CN110475892B

CN110475892B - High-strength cold-rolled steel sheet and method for producing same

Info

Publication number: CN110475892B
Application number: CN201880020730.4A
Authority: CN
Inventors: 田中孝明; 田路勇树
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2017-04-05
Filing date: 2018-04-02
Publication date: 2022-01-14
Anticipated expiration: 2038-04-02
Also published as: WO2018186335A1; CN110475892A

Abstract

The invention provides a high-strength cold-rolled steel sheet having a tensile strength of 980MPa or more, excellent ductility and stretch-flangeability, and a low fraction defective in a hole expansion test. The high-strength cold-rolled steel sheet has the following composition: contains, in mass%, C: more than 0.15% and 0.45% or less, Si: 0.50% -2.50%, Mn: 1.50% -3.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010% -0.100%, N: 0.0100% or less, the balance being Fe and unavoidable impurities, wherein the total of ferrite and bainitic ferrite is 20 to 80%, the residual austenite is more than 10% and 40% or less, the martensite is more than 0% and 50% or less, the proportion of the residual austenite having an aspect ratio of 0.5 or less in the residual austenite is 75% or more, and the proportion of the residual austenite present at the boundary of the Bain group is 50% or more in the residual austenite having an aspect ratio of 0.5 or less.

Description

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

Technical Field

The present invention relates to a high-strength cold-rolled steel sheet and a method for manufacturing the same. More specifically, the present invention relates to a steel sheet having a Tensile Strength (TS): a high-strength, high-strength cold-rolled steel sheet having a strength of 980MPa or more and a method for producing the same.

Background

Conventionally, high-strength cold-rolled steel sheets have been used for automobile body parts and the like (for example, patent documents 1 to 2).

In recent years, from the viewpoint of global environmental conservation, it has been desired to improve fuel efficiency of automobiles and promote the use of high-strength cold-rolled steel sheets having a tensile strength of 980MPa or more.

In recent years, there has been an increasing demand for improving the collision safety of automobiles, and from the viewpoint of ensuring the safety of occupants at the time of collision, application of a high-strength cold-rolled steel sheet having a tensile strength of 1180MPa or more and an extremely high strength as a structural member such as a frame portion of a vehicle body has been studied.

Documents of the prior art

Patent document

Patent document 1 International publication No. 2016/132680

Patent document 2 International publication No. 2016/021193

Disclosure of Invention

The ductility of the steel sheet decreases with increasing strength. Since a steel sheet having low ductility is cracked during press forming, it is necessary to have both high strength and high ductility in order to process a high-strength steel sheet into an automobile part.

In addition, as one of the indicators of formability, steel sheets are sometimes required to have excellent stretch flangeability. The stretch flangeability was evaluated to be good when the average value of hole expansibility obtained by a predetermined hole expansion test is large, for example.

However, even in a steel sheet having an excellent average value of the hole expansibility (average hole expansibility), when the number of tests increases, a value significantly lower than the average value can be measured in a very small number of cases. The probability of such a measurement to a value significantly lower than the average value was taken as the failure rate of the hole expanding test.

A steel sheet having a high defective rate in the hole expansion test has a high probability of becoming defective during actual pressing. Such a disadvantage cannot be ignored in mass production and mass molding of parts. In order to reduce the fraction defective in press forming, a steel sheet having a low fraction defective in a hole expansion test is required.

Therefore, a steel sheet having a tensile strength of 980MPa or more, a high strength, excellent ductility and stretch flangeability, and a reduced fraction defective in the hole expansion test has been demanded.

However, in the conventional cold-rolled steel sheet, any one of the above properties may be insufficient.

Accordingly, an object of the present invention is to provide a high-strength cold-rolled steel sheet having a tensile strength of 980MPa or more, excellent ductility and stretch-flange formability, and a low fraction defective in a hole expansion test, and a method for manufacturing the same.

The present inventors have conducted intensive studies to achieve the above object. The following findings were obtained: when a large amount of block-like retained austenite having a large aspect ratio (aspect ratio) contained in the steel sheet is exposed at the punched end face in punching before the hole expansion test, end face cracks are induced, and the hole expansion ratio is greatly reduced.

The present inventors have also found that the presence of acicular retained austenite having a small aspect ratio at the Bain group boundary has the effect of suppressing the occurrence of the above-described end face cracks.

The present inventors have found that a steel sheet having a microstructure in which the fraction of retained austenite is high and most of the retained austenite is present at the Bain group boundary has needle-like shapes with a small aspect ratio, and that the fraction defective in the hole expansion test is significantly small.

The present inventors have further studied repeatedly, and as a result, have found that: when the heat treatment (annealing step) of the steel sheet is performed 2 times, and particularly the heat history in the 1 st annealing step is optimized, the microstructure of the steel sheet can be stably formed into the above-described microstructure.

The present inventors have further studied based on the above findings, and have completed the present invention.

Namely, the present invention provides the following [1] to [6 ].

[1] A high-strength cold-rolled steel sheet having the following composition, in mass%, containing C: more than 0.15% and 0.45% or less, Si: 0.50% -2.50%, Mn: 1.50% -3.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010% -0.100% and N: 0.0100% or less, the remainder being composed of Fe and unavoidable impurities, wherein the total of the area ratios of ferrite and bainitic ferrite in the microstructure is 20% to 80%, the area ratio of retained austenite exceeds 10% and is 40% or less, the area ratio of martensite exceeds 0% and is 50% or less, the proportion of retained austenite having an aspect ratio of 0.5 or less in the retained austenite is 75% or more by area ratio, the proportion of retained austenite present at the Bain group boundary is 0.5 or less in the retained austenite, and the proportion of retained austenite present at the Bain group boundary is 50% or more by area ratio.

[2] The high-strength cold-rolled steel sheet according to the above [1], wherein the composition further contains, in mass%, a metal selected from the group consisting of Ti: 0.005% -0.035%, Nb: 0.005% -0.035%, V: 0.005-0.035%, Mo: 0.005% -0.035%, B: 0.0003% -0.0100%, Cr: 0.05% -1.00%, Ni: 0.05 to 1.00%, Cu: 0.05% -1.00%, Sb: 0.002% -0.050%, Sn: 0.002% -0.050%, Ca: 0.0005% -0.0050%, Mg: 0.0005% -0.0050% and REM: 0.0005% -0.0050% of at least 1 element.

[3] The high-strength cold-rolled steel sheet according to the above [1] or [2], wherein C and Mn of the above composition satisfy the following formula (z) in mass%:

7.5×C+Mn＜5.0···(z)

in the formula (z), C and Mn represent the contents of the respective elements.

[4] The high-strength cold-rolled steel sheet according to any one of the above [1] to [3], wherein the surface has a plated layer.

[5]A method for producing a high-strength cold-rolled steel sheet, which comprises the step of [1]]～[4]The method for producing a high-strength cold-rolled steel sheet according to any one of the above processes, comprising: hot rolling step for the steel sheet having the above [1]]～[3]Hot rolling a steel slab having any one of the compositions to obtain a hot-rolled sheet; a pickling step of pickling the hot rolled sheet; a cold rolling step of cold rolling the hot-rolled sheet subjected to the acid pickling at a reduction of 30% or more to obtain a cold-rolled sheet; annealing step in stage 1, in which the cold-rolled sheet is subjected to Ac₃Annealing temperature T of point-950 DEG C₁Heating to over 10 deg.CThe average cooling rate in DEG C/s is set from the above annealing temperature T₁Cooling to a cooling stop temperature T of 250 ℃ or higher and less than 350 DEG C₂At the above-mentioned cooling stop temperature T₂Maintaining for 10s or more, thereby obtaining a stage 1 cold rolled annealed sheet; and a 2 nd stage annealing step of annealing the 1 st stage cold-rolled annealed sheet at an annealing temperature T of 700 to 850 DEG C₃Heating from the above annealing temperature T₃Cooling to a cooling stop temperature T of 300-500 DEG C₄Thereby obtaining a stage 2 cold-rolled annealed sheet.

[6] The method of manufacturing a high-strength cold-rolled steel sheet according to [5], further comprising a plating step of subjecting the 2 nd-stage cold-rolled annealed sheet to a hot-dip galvanizing treatment, an alloying treatment, or an electrogalvanizing treatment.

According to the present invention, a high-strength cold-rolled steel sheet having a tensile strength of 980MPa or more, excellent ductility and stretch-flangeability, and a low fraction defective in a hole expansion test, and a method for manufacturing the same can be provided.

The high-strength cold-rolled steel sheet of the present invention is suitable for structural steel materials such as parts of transportation machines including automobiles and structural steel materials. According to the present invention, the range of applications of the high-strength cold-rolled steel sheet can be further expanded, and industrially significant effects can be obtained.

Drawings

FIG. 1 is a schematic view showing a part of the microstructure of a steel sheet (a region considered to be generated from one prior austenite grain).

Fig. 2 is a graph showing the influence of the proportion of retained austenite having an aspect ratio of 0.5 or less in the retained austenite and the proportion of retained austenite present at the Bain group boundary in the retained austenite having an aspect ratio of 0.5 or less on the fraction defective in the hole expansion test.

Detailed Description

[ high-Strength Cold-rolled Steel sheet ]

The high-strength cold-rolled steel sheet of the present invention is a high-strength cold-rolled steel sheet having a composition and a microstructure, the composition containing, in mass%, C: more than 0.15% and 0.45% or less, Si: 0.50% -2.50%, Mn: 1.50% -3.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010% -0.100% and N: 0.0100% or less, and the balance being Fe and unavoidable impurities, wherein the microstructure has a total of ferrite and bainitic ferrite area ratios of 20% to 80%, an area ratio of retained austenite of more than 10% and 40% or less, an area ratio of martensite of more than 0% and 50% or less, a proportion of retained austenite having an aspect ratio of 0.5 or less in the retained austenite of 75% or more in terms of area ratio, and a proportion of retained austenite having a Bain group boundary of 0.5 or less in the retained austenite of 50% or more in terms of area ratio.

The high-strength cold-rolled steel sheet of the present invention has a thickness of, for example, 5mm or less.

Composition

First, the composition (component composition) of the high-strength cold-rolled steel sheet of the present invention will be described below. The unit of the content of the elements in the component composition is "mass%", and hereinafter, unless otherwise specified, it is represented by "%".

C: more than 0.15% and less than 0.45% >

C stabilizes austenite, ensures a desired area fraction of retained austenite, and contributes effectively to improvement of ductility, as well as to improvement of hardness of martensite and increase of strength. In order to sufficiently obtain such an effect, the content of C needs to exceed 0.15%.

On the other hand, if the content exceeds 0.45% and the content is large, the toughness, weldability, and delayed fracture resistance may deteriorate, and the amount of martensite produced may be excessive, resulting in a decrease in ductility and stretch flangeability.

Therefore, the content of C is more than 0.15% and 0.45% or less, preferably 0.18% to 0.42%, more preferably 0.20% to 0.40%.

《Si：0.50％～2.50％》

Si suppresses the formation of carbides (cementite), promotes the enrichment of C into austenite to stabilize austenite, and contributes to the improvement of ductility of the steel sheet. Si dissolved in ferrite increases work hardening energy and contributes to improvement of ductility of ferrite itself. In order to sufficiently obtain such an effect, Si needs to be contained by 0.50% or more.

On the other hand, if Si exceeds 2.50%, not only is the effect of suppressing the formation of carbides (cementite) and contributing to the stabilization of the retained austenite saturated, but also ductility is reduced because the amount of Si dissolved in the ferrite becomes excessive.

Therefore, the content of Si is 0.50% to 2.50%, preferably 0.80% to 2.30%, more preferably 1.00% to 2.10%.

《Mn：1.50％～3.00％》

Mn is an austenite stabilizing element, and contributes to improvement of ductility by stabilizing austenite, and contributes to increase of strength of a steel sheet by promoting generation of martensite by improving hardenability. In order to sufficiently obtain such an effect, Mn needs to be contained by 1.50% or more.

On the other hand, if Mn exceeds 3.00%, martensite is excessively generated, and ductility and stretch-flange formability are deteriorated.

Accordingly, the Mn content is 1.50% to 3.00%, preferably 1.80% to 2.70%.

P: 0.050% or less

P is a harmful element that segregates in grain boundaries to reduce elongation, induces cracks during processing, and deteriorates impact resistance. Therefore, the P content is set to 0.050% or less. Preferably 0.010% or less.

However, excessive dep leads to an increase in refining time, an increase in cost, and the like, and therefore the P content is preferably 0.002% or more.

S: 0.0100% or less

S exists as MnS in steel, promotes the generation of voids during punching, and further serves as a starting point of the generation of voids during machining, thereby lowering stretch-flange formability. Therefore, the S amount is preferably reduced as much as possible to 0.0100% or less. Preferably 0.0050% or less.

However, since excessive S removal leads to an increase in refining time, an increase in cost, and the like, the S content is preferably 0.0002% or more.

《Al：0.010％～0.100％》

Al is an element that functions as a deoxidizer. In order to obtain such an effect, 0.010% or more of Al is contained.

However, if the Al content is excessive, the Al oxide remains in the steel sheet, and the Al oxide is easily aggregated and coarsened, which causes deterioration in stretch flangeability. Therefore, the Al content is set to 0.100% or less.

N: 0.0100% or less

N is present in steel as AlN, promotes the generation of coarse voids during punching, and further becomes a starting point of the generation of coarse voids during machining, thereby lowering stretch-flange formability. Therefore, it is preferable to reduce the amount of N as much as possible to make the N content 0.0100% or less. Preferably 0.0060% or less.

However, since excessive de-N increases refining time and cost, the N content is preferably 0.0005% or more.

《7.5×C+Mn》

Both C and Mn are elements contributing to the formation of hard martensite, but when the contents of the respective elements are within the above ranges, 7.5 × C + Mn is less than 5.0, the stretch flangeability tends to be more excellent. This is because C, Mn do not determine the properties of martensite individually but affect each other, but when 7.5 × C + Mn is less than 5.0, excessive hardening of martensite is suppressed, and stretch flangeability is more excellent.

Therefore, it is preferable that C and Mn satisfy the following formula (z) in mass%.

7.5×C+Mn＜5.0···(z)

In the formula (z), C and Mn represent the contents of the respective elements.

The lower limit is not particularly limited, and for example, 7.5 × C + Mn is preferably 3.0 or more, and more preferably 3.5 or more.

Other ingredients (elements)

The high-strength cold-rolled steel sheet of the present invention further contains, as necessary, a component selected from the group consisting of Ti: 0.005% -0.035%, Nb: 0.005% -0.035%, V: 0.005-0.035%, Mo: 0.005% -0.035%, B: 0.0003% -0.0100%, Cr: 0.05% -1.00%, Ni: 0.05 to 1.00%, Cu: 0.05% -1.00%, Sb: 0.002% -0.050%, Sn: 0.002% -0.050%, Ca: 0.0005% -0.0050%, Mg: 0.0005% -0.0050% and REM: 0.0005% -0.0050% of at least 1 element.

(Ti：0.005％～0.035％)

Ti forms carbonitrides, and increases the strength of the steel by precipitation strengthening. When Ti is added, the Ti content is preferably 0.005% or more in order to effectively exhibit the above-mentioned effects. On the other hand, when Ti is excessive, precipitates are excessively formed, and ductility may be lowered.

Therefore, the content of Ti is preferably 0.005% to 0.035%, more preferably 0.005% to 0.020%.

(Nb：0.005％～0.035％)

Nb forms carbonitrides, and increases the strength of the steel by precipitation strengthening. When Nb is added, the Nb content is preferably 0.005% or more in order to effectively exhibit the above-described effects. On the other hand, if Nb is excessive, precipitates are excessively formed, and ductility may be reduced.

Therefore, the content of Nb is preferably 0.005% to 0.035%, more preferably 0.005% to 0.030%.

(V：0.005％～0.035％)

V forms carbonitrides, and increases the strength of the steel by precipitation strengthening. When V is added, the content of V is preferably 0.005% or more in order to effectively exhibit the above-mentioned effects. On the other hand, when V is excessive, precipitates are excessively formed, ductility may be lowered,

therefore, the content of V is preferably 0.005% to 0.035%, more preferably 0.005% to 0.030%.

(Mo：0.005％～0.035％)

Mo forms carbonitrides, and increases the strength of the steel by precipitation strengthening. When Mo is added, the content of Mo is preferably 0.005% or more in order to effectively exhibit the above-described effects. On the other hand, when Mo is excessive, precipitates are excessively formed, and ductility may be lowered.

Therefore, the content of Mo is preferably 0.005% to 0.035%, more preferably 0.005% to 0.030%.

(B：0.0003％～0.0100％)

B has the action of improving hardenability and promoting the formation of martensite, and is therefore useful as a reinforcing element for steel. In order to effectively exhibit the above-described effects, the content of B is preferably 0.0003% or more. On the other hand, if B is excessive, martensite is excessively generated, and ductility may be reduced.

Therefore, the content of B is preferably 0.0003% to 0.0100%.

(Cr：0.05％～1.00％)

Cr has an action of enhancing hardenability and promoting the formation of martensite, and is therefore useful as a reinforcing element for steel. In order to effectively exhibit the above-described effects, the Cr content is preferably 0.05% or more. On the other hand, if Cr is excessive, martensite is excessively generated, and ductility may be reduced.

Therefore, the content of Cr is preferably 0.05% to 1.00%.

(Ni：0.05％～1.00％)

Ni has an action of enhancing hardenability and promoting the formation of martensite, and is therefore useful as a reinforcing element for steel. In order to effectively exhibit the above-described effects, the Ni content is preferably 0.05% or more. On the other hand, if Ni is excessive, martensite is excessively generated, and ductility may be reduced.

Therefore, the content of Ni is preferably 0.05% to 1.00%.

(Cu：0.05％～1.00％)

Cu has an action of improving hardenability and promoting the formation of martensite, and is therefore useful as a reinforcing element for steel. In order to effectively exhibit the above-described effects, the Cu content is preferably 0.05% or more. On the other hand, if Cu is excessive, martensite is excessively generated, and ductility may be reduced.

Therefore, the content of Cu is preferably 0.05% to 1.00%.

(Sb：0.002％～0.050％)

Sb has an action of suppressing decarburization of the surface layer (region of about several tens μm) of the steel sheet due to nitriding and oxidation of the surface of the steel sheet. This prevents a decrease in the amount of austenite produced on the surface of the steel sheet, and is effective for ensuring desired ductility. In order to effectively exhibit the above-described effects, the Sb content is preferably 0.002% or more. On the other hand, if Sb is excessive, toughness may be reduced.

Therefore, the content of Sb is preferably 0.002% to 0.050%.

(Sn：0.002％～0.050％)

Sn has an effect of suppressing decarburization of the steel sheet surface layer (region of about several tens μm) due to nitriding and oxidation of the steel sheet surface. This prevents a decrease in the amount of austenite produced on the surface of the steel sheet, and is effective for ensuring desired ductility. In order to effectively exhibit the above-described effects, the Sn content is preferably 0.002% or more. On the other hand, when Sn is excessive, toughness may be reduced.

Therefore, the content of Sn is preferably 0.002% to 0.050%.

(Ca：0.0005％～0.0050％)

Ca has an effect of controlling the form of sulfide-based inclusions, and is effective in suppressing a local reduction in ductility. When Ca is added, the Ca content is preferably 0.0005% or more in order to obtain the above effects. On the other hand, if the Ca content is excessive, the effect may be saturated.

Therefore, the content of Ca is preferably 0.0005% to 0.0050%.

(Mg：0.0005％～0.0050％)

Mg has an effect of controlling the form of sulfide-based inclusions, and is effective in suppressing a local reduction in ductility. When Mg is added, the Mg content is preferably 0.0005% or more in order to obtain the above effects. On the other hand, if the Mg content is excessive, the effect may be saturated.

Therefore, the content of Mg is preferably 0.0005% to 0.0050%.

(REM：0.0005％～0.0050％)

REM (rare earth element) has an effect of controlling the form of sulfide-based inclusions, and is effective in suppressing a local reduction in ductility. When REM is added, in order to obtain the above effects, the REM content is preferably 0.0005% or more. On the other hand, when the REM content is excessive, the effect may be saturated.

Therefore, the content of REM is preferably 0.0005% to 0.0050%.

The remainder of Fe and unavoidable impurities

In the above composition, the remainder other than the above components is composed of Fe (balance Fe) and unavoidable impurities.

Microstructure of steel plate

Next, the microstructure of the high-strength cold-rolled steel sheet of the present invention will be described.

Sum of area ratios of ferrite + bainitic ferrite: 20% -80% of the Chinese medicinal herbs

Ferrite and bainitic ferrite are soft steel structures and contribute to improvement in ductility of the steel sheet. Since carbon is less likely to be dissolved in solid in these structures, C is discharged into austenite, and the stability of austenite is improved, contributing to improvement of ductility.

In order to impart necessary ductility to a steel sheet, the total area ratio of ferrite and bainitic ferrite needs to be 20% or more.

On the other hand, when the sum of the area ratios of ferrite and bainitic ferrite exceeds 80%, it is difficult to secure tensile strength of 980MPa or more.

Therefore, the sum of the area ratios of ferrite and bainitic ferrite is 20% to 80%.

Area ratio of retained austenite: more than 10% and less than 40% > (see the description of the examples)

The retained austenite itself is a structure rich in ductility, and a structure in which strain-induced transformation occurs to contribute to further improvement in ductility. In order to obtain such an effect, the retained austenite area ratio needs to exceed 10%.

On the other hand, if the area ratio of the retained austenite exceeds 40%, the stability of the retained austenite is lowered, and thus strain-induced transformation occurs early, and ductility is lowered.

Therefore, the area ratio of the retained austenite exceeds 10% and is 40% or less.

In the present specification, the volume fraction of retained austenite is calculated by the method described later, and this is used as the area fraction.

Area ratio of martensite: more than 0% and less than 50% > (see the description of the examples)

The term "martensite" as used herein is intended to include both fresh martensite and tempered martensite.

Martensite is a very hard structure and contributes to increasing the strength of the steel sheet. For the purpose of making the steel sheet high in strength, the area ratio of martensite exceeds 0% (excluding 0%), preferably 3% or more.

On the other hand, if the area ratio exceeds 50%, the desired ductility and stretch flangeability cannot be secured.

Therefore, the sum of the area ratios of martensite exceeds 0% and is 50% or less, preferably 3% to 50%.

The microstructure of the high-strength cold-rolled steel sheet of the present invention includes a case where the total area ratio of the ferrite and bainitic ferrite, the retained austenite, and the martensite is 100%, and a case where the area ratio of the pearlite and the like is 100% in addition to the above.

The ratio of retained austenite having an aspect ratio of 0.5 or less: area ratio of 75% or more

The retained austenite increases the ductility of the steel sheet, but contributes differently to the improvement in ductility due to its shape. The retained austenite having an aspect ratio of 0.5 or less is more stable in processing and has a greater ductility improving effect than the retained austenite having an aspect ratio of more than 0.5.

The retained austenite having a low work stability and an aspect ratio exceeding 0.5 becomes hard martensite at an early stage in the piercing before the hole expansion test, and therefore coarse voids are easily formed around the former. In particular, when a large number of punched end faces are exposed, end face cracks are induced, which causes a failure in the hole expansion test and increases the failure rate in the hole expansion test.

On the other hand, the retained austenite having an aspect ratio of 0.5 or less deforms in the flow direction of the microstructure, and it is difficult to form a void in the periphery.

In order to sufficiently reduce the fraction defective in the hole expansion test while ensuring the desired ductility, the ratio of the retained austenite having an aspect ratio of 0.5 or less may be 75% or more in terms of area ratio. Preferably 80% or more.

The upper limit of the proportion is not particularly limited, and may be 100%.

The ratio of retained austenite present at the Bain group boundary among retained austenite having an aspect ratio of 0.5 or less: area ratio of more than 50%

First, the retained austenite existing at the boundary of the Bain group will be described.

In martensite and bainite, 24 variants having the relationship of Kurdjumov-Sachs (K-S) were generated from 1 prior austenite grain. The modifications produced from 1 prior austenite grain are divided into 3 Bain groups (for example, refer to "dynasty benglan, other 3 names," crystallographic limitation of martensite/bainite transformation of steel ", japan society of metals, japan society of public welfare society, japan society of metals, 2015 7, volume 79, No. 7, p.339-347).

As will be described later, since the high-strength cold-rolled steel sheet of the present invention is obtained through a plurality of annealing steps, the microstructure of the steel sheet is different from martensite and bainite transformed from an austenite single phase, but the same grouping as described above can be performed for the portion judged to be the bcc phase.

Fig. 1 is a schematic diagram showing a part of the microstructure of a steel sheet (region considered to be generated from 1 prior austenite grain). The microstructure of the steel sheet shown in fig. 1 is composed of 3 Bain groups (B1 to B3). The same identification is depicted for the same Bain group.

The microstructure of the steel sheet shown in fig. 1 also contains retained austenite. By the symbol "RA₂"the retained austenite is present in the interior of 1 Bain group B2. In contrast, the symbol "RA" is used₁"indicates that the retained austenite exists at the boundary of Bain group B1 and another Bain group B3.

By the symbol "RA₁"indicates that the retained austenite belongs to the retained austenite existing at the boundary of the Bain group.

When retained austenite having an aspect ratio of 0.5 or less exists at the Bain group boundary, even if retained austenite having an aspect ratio exceeding 0.5 exists, the occurrence of cracks at the punched end face caused by the residual austenite is suppressed, and the fraction defective in the hole expansion test is significantly reduced.

The reason is not clear, but the inventors consider the following: in the Bain group boundary where the difference in orientation is large and stress is likely to concentrate, retained austenite having an aspect ratio of 0.5 or less is present so as to cover the boundary, and stress concentrated by deformation of the retained austenite or work-induced martensite transformation can be relaxed. As a result, stress concentration around the retained austenite having an aspect ratio exceeding 0.5 existing in the vicinity is reduced, and generation of voids and cracks is suppressed.

In order to sufficiently reduce the fraction defective in the hole expansion test, the proportion of the retained austenite having an aspect ratio of 0.5 or less existing at the Bain group boundary may be 50% or more, preferably 65% or more, in terms of area ratio.

The upper limit of the proportion is not particularly limited, and may be 100%. Preferably 95% or less.

Coating (plating)

The high-strength cold-rolled steel sheet of the present invention may further have a plating layer on the surface thereof from the viewpoint of improving corrosion resistance and the like. As the plating layer, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electro-galvanized layer is preferable.

The hot-dip galvanized layer, the alloyed hot-dip galvanized layer and the electrogalvanized layer are not particularly limited, and conventionally known hot-dip galvanized layers, conventionally known alloyed hot-dip galvanized layers and conventionally known electrogalvanized layers are preferably used.

The zinc electroplating layer may be a zinc alloy plating layer in which an appropriate amount of an element such as Fe, Cr, Ni, Mn, Co, Sn, Pb, or Mo is added to Zn depending on the purpose.

[ method for producing high-strength Cold-rolled Steel sheet ]

Next, a preferred embodiment of the method for producing a high-strength cold-rolled steel sheet according to the present invention (hereinafter, simply referred to as "the production method of the present invention") will be described.

In summary, the manufacturing method of the present invention is a method for obtaining the high-strength cold-rolled steel sheet of the present invention by sequentially performing hot rolling, pickling, cold rolling, and annealing on a steel slab having the above-described composition. In the manufacturing method of the present invention, the annealing step is performed in 2 steps.

Steel blank

The steel material is not particularly limited as long as it has the above-described composition.

The method of melting the steel material is not particularly limited, and a known melting method using a converter, an electric furnace, or the like can be used. In view of productivity and the like, it is preferable to produce a slab (steel material) by a continuous casting method after melting, but a slab may be produced by a known casting method such as an ingot-cogging rolling method or a thin slab continuous casting method.

Hot rolling process

The hot rolling step is a step of hot rolling the steel slab having the above composition to obtain a hot rolled sheet.

The hot rolling step is not particularly limited as long as the hot rolling step is a step of heating a steel slab having the above composition and hot rolling the heated slab to obtain a hot rolled sheet having a predetermined size.

As a general hot rolling process, for example, the following hot rolling process can be exemplified: the hot rolled steel sheet is produced by heating a steel billet at a heating temperature of 1100 to 1300 ℃, hot rolling the heated steel billet at a finish rolling exit temperature of 850 to 950 ℃, cooling after the hot rolling is appropriately performed (specifically, for example, after cooling at an average cooling rate of 20 to 100 ℃/s in a temperature range of 450 to 950 ℃), and coiling at a coiling temperature of 400 to 700 ℃ to produce a hot rolled sheet having a predetermined dimensional 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 is a step capable of pickling the hot rolled sheet to such an extent that cold rolling can be performed, and for example, a common pickling step using hydrochloric acid, sulfuric acid, or the like can be applied.

Cold rolling process

The cold rolling step is a step of cold rolling the hot-rolled sheet having undergone the pickling step. More specifically, the cold rolling step is a step of cold rolling the pickled hot-rolled sheet at a reduction of 30% or more to obtain a cold-rolled sheet having a predetermined thickness.

Reduction ratio of cold rolling: more than 30%)

The reduction rate of cold rolling is 30% or more. When the reduction ratio is less than 30%, the amount of work is insufficient and the austenite nucleation sites become small. Therefore, austenite becomes coarse and uneven in the 1 st-stage annealing step in the next step, and lower bainite transformation in the holding process in the subsequent 1 st-stage annealing step is suppressed, so that martensite is excessively generated. As a result, the microstructure of the steel sheet after the 1 st-stage annealing step cannot be a microstructure mainly composed of lower bainite. The portion of martensite after the 1 st-stage annealing process is likely to generate retained austenite having an aspect ratio of more than 0.5 in the subsequent 2 nd-stage annealing process.

On the other hand, the upper limit of the reduction is determined by the capacity of the cold rolling mill, but if the reduction is too high, the rolling load becomes high, and the productivity may be lowered. Therefore, the rolling reduction is preferably 70% or less.

The number of rolling passes and the reduction per pass are not particularly limited.

Annealing process

The annealing step is a step of annealing the cold-rolled sheet obtained through the cold-rolling step, and more specifically, includes a stage 1 annealing step and a stage 2 annealing step described later.

Annealing step at stage 1

The annealing step in the 1 st stage is to subject the cold-rolled sheet obtained in the cold-rolling step to Ac₃Annealing temperature T of point-950 DEG C₁Heating from the annealing temperature T at an average cooling rate of 10 ℃/s₁Cooling to a cooling stop temperature T of 250 ℃ or higher and less than 350 DEG C₂At a cooling stop temperature T₂And maintaining the temperature for 10 seconds or more to obtain a 1 st stage cold-rolled annealed sheet.

The purpose of this step is to make the microstructure of the steel sheet at the end of the 1 st-stage annealing step lower bainite. In particular, the portion of martensite after the 1 st-stage annealing step is likely to generate retained austenite having an aspect ratio exceeding 0.5 in the subsequent 2 nd-stage annealing step, and therefore, when martensite is excessively generated in the 1 st-stage annealing step, it is difficult to obtain a desired microstructure of the steel sheet.

If the production conditions are controlled within the above ranges, a steel sheet having a microstructure mainly composed of lower bainite can be obtained, and the microstructure of the steel sheet after the 2 nd-stage annealing step can be made to be a desired microstructure.

(Ac₃Dot)

Ac₃The point (unit:. degree. C.) can be obtained by the following equation of Andrews et al.

Ac₃＝910－203[C]^1/2+45[Si]－30[Mn]－20[Cu]－15[Ni]+11[Cr]+32[Mo]+104[V]+400[Ti]+460[Al]

The brackets in the above formula represent the content (unit: mass%) of the elements in brackets in the steel sheet. When no element is contained, 0 is calculated.

(annealing temperature T)₁：Ac₃Point-950 ℃ C.)

Annealing temperature T₁Below Ac of₃In this case, ferrite remains in the annealing, and ferrite grows with the remaining ferrite as a core in the subsequent cooling process. As a result, C is distributed in austenite, so that the lower bainite transformation is suppressed in the subsequent holding process, martensite is excessively generated, and the microstructure of the steel sheet after the 1 st-stage annealing step cannot be made a microstructure mainly composed of lower bainite.

On the other hand, the annealing temperature T₁When the temperature exceeds 950 ℃, the austenite grains are excessively coarsened, and the formation of lower bainite in the holding process after cooling is suppressed, so that martensite is excessively formed, and the microstructure of the steel sheet after the 1 st-stage annealing step cannot be made to be a microstructure mainly composed of lower bainite.

After the 1 st-stage annealing step, the portion belonging to martensite is likely to form retained austenite having an aspect ratio of more than 0.5 in the subsequent 2 nd-stage annealing step.

Thus, the annealing temperature T₁Is Ac₃Point-950 ℃.

At an annealing temperature T₁The holding time of (b) is not particularly limited, and is, for example, 10s to 1000 s.

(from annealing temperature T)₁To a cooling stop temperature T₂Average cooling rate of (2): more than 10 ℃/s)

If it is from the annealing temperature T₁To a cooling stop temperature T₂When the average cooling rate of (2) is 10 ℃/s or less, ferrite is generated during cooling. As a result, C is distributed in austenite, so that the lower bainite transformation is suppressed in the subsequent holding process, martensite is excessively generated, and the microstructure of the steel sheet after the 1 st-stage annealing step cannot be made a microstructure mainly composed of lower bainite. After the 1 st-stage annealing step, the portion belonging to martensite is likely to form retained austenite having an aspect ratio of more than 0.5 in the subsequent 2 nd-stage annealing step.

Thus, from the annealing temperature T₁To a cooling stop temperature T₂The average cooling rate of (2) is more than 10 ℃/s, preferably 15 ℃/s.

The upper limit of the average cooling rate is not particularly limited, and an excessively large cooling apparatus is required to secure an excessively high cooling rate, and therefore, from the viewpoint of production technology, equipment investment, and the like, the average cooling rate is preferably 50 ℃/s or less.

The cooling is preferably gas cooling, but may be performed by combining furnace cooling and water mist cooling.

(Cooling stop temperature T)₂: above 250 ℃ and below 350 ℃)

Cooling stop temperature T₂At temperatures below 250 ℃, martensite is excessively generated in the microstructure of the steel sheet. After the 1 st-stage annealing step, the portion belonging to martensite is likely to form retained austenite having an aspect ratio of more than 0.5 in the subsequent 2 nd-stage annealing step.

On the other hand, if the cooling stop temperature T₂When the temperature is 350 ℃ or higher, upper bainite is formed instead of lower bainite. Since the upper bainite is significantly coarser than the lower bainite in the same Bain group, a large amount of retained austenite having an aspect ratio of 0.5 or less is formed in the same Bain group after the subsequent 2 nd-stage annealing step, and the microstructure of the steel sheet after the 2 nd-stage annealing step does not become a desired microstructure.

Therefore, the temperature of the molten metal is controlled,cooling stop temperature T₂Is 250 ℃ or higher and lower than 350 ℃. More preferably from 270 ℃ to 340 ℃.

(at the cooling stop temperature T₂The retention time of (c): more than 10 s)

If at the cooling stop temperature T₂The holding time of (2) is less than 10s (sec), the lower bainite transformation cannot be sufficiently completed. Therefore, martensite is excessively generated, and a desired microstructure cannot be obtained in the subsequent 2 nd-stage annealing step. After the 1 st-stage annealing step, the portion belonging to martensite is likely to form retained austenite having an aspect ratio of more than 0.5 in the subsequent 2 nd-stage annealing step.

Therefore, at the cooling stop temperature T₂The retention time of (3) is 10s or more. Preferably 30 seconds or more.

At the cooling stop temperature T₂The upper limit of the holding time of (2) is not particularly limited, but if the holding time is too long, a long and large production facility is required, and the productivity of the steel sheet is remarkably lowered, so that 1800s or less is preferable.

At the cooling stop temperature T₂After the holding, the second-stage annealing step may be performed, for example, by cooling to room temperature or by continuing the heating without cooling until the second-stage annealing step in the next step. In order to continue the process from the 1 st-stage annealing process to the 2 nd-stage annealing process without cooling to room temperature, 2 furnaces of a general continuous annealing facility (CAL) were required in 1 production line, and actually the 1 st-stage annealing process was performed by CAL, and then the 2 nd-stage annealing process was performed by passing the sheet through CAL 1 time.

Annealing step at stage 2

The 2 nd stage annealing step is to anneal the 1 st stage cold-rolled annealed sheet obtained by the 1 st stage annealing step at an annealing temperature T of 700 to 850 DEG C₃Heating (reheating) from the annealing temperature T₃Cooling to a cooling stop temperature T of 300-500 DEG C₄And a step of obtaining a 2 nd stage cold-rolled annealed sheet.

(annealing temperature T)₃：700℃～850℃)

Annealing temperature T₃When the temperature is less than 700 ℃, the annealing temperature is not sufficiently highBecause of the amount of austenite, a desired amount of retained austenite cannot be secured in the microstructure of the steel sheet after the 2 nd-stage annealing step, and ferrite is excessively increased.

On the other hand, if the annealing temperature T is set₃When the temperature exceeds 850 ℃, austenite is excessively generated, and the effect of microstructure control before the 2 nd-stage annealing is initialized. Therefore, it is difficult to set the ratio of retained austenite having an aspect ratio of 0.5 or less and the ratio of retained austenite present at the Bain group boundary among the retained austenite having an aspect ratio of 0.5 or less to desired values.

Thus, the annealing temperature T₃At a temperature of 700 ℃ to 850 ℃, preferably 710 ℃ to 830 ℃.

At an annealing temperature T₃The holding time of (b) is not particularly limited, and is, for example, 10s to 1000 s.

From the annealing temperature T₃To a cooling stop temperature T₄The average cooling rate of (2) is not particularly limited, but is, for example, 5 to 50 ℃/s.

(Cooling stop temperature T)₄：300℃～500℃)

Cooling stop temperature T₄When the temperature is less than 300 ℃, C enrichment into austenite becomes insufficient, the amount of retained austenite decreases, and a large amount of martensite is formed, so that a desired microstructure of the steel sheet cannot be obtained.

On the other hand, if the cooling stop temperature T₄When the temperature exceeds 500 ℃, ferrite and bainitic ferrite are generated in a large amount, and pearlite is generated from austenite, so that the amount of retained austenite is reduced, and a desired microstructure of the steel sheet cannot be obtained.

At the cooling stop temperature T₄The holding time of (b) is not particularly limited, and is, for example, 10s to 1800 s.

At the cooling stop temperature T₄The retained 2 nd stage cold rolled annealed sheet is preferably cooled. The cooling is not particularly limited, and the cooling may be carried out to a desired temperature such as room temperature by any method such as cooling.

The 2 nd-stage cold-rolled annealed sheet obtained through the 2 nd-stage annealing step becomes the high-strength cold-rolled steel sheet of the present invention without performing the plating step described later.

Coating Process

The 2 nd-stage cold-rolled annealed sheet obtained through the 2 nd-stage annealing step may be further subjected to plating treatment to form a plated layer on the surface thereof. In this case, the 2 nd stage cold-rolled annealed sheet having the plated layer formed on the surface thereof becomes the high-strength cold-rolled steel sheet of the present invention.

As the plating treatment, a hot dip galvanizing treatment, and an alloying treatment, or an electrogalvanizing treatment is preferable. The hot dip galvanizing treatment, the hot dip galvanizing treatment and alloying treatment, and the electrogalvanizing treatment are not particularly limited, and conventionally known hot dip galvanizing treatment, conventionally known hot dip galvanizing treatment and alloying treatment, and conventionally known electrogalvanizing treatment are preferably used.

Before the plating treatment, pretreatment such as degreasing and phosphate treatment may be performed.

As the hot-dip galvanizing treatment, for example, it is preferable to use a general-purpose continuous hot-dip galvanizing line, and dip-galvanizing the 2 nd-stage cold-rolled annealed sheet in a hot-dip galvanizing bath to form a predetermined amount of a hot-dip galvanized layer on the surface.

When the sheet is immersed in the hot dip galvanizing bath, it is preferable to adjust the temperature of the cold rolled and annealed sheet in the 2 nd stage to a temperature not lower than the temperature of the hot dip galvanizing bath minus 50 ℃ and not higher than the temperature of the hot dip galvanizing bath plus 80 ℃ by reheating or cooling.

The temperature of the hot dip galvanizing bath is preferably 440 ℃ to 500 ℃.

The hot dip galvanizing bath may contain Al, Fe, Mg, Si, or the like in addition to pure zinc.

The amount of the hot-dip galvanized layer is preferably 45g/m per one surface, although the amount of the hot-dip galvanized layer may be adjusted to a desired amount by gas wiping or the like²Left and right.

The plating layer (hot-dip galvanized layer) formed by the hot-dip galvanizing treatment may be subjected to a usual alloying treatment as needed to prepare an alloyed hot-dip galvanized layer.

The temperature of the alloying treatment is preferably 460 to 600 ℃.

In the case of producing an alloyed hot-dip galvanized layer, the effective Al concentration in the hot-dip galvanizing bath is preferably adjusted to a range of 0.10 to 0.22 mass% from the viewpoint of ensuring a desired plating appearance.

As the electrogalvanizing treatment, for example, a treatment of forming a predetermined amount of an electrogalvanized layer on the surface of the cold-rolled annealed sheet in the 2 nd stage using a general electrogalvanizing line is preferable.

The amount of the zinc plating layer deposited can be adjusted to a predetermined amount by adjusting the speed of passing through the plate, the current value, etc., and is preferably 30g/m per one surface²Left and right.

Examples

The present invention will be specifically described below with reference to examples. However, the present invention is not limited thereto.

Production of Cold-rolled Steel sheet

Molten steels having compositions shown in Table 1 below were melted by a generally known method and continuously cast into slabs (steel materials) having a wall thickness of 300 mm. The obtained slab was hot-rolled to obtain a hot-rolled sheet. The obtained hot-rolled sheet was pickled by a generally known method, and then cold-rolled at the reduction ratios shown in tables 2 to 3 below to obtain a cold-rolled sheet (sheet thickness: 1.4 mm).

The obtained cold-rolled sheet was annealed under the conditions shown in tables 2 to 3 below to obtain a 2 nd stage cold-rolled annealed sheet.

The annealing step is a 2-stage step consisting of a 1 st-stage annealing step and a 2 nd-stage annealing step.

And cooling to room temperature between the annealing process of the 1 st stage and the annealing process of the 2 nd stage.

Annealing temperature T in the 1 st stage annealing process₁The holding time of (2) is 100 s.

Annealing temperature T in the 2 nd stage annealing process₃Has a holding time of 100s from the annealing temperature T₃To a cooling stop temperature T₄At a cooling stop temperature T of 20 ℃/s₄The holding time of (3) was 250 s.

After the annealing is completed, a hot-dip galvanizing treatment is further performed on a part of the cold-rolled and annealed sheet in the 2 nd stage, thereby forming a hot-dip galvanized layer on the surface and producing a hot-dip galvanized steel sheet.

In the hot-dip galvanizing treatment, the cold-rolled annealed sheet of the 2 nd stage is reheated at a temperature in the range of 430 to 480 ℃ as required using a continuous hot-dip galvanizing line, and dipped in a hot-dip galvanizing bath (bath temperature: 470 ℃) so that the amount of plating adhesion is 45g/m per surface²The manner of (2) is adjusted. The bath composition was Zn-0.18 mass% Al.

At this time, in some of the hot-dip galvanized steel sheets, the bath composition was made to be Zn — 0.14 mass% Al, and after the plating treatment, alloying treatment was performed at 520 ℃.

The Fe concentration in the plating layer is 9-12 mass%.

For the other 2 nd stage cold rolled annealed sheet, after the annealing was completed, an electrogalvanizing line was further used so that the plating adhesion amount per one side was 30g/m²The electrogalvanizing treatment is performed to produce an electrogalvanized steel sheet.

In tables 4 to 5 below, the 2 nd-stage cold-rolled annealed sheet on which no plating layer was formed was denoted as "CR", the hot-dip galvanized steel sheet was denoted as "GI", the alloyed hot-dip galvanized steel sheet was denoted as "GA", and the electrogalvanized steel sheet was denoted as "EG".

Hereinafter, the 2 nd stage cold rolled annealed sheet on which the plating layer is not formed and the 2 nd stage cold rolled annealed sheet on which the plating layer is formed (hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and electrogalvanized steel sheet) are collectively referred to as "cold-rolled steel sheets".

The cold rolled steel sheet is manufactured as described above.

Evaluation

Test pieces were collected from the obtained cold-rolled steel sheets, and microstructure observation, measurement of the retained austenite area ratio, tensile test, and hole expansion test were performed. The test method is as follows.

Observation of microscopic tissues

First, a test piece for microstructure observation was collected from a cold-rolled steel sheet.

Next, the sampled test piece was polished so that a position corresponding to 1/4 of the sheet thickness in the rolling direction cross section (L cross section) was an observation plane. After the observation surface was etched (1 vol% nitric acid alcohol solution etching), 10-field observation was performed in a field of view of 30 μm × 35 μm using a scanning electron microscope (SEM, magnification: 3000 times), and an SEM image was obtained.

The area ratio of each tissue was determined by image analysis using the obtained SEM image.

The area ratio is an average of 10 fields. In the SEM image, ferrite and bainitic ferrite are gray, and martensite and retained austenite are white, so each structure is judged from its color tone. It is difficult to accurately distinguish ferrite from bainitic ferrite, but it is important to sum up these structures, and the area ratio of the sum of ferrite and bainitic ferrite is determined without particularly distinguishing each structure.

The area ratio of the retained austenite obtained by X-ray diffraction was subtracted from the area ratio of the white microstructure to obtain the area ratio of martensite. The volume fraction of austenite obtained by X-ray diffraction was regarded as equal to the area fraction.

Then, the test piece was polished by colloidal silica vibration polishing so that the position of the cross section in the rolling direction (L cross section) corresponding to 1/4 of the sheet thickness was the observation plane. The observation surface is a mirror surface. Next, after removing the machining phase change phase of the observed surface due to the polishing strain by the extremely low accelerated ion milling, Electron Back Scattering Diffraction (EBSD) measurement was performed to obtain local crystal orientation data. In this case, the SEM magnification was 1500 times, the step size was 0.04 μm, the measurement region was 40 μm square, and the WD was 15 mm. Using the analysis software: the OIM Analysis 7 analyzes the obtained local orientation data. Analysis was performed for 3 fields, and the average value was used.

Before data analysis, the cleaning process by the Grain comparison function (Grain distance Angle: 5, Minimum Grain Size: 5, Single execution: ON) and the Grain CI standardization function (Grain distance Angle: 5, Minimum Grain Size: 5) of the analysis software was performed 1 time each in sequence. Thereafter, only the measurement points with a CI value > 0.1 were used for the analysis.

The fcc-phase data was analyzed using the Area Fraction of the gain Shape Aspect Ratio diagram, and the proportion (Area Fraction) of retained austenite having an Aspect Ratio of 0.5 or less among the retained austenite was determined. In the above analysis, Method 2 was used as the Grain shape calculation Method.

In addition, for the bcc phase data, the regions belonging to the same Bain group are colored in the same color using the highlighting function, and then the ratio of the retained austenite in the boundary of the Bain group (including the prior austenite grain boundary) which is the boundary existing in the region colored in a different color among the retained austenite having the aspect ratio of 0.5 or less which is previously obtained, that is, the Bain group boundary is obtained in the area ratio.

Determination of residual Austenite area Rate

A test piece for X-ray diffraction was taken from a cold-rolled steel sheet, and the test piece was ground and polished so that a position corresponding to 1/4 in the sheet thickness became a measurement surface, and the volume fraction of retained austenite was determined from the intensity of diffracted X-rays by an X-ray diffraction method. CoK α rays were used as incident X-rays.

When the volume fraction of the retained austenite is calculated, the intensity ratio is calculated for all combinations of integrated intensities of peaks of {111}, {200}, {220} and {311} planes of the fcc phase (retained austenite) and {110}, {200} and {211} planes of the bcc phase, and the volume fraction of the retained austenite is calculated by obtaining the average value of the intensity ratios.

The volume fraction of austenite thus obtained was defined as an area fraction.

Tensile test

Tensile test pieces (JIS Z2001) No. JIS5 were sampled from cold-rolled steel sheets with the direction (C direction) perpendicular to the rolling direction as the tensile direction, and a tensile test was performed in accordance with JIS Z2241 to measure the Tensile Strength (TS) and the elongation (El).

(Strength)

The high strength was evaluated when the TS was 980MPa or more.

(ductility)

When the TS was 980MPa or more and less than 1180MPa, the El was 25% or more, and when the TS was 1180MPa or more, the El was 18% or more, and the ductility was evaluated to be high (good ductility).

Examination of hole enlargement

Test pieces (size: 100 mm. times.100 mm) were collected from cold-rolled steel sheets and placed on the test piecesThe initial diameter d was formed by punching (gap: 12.5% of the thickness of the test piece)₀: a hole of 10mm phi. Using these test pieces, a hole expansion test was performed. I.e. at the initial diameter d₀: the 10mm phi hole was inserted from the punch side at the time of punching with an apex angle: when the hole was expanded by a 60-degree conical punch to make a crack penetrate through the steel sheet (test piece), the hole diameter d (unit: mm) was measured, and the hole expansion ratio λ (unit:%) was calculated by the following formula.

The hole expansion ratio λ { (d-d)₀)/d ₀}×100

Each steel sheet was subjected to 100 hole expansion tests, and the average value thereof was defined as the average hole expansion rate λ (unit:%). The average porosity λ is also referred to as "average λ" below.

The probability that the value of the hole expansion ratio λ is equal to or less than half of the average hole expansion ratio λ was obtained and used as the fraction defective (unit:%) in the hole expansion test.

(stretch flangeability)

The stretch flangeability was evaluated to be good when the average λ was 20% or more when the TS was 980MPa or more and less than 1180MPa, and when the average λ was 15% or more when the TS was 1180MPa or more.

(defective rate of hole expanding test)

The case where the defective rate in the hole expanding test was 4% or less was evaluated as a low defective rate in the hole expanding test.

TABLE 1

TABLE 2

TABLE 3

TABLE 4

The ratio of retained austenite of which the aspect ratio is 0.5 or less in retained austenite of 1

The ratio of retained austenite existing at the boundary of Bain group in retained austenite having aspect ratio of 2 of 0.5 or less

TABLE 5

Fig. 2 is a graph plotting a part of the results in tables 4 to 5. More specifically, fig. 2 is a graph showing the influence of the ratio of retained austenite having an aspect ratio of 0.5 or less among retained austenite and the ratio of retained austenite present at the Bain group boundary among retained austenite having an aspect ratio of 0.5 or less on the fraction defective in the hole expansion test.

As is clear from the graph of fig. 2, only when the proportion of retained austenite having an aspect ratio of 0.5 or less is 75% or more and the proportion of retained austenite present at the Bain group boundary among the retained austenite having an aspect ratio of 0.5 or less is 50% or more, a steel sheet having a low fraction defective in the hole expansion test was obtained.

As is clear from tables 1 to 5 and fig. 2, the cold-rolled steel sheets of the present invention all had high strength with a Tensile Strength (TS) of 980MPa or more, good ductility and stretch-flangeability at the same time, and a small fraction defective in the hole expansion test.

In contrast, in the comparative example, any of the above characteristics was insufficient.

Description of the symbols

B1, B2, B3: bain group

RA₁、RA₂: retained austenite

Claims

1. A high strength cold rolled steel sheet having a composition of: contains, in mass%, C: more than 0.15% and 0.45% or less, Si: 0.50% -2.50%, Mn: 1.50% -3.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010% -0.100% and N: 0.0005 to 0.0100%, the remainder being Fe and unavoidable impurities,

in the microstructure, the total area ratio of ferrite and bainitic ferrite is 20 to 80%, the area ratio of retained austenite exceeds 10% and is 40% or less, and the area ratio of martensite exceeds 0% and is 50% or less,

the ratio of retained austenite having an aspect ratio of 0.5 or less is 75% or more in terms of area percentage,

among the retained austenite having an aspect ratio of 0.5 or less, the retained austenite present at the Bain group boundary is 50% or more in terms of area ratio, and the Bain group boundary means: in the region of the microstructure of the steel sheet where 1 prior austenite grain is generated, the microstructure of the steel sheet is composed of 3 Bain groups, which are boundaries with each other.

2. The high strength cold rolled steel sheet as claimed in claim 1, wherein the composition further comprises, in mass%, a metal selected from the group consisting of Ti: 0.005% -0.035%, Nb: 0.005% -0.035%, V: 0.005-0.035%, Mo: 0.005% -0.035%, B: 0.0003% -0.0100%, Cr: 0.05% -1.00%, Ni: 0.05 to 1.00%, Cu: 0.05% -1.00%, Sb: 0.002% -0.050%, Sn: 0.002% -0.050%, Ca: 0.0005% -0.0050%, Mg: 0.0005% -0.0050% and REM: 0.0005% -0.0050% of at least 1 element.

3. The high-strength cold-rolled steel sheet according to claim 1 or 2, wherein C and Mn of said composition satisfy the following formula (z) in mass%,

7.5×C+Mn＜5.0···(z)

in the formula (z), C and Mn represent the contents of the respective elements.

4. The high strength cold rolled steel sheet as claimed in claim 1 or 2, wherein the surface has a plating layer.

5. The high strength cold rolled steel sheet as claimed in claim 3, wherein the surface has a plating layer.

6. A method for manufacturing a high-strength cold-rolled steel sheet according to any one of claims 1 to 5, comprising:

a hot rolling step of hot rolling a steel slab having the composition according to any one of claims 1 to 3 to obtain a hot rolled sheet;

a pickling step of pickling the hot-rolled sheet;

a cold rolling step of cold-rolling the hot-rolled sheet subjected to the acid pickling at a reduction of 30% or more to obtain a cold-rolled sheet;

a 1 st stage annealing step of annealing the cold-rolled sheet at Ac₃Annealing temperature T of point-950 DEG C₁Heating from said annealing temperature T at an average cooling rate exceeding 10 ℃/s₁Cooling to a cooling stop temperature T of 250 ℃ or higher and less than 350 DEG C₂At said cooling stop temperature T₂Maintaining for 10s or more, thereby obtaining a stage 1 cold rolled annealed sheet; and

a 2 nd stage annealing step of annealing the 1 st stage cold-rolled annealed sheet at an annealing temperature T of 700 to 850 DEG C₃Heating from said annealing temperature T₃Cooling to a cooling stop temperature T of 300-500 DEG C₄Thereby obtaining a stage 2 cold-rolled annealed sheet.

7. The method of manufacturing a high-strength cold-rolled steel sheet according to claim 6, comprising a plating step of performing hot-dip galvanizing, hot-dip galvanizing and alloying, or electrogalvanizing on the 2 nd-stage cold-rolled annealed sheet.