JPWO2014057519A1 - Cold-rolled steel sheet having excellent shape freezing property and manufacturing method thereof - Google Patents

Abstract

A cold-rolled steel sheet having excellent shape freezing properties and a method for producing the same are provided. In mass%, C: 0.0010 to 0.0030%, Si: 0.05% or less, Mn: 0.1 to 0.5%, Ti: 0.021 to 0.060%, B: 0.0005 to 0.0050%, and B and C, A steel material having a composition contained so as to satisfy 0.5 or more is subjected to finish rolling at a finish rolling finish temperature of 870 to 950 ° C and wound at a winding temperature of 450 to 630 ° C, and a cold rolling reduction ratio : 90% or less of the cold rolling process, and after the cold rolling process, the temperature range of 600 ° C. or higher is heated to a soaking temperature in the range of 700 to 850 ° C. at an average heating rate of 1 to 30 ° C./s, 30 After holding for ˜200 s, an annealing step is performed in which the temperature range up to 600 ° C. is cooled at an average cooling rate of 3 ° C./s or more. Thereby, a cold-rolled steel sheet having a structure mainly composed of ferrite having an average particle diameter of 10 to 30 μm and having a proportional limit of 100 MPa or less and excellent in shape freezing property is obtained.

Description

  The present invention relates to a cold-rolled steel sheet excellent in formability and suitable for use as a member for parts having severe shape accuracy in the fields of electric machinery, automobiles, building materials, and the like, and in particular, has a shape fixability. Regarding improvement.

In recent years, in order to preserve the global environment, reduction of automobile fuel consumption has been demanded from the viewpoint of reducing the amount of CO ₂ emissions. In response to such demands for reducing fuel consumption, the weight reduction of automobile bodies is aimed at, and coupled with the demand for cost reduction, there is a growing demand for reducing the amount of steel used by reducing the thickness of steel used. However, if the thickness of the steel (steel plate) is reduced, the rigidity of the parts decreases, and problems such as part deflection, stickiness, and warp become obvious. Furthermore, in the field of home appliances such as AV and OA equipment, the demand for the dimensional accuracy of parts has become stricter, and the demand for a steel sheet having excellent shape freezing properties has been increasing.

In response to such a demand, for example, Patent Document 1 describes a ferritic thin steel sheet having excellent shape freezing properties. In the technique described in Patent Document 1, in mass%, C: 0.0001 to 0.05%, Si: 0.01 to 1.0%, Mn: 0.01 to 2.0%, P: 0.15% or less, S: 0.03% or less, Al: 0.01 %, N: 0.01% or less, O: 0.007% or less of steel composition with hot rolling at 950 ° C or less Ar ₃ transformation point or more total rolling reduction of 25% or more and 950 ° C or less The hot rolling is finished at the Ar ₃ transformation point or higher so that the friction coefficient in the steel is 0.2 or less, and after cooling, it is wound up at a temperature below a predetermined critical temperature, and thus {100} parallel to the plate surface It is said that a thin steel sheet having a ratio of the plane to the {111} plane of 1.0 or more can be obtained. In such a thin steel plate, the slip system during bending can be controlled, and spring back can be suppressed in forming mainly bending.

  Patent Document 2 describes a press molding method with excellent dimensional accuracy of a molded product. In the technique described in Patent Document 2, a steel plate having a ratio of {100} plane and {111} plane parallel to the plate surface of 1.0 or more is used, and a material tensile strength of 40 is applied to the vertical wall portion of the hat-shaped member. A press molding method that performs molding while applying a tensile stress of ˜100% and that is excellent in dimensional accuracy of a molded product is described. According to the technique described in Patent Document 2, a hat bending workability is remarkably improved, a springback amount is small, and a member excellent in shape freezing property can be provided.

International Publication WO 00/06791 JP 2002-66637 A

  However, in the technique described in Patent Document 1, when press molding other than bending is performed, the degree of improvement in shape freezing property is small, and also in the case of bending, due to the effects of grain boundary sliding and the like. There was a problem that the springback might become large. In addition, in the technique described in Patent Document 2, when press molding other than hat molding is performed, there is no effect of increasing the dimensional accuracy of the molded product. In order to apply the stress, it is necessary to increase the wrinkle pressing pressure. Therefore, it is necessary to greatly increase the capacity of the press machine, leading to an increase in cost.

  An object of the present invention is to solve such problems of the prior art, and particularly to provide a cold-rolled steel sheet excellent in shape freezing property and a manufacturing method thereof, in which a large distortion does not occur in a flat part after forming.

In order to achieve the above-mentioned object, the present inventors diligently studied the factors affecting the shape freezing property, in particular, the distortion of the flat part after molding. As a result, it has been conceived that the deformation of the flat part after forming is greatly influenced by the proportional limit of the steel sheet used. In particular, when the proportional limit exceeds 100 MPa, it has been found that the deformation of the flat part after molding is significantly increased. As a result of further research, in order to set the proportional limit to 100 MPa or less, as a composition containing Ti and B as an essential component in an extremely low carbon system, the ratio of B content to C content, B / C , Found that it is necessary to adjust to satisfy 0.5 or more.

  First, the experimental results on which the present invention is based will be described.

  Steel with a composition containing 0.0010 to 0.0035% C, 0.01 to 0.03% Si, 0.10 to 0.45% Mn, 0.03 to 0.08% Al, 0.022 to 0.060% Ti, 0.0003 to 0.0048% B, and 0.0015 to 0.0040% N by mass The material (slab) was subjected to hot rolling, cold rolling, and annealing with various heating soaking and cooling conditions changed to obtain a cold-rolled annealing plate.

  From the obtained cold-rolled annealed plate, a JIS No. 5 test piece was sampled so that the tensile direction was the rolling direction, and the proportional limit was obtained. A strain gauge with a length of 5 mm is affixed to the parallel part of the tensile test piece, a tensile test is performed at a tensile speed of 1 mm / min, and the stress at which the slope of the stress-strain curve begins to decrease is proportionally limited. It was.

  Further, a test material (size: 120 × 120 mm) was collected from the obtained cold-rolled annealed plate and subjected to stretch forming. The overhang forming was press forming in which a central portion of the test material was overlaid by 8 mm with a ball head punch having a diameter of 20 mm. In the stretch forming, as shown in FIG. 1, the region (shaded portion) having a diameter of 28 to 54 mm was pressed with a load of 100 kN. Next, as schematically shown in FIG. 2, the molded test material was placed on a surface plate, and the maximum distortion height of the flange portion was measured. In addition, although the structure | tissue was observed about the obtained cold-rolled annealing board, all the cold-rolling annealing boards were the structures | tissues which have ferrite as a main component.

  The obtained results are shown in FIGS. FIG. 3 shows the relationship between the maximum distortion height of the flange portion and the proportional limit, and FIG. 4 shows the relationship between the proportional limit and B / C.

  FIG. 3 shows that when the proportional limit exceeds 100 MPa, the maximum distortion height of the flange portion increases rapidly. FIG. 4 also shows that B / C needs to be 0.5 or more in order to set the proportional limit to 100 MPa or less.

  For this reason, by using as a raw material a steel sheet having a composition containing Ti and B as essential, B / C being 0.5 or more, and a structure mainly composed of ferrite and having a proportional limit of 100 MPa or less. It has been found that the shape freezing property of the pressed parts is improved, and in particular, the distortion of the flat portion of the member after molding is significantly reduced. Further, according to further studies by the present inventors, the hot rolling conditions are optimized so that C is in a solid solution state, further cold rolling is performed, and further, the coarseness of B containing C and Fe during annealing. It has been found that it is effective to improve the shape freezing property by precipitating the precipitates at the grain boundaries and further within the grains. With such a structure, the coarse precipitates of B that have been dispersed and precipitated moderately fix dislocations during press working, concentrate strain around the precipitates, and prevent dislocations from concentrating on the grain boundaries. It is considered that the dislocations are prevented from being entangled with each other, which greatly reduces the springback, lowers the proportional limit, and remarkably improves the shape freezing property.

The present invention has been completed based on such findings and further investigations. That is, the gist of the present invention is as follows.
(1) In mass%,
C: 0.0010 to 0.0030%, Si: 0.05% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: 0.02% or less, Al: 0.10% or less,
N: 0.0050% or less, Ti: 0.021 to 0.060%,
B: Containing 0.0005 to 0.0050%, and containing B and C so that B / C satisfies 0.5 or more, the balance consisting of Fe and inevitable impurities, and ferrite having an average particle size of 10 to 30 μm A cold-rolled steel sheet having an excellent shape freezing property and having a proportional limit of 100 MPa or less.
(2) The cold-rolled steel sheet according to (1), further containing Nb: 0.009% or less in mass% in addition to the composition.
(3) The cold-rolled steel sheet according to (1), further containing Cr: 0.06% or less in mass% in addition to the composition.
(4) The cold-rolled steel sheet according to (1), further containing Nb: 0.009% or less and Cr: 0.06% or less in mass% in addition to the composition.
(5) The cold-rolled steel sheet according to (2), wherein the Nb content is 0.001 to 0.009% by mass.
(6) The cold-rolled steel sheet according to (3), wherein the Cr content is 0.001 to 0.06% by mass.
(7) The cold-rolled steel sheet according to (1), wherein the B / C is 0.5 or more and 5 or less.
(8) The cold-rolled steel sheet according to (7), wherein the B / C is 1.0 or more and 3.3 or less.
(9) The cold-rolled steel sheet according to (8), wherein the B / C is 1.5 or more and 3.3 or less.
(10) The cold-rolled steel sheet according to (1), wherein the proportional limit is 40 MPa or more and 100 MPa or less.
(11) The cold-rolled steel sheet according to (1), wherein the structure mainly composed of ferrite is a structure containing 95% or more of ferrite by area ratio.
(12) In the method of manufacturing a cold-rolled steel sheet, in which a hot rolling process, a pickling process, a cold rolling process, and an annealing process are sequentially performed on the steel material,
The steel material in mass%,
C: 0.0010 to 0.0030%, Si: 0.05% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: 0.02% or less, Al: 0.10% or less,
N: 0.0050% or less, Ti: 0.021 to 0.060%,
B: 0.0005-0.0050%
And B and C are contained so that B / C satisfies 0.5 or more, and a steel material having a composition composed of the balance Fe and inevitable impurities,
The hot rolling step is a step of heating the steel material, subjecting it to rough rolling and finish rolling to a finish rolling finish temperature of 870 to 950 ° C, and winding at a winding temperature of 450 to 630 ° C.
The cold rolling step is a step of performing cold rolling with a rolling reduction of 90% or less,
The annealing step is heated to a soaking temperature in the range of 700 to 850 ° C. at a heating rate of 1 to 30 ° C./s on average over a temperature range of 600 ° C. or more and held at the soaking temperature for 30 to 200 s. After that, the temperature range up to 600 ℃ on average, it is a process of cooling at a cooling rate of 3 ℃ / s,
A method for producing a cold-rolled steel sheet having excellent shape freezing properties.
(13) The method for producing a cold-rolled steel sheet according to (12), further containing Nb: 0.009% or less in mass% in addition to the composition.
(14) The method for producing a cold-rolled steel sheet according to (12), further containing, in addition to the above composition, Cr: 0.06% or less by mass%.
(15) The method for producing a cold-rolled steel sheet according to (12), further containing Nb: 0.009% or less and Cr: 0.06% or less in mass% in addition to the composition.
(16) The method for producing a cold-rolled steel sheet according to (13), wherein the Nb content is 0.001 to 0.009% by mass.
(17) The method for producing a cold-rolled steel sheet according to (14), wherein the Cr content is 0.001 to 0.06% by mass.
(18) The method for producing a cold-rolled steel sheet according to (12), wherein the B / C is 0.5 or more and 5 or less.
(19) The method for producing a cold-rolled steel sheet according to (18), wherein the B / C is 1.0 or more and 3.3 or less.
(20) The method for producing a cold-rolled steel sheet according to (19), wherein the B / C is 1.5 or more and 3.3 or less.

  According to the present invention, a cold-rolled steel sheet having a significantly reduced proportional limit and excellent shape freezing property after forming can be manufactured easily and inexpensively, and a remarkable industrial effect can be achieved. Moreover, according to this invention, there also exists an effect that the thinning of a member can be accelerated | stimulated.

It is explanatory drawing which shows typically the test piece for bulge forming, and the flange holding | suppressing area | region (hatched part) at the time of a forming test. It is explanatory drawing which shows typically the measuring method of the maximum distortion height after an overhanging test. It is a graph which shows the relationship between the maximum distortion height and a proportional limit. It is a graph which shows the relationship between a proportional limit and B / C.

  First, the reason for limiting the composition of the cold-rolled steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: 0.0010 to 0.0030%
C is an element that dissolves and promotes the formation of coarse precipitates of B and contributes to the reduction of the proportional limit. Such an effect becomes remarkable when the content is 0.0010% or more. On the other hand, a large content exceeding 0.0030% increases the solid solution C and carbide, increases the strength too much, and causes a decrease in ductility. For this reason, C was limited to the range of 0.0010 to 0.0030%. In addition, Preferably it is 0.0020% or less.

Si: 0.05% or less
When Si is contained in a large amount, workability deteriorates due to hardening, or Si oxide is generated during annealing, thereby impairing the plateability. In addition, when a large amount of Si is contained, the austenite (γ) → ferrite (α) transformation point is set to a high temperature, so that it is difficult to finish rolling in the γ region during hot rolling. For this reason, Si was limited to 0.05% or less.

Mn: 0.1-0.5%
Mn combines with the harmful S in steel, which significantly reduces the hot ductility, forms MnS, contributes to detoxification of S, and hardens the steel. In order to obtain such an effect, a content of 0.1% or more is required. On the other hand, a large content exceeding 0.5% suppresses deterioration of ductility due to hardening and recrystallization of ferrite during annealing. For this reason, Mn was limited to the range of 0.1 to 0.5%. In addition, Preferably it is 0.3% or less, More preferably, it is 0.2% or less.

P: 0.05% or less P has a function of segregating at grain boundaries and reducing ductility. Therefore, P is preferably reduced as much as possible in the present invention, but up to 0.05% is acceptable. Therefore, P is limited to 0.05% or less. In addition, Preferably it is 0.03% or less, More preferably, it is 0.02% or less.

S: 0.02% or less It is desirable to reduce S as an impurity element as much as possible. S has the adverse effect of significantly reducing hot ductility, inducing hot cracking, and significantly deteriorating the surface properties. Further, S does not contribute to the strength but also forms coarse MnS to form ductility. Reduce. Such a phenomenon becomes remarkable when it exceeds 0.02%, so in the present invention, S is limited to 0.02% or less. In addition, Preferably it is 0.01% or less.

Al: 0.10% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.02% or more. On the other hand, Al has the effect of increasing the γ → α transformation point of steel, so a large content exceeding 0.10% makes it difficult to finish rolling in the γ region during hot rolling. For this reason, Al was limited to 0.10% or less.

N: 0.0050% or less N is an element that combines with a nitride-forming element to form nitride and harden the steel by precipitation strengthening. A large content exceeding 0.0050% only reduces ductility. In addition, slab cracking may occur during hot rolling, and surface defects may occur frequently. For this reason, N was limited to 0.0050% or less. In addition, Preferably it is 0.0030% or less, More preferably, it is 0.0020% or less.

Ti: 0.021 to 0.060%
Ti is an element having an action of fixing N as nitride and suppressing hardening and aging deterioration due to solute N. In order to obtain such an effect, a content of 0.021% or more is required. On the other hand, a large content exceeding 0.060% promotes the precipitation of carbides and reduces the solid solution C, and therefore suppresses the formation of coarse precipitates of B containing C and Fe. Can not be achieved. For these reasons, Ti is limited to the range of 0.021 to 0.060%. In addition, Preferably, it is 0.050% or less.

B: 0.0005-0.0050%
B is an important element in the present invention, and contributes to the reduction of the proportional limit by forming coarse B precipitates. In order to acquire such an effect, 0.0005% or more needs to be contained. On the other hand, a large content exceeding 0.0050% causes slab cracking. For this reason, B was limited to the range of 0.0005 to 0.0050%. In addition, Preferably it is 0.0010% or more, More preferably, it is 0.0020% or more, More preferably, it is 0.0030% or more.

B / C: 0.5 or more In the present invention, the C and B contents are adjusted so as to include C and B in the above range, and further, the ratio of B content to C content, and B / C to satisfy 0.5 or more. To do. If B / C is less than 0.5, it becomes difficult to form coarse precipitates of B. For this reason, B / C was limited to 0.5 or more. In addition, Preferably it is 1.0 or more, More preferably, it is 1.5 or more, More preferably, it is 2.0 or more.

  The above-mentioned components are basic components, but in the present invention, in addition to the basic composition, Nb: 0.009% or less and / or Cr: 0.06% or less may be further included as a selection element as necessary. it can.

Nb: 0.009% or less
Nb, like Ti, forms a nitride by bonding with N, fixes N, suppresses hardening and aging deterioration due to solute N, and contributes to improvement of shape freezing property. Can be contained. In order to acquire such an effect, it is desirable to contain 0.001% or more. However, if it contains more than 0.009%, crystal grains become finer. For this reason, when it contains, it is preferable to limit Nb to 0.009% or less.

Cr: 0.06% or less
Cr is an element that destabilizes C in a solid solution state and promotes the formation of a coarse precipitate of B containing C, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.001% or more. On the other hand, a large amount of Cr exceeding 0.06% inhibits the formation of coarse precipitates of B containing C. For this reason, when contained, Cr is preferably limited to 0.06% or less.
The balance other than the components described above consists of Fe and inevitable impurities.

  Next, the reason for limiting the structure of the cold-rolled steel sheet of the present invention will be described.

The cold-rolled steel sheet of the present invention has a structure mainly composed of ferrite having an average particle diameter of 10 to 30 μm. By making the structure mainly composed of ferrite, the steel sheet is softened and the workability can be improved. Here, the “structure mainly composed of ferrite” refers to a structure in which ferrite (polygonal ferrite) occupies 95% or more, preferably 100% in area ratio.
The second phase other than ferrite is preferably cementite or bainite. Further, by setting the average grain size of ferrite to 10 μm or more, it is possible to suppress the concentration of strain on the grain boundary during molding, to concentrate the strain around the precipitate, and to reduce the proportional limit. On the other hand, when the average grain size of ferrite exceeds 30 μm, surface patterns such as rough skin become apparent during press working.
For this reason, the average particle diameter of the ferrite was limited to the range of 10 to 30 μm. In addition, Preferably it is 15-25 micrometers.

  Next, a preferred method for producing the cold-rolled steel sheet of the present invention will be described.

  A steel material (slab) having the above composition is used as a starting material.

  The method for producing the steel material is not particularly limited, but after melting the molten steel having the above composition in a conventional converter, electric furnace, etc., the conventional continuous casting method or ingot-splitting rolling method is used. It is preferable to use a slab (steel material). If the slab is made of continuous casting, if it has heat that can be hot-rolled after casting, it is kept in the heating furnace as it is or without being cooled to room temperature. After or after cooling to room temperature, it is preferably charged in a heating furnace and preferably reheated to a temperature in the range of 1100 to 1250 ° C. and then hot rolled.

  The heated steel material is then subjected to a hot rolling process.

  In the hot rolling step, hot rolling consisting of rough rolling and finish rolling is performed and then wound.

  In rough rolling, it is only necessary to secure a sheet bar having a desired size and shape, and the conditions are not particularly limited. Next, the sheet bar is subjected to finish rolling to form a hot-rolled sheet.

  In the finish rolling, the finish rolling finish temperature is 870 to 950 ° C.

  When the finish rolling finish temperature is lowered to less than 870 ° C, the structure changes from austenite to ferrite during rolling, and the load control of the rolling mill becomes difficult, which increases the risk of breakage or the like in the plate. . Note that if rolling is performed in the ferrite region from the entry side of finish rolling, the above-described breakage in the sheet-passing can be avoided, but due to the reduction in rolling temperature, the structure of the hot-rolled sheet becomes non-recrystallized ferrite, and cold There is a problem that the load during rolling increases. On the other hand, when the finish rolling finish temperature exceeds 950 ° C., the ferrite grain size of the hot-rolled sheet increases. For this reason, the ferrite grain size of the cold-rolled annealed plate is too large. For this reason, the finish rolling end temperature is limited to a temperature in the range of 870 to 950 ° C. After finishing rolling, the hot rolled sheet is wound up. In addition, although it does not specifically limit cooling after finishing rolling, although it is sufficient if there is a cooling rate more than air cooling, even if it performs rapid cooling of 100 degrees C / s or more as needed, a problem is especially problematic. Absent.

  The coiling temperature after finishing rolling is set to a temperature in the range of 450 to 630 ° C.

  When the coiling temperature is less than 450 ° C., acicular ferrite is generated, the steel plate becomes hard, and the subsequent cold rolling load becomes high, resulting in hot rolling operation difficulties. On the other hand, when the coiling temperature is higher than 630 ° C., the precipitation of carbide is promoted, the solid solution C amount is lowered, and a desired solid solution C amount cannot be secured in the hot rolling stage. For this reason, the coiling temperature was limited to a temperature in the range of 450 to 630 ° C.

  The wound hot rolled sheet is then subjected to a normal pickling process, followed by a cold rolling process to form a cold rolled sheet.

  In the cold rolling step, cold rolling at a cold rolling reduction ratio of 90% or less is performed to obtain a cold rolled sheet.

  When the cold rolling reduction ratio exceeds 90%, the recrystallized ferrite grains after annealing become finer, but at the same time, the cold rolling load increases, resulting in operational difficulties in cold rolling. For this reason, the cold rolling reduction ratio is limited to 90% or less. In addition, Preferably it is 80% or less. On the other hand, the lower limit of the cold rolling reduction ratio is not particularly specified, but when the cold rolling reduction ratio is small, it is necessary to reduce the thickness of the hot rolled sheet relative to the determined product thickness. Therefore, the cold rolling reduction ratio is preferably 50% or more.

  The cold-rolled sheet is then subjected to an annealing step to become a cold-rolled annealed sheet.

  In the annealing step, the temperature range of 600 ° C. or higher was heated to a soaking temperature in the range of 700 to 850 ° C. at a heating rate of 1 to 30 ° C./s on average, and held at the soaking temperature for 30 to 200 s. Then, it is set as the process of cooling to 600 degrees C or less with the cooling rate of 3 degrees C / s or more. In the annealing step, the cold-rolled processed ferrite is recrystallized to obtain a ferrite having a desired average grain size, and coarse B precipitates containing C and Fe are dispersed and precipitated in the grain boundaries and grains.

Heating rate: 1-30 ° C / s
If the average heating rate in the temperature range from 600 ° C. to the soaking temperature is less than 1 ° C./s, the ferrite grains grow remarkably, and it becomes impossible to obtain a ferrite having a desired average particle diameter. On the other hand, when the heating rate is higher than 30 ° C./s, TiC precipitates instead of forming B precipitates during heating, and it becomes difficult to form desired B coarse precipitates. For this reason, the heating rate in the temperature range of 600 ° C. or higher was limited to the range of 1 to 30 ° C./s on average. In addition, Preferably it is 5 degrees C / s or more, More preferably, it is 10 degrees C / s or more.

Soaking temperature: 700-850 ° C
In the annealing process, it is necessary to complete recrystallization of cold-worked ferrite, so the soaking temperature is 700 ° C. or higher. On the other hand, if the soaking temperature is higher than 850 ° C., the ferrite grains become coarse and it becomes impossible to obtain a ferrite having a desired average particle diameter. For this reason, the soaking temperature was limited to 700 to 850 ° C.

Soaking time: 30-200s
In order to complete recrystallization of cold-worked ferrite, the soaking time is set to 30 seconds or more.
When the soaking time is short, recrystallization is not completed or ferrite grains remain fine. On the other hand, when the soaking time is longer than 200 s, the ferrite grains grow too much. For this reason, the soaking time was limited to 30 to 200 s.

Cooling rate: 3 ° C./s or more If the cooling rate after soaking is small, the growth of ferrite grains is promoted. For this reason, the average cooling rate in the temperature range from the soaking temperature to 600 ° C. is limited to 3 ° C./s or more. The upper limit of the cooling rate is not particularly limited, and is determined depending on the capacity of the cooling facility. If it is a normal cooling facility, the upper limit of the cooling rate is about 30 ° C./s.

  In addition, if it cools to 600 degreeC, the coarsening of the structure | tissue by the grain growth of a ferrite can be suppressed, and it becomes possible to obtain the structure | tissue which mainly has the ferrite which has a desired average particle diameter. Further, the cooling conditions of 600 ° C. or lower are not particularly limited, and there is no particular problem with arbitrary cooling.

  Note that after the cooling is stopped, galvanizing at around 480 ° C. may be performed as necessary. Further, after hot dip galvanization, the hot dip galvanization may be alloyed by reheating to 500 ° C. or higher. Further, a heat history such as holding during cooling may be applied. Furthermore, you may perform temper rolling of about 0.5 to 2% as needed. Further, when plating is not performed, electrogalvanization or the like may be performed in order to improve corrosion resistance. Further, a film may be formed on the cold-rolled steel plate or the plated steel plate by chemical conversion treatment or the like.

  Hereinafter, based on an Example, this invention is demonstrated further.

  A steel material (slab) having the composition shown in Table 1 was used as a starting material. After these slabs were heated to 1200 ° C., a hot rolling process, a pickling process, a cold rolling process, and an annealing process were sequentially performed on the slab to obtain a cold-rolled annealed sheet. In the hot rolling step, the steel material is subjected to rough rolling to form a sheet bar, and then the sheet bar is subjected to finish rolling at which the finish rolling finish temperature (FT) is shown in Table 2, and the winding temperature shown in Table 2 is applied. It was wound up with (CT) to obtain a hot-rolled sheet having a thickness shown in Table 2. Next, the hot-rolled sheet was subjected to a pickling process, and then cold-rolled at the cold rolling reduction shown in Table 2 to obtain cold-rolled sheets having the thickness shown in Table 2.

  Subsequently, the cold-rolled sheet was subjected to an annealing process to obtain a cold-rolled sheet. In the annealing step, annealing was performed at the heating rate, the soaking temperature, the soaking time, and the cooling rate shown in Table 2. In addition, about 600 degrees C or less, it cooled to room temperature with the same cooling rate. In addition, after performing the annealing process, temper rolling with a rolling reduction of 1.0% was performed.

The obtained cold-rolled annealed plate (cold-rolled steel plate) was subjected to a structure observation, a tensile test, and a stretch forming test. The test method was as follows.
(1) Structure observation From the obtained cold-rolled annealed plate, a structure observation specimen is collected, the rolling direction cross section (L cross section) is polished and corroded, and an optical microscope (magnification: 100 times) and scanning electron microscope are used. Using (magnification: 1000 times), the structure was observed, imaged, and the average particle diameter of ferrite, the fraction of ferrite, the type and fraction of the second phase were measured by image analysis. For ferrite, in the region of 300 x 300 μm, calculate the average intercept length in the rolling direction and the thickness direction of the ferrite grains, and let A and B be the values of 2 / (1 / A + 1 / B). The average particle size was taken. Further, the ferrite fraction was measured in the region of 300 × 300 μm.
(2) Tensile test A JIS No. 5 test piece was collected from the obtained cold-rolled annealed plate so that the tensile direction was the rolling direction, and the proportional limit was determined. A strain gauge was attached to the parallel part of the tensile test piece, and a tensile test was performed at a tensile rate of 1 mm / min to obtain tensile properties (proportional limit, tensile strength, elongation). The proportional limit is a stress at which the slope of the stress-strain curve starts to decrease.
(3) Stretching test A test material (size: 120 × 120 mm) was sampled from the obtained cold-rolled annealed plate and stretched. The overhang forming was press forming in which a central portion of the test material was overlaid by 8 mm with a ball head punch having a diameter of 20 mm. In the stretch forming, as shown in FIG. 1, the region (shaded portion) having a diameter of 28 to 54 mm was pressed with a load of 100 kN. After the molding, as schematically shown in FIG. 2, the test material was placed on a surface plate, and the maximum distortion height of the flange portion was measured.
The obtained results are shown in Table 3.

  Each of the examples of the present invention has a low proportional limit of 100 MPa or less, the flat part maximum distortion height of the stretch-formed member is 0.8 mm or less, and is a cold-rolled steel sheet excellent in shape freezing property. On the other hand, in the comparative example out of the scope of the present invention, the proportional limit exceeds 100 MPa, or the maximum distortion height exceeds 0.8 mm, and the shape freezing property is reduced.

Claims (20)

% By mass
C: 0.0010 to 0.0030%, Si: 0.05% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: 0.02% or less, Al: 0.10% or less,
N: 0.0050% or less, Ti: 0.021 to 0.060%,
B: Containing 0.0005 to 0.0050%, and containing B and C so that B / C satisfies 0.5 or more, the balance consisting of Fe and inevitable impurities, and ferrite having an average particle size of 10 to 30 μm A cold-rolled steel sheet having an excellent shape freezing property and having a proportional limit of 100 MPa or less.   The cold-rolled steel sheet according to claim 1, further comprising Nb: 0.009% or less by mass% in addition to the composition.   The cold-rolled steel sheet according to claim 1, further comprising Cr: 0.06% or less by mass% in addition to the composition.   The cold-rolled steel sheet according to claim 1, further comprising, by mass%, Nb: 0.009% or less and Cr: 0.06% or less in addition to the composition.   The cold-rolled steel sheet according to claim 2, wherein the Nb content is 0.001 to 0.009% in mass%. The cold-rolled steel sheet according to claim 3, wherein the Cr content is 0.001 to 0.06% by mass%.
The cold-rolled steel sheet according to claim 1, wherein the B / C is 0.5 or more and 5 or less. The cold-rolled steel sheet according to claim 7, wherein the B / C is 1.0 or more and 3.3 or less. The cold-rolled steel sheet according to claim 8, wherein the B / C is 1.5 or more and 3.3 or less. The cold-rolled steel sheet according to claim 1, wherein the proportional limit is 40 MPa or more and 100 MPa or less. The cold-rolled steel sheet according to claim 1, wherein the structure mainly composed of ferrite is a structure containing 95% or more of ferrite by area ratio. In the method of manufacturing a cold-rolled steel sheet, in which a hot rolling process, a pickling process, a cold rolling process, and an annealing process are sequentially performed on a steel material,
The steel material in mass%,
C: 0.0010 to 0.0030%, Si: 0.05% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: 0.02% or less, Al: 0.10% or less,
N: 0.0050% or less, Ti: 0.021 to 0.060%,
B: 0.0005-0.0050%
And B and C are contained so that B / C satisfies 0.5 or more, and a steel material having a composition composed of the balance Fe and inevitable impurities,
The hot rolling step is a step of heating the steel material, subjecting it to rough rolling and finish rolling to a finish rolling finish temperature of 870 to 950 ° C, and winding at a winding temperature of 450 to 630 ° C.
The cold rolling step is a step of performing cold rolling with a rolling reduction of 90% or less,
The annealing step is heated to a soaking temperature in the range of 700 to 850 ° C. at a heating rate of 1 to 30 ° C./s on average over a temperature range of 600 ° C. or more and held at the soaking temperature for 30 to 200 s. After that, the temperature range up to 600 ℃ on average, it is a process of cooling at a cooling rate of 3 ℃ / s,
A method for producing a cold-rolled steel sheet having excellent shape freezing property   The method for producing a cold-rolled steel sheet according to claim 12, further comprising Nb: 0.009% or less by mass% in addition to the composition.   The method for producing a cold-rolled steel sheet according to claim 12, further comprising Cr: 0.06% or less by mass% in addition to the composition.   The method for producing a cold-rolled steel sheet according to claim 12, further comprising Nb: 0.009% or less and Cr: 0.06% or less in mass% in addition to the composition.   The method for producing a cold-rolled steel sheet according to claim 13, wherein the Nb content is 0.001 to 0.009% in mass%.   The method for producing a cold-rolled steel sheet according to claim 14, wherein the Cr content is 0.001 to 0.06% by mass. The method for producing a cold-rolled steel sheet according to claim 12, wherein the B / C is 0.5 or more and 5 or less. The method for producing a cold-rolled steel sheet according to claim 18, wherein the B / C is 1.0 or more and 3.3 or less. The method for producing a cold-rolled steel sheet according to claim 19, wherein the B / C is 1.5 or more and 3.3 or less.

Images