COLD-ROLLED STEEL SHEET WITH SUPERIOR SHAPE FIXABILITY AND MANUFACTURING METHOD THEREFOR

Abstract

 Provided is a cold-rolled steel sheet with excellent shape fixability and a method for manufacturing the same. A steel material having a chemical composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more is subjected to a hot rolling step in which the steel material is finish-rolled at a finishing delivery temperature of 870°C to 950°C and is coiled at a coiling temperature of 450°C to 630°C; a cold-rolling step in which cold rolling is performed at a rolling reduction of 90% or less; and an annealing step in which heating is performed up to a holding temperature in the range of 700°C to 850°C at an average heating rate of 1 °C/s to 30 °C/s in a temperature region not lower than 600°C after the cold rolling step, retention is performed for 30 s to 200 s, and cooling is then performed at a cooling rate of 3 °C/s or more in a temperature region down to 600°C, whereby a cold-rolled steel sheet having a microstructure dominated by ferrite with an average grain size of 10 µm to 30 µm, a proportional limit of 100 MPa or less, and excellent shape fixability is obtained.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet which is suitable for members of parts requiring strict dimensional accuracy in the electrical, automotive, building material, and other fields and which has excellent shape fixability and also relates to a method for manufacturing the same. The present invention particularly relates to the enhancement of shape fixability.

Background Art

[0002] In recent years, in order to protect the global environment, the reduction of automotive fuel consumption has been required from the viewpoint of reducing CO₂ emissions. For such a request to reduce fuel consumption, the reduction in weight of automotive bodies has been attempted. Furthermore, demands to reduce the gauge of steel used and the amount of steel used have been growing in association with a requirement for cost reduction. However, the reduction in gauge of steel materials (steel sheets) reduces the rigidity of parts to cause problems such as the deflection, dent, and warpage of the parts. Furthermore, in the field of consumer electrical appliances such as AV devices and OA machines, requirements for the dimensional accuracy of parts have become strict and therefore demands for steel sheets with excellent shape fixability have been increasingly growing.

[0003] For such requirements, for example, Patent Literature 1 discloses a ferritic steel sheet with excellent shape fixability. In a technique described in Patent Literature 1, steel having a composition containing 0.0001% to 0.05% C, 0.01% to 1.0% Si, 0.01% to 2.0% Mn, 0.15% or less P, 0.03% or less S, 0.01% or less Al, 0.01% or less N, and 0.007% or less O on a mass basis is hot-rolled such that the sum of rolling reductions at a temperature of not lower than the Ar₃ transformation temperature to 950°C is 25% or more and the coefficient of friction during hot rolling at 950°C or lower is 0.2 or less, hot rolling is completed at a temperature not lower than the Ar₃ transformation temperature, and coiling is performed at a temperature not higher than a predetermined critical temperature after cooling, whereby a steel sheet in which the ratio of the {100} plane to {111} plane parallel to a sheet surface is 1.0 or more is obtained. In the steel sheet, a slip system can be controlled during bending and springback can be suppressed during bending-dominated forming.

[0004] Patent Literature 2 discloses a method for press-forming a formed product with excellent dimensional accuracy. In a technique described in Patent Literature 2, forming is performed using a steel sheet in which the ratio of the {100} plane to {111} plane parallel to a sheet surface is 1.0 or more in such a manner that a tensile stress equal to 40% to 100% of the tensile strength of material is applied to a vertical wall portion of a hat-shaped member. According to the technique described in Patent Literature 2, a member having significantly increased hat bendability, small springback, and excellent shape fixability can be provided.

Citation List

Patent Literature

[0005] 

PTL 1: WO 00/06791

PTL 2: Japanese Unexamined Patent Application Publication No. 2002-66637

Summary of Invention

Technical Problem

[0006] However, the technique described in Patent Literature 1 has problems such as: the degree of improvement in shape fixability is small in the case of performing press forming other than bending, and springback may be large due to the influence of grain boundary sliding or the like even in the case of performing bending. Furthermore, the technique described in Patent Literature 2 has a problem that the effect of improving the dimensional accuracy of a formed product is not obtained in the case of performing press forming other than hat forming and a problem that the blank holding pressure needs to be large in order to apply stress to a vertical wall in the case of performing hat forming and therefore the power of a press needs to be increased, leading to an increase in cost.

[0007] The present invention solves the problems with the conventional techniques. It is an object of the present invention to provide a cold-rolled steel sheet which has excellent shape fixability and which causes no significant strain in a flat portion of a formed member and a method for manufacturing the same.

Solution to Problem

[0008] The inventors have intensively investigated shape fixability, particularly factors affecting the strain of flat portions of formed members, to achieve the above object. As a result, the inventors have appreciated that the strain of a flat portion of a formed member is significantly affected by the proportional limit of a steel sheet used. The inventors have found that the strain of a flat portion of a formed member is significantly increased particularly when the proportional limit is more than 100 MPa. As a result of further investigations, the inventors have found that an ultra-low carbon based chamical composition essentially containing Ti and B needs to be adjusted such that the ratio, B/C, of the content of B to the content of C satisfies 0.5 or more, in order that the proportional limit is 100 MPa or less.

[0009] First, experiment results underlying the present invention are described.

[0010] Steel materials (slabs) having a composition containing 0.0010% to 0.035% 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 on a mass basis were subjected to hot rolling and cold rolling and were further subjected to annealing under various heating, holding, and cooling conditions, whereby cold-rolled annealed sheets were obtained.

[0011] A JIS #5 test specimen was taken from each obtained cold-rolled annealed sheet such that a tensile direction coincided with a rolling direction, followed by determining the proportional limit thereof. A 5 mm strain gauge was attached to a parallel portion of the tensile test specimen and tensile testing was performed at a cross head speed of 1 mm/min. The stress at which the slope of the stress-strain curve thereof began to decrease was defined as the proportional limit thereof.

[0012] A test specimen (a size of 120 mm x 120 mm) was taken from each obtained cold-rolled annealed sheet and was then punch stretch formed. Punch stretch forming was performed by press forming in such a manner that a central portion of the test specimen was stretched by 8 mm using a spherical punch with a diameter of 20 mm. Incidentally, in punch stretch forming, a region (hatched portion) with a diameter of 28 mm to 54 mm was pressed with a load of 100 kN and was formed as shown in Fig. 1. Next, as shown in Fig. 2, the formed test specimen was placed on a platen and a flange portion thereof was measured for maximum strain height. Incidentally, the observation of the obtained cold-rolled annealed sheets showed that all the cold-rolled annealed sheets had a microstructure dominated by ferrite.

[0013] Obtained results are shown in Figs. 3 and 4. Fig. 3 shows the relationship between the proportional limit and maximum strain height of each flange portion. Fig. 4 shows the relationship between B/C and the proportional limit.

[0014] As is clear from Fig. 3, as the proportional limit exceeds 100 MPa, the maximum strain height of the flange portion increases sharply. As is clear from Fig. 4, in order to adjust the proportional limit to 100 MPa or less, B/C needs to be 0.5 or more.

[0015] From this, the inventors have found that the shape fixability of a pressed part is increased and particularly the strain of a flat portion of a formed member is significantly reduced by using a steel sheet having a composition which essentially contains Ti and B and in which B/C is 0.5 or more, a microstructure dominated by ferrite, and a proportional limit of 100 MPa or less as material. According to further investigations, the inventors have found that it is effective in enhancing shape fixability that hot rolling conditions are optimized such that C forms a solid solution, cold rolling is performed, and coarse B precipitates containing C and Fe are formed at grain boundaries and also in grains during annealing. The inventors have thought that, in such a microstructure, distributed coarse B precipitates adequately anchor dislocations during press forming to concentrate strain around the precipitates and suppress the intertwining of the dislocations by prevent the dislocations from gathering at grain boundaries, whereby springback is significantly reduced, the proportional limit is reduced, and shape fixability is remarkably enhanced.

[0016] The present invention has been completed on the basis of these findings and further investigations. That is, the scope of the present invention is as described below.

(1) A cold-rolled steel sheet with excellent shape fixability, has a chemical composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, the remainder being Fe and incidental impurities; a microstructure dominated by ferrite with an average grain size of 10 µm to 30 µm; and a proportional limit of 100 MPa or less.

(2) The cold-rolled steel sheet specified in (1) further contains 0.009% or less Nb on a mass basis in addition to the chemical composition.

(3) The cold-rolled steel sheet specified in (1) further contains 0.06% or less Cr on a mass basis in addition to the chemical composition.

(4) The cold-rolled steel sheet specified in (1) further contains 0.009% or less Nb and 0.06% or less Cr on a mass basis in addition to the chemical composition.

(5) In the cold-rolled steel sheet specified in (2), the content of Nb is 0.001% to 0.009% on a mass basis.

(6) In the cold-rolled steel sheet specified in (3), the content of Cr is 0.001% to 0.06% on a mass basis.

(7) In the cold-rolled steel sheet specified in (1), the B/C is greater than or equal to 0.5 and less than or equal to 5.

(8) In the cold-rolled steel sheet specified in (7), the B/C is greater than or equal to 1.0 and less than or equal to 3.3.

(9) In the cold-rolled steel sheet specified in (8), the B/C is greater than or equal to 1.5 and less than or equal to 3.3.

(10) In the cold-rolled steel sheet specified in (1), the proportional limit is greater than or equal to 40 MPa and less than or equal to 100 MPa.

(11) In the cold-rolled steel sheet specified in (1), the microstructure dominated by ferrite contains 95% or more ferrite in terms of area fraction.

(12) A method for manufacturing a cold-rolled steel sheet with excellent shape fixability includes subjecting a steel material to a hot-rolling step, a pickling step, a cold-rolling step, and an annealing step in that order. The steel material has a chemical composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, the remainder being Fe and incidental impurities. The hot rolling step is a step in which the steel material is heated, is roughly rolled, is finish-rolled at a finishing delivery temperature of 870°C to 950°C, and is coiled at a coiling temperature of 450°C to 630°C. The cold-rolling step is a step in which cold rolling is performed at a rolling reduction of 90% or less. The annealing step is a step in which heating is performed up to a holding temperature in the range of 700°C to 850°C at an average heating rate of 1 °C/s to 30 °C/s in a temperature region not lower than 600°C, retention is performed at the holding temperature for 30 s to 200 s, and cooling is then performed at a cooling rate of 3 °C/s or more in a temperature region down to 600°C.

(13) In the method for manufacturing the cold-rolled steel sheet specified in (12), it further contains 0.009% or less Nb on a mass basis in addition to the chemical composition.

(14) In the method for manufacturing the cold-rolled steel sheet specified in (12), it further contains 0.06% or less Cr on a mass basis in addition to the chemical composition.

(15) In the method for manufacturing the cold-rolled steel sheet specified in (12), it further contains 0.009% or less Nb and 0.06% or less Cr on a mass basis in addition to the chemical composition.

(16) In the method for manufacturing the cold-rolled steel sheet specified in (13), the content of Nb is 0.001% to 0.009% on a mass basis.

(17) In the method for manufacturing the cold-rolled steel sheet specified in (14), the content of Cr is 0.001% to 0.06% on a mass basis.

(18) In the method for manufacturing the cold-rolled steel sheet specified in (12), the B/C is greater than or equal to 0.5 and less than or equal to 5.

(19) In the method for manufacturing the cold-rolled steel sheet specified in (18), the B/C is greater than or equal to 1.0 and less than or equal to 3.3.

(20) In the method for manufacturing the cold-rolled steel sheet specified in (19), the B/C is greater than or equal to 1.5 and less than or equal to 3.3.

Advantageous Effects of Invention

[0017] According to the present invention, a cold-rolled steel sheet having a significantly reduced proportional limit and excellent shape fixability after forming can be readily manufactured at low cost. This is industrially particularly advantageous. Furthermore, according to the present invention, there is an effect that the reduction in gauge of a member can be accelerated.

Brief Description of Drawings

[0018] 

[Fig. 1] Fig. 1 is a schematic view showing a test specimen for punch stretch forming and a flange-suppressing region (hatched portion) during a forming test.

[Fig. 2] Fig. 2 is a schematic view showing a method for measuring the maximum strain height after a punch stretch forming test.

[Fig. 3] Fig. 3 is a graph showing the relationship between the proportional limit and the maximum strain height.

[Fig. 4] Fig. 4 is a graph showing the relationship between B/C and the proportional limit.

Description of Embodiments

[0019] First, reasons for limiting the composition (chemical composition) of a cold-rolled steel sheet according to the present invention are described. Incidentally, mass percent is hereinafter simply represented by % unless otherwise specified.

C: 0.0010% to 0.0030%

[0020] C is an element which forms a solid solution to promote the formation of coarse B precipitates and which contributes to a reduction in proportional limit. Such an effect is remarkable when the content thereof is 0.0010% or more. However, when the content thereof is high, more than 0.0030%, the reduction of ductility is caused because the amount of solute C and/or carbides is large and the strength is excessively high. Therefore, C is limited to the range of 0.0010% to 0.0030%. Incidentally, it is preferably 0.0020% or less.

Si: 0.05% or less

[0021] When a large amount of Si is contained, workability is deteriorated by hardening, and Si oxides are produced during annealing and thereby wettability is impaired. Furthermore, since high Si content increases the austenite (γ)-to-ferrite (α) transformation temperature, it is difficult to complete rolling in a γ-region during hot rolling. Therefore, Si is limited to 0.05% or less.

Mn: 0.1% to 0.5%

[0022] Mn combines with S, where S significantly reduces hot ductility and is harmful, in steel to form MnS, contributes to rendering S harmless, and has the effect of hardening steel. In order to achieve such effects, the content thereof needs to be 0.1% or more. However, when the content thereof is high, more than 0.5%, ductility is reduced by hardening and the recrystallization of ferrite is suppressed during annealing. Therefore, Mn is limited to the range of 0.1% to 0.5%. Incidentally, it is preferably 0.3% or less and more preferably 0.2% or less.

P: 0.05% or less

[0023] P segregates at grain boundaries and has the function of reducing ductility. Therefore, in the present invention, P is preferably minimized and up to 0.05% is acceptable. Hence, P is limited to 0.05% or less. Incidentally, it is preferably 0.03% or less and more preferably 0.02% or less.

S: 0.02% or less

[0024] S is an impurity element and is preferably minimized. S significantly reduces hot ductility, causes hot cracking, significantly deteriorates surface properties, and has adverse influences. Furthermore, S hardly contributes to strength and forms coarse MnS to reduce ductility. This becomes significant when S is more than 0.02%. Therefore, in the present invention, S is limited to 0.02% or less. Incidentally, it is preferably 0.01% or less.

Al: 0.10% or less

[0025] Al is an element acting as a deoxidizer. In order to achieve such an effect, 0.02% or more is preferably contained. On the other hand, Al has the function of increasing the γ-to-α transformation temperature of steel. Therefore, when the content is high, more than 0.10%, it is difficult to complete rolling in a γ-region during hot rolling. Therefore, Al is limited to 0.10% or less.

N: 0.0050% or less

[0026] N is an element which combines with a nitride-forming element to form a nitride and which has the function of hardening steel by precipitation hardening. When the content is high, more than 0.0050%, not only a reduction in ductility but also slab cracking during hot rolling are caused and many surface flaws may possibly be caused. Therefore, N is limited to 0.0050% or less. Incidentally, it is preferably 0.0030% or less and more preferably 0.0020% or less.

Ti: 0.021% to 0.060%

[0027] Ti is an element which fixes N in the form of a nitride and which has the function of suppressing hardening and aging deterioration due to solute N. In order to achieve such effects, 0.021% or more needs to be contained. However, when the content is high, more than 0.060%, the precipitation of carbides is promoted and the amount of solute C is reduced; hence, the production of coarse B precipitates containing C and Fe is suppressed. Therefore, a desired reduction in proportional limit cannot be achieved. Thus, Ti is limited to the range of 0.021% to 0.060%. Incidentally, it is preferably 0.050% or less.

B: 0.0005% to 0.0050%

[0028] B is an element important in the present invention and forms coarse B precipitates to contribute to a reduction in proportional limit. In order to achieve such an effect, 0.0005% or more needs to be contained. However, when the content is high, more than 0.0050%, slab cracking is caused. Therefore, B is limited to the range of 0.0005% to 0.0050%. Incidentally, it is preferably 0.0010% or more, more preferably 0.0020% or more, and further more preferably 0.0030% or more.

B/C: 0.5 or more

[0029] In the present invention, C and B are contained in the above ranges and the contents of C and B are adjusted such that the ratio, B/C, of the content of B to the content of C satisfies 0.5 or more. When B/C is less than 0.5, it is difficult to form coarse B precipitates. Therefore, B/C is limited to 0.5 or more. Incidentally, it is preferably 1.0 or more, more preferably 1.5 or more, and further more preferably 2.0 or more.

[0030]  The above components are fundamental components. In the present invention, 0.009% or less Nb and/or 0.06% or less Cr may be contained as a selective element in addition to the fundamental components as required.

Nb: 0.009% or less

[0031] Nb, as well as Ti, is an element which combines with N to form a nitride, which fixes N, which suppresses hardening and aging deterioration due to solute N, and which contributes to the enhancement of shape fixability and may be contained as required. In order to achieve such effects, 0.001% or more is preferably contained. However, the content is high, more than 0.009%, grains become fine. Therefore, when Nb is contained, Nb is preferably limited to 0.009% or less.

Cr: 0.06% or less

[0032] Cr is an element which destabilizes C in a solid solution to promote the production of coarse B precipitates containing C and may be contained as required. In order to achieve such an effect, 0.001% or more is preferably contained. However, when the content of Cr is high, more than 0.06%, the production of the coarse B precipitates containing C is inhibited instead. Therefore, when Cr is contained, Cr is preferably limited to 0.06% or less. The remainder other than the above components are Fe and incidental impurities.

[0033] Next, reasons for limiting the microstructure of the cold-rolled steel sheet according to the present invention are described.

[0034] The cold-rolled steel sheet according to the present invention has a microstructure dominated by ferrite with an average grain size of 10 µm to 30 µm. The microstructure dominated by ferrite allows the steel sheet to be soft and therefore allows the workability thereof to be enhanced. Incidentally, the term "microstructure dominated by ferrite" as used herein refers to a microstructure in which ferrite (polygonal ferrite) accounts for 95% or more, and more preferably 100%, in terms of area fraction. A secondary phase other than ferrite is preferably cementite or bainite. If the average grain size of ferrite is 10 µm or more, the concentration of strain at grain boundaries can be suppressed, strain can be concentrated around precipitates, and the proportional limit can be reduced. However, when the average grain size of ferrite is large, more than 30 µm, surface markings such as orange peeling become obvious during press working. Therefore, the average grain size of ferrite is limited to the range of 10 µm to 30 µm.
Incidentally, it is preferably 15 µm to 25 µm.

[0035] Next, a preferred method for manufacturing the cold-rolled steel sheet according to the present invention is described.

[0036] A steel material (slab) with the above composition is used as a starting material.

[0037] A method for manufacturing the steel material is not particularly limited. Molten steel with the above composition is preferably produced in a regular converter, an electric furnace, or the like and is then solidified into a slab (steel material) by a continuous casting process or an ingot casting-blooming process. If the slab is manufactured by continuous casting, the slab is preferably directly hot-rolled without cooling the slab to room temperature when having heat sufficient for hot rolling. Alternatively, the slab is preferably hot-rolled after the slab is temporally charged into a furnace and is heat-retained or the slab is cooled to room temperature and is then reheated to a temperature of 1,100°C to 1,250°C by charging the slab into a furnace.

[0038] The heated steel material is subjected to a hot rolling step.

[0039] In the hot rolling step, hot rolling including rough rolling and finish rolling is performed and coiling is then performed.

[0040] In rough rolling, conditions are not particulary limited as far as a sheet bar having a desired size and shape is obtained. Next, the sheet bar is finish-rolled, whereby a hot-rolled sheet is obtained.

[0041] Finish rolling is performed at a finishing delivery temperature of 870°C to 950°C.

[0042] When the finishing delivery temperature is low, lower than 870°C, the microstructure is transformed from austenite into ferrite in the course of rolling and therefore it is difficult to control the load of a rolling machine; hence, the risk of causing fracture or the like during processing increases. Incidentally, if rolling is performed from the finishing entry side in a ferrite region, the fracture or the like during processing can be avoided; however, there is a problem in that the microstructure of the hot-rolled sheet is transformed into unrecrystallized ferrite because of the decrease of the rolling temperature and therefore the load for cold rolling is increased. On the other hand, when the finishing delivery temperature is high, higher than 950°C, the hot-rolled sheet has a large ferrite grain size. Therefore, a cold-rolled annealed sheet has an excessively large ferrite grain size. Thus, the finishing delivery temperature is limited to the range of 870°C to 950°C. After finish rolling is completed, the hot-rolled sheet is coiled. Cooling until coiling after finish rolling is not particularly limited and it is sufficient that the rate of cooling is higher than that of air cooling. There is no particular problem even if quenching is performed at 100 °C/s or more as required.

[0043] The coiling temperature after the completion of finish rolling ranges from 450°C to 630°C.

[0044] When the coiling temperature is lower than 450°C, acicular ferrite is produced and a steel sheet is hardened; hence, the load for subsequent cold rolling is increased, and also leading to the difficulty in operating hot rolling. However, when the coiling temperature is high, higher than 630°C, the precipitation of carbides is promoted, the amount of solute C is reduced, and therefore a desired amount of solute C cannot be ensured during hot rolling process. Thus, the coiling temperature is limited to the range of 450°C to 630°C.

[0045] The coiled hot-rolled sheet is subjected to an ordinary pickling step and is then subjected to a cold-rolling step, whereby a cold-rolled sheet is obtained.

[0046] In the cold-rolling step, the cold-rolled sheet is obtained by performing cold rolling at a cold-rolling reduction of 90% or less.

[0047] When the cold-rolling reduction is large, more than 90%, recrystallized ferrite grains after annealing become fine. At the same time, the load for cold rolling is increased, leading to the difficulty in operating cold rolling. Thus, the cold-rolling reduction is limited to 90% or less. Incidentally, it is preferably 80% or less. The lower limit of the cold-rolling reduction is not particularly limited. However, when the cold rolling reduction is low, the thickness of the hot-rolled sheet needs to be reduced with respect to the predetermined thickness of products and therefore the productivity of hot rolling and pickling is reduced. Hence, the cold-rolling reduction is preferably 50% or more.

[0048] The cold-rolled sheet is subjected to an annealing step, whereby a cold-rolled annealed sheet is obtained.

[0049] The annealing step is a step in which heating is performed up to a holding temperature in the range of 700°C to 850°C at an average heating rate of 1 °C/s to 30 °C/s in a temperature region not lower than 600°C, retention is performed at the holding temperature for 30 s to 200 s, and cooling is then performed at a cooling rate of 3 °C/s or more down to 600°C or lower. In the annealing step, cold-rolled worked ferrite is recrystallized so as to have a desired average grain size and coarse B precipitates containing C and Fe are distributed at grain boundaries and in grains.

Heating rate: 1 °C/s to 30 °C/s

[0050] When the average heating rate in a temperature region ranging from 600°C to the holding temperature is less than 1 °C/s, ferrite grains grow significantly and therefore ferrite with a desired average grain size cannot be obtained. However, when the heating rate is high, more than 30 °C/s, TiC is precipitated during heating instead of the production of B precipitates and therefore it is difficult to form desired coarse B precipitates. Thus, the average heating rate in a temperature region not lower than 600°C is limited to the range of 1 °C/s to 30 °C/s. Incidentally, it is preferably 5 °C/s or more and more preferably 10 °C/s or more.

Holding temperature: 700°C to 850°C

[0051] In the annealing step, the holding temperature is 700°C or higher because the recrystallization of cold-worked ferrite needs to be completed. However, when the holding temperature is high, higher than 850°C, ferrite grains become coarse and therefore ferrite with a desired average grain size cannot be obtained. Thus, the holding temperature is limited to the range of 700°C to 850°C.

Holding time: 30 s to 200 s

[0052] In order to complete the recrystallization of cold-worked ferrite, the holding time is 30 s or more. When the holding time is short, the recrystallization thereof is not completed or ferrite grains remain fine. However, when the holding time is long, more than 200 s, ferrite grains grow excessively. Thus, the holding time is limited to the range of 30 s to 200 s.

Cooling rate: 3 °C/s or more

[0053] When the cooling rate after holding is low, the growth of ferrite grains is promoted. Thus, the average cooling rate in a temperature region ranging from the holding temperature to 600°C is limited to 3 °C/s or more. Incidentally, the upper limit of the cooling rate need not be particularly limited and is determined depending on the capacity of a cooling facility. In ordinary cooling facilities, the upper limit of the cooling rate is about 30 °C/s.

[0054] The coarsening of a microstructure due to the growth of ferrite grains can be suppressed by cooling to 600°C, whereby a microstructure dominated by ferrite with a desired average grain size can be obtained. Conditions for cooling to 600°C or less need not be particularly limited and arbitrary cooling is not particularly problematic.

[0055] After cooling is stopped, galvanizing may be performed at about 480°C as required. After galvanizing, galvannealing may be performed by reheating to 500°C or higher. Thermal history including retention during cooling may be performing. Furthermore, temper rolling may be performed at about 0.5% to 2% as required. In the case of not performing plating, electrogalvanizing may be performed for the purpose of enhancing corrosion resistance. Furthermore, a coating may be provided on the cold-rolled steel sheet or a plated steel sheet using chemical conversion or the like.

[0056] The present invention is further described below in detail on the basis of examples.

EXAMPLES

[0057] Steel materials (slabs) having a chemical composition shown in Table 1 were used as starting materials. After the slabs were heated to 1,200°C, the slabs were subjected to a hot-rolling step, a pickling step, a cold-rolling step, and an annealing step in that order, whereby cold-rolled annealed sheets were obtained. In the hot-rolling step, each steel material was roughly rolled into a sheet bar and the sheet bar was finish-rolled at a finishing delivery temperature equal to a temperature (FT) shown in Table 2 and was then coiled at a coiling temperature (CT) shown in Table 2, whereby a hot-rolled sheet with a thickness shown in Table 2. Next, after the hot-rolled sheet was subjected to the pickling step, the hot-rolled sheet was subjected to cold rolling at a cold-rolling reduction shown in Table 2, whereby a cold-rolled sheet with a thickness shown in Table 2 was obtained.

[0058] Next, the cold-rolled sheet is subjected to the annealing step, whereby a cold-rolled annealed sheet was obtained. In the annealing step, annealing was performed at a heating rate, a holding temperature, a holding time, and a cooling rate as shown in Table 2. Incidentally, cooling from 600°C or lower to room temperature was performed at a similar cooling rate. After the annealing step was performed, temper rolling was performed at a rolling reduction of 1.0%.

[0059] The obtained cold-rolled annealed sheets (cold-rolled steel sheets) were subjected to microstructure observation, a tensile test, and a punch stretch forming test. Testing methods were as described below.

(1) Microstructure observation

[0060] A test specimen for microstructure observation was taken from each obtained cold-rolled annealed sheet; a cross section (L-cross section) in a rolling direction was polished and was etched; the microstructure thereof was observed and photographed using an optical microscope (a magnification of 100 times) and a scanning electron microscope (a magnification of 1,000 times); and the average grain size of ferrite, the fraction of ferrite, and the type and fraction of a secondary phase were determined by image analysis. For ferrite, the average intercept length of ferrite grains in a 300 µm x 300 µm region was determined in the rolling direction and a thickness direction and the value of 2/(1/A + 1/B) was defined as the average grain size, where A is the average intercept length of the ferrite grains in the rolling direction and B is the average intercept length of the ferrite grains in the thickness direction. The fraction of ferrite was measured in a 300 µm x 300 µm region.

(2) Tensile test

[0061] A JIS #5 test specimen was taken from each obtained cold-rolled annealed sheet such that a tensile direction coincided with the rolling direction, followed by determining the proportional limit thereof. A strain gauge was attached to a parallel portion of the tensile test specimen and tensile testing was performed at a cross head speed of 1 mm/min, whereby tensile properties (proportional limit, tensile strength, and elongation) were determined. Incidentally, the proportional limit was defined as the stress at which the slope of the stress-strain curve thereof began to decrease.

(3) Punch stretch forming test

[0062] A test specimen (a size of 120 mm x 120 mm) was taken from each obtained cold-rolled annealed sheet and was then punch stretch formed. Punch stretch forming was performed by press forming in such a manner that a central portion of the test specimen was stretched by 8 mm using a spherical punch with a diameter of 20 mm. Incidentally, in punch stretch forming, a region (hatched portion) with a diameter of 28 mm to 54 mm was depressed with a load of 100 kN and was formed as shown in Fig. 1. After forming, as shown in Fig. 2, the test specimen was placed on a platen and a flange portion thereof was measured for maximum strain height. Obtained results are shown in Table 3.

[Table 1]

Steel Material ID	Chemical components (weight percent)												Remarks
	C	Si	Mn	P	S	Al	N	Ti	B	Nb	Cr	B/C
A	0.0015	0.01	0.15	0.01	0.01	0.03	0.0020	0.040	0.0029	-	-	1.9	Adequate example
B	0.0013	0.03	0.35	0.04	0.01	0.05	0.0040	0.022	0.0018	0.005		1.4	Adequate example
C	0.0016	0.02	0.45	0.02	0.02	0.08	0.0030	0.058	0.0009	0.008	0.01	0.6	Adequate example
D	0.0028	0.05	0.25	0.01	0.01	0.04	0.0020	0.035	0.0048	-	0.05	1.7	Adequate example
E	0.0012	0.01	0.15	0.01	0.01	0.05	0.0015	0.031	0.0025	-	-	2.1	Adequate example
F	0.0013	0.01	0.15	0.01	0.01	0.04	0.0025	0.055	0.0035	-	0.01	2.7	Adequate example
G	0.0012	0.01	0.10	0.01	0.01	0.05	0.0015	0.060	0.0040	-	-	3.3	Adequate example
H	0.0025	0.01	0.10	0.01	0.01	0.04	0.0020	0.035	0.0023	-	-	0.9	Adequate example
I	0.0015	0.01	0.15	0.01	0.01	0.05	0.0020	0.045	0.0008	-	-	0.5	Adequate example
J	0.0035	0.02	0.25	0.02	0.01	0.05	0.0025	0.032	0.0015	-	-	0.4	Comparative example
K	0.0010	0.01	0.20	0.02	0.01	0.06	0.0021	0.025	0.0003	-	-	0.3	Comparative example
L	0.0020	0.01	0.18	0.01	0.02	0.05	0.0023	0.035	0.0010	0.003	-	0.5	Adequate example
M	0.0011	0.02	0.15	0.02	0.01	0.04	0.0030	0.030	0.0020	-	-	1.8	Adequate example
N	0.0025	0.02	0.20	0.01	0.01	0.04	0.0030	0.040	0.0020	-	-	0.8	Adequate example
O	0.0015	0.01	0.15	0.01	0.01	0.04	0.0030	0.005	0.0030	-	-	2.0	Comparative example

[Table 2]

Steel sheet ID	Steel Material No.	Hot-rolling step				Cold-rolling step		Annealing step				Remarks
		Heating temperature (°C)	Finishing delivery temperature (°C)	Coiling temperature (°C)	Thickness (mm)	Cold-rolling reduction (%)	Thickness (mm)	Heating rate (°C/s)*	Holding temperature (°C)	Holding time (s)	Cooling rate (°C/s)**
1	A	1200	890	560	2.5	76	0.6	11	770	130	20	Example of present invention
2	B	1200	920	620	2.7	78	0.6	6	720	40	5	Example of present invention
3	C	1200	940	460	1.5	60	0.6	3	840	180	12	Example of present invention
4	D	1200	900	500	1.3	55	0.6	20	780	80	25	Example of present invention
5	E	1200	890	600	2.0	70	0.6	28	800	100	15	Example of present invention
6	F	1200	930	580	2.4	75	0.6	15	830	150	10	Example of present invention
7	G	1200	920	570	2.9	79	0.6	12	850	180	8	Example of present invention
8	H	1200	910	580	2.4	75	0.6	10	800	150	10	Example of present invention
9	I	1200	890	560	2.7	78	0.6	10	800	130	18	Example of present invention
10	J	1200	890	600	2.4	75	0.6	12	830	130	10	Comparative example
11	K	1200	880	590	2.5	76	0.6	10	820	120	11	Comparative example
12	L	1200	910	650	2.7	78	0.6	15	800	140	15	Comparative example
13	M	1200	890	590	2.4	75	0.6	0.4	860	150	10	Comparative example
14	N	1200	880	560	2.2	73	0.6	12	750	20	15	Comparative example
15	O	1200	890	560	2.4	75	0.6	10	750	100	15	Comparative example

(*) Average in a temperature region not lower than 600°C. (**) Average from a holding temperature to 600°C.

[Table 3]

Steel sheet No.	Microstructure			Tensile properties			Shape fixability	Remarks
	Type*	Ferrite		Proportional limit (MPa)	Tensile strength TS (MPa)	Elongation El (%)	Maximum strain height (mm)
		Average grain size (µm)	Fraction (area percent)
1	F	16	100	80	330	50	0.4	Example of present invention
2	F	12	100	85	340	49	0.6	Example of present invention
3	F+C	11	98	100	350	48	0.7	Example of present invention
4	F	13	100	80	355	47	0.5	Example of present invention
5	F	16	100	70	320	51	0.3	Example of present invention
6	F	23	100	50	310	51	0.2	Example of present invention
7	F	28	100	40	300	52	0.2	Example of present invention
8	F	12	100	95	330	50	0.7	Example of present invention
9	F	13	100	100	320	51	0.8	Example of present invention
10	F	10	100	125	360	46	2.0	Comparative example
11	F	12	100	130	320	51	2.2	Comparative example
12	F	11	100	120	340	49	1.9	Comparative example
13	F	35	100	100	290	53	0.8	Comparative example
14	F+C	8	97	130	330	50	2.3	Comparative example
15	F	15	100	140	340	48	2.4	Comparative example

(*) F represents ferrite, C represents cementite, and B represents bainite.

[0063] In all examples of the present invention, cold-rolled steel sheets have excellent shape fixability with a low proportional limit of 100 MPa or less and flat portions of punch stretch formed members having a maximum strain height of 0.8 mm or less. However, in comparative examples which are outside the scope of the present invention, the proportional limit is more than 100 MPa or the maximum strain height is large, more than 0.8 mm, and shape fixability is low.

Claims

1. A cold-rolled steel sheet with excellent shape fixability, having a chemical composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, the remainder being Fe and incidental impurities; a microstructure dominated by ferrite with an average grain size of 10 µm to 30 µm; and a proportional limit of 100 MPa or less.

2. The cold-rolled steel sheet according to Claim 1, further containing 0.009% or less Nb on a mass basis in addition to the chemical composition.

3. The cold-rolled steel sheet according to Claim 1, further containing 0.06% or less Cr on a mass basis in addition to the chemical composition.

4. The cold-rolled steel sheet according to Claim 1, further containing 0.009% or less Nb and 0.06% or less Cr on a mass basis in addition to the chemical composition.

5. The cold-rolled steel sheet according to Claim 2,
wherein the content of Nb is 0.001% to 0.009% on a mass basis.

6. The cold-rolled steel sheet according to Claim 3,
wherein the content of Cr is 0.001% to 0.06% on a mass basis.

7. The cold-rolled steel sheet according to Claim 1,
wherein the B/C is greater than or equal to 0.5 and less than or equal to 5.

8. The cold-rolled steel sheet according to Claim 7,
wherein the B/C is greater than or equal to 1.0 and less than or equal to 3.3.

9. The cold-rolled steel sheet according to Claim 8,
wherein the B/C is greater than or equal to 1.5 and less than or equal to 3.3.

10. The cold-rolled steel sheet according to Claim 1,
wherein the proportional limit is greater than or equal to 40 MPa and less than or equal to 100 MPa.

11. The cold-rolled steel sheet according to Claim 1,
wherein the microstructure dominated by ferrite contains 95% or more ferrite in terms of area fraction.

12. A method for manufacturing a cold-rolled steel sheet with excellent shape fixability, comprising subjecting a steel material to a hot-rolling step, a pickling step, a cold-rolling step, and an annealing step in that order, wherein the steel material has a composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, the remainder being Fe and incidental impurities; the hot rolling step is a step in which the steel material is heated, is roughly rolled, is finish-rolled at a finishing delivery temperature of 870°C to 950°C, and is coiled at a coiling temperature of 450°C to 630°C; the cold-rolling step is a step in which cold rolling is performed at a rolling reduction of 90% or less; and the annealing step is a step in which heating is performed up to a holding temperature in the range of 700°C to 850°C at an average heating rate of 1 °C/s to 30 °C/s in a temperature region not lower than 600°C, retention is performed at the holding temperature for 30 s to 200 s, and cooling is then performed at a cooling rate of 3 °C/s or more in a temperature region down to 600°C.

13. The method for manufacturing the cold-rolled steel sheet according to Claim 12, wherein it further contains 0.009% or less Nb on a mass basis in addition to the chemical composition.

14. The method for manufacturing the cold-rolled steel sheet according to Claim 12, wherein it further contains 0.06% or less Cr on a mass basis in addition to the chemical composition.

15. The method for manufacturing the cold-rolled steel sheet according to Claim 12, wherein it further contains 0.009% or less Nb and 0.06% or less Cr on a mass basis in addition to the chemical composition.

16. The method for manufacturing the cold-rolled steel sheet according to Claim 13, wherein the content of Nb is 0.001% to 0.009% on a mass basis.

17. The method for manufacturing the cold-rolled steel sheet according to Claim 14, wherein the content of Cr is 0.001% to 0.06% on a mass basis.

18. The method for manufacturing the cold-rolled steel sheet according to Claim 12, wherein the B/C is greater than or equal to 0.5 and less than or equal to 5.

19. The method for manufacturing the cold-rolled steel sheet according to Claim 18, wherein the B/C is greater than or equal to 1.0 and less than or equal to 3.3.

20. The method for manufacturing the cold-rolled steel sheet according to Claim 19, wherein the B/C is greater than or equal to 1.5 and less than or equal to 3.3.

Drawing