The present invention relates to a steel sheet suitable for automotive parts and the like, and particularly to a steel sheet with excellent formability and quench hardenability and a method for manufacturing the same.

In many cases, a steel sheet for use in tools, automotive parts (gear, transmission), etc., is formed into a desired shape, and subjected to heat treatment, such as hardening annealing, for use. Such a steel sheet is processed into various complicated shapes, and thus is required to have excellent formability. In recent years, reduction in manufacturing cost has been strongly demanded in such parts. Thus, processing techniques in which omission of a processing process or alteration of a processing manner is intended, e.g., a double-acting processing technique which allows thickening of automobile driving parts using a high carbon steel sheet, and achieves sharp reduction in the number of processes, have been developed, and some of them have been put into practical use. In accordance therewith, the steel sheets for use in automotive parts have been strictly required to have high formability, and the steel sheets have been demanded to be softer and have high ductility. For example, when processed by cold forging, lower yield stress has been demanded. Furthermore, when hole expanding (burring) is performed after punching, excellent stretch-flangeability is desired.

In order to satisfy such demands, a technique has been examined which is intended to graphitize c in steel for improving formability. For example, Patent Document 1 discloses a steel sheet suitable as a tiller claw: containing, by mass %, C: 0.40 to 0.80%, Si: 0.20 to 2.00%, Mn: 0.20 to 1.50%, Al: 0.001 to 0.150%, P: 0.018% or lower, S: 0.010% or lower, N: 0.0050% or lower, balance Fe, and inevitable impurities; having a microstructure containing a ferrite phase and a graphite as a main body; has a soft material of TS ≤ 60kgf/mm²; and having excellent formability, tenacity, and quench hardenability, and a method for manufacturing the same. Patent Document 2 discloses a method for manufacturing a medium carbon steel sheet with excellent formability, including: holding a hot rolled steel sheet containing, by mass%, C: 0.10 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.50%, Nb: 0.005 to 0.1%, Al: 0.01 to 1.00%, N: 0.002 to 0.010%, B:3 to 50 ppm, Ca: 0.001 to 0.01%, Ni: 0 to 2.00%, the balance being Fe and inevitable impurities, P in the impurities of 0.012% or lower, and S in the impurities of 0.008% or lower within a temperature range of from Ac₁ to Ac₃ for 0.1 to 10 hr; cooling the resultant at a cooling rate of from 20 to 100°C/hr; and box annealing the resultant within the temperature range of from 650 to 750°C to thereby graphitize 50 area% or more of cementite in the steel.

Patent Document 3 discloses a high carbon steel sheet with excellent formability containing a chemical composition including, by mass %, C: 0.20 to 1.00%, Si: 0.20% or more and 1.20% or lower, Mn: 0.05 to 0.50%, N: 0.005 to 0.015%, B: 0.2 × N% to 0.8 × N%, and Al: lower than 0.05% and satisfying 1.0 × (N - B)% to 5.0 × (N - B)%, balance Fe, inevitable impurities, P in the impurities of 0.020% or lower, and S in the impurities being 0.010% or lower, and a microstructure containing ferrite, graphite, and cementite, and a method for manufacturing the same.

• Patent-Document 1: Japanese Patent Application Laid-Open (JP-A) No. 1-025946
• Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 7-258743
• Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No. 4-202744

Conventionally, addition of a large amount of Si has been essential to graphitize c in steel for improving formability as described in, for example, Patent Documents 1 and 3. However, adding of Si hardens ferrite itself, which makes it difficult to obtain favorable formability. Moreover, as described in Patent Document 2, a technique has been developed which achieves graphitization and an increase in ductility by forming a component system containing B and Nb, and performing annealing twice under predetermined conditions, even when the addition amount of Si is not always large. However, performing annealing twice increases cost. Here, Patent Document 2 relates to a technique which is intended to graphitize 50% or more of cementite in steel. As a component composition of steel disclosed in Examples of Patent Document 2, the amount of Si is large and exceeds 0.20%. Although the steel sheets described in Patent Documents 1 to 3 are soft and excellent in bending properties and stretching properties in a tensile test, graphite and cementite may not fully dissolve at the time of hardening treatment of a steel sheet depending on heat conditions, resulting in poor hardening in some cases. Although the steel sheets described in Patent Documents 1 to 3 are soft, the steel sheets have had a problem that they are not always excellent in stretch-flangeability which is an index of hole expanding formability after punching.

The present invention aims to provide a steel sheet which is soft and has excellent formability and quench hardenability and a steel sheet with excellent formability having excellent stretch-flangeability, and a method for manufacturing the same.

The present inventors have conducted intensive studies on the above-described problems of the prior-art techniques. As a result, the present inventors found that, even in the case where the content of Si in a high carbon steel is very low, specifically 0.1% or lower, excellent formability can be achieved and excellent quench hardenability and stretch-flangeability can be secured by controlling the distributions of graphite and cementite even when a graphitization ratio is not always high. More specifically, the present inventors have conduced intensive studies on influences of the microstructure of a steel sheet containing C: 0.3 to 0.7 mass% on strength, quench hardenability, and stretch-flangeability thereof, and, as a result, found the following findings:

1. (1) For softening, it is effective to form a microstructure containing ferrite, graphite, and cementite and to adjust the total volume ratio of ferrite, graphite, and cementite to 95% or more based on the whole microstructure and adjust the volume ratio of graphite based on the total of graphite and cementite to 5% or more.
2. (2) It is necessary to adjust the mean grain diameter of graphite and cementite to 5 µm or lower for improvement in quench hardenability.
3. (3) For control of the grain diameters of graphite and cementite, cooling conditions after hot rolling are very important.
4. (4) For improvement in stretch-flangeability, it is necessary to adjust the total volume ratio of graphite and cementite present in ferrite grains to 15% or lower based on the total of graphite and cementite.
5. (5) For control of the volume ratios of graphite and cementite present in ferrite grains, cooling conditions after hot rolling are very important.

The present invention has been made based on such findings, and provides a steel sheet, containing: a composition containing, by mass%, C: 0.3 to 0.7%, Si: 0.1% or lower, Mn: 0.20% or lower, P: 0.01% or lower, S: 0.01% or lower, Al: 0.05% or lower, N: 0.0050% or lower, balance Fe, and inevitable impurities and a microstructure containing ferrite, graphite, and cementite, in which the total volume ratio of ferrite, graphite, and cementite based on the whole microstructure is 95% or more, the volume ratio of graphite (ratio of graphite) based on the total of graphite and cementite is 5% or more, and the mean grain diameter of graphite and cementite is 5 µm or lower.

It is preferable for the steel sheet of the present invention to contain at least one member selected from Ni: 3.0% or lower, B: 0.005% or lower, and Cu: 0.1% or lower (by mass%).

The steel sheet of the present invention can be obtained by a method, including: hot rolling the steel having the above-described composition at a finishing temperature of from 800 to 950°C to manufacture a hot rolled sheet, cooling the hot rolled sheet at a mean cooling rate of 50°C/s or more to a cooling temperature of 500°C or lower, winding the resultant at a winding temperature of 450°C or lower, and then annealing the wound hot rolled sheet at an annealing temperature of 720°C or lower.

The present invention provides a steel sheet, containing: a composition containing, by mass %, C: 0.3 to 0.7%, Si: 0.1% or lower, Mn: lower than 0.15%, P: 0.01% or lower, S: 0.01% or lower, Al: 0.05% or lower, N: 0.0050% or lower, balance Fe, and inevitable impurities; and a microstructure containing ferrite, graphite, and cementite, in which the total volume ratio of ferrite, graphite, and cementite based on the whole microstructure is 95% or more, the volume ratio of graphite (ratio of graphite) based on the total of graphite and cementite is 5% or more, and the total volume ratio of graphite and cementite present in ferrite grains based on the total of graphite and cementite is 15% or lower.

It is preferable for the steel sheet of the present invention to contain at least one member selected from Ni: 3.0% or lower, B: 0.005% or lower, and Cu: 0.1% or lower (by mass%).

The steel sheet of the present invention can be obtained by a method, including: hot rolling the steel having the above-described composition at a finishing temperature of from 800 to 950°C to manufacture a hot rolled sheet, cooling the hot rolled sheet at a mean cooling rate of 50°C/s or more to a cooling temperature of 600°C or lower, winding the resultant at a winding temperature of 550°C or lower, and then annealing the wound hot rolled sheet at an annealing temperature of 720°C or lower.

The present invention has made it possible to manufacture a steel sheet which is soft and has excellent formability and quench hardenability. In particular, the steel sheet of the present invention can be easily manufactured at low cost because components and cooling conditions after hot rolling may be merely controlled. Moreover, the steel sheet of the present invention is soft and excellent in formability, and thus is suitable for thickening of automobile driving parts. Even when applied to complicated-shaped parts, processing and welding of a plurality of parts become unnecessary, and thus an increase in productivity and cost reduction of automotive parts can be achieved. Furthermore, in the steel sheet of the present invention, poor hardening due to non-dissolution of graphite and cementite at the time of heating with high frequency does not occur.

The present invention has made it possible to manufacture a steel sheet which is soft and is excellent in formability, such as stretch-flangeability. In particular, the steel sheet of the present invention can be easily manufactured at low cost because components and cooling conditions after hot rolling may be merely controlled. Moreover, the steel sheet of the present invention is soft and excellent in formability, such as stretch-flangeability, and thus is suitable for thickening of automobile driving parts. Even when applied to complicated-shaped parts, processing and welding of a plurality of parts become unnecessary, and thus an increase in productivity and cost reduction of automotive parts can be achieved.

• [Fig. 1] Fig. 1 is a diagram illustrating the relationship between the mean particle diameter d and ΔHv of cementite and graphite.
• [Fig. 2] Fig. 2 is a diagram illustrating the relationship between the volume ratio S and the mean λ of cementite and graphite present in ferrite grains.

Hereinafter, a steel sheet excellent in formability and a method for manufacturing the same of the present invention will be described in detail. It should be noted that "%" indicating the amount of a component is "mass%" unless otherwise specified.

C is an element forming graphite. When the amount of C is lower than 0.3%, hardness after quench hardening cannot be secured. When the amount of C exceeds 0.7%, a steel sheet is hardened, resulting in reduced formability, even when graphitized. Therefore, the amount of C is adjusted to 0.3 to 0.7%.

When the amount of Si exceeds 0.1%, ferrite is hardened, resulting in reduced formability. Therefore, the amount of Si is adjusted to 0.1% or lower, and preferably 0.05% or lower.

When the amount of Mn exceeds 0.20%, graphite formation is impeded. Thus, Mn is adjusted to 0.20% or lower, and preferably 0.10% or lower.

Since P is segregated on grain boundaries or the like to reduce formability and has an action of stabilizing cementite to impede graphite formation, the amount of P is preferably reduced as much as possible. Therefore, the amount of P is adjusted to 0.01% or lower, and preferably 0.008% or lower.

Since S forms sulfide, such as MnS, to reduce formability and has an action of stabilizing cementite to impede graphite formation, the amount of S is preferably reduced as much as possible. Therefore, the amount of S is adjusted to 0.01% or lower, and preferably 0.007% or lower.

Al is an element which is combined with solid solution N to form AlN, thereby rendering the adverse effects of solid solution N, which has an action of impeding graphite formation, harmless and which promotes graphite formation with AlN as the nucleus.

Therefore, it is preferable to adjust the amount of Al to 0.003% or more. When the amount of Al exceeds 0.05%, cleanliness of steel decreases to deteriorate formability. Thus, the amount of Al is 0.05% or lower, and preferably 0.04% or lower.

When the amount of N exceeds 0.0050%, the action of solid solution N of stabilizing cementite becomes remarkable, and graphite formation is impeded. Therefore, the amount of N is adjusted to 0.0050%, and preferably 0.0040% or lower.

The balance contains Fe and inevitable impurities, and it is preferable that at least one member selected from Ni: 30% or lower, B: 0.005% or lower, and Cu: 0.1% or lower be contained for the following reasons.

Ni is an element which promotes graphite formation and which is effective in improvement in quench hardenability. In order to obtain such effects, it is preferable to contain 0.1% or more of Ni. However, when the amount of Ni exceeds 3.0%, the effects are saturated. Therefore, the amount of Ni is adjusted to 3.0% or lower, preferably 0.1 to 3.0%, and more preferably 0.3 to 1.0%.

B is a useful element which is combined with N to form BN, and acts as the nucleus of graphite formation and which effectively acts in improvement in quench hardenability. In order to obtain such effects, it is preferable to contain 0.0005% or more of B. When the amount of B exceeds 0.005%, the effects are saturated. Therefore, the amount of B is adjusted to 0.005% or lower, preferably 0.0005 to 0.005%, and more preferably 0.0010 to 0.0040%.

Cu is an element which promotes graphite formation and which is effective in improvement in quench hardenability. In order to obtain such effects, Cu is contained in a proportion of 0.01% or more, and more preferably 0.02% or more. However, when the amount of Cu exceeds 0.1%, the effects are saturated. Therefore, the amount of Cu is adjusted to 0.1% or lower, and preferably 0.07% or lower.

In order to soften a steel sheet and to increase bending properties and stretch properties in a tensile test, it is necessary to form a microstructure containing ferrite, graphite, and cementite, to adjust the total volume ratio of ferrite, graphite, and cementite based on the whole microstructure to 95% or more, and to adjust the ratio of graphite based on the total of graphite and cementite to 5% or more. The present invention includes the case where the ratio of graphite is 100%, i.e., cementite being thoroughly graphitized, because the same effects are obtained. When the total volume ratio of ferrite, graphite, and cementite is lower than 95%, i.e., when the volume ratio of a phase other than ferrite, graphite, and cementite exceeds 5%, formability decreases. When the ratio of graphite is lower than 5%, formability decreases.

Here, the volume ratio of ferrite, graphite, and cementite is determined as follows. More specifically, a steel sheet is ground at 1/4 position of the sheet thickness of a through-thickness section in the rolling direction of the steel sheet, and subjected to nital corrosion. Then, the resultant is observed under an optical microscope (400x magnification) for 5 parts per visual field, i.e., 10 visual fields (Total: 50 parts). These images are subjected to image analysis with an image-analysis software "Image Pro Plus ver. 4.0" manufactured by Media Cybernetics. Then, areas of ferrite, graphite, and cementite are measured, and the proportions (area ratios) based on the whole observed area are defined as the volume ratio of each of ferrite, graphite, and cementite. Moreover, the proportion (area ratio) of the graphite area (Sgr) based on the sum of the graphite area (Sgr) and the cementite area (Scm) is defined as the volume ratio of graphite (ratio of graphite). More specifically, the ratio of graphite (%) can be represented by the following equation. $$\mathrm{Ratio\; of\; graphite}=\left\{\mathrm{Sgr}/\left(\mathrm{Sgr}+\mathrm{Scm}\right)\right\}\times 100$$

Even when the total volume ratio of ferrite, graphite, and cementite and the ratio of graphite are merely controlled, excellent quench hardenability, especially quench hardenability at the time of performing induction quench hardening, is not always obtained. More specifically, in the present invention, in order to secure excellent quench hardenability, it is necessary to adjust the mean particle diameter of cementite and graphite to 5 µm or lower. More preferably, the mean particle diameter of cementite and graphite is adjusted to 3 µm or lower.

The present inventors have conducted various studies in order to obtain excellent quench hardenability. Hereinafter, an example of the studies will be described. More specifically, a steel slab containing C: 0.55%, Si: 0.01%, Mn: 0.10%, P: 0.003%, S: 0.0006%, Al: 0.005%, N: 0.0018%, Ni: 0.50%, B: 0.0013%, balance Fe, and inevitable impurities is heated to 1,150°C. Then, the resultant is subjected to rough rolling of 5 passes, and then subjected to finish rolling of 7 passes at a finishing temperature of 880°C to form a hot rolled sheet with a sheet thickness of 4.0 mm. Then, the hot rolled sheet is wound at a winding temperature of 430°C, washed with acid, and then subjected to batch annealing at 720°C for 40 hr. At this time, in order to change the grain diameters of cementite and graphite, cooling is performed after finish rolling while controlling the temperature range to the winding temperature at a mean cooling rate of from air-cooling (5°C/(s)) to 200 °C/s. Then, the microstructure and quench hardenability are examined as follows.

Similarly as described above, a steel sheet is ground at 1/4 position of the sheet thickness of a cross section parallel to the rolling direction of the steel sheet, and subjected to nital corrosion. Then, the cross section is observed under a scanning electron microscope (1,500x magnification) for 5 parts per visual field, i.e., 10 visual fields (Total: 50 parts). Using the above-mentioned image-analysis software, the diameter passing through two points on the outer circumference of cementite or graphite and the center of gravity of a substantially oval shape of cementite or graphite (ellipse having the same area as cementite and graphite and having the same primary and second moments as cementite and graphite) is measured twice, and then averaged to thereby determine each grain diameter. Then, grain diameters of cementite and graphite measured by observing 50 visual fields are averaged to be used as mean grain diameters of cementite and graphite.

Quench hardenability: A disc test sample having a diameter of 100 mm is extracted, the peripheral end of the disc test sample is heated to 1000°C using an induction heat treatment apparatus at a frequency of 100 kHz, and then the resultant is immediately water cooled. Then, the disc test sample after heat treatment is measured for Vickers hardness [Load: 49N (= 5 kgf)] of the front and rear surfaces 1.5 mm inside from the peripheral end at 8 places along the circumferential direction to obtain a difference ΔHv between the maximum Hv and the minimum Hv. When the ΔHv is 8 or lower, it can be said that the quench hardenability is excellent.

Fig. 1 shows the relationship between the mean grain diameter d and ΔHv of cementite and graphite. When the mean grain diameter d of cementite and graphite becomes 5 µm or lower, ΔHv becomes 8 or lower, which shows that excellent quench hardenability is obtained.

The present inventors have conducted various studies based on the above studies, and as a result, found that, in order to secure excellent quench hardenability, the mean grain diameter of cementite and graphite needs to be 5 µm or lower, and preferably 3 µm or lower. Thus, a reason why excellent quench hardenability is obtained by specifying a microstructure is considered as follows. More specifically, it is considered that, when the mean grain diameter of cementite and graphite become 5 µm or lower, cementite and graphite nearly thoroughly dissolve at the time of high frequency heating, and thus hardness after quench hardening is equalized.

Hereinafter, preferable manufacturing conditions of the steel sheet of the present invention will be described. It should be noted that the method for manufacturing a steel sheet of the present invention is not limited to the following methods.

When a finishing temperature at the time of hot rolling is lower than 800°C, a rolling load sharply increases. When the finishing temperature exceeds 950°C, a scale to be generated is thickened, pickling properties decrease, and a decarburized layer is manufactured on a steel sheet surface layer in some cases. Thus, the finishing temperature at the time of hot rolling is adjusted to 800 to 950°C.

A steel sheet after hot rolling is immediately cooled to a cooling stop temperature mentioned later at a mean cooling rate of 50°C/s or more. When the mean cooling rate is lower than 50°C/s, ferrite grains easily grow during cooling to form large ferrite grains. It is considered that, at the time of annealing performed thereafter, graphite or cementite is formed with ferrite grain boundaries, inclusions, etc., as the nucleus. Thus, when ferrite grains are large, graphite or cementite which is formed with grain boundaries as the nucleus is coarsened, resulting in reduced quench hardenability. When the mean cooling rate is low, pearlites with coarse carbides are generated. Since graphite or cementite is formed through fragmentation, agglomeration, and coarsening of carbides in pearlites, graphite or cementite is coarsened, resulting in reduced quench hardenability. It should be noted that there are merits that, when the mean cooling rate is adjusted to 50°C/s or higher, rolling distortion introduced into austenite by hot rolling easily remains in a microstructure after modification to increase dislocation density, and graphite formation with such dislocation as the nucleus is promoted at the time of annealing. As described above, the mean cooling rate is 50°C/s or higher, and preferably 80°C/s or higher. The upper limit of the mean cooling rate is not necessary specified, and is preferably 200°C/s or lower so as to suppress deterioration of the shape of a steel sheet to secure the shape of the steel sheet.

When the lowest temperature which needs to be cooled at the above-mentioned cooling rate, i.e., cooling stop temperature, exceeds 500°C, pro-eutectoid ferrite generates during cooling until winding and a coarse pearlite generates. Thus, cementite or graphite is coarsened at the time of annealing after winding, reducing quench hardenability.

Thus, the cooling stop temperature is adjusted to 500°C or lower, and preferably 470°C or lower. The lower limit of the cooling stop temperature is not necessary specified, and is preferably 200°C or higher so as to secure the shape of a steel sheet.

A hot rolled sheet after cooling is immediately wound. At the time of winding, when a winding temperature exceeds 450°C, a coarse pearlite generates, and thus cementite or graphite is coarsened at the time of annealing, resulting in reduced quench hardenability. Therefore, the winding temperature is adjusted to 450°C or lower. It should be noted that the winding temperature is preferably lower than the cooling stop temperature so as to sufficiently obtain the above-described cooling effects after hot rolling. Moreover, since the shape of a hot rolled sheet easily deteriorates, the winding temperature is preferably adjusted to 200°C or higher.

A hot rolled sheet after winding is washed with acid or the like to remove scales, and is annealed so as to promote spheroidizing or graphitization of cementite for softening. During the process, when the annealing temperature exceeds 720°C, a coarse pearlite generates during cooling, resulting in reduced quench hardenability. Thus, the annealing temperature is adjusted to 720°C or lower. When the annealing temperature is lower than 600°C, annealing time is excessively prolonged. Thus, the annealing temperature is preferably adjusted to 600°C or higher.

It should be noted that the annealing time is not necessary limited, and is preferably 8 hr or more so as to form graphite or 100 hr or lower because there is a possibility that ferrite grains may be excessively coarsened, resulting in reduced ductility.

For melting the steel of the present invention, both a converter and an electric furnace are usable. The steel thus melted is formed into a slab by ingot making-slabbing or continuous casting. A slab is generally hot rolled after heating (reheating). It should be noted that, in the case of a slab manufactured by continuous casting, the slab can be used as it is or may be subjected to direct rolling in which rolling is performed while maintaining heat so as to suppress reduction in temperature. When reheating a slab for hot rolling, it is preferable to adjust a slab heating temperature to 1,280°C or lower so as to avoid deterioration of the surface condition due to scales. The hot rolling can be carried out merely by finish rolling while omitting rough rolling. In order to secure a finishing temperature, a material to be rolled may be heated with a heating member, such as a sheet bar heater, during hot rolling. The sheet thickness of a hot rolled sheet is not limited insofar as the manufacturing conditions of the present invention can be maintained, and is preferably from 1.0 to 10.0 mm. The steel sheet after annealing can be subjected to temper rolling as required. A working example will be described in Example 1.

Simply by controlling the total volume ratio of ferrite, graphite, and cementite and the ratio of graphite, excellent stretch-flangeability is not always obtained. More specifically, in the present invention, the total volume ratio of cementite and graphite present in ferrite grains needs to be adjusted to 15% or lower in order to secure excellent stretch-flangeability. More preferably, the total volume ratio thereof is adjusted to 10% or lower.

The present inventors have conducted various studies in order to obtain excellent stretch-flangeability. An example of the studies will be described below. A steel slab containing C: 0.55%, Si: 0.01%, Mn: 0.10%, P: 0.003%, S: 0.0006%, Al: 0.005%, N: 0.0018%, Ni: 0.50%, B: 0.0013%, balance Fe, and inevitable impurities is heated to 1,150°C, subjected to rough rolling of 5 passes, subjected to finish rolling of 7 passes at a finishing temperature of 870°C to manufacture a hot rolled sheet having a sheet thickness of 4.0 mm. Then, the hot rolled sheet is wound at a winding temperature of 520°C, washed with acid, and subjected to batch annealing at 720°C for 40 hr. At this time, for the purpose of changing the amounts and distribution states of cementite and graphite, cooling is performed after finish rolling while changing the temperature range to the winding temperature at a mean cooling rate of from air-cooling (5°C/(s)) to 200°C/s. Then, the microstructure and stretch-flangeability are examined as follows.

Similarly as the above, a steel sheet is ground at 1/4 position of the sheet thickness of a cross section parallel to the rolling direction of the steel sheet, and subjected to nital corrosion. Then, the cross section is observed under an optical microscope (400x magnification) for 5 parts of the cross section, i.e., 10 visual fields (Total: 50 parts). Using the above-mentioned image-analysis software, cementite and graphite present on ferrite grain boundaries and cementite and graphite present in ferrite grains are distinguished. The occupation area S_on of cementite and graphite present on ferrite grain boundaries and the occupation area S_in of cementite and graphite present in ferrite grains are measured. The area ratio of cementite and graphite present in ferrite grains is measured according to the following equation to be used as a volume ratio S(%) of cementite and graphite present in ferrite grains based on the total of cementite and graphite. More specifically, S(%) can be represented by the following equation. $$\mathrm{S}\mathrm{=}\{{\mathrm{S}}_{\mathrm{in}}\mathrm{/}\left({\mathrm{S}}_{\mathrm{on}}\mathrm{+}{\mathrm{S}}_{\mathrm{in}}\right)\mathrm{\times }\mathrm{100}$$

It should be noted that, with respect to cementite grains or graphite grains having a part present on ferrite grain boundaries, the whole area of each cementite grain or each graphite grain is measured as an occupation area of cementite grains or graphite grains present on ferrite grain boundaries. Moreover, the area of cementite grains or graphite grains not having a part present on ferrite grain boundaries is measured as an occupation area of cementite grains or graphite grains present in ferrite grains.

Stretch-flangeability: A test sample for a hole expanding test (100 × 100 mm) is extracted, and is punched using a punching tool having a punch diameter of 10 mm and a die diameter of 11.6 mm (clearance: sheet thickness of 20%) at the center of the test sample. Thereafter, the punched hole is pushed up using a cylindrical flat bottomed punch (diameter: 50 mmΦ, shoulder R: 8 mm) for hole expanding. Then, the hole diameter d (mm) at the time when through thickness cracks are formed at the hole edge is measured. Then, the hole expanding ratio λ(%) is calculated according to the equation. The same test is carried out 6 times to thereby obtain the mean ratio λ(%). $$\mathrm{\lambda }\mathrm{=}\mathrm{100}\mathrm{\times }\left(\mathrm{d}\mathrm{-}\mathrm{10}\right)\mathrm{/}\mathrm{10}$$

Fig. 2 represents the relationship between the volume ratio S and the mean λ of cementite and graphite present in ferrite grains. It is revealed that when the volume ratio S of cementite and graphite present in ferrite grains becomes 15% or lower, the mean λ becomes 60% or more, and excellent stretch-flangeability is obtained.

The present inventors have conducted various studies based on the above studies, and, as a result, fount that, in order to secure excellent stretch-flangeability, the total volume ratio of cementite and graphite present in ferrite grains needs to be adjusted to 15% or lower, and preferably 10% or lower. The reason why excellent stretch-flangeability is obtained by specifying the microstructure as described above is considered as follows. More specifically, when a large amount of cementite or graphite is present in ferrite grains, fine cracks are likely to form at the interfaces between cementite or graphite and ferrite at the time of punching, and propagation and coalescence of cracks occur from the first stage of a hole expanding test, easily resulting in the formation of through thickness cracks. In contrast, the diffusion rate of carbon on ferrite grain boundaries is high, and thus an increase in agglomeration is promoted rather than inside ferrite grains. Thus, cementite or graphite on ferrite grain boundaries is likely to coarsen rather than cementite or graphite in ferrite grains, and the gap between each cementite grain and each graphite grain is likely to become broad. Therefore, cementite or graphite on ferrite grain boundaries slows down crack propagation compared with cementite or graphite in ferrite grains.

Hereinafter, preferable manufacturing conditions of the steel sheet of the present invention will be described. It should be noted that a method for manufacturing a steel sheet of the present invention is not limited to the following methods.

When the finishing temperature at the time of hot rolling is lower than 800°C, a rolling load sharply increases. When the finishing temperature at the time of hot rolling exceeds 950°C, a scale to be generated becomes thick, pickling properties decrease, and a decarburized layer may be formed on a steel sheet surface layer. Thus, the finishing temperature at the time of hot rolling is adjusted to 800 to 950°C.

When a steel sheet after hot rolling is immediately cooled to a cooling stop temperature mentioned later at a mean cooling rate of 50°C/s or more, formation of pro-eutectoid ferrite is suppressed and ferrite and cementite are finely precipitated. Therefore, c is likely to diffuse on ferrite grain boundaries at the time of annealing performed after winding, agglomeration and coarsening of cementite present on ferrite grain boundaries and graphitization thereof are promoted, cementite or graphite in ferrite grains decrease, and stretch-flangeability increases. Moreover, rolling distortion introduced into austenite with hot rolling is likely to remain in the microstructure after modification, resulting in an increase in dislocation density. As a result, the formation of graphite with dislocation as the nucleus becomes easy at the time of annealing and softening proceeds, resulting in increased formability. Considering the above, the mean cooling rate is adjusted to 50°C/s or more, and preferably 80°C/s or more. The upper limit of the mean cooling rate does not need to be specified, and is preferably adjusted to 200°C/s or lower in order to suppress deterioration of the shape of a steel sheet and secure the shape of a steel sheet.

The lowest temperature which needs to be cooled at the above-mentioned cooling rate, i.e., cooling stop temperature, exceeds 600°C, a pro-eutectoid ferrite generates during cooling to winding, a pearlite generates, cementite or graphite present in ferrite grains increases at the time of annealing after winding, and stretch-flangeability deceases. Thus, the cooling stop temperature is adjusted to 600°C or lower, and preferably 550°C or lower. The lower limit of the cooling stop temperature does not need to be specified, and is preferably adjusted to 200°C or higher in order to secure the shape of a steel sheet.

A hot rolled sheet after cooling is immediately wound. When the winding temperature exceeds 550°C, a pearlite generates, cementite or graphite present in ferrite grains at the time of annealing increases, and stretch-flangeability decreases. Therefore, the winding temperature is adjusted to 550°C or lower. It should be noted that, in order to fully obtain the effects of cooling after hot rolling, it is preferable for the winding temperature to be lower than the cooling stop temperature. Since the shape of a hot rolled sheet is likely to deteriorate, in view of securing the shape of a steel sheet, the winding temperature is adjusted to preferably 200°C or higher, and more preferably exceeding 450°C.

A hot rolled sheet after winding is washed with acid to remove scales, and then is annealed in order to promote spheroidizing and graphitization of cementite for softening. When the annealing temperature exceeds 720°C, a pearlite generates during cooling and stretch-flangeability deceases. Thus, the annealing temperature is adjusted to 720°C or lower. When the annealing temperature is lower than 600°C, there is a tendency that cementite or graphite present in ferrite grains increases and stretch-flangeability deteriorates. Thus, the annealing temperature is adjusted to 600°C or higher.

It should be noted that the annealing time does not need to be specified, and is preferably 8 hr or more for forming graphite and reducing cementite or graphite present in ferrite grains. Moreover, there is a possibility that ferrite grains are excessively coarsened to reduce ductility, and thus the annealing time is preferably 100 hr or lower.

For melting the steel of the present invention, both a converter and an electric furnace are usable. The steel thus melted is formed into a slab by ingot making-slabbing or continuous casting. A slab is generally hot rolled after heating (reheating). It should be noted that, in the case of a slab manufactured by continuous casting, the slab can be used as it is, or may be subjected to direct rolling in which rolling is performed while maintaining heat so as to suppress reduction in temperature. When reheating a slab for hot rolling, it is preferable to adjust the slab heating temperature to 1,280°C or lower so as to avoid deterioration of the surface condition due to scales. The hot rolling can be carried out merely by finish rolling while omitting rough rolling. In order to secure a finishing temperature, a material to be rolled may be heated with a heating member, such as a sheet bar heater, during hot rolling. The sheet thickness of a hot rolled sheet is not limited insofar as the manufacturing conditions of the present invention can be maintained, and is preferably from 1.0 to 10.0 mm. The hot rolled sheet is washed with acid or subjected to shot blasting to remove scales on the surface, and then annealed. The steel sheet after annealing can be subjected to temper rolling as required. A working example will be described in Example 2.

Slabs of No. A to S steels having the compositions shown in Table 1 were heated to 1,250°C, hot rolled under the conditions shown in Table 2, washed with acid, and annealed under the conditions shown in Table 2 to manufacture No. 1 to 22 steel sheets having a sheet thickness of 4.0 mm. Then, a graphite ratio, a mean grain diameter of cementite and graphite, and ΔHv for evaluating quench hardenability were measured. Separately, a JIS No. 5 test piece for a tensile test was extracted along the rolling direction. Then, a tensile test was carried out, and a yield stress YP, a tensile strength Ts, and elongation El were measured.

The results are shown in Table 3. It is revealed that all the steel sheets of this example of the present invention have low YP, low TS, high El, and low ΔHv, are soft, and are excellent in formability and quench hardenability. It has been confirmed that the microstructure of each steel sheet of this example of the present invention basically contains ferrite, cementite, and graphite as shown in Table 3, and that the total volume ratio thereof needs to be 95% or more. (mass%) Steel No. C Si Mn P S Al N Ni B Cu Remarks A 0.35 0.08 0.06 0.010 0.0033 0.037 0.0031 - - - Within the scope of the present invention B 0.33 0.05 0.09 0.006 0.0032 0.023 0.0037 0.45 0.0023 - Within the scope of the present invention C 0.32 0.09 0.14 0.009 0.0040 0.026 0.0028 - 0.0017 - Within the scope of the present invention D 0.37 0.01 0.08 0.005 0.0037 0.025 0.0039 0.63 - - Within the scope of the present invention E 0.36 0.02 0.15 0.007 0.0035 0.023 0.0042 0.47 0.0021 0.08 Within the scope of the present invention F 0.33 0.07 0.11 0.009 0.0059 0.045 0.0035 - - 0.07 Within the scope of the present invention G 0.44 0.05 0.10 0.005 0.0041 0.029 0.0026 0.46 0.0033 0.06 Within the scope of the present invention H 0.45 0.06 0.14 0.008 0.0044 0.036 0.0045 0.57 - - Within the scope of the present invention I 0.43 0.01 0.09 0.010 0.0025 0.010 0.0034 - 0.0029 - Within the scope of the present invention the of the invention J 0.46 0.05 0.08 0.005 0.0036 0.032 0.0033 - - 0.06 Within scope present the of the invention K 0.47 0.03 0.07 0.003 0.0028 0.005 0.0029 0.43 0.0024 - Within scope present L 0.44 0.08 0.10 0.032 0.0034 0.043 0.0041 - - - Outside the scope of the present invention the invention M 0.43 0.05 0.08 0.008 0.0044 0.035 0.0035 - - - Within the scope of present the invention N 0.54 0.03 0.07 0.007 0.0026 0.003 0.0017 0.52 0.0025 - Within the scope of present O 0.55 0.10 0.76 0.009 0.0037 0.023 0.0044 - - - Outside the scope of the present invention P 0.51 0.07 0.04 0.008 0.0021 0.025 0.0047 0.77 0.0015 0.03 Within the scope of the present invention Q 0.58 0.07 0.09 0.007 0.0033 0.034 0.0029 - - - Within the scope of the present invention R 0.63 0.10 0.13 0.010 0.0046 0.028 0.0049 0.61 0.0026 - Within the scope of the present invention S 0.69 0.04 0.07 0.007 0.0040 0.031 0.0032 - - - Within the scope of the present invention Steel sheet No. Steel No. Hot rolling conditions Annealing conditions Remarks Finishing temperature
(°C) Mean cooling rate
(°C/s) Cooling stop temperature
(°C) Winding temperature
(°C) Temperature
(°C) Time
(hr) 1 A 855 75 500 430 710 50 Example of the present invention 2 B 830 85 470 440 720 45 Example of the present invention 3 C 865 55 485 430 700 40 Example of the present invention 4 D 850 100 460 410 720 40 Example of the present invention 5 D 855 40 480 435 720 40 Comparative example 6 D 850 90 595 420 720 40 Comparative example 7 D 855 95 490 470 720 40 Comparative example 8 E 880 80 485 430 650 80 Example of the present invention 9 F 840 95 475 445 690 50 Example of the present invention 10 G 875 100 450 400 720 50 Example of the present invention 11 H 920 70 475 425 715 40 Example of the present invention 12 I 865 50 500 450 680 60 Example of the present invention 13 J 855 85 470 410 720 40 Example of the present invention 14 K 875 115 450 430 710 40 Example of the present invention 15 L 840 95 445 405 720 50 Comparative example 16 M 855 85 460 430 720 40 Example of the present invention 17 N 850 90 460 400 720 40 Example of the present invention 18 O 845 95 440 415 720 50 Comparative example 19 P 865 55 465 435 700 60 Example of the present invention 20 Q 845 100 440 410 670 40 Example of the present invention 21 R 850 60 495 420 630 70 Example of the present invention 22 S 890 85 430 400 700 50 Example of the present invention Steel sheet No. Steel No. Microstructure Tensile properties ΔHv Remarks Composition* Ratio of graphite
(%) Mean grain diameter of cementite and graphite (µm) YP
(MPa) TS
(MPa) El
(%) 1 A F+G+C 16 4.5 152 330 53.8 6.6 Example of the present invention 2 B F+G+C 25 3.0 138 314 53.7 4.7 Example of the present invention 3 C F+G+C 19 4.2 153 333 51.0 7.1 Example of the present invention 4 D F+G+C 16 2.1 137 311 54.5 1.4 Example of the present invention 5 D F+G+C 3 7.8 195 389 40.2 14.5 Comparative example 6 D F+G+C 14 8.3 177 354 44.0 15.3 Comparative example 7 D F+G+C 13 10.4 175 357 46.3 16.0 Comparative example 8 E F+G+C 23 4.1 158 343 49.4 6.5 Example of the present invention 9 F F+G+C 25 4.7 156 339 49.1 7.5 Example of the present invention 10 G F+G+C 37 2.7 145 323 50.5 3.3 Example of the present invention 11 H F+G+C 42 3.6 163 354 47.3 6.4 Example of the present invention 12 I F+G+C 35 3.9 172 367 48.6 6.9 Example of the present invention 13 J F+G+C 41 2.5 144 320 51.2 2.5 Example of the present invention 14 K F+G+C 44 2.8 145 322 51.0 3.4 Example of the present invention 15 L F+C 3 8.4 361 555 35.8 16.2 Comparative example 16 M F+G+C 28 2.3 146 325 49.9 3.0 Example of the present invention 17 N F+G+C 63 1.9 167 334 46.2 1.8 Example of the present invention 18 O F+C 2 9.1 383 598 32.5 14.9 Comparative example 19 P F+G+C 72 4.4 179 366 45.9 6.2 Example of the present invention 20 Q F+G+C 50 2.2 157 335 43.7 2.5 Example of the present invention 21 R F+G+C 74 3.7 176 382 41.3 6.0 Example of the present invention 22 S F+G+C 67 2.6 152 345 40.8 2.7 Example of the present invention *: F: ferrite, G: graphite, C: cementite

Slabs of No. AA to AS steels having the compositions shown in Table 4 were heated to 1,250°C, hot rolled under the conditions shown in Table 5, washed with acid, and annealed under the conditions shown in Table 5 to manufacture No. 101 to 122 steel sheets having a sheet thickness of 4.0 mm. Then, a ratio of graphite, the volume ratio S of cementite and graphite present in ferrite grains based on the total of cementite and graphite, and a mean λ which is an index of stretch-flangeability were measured by the above-mentioned method. Separately, a JIS No. 5 test piece for a tensile test was extracted along the rolling direction. Then, a tensile test was carried out, and a yield stress YP, a tensile strength Ts, and elongation El were measured. It should be noted that the same test was carried out twice for every test piece to obtain the mean value. Then, the mean value was defined as a property value of the steel sheet.

The results are shown in Table 6. It is revealed that all the steel sheets of this example of the present invention have low YP, low TS, high El, and high λ, are soft, and are excellent in formability including stretch-flangeability. It has been confirmed that the microstructure of each steel sheet of this example of the present invention basically contains ferrite, cementite, and graphite as shown in Table 6, and that the total volume ratio thereof needs to be 95% or more. Steel No. C Si Mn P S Al N Ni B Cu Remarks AA 0.36 0.01 0.01 0.008 0.0010 0.027 0.0023 - - - Within the scope of the present invention AB 0.32 0.08 0.08 0.007 0.0030 0.020 0.0034 0.46 0.0020 - Within the scope of the present invention AC 0.31 0.05 0.13 0.010 0.0033 0.028 0.0029 - 0.0015 - Within the scope of the present invention AD 0.36 0.01 0.10 0.006 0.0042 0.024 0.0037 0.59 - - Within the scope of the present invention AE 0.35 0.03 0.14 0.008 0.0031 0.026 0.0039 0.51 0.0018 0.06 Within the scope of the present invention AF 0.34 0.09 0.13 0.010 0.0067 0.042 0.0045 - - 0.08 Within the scope of the present invention AG 0.46 0.04 0.05 0.006 0.0038 0.025 0.0033 0.43 0.0024 0.05 Within the scope of the present invention AH 0.44 0.09 0.12 0.010 0.0054 0.039 0.0042 0.55 - - Within the scope of the present invention Al 0.47 0.03 0.10 0.009 0.0040 0.009 0.0035 - 0.0030 - Within the scope of the present invention AJ 0.45 0.05 0.07 0.007 0.0029 0.026 0.0028 - - 0.07 Within the scope of the present invention AK 0.46 0.01 0.07 0.002 0.0027 0.002 0.0035 0.40 0.0023 - Within the scope of the present invention AL 0.45 0.06 0.09 0.035 0.0030 0.034 0.0040 - - - Outside the scope of the present invention AM 0.45 0.04 0.10 0.008 0.0046 0.033 0.0039 - - - Within the scope of the present invention AN 0.55 0.01 0.10 0.001 0.0006 0.004 0.0016 0.50 0.0013 - Within the scope of the present invention AO 0.54 0.09 0.70 0.006 0.0039 0.025 0.0038 - - - Outside the scope of the present invention AP 0.53 0.08 0.03 0.007 0.0019 0.027 0.0044 0.67 0.0010 0.02 Within the scope of the present invention AQ 0.57 0.10 0.10 0.006 0.0027 0.030 0.0039 - - - Within the scope of the present invention AR 0.62 0.09 0.14 0.010 0.0044 0.022 0.0047 0.60 0.0021 - Within the scope of the present invention AS 0.67 0.05 0.10 0.008 0.0037 0.029 0.0034 - - - Within the scope of the present invention Steel sheet No. Steel No. Hot rolling conditions Annealing Remarks Finishing temperature
(°C) Mean cooling rate
(°C/s) Cooling stop temperature
(°C) Winding temperature
(°C) Temperature
(°C) Time
(hr) 101 AA 855 75 600 540 710 50 Example of the present invention 102 AB 830 85 540 520 720 45 Example of the present invention 103 AC 865 55 590 550 700 40 Example of the present invention 104 AD 850 100 550 530 720 40 Example of the present invention 105 AD 855 40 585 540 720 40 Comparative example 106 AD 850 90 660 535 720 40 Comparative example 107 AD 855 95 595 580 720 40 Comparative example 108 AE 880 80 580 525 650 80 Example of the present invention 109 AF 840 95 575 455 690 50 Example of the present invention 110 AG 875 100 530 500 720 50 Example of the present invention 111 AH 920 70 580 550 715 40 Example of the present invention 112 Al 865 50 595 530 680 60 Example of the present invention 113 AJ 855 85 545 510 720 40 Example of the present invention 114 AK 875 115 540 510 710 40 Example of the present invention 115 AL 840 95 530 510 720 50 Comparative example 116 AM 855 85 550 530 720 40 Example of the present invention 117 AN 850 90 530 520 720 40 Example of the present invention 118 AO 845 95 545 505 720 50 Comparative example 119 AP 865 55 585 545 700 60 Example of the present invention 120 AQ 845 100 550 525 670 40 Example of the present invention 121 AR 850 60 570 550 630 70 Example of the present invention 122 AS 890 85 540 515 700 50 Example of the present invention Steel sheet No. Microstructure* Ratio of graphite S Tensile properties Average λ Remarks YP TS El (%) (%) (MPa) (MPa) (%) (%) 101 F+G+C 14 11 153 332 52.4 70 Example of the present invention 102 F+G+C 23 7 146 325 53.5 75 Example of the present invention 103 F+G+C 21 14 155 337 49.8 68 Example of the present invention 104 F+G+C 18 8 147 335 54.3 77 Example of the present invention 105 F+G+C 3 30 295 483 35.5 49 Comparative example 106 F+G+C 13 35 173 345 51.7 46 Comparative example 107 F+G+C 12 25 168 343 52.6 53 Comparative example 108 F+G+C 25 12 157 341 49.6 71 Example of the present invention 109 F+G+C 19 12 159 346 48.2 70 Example of the present invention 110 F + G + C. 46 9 157 348 49.9 69 Example of the present invention 111 F+G+C 39 13 164 356 45.3 62 Example of the present invention 112 F+G+C 43 13 165 350 47.4 61 Example of the present invention 113 F+G+C 37 10 156 347 50.7 65 Example of the present invention 114 F+G+C 48 8 155 344 51.5 67 Example of the present invention 115 F+C 2 55 359 553 33.9 36 Comparative example 116 F+G+C 35 9 157 349 49.6 66 Example of the present invention 117 F+G+C 67 8 193 385 45.1 64 Example of the present invention 118 F+C 1 40 364 568 31.2 32 Comparative example 119 F+G+C 77 14 179 365 43.5 60 Example of the present invention 120 F+G+C 52 7 178 379 42.7 63 Example of the present invention 121 F+G+C 76 13 175 381 40.6 55 Example of the present invention 122 F+G+C 69 10 167 380 41.3 57 Example of the present invention *: F: ferrite, G: graphite, C: cementite