 5% to 40% bainitic ferrite, 15% to 40% residual austenite, and 0% to 30% martensite A structure having a site, the average grain size of the polygonal ferrite is 10.0 μm or less, the aspect ratio of the polygonal ferrite is 1.5 or more, and the average grain size of the retained austenite is Particle size at 2.0μm or less, and is the aspect ratio of the residual austenite is 2.0 or more, high strength cold rolled steel sheets.
[2] The above composition is further mass%, Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, B: 0.0001% to 0.0050% Hereinafter, Cr: 0.05% to 0.20%, Cu: 0.05% to 0.20%, Sb: 0.002% to 0.050%, Sn: 0.002% to 0.000%. 050% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005 The high-strength cold-rolled thin steel sheet according to [1] above, which contains at least one element selected from the group consisting of% or more and 0.0050% or less.
[3] The high-strength cold-rolled thin steel sheet according to [1] or [2], which has a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer on the surface.
[4] A method for producing a high-strength cold-rolled thin steel sheet according to any one of [1] to [3], wherein a steel material having the composition according to [1] or [2] A rolling reduction ratio of 30 is applied to the hot rolling step of obtaining a hot rolled sheet by performing hot rolling, the pickling step of performing pickling treatment on the hot rolled plate, and the hot rolled plate subjected to the pickling treatment. % Of cold rolling to obtain a thin cold rolled sheet, and the thin cold rolled sheet is heated at an annealing temperature T ₁ of 800 ° C. or more and 950 ° C. or less, and the annealing temperature T _1. The first stage having a structure in which the sum of martensite and bainite is 80% or more by volume ratio by cooling to a cooling stop temperature T ₂ of 500 ° C. or less at an average cooling rate of 5 ° C./s or more. A first-stage annealing step for obtaining a cold-rolled annealed plate and the first-stage cold-rolled annealed plate at 700 ° C. or higher and 850 ° C. or lower At annealing temperature _{T 3,} and held 10s or 900s or less, from the annealing temperature _{T 3,} at an average cooling rate of 50 ° C. / s or less 5 ° C. / s or higher, up to 200 ° C. or higher 500 ° C. or less of the cooling stop temperature _{T 4} cooling, in the cooling stop temperature T _4, by holding 10s or 1800s or less, the method of producing a high strength cold rolled steel sheets having a second-stage annealing process of obtaining a second Danhiyanobe annealed sheet, a.
[5] The high strength according to [4], further comprising a plating step of subjecting the second-stage cold-rolled annealed plate to a hot dip galvanizing treatment, a hot dip galvanizing treatment and an alloying treatment, or an electrogalvanizing treatment. A method for producing a cold-rolled thin steel sheet.

According to the present invention, it is possible to provide a high-strength cold-rolled thin steel sheet having a tensile strength of 980 MPa or more and having high ductility and high stretch flangeability, and a method for producing the same.
By applying the high-strength cold-rolled thin steel sheet of the present invention to, for example, an automobile structural member, it is possible to greatly contribute to the weight reduction of the automobile body and to greatly contribute to the improvement of the fuel consumption of the automobile.

[High strength cold-rolled thin steel sheet]
The high-strength cold-rolled thin steel sheet of the present invention has a composition and a volume ratio of 10% to 70% polygonal ferrite, 5% to 40% bainitic ferrite, 15% to 40% in terms of the composition described later. Residual austenite and a structure having martensite of greater than 0% and less than 30%, the average crystal grain size of the polygonal ferrite is 10.0 μm or less, and the aspect ratio of the polygonal ferrite is 1 A high-strength cold-rolled thin steel sheet having an average crystal grain size of 2.0 μm or less and an aspect ratio of the retained austenite of 2.0 or more.

Hereinafter, the composition of the high-strength cold-rolled steel sheet of the present invention will be described first, and then the structure of the high-strength cold-rolled steel sheet of the present invention will be described.

<composition>
The high-strength cold-rolled thin steel sheet of the present invention is, in mass%, C: more than 0.15% and 0.45% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more. 50% or less, P: 0.001% or more and 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, and Al: 0.010% or more and 1.00%, the balance It has a composition consisting of Fe and inevitable impurities.
First, the reason for composition limitation will be described. Hereinafter, unless otherwise specified, “mass%” is simply expressed as “%”.

<< C: more than 0.15% and less than 0.45% >>
C has a high solid solution strengthening ability, contributes to increase in strength, stabilizes retained austenite, secures retained austenite having a desired volume ratio, and contributes effectively to improving ductility. In order to acquire such an effect, C needs to contain more than 0.15%.
On the other hand, a large content exceeding 0.45% invites concern about deterioration of toughness and weldability and occurrence of delayed fracture. In addition, ductility and stretch flangeability are reduced.
Therefore, the C content is more than 0.15% and 0.45% or less, preferably 0.18% or more and 0.42% or less, and more preferably 0.20% or more and 0.40% or less.

<< Si: 0.50% or more and 2.50% or less >>
Si is a useful element that has a high solid solution strengthening ability in ferrite, contributes to an increase in strength, suppresses the formation of carbide (cementite), and contributes to stabilization of retained austenite. Moreover, Si has the effect | action which discharges C (solid solution) in a ferrite to austenite, cleans a ferrite, and contributes to a ductile improvement. Moreover, Si dissolved in ferrite improves work hardening ability and contributes to improvement of ductility of ferrite itself. In order to acquire such an effect, Si needs to contain 0.50% or more.
On the other hand, if Si exceeds 2.50%, not only the effect of suppressing the formation of carbide (cementite) and stabilizing the retained austenite is saturated, but the amount of Si dissolved in ferrite becomes excessive. Therefore, ductility is reduced.
For this reason, the Si content is 0.50% or more and 2.50% or less, preferably 0.80% or more and 2.00% or less, and more preferably 1.00% or more and 1.80% or less.

<< Mn: 1.50% to 3.50% >>
Mn is an element that contributes effectively to an increase in strength through solid solution strengthening or improvement in hardenability and stabilizes austenite, and is an indispensable element for securing a desired amount of retained austenite. In order to acquire such an effect, Mn needs to contain 1.50% or more.
On the other hand, when Mn exceeds 3.50%, it becomes difficult to obtain a desired amount of retained austenite, and martensite is excessively generated.
For this reason, content of Mn is 1.50% or more and 3.50% or less, and 2.30% or more and 3.00% or less are preferable.

<< P: 0.001% to 0.050% >>
P is an element contributing to an increase in strength by solid solution strengthening, and can be contained in an appropriate amount according to a desired strength. P is an element that has an action of promoting ferrite transformation and is effective in forming a composite structure. In order to acquire such an effect, P needs to contain 0.001% or more.
On the other hand, if P exceeds 0.050%, weldability is deteriorated and grain boundary fracture due to grain boundary segregation is promoted.
For this reason, content of P is 0.001% or more and 0.050% or less, and 0.005% or more and 0.030% or less are preferable.

<< S: 0.0100% or less >>
S is an element that segregates at the grain boundaries and embrittles the steel during hot working, and is present in the steel as a sulfide to reduce local deformability, and is preferably reduced as much as possible. % Or less, the above-mentioned adverse effects are acceptable.
For this reason, content of S is 0.0100% or less, and 0.0050% or less is preferable. Since excessively reducing S leads to restrictions on production technology or an increase in refining costs, the S content is preferably 0.0001% or more.

<< N: 0.0100% or less >>
N is an element that lowers the aging resistance of steel and is preferably reduced as much as possible. However, if it is 0.0100% or less, the above-described adverse effects can be tolerated.
For this reason, content of N is 0.0100% or less, and 0.0070% or less is preferable. Since excessively reducing N leads to restrictions on production technology or an increase in refining costs, the N content is preferably 0.0005% or more.

<< Al: 0.010% or more and 1.00% or less >>
Al is a ferrite-forming element and is an element that improves the balance between strength and ductility (strength-ductility balance). In order to acquire such an effect, it is necessary to contain Al 0.010% or more.
On the other hand, the content of Al exceeding 1.00% causes a decrease in surface properties.
For this reason, content of Al is 0.010% or more and 1.00% or less, 0.030% or more and 0.500% or less are preferable, and 0.050% or more and 0.450% or less are more preferable.

<Other components (elements)>
The above composition is a basic composition, but the composition is further Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, B: 0.0001% to 0 .0050% or less, Cr: 0.05% to 0.20%, Cu: 0.05% to 0.20%, Sb: 0.002% to 0.050%, Sn: 0.002% 0.050% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: It may contain at least one element selected from the group consisting of 0.0005% or more and 0.0050% or less.

(Ti and Nb)
Ti and Nb are both effective elements that suppress the coarsening of crystal grains during heating in the annealing step or the like and contribute to the refinement and homogenization of the steel sheet structure after annealing. In order to obtain such effects, it is preferable to contain Ti: 0.005% or more and Nb: 0.005% or more, respectively.
On the other hand, when the content exceeds Ti: 0.030% and Nb: 0.030%, respectively, Ti-based and Nb-based precipitates are excessively generated in the ferrite, so that the ductility may be lowered.
For this reason, the content of Ti is preferably 0.005% or more and 0.030% or less, and more preferably 0.010% or more and 0.020% or less. The Nb content is preferably 0.005% or more and 0.030% or less, and more preferably 0.010% or more and 0.020% or less.

(B)
B is an effective element that contributes to strengthening of the steel sheet through improvement of hardenability. In order to acquire such an effect, it is preferable to contain 0.0001% or more.
On the other hand, if the content exceeds 0.0050%, the content of martensite is excessively increased, and the increase in strength is excessively increased, which may cause a decrease in ductility.
For this reason, when B is contained, the content of B is preferably 0.0001% or more and 0.0050% or less, and more preferably 0.0005% or more and 0.0030% or less.

(Cr)
Cr contributes to the strengthening of the steel sheet by solid solution strengthening, stabilizes austenite during cooling in the annealing process, and facilitates complexation of the structure.
In order to obtain such an effect, the content is preferably 0.05% or more.
On the other hand, when it contains more than 0.20%, moldability may fall.
For this reason, when Cr is contained, the content of Cr is preferably 0.05% or more and 0.20% or less.

(Cu)
Cu contributes to the strengthening of the steel sheet by solid solution strengthening, stabilizes austenite during cooling in the annealing process, and facilitates complexation of the structure.
In order to obtain such an effect, the content is preferably 0.05% or more.
On the other hand, when it contains more than 0.20%, moldability may fall.
For this reason, when Cu is contained, the content of Cu is preferably 0.05% or more and 0.20% or less.

(Sb and Sn)
Sb and Sn have an effect of suppressing decarburization of the steel sheet surface layer (a region of several tens of μm) caused by nitriding and oxidation of the steel sheet surface. By suppressing such nitriding and oxidation of the steel sheet surface layer, it is possible to prevent a reduction in the amount of martensite produced on the steel sheet surface. As a result, it is effective to secure a desired strength. In addition, variations in strength and elongation due to temperature fluctuations during annealing can be reduced, which is also effective in ensuring manufacturing stability.
In order to obtain such an effect, it is preferable to contain 0.002% or more of Sb and Sn, respectively.
On the other hand, when Sb and Sn are respectively contained excessively exceeding 0.050%, the toughness may be lowered.
For this reason, when Sb and / or Sn are contained, the content of Sb and Sn is preferably 0.002% or more and 0.050% or less, respectively.

(Ta)
Ta generates carbides and carbonitrides and contributes to increasing the strength of the steel sheet. In order to obtain such an effect, the content is preferably 0.001% or more.
On the other hand, if the content exceeds 0.100%, the material cost increases, and an effect commensurate with the content cannot be expected, which may be economically disadvantageous.
For this reason, when Ta is contained, the content of Ta is preferably 0.001% or more and 0.100% or less.

(Ca, Mg and REM)
Ca, Mg, and REM (rare earth elements) are all elements used for deoxidation, and have an effect of improving the adverse effect on the local ductility and stretch flangeability of sulfides by making the shape of sulfides spherical. Yes, it can contain 1 type or 2 types or more as needed.
In order to obtain such an effect, Ca, Mg, and REM are each preferably contained in a content of 0.0005% or more.
On the other hand, if the content exceeds 0.0050%, inclusions and the like increase, and surface defects and internal defects may occur.
When Ca, Mg and REM are contained, the content of Ca, Mg and REM is preferably 0.0005% or more and 0.0050% or less, respectively.

<< Remainder Fe and inevitable impurities >>
In the above composition, the balance other than the above components consists of Fe (remainder Fe) and inevitable impurities.

<Organization>
Next, the structure limitation of the high-strength cold-rolled thin steel sheet of the present invention will be described.
The high-strength cold-rolled thin steel sheet of the present invention has a structure (composite structure) composed of polygonal ferrite, bainitic ferrite, retained austenite, and martensite. Specifically, the high-strength cold-rolled steel sheet of the present invention has a volume ratio of 10% or more and 70 at a position corresponding to ¼ of the plate thickness in the plate thickness direction from the surface (plate thickness ¼ position). % Of polygonal ferrite, 5% to 40% bainitic ferrite, 15% to 40% residual austenite, and 0% to 30% martensite.

<< Volume ratio of polygonal ferrite: 10% to 70% >>
Polygonal ferrite contributes to improvement of ductility (elongation). For this reason, it is set as the structure | tissue containing 10% or more polygonal ferrite by volume ratio. If the polygonal ferrite is less than 10% by volume, it is difficult to ensure desired ductility.
On the other hand, if the polygonal ferrite exceeds 70% by volume, the desired high strength cannot be ensured.
For this reason, the volume fraction of polygonal ferrite is 10% or more and 70% or less, and preferably 15% or more and 65% or less.

<< Volume ratio of bainitic ferrite: 5% to 40% >>
Bainitic ferrite has a high dislocation density and contributes not only to an increase in strength but also to an improvement in stretch flangeability (hole expansion ratio). Furthermore, in order to concentrate C in untransformed austenite, it is necessary to secure desired retained austenite. In order to acquire such an effect, bainitic ferrite is made into 5% or more by volume ratio.
On the other hand, if the bainitic ferrite exceeds 40% by volume, the desired high strength cannot be ensured.
For this reason, the volume ratio of bainitic ferrite is 5% or more and 40% or less.
The “bainitic ferrite” referred to here is a ferrite generated by the upper bainite transformation, and has a higher dislocation density than polygonal ferrite.

<< Volume ratio of retained austenite: more than 15% and 40% or less >>
Residual austenite itself is rich in ductility, but is a structure that contributes to further improving ductility by strain-induced transformation, and contributes to improving ductility and improving the strength-ductility balance. In order to obtain such an effect, the retained austenite needs to exceed 15% by volume.
On the other hand, if the retained austenite increases in volume ratio exceeding 40%, the strength is lowered and the desired high strength cannot be ensured.
For this reason, the volume ratio of retained austenite is 15% to 40%, preferably 17% to 40%.

<< Volume ratio of martensite: 0% to 30% >>
The “martensite” here includes fresh martensite and tempered martensite.
When martensite exceeds 30% in volume ratio, desired ductility and stretch flangeability cannot be secured.
On the other hand, in order to ensure a desired high strength, martensite is preferably in a volume ratio exceeding 0% (not including 0%) and 3% or more.
For this reason, the volume ratio of martensite is more than 0% and 30% or less, and preferably 3% or more and 30% or less.

<< Average crystal grain size of polygonal ferrite: 10.0 μm or less >>
By setting the average crystal grain size of polygonal ferrite to 10.0 μm or less, the hard austenite and martensite and other hard structures are uniformly dispersed and contribute to improvement of stretch flangeability. When the average crystal grain size of polygonal ferrite exceeds 10.0 μm, the dispersion of retained austenite and martensite becomes non-uniform, and the desired stretch flangeability cannot be secured. The average crystal grain size of polygonal ferrite is preferably 8.0 μm or less.
On the other hand, the lower limit of the average grain size of polygonal ferrite is not particularly limited, but is, for example, 3.0 μm or more.

<< Aspect ratio of polygonal ferrite: 1.5 or more >>
By setting the aspect ratio of polygonal ferrite to 1.5 or more, a desired amount of retained austenite can be secured, which contributes to improvement of ductility and improvement of strength-ductility balance. In addition, it suppresses crack extension during the hole expansion test and contributes to the improvement of stretch flangeability. For this reason, the aspect ratio of polygonal ferrite is 1.5 or more, and preferably 2.0 or more.
On the other hand, the upper limit of the aspect ratio of polygonal ferrite is not particularly limited, but is, for example, 4.0 or less.

<< Average crystal grain size of retained austenite: 2.0 μm or less >>
By reducing the crystal grains of retained austenite, ductility is improved. Therefore, in order to ensure good ductility, the average crystal grain size of retained austenite needs to be 2.0 μm or less. In order to ensure better ductility, the average crystal grain size of retained austenite is preferably 1.7 μm or less.
On the other hand, the lower limit of the average crystal grain size of retained austenite is not particularly limited, but is, for example, 0.3 μm or more.

<< Aspect ratio of retained austenite: 2.0 or more >>
By setting the aspect ratio of retained austenite to 2.0 or more, good ductility and strength-ductility balance are ensured, and further, crack extension during the hole expansion test is suppressed, contributing to improvement of stretch flangeability. To do. For this reason, the aspect ratio of retained austenite is 2.0 or more, and preferably 2.3 or more.
On the other hand, the upper limit of the aspect ratio of retained austenite is not particularly limited, but is, for example, 5.0 or less.

In the above structure, non-recrystallized ferrite, pearlite, cementite and the like may be further generated. However, the volume ratio is preferably 10% or less for non-recrystallized ferrite, 5% or less for pearlite, and 5% or less for cementite.

<Plating layer>
The high-strength cold-rolled thin steel sheet of the present invention having the above composition and the above structure may further have a plating layer on its surface in order to improve corrosion resistance. As the plating layer, a hot dip galvanized layer, an alloyed hot dip galvanized layer, or an electrogalvanized layer is preferable.
The hot-dip galvanized layer, the alloyed hot-dip galvanized layer, and the electrogalvanized layer are not particularly limited, and are conventionally known hot-dip galvanized layer, conventionally known alloyed hot-dip galvanized layer, and conventionally known, respectively. The electrogalvanized layer is preferably used.
The electrogalvanized layer may be a zinc alloy plated layer obtained by adding an appropriate amount of elements such as Fe, Cr, Ni, Mn, Co, Sn, Pb, or Mo to Zn according to the purpose. .

[Method for producing high-strength cold-rolled thin steel sheet]
Next, a preferred embodiment of the method for producing a high-strength cold-rolled thin steel sheet of the present invention (hereinafter also simply referred to as “the production method of the present invention”) will be described.
The manufacturing method of the present invention generally includes the above-described high-strength cold-rolled thin film of the present invention by sequentially performing hot rolling, pickling, cold rolling, and annealing on a steel material having the above composition. It is a method of obtaining a steel plate. And in the manufacturing method of this invention, the process of annealing is divided into two processes.

<Steel material>
The steel material is not particularly limited as long as it is a steel material having the above composition. For example, the steel material having the above composition is melted by a conventional melting method using a converter or the like, and a continuous casting method is used. The obtained slab having a predetermined dimension is preferably used. A steel piece (steel material) may be manufactured by ingot-bundling rolling.

<Hot rolling process>
A hot rolling process is a process of obtaining a hot-rolled sheet by hot-rolling the steel raw material which has the said composition.
The hot rolling process is not particularly limited as long as it is a process in which a steel material having the above composition is heated and subjected to hot rolling to obtain a hot rolled sheet having a predetermined size, and a normal hot rolling process is applied. it can.
As a normal hot rolling process, for example, a steel material is heated to a heating temperature of 1100 ° C. or more and 1250 ° C. or less, and the heated steel material is hot rolled at a hot rolling outlet temperature of 850 ° C. or more and 950 ° C. or less. After rolling and hot rolling is completed, an appropriate post-rolling cooling (specifically, for example, an average cooling rate of 40 ° C./s to 100 ° C./s in a temperature range of 450 ° C. to 950 ° C.) In the hot rolling step, the steel sheet is wound at a coiling temperature of 450 ° C. or higher and 650 ° C. or lower to obtain a hot-rolled sheet having a predetermined size and shape.

<Pickling process>
The pickling step is a step of pickling the hot-rolled sheet obtained through the hot rolling step.
The pickling step is not particularly limited as long as it can be pickled to such an extent that cold rolling can be performed on the hot-rolled sheet. For example, a conventional pickling step using hydrochloric acid or sulfuric acid can be applied.

<Cold rolling process>
The cold rolling process is a process of performing cold rolling on the hot-rolled sheet that has undergone the pickling process. More specifically, the cold rolling step is a step of obtaining a thin cold rolled plate having a predetermined thickness by subjecting the hot rolled plate subjected to pickling to cold rolling with a rolling reduction of 30% or more.

<Cold rolling reduction: 30% or more>
The rolling reduction of cold rolling is 30% or more. When the rolling reduction is less than 30%, the processing amount is insufficient, and recrystallization of the processed ferrite cannot be sufficiently achieved in the subsequent annealing step.
On the other hand, the upper limit of the rolling reduction is determined by the capability of the cold rolling mill, but if the rolling reduction is too high, the rolling load increases and the productivity may decrease. For this reason, the rolling reduction is preferably 70% or less.
The number of rolling passes and the rolling reduction per pass are not particularly limited.

<Annealing process>
An annealing process is a process which anneals the thin cold-rolled sheet obtained through the cold rolling process, and is a process including the 1st stage annealing process and 2nd stage annealing process mentioned later in detail.

<< First stage annealing process >>
The first stage annealing process, a thin cold-rolled sheet is heated at an annealing temperature T ₁ of the 800 ° C. or higher 950 ° C. or less, from the annealing temperatures T _1, at 5 ° C. / s or more average cooling rate, cooling below 500 ℃ by cooling to stop temperature T _2, which is a step of obtaining a first Danhiyanobe annealed sheets having tissue total of martensite and bainite is not less than 80% by volume.

(Annealing temperature T ₁ : 800 ° C. or higher and 950 ° C. or lower)
When the annealing temperature T ₁ is less than 800 ° C., too much amount of generated ferrite during annealing, it can not be secured on the total amount of the desired martensite and bainite.
On the other hand, when it exceeds annealing temperature T ₁ is 950 ° C., and the austenite grains are excessively coarsened, since the formation of ferrite is suppressed in the second stage annealing process, a second Danhiyanobe obtained through the second-stage annealing process Martensite is excessively generated in the annealed plate.
Thus, annealing temperatures _{T 1} is 800 ° C. or higher 950 ° C. or less.

Holding time at the annealing temperatures _{T 1} is not particularly limited, for example, is 10s or 900s or less.

(Average cooling rate: 5 ° C / s or more)
If the average cooling rate from the annealing temperature T ₁ of to the cooling stop temperature T ₂ is less than 5 ° C. / s, ferrite and pearlite generates during cooling, it is difficult to ensure a desired amount of martensite and bainite .
The upper limit of the average cooling rate is not particularly limited, but an excessively large cooling device is required to ensure an excessively high cooling rate, and the average cooling rate is 50 ° C./s from the viewpoint of production technology and capital investment. The following is preferred.
The cooling is preferably gas cooling, but may be performed in combination with furnace cooling and mist cooling.

(Cooling stop temperature T ₂ : 500 ° C. or less)
In tissue after cooling, to 80% or more by volume of the total of martensite and bainite, the cooling stop temperature T ₂ and the temperature of the temperature range below 500 ℃. Cooling stop temperature _{T 2} is preferably 300 ° C. or higher 480 ° C. or less.
After the cooling is stopped, the second-stage annealing process may be continued. After cooling is stopped, it is allowed to cool, and after cooling to room temperature, it may be shifted to the second stage annealing step.

(Total volume ratio of martensite and bainite: 80% or more)
In the structure of the first stage cold-rolled annealed sheet obtained through the first stage annealing step, the second stage obtained through the second stage annealing step when the sum of martensite and bainite is less than 80% in volume ratio. In a cold-rolled annealed sheet, it becomes difficult to secure desired retained austenite and it is difficult to secure polygonal ferrite having a desired shape (aspect ratio).
Here, “bainite” includes upper bainite and lower bainite.

<< Second stage annealing process >>
The second stage annealing process, a first Danhiyanobe annealed sheets obtained through the first-stage annealing process, at 700 ° C. or higher 850 ° C. below the annealing temperature _{T 3,} and held 10s or 900s or less, the annealing temperature _{T 3} from an average cooling rate of 50 ° C. / s or less 5 ° C. / s or more, cooled to 200 of the cooling stop ° C. or higher 500 ° C. or less temperature _{T 4,} the cooling stop temperature _{T 4,} by holding 10s or 1800s or less, This is a step of obtaining a second-stage cold-rolled annealed plate.

(Annealing temperature _T 3: 700 ℃ more than 850 ℃ or less)
When the annealing temperature T ₃ is lower than 700 ° C., can not be secured a sufficient amount of austenite during annealing, final desired amount of retained austenite and bainitic ferrite can not be secured.
On the other hand, if the annealing temperature T ₃ is higher than 850 ° C., since the austenite single-phase region, and finally, after that can not be generated residual austenite desired amount, residual austenite with a desired aspect ratio, and a desired aspect ratio In addition, it becomes difficult to secure polygonal ferrite having an average crystal grain size, and martensite is excessively generated.
Therefore, the annealing temperature _{T 3} is at 700 ° C. or higher 850 ° C. or less, preferably 720 ° C. or higher 830 ° C. or less.

(Retention time at the annealing temperature _{T 3:} 10s or 900s or less)
Holding time at the annealing temperature T ₃ is less than 10s, can not be ensured a sufficient amount of austenite during annealing, final desired amount of retained austenite and bainitic ferrite can not be secured.
On the other hand, the holding time at the annealing temperature T ₃ is when it comes to long beyond 900s, resulting grain coarsening, eventually no longer able to generate residual austenite desired amount.
Therefore, the holding time at the annealing temperature _{T 3} is 10s or 900s or less.

(Average cooling rate: 5 ° C / s or more and 50 ° C / s or less)
If the average cooling rate from the annealing temperature T ₃ to a cooling stop temperature T ₄ is lower than 5 ° C. / s, and generates a large amount of polygonal ferrite and pearlite during cooling, it can be ensured desired amount of bainitic ferrite Disappear.
On the other hand, if the average cooling rate from the annealing temperature T ₃ to a cooling stop temperature T ₄ is greater than 50 ° C. / s, the low temperature transformation structure such as martensite is generated excessively.
Therefore, the average cooling rate from the annealing temperature T ₃ to a cooling stop temperature T ₄ is 50 ° C. / s or less 5 ° C. / or.
The cooling is preferably gas cooling, but can be performed by combining furnace cooling and mist cooling.

(Cooling stop temperature T ₄ : 200 ° C. or more and 500 ° C. or less)
If the cooling stop temperature T ₄ is lower than 200 ° C., during retention after cooling down, a large amount of martensite is produced, it can not be ensured the desired tissue (volume ratio and aspect ratio of retained austenite).
On the other hand, the cooling stop temperature T ₄ is more than 500 ° C., during retention after cooling down, to generate a large amount of polygonal ferrite and pearlite, it can not be ensured the desired tissue. Specifically, for polygonal ferrite, the volume ratio is excessive, while the aspect ratio is excessive. Moreover, the volume fraction of bainitic ferrite becomes too small. Furthermore, the volume fraction and aspect ratio of retained austenite are too small.
Therefore, the cooling stop temperature _{T 4} is 200 ° C. or higher 500 ° C. or less.

(Cooling stop temperature _T retention time in the _4: 10s or 1800s or less)
If the holding time at the cooling stop temperature T ₄ is less than 10s, it is insufficient time for the concentration of C into austenite, and finally it is difficult to secure a retained austenite desired amount, also, Residual austenite having the desired aspect ratio cannot be obtained.
On the other hand, even if retained for longer than 1800 s, the increase in retained austenite is small, and part of retained austenite decomposes into ferrite and cementite, making it difficult to secure a desired amount of retained austenite.
Therefore, the holding time at the cooling stop temperature _{T 4} is 10s or 1800s or less.
Here, "holding", in addition to isothermal hold, the category includes slow cooling or heating in the temperature range of the cooling stop temperature T _4.

Second Danhiyanobe annealed sheet after holding in the cooling stop temperature T ₄ cools. This cooling is not particularly limited, and the cooling can be performed to a desired temperature such as room temperature by an arbitrary method such as cooling.

When the plating process described later is not performed, the second-stage cold-rolled annealed sheet obtained through the second-stage annealing process becomes the high-strength cold-rolled thin steel sheet of the present invention.
Hereinafter, the second-stage cold-rolled annealed sheet may be referred to as “cold-rolled thin steel sheet”.

<Plating process>
The second-stage cold-rolled annealed plate (cold-rolled thin steel plate) obtained through the second-stage annealing step may be further subjected to a plating treatment to form a plating layer on the surface thereof. In this case, the second-stage cold-rolled annealed plate having a plating layer formed on the surface is the high-strength cold-rolled thin steel plate of the present invention.

As the plating treatment, hot dip galvanizing treatment, hot dip galvanizing treatment and alloying treatment, or electrogalvanizing treatment is preferable. The hot dip galvanizing treatment, the hot dip galvanizing treatment and the alloying treatment, and the electrogalvanizing treatment are not particularly limited, and are conventionally known hot dip galvanizing treatment, conventionally known hot dip galvanizing treatment and alloying treatment, respectively. In addition, a conventionally known electrogalvanizing treatment is preferably used.
Prior to the plating treatment, pretreatment such as degreasing and phosphate treatment may be performed.

As the hot dip galvanizing treatment, for example, a conventional continuous hot dip galvanizing line is used to immerse the second stage cold-rolled annealing plate in a hot dip galvanizing bath and form a predetermined amount of hot dip galvanized layer on the surface. It is preferable that
When immersed in a hot dip galvanizing bath, the temperature of the second-stage cold-rolled annealed plate is not less than the temperature of the hot dip galvanizing bath temperature −50 ° C. and not more than the temperature of the hot dip galvanizing bath temperature + 80 ° C. by reheating or cooling. It is preferable to adjust within the range.
The temperature of the hot dip galvanizing bath is preferably 440 ° C or higher, and more preferably 500 ° C or lower.
The hot dip galvanizing bath may contain Al, Fe, Mg, Si or the like in addition to pure zinc.
The adhesion amount of the hot-dip galvanized layer can be adjusted to a desired adhesion amount by adjusting gas wiping or the like, and is preferably about 45 g / m ² per side.

The plated layer (hot galvanized layer) formed by the hot dip galvanizing process may be an alloyed hot dip galvanized layer by performing a usual alloying process as necessary.
The temperature for the alloying treatment is preferably 460 ° C. or more and 600 ° C. or less.
In the case of an alloyed hot dip galvanized layer, adjusting the effective Al concentration in the hot dip galvanizing bath to a range of 0.10% by mass or more and 0.22% by mass or less from the viewpoint of securing a desired plating appearance. preferable.

The electrogalvanizing treatment is preferably, for example, a treatment of forming a predetermined amount of electrogalvanized layer on the surface of the second stage cold-rolled annealed plate using a conventional electrogalvanizing line.
The adhesion amount of the electrogalvanized layer can be adjusted to a predetermined adhesion amount by adjusting the sheet passing speed or the current value, and is preferably about 30 g / m ² per side.

Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to these.

<Manufacture of cold rolled steel sheet>
Molten steel having the composition shown in Table 1 below was melted in a converter, and a slab (thickness: 230 mm) as a steel material was obtained by a continuous casting method. A hot-rolled sheet was obtained by subjecting the obtained slab to hot rolling. The obtained hot-rolled sheet was pickled using hydrochloric acid, and then cold-rolled at the rolling reduction shown in Table 2 below to obtain a thin cold-rolled sheet (sheet thickness: 1.4 mm).
The obtained thin cold-rolled sheet was annealed under the conditions shown in Table 2 below to obtain a second-stage cold-rolled annealed sheet (cold-rolled thin steel sheet). The annealing process was a two-stage process consisting of a first stage annealing process and a second stage annealing process. Holding time at the annealing temperature T ₁ of the first stage annealing process was 100s. After the first stage annealing step, a specimen for observing the structure was collected from the first stage cold-rolled annealed sheet, and the structure was observed.

For some second-stage cold-rolled annealed plates (cold-rolled thin steel plates), a hot-dip galvanized layer is formed on the surface of the hot-dip galvanized thin steel plate after the annealing has been completed. It was.
In the hot dip galvanizing process, using a continuous hot dip galvanizing line, the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) is reheated to a temperature in the range of 430 ° C. or higher and 480 ° C. or lower as necessary. It was immersed in a hot dip galvanizing bath (bath temperature: 470 ° C.). It adjusted so that the adhesion amount of a plating layer might be 45 g / m < ² > per single side | surface. The bath composition was Zn-0.18 mass% Al.
At this time, in some hot-dip galvanized steel sheets, the bath composition was Zn-0.14 mass% Al, and after the plating treatment, alloying treatment was performed at 520 ° C. to obtain an alloyed hot-dip galvanized thin steel sheet. .
The Fe concentration in the plating layer was 9% by mass or more and 12% by mass or less.
For some second-stage cold-rolled annealed plates (cold-rolled thin steel plates), an electric galvanizing line is used after the annealing, so that the amount of plating is 30 g / m ² per side. A galvanizing treatment was performed to obtain an electrogalvanized sheet steel.

In Table 3 below, the second-stage cold-rolled annealed sheet (cold-rolled sheet steel) that does not form a plating layer is “CR”, the hot-dip galvanized sheet steel is “GI”, and the galvannealed sheet steel is “GA”. The electrogalvanized sheet steel was denoted as “EG”.

<Evaluation>
From the obtained cold-rolled thin steel sheet (including hot-dip galvanized thin steel sheet, alloyed hot-dip galvanized thin steel sheet, and electrogalvanized thin steel sheet), specimens were collected and subjected to structure observation and tensile tests. The test method was as follows.

<< Organizational observation >>
First, from the first-stage cold-rolled annealed plate and the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) or the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) on which the plating layer is formed, for structure observation The test piece was collected.
Next, the collected specimen was polished so that the position corresponding to 1/4 of the plate thickness in the rolling direction cross section (L cross section) was the observation surface. The polished specimen was corroded using 3% by volume nital solution. Then, using the scanning electron microscope (SEM) (magnification: 2000 times), the structure | tissue of the test piece was observed in the visual field of the range of 40 micrometers x 40 micrometers, respectively, and it imaged and obtained the SEM image.
Using the obtained SEM image, the fraction (area ratio) of each tissue was determined by image analysis. The obtained value was treated as a volume fraction and used as a fraction of each tissue. For image analysis, “Image-Pro” (trade name) manufactured by Media Cybernetics was used as analysis software.
In the SEM image, polygonal ferrite is gray, martensite and retained austenite are white, so each structure was judged from the color tone.
A structure in which retained austenite and cementite are observed in fine lines or dots in ferrite is bainite.
The volume ratio of the retained austenite obtained separately was subtracted from the volume ratio of the structure exhibiting white to obtain the volume ratio of martensite.
Using the obtained SEM image, the major axis and minor axis of each polygonal ferrite were determined by image analysis, the area was calculated from the determined major axis and minor axis, and the equivalent circle diameter was calculated from the calculated area. The values were arithmetically averaged to obtain the average crystal grain size of polygonal ferrite.
From the obtained major axis and minor axis, the aspect ratio of each polygonal ferrite was calculated, and the obtained values were arithmetically averaged to obtain the polygonal ferrite aspect ratio (average).

Test specimens for observation with a transmission electron microscope were collected from the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) or the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) on which the plating layer was formed. The collected specimen was ground and polished (mechanical polishing and electrolytic polishing) so that a position corresponding to 1/4 of the plate thickness was an observation position, and a thin film sample was obtained.
About the obtained thin film sample, the structure | tissue was observed using the transmission electron microscope (TEM) (magnification: 15000 times), and 20 or more visual fields were imaged in the visual field of the range of 3 micrometers x 3 micrometers, and the TEM image was obtained.
Using the obtained TEM image, the volume fraction of bainitic ferrite and the average crystal grain size and aspect ratio (average) of retained austenite were determined by image analysis.
The average crystal grain size of retained austenite was obtained by calculating the area of each retained austenite, calculating the equivalent circle diameter from the determined area, and arithmetically averaging the obtained values to obtain the average crystal grain size of retained austenite.
Using the obtained TEM image, the major axis and the minor axis of each retained austenite are obtained by image analysis, the aspect ratio of each retained austenite is calculated, the obtained value is arithmetically averaged, and the aspect ratio of the retained austenite (average) ).
In the image analysis of the TEM image, “Image-Pro” (trade name) of Media Cybernetics was used as the analysis software in the same manner as the image analysis of the SEM image.

From the first-stage cold-rolled annealed plate and the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) or the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) on which the plating layer is formed, Test specimens were collected. The collected specimen was ground and polished so that the position corresponding to 1/4 of the plate thickness was the measurement surface. About the test piece which grind | polished and grind | polished, the volume ratio of the retained austenite was calculated | required from the diffraction X-ray intensity | strength by the X ray diffraction method. CoKα rays were used as incident X-rays.
In calculating the volume fraction of retained austenite, the integrated intensity of the peaks of the {111}, {200}, {220} and {311} faces of austenite and the {110}, {200} and {211} faces of ferrite The intensity ratio was calculated for all the combinations. The average value thereof was obtained, and the volume fraction of retained austenite was calculated.

<Tensile test>
From the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) or the second-stage cold-rolled annealed plate (cold-rolled thin steel plate) on which the plating layer is formed, the tensile direction becomes the direction (C direction) perpendicular to the rolling direction. Thus, a JIS No. 5 tensile test piece was sampled, and a tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011) to determine the tensile strength (TS) and elongation at break (El). -The ductility balance (TS x El) was calculated. The results are shown in Table 3 below.

(Strength)
If TS is 980 MPa or more, it can be evaluated as high strength.

(Ductility)
When TS is 980 MPa or more and less than 1180 MPa, El is 25% or more. When TS is 1180 MPa or more, if El is 20% or more, high ductility can be evaluated.

(Strength-ductility balance)
When TS is 980 MPa or more and less than 1180 MPa, TS × El is 24500 MPa ·% or more, and when TS is 1180 MPa or more, if TS × El is 23600 MPa ·% or more, it can be evaluated that the strength-ductility balance is good.

《Hole expansion test》
A test piece having a size of 100 mmW × 100 mmL was taken from a second-stage cold-rolled annealed plate (cold-rolled thin steel plate) or a second-stage cold-rolled annealed plate (cold-rolled thin steel plate) on which a plating layer was formed. A 10 mmφ hole was punched into the collected test piece with a clearance of 12.5% in accordance with the provisions of JIS Z 2256 (2010). The hole was then widened by raising the 60 ° conical punch. At that time, the rise of the conical punch was stopped when the crack penetrated the plate thickness direction. The hole expansion ratio λ [%] was determined from the hole diameter after the crack penetration and the hole diameter before the test. The results are shown in Table 3 below.

(Stretch flangeability)
When TS is 980 MPa or more and less than 1180 MPa, λ is 30% or more. When TS is 1180 MPa or more, if λ is 20% or more, it can be evaluated that the stretch flangeability is good (high stretch flangeability).

In Tables 1 to 3, the underlined portion indicates outside the scope of the present invention.
Each of the cold-rolled thin steel sheets (including the cold-rolled thin steel sheet on which the plating layer is formed) of the present invention has high strength, high ductility, excellent strength-ductility balance, and elongation. Flangeability was also good.
On the other hand, the cold-rolled thin steel sheet of the comparative example (including the cold-rolled thin steel sheet on which the plating layer is formed) has ductility and / or stretch flange even if the strength is insufficient or the strength is sufficient. Sex was insufficient.

Claims

In mass%, C: more than 0.15% and not more than 0.45%, Si: 0.50% to 2.50%, Mn: 1.50% to 3.50%, P: 0.001% or more 0.055% or less, S: 0.0100% or less, N: 0.0100% or less, and Al: 0.010% or more and 1.00% or less, and the balance Fe and inevitable impurities,
10% to 70% polygonal ferrite, 5% to 40% bainitic ferrite, 15% to 40% residual austenite, and 0% to 30% martensite An organization, and
The average grain size of the polygonal ferrite is 10.0 μm or less, and the aspect ratio of the polygonal ferrite is 1.5 or more,
A high-strength cold-rolled steel sheet having an average crystal grain size of the retained austenite of 2.0 µm or less and an aspect ratio of the retained austenite of 2.0 or more.
The composition is further, in mass%, Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, B: 0.0001% to 0.0050%, Cr : 0.05% to 0.20%, Cu: 0.05% to 0.20%, Sb: 0.002% to 0.050%, Sn: 0.002% to 0.050% Ta: 0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0 The high-strength cold-rolled thin steel sheet according to claim 1, comprising at least one element selected from the group consisting of .0050% or less.
The high-strength cold-rolled thin steel sheet according to claim 1 or 2, wherein the surface has a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
A method for producing the high-strength cold-rolled thin steel sheet according to any one of claims 1 to 3,
A hot rolling step for obtaining a hot-rolled sheet by subjecting the steel material having the composition according to claim 1 or 2 to hot rolling,
A pickling step of subjecting the hot-rolled sheet to a pickling treatment;
A cold rolling step of obtaining a thin cold-rolled sheet by subjecting the hot-rolled sheet subjected to the pickling treatment to cold rolling with a rolling reduction of 30% or more;
The thin cold-rolled sheet is heated at an annealing temperature T 1 of 800 ° C. or more and 950 ° C. or less, and from the annealing temperature T 1 to a cooling stop temperature T 2 of 500 ° C. or less at an average cooling rate of 5 ° C./s or more. A first-stage annealing step of obtaining a first-stage cold-rolled annealed sheet having a structure in which the sum of martensite and bainite is 80% or more by volume, by cooling;
Said first Danhiyanobe annealed sheets at 700 ° C. or higher 850 ° C. below the annealing temperature T 3, and held 10s or 900s or less, wherein the annealing temperature T 3, 5 ℃ / s or higher 50 ° C. / s or less in average cooling at a rate, and cooled to 200 ° C. or higher 500 ° C. or less of the cooling stop temperature T 4, the at cooling stop temperature T 4, by holding 10s or 1800s or less, the second-stage annealing process of obtaining a second Danhiyanobe annealed sheets And a method for producing a high-strength cold-rolled thin steel sheet.
5. The high-strength cold-rolled steel sheet according to claim 4, further comprising a plating step of subjecting the second stage cold-rolled annealed plate to a hot-dip galvanizing treatment, a hot-dip galvanizing treatment and an alloying treatment, or an electrogalvanizing treatment. Manufacturing method.