JP2016000850A - Cold rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet having a tensile strength of 590 MPa or more and excellent in processability with good elongation by using precipitation strengthening of a fine carbide and a manufacturing method therefor.SOLUTION: There is provided a cold rolled steel sheet containing, by mass%, C:0.04 to 0.06%, Si+Al:0.05 to 0.10%, Mn:0.15 to 0.45%, P:less than 0.01%, Ti:0.12 to 0.16% with Tidefined by the following (1) formula having a composition satisfying the following (2) formula, volume fraction of a ferrite phase:90% or more, volume fraction of a cementite:less than 5%, recrystallization fraction of the ferrite phase:70% or more, Ti carbide (TiC) depositing in the ferrite phase with a structure having an average length of less than 15 nm and Ti deposition fraction of 90% or more and tensile strength of 590 MPa or more. Ti=Ti-(3.43N+1.5S) (1)1.2≤4C/Ti≤2.4(2)

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in workability such as elongation, suitable for automobile frame parts and the like, in particular, a cold-rolled steel sheet having a tensile strength (TS) of 590 MPa or more and a method for producing the same.

  In recent years, high-strength steel sheets are being frequently used for automobile frame parts and underbody parts in order to reduce the weight of automobile bodies. In addition, regarding the component composition of the steel sheet, it is required to use rare alloy elements as much as possible in response to the global procurement of raw materials.

Here, in order to increase the strength of hot-rolled steel sheets, precipitation strengthening methods for forming carbonitrides such as Ti, Nb, and V in the ferrite phase have been actively utilized. For example, Patent Document 1 discloses that, by weight, C: 0.02 to 0.10%, Ti: 0.3% or less, Nb: 0.2% or less, and 0.50 <{(Ti− 3.43N-1.5S) /4+Nb/7.75} / C in an amount of at least one of Ti and Nb, Fe as a main component, an acicular ferrite structure, and fine TiC and A high-strength hot-rolled steel sheet excellent in stretch flangeability characterized by having a structure in which NbC is precipitated has been proposed. Patent Document 2 includes, in mass%, C <0.10%, Ti: 0.03 to 0.10%, Mo: 0.05 to 0.6%, Fe as a main component, and ferrite single element. A thin steel sheet excellent in press workability with a TS of 550 MPa or more, in which fine precipitates having a particle size of less than 10 nm are dispersed and precipitated in a matrix of a phase structure, has been proposed. Further, Patent Document 3 contains C: 0.02 to 0.08% and Ti: 0.03 to 0.06% in mass%, and further, the total content of Nb, Mo and V is 0. 0.01% or less, the ratio of Ti amount to C amount is Ti / C: 0.375 to 1.6, and the average diameter of TiC precipitates in the crystal grains is 0.8 to 3 nm, An alloy-saving high-strength hot-rolled steel sheet having an average number density of 1 × 10 ¹⁷ [pieces / cm ³ ] or more and a tensile strength of 540 to 650 MPa has been proposed.

JP-A-7-11382 JP 2002-322539 A JP 2011-26690 A JP 2009-31269 A

  However, in hot-rolled steel sheets, as described above, although techniques have been developed to finely control carbides such as TiC and obtain a steel sheet with an excellent balance between strength and ductility, the sheet thickness is only up to about 2 mm. It is intended for steel plates. In the manufacture of thin steel sheets that are thinner than these hot-rolled steel sheets, a process of further cold rolling after hot rolling and recrystallization annealing is generally carried out, and carbides are used during recrystallization annealing. Therefore, it is difficult to achieve a strength exceeding 440 MPa in a structure mainly composed of ferrite utilizing precipitation strengthening. For this reason, when manufacturing a cold-rolled steel sheet having a tensile strength of 590 MPa or more, it is necessary to add a solid solution strengthening element such as Si or Mn, resulting in high costs and inferior plating properties.

  Therefore, in the present invention, from the viewpoint of raw material costs, plating properties, etc., it has a tensile strength of 590 MPa or more by utilizing precipitation strengthening of fine carbides, regardless of solid solution strengthening of Si, Mn, etc. It aims at providing the cold-rolled steel plate which has elongation and is excellent in workability, and its manufacturing method.

  Under the above problems, the present inventors have made various studies on utilizing precipitation strengthening using Ti carbide (TiC) as fine carbide.

  First, in the ferrite phase of the hot-rolled steel sheet, which is the pre-stage of cold-rolled annealed steel plate, Ti carbide is finely precipitated to increase the strength of the hot-rolled steel sheet as much as possible. We considered increasing the strength of the cold-rolled steel sheet by annealing. In this case, the very fine carbides present at the stage of the hot-rolled steel sheet significantly delay the subsequent recrystallization. For this reason, promotion of recrystallization by increasing the rolling reduction (also called rolling rate) of cold rolling was examined. As a result, contrary to expectation, it was found that recrystallization was significantly delayed even when the rolling rate was increased. Furthermore, it has been found that during cold rolling, TiC precipitated in the hot-rolled steel sheet is re-dissolved or refined as the cold rolling rate increases. Therefore, as a result of further investigation, the strength at the time of hot-rolled steel sheet is not so important, and the minimum strength that can achieve the desired strength when cold-rolled and annealed to make a cold-rolled steel sheet By ensuring the above, it has been found that recrystallization after cold rolling proceeds rapidly, and a cold-rolled steel sheet having a tensile strength of 590 MPa or more and excellent workability can be produced advantageously.

  The present invention has been made on the basis of the above-described findings, and the gist thereof is as follows.

[1] By mass%, C: 0.04 to 0.06%, Si + Al: 0.05 to 0.10%, Mn: 0.15 to 0.45%, P: less than 0.01%, S: Ti ^* containing 0.004% or less, N: 0.0045% or less, Ti: 0.12 to 0.16%, the balance consisting of Fe and inevitable impurities, and defined by the following formula (1) Having a composition satisfying the following formula (2): volume fraction of ferrite phase: 90% or more, volume fraction of cementite: less than 5%, recrystallization rate of the ferrite phase: 70% or more, and the ferrite Ti carbide (TiC) is precipitated in the phase, the Ti carbide has an average length of less than 15 nm, a Ti precipitation rate of 90% or more, and a tensile strength of 590 MPa or more. Cold-rolled steel sheet;
Ti ^* = Ti− (3.43N + 1.5S) (1)
1.2 ≦ 4C / Ti ^* ≦ 2.4 (2)
However, Ti, N, S, and C in the formula represent the content (mass%) of each element.
Moreover, the average length of Ti carbide is a value obtained by arithmetically averaging the lengths of TiC observed in the [001] orientation of the ferrite phase as a matrix with respect to 200 or more by a transmission electron microscope.

  [2] The cold-rolled steel sheet according to [1], further containing B: 0.0003 to 0.0010% by mass.

  [3] The steel having the composition described in [1] or [2] is heated to 1150 ° C. to 1250 ° C. and then hot-rolled at a finish rolling end temperature: 880 to 930 ° C., and an average cooling rate: 10 ° C. Cooling at / s or more and less than 30 ° C / s, winding at winding temperature: 655 to 685 ° C, pickling, cold rolling at a reduction ratio of 50% or more, and heating rate: 10 ° C / s or less A method for producing a cold-rolled steel sheet, comprising: annealing at 90 to 180 s after heating to 750 to 800 ° C., and subsequently performing an overaging treatment at 280 to 350 ° C. for 240 s or more.

  According to the present invention, a thin high-strength cold-rolled steel sheet having excellent workability such as elongation, for example, a cold-rolled steel sheet having good workability having a TS of 1.5 mm or less and a TS of 590 MPa or more can be manufactured at low cost. Became. The steel sheet of the present invention is suitable for automobile frame parts and the like.

  In the present invention, a large number of fine plate-like TiC having a side of less than 15 nm in the recovered / recrystallized ferrite phase is precipitated with a Ti precipitation rate of 90% or more, thereby increasing the strength without deteriorating the elongation characteristics. I am trying. Instead of carbides containing Ti and Mo as disclosed in Patent Document 2, it is not necessary to use expensive Mo because only TiC is used, and Si and Mn frequently used as solid solution strengthening elements are reduced. By doing so, the alloy cost is greatly reduced.

  In the present invention, the most important point in maintaining the strength / elongation balance after annealing is to sufficiently advance carbide precipitation at the stage of the hot-rolled steel sheet. In the present invention, by optimizing the balance between the amount of C and the amount of Ti and setting the coiling temperature within a predetermined range, TiC at the stage of the hot-rolled steel sheet was successfully aligned in a plate-like form with a thickness of about 5 nm. . Ensuring this initial precipitation form is an important point that the work structure formed by cold rolling is rapidly recovered and recrystallized by annealing after cold rolling of 50% or less mainly for adjusting the plate thickness. is there. Finally, plate-like TiC with a side length of around 15 nm is dispersed in the recovered / recrystallized ferrite with a recrystallization rate of 70% or more, and a TS of 590 MPa or more and an elongation characteristic of about 20% or more are obtained. This realizes a steel sheet that achieves both high strength and good workability.

  TiC that has a plate-like form at the stage of the hot-rolled steel sheet and the parent phase ferrite has a Baker-Nutting orientation relationship (Baker-Nutting orientation relationship: the parent phase Fe having a body-centered cubic structure (bcc)). , (100) bcc // (100) MC, [001] bcc // [011] MC), which is a typical orientation relationship when NaCl type MC carbide precipitates in a coherent manner. In general, in the annealing process after cold rolling, TiC is subjected to Ostwald growth, whereby the precipitation strengthening ability of TiC is drastically reduced. For this reason, as mentioned above, in anticipation of a decrease in strength at the time of annealing in advance, the precipitation of TiC at the hot-rolled steel plate stage should be made fine enough to increase the strength of the hot-rolled steel plate as much as possible. However, it was considered effective in increasing the strength of the cold-rolled steel sheet. Therefore, in order to improve the delay of recrystallization due to fine precipitates, optimization of the rolling rate aimed at promoting recrystallization by increasing the cold rolling rate was investigated. As a result, the phenomenon that TiC precipitated in the hot-rolled steel sheet is re-dissolved or refined as the cold rolling rate increases, and TiC finely precipitated in the hot-rolled steel sheet is mechanically cooled by cold rolling. It was found that the material was further refined and the recovery and recrystallization were significantly delayed. As a result of further investigation by the inventors based on this knowledge, in the hot-rolled steel sheet, TiC is grown to a stable size where it can exist stably, suppressing the refinement of TiC in cold rolling, By making recovery and recrystallization possible, the strength of the resulting cold-rolled steel sheet is not greatly reduced from the initial strength (strength of the hot-rolled steel sheet), and has a tensile strength of 590 MPa or more and good elongation. The conclusion that the present invention is effective in obtaining a cold-rolled steel sheet having excellent workability and has completed the present invention.

  Details of the present invention will be described below. Note that “%” representing the content of each component element means “% by mass” unless otherwise specified.

1) Component composition C: 0.04 to 0.06%
C is an important element that precipitates finely in the ferrite phase as TiC and contributes to high strength. If the amount of C is less than 0.04%, sufficient strength cannot be obtained at the stage of hot-rolled steel sheets, and TS of 590 MPa or more cannot be secured after cold rolling annealing. On the other hand, if it exceeds 0.06%, a large amount of fine TiC precipitates in the hot-rolled steel sheet, the strength of the hot-rolled steel sheet becomes too high, and recrystallization is significantly delayed in the annealing after cold rolling, The transformation structure such as cementite or bainite increases and the workability is lowered. Therefore, the C content is 0.04 to 0.06%.

Si + Al: 0.05-0.10%
Si and Al are elements (deoxidation elements) necessary for deoxidation during steelmaking. Since this steel is used to precipitate Ti, which is also a deoxidizing element, as TiC, deoxidation with Si and Al is essential. For this reason, it is necessary to make Si + Al which is the total content of Si and Al 0.05% or more. On the other hand, both Al and Si are elements that raise the A3 transformation temperature. If the total content exceeds 0.10%, it is necessary to increase the hot rolling finishing temperature in order to avoid two-phase region rolling. For this reason, the slab heating temperature before hot rolling must be increased, resulting in high manufacturing costs. Furthermore, when the transformation point rises, it is necessary to increase the coiling temperature in order to deposit TiC, leading to coarsening of TiC. Therefore, the Si amount + Al amount is set to 0.10% or less.
The Si amount is preferably 0.03% or more in order to obtain the effect of deoxidation. On the other hand, since it is one of the elements that adversely affects the plating properties, the Si amount should be 0.08% or less. preferable. In order to obtain the deoxidation effect, the Al amount is preferably 0.02% or more. However, even if added in excess, the deoxidation effect is saturated and the transformation point is unnecessarily increased. The following is preferable.

Mn: 0.15 to 0.45%
Mn is an element with a high solid solution strengthening ability, so it is effective for adjusting the strength level of steel, and for reducing the transformation temperature of the steel, so it is effective for lowering the heating temperature before hot rolling. Therefore, the Mn content is 0.15% or more. On the other hand, excessive addition increases the cost and tends to form a band-like structure due to segregation, leading to a decrease in elongation and further a decrease in stretch flangeability, so the Mn amount is set to 0.45% or less.

P: Less than 0.01% P is an effective element frequently used as a solid solution strengthening element in increasing the strength of a steel sheet. On the other hand, it often segregates at the grain boundaries, leading to embrittlement during processing. Moreover, in galvanized steel sheets, alloying is significantly delayed. However, if the amount of P is less than 0.01%, these workability and plating properties are not adversely affected. In the present invention, the strength of the cold-rolled steel sheet is achieved by precipitation of TiC, and workability is emphasized. Therefore, the P content is less than 0.01%, which does not affect the plateability and workability.

S: 0.004% or less In the present invention, S is mainly fixed as TiS and leads to a decrease in Ti ^* (effective Ti amount) defined by the formula (1). Up to about 004% is acceptable. For this reason, the upper limit of the amount of S is made 0.004%.

N: 0.0045% or less When the N amount exceeds 0.0045%, coarse TiN is formed, resulting in a decrease in the effective Ti amount, which will be described later, and precipitation strengthening due to TiC cannot be sufficiently exhibited, and the elongation decreases. Furthermore, stretch flangeability is also reduced. Therefore, the N amount is set to 0.0045% or less. On the other hand, in order to control the amount of N to less than 0.0020%, a special process is required in the steel making process, resulting in an increase in production cost. Therefore, the lower limit of the amount of N is preferably about 0.0020%.

Ti: 0.12-0.16%
In the present invention, Ti is an element that precipitates TiC and is the most important element used for precipitation strengthening. In the present invention, the lower limit of the Ti amount is set to 0.12% in order to ensure a sufficient precipitation strengthening amount at the stage of the hot-rolled steel sheet. On the other hand, when Ti is added excessively, the structure recovery at the time of annealing due to solute Ti and a large amount of TiC, recrystallization is delayed, so high temperature annealing is required, and the target strength cannot be reached by TiC coarsening, The upper limit is 0.16%.

1.2 ≦ 4C / Ti ^* ≦ 2.4 (where Ti and C are the contents of each element)
In the present invention, the amount of precipitation as TiC is governed by the effective amount of Ti in consideration of the amount fixed as TiN and the amount fixed as TiS among the amount of Ti contained, that is, Ti ^* . Here, the effective Ti amount (Ti ^* ) is defined by the above formula (1), and Ti ^* = Ti− (3.43N + 1.5S) (where Ti, N, and S are the contents of each element) ).
In the present invention, in order to optimize the size of TiC, it is necessary to satisfy the relationship of 1.2 ≦ 4C / Ti ^* ≦ 2.4. Here, 4C / Ti ^* represents the atomic concentration ratio of C and effective Ti. In the present invention, 4C / Ti ^* = 1, which is the minimum necessary when all the C in the steel is formed as TiC. Also, 1.2 ≦ 4C / Ti ^* , which is an excess of carbon. This is to utilize the delay of the γ → α transformation caused by solute carbon, and as a result, to lower the TiC precipitation temperature range and to precipitate and refine TiC in the hot-rolled steel sheet. On the other hand, on the carbon excess side where 4C / Ti ^* exceeds 2.4, precipitation and refinement of TiC further progresses, and the strength of the hot-rolled steel sheet becomes too high, so 4C / Ti ^* ≦ 2.4 needs to be satisfied. .

  The balance is Fe and inevitable impurities, but it is preferable to contain B: 0.0003 to 0.0010% for the following reason.

B: 0.0003 to 0.0010%
B is an element effective in making the initial TiC precipitation fine in the stage of a hot-rolled steel sheet in the present invention. In order to obtain this effect, the B content needs to be 0.0003% or more. On the other hand, if the amount of B exceeds 0.0010%, not only will the effect be saturated, but also the load during hot rolling will increase, so the amount of B is made 0.0010% or less. In the present invention, when B is less than 0.0003% (B <0.0003%), B is an inevitable impurity.

2) Microstructure The cold rolled steel sheet of the present invention has a ferrite phase volume ratio of 90% or more, a cementite volume ratio of less than 5%, and a recrystallization rate of the ferrite phase of 70% or more. Ti carbide (TiC) is precipitated, and the average length of the Ti carbide is less than 15 nm, and the Ti precipitation rate is 90% or more.

Volume ratio of ferrite phase: 90% or more, recrystallization rate of ferrite phase: 70% or more In the present invention, TiC is precipitated in the ferrite phase, and desired steel sheet characteristics are obtained by utilizing precipitation strengthening by TiC. . In order to ensure good workability, the recrystallization rate of the ferrite phase needs to be 70% or more. When the recrystallization rate of the ferrite phase is less than 70%, not only good elongation characteristics cannot be obtained, but also the elongation flange characteristics often required for automobile members deteriorate. On the other hand, when the volume fraction of the ferrite phase having a recrystallization ratio of 70% or more is less than 90%, the influence of the phases other than the ferrite phase is increased, and good elongation characteristics cannot be obtained, and the workability is excellent. A rolled steel sheet cannot be obtained. For this reason, the volume ratio of the ferrite phase is 90% or more, and the recrystallization ratio of the ferrite phase is 70% or more.

  Here, the recrystallization rate is determined by observing a cross-sectional structure (L cross-sectional structure) along the rolling direction with a scanning transmission electron microscope (STEM) for the cold-rolled steel sheet after annealing, and equiaxial with respect to the area of all ferrite in the observation field. The ratio of the area of the recrystallized ferrite (area ratio) was obtained, and this was used as the recrystallization ratio.

Cementite volume fraction: less than 5% Cementite having a volume fraction of less than 5% appears unavoidably, and if this amount (less than 5%) is used, the material is not particularly affected. The cementite present in the hot-rolled steel sheet is once solid-dissolved by annealing after cold rolling, but solid-solution carbon is fixed by an overaging treatment described later, which is useful for securing elongation characteristics. On the other hand, if the volume fraction of cementite becomes too large, the precipitation of TiC will be suppressed, and the desired ferrite content cannot be secured, so the cementite volume fraction is set to less than 5%. The volume fraction of cementite may be 0%.

When the average length of Ti carbides in the ferrite phase is less than 15 nm and the precipitation rate of Ti is 90% or more When the average length of Ti carbides precipitated in the ferrite is 15 nm or more, the precipitation strengthening ability of TiC decreases. It becomes difficult to obtain a strength of 590 MPa or more. Moreover, if the precipitation rate of Ti is less than 90%, the amount of precipitates is insufficient, and it becomes difficult to obtain a strength of 590 MPa or more. In the steel sheet of the present invention, in order to obtain a strength of 590 MPa or more by precipitating many fine Ti carbides (TiC) in ferrite, the average length of Ti carbides (TiC) is less than 15 nm, and the Ti precipitation rate Is 90% or more.

  The average length of Ti carbide (TiC) is a value obtained by arithmetically averaging the lengths of TiC observed in the [001] orientation of the ferrite phase as a matrix with respect to 200 or more by a transmission electron microscope. . In the present invention, the TiC plate-like form deposited in a Baker-Nutting orientation relationship with the parent phase ferrite at the stage of the hot-rolled steel sheet is substantially maintained even in the cold-rolled steel sheet. For this reason, the TiC observed in the [001] orientation of the ferrite phase with a transmission electron microscope in the cold-rolled steel sheet is a TiC side length (one side length) having a square plate-like form deposited on the hot-rolled steel sheet. ) Is the length, and the thickness is the width, and is observed as a substantially rectangular shape (rod-like shape). In the present invention, the length of one side of Ti carbide in the cold-rolled steel sheet is measured by measuring the length of TiC observed in this substantially rectangular shape.

The Ti precipitation rate was determined by the following method.
First, the amount of Ti deposited was determined according to the method described in Patent Document 4. In other words, since the target precipitate is very fine, accuracy is not obtained in the general method for determining the amount of precipitate directly quantifying the extracted precipitate. After only electrolysis, the remainder of the metal sample was removed from the electrolytic solution, and then a part of this electrolytic solution was collected and used as an analytical solution, and the concentration of Ti as a comparative element and Fe in the liquid was measured using ICP mass spectrometry. Based on the obtained concentration, the concentration ratio of Ti to Fe was calculated, and further, the amount of solid solution Ti (% by mass) was obtained by multiplying by the amount of Fe (% by mass) in the sample. The amount (% by mass) of Fe in the sample can be obtained by subtracting the total of composition values other than Fe from 100% by mass. The precipitation amount of Ti was calculated | required by subtracting the amount of solid solution Ti calculated | required in this way from Ti content of steel. And the precipitation rate of Ti was calculated | required as Ti precipitation rate = (Ti precipitation amount) / (Ti content of steel).

  The remaining structure other than those specified above is a transformation structure such as bainite, martensite, and retained austenite. However, with respect to such a remaining structure, if the volume ratio is 10% or less, the effect of the present invention is impaired. Since it is not a thing, you may contain.

3) Manufacturing conditions The manufacturing method of the cold rolled steel sheet of this invention is demonstrated. In the present invention, the steel having the above-described composition is heated to 1150 ° C. to 1250 ° C., and then hot-rolled at a finish rolling finish temperature of 880 to 930 ° C., and an average cooling rate of 10 ° C./s to 30 ° C./s. After cooling at a temperature below 655 to 685 ° C., pickling, cold rolling at a reduction rate of 50% or more, and a temperature increase rate of 10 ° C./s or less to 750 to 800 ° C. After heating, annealing is performed for 90 to 180 s, followed by over-aging treatment for 280 to 350 ° C. for 240 s or more.

Heating temperature before hot rolling: 1150 ° C to 1250 ° C
In order to increase the strength by precipitating fine TiC in the ferrite phase after hot rolling, hot rolling the coarse TiC precipitated in the steel in steel materials such as steel slabs used for hot rolling. It must be heated and dissolved before. In the present invention, TiC precipitated in the steel before hot rolling can be sufficiently dissolved by heating at 1150 ° C. or higher. Therefore, the heating temperature before hot rolling is 1150 ° C. or higher. In addition, it is necessary to heat at the temperature of 1250 degrees C or less from a viewpoint of reducing manufacturing cost. Therefore, heating temperature shall be 1150 degreeC-1250 degreeC. A more preferable heating temperature range is 1150 ° C to 1200 ° C.

Finishing rolling finish temperature of hot rolling: 880 ° C to 930 ° C
When the finish rolling finish temperature is less than 880 ° C., the rolled structure remains, the elongation deteriorates, and the stretch flangeability also deteriorates. Accordingly, the finish rolling end temperature is set to 880 ° C. or higher. On the other hand, when the finish rolling finish temperature exceeds 930 ° C., the transformation is from a sufficiently recrystallized austenite structure, so that the ferrite structure becomes coarse and the strength of the ferrite phase as the matrix structure is significantly reduced. For this reason, finish rolling completion temperature shall be 930 degrees C or less.

  In addition, in this invention, the technique of the direct feed rolling which hot-rolls the steel (steel raw material) after continuous casting as it is can also be applied. At this time, in order to secure a finish rolling finish temperature of 880 ° C. or higher, auxiliary heating can be performed before hot rolling.

Average cooling rate after hot rolling: 10 ° C./s or more and less than 30 ° C./s If the average cooling rate until winding after hot rolling is less than 10 ° C./s, pearlite transformation is likely to occur during cooling, The formation of grain boundary cementite is prioritized, and the formation of TiC, which is a precipitation strengthening factor, is insufficient, so that the required strength in the hot-rolled steel sheet cannot be ensured, and consequently the desired strength cannot be ensured in the cold-rolled steel sheet. In the present invention, in order to ensure proper initial nucleation of TiC so that TiC is stably present during cold rolling that is subsequently performed after hot rolling and does not grow rapidly during annealing, The average cooling rate is 10 ° C./s or more and less than 30 ° C./s.

Winding temperature: 655-685 ° C
In the present invention, it is important not to increase the strength excessively at the stage of the hot-rolled steel sheet, and the optimum amount and size of TiC is precipitated in the steel at the stage of winding after hot rolling. For this reason, it is necessary to control the coiling temperature with high accuracy. The precipitation of TiC is greatly affected by the γ → α transformation, and it is particularly important to control the coiling temperature in accordance with the Mn content that lowers the transformation point. In the present invention, as described above, the amount of Mn is reduced to 0.15 to 0.45%, and it is necessary to set the coiling temperature to 600 ° C. or higher for TiC precipitation. In order to stably precipitate TiC in steel and ensure the strength of the hot-rolled steel sheet, specifically, to ensure the strength of the hot-rolled steel sheet of 650 MPa or more, the lower limit of the coiling temperature is 655 ° C. On the other hand, when the coiling temperature exceeds 685 ° C., the recovery of the ferrite structure and the coarsening of TiC are promoted, and the strength of the hot-rolled steel sheet is lowered. Therefore, it is possible to ensure the strength after the subsequent cold rolling and annealing. It becomes impossible. Therefore, the winding temperature is 655 to 685 ° C.

  The steel sheet after winding is pickled according to a conventional method. The conditions for pickling are not particularly limited, and may be performed according to a conventionally known method such as pickling with hydrochloric acid.

Cold rolling reduction: 50% or more The hot-rolled steel sheet after pickling is subjected to cold rolling in order to reduce the thickness and obtain a thin steel sheet (cold-rolled steel sheet) having a desired thickness. For example, in producing a thin steel sheet having a thickness of 1 mm or less, if the thickness of the hot-rolled steel sheet after pickling is 2.0 mm, the cold rolling rate (cold rolling reduction) needs to be 50% or more. It becomes. In general, in a steel sheet excessively precipitation strengthened by TiC less than several nanometers, recrystallization after cold rolling is significantly delayed, so that high temperature annealing is essential, and as a result, there is a great concern that the final strength is lowered. In the hot-rolled steel sheet of the present invention, the average precipitation size of TiC is controlled to about 10 nm, and these initial precipitation forms are not greatly changed by rolling. For this reason, it is important to promote recrystallization by strain accumulation during rolling, and the lower limit of the cold rolling reduction is set to 50% in order to reduce the annealing temperature.

  The steel sheet after the cold rolling is heated to 750 ° C. to 800 ° C. at a rate of temperature increase of 10 ° C./s or less, and then annealed for 90 to 180 s, and subsequently maintained at 280 ° C. to 350 ° C. for 240 s or more. Apply aging treatment. In the present invention, annealing under appropriate conditions is extremely important for achieving both strength and elongation characteristics. Although the steel plate adjusted to the desired plate thickness by cold rolling has high strength, the workability is lowered, and it is necessary to perform an appropriate annealing treatment by a continuous annealing process or the like. The annealing treatment is performed to recover the processed structure, and these changes in structure are largely controlled by the annealing conditions as well as the cold rolling rate.

Annealing conditions: heating rate: 10 ° C./s or less, heated to 750 ° C. to 800 ° C., and maintained for 90 to 180 s In the present invention, the annealing temperature is set to 750 ° C. to 800 ° C., and is heated to the annealing temperature. This annealing temperature range is an appropriate annealing temperature for obtaining a ferrite structure having a recrystallization rate of 70% or more after cold rolling of 50% or more. When the annealing temperature is less than 750 ° C., recrystallization does not proceed, the strength becomes too high, and the elongation becomes small. On the other hand, if the annealing temperature exceeds 800 ° C., the coarsening of ferrite grains and the coarsening of TiC, which is a precipitation strengthening factor, progress rapidly, and the strength suddenly decreases, making it difficult to ensure TS ≧ 590 MPa.
Further, the holding time at the annealing temperature (annealing time) is set to 90 s (90 seconds) or longer in order to sufficiently recover in the annealing temperature range and secure the elongation characteristics. On the other hand, if the annealing time is too long, the productivity is lowered, so the annealing time is 180 s or less.
The heating rate at the time of heating to the above-described annealing temperature is set to 10 ° C./s or less in order to promote the tissue recovery during the heating. When the rate of temperature rise exceeds 10 ° C./s, the reverse transformation to γ is partially promoted, and a hardened structure is included, leading to a decrease in elongation. In addition, although the minimum of a temperature increase rate is not prescribed | regulated in particular, it is preferable to set it as 5 degrees C / s or more from a viewpoint of ensuring productivity, and avoiding the growth of the fine TiC in an initial stage hot-rolled steel plate.

Overaging treatment: 280 ° C. or higher and 350 ° C. or lower and holding for 240 s or longer In the present invention, 1.2 ≦ 4C / Ti ^* ≦ 2.4 is designed to be excessive in carbon and slightly precipitated at the stage of hot-rolled steel sheet. The cementite partially dissolves in the annealing process after the cold rolling. When solute carbon is present in the steel sheet, not only the workability is affected, but also aging after processing becomes a problem. For this reason, in order to fix the solid solution carbon to cementite, an overaging treatment is performed after annealing. When the temperature of the overaging treatment is less than 280 ° C., it takes a long time to fix the solid solution carbon, and thus the productivity is lowered. On the other hand, when the temperature of the overaging treatment exceeds 350 ° C., the temperature is too high, and it becomes difficult to sufficiently fix the carbon. For this reason, the temperature of an overaging process shall be 280 degreeC or more and 350 degrees C or less. Further, in order to sufficiently fix the solute carbon, the holding time in overaging is set to 240 s or more. Note that even if the holding time is excessively long, the effect is saturated and the productivity is lowered, so the holding time is preferably 600 s or less. In addition, it is preferable to perform the above-described annealing and overaging treatment continuously in a continuous annealing line from the viewpoint of productivity.

  Steel having the component composition shown in Table 1 was melted in a 50 kg vacuum melting furnace to form a steel slab, and then hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Next, this hot-rolled steel sheet was pickled with hydrochloric acid to obtain a steel sheet having a thickness of 2.5 mm. After this was cold-rolled under the conditions shown in Table 2, it was heated to the annealing temperature shown in Table 2 at the rate of temperature rise shown in Table 2, and subjected to annealing treatment for 90 to 180 s, followed by 300 s at 350 ° C. Was over-aged to obtain a cold-rolled steel sheet. Test pieces were sampled from the obtained cold-rolled steel sheets and investigated as follows. The obtained results are shown in Table 3.

Volume ratio of ferrite phase and cementite The cross-sectional structure (L cross-sectional structure) along the rolling direction was observed with a scanning electron microscope at 3000 magnifications in three fields of view, and the area ratio of the ferrite phase and cementite was obtained to obtain the volume ratio.

Recrystallization rate of ferrite phase Using a sample collected from the central part of the plate thickness, a thin film sample for a transmission electron microscope was prepared, and a dark field method of a scanning transmission electron microscope (STEM) attached to the transmission electron microscope By observing 5 fields at a magnification of 5000, the ratio (area ratio) of the recrystallized ferrite area to the total ferrite area in the observed field was determined, and this was defined as the recrystallization ratio of the ferrite phase. Here, the recrystallized ferrite is recognized as an equiaxed crystal with few dislocations in the STEM image.

Average length of Ti carbide Using the sample used for the STEM observation described above, a transmission electron microscope was used to observe five fields of a region of 300,000 times and a thickness of about 100 nm to measure the length of TiC. At this time, the TiC to be evaluated is a plate-like form TiC having a thickness of 1 to 2 nm observed in the [001] direction of the ferrite phase as a parent phase, and the length of each TiC observed in a substantially rectangular shape. Was measured and obtained as an average value of 200 or more observation results.

Tensile properties JIS No. 5 tensile test specimens were taken from the cold-rolled steel sheet produced as described above in parallel with the rolling direction, and subjected to a tensile test at a crosshead speed of 10 mm / min in accordance with JISZ2241, yield strength (YS ), Tensile strength (TS) and total elongation (El). Moreover, the tensile test was similarly performed about the hot-rolled steel plate, and the tensile strength (TS) was calculated | required.

  From the results shown in Table 3, the present invention example in which the component composition and production conditions are within the scope of the present invention has a tensile strength of 590 MPa or more and a total elongation of 20% or more, and both high strength and good workability are compatible. It turns out that it is a cold-rolled steel plate. In addition, the structure of the cold-rolled steel sheet was bainite except for the ferrite and cementite shown in Table 3.

  On the other hand, steel plate No. A1 has a small amount of C and a small precipitation rate of TiC, and an absolute precipitation amount of TiC is insufficient, so that the tensile strength is less than 590 MPa. Steel plate No. Since B1 has a large amount of C, the strength of the hot-rolled steel sheet becomes excessive, the recrystallization rate of the annealed sheet decreases, and high elongation cannot be obtained. Steel plate No. F1, No. G1 and No. J1 is inadequate in workability because the amount of Ti and the amount of Mn are not appropriate, so that the recrystallization is not sufficiently promoted and the total elongation is less than 20%. Steel No. H1 and No. I1 has a high transformation point, and TiC growth in the hot-rolled steel sheet cannot be suppressed, and the TiC size in the cold-rolled steel sheet after annealing becomes coarse and the strength decreases. Steel No. C2, D2 and E2 are out of hot rolling conditions, and precipitation and structure optimization are not achieved. Steel No. E3 has a high annealing temperature, leading to insufficient strength. Steel no. K2 has a low annealing temperature, insufficient recrystallization promotion, and is inferior in workability.

Claims (3)

% By mass
C: 0.04 to 0.06%,
Si + Al: 0.05 to 0.10%,
Mn: 0.15 to 0.45%,
P: less than 0.01%,
S: 0.004% or less,
N: 0.0045% or less,
Ti: 0.12 to 0.16% is contained,
The balance is composed of Fe and inevitable impurities, and Ti ^* defined by the following formula (1) has a composition satisfying the following formula (2), the volume fraction of the ferrite phase: 90% or more, the volume of cementite Rate: less than 5%, recrystallization rate of the ferrite phase: 70% or more, Ti carbide (TiC) is precipitated in the ferrite phase, the average length of the Ti carbide is less than 15 nm, the precipitation rate of Ti A cold-rolled steel sheet having a structure of 90% or more and a tensile strength of 590 MPa or more;
Ti ^* = Ti− (3.43N + 1.5S) (1)
1.2 ≦ 4C / Ti ^* ≦ 2.4 (2)
However, Ti, N, S, and C in the formula represent the content (mass%) of each element.
Further, the average length of Ti carbide is a value obtained by arithmetically averaging the length of TiC observed in the [001] orientation of the ferrite phase as a matrix with respect to 200 or more by a transmission electron microscope.   The cold-rolled steel sheet according to claim 1, further comprising, by mass%, B: 0.0003 to 0.0010%.   The steel having the composition according to claim 1 or 2 is heated to 1150 ° C to 1250 ° C, and then hot-rolled at a finish rolling finish temperature of 880 to 930 ° C, and an average cooling rate of 10 ° C / s to 30 ° C / s. Cooling at less than s, winding at winding temperature: 655-685 ° C., pickling, cold rolling at a reduction rate of 50% or more, heating rate: 750 ° C.-800 ° C. at 10 ° C./s or less The method of manufacturing a cold-rolled steel sheet is characterized by performing annealing for 90 to 180 s after heating and subsequently performing an overaging treatment for holding for 240 s or more at 280 ° C. or more and 350 ° C. or less.