JP2007177271A - High-strength cold-rolled steel sheet superior in hole expandability, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet which has a tensile strength of 1,100 MPa or higher and has such superior hole-expandability as 40% or higher by a hole-expansion rate, and to provide a manufacturing method therefor. <P>SOLUTION: This high-strength cold-rolled steel sheet has a composition comprising, by mass%, 0.05-0.22% C, 0.001-0.8% Si, 2.0-3.0% Mn, 0.001-0.1% P, 0.0001-0.01% S, 0.001-0.2% Al, 0.0001-0.01% B, 0.005-0.3% Ti, and the balance Fe with unavoidable impurities; and has a microstructure comprising a martensite phase of 50-90 vol.%, a hard bainite phase of 5-35 vol.%, a soft bainite phase of 35% or less and a retained austenite of 0.1-5 vol.%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in hole expansibility suitable for applications such as building materials, home appliances, and automobiles, and a method for producing the same.

  In recent years, especially in automobile bodies, demand for high-strength steel sheets with excellent workability is increasing from the viewpoint of improving fuel efficiency and durability. In particular, steel sheets of a 780 MPa class or higher in tensile strength are being used for some members such as reinforcement because of the need for collision safety and expansion of cabin space.

  When a member is assembled using such a high-strength material, ductility, bendability, hole expansibility, and the like are important. For this reason, conventionally, various measures have been taken to improve these properties in high-strength steel sheets having a tensile strength of up to about 590 MPa (for example, see Non-Patent Document 1). For example, in Non-Patent Document 1, the hole expandability can be improved by using bainite as the main phase, and by forming retained austenite in the second phase, the same stretched forming as that of the current retained austenitic steel. It is disclosed that the property can be obtained. Further, in Non-Patent Document 1, when austempering is performed at a temperature equal to or lower than the Ms temperature to generate a retained austenite having a volume ratio of 2 to 3%, the product of the tensile strength and the hole expansion ratio is maximized, and stretch flangeability is obtained. It has also been shown that it can be improved.

  Also, in conventional high-strength steel sheets, the composite structure is actively used to increase the ductility of high-strength materials. However, if the second phase is martensite or retained austenite, the hole expandability There is a problem that remarkably decreases. Therefore, conventionally, a technique has been disclosed in which the hole expandability is improved by reducing the hardness difference between the main phase and the second phase by using ferrite as the main phase and martensite as the second phase (for example, Non-patent document 2).

Furthermore, there are also disclosed examples in which the hole expandability is improved for high-strength steel sheets having a tensile strength of 900 MPa or more (see, for example, Patent Documents 1 to 5). In the method described in Patent Document 1, the finish rolling temperature during hot rolling is in the range of Ar ₁ point to Ar ₃ point, and the annealing conditions of the cold-rolled steel sheet are the heating temperature: Ac ₁ point to 850 ° C., heating time. : 10 seconds or more, average cooling rate: 10 ° C./second or more, and further, the alloying temperature after plating is in the range of 450 to 600 ° C. to transform the austenite phase into a hard phase. The tensile strength of the alloyed hot-dip galvanized high-strength cold-rolled steel sheet is 680 MPa (70 kgf / mm ² ) or more, and the yield ratio is 0.50 or less.

Further, in the method described in Patent Document 2, the steel composition is defined and the cooling temperature from the recrystallization annealing heating temperature to the galvanizing temperature is controlled, so that austenite having a low C concentration is finely composed mainly of bainite. High-strength steel sheet having a uniform ferrite-bainite composite structure or a ferrite-bainite-martensite composite structure, and a tensile strength of 490 MPa (50 kgf / mm ² ) or more, particularly 580-1170 MPa (60-120 kgf / mm ² ) class The hole expansibility is improved.

  On the other hand, in Patent Document 3, the austenite phase is stabilized by setting the C content to 0.1 to 0.2% by mass and the Mn content to 2.0 to 3.0% by mass, and recrystallized in the plating line. A method for improving the strength and workability of a hot-dip galvanized steel sheet is disclosed by annealing at a temperature higher than that, and then maintaining the temperature low at 480 to 560 ° C. and hot-dip galvanizing to leave the austenite phase.

  In addition, the high-strength galvannealed steel sheet described in Patent Document 4 defines the steel composition, has a crystal grain size of 20 μm or less at 80% or more, and further has a single phase of bainitic ferrite. Thus, the strength is set to 490 MPa or more and the hole expansion rate is set to 80% or more. In the example of Patent Document 4, for example, C: 0.05% by mass, Si: 0.55% by mass, and Mn: 1.59% by mass, and trace amounts of Mo, Ti, Cr, Nb, B, V, and the like are contained. To obtain a high-strength galvannealed steel sheet (test number 27) with a hole expansion ratio of 93%.

Furthermore, Patent Document 5 discloses that the proper range of annealing temperature can be expanded by containing appropriate amounts of Nb and Ti that can form carbides that can exist stably even in the austenite region, and the regulation of manufacturing conditions can be relaxed. When the components Mn, Cr and B are contained in proper amounts, the composite structure can be obtained by retaining the component in the temperature range of about 500 ° C. for several minutes without adding a component that promotes ferrite transformation such as Si. It is disclosed that by controlling the cooling rate after the retention and further retention, the generated second phase structure can be prevented from being hardened more than necessary, and as a result, stretch flangeability is improved. In Patent Document 5, for example, C: 0.16% by mass, Mn: 2.3% by mass, Ti: 0.05% by mass, Al: 0.03% by mass, and B, P, and S in trace amounts. In addition, an example (sample number No. 10) in which the strength is 1216 MPa (124 kgf / mm ² ) and the hole expansion rate is 23% is disclosed.

Japanese Patent No. 2607950 Japanese Patent No. 2862187 JP-A-1-198459 JP 2001-355043 A Japanese Patent No. 3037767 Sugimoto Koichi, “Stretch Flangeability of TRIP Type Bainitic Cold Rolled Steel Sheet”, CAMP-ISIJ, 2000, Vol. 13, p. 395-398 Nobuyuki Nakamura, “Effect of Composition on Stretch Flange Formability of Ultra High Strength Cold Rolled Steel Sheet”, CAMP-ISIJ, 2000, Vol. 13, p. 391-394

  However, the conventional techniques described above have the following problems. First, the technique described in Non-Patent Document 1 is intended for a steel plate having a tensile strength of about 590 MPa, and the same effect cannot be obtained even if it is simply applied to a high-strength steel plate having a tensile strength of 900 MPa or more. On the other hand, in the methods described in Patent Document 1 and Patent Document 2, a high-strength steel sheet having a tensile strength of 900 MPa or more is obtained. However, since the main structure is a composite structure, a hardness difference occurs between the phases, and the hole expansion ratio is 30. % Or less. Similarly, since the high-strength galvannealed steel sheet described in Patent Document 5 is based on a composite structure, a difference in hardness occurs between the phases, and the hole expansion rate is only about 40% at the maximum.

  Moreover, since the austenite phase remains in the hot dip galvanized steel sheet described in Patent Document 3, it tends to be a non-uniform composite structure, and an improvement in the hole expansion rate cannot be expected. Furthermore, the high-strength galvannealed steel sheet described in Patent Document 4 has a lower Mn content than conventional products, so it is not suitable when the heating temperature during heat treatment is low or when the cooling rate after heat treatment is low. It becomes a uniform composite structure, and it is difficult to stably obtain a steel sheet having excellent hole expansibility. As described above, the conventional technology mainly focuses on creating a composite structure having an appropriate phase fraction, so that a difference in hardness occurs between different phases, and as a result, the hole expansion rate is reduced. It is difficult to obtain a high steel plate.

  On the other hand, in the technique described in Non-Patent Document 2, the hole expandability is improved by reducing the difference in hardness between the main phase and the second phase. Even if the tensile strength is 1000 to 1070 MPa and the steel sheet has a tensile strength higher than that, the same effect is not always obtained. Moreover, the steel plate obtained by the technique described in Non-Patent Document 2 has a hole expansion rate of about 65% at the maximum, and an effect of significantly improving the hole expansion property as compared with the conventional product cannot be expected.

  Therefore, these conventional techniques have a problem that it is difficult to efficiently produce a cold-rolled thin steel sheet having a tensile strength of 1100 MPa or more and a hole expansion ratio of 40% or more.

  The present invention has been devised in view of the above-mentioned problems, a high-strength cold-rolled steel sheet excellent in hole expansibility having a tensile strength of 1100 MPa or more and a hole expansion ratio of 40% or more, and its production It aims to provide a method.

  The high-strength cold-rolled steel sheet excellent in hole expansibility according to the present invention is mass%, C: 0.05 to 0.22%, Si: 0.001 to 0.8%, Mn: 2.0 to 3.0%, P: 0.001-0.1%, S: 0.0001-0.01%, Al: 0.001-0.2%, B: 0.0001-0.01%, Ti : 0.005 to 0.3% contained, the balance is composed of Fe and inevitable impurities, and the microstructure is martensite phase: 50 to 90% by volume, hard bainite phase: 5 to 35% by volume Soft bainite phase: 35% or less and residual austenite: 0.1-5% by volume, having a tensile strength of 1100 MPa or more and a hole expansion ratio of 40% or more.

In the present invention, a steel plate having a thickness of 3 mm or less is defined as a thin steel plate. In addition, the hole expansion rate λ (%) specified in the present invention was subjected to the hole expansion test described in the Steel Federation Standard, the diameter of the initial hole before the test was d ₀ , and cracks were generated by the test. when the diameter of the point hole of a d _f, is the value obtained by the following equation (1).

  The other high strength cold-rolled thin steel sheets excellent in hole expansibility according to the present invention are in mass%, C: 0.05 to 0.22%, Si: 0.001 to 0.8%, Mn: 1.%. 5-3.0%, P: 0.001-0.1%, S: 0.0001-0.01%, Al: 0.001-0.2%, B: 0.0001-0.01% , Ti: 0.005 to 0.3% and Cr, the balance being Fe and inevitable impurities, Mn content (%) [Mn], Cr content (%) [ Cr], the composition satisfies the following formula (2), and the microstructure is martensite phase: 50 to 90% by volume, hard bainite phase: 5 to 35% by volume, soft bainite phase: 35% or less, And retained austenite: 0.1 to 5% by volume, a tensile strength of 1100 MPa or more, and a hole expansion ratio of 4 Characterized in that at least%.

  These high-strength cold-rolled steel sheets excellent in hole expansibility were further selected from the group consisting of Mo: 0.11 to 1.0% and Nb: 0.003 to 0.3% by mass%. One or two kinds of elements may be contained.

  Furthermore, it is also possible to contain one or two elements selected from the group consisting of Co: 0.01 to 1% and W: 0.01 to 0.3% by mass%.

  Furthermore, 0.001 to 1% in total of one or more elements selected from the group consisting of Zr, Hf, Ta, and V may be contained by mass%.

  Further, it may contain 0.0001 to 0.5% in total of one or more elements selected from the group consisting of Ca, Mg, and Rem in mass%.

The manufacturing method of the high intensity | strength cold-rolled thin steel plate excellent in the hole expansibility which concerns on this invention is the mass%, C: 0.05-0.22%, Si: 0.001-0.8%, Mn: 2 0.0-3.0%, P: 0.001-0.1%, S: 0.0001-0.01%, Al: 0.001-0.2%, B: 0.0001-0.01 %, Ti: 0.005 to 0.3%, with the balance consisting of Fe and unavoidable impurities, directly or once cooled to 1000 ° C. or lower and then heated again and hot rolled A step of obtaining a hot-rolled coil, a step of winding the hot-rolled coil, pickling, and cold-rolling to obtain a cold-rolled steel plate, and a maximum temperature of the cold-rolled steel plate equal to or higher than the Ac ₃ transformation temperature. After heat treatment at 860 ° C. or lower, the sample is cooled to a first temperature range of 680 to 780 ° C. at a cooling rate of 0.1 to 20 ° C./second, and pulled. And continuously cooling to a second temperature range of 250 to 350 ° C. at a cooling rate of 40 to 70 ° C./second, and then maintaining the second temperature range for 10 to 500 seconds. .

The manufacturing method of the other high strength cold-rolled thin steel sheet excellent in hole expansibility according to the present invention is mass%, C: 0.05 to 0.22%, Si: 0.001 to 0.8%, Mn : 1.5-3.0%, P: 0.001-0.1%, S: 0.0001-0.01%, Al: 0.001-0.2%, B: 0.0001-0 .01%, Ti: 0.005 to 0.3% and Cr, with the balance being Fe and inevitable impurities, Mn content (%) is [Mn], Cr content (% ) Is [Cr], a casting slab having a composition satisfying the following formula (3) is directly or once cooled to 1000 ° C. or less and then heated and hot-rolled to obtain a hot-rolled coil; winding the hot-rolled coil, after pickling, obtaining a cold-rolled steel sheet by cold rolling, the cold-rolled steel sheet, Ac ₃ varying the maximum temperature After heat treatment at a state temperature of 860 ° C. or less, the sample is cooled to a first temperature range of 680 to 780 ° C. at a cooling rate of 0.1 to 20 ° C./second, and then 250 at a cooling rate of 40 to 70 ° C./second. And after cooling to a second temperature range of ˜350 ° C., the step of holding in the second temperature range for 10 to 500 seconds.

  In these high-strength steel sheet manufacturing methods excellent in hole expansibility, the cast slab further comprises, in mass%, Mo: 0.11 to 1.0% and Nb: 0.003 to 0.3%. One or two elements selected from the group may be contained.

  Moreover, the said casting slab contains 1 type or 2 types of elements further selected from the group which consists of Co: 0.01-1% and W: 0.01-0.3% by the mass%. You can also.

  Further, the cast slab can contain 0.001 to 1% in total of one or more elements selected from the group consisting of Zr, Hf, Ta, and V in mass%.

  Furthermore, the cast slab may contain 0.0001 to 0.5% in total of one or more elements selected from the group consisting of Ca, Mg, and Rem in mass%.

  According to the present invention, since the steel composition is optimized and the volume fraction of each phase in the microstructure is optimized, the hole expandability is excellent with a tensile strength of 1100 MPa and a hole expansion ratio of 40%. Furthermore, it is possible to obtain a high-strength cold-rolled thin steel sheet that suppresses softening of the weld heat-affected zone and is excellent in fatigue durability of the weld zone.

Hereinafter, the best mode for carrying out the present invention will be described in detail. In order to obtain a high-strength steel sheet with a high hole expansion rate, it is important to make the microstructure a single phase and a uniform structure with no hardness difference. As a method of obtaining such a homogeneous phase, for example, a steel sheet after cold rolling Ac ₃ transformation temperature (e.g., W.C.Leslie al, Nariyasu Koda translation supervisor, "Steels Studies", Maruzen, P273 reference.) There is a method in which the austenite phase is made uniform martensite by heat-treating at a higher temperature to make the structure once a uniform austenite phase and then rapidly cooling the steel sheet.

However, in order to obtain a uniform martensite single phase with a high hole expansion ratio by the above-described method, it is necessary to heat to a temperature higher by 40 ° C. than the Ac ₃ transformation temperature, and the specific heat treatment temperature at this time is Depending on the components of the steel sheet to be manufactured, the temperature becomes as high as about 860 to 1000 ° C. When such a high-temperature heat treatment is performed, the energy cost increases and the production efficiency also decreases. In addition, the ultimate temperature in the rapid cooling after the high-temperature heat treatment is also important. By holding at the ultimate temperature for a certain time after the rapid cooling, the martensitic transformation stops, and the untransformed retained austenite undergoes bainite transformation. At this time, the volume fraction of the bainite phase produced | generated becomes large, so that the ultimate temperature at the time of rapid cooling is high. Furthermore, such high-temperature heat treatment has an effect of tempering the martensite structure at the same time.

  The bainite phase produced here has a Vickers hardness Hv (hereinafter simply referred to as Vickers hardness Hv) of 340 to 360 when the load is 100 to 1000 g. Moreover, the Vickers hardness Hv of the bainite phase produced | generated when it cools slowly from a higher temperature is 300-340. Hereinafter, a bainite phase having a Vickers hardness Hv of less than 340 generated by low-speed cooling or the like is referred to as a soft bainite phase, and a bainite phase having a Vickers hardness Hv of 340 or more is referred to as a hard bainite phase.

  The steel sheet produced by the above-described method is essentially not a single structure of martensite phase but a mixed structure of tempered martensite phase and hard bainite phase. For these reasons, the ultimate temperature in the rapid cooling after the heat treatment has an important influence on the mechanical properties of the steel sheet. For this reason, when the ultimate temperature is as low as about 200 to 290 ° C., the strength is obtained, but the uniformity of the structure is disturbed due to the formation of martensite carbide, etc., resulting in a hardness difference with the hard bainite phase, The spread rate decreases. Moreover, if the temperature reached at the time of rapid cooling is low, the difference from the maximum temperature achieved during heat treatment becomes large, increasing the energy cost and adversely affecting production efficiency. On the other hand, when the ultimate temperature at the time of rapid cooling is increased to improve the production efficiency, the hardness of the martensite phase is lowered and the strength of the steel sheet is lowered. Furthermore, when the thickness of the steel sheet is 1.6 mm or more, the heat capacity becomes large, so that a sufficient cooling rate cannot be obtained, and it is difficult to perform stable rapid cooling. As a result, the portion is slowly cooled, and the slowly cooled portion does not undergo martensitic transformation and a bainite phase or the like is generated, and a uniform structure cannot be obtained.

  Therefore, the present inventor has conducted various studies to solve the above-described problems, and as a result, in addition to the steel composition, by optimizing the volume fraction of each phase constituting the microstructure, the tensile strength Is 1100 MPa or more, and found that a high-strength cold-rolled thin steel sheet having excellent hole expansibility can be obtained, leading to the present invention.

  First, the high-strength cold-rolled thin steel sheet according to the first embodiment of the present invention will be described. In the following description, mass% in the composition is simply described as%. The high-strength cold-rolled thin steel plate of this embodiment is a thin steel plate having a plate thickness of 3 mm or less, a tensile strength of 1100 MPa or more, and a hole expansion rate of 40% or more. And the composition is C: 0.05-0.22%, Si: 0.001-0.8%, Mn: 2.0-3.0%, P: 0.001-0.1%, S: 0.0001 to 0.01%, Al: 0.001 to 0.2%, B: 0.0001 to 0.01%, Ti: 0.005 to 0.3%, the balance being Fe And inevitable impurities. Further, the microstructure contains 50 to 90% by volume of martensite phase, 5 to 35% by volume of hard bainite phase and 0.1 to 5% by volume of retained austenite, and the soft bainite phase to 35% or less. It is regulated.

  Hereinafter, the reason for limiting the numerical values of the steel components in the high-strength cold-rolled thin steel sheet of this embodiment will be described.

C: 0.05 to 0.22%
C is an element added to control the volume fraction of the martensite phase and to improve the balance between the strength of the steel sheet and the hole expandability. C also affects the uniform fineness of the substrate. However, when the C content is less than 0.05%, sufficient strength cannot be obtained. On the other hand, when the C content exceeds 0.22%, the hole expansibility decreases and the strength of the welded portion tends to deteriorate. Therefore, the C content is set to 0.05 to 0.22%. Note that the C content is preferably 0.08% or more, and more preferably 0.1 to 0.16%.

Si: 0.001 to 0.8%
Si is an element added for the purpose of suppressing the formation of relatively coarse carbides that deteriorate the balance between the strength and ductility of the steel sheet. However, if the Si content exceeds 0.8%, the plateability is significantly deteriorated and the weldability is also adversely affected. On the other hand, ultra-low Si such that the Si content is less than 0.001% leads to an increase in manufacturing cost. Therefore, the Si content is set to 0.001 to 0.8%. In addition, it is preferable that Si content shall be 0.6% or less, and thereby surface property can be made favorable.

Mn: 2.0 to 3.0%
Mn has an effect of suppressing ferrite transformation, and by adding an appropriate amount of Mn, the main phase, which is the phase with the largest volume fraction, can be converted into martensite and a uniform structure can be obtained. However, when the Mn content is less than 2.0%, the effect of suppressing the precipitation of carbides and the formation of pearlite that causes a decrease in strength and deterioration of hole expansibility cannot be obtained. On the other hand, when Mn is added excessively, specifically, when the Mn content exceeds 3.0%, segregation or the like occurs, and ductility and hole expandability are remarkably lowered. Therefore, the Mn content is set to 2.0 to 3.0%. In addition, it is preferable to make Mn content 2.3% or more, and it is more preferable to exceed 2.4%.

P: 0.001 to 0.1%
P is a strengthening element that increases the strength of the steel sheet. However, the addition of such a large amount that the P content exceeds 0.1% adversely affects weldability, manufacturability during casting and hot rolling, and hole expandability. On the other hand, when the P content is reduced, hole expansibility is improved, but extremely low P such that the P content is less than 0.001% is also economically disadvantageous. Therefore, the P content is set to 0.001 to 0.1%.

S: 0.0001 to 0.01%
S is an unavoidable impurity, and if the S content exceeds 0.01%, the hole expandability deteriorates. On the other hand, lowering the S content to reduce the S content is effective for improving the hole expansibility, but extremely lower S to make the S content less than 0.0001% is economically disadvantageous. Therefore, the S content is set to 0.0001 to 0.01%. The S content is preferably 0.003% or less.

Al: 0.001 to 0.2%
Al is an effective element as a deoxidizing element. However, if the Al content is less than 0.001% by mass, the effect cannot be obtained. On the other hand, when Al is added excessively, specifically, when the Al content exceeds 0.2%, hole expansibility, weldability and plating wettability are impaired. Therefore, the Al content is 0.001 to 0.2%. The Al content is preferably in the range of 0.005 to 0.08%.

B: 0.0001 to 0.01% and Ti: 0.005 to 0.3%
B and Ti are extremely important elements for the high-strength cold-rolled thin steel sheet of the present invention. Specifically, B is an element effective for strengthening grain boundaries and increasing the strength of steel materials, and improves hardenability and strongly suppresses the formation of bainite phase and the like, contributing to the formation of a martensite single phase. It is also an element. Further, Ti has an effect of preventing the effect of addition of B from being lost due to the combination of N and B which are inevitably present in the steel, and the hardenability being lowered. Furthermore, Ti has the effect of suppressing the ferrite transformation by using martensite as the main phase. Therefore, in the present invention, B and Ti are added in combination, and the Mn content is set to 2.0% or more as described above to improve the hole expandability.

  However, when the B content is less than 0.0001%, the above-described addition effect cannot be obtained. On the other hand, if the B content exceeds 0.01%, not only the effect is saturated but also the hot workability is lowered. On the other hand, when the Ti content is less than 0.005%, the effect of preventing the deterioration of hardenability and suppressing the ferrite transformation cannot be obtained. On the other hand, when Ti is added excessively, specifically, when the Ti content exceeds 0.3%, the hole expandability deteriorates. Therefore, the B content is 0.0001 to 0.01%, and the Ti content is 0.005 to 0.3%.

  Moreover, the high-strength cold-rolled thin steel sheet of this embodiment can add Mo and / or Nb in addition to the above components. Thereby, the balance between strength and hole expandability can be further improved. Mo also has the effect of improving the hardenability and preventing softening of the heat affected zone during welding. However, when the Mo content is less than 0.11%, the above-described addition effect cannot be obtained. On the other hand, when the Mo content exceeds 1.0%, the manufacturing cost increases. Furthermore, Nb is a very effective element for strengthening steel sheets by forming fine carbides, nitrides or carbonitrides. Furthermore, Nb delays the ferrite transformation, promotes the formation of bainite and bainitic ferrite, and is effective in suppressing softening of the weld heat affected zone. However, when the Nb content is less than 0.003%, the above-described effects cannot be obtained. On the other hand, when Nb is added excessively, specifically, when the Nb content exceeds 0.3%, ductility and hot workability deteriorate. Therefore, when adding Mo and / or Nb, the Mo content is 0.11 to 1.0% and the Nb content is 0.003 to 0.3%.

  Furthermore, the high-strength cold-rolled thin steel sheet of this embodiment may contain Co and / or W in addition to the above components. Co has the effect of controlling the bainite transformation and improving the balance between strength and hole expansibility. However, when the Co content is less than 0.01%, the effect cannot be obtained. On the other hand, the upper limit of the Co content is not particularly limited, but Co is an expensive element, and if added in a large amount, the economic efficiency is impaired. Therefore, the Co content is preferably 1.0% or less. W is a strengthening element that increases the strength of the steel. However, when the W content is less than 0.01%, the effect cannot be obtained. On the other hand, when the W content exceeds 0.3%, workability deteriorates. Therefore, when adding Co and / or W, the Co content is 0.01 to 1.0% and the W content is 0.01 to 0.3%.

  Furthermore, the high-strength cold-rolled steel sheet of the present embodiment includes one or more elements selected from the group consisting of Zr, Hf, Ta, and V, which are strong carbide forming elements, in addition to the above components. May be added. Thereby, the balance between strength and hole expandability can be further improved. In addition, when the total content of these elements is less than 0.001%, the above-described addition effect cannot be obtained. On the other hand, when the total content of these elements exceeds 1.0%, ductility and hot workability deteriorate. Therefore, when adding Zr, Hf, Ta and / or V, the total content is made 0.001 to 1.0%.

  Furthermore, the high-strength cold-rolled thin steel sheet of the present embodiment may contain one or more elements selected from the group consisting of Ca, Mg, and Rem, in addition to the above components. Ca, Mg, and Rem can control inclusions by adding appropriate amounts, and particularly contribute to fine dispersion of inclusions. However, when the total content of Ca, Mg and Rem is less than 0.0001%, the addition effect cannot be obtained. On the other hand, when these elements are added excessively, the manufacturability such as castability and hot workability, and the ductility of the steel sheet product are lowered. Therefore, when adding Ca, Mg and / or Rem, the total content is made 0.0001 to 0.5%.

  Note that the balance of the high-strength cold-rolled thin steel sheet of the present embodiment is Fe and inevitable impurities. Examples of this unavoidable impurity include N and Sn, but the high-strength cold-rolled thin steel sheet of the present embodiment has the above-described effects even when these elements are contained in a total range of 0.2% or less. Will not be damaged.

  Next, the definition of the microstructure in the high-strength cold-rolled thin steel sheet of this embodiment will be described. The martensite phase and the bainite phase are phases formed by transformation of the austenite phase by cooling. Specifically, the martensite phase is generated below the martensite start temperature (around 290 to 300 ° C. in the case of the high-strength cold-rolled thin steel sheet of the present embodiment) in the cooling process, and is generated before or after the end of cooling. finish. This martensite phase has a high hardness and a Vickers hardness Hv of about 400 to 430. Further, the soft bainite phase is generated at a temperature higher than the martensite start temperature described above in the cooling process, and the Vickers hardness Hv is 300 or more and less than 340 as described above. Further, the hard bainite phase is generated in the isothermal holding process (aging treatment) after cooling, and the Vickers hardness Hv is 340 to 360. And the austenite phase which remains without transformation is the retained austenite phase, which inevitably remains.

  Next, the reason for limiting the numerical value of each phase in the microstructure of the high-strength cold-rolled thin steel sheet of the present embodiment will be described.

Martensite phase: 50-90% by volume
The martensite phase has a high hardness and is a phase necessary for ensuring the tensile strength of the steel sheet. Basically, the more the martensite phase, the higher the tensile strength. If the volume fraction of the martensite phase is less than 50%, sufficient tensile strength cannot be obtained. Therefore, the volume fraction of the martensite phase is 50% by volume or more. In addition, although the upper limit of the volume fraction of a martensite phase is decided by the volume fraction of a hard bainite phase and a soft bainite phase, it is 90 volume% in this invention. In addition, it is preferable that the upper limit of a martensite phase shall be 60 volume%.

Hard bainite phase: 5-35% by volume
In the high-strength cold-rolled thin steel sheet of this embodiment, the hard bainite phase is generated during the aging treatment performed after the cold-rolled steel sheet is annealed and cooled. This aging treatment can remove dislocations in the martensite phase to some extent and thereby improve the toughness. However, when the bainite phase appears in the steel, the effect is amplified and the hole expandability is improved. However, when the hard bainite phase is less than 5% by volume, the effect cannot be obtained. On the other hand, when the ratio of the hard bainite phase is too large, specifically, when the hard bainite phase is contained in an amount of 35% by volume or more, the tensile strength is lowered. Therefore, the volume fraction of the hard bainite phase is 5 to 35% by volume. In addition, it is preferable that the volume fraction of a hard bainite phase shall be 10 volume% or more, More preferably, it is 15 volume% or more.

Soft bainite phase: 35% by volume or less In the high-strength cold-rolled thin steel sheet of the present embodiment, the soft bainite phase is generated during cooling after annealing. The appearance of the soft bainite phase decreases the tensile strength of the steel sheet. Therefore, in the present invention, the soft bainite phase is regulated to 35% by volume or less. As will be described later, in the high-strength cold-rolled thin steel sheet of the present embodiment, by adjusting the cooling conditions, the generation of soft bainite can be suppressed, and as a result, the volume content of the soft bainite phase is reduced. It is also possible to make it 0%. For this reason, in this invention, although the case where a soft bainite phase is 0% is included, from a viewpoint of manufacturing cost etc., it is preferable that the lower limit of a soft bainite phase shall be 5 volume%.

Residual austenite: 0.1 to 5% by volume
In the high-strength cold-rolled thin steel sheet of this embodiment, there is no important role for the retained austenite phase. However, the austenite phase generated by the transformation during the annealing heat treatment remains without being transformed in the martensitic transformation in the subsequent cooling process. Then, in the subsequent aging treatment, it is transformed into hard bainite, but at this time, not all is transformed, and an amount of 0.1% by volume or more inevitably remains. If the amount of retained austenite is too large, specifically, if the amount of retained austenite exceeds 5% by volume, the uniformity of the structure is lowered and the hole expansibility is lowered. Therefore, the volume fraction of retained austenite is 5% by volume or less. In addition, it is preferable that the volume fraction of a retained austenite is 3 volume% or less.

  Moreover, the volume fraction of each phase in a microstructure can be measured by the method shown below. When the microstructure of the steel transforms from the ferrite phase to the austenite phase during the temperature raising process, the steel volume shrinks. Further, in the cooling process, when the austenite phase is transformed into a martensite phase or a soft bainite phase, volume expansion occurs in the steel sheet. Furthermore, the martensitic transformation and the soft bainite transformation have different transformation temperatures and transformation speeds. Furthermore, during the isothermal heat treatment (aging treatment) after cooling, volume expansion occurs due to the hard bainite transformation. Therefore, measure the volume ratio of martensite phase, soft bainite phase and hard bainite phase by observing volume change with total heat treatment cycle for temperature rise, annealing, cooling and isothermal heat treatment (aging treatment). Can do.

Specifically, in the volume expansion curve, when the temperature rises to the Ac ₁ transformation point, the volume of the steel sheet linearly expands as the temperature increases due to the thermal expansion coefficient of the ferrite phase. This region is referred to as region 1. When the temperature exceeds the Ac ₁ transformation point, the austenite transformation starts. Since the thermal expansion coefficient of the austenite phase is smaller than that of the ferrite phase, the volume expansion coefficient decreases, and the volume of the steel sheet changes from expansion to contraction as the fraction of the austenite phase increases. Here, since the volume shrinkage ratio changes depending on the ratio of the austenite phase, the steel sheet shrinks nonlinearly.

Next, when the temperature reaches the Ac ₃ transformation temperature, the structure becomes an austenite single phase, so that the volume expansion coefficient becomes constant, and the volume expansion curve starts to linear expansion again. This region is referred to as region 2. Thereafter, the volume of the steel sheet linearly expands until reaching the heat treatment (annealing) temperature range. And if it hold | maintains at holding temperature (maximum reach | attainment temperature), a volume will be hold | maintained uniformly, and if cooling is started after that, a volume will shrink | contract linearly.

  When the cooling is further continued, in the high-strength cold-rolled thin steel sheet of this embodiment, a soft bainite phase starts to precipitate in a temperature range of 300 to 500 ° C. depending on conditions. Since the soft bainite phase has a large volume per mole, the volume of the entire steel sheet begins to expand. At this time, the volume of the steel sheet expands relatively slowly as the temperature decreases. Thereafter, a martensite phase precipitates at about 290 to 300 ° C. Since the precipitation of this martensite phase proceeds rapidly, rapid volume expansion is observed. And if it hold | maintains for a fixed time at the aging treatment temperature after cooling, since the remaining austenite phase will decompose | disassemble into a carbide | carbonized_material and a ferrite phase and a hard bainite phase will precipitate, the volume of a steel plate will expand further.

  By measuring the volume change of the steel sheet in all of the above heat treatment processes, and algebraically treating the result and the volume change straight line of the ferrite single phase in the region 1 and the volume change straight line in the region 2 described above, The volume fraction value of each phase can be obtained.

  Moreover, about a retained austenite phase, the volume fraction absolute value can be calculated | required by analyzing the steel after heat processing by a X ray diffraction method. Therefore, the volume fraction of these phases can be obtained by combining the volume change of the steel sheet during the heat treatment and the X-ray diffraction result of the steel sheet after the heat treatment.

Next, a method for manufacturing the high-strength cold-rolled thin steel plate of the present embodiment configured as described above will be described. First, a cast slab adjusted so that the steel component is in the above-described range is directly or once cooled to 1000 ° C. or less and then reheated and hot-rolled to obtain a hot-rolled coil. At that time, when the cast slab is heated and hot-rolled as it is, the heating unit can be reduced. On the other hand, when the cast slab is once cooled to 1000 ° C. or less and then reheated and hot-rolled, the ductility of the final product can be improved. The reheating temperature at this time is desirably 1100 to 1300 ° C. When this reheating temperature becomes high, it becomes coarse and a thick oxide scale is formed. On the other hand, when the reheating temperature is low, deformation resistance during rolling becomes high. The end temperature of hot rolling is generally carried out at an Ar ₃ transformation temperature or higher determined by the steel composition, but it is thin as long as it is within a range of about 100 ° C. lower than the Ar ₃ transformation temperature. The properties of the steel sheet do not deteriorate.

  Next, the hot-rolled coil is wound, pickled, and then cold-rolled to obtain a cold-rolled steel sheet. At this time, the coiling temperature of the hot-rolled coil after cooling is preferably not less than the bainite transformation start temperature determined by the steel component. Thereby, it can prevent that the load at the time of cold rolling becomes higher than necessary. However, this is not the case when the total rolling reduction during cold rolling is as small as 40% or less, and the properties of the finally obtained thin steel sheet will not deteriorate even if the steel sheet is wound below the bainite transformation temperature of the steel. Further, after hot rolling, the surface scale is removed by a high-pressure descaling apparatus or pickling. Thereby, the surface state in a product can be improved. Further, the total rolling reduction in the cold rolling performed after pickling is set based on the relationship between the sheet thickness and the cold rolling load in the final product, but 30% or more is sufficient for recrystallization, and this range. If so, the properties of the finally obtained thin steel sheet will not deteriorate.

Then, the cold rolled steel sheet, the highest temperature after heat treatment in the range of Ac ₃ transformation temperature ～860 ° C., to a first temperature range of six hundred and eighty to seven hundred and eighty ° C. at a cooling rate of 0.1 to 20 ° C. / sec After cooling and subsequently cooling to a second temperature range of 250 to 350 ° C. at a cooling rate of 40 to 70 ° C./second, the temperature is held for 10 to 500 seconds in this second temperature range.

Generally, when heat treatment is performed with the maximum temperature reached below the Ac ₃ transformation temperature for a cold-rolled steel sheet, the amount of austenite obtained during heating decreases, and the structure of the finally obtained thin steel sheet does not become uniform. Further, the conventional product, in a temperature range immediately above the Ac ₃ transformation temperature, uniformity is not sufficient austenite structure had to be heat-treated at a temperature of about (Ac ₃ transformation temperature + 30 ° C.). On the other hand, according to the manufacturing method of the present invention, a thin steel plate having a good balance between strength and hole expansibility can be obtained even when the maximum temperature reached is set low. That is, the feature of the high-strength cold-rolled thin steel sheet of the present embodiment is the ability to lower the maximum reached temperature to Ac ₃ transformation temperature. On the other hand, in order to reduce energy costs, it is desirable that the maximum temperature reached during heat treatment is low. Therefore, the upper limit of the maximum temperature reached during heat treatment is 860 ° C.

  Moreover, in this heat processing, in order to make the temperature of a steel plate uniform, it is preferable to hold | maintain for 1 second or more in the temperature range mentioned above. On the other hand, if the holding time exceeds 10 minutes, the production of grain boundary oxidized phases is promoted and the cost may increase. Therefore, the heat treatment time after cold rolling is preferably 1 second to 10 minutes. As described above, in the manufacturing method of the high-strength cold-rolled thin steel sheet according to the present embodiment, the maximum ultimate temperature in the heat treatment after cold rolling is lower than in the conventional method, so that the energy cost is reduced and the efficiency is high. Production becomes possible.

  Furthermore, the cooling step after the heat treatment is an important step for obtaining a mixed phase structure of the martensite phase and the hard untransformed ferrite phase. In the present embodiment, because of the structure of the line, rapid cooling cannot be performed immediately after the heat treatment, so rapid cooling is performed after slow cooling. First, in slow cooling, it is necessary to sufficiently suppress the formation of ferrite. When the cooling rate during this slow cooling is less than 0.1 ° C./second, the formation of ferrite and pearlite is promoted, and the strength of the thin steel sheet is lowered. On the other hand, if the cooling rate during slow cooling exceeds 20 ° C./second, the start temperature of rapid cooling becomes too low. When the rapid cooling start temperature is less than 680 ° C., ferrite transformation is started, and there is a concern that desired characteristics cannot be obtained. On the other hand, although it is advantageous that the rapid cooling start temperature is high, this temperature is affected by the maximum temperature reached in the heat treatment, and therefore it is difficult to make the rapid cooling start temperature higher than 780 ° C. Therefore, in the slow cooling after the heat treatment, the cooling rate is set to 0.1 to 20 ° C./second, and the cooling is performed to the temperature range of 680 to 780 ° C. which is the rapid cooling start temperature.

  Rapid cooling is performed after such slow cooling. The cooling rate in this rapid cooling is an important condition for suppressing the formation of a soft bainite phase and obtaining a martensite phase. When the cooling rate during rapid cooling is less than 40 ° C./second, the formation of soft bainite becomes significant. On the other hand, when manufacturing a steel plate having a thickness exceeding 1.6 mm, if the cooling rate at the time of rapid cooling exceeds 70 ° C./second, the cooling cost is required and it is not practical. Therefore, the cooling rate during rapid cooling is set to 40 to 70 ° C./second. In this two-stage cooling process of slow cooling and rapid cooling, a hard martensite phase and a residual austenite phase are generated.

  Subsequently, overaging treatment is performed with the temperature reached at the time of rapid cooling as the overaging treatment temperature. That is, the cold-rolled steel sheet after the heat treatment is held for 10 to 500 seconds with the temperature reached at the time of rapid cooling. During this overaging treatment, the retained austenite is transformed into hard bainite and the martensite phase is tempered. At this time, if the overaging temperature exceeds 350 ° C., the strength of the thin steel sheet is lowered. On the other hand, when the overaging temperature is less than 250 ° C., the production efficiency is lowered. Therefore, the overaging treatment temperature is set to 250 to 350 ° C. In addition, it is preferable that the lower limit of overaging temperature is 270 degreeC. Further, when the holding time in this temperature range is less than 10 seconds, the hard bainite transformation is not sufficiently performed. On the other hand, if the holding time is 10 seconds or more, the martensite phase is tempered to make the structure uniform, and the hole expandability is improved. However, if the holding time is too long, specifically, the holding time is 500 seconds. If it exceeds, the strength of the thin steel sheet will decrease. Therefore, the holding time at the overaging temperature is 10 to 500 seconds.

  By the manufacturing method described above, a high-strength cold-rolled steel sheet having excellent hole expandability with a tensile strength of 1100 MPa or more and a hole expansion rate of 40% or more is obtained. In the high-strength cold-rolled steel sheet of this embodiment, the tensile strength is preferably 1180 MPa or more. Further, the upper limit of the tensile strength is not particularly limited, but it is technically difficult to set it to 1600 MPa or more. Therefore, it is economical to set this value as the upper limit. On the other hand, the hole expansibility is preferably 45% or more, more preferably 50% or more. And since it is technically difficult to make a hole expansion rate 200% or more, this value is a substantial upper limit.

  Next, a high-strength cold-rolled steel sheet according to the second embodiment of the present invention will be described. The steel-strength cold-rolled thin steel plate of this embodiment is obtained by further adding Cr in addition to the components in the high-strength cold-rolled steel plate of the first embodiment described above. Since Cr has the effect of suppressing ferrite transformation like Mn, substitution of Mn with Cr can reduce the Mn content and improve the balance between strength and hole expansibility. Thus, when Mn and Cr are added together, the Mn equivalent due to Cr substitution is expressed as ([Mn] + 1.06 × [Cr]). Here, [Mn] is the Mn content (%), and [Cr] is the Cr content (%).

  As in the case of the steel strength cold-rolled thin steel sheet of the first embodiment described above, when adding Mn alone without adding Cr, the Mn content needs to be 2.0% or more, In the high-strength cold-rolled thin steel sheet of the present embodiment, since Cr is added in a composite manner, the Mn content can be reduced to less than 2.0%. However, when the Mn content is excessively reduced, specifically, when the Mn content is less than 1.5%, the strength of the steel sheet decreases. On the other hand, as in the case of adding Mn alone, when Mn is added in excess of 3.0%, segregation or the like occurs, and ductility and hole expansibility are significantly reduced. Therefore, when Cr and Mn are added in combination, the Mn content is set to 1.5 to 3.0%.

Furthermore, in the high-strength cold-rolled thin steel sheet of the present embodiment, the Mn content is within the above range, and the Mn equivalent ([Mn] + 1.06 × [Cr]) by Cr substitution satisfies the following formula (4). Like that. When the Mn equivalent ([Mn] + 1.06 × [Cr]) is less than 2.4%, sufficient strength cannot be ensured. On the other hand, when the Mn equivalent ([Mn] + 1.06 × [Cr]) exceeds 4.5%, scale is generated on the surface of the steel sheet. Therefore, the Mn equivalent ([Mn] + 1.06 × [Cr]) is set to 2.4 to 4.5%. Thereby, the balance between strength and hole expansibility can be improved.

  In addition, the structure other than the above in the steel strength cold-rolled thin steel plate of this embodiment, an effect, and a manufacturing method are the same as that of the steel strength cold-rolled thin steel plate of the above-mentioned 1st Embodiment.

  Moreover, the high-strength cold-rolled steel sheets of the first and second embodiments described above are also excellent in weldability. As the welding method, a welding method that is usually performed, such as arc welding, spot welding, TIG welding, MIG welding, mash welding, laser welding, or the like can be applied.

Examples of the present invention will be described below. First, as Example 1 of the present invention, an example within the scope of the present invention and a comparative cold-rolled thin steel sheet that is out of the scope of the present invention were produced, and its microstructure was observed, a hole expansion test prescribed by the Federation of Steels, and A tensile test based on JIS was performed. Specifically, a cast slab having the composition shown in Table 1 and Table 2 below is once cooled to room temperature, heated to 1200 ° C. and hot-rolled, and a range of 880 to 910 ° C. which is not lower than the Ar ₃ transformation temperature. Then, the hot rolling was completed to form a hot rolled coil having a thickness of 4 mm. Next, after this hot-rolled coil is cooled, it is wound at 550 ° C., which is higher than the bainite transformation start temperature determined by the chemical composition of each steel, and the surface is pickled and cold-rolled, and the thickness is 2 mm. A cold-rolled steel sheet was produced. Next, each cold-rolled steel sheet was heated to a temperature range of (Ar ₃ transformation temperature + 50 ° C.) to (Ar ₃ transformation temperature + 10 ° C.) at a heating rate of 2.8 ° C./second. Held for 140 seconds. Thereafter, it was cooled to 700 ° C. at a cooling rate of 2.7 ° C./second, and subsequently cooled to 300 ° C. at a cooling rate of 50 ° C./second. Furthermore, after carrying out an overaging process by hold | maintaining at the same temperature, ie, 300 degreeC, for 150 second, it cooled and it was set as the cold rolled sheet steel of the Example and the comparative example. The balance in the steel compositions shown in Tables 1 and 2 below is Fe and inevitable impurities. In Tables 1 and 2 below, conditions outside the scope of the present invention are underlined. Further, Tables 1 and 2 below also show the Ar ₃ transformation temperature of each steel and the heat treatment temperature (maximum ultimate temperature) after cold rolling.

  Next, the volume fraction of each phase in the microstructure was measured for each cold-rolled thin steel sheet of Examples and Comparative Examples by the method described above. Moreover, a JIS No. 5 tensile test piece was collected from each cold-rolled thin steel sheet, and its tensile strength and elongation were measured. Moreover, the hole expansion test based on the steel federation standard was done, and the hole expansion rate of each cold-rolled thin steel plate was calculated | required. The above results are summarized in Table 3 below. In Table 3 below, the tensile strength is less than 1100 MPa, the hole expansion rate is less than 40%, or the volume fraction in the microstructure is out of the scope of the present invention, is underlined.

  As shown in Table 3 above, Comparative Example No. 4 (steel grade D) and No. 4 The cold rolled thin steel sheet No. 6 (steel type F) had a Mn content exceeding the range of the present invention, so that the hole expandability was less than 40%. This No. The cold rolled steel sheet No. 4 (steel type D) also had a volume fraction of hard bainite that was less than the range of the present invention. On the other hand, no. Since the cold rolled steel sheet of No. 7 (steel type G) has a Mn content smaller than the range of the present invention, the hole expandability was less than 40%. No. The cold rolled thin steel sheet of No. 12 (steel type L) had a C content less than the range of the present invention, so that the hole expandability was less than 40% and the tensile strength was also less than 1100 MPa. On the other hand, no. 15 (steel grade O) and No. Since the C content of the cold rolled steel sheet of 16 (steel type P) exceeded the range of the present invention, the hole expandability was less than 40%. This No. The cold rolled thin steel plate of 16 (steel type P) also had a volume fraction of hard bainite that was less than the range of the present invention.

  No. The cold rolled thin steel sheet of No. 17 (steel type Q) has a steel composition within the scope of the present invention, but the martensite volume fraction is 37%, which is less than the scope of the present invention, so that the hole expandability is less than 40%. The tensile strength was less than 1100 MPa. No. The cold rolled thin steel plate of 19 (steel type S) is a comparative example showing the effect of addition of B. Since this cold-rolled thin steel sheet had no B added, the hole expandability was less than 40%. Furthermore, no. 26 (steel grade Z) and No. The cold rolled thin steel plate No. 27 (steel type AA) had a hole expandability of less than 40% because the Si content exceeded the range of the present invention. This No. The cold-rolled thin steel sheet of No. 26 (steel type Z) also had soft bainite and martensite volume fractions outside the scope of the present invention. Furthermore, no. 31 (steel type AE) has a C content less than the range of the present invention, and the volume fraction of soft bainite and martensite in the microstructure is out of the range of the present invention. The property was less than 40%.

  No. The cold-rolled steel sheet of 32 (steel type AF) has a C content and a Mn content exceeding the scope of the present invention, and the volume fraction of soft bainite and martensite is outside the scope of the present invention. The expansibility was less than 40% and the tensile strength was also less than 1100 MPa. Moreover, No. 33 (steel type AG) cold-rolled thin steel sheet is a comparative example showing the effect of addition of Ti. Since this cold-rolled thin steel sheet was not added with Ti and the volume fraction of martensite was also outside the scope of the present invention, the hole expandability was less than 40% and the tensile strength was less than 1100 MPa. Furthermore, the cold rolled thin steel sheet of No. 35 (steel grade AI) has a C content, a Ti content, a volume fraction of soft bainite and a volume fraction of martensite that are out of the scope of the present invention. The tensile strength was less than 1100 MPa.

  On the other hand, Example No. satisfying the requirements of the present invention. 1-No. 3, no. 5, no. 8-No. 11, no. 13, no. 14, no. 18, no. 20-No. 28-No. 30, no. 34, no. 36-No. All 45 cold-rolled thin steel sheets had a hole expansibility of 40% or more and a tensile strength of 1100 MPa or more, and the balance between strength and hole expansibility was excellent. In particular, No. with both Mo and Nb added. 1 to 3 (steel types A to C), No. 1 5 (steel grade E), No. 8 (steel grade H), No. 8 9 (steel type I), No. 9 13 (steel grade M), No. 14 (steel type N), No. 14 18 (steel type R), No. 18 20-No. 24 (steel types T to X) was excellent in hole expansibility as compared with the cold-rolled thin steel sheets of other examples. This is because, among the cold-rolled thin steel plates of the examples, Mo and Nb are not added, or only one of them is added in comparison with the case where both Mo and Nb are added in the cooling process. This is because non-uniformity is likely to occur in the tissue. This result does not pose any problem in practical use, and indicates that it is preferable to add Mo and Nb within the scope of the present invention.

  Next, as Example 2 of the present invention, a cast slab having the composition shown in Table 4 below was used, and cold-rolled thin steel sheets of Examples and Comparative Examples were produced in the same manner and conditions as in Example 1 described above. . And about these cold-rolled thin steel plates, the tensile strength, the elongation, the hole expansion rate, and the volume fraction of each phase in the microstructure were measured in the same manner as in Example 1 described above. The results are summarized in Table 5 below. The balance in the steel composition shown in Table 4 below is Fe and inevitable impurities. In Table 4 below, conditions outside the scope of the present invention are underlined, and in Table 5 below, the tensile strength is less than 1100 MPa, the hole expansion rate is less than 40%, and the volume in the microstructure. Those whose fractions fall outside the scope of the present invention are underlined.

  As shown in Table 5 above, by adding Cr and Mn in combination, even if the Mn content is less than 2.0%, the hole expansion rate can be made 40% or more, and sufficient strength and hole can be obtained. A balance with expansibility was obtained. On the other hand, the cold rolled thin steel sheet of Comparative Example No. 57 (steel type AZ) had poor hole expansibility because [Mn] + 1.06 × [Cr] was less than the range of the present invention.

Claims (12)

% By mass
C: 0.05 to 0.22%,
Si: 0.001 to 0.8%,
Mn: 2.0 to 3.0%,
P: 0.001 to 0.1%,
S: 0.0001 to 0.01%,
Al: 0.001 to 0.2%,
B: 0.0001 to 0.01%
Ti: 0.005 to 0.3% is contained,
The balance has a composition consisting of Fe and inevitable impurities,
The microstructure consists of martensite phase: 50-90% by volume, hard bainite phase: 5-35% by volume, soft bainite phase: 35% or less, and retained austenite: 0.1-5% by volume,
The tensile strength is 1100 MPa or more,
A high-strength cold-rolled thin steel sheet excellent in hole expandability, characterized by having a hole expansion ratio of 40% or more. % By mass
C: 0.05 to 0.22%,
Si: 0.001 to 0.8%,
Mn: 1.5-3.0%
P: 0.001 to 0.1%,
S: 0.0001 to 0.01%,
Al: 0.001 to 0.2%,
B: 0.0001 to 0.01%
While containing Ti: 0.005 to 0.3%,
Containing Cr,
The balance consists of Fe and inevitable impurities,
When the Mn content (%) is [Mn] and the Cr content (%) is [Cr], the composition satisfies the following mathematical formula (A).
The microstructure consists of martensite phase: 50-90% by volume, hard bainite phase: 5-35% by volume, soft bainite phase: 35% or less, and retained austenite: 0.1-5% by volume,
The tensile strength is 1100 MPa or more,
And the high-strength cold-rolled thin steel plate excellent in hole expandability characterized by having a hole expansion ratio of 40% or more.

Furthermore, in mass%,
The element according to claim 1 or 2, comprising one or two elements selected from the group consisting of Mo: 0.11 to 1.0% and Nb: 0.003 to 0.3%. High-strength cold-rolled thin steel sheet with excellent hole expandability. Furthermore, in mass%,
4. One or more elements selected from the group consisting of Co: 0.01 to 1% and W: 0.01 to 0.3% are contained. A high-strength cold-rolled thin steel sheet excellent in hole expansibility described in the item. Furthermore, in mass%,
5. One or more elements selected from the group consisting of Zr, Hf, Ta, and V are contained in a total amount of 0.001 to 1%. 5. High-strength cold-rolled thin steel sheet with excellent hole expandability. Furthermore, in mass%,
The total content of one or more elements selected from the group consisting of Ca, Mg, and Rem is 0.0001 to 0.5%. High-strength cold-rolled thin steel sheet with excellent hole expandability. In mass%, C: 0.05 to 0.22%, Si: 0.001 to 0.8%, Mn: 2.0 to 3.0%, P: 0.001 to 0.1%, S: 0.0001-0.01%, Al: 0.001-0.2%, B: 0.0001-0.01%, Ti: 0.005-0.3%, the balance being Fe and inevitable A step of obtaining a hot-rolled coil by directly or directly cooling the cast slab having a composition composed of impurities to 1000 ° C. or less and then hot-rolling it again.
Winding the hot rolled coil, pickling, and cold rolling to obtain a cold rolled steel sheet; and
The cold-rolled steel sheet is heat-treated at a maximum temperature of Ac ₃ transformation temperature or more and 860 ° C. or less, and then cooled to a first temperature range of 680 to 780 ° C. at a cooling rate of 0.1 to 20 ° C./second, And subsequently cooling to a second temperature range of 250 to 350 ° C. at a cooling rate of 40 to 70 ° C./second, and then maintaining the second temperature range for 10 to 500 seconds. A method for producing a high-strength cold-rolled steel sheet with excellent spreadability. In mass%, C: 0.05 to 0.22%, Si: 0.001 to 0.8%, Mn: 1.5 to 3.0%, P: 0.001 to 0.1%, S: 0.0001-0.01%, Al: 0.001-0.2%, B: 0.0001-0.01%, Ti: 0.005-0.3% and Cr When the balance is Fe and inevitable impurities, the Mn content (%) is [Mn] and the Cr content (%) is [Cr], a cast slab having a composition satisfying the following formula (A) is directly Or, once cooled to 1000 ° C. or lower, heated again and hot rolled to obtain a hot rolled coil,
Winding the hot rolled coil, pickling, and cold rolling to obtain a cold rolled steel sheet; and
The cold-rolled steel sheet is heat-treated at a maximum temperature of Ac ₃ transformation temperature or more and 860 ° C. or less, and then cooled to a first temperature range of 680 to 780 ° C. at a cooling rate of 0.1 to 20 ° C./second, And subsequently cooling to a second temperature range of 250 to 350 ° C. at a cooling rate of 40 to 70 ° C./second, and then maintaining the second temperature range for 10 to 500 seconds. A method for producing a high-strength cold-rolled steel sheet with excellent spreadability.

  The cast slab further contains one or two elements selected from the group consisting of Mo: 0.11 to 1.0% and Nb: 0.003 to 0.3% by mass%. The method for producing a high-strength cold-rolled thin steel sheet excellent in hole expansibility according to claim 7 or 8.   The cast slab further contains one or two elements selected from the group consisting of Co: 0.01 to 1% and W: 0.01 to 0.3% by mass%. The manufacturing method of the high intensity | strength cold-rolled thin steel plate excellent in the hole expansibility of any one of Claim 7 thru | or 9.   The cast slab further contains 0.001 to 1% in total of one or more elements selected from the group consisting of Zr, Hf, Ta and V in mass%. Item 11. A method for producing a high-strength cold-rolled thin steel sheet excellent in hole expansibility according to any one of Items 7 to 10.   The cast slab further contains 0.0001 to 0.5% in total of one or more elements selected from the group consisting of Ca, Mg, and Rem in mass%. The manufacturing method of the high intensity | strength cold-rolled thin steel plate excellent in the hole expansibility of any one of claim | item 7 thru | or 11.