JP2008081823A - Steel plate having excellent fine blanking workability, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel plate capable of continuous blanking on a commercial scale without causing the reduction in service life of a die, having excellent FB workability and further having excellent formability after FB working. <P>SOLUTION: The steel plate has a composition comprising, by mass, 0.1 to 0.5% C, 0.2 to 1.5% Si and Mn, and Si, P and S controlled into an appropriate range, and comprising one or two kinds selected from 0.0005 to 0.005% Ca and 0.001 to 0.02% rare earth metals so as to satisfy the specified relation with the S content, and has a structure where the average particle diameter of ferrite is 1 to 10 μm, the spheroidized rate of carbides is 80% or more, and the amount of carbides in grain boundaries of ferrite defined by the expression of S<SB>gb</SB>(%)[=äS<SB>on</SB>/(S<SB>on</SB>+S<SB>in</SB>)}×100 (wherein, S<SB>on</SB>represents the total occupation area of carbides existing in the grain boundaries among the carbides existing in a unit area; and S<SB>in</SB>represents the total occupation area of carbides existing in the grains among the carbides existing in the unit area)] is more than 40%. Then, the steel plate can be subjected to continuous blanking on a commercial scale, and acquires superior FB workability and formability after FB working. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel plate suitable for use in automobile parts and the like, and is particularly excellent in fine blanking workability suitable for use in which precision punching (hereinafter also referred to as fine blanking or FB processing) is performed. Related to the steel plate.

When manufacturing complex machine parts, it is known that fine blanking is a very advantageous processing method compared to cutting from the viewpoint of improving dimensional accuracy and shortening the manufacturing process.
In normal punching, the clearance between tools is about 5 to 10% of the thickness of the metal plate that is the material to be punched. Unlike normal punching, fine blanking is the clearance between tools. Is set to an extremely small value (actually about 2% or less of the thickness of the metal plate that is the material to be punched), and punching is performed by applying a compressive stress to the material near the tool cutting edge. And fine blanking is
(1) Suppression of cracks from the tool cutting edge is suppressed, the fracture surface seen in normal punching is almost zero, and a smooth machined surface with a machined surface (punched end surface) of almost 100% shear surface is obtained. ,
(2) Good dimensional accuracy,
(3) It has a feature such that a complicated shape can be overcome in one step. However, in the fine blanking process, the degree of processing that the material (metal plate) receives is extremely severe. Further, in the fine blanking process, since the clearance between tools is almost zero, there is a problem that the load on the mold becomes excessive and the mold life is shortened.

For this reason, materials to which fine blanking processing is applied have been required to have excellent fine blanking workability and to prevent a decrease in mold life.
In response to such a request, for example, Patent Document 1 includes a composition containing C: 0.15 to 0.90 wt%, Si: 0.4 wt% or less, Mn: 0.3 to 1.0 wt%, and a spheroidization rate of 80% or more, A high-carbon steel sheet having a structure in which carbide having an average particle diameter of 0.4 to 1.0 μm is dispersed in a ferrite matrix and having a notch tensile elongation of 20% or more and excellent in precision punching workability has been proposed. According to the technique described in Patent Document 1, the precision punchability is improved, and the die life is also improved. However, the high carbon steel sheet described in Patent Document 1 has a problem that the formability after fine blanking is inferior.

  Patent Document 2 includes C: 0.08 to 0.19%, Si, Mn, and Al in appropriate amounts, Cr: 0.05 to 0.80%, and B: 0.0005 to 0.005%. A steel sheet for precision punching that has been rolled into a steel sheet has been proposed. The steel sheet described in Patent Document 2 has low yield strength, high impact value, excellent fine blanking workability, high low strain region n value, excellent composite forming workability, and quick workability in a short time. It is said to be an excellent steel plate. However, Patent Document 2 does not show a specific evaluation for fine blanking workability. Moreover, the steel plate described in Patent Document 2 has a problem that the formability after fine blanking is inferior.

  Patent Document 3 contains C: 0.15 to 0.45%, has a composition in which the contents of Si, Mn, P, S, Al, and N are adjusted to an appropriate range, and further has a pearlite + cementite fraction of 10 %, And a high carbon steel sheet having a structure in which the average grain size of ferrite grains is 10 to 20 μm and excellent in formability in rolling and fine blanking is proposed. The high-carbon steel sheet described in Patent Document 3 is excellent in fine blanking workability and further improves the mold life in fine blanking work. However, the high carbon steel sheet described in Patent Document 3 has a problem that the formability after fine blanking is inferior.

  Furthermore, all of the steel sheets described in Patent Document 1, Patent Document 2, and Patent Document 3 have satisfactory fine blanking workability in fine blanking processing under recent severe processing conditions. However, the mold life has not been improved sufficiently, and the moldability after fine blanking has been poor.

  Initially, fine blanking has been applied to parts that are not processed after fine blanking, such as gear parts. Recently, however, the application of fine blanking to automotive parts (reclining parts, etc.) has been expanding. Considering application to parts that require stretch flange processing or overhanging after fine blanking. Has been. For this reason, as an automobile part, a steel sheet that is excellent in fine blanking workability and excellent in forming workability such as stretch flange processing and overhang processing after fine blanking processing is eagerly desired.

  Many proposals have been made as techniques for improving stretch flangeability. For example, Patent Document 4 includes C: 0.20 to 0.33%, a composition containing Si, Mn, P, S, sol. Al, and N in an appropriate range, and further containing Cr: 0.15 to 0.7%. There has been proposed a hot-rolled steel sheet for wear resistance that has a mixed structure of ferrite and bainite that may contain pearlite and has excellent stretch flangeability. In the hot-rolled steel sheet described in Patent Document 4, the above-described structure increases the hole expansion rate and improves stretch flangeability. Further, Patent Document 5 has a composition containing C: 0.2 to 0.7%, a structure in which the average particle size of carbide is 0.1 μm or more and less than 1.2 μm, and the volume fraction of ferrite grains not containing carbide is 15% or less. There has been proposed a high carbon steel sheet having excellent stretch flangeability. In the high carbon steel sheet described in Patent Document 5, the generation of voids at the end face during punching can be suppressed, the growth of cracks in the hole expanding process can be slowed, and the stretch flangeability is improved.

Patent Document 6 discloses a punching property having a composition containing C: 0.2% or more, having a structure mainly composed of ferrite and carbide, having a carbide particle size of 0.2 μm or less and a ferrite particle size of 0.5 to 1 μm. A high carbon steel plate excellent in hardenability has been proposed. Thereby, both the punchability determined by the burr height and the mold life and the hardenability are improved.
Patent Document 7 proposes a method for producing a medium-high carbon hot-rolled steel strip suitable for precision punching. In the technique described in Patent Document 7, C: 0.2 to 0.7%, Ti: 0.010 to 0.50%, N: 0.0030% are included, Ti / N is set to 3 or more, and Si, Mn, P, and S are appropriate amounts. Or rolled to steel containing rare earth elements, Ca, etc. so that the total rolling reduction is 50% or more in a temperature range of 1030 to 700 ° C, and then cooled at an average cooling rate of 10 ° C / s or more. Then, it is characterized by spheroidizing annealing. Thereby, dispersibility of cementite is improved, spheroidization of cementite is promoted, and FB workability is improved.
JP 2000-265240 A JP 59-76861 A Japanese Patent Laid-Open No. 2001-140037 Japanese Unexamined Patent Publication No. 9-49065 JP 2001-214234 A Japanese Patent Laid-Open No. 9-316595 Japanese Unexamined Patent Publication No. 55-50427

  However, both of the techniques described in Patent Document 4 and Patent Document 5 are based on the premise that the conventional punching process is performed, and the application of the fine blanking process in which the clearance is almost zero is not considered. Absent. Therefore, it is difficult to ensure the same stretch flangeability after severe fine blanking, and there is a problem that the mold life is shortened even if it can be ensured.

Moreover, in the technique described in Patent Document 6, it is necessary to make the ferrite grain size in the range of 0.5 to 1 μm, and it is difficult to industrially manufacture a steel plate having such a ferrite grain size. There has been a problem that it leads to a decrease in product yield.
In addition, the technique described in Patent Document 7 evaluates precision punchability (FB workability) by precision punching about 100 times, and commercial processing in which punching processing (FB processing) exceeding several thousand times is performed. It is hard to say that the technology is compatible with production.

  The present invention has been made in view of the above-described problems of the prior art, and is excellent in fine blanking workability, capable of continuous punching on a commercial scale, without being accompanied by a decrease in mold life. It aims at providing the steel plate excellent also in the formability after ranking processing, and its manufacturing method.

In order to achieve the above-mentioned object, the present inventors firstly examined the influence of the metal structure on the fine blanking workability (hereinafter abbreviated as FB workability), especially the influence of the morphology and distribution state of ferrite and carbide. Researched earnestly.
As a result, it was found that FB workability and mold life are closely related to carbides present in ferrite grains and ferrite grain size. The steel material having a composition in a predetermined range is subjected to hot rolling finish rolling conditions and subsequent cooling as appropriate conditions to obtain a hot rolled steel sheet having a nearly 100% pearlite structure, and further subjected to appropriate conditions of hot rolled sheet annealing. Applied, the ferrite has an average ferrite grain size of 10 μm or less, a carbide spheroidization rate of 80% or more, and a carbide area present at the ferrite grain boundary of 40% or more in terms of the total carbide area. It has been found that FB workability and mold life are remarkably improved by using a ferrite + spheroidized cementite (spherical carbide) structure in which the amount of carbide in the grain is limited. In addition, it has been newly found that by restricting the amount of carbide in the ferrite grains, the formability after FB processing is remarkably improved.

  In FB processing, materials are processed with zero clearance and compressive stress. Therefore, a crack occurs in the material after undergoing a large deformation. If a large number of cracks occur during large deformation, the FB workability will be greatly reduced. It is said that spheroidization of carbide and refinement of the grain size of carbide are important for preventing the occurrence of cracks. However, in FB processing, even if fine carbides are made into 100% spheroids, microcracks are inevitable if they are present in ferrite grains. Therefore, the inventors considered that when stretch flange processing is further performed after FB processing, microcracks generated during FB processing are connected to each other, resulting in a decrease in stretch flangeability. In addition, regarding the mold life, the present inventors have inferred that if a large number of carbides are present in the ferrite grains, wear of the tool cutting edge is promoted and the mold life is reduced.

Furthermore, the present inventors have come to realize that in order to further improve the FB workability, in addition to the adjustment of the metal structure described above, it is also important to control the form of inclusions.
For example, in the case where inclusions extending in the rolling direction, such as MnS, are present, the structure around the extended inclusions is torn during FB processing. For this reason, the deterioration of the mold is promoted, the surface roughness of the processed surface becomes rough after about 500 times of punching, and a decrease in FB workability is recognized. We found that it began to decline. As a result of further research, we found that the elongation length (major axis) of inclusions in the rolling direction is closely related to the degree of mold deterioration, and to further improve FB workability Has conceived that it is necessary to control the shape of the inclusions by reducing the elongated inclusions that greatly affect the reduction in the mold life and controlling the long diameter of the inclusions to a desired value or less. And in this case, it was found that extremely low sulfidation and addition of appropriate amounts of Ca and / or REM are effective as form control of inclusions.

First, the experimental results on which the present invention is based will be described.
High carbon steel slab (corresponding to S35C) containing 0.34% C-0.2% Si-0.8% Mn in mass%, S with two levels of 0.0011% and 0.0045%, and various Ca contents. After heating to ° C., hot rolling consisting of 5 passes of rough rolling and 7 passes of finish rolling was performed to obtain a hot rolled steel plate having a thickness of 6.0 mm. In the hot rolling finish rolling, the rolling end temperature is 780 ° C., the finish cooling is performed at an average cooling rate of 60 ° C./s to a temperature of 680 to 720 ° C., and the winding temperature is 670 ° C. It was. Then, after pickling these hot-rolled steel sheets, batch annealing (720 ° C. × 40 h) was performed as hot-rolled sheet annealing.

For these hot-rolled sheet annealed steel sheets, the form of inclusions was observed and the FB workability was evaluated.
Sample specimens for structure observation were collected from the obtained hot-rolled steel sheet, the thickness section parallel to the rolling direction of the specimen was buffed, and the region of the thickness 1/4 to 3/4 was interposed with an optical microscope. The form of the object was observed (magnification: 400 times), and the length (major axis) of inclusions extending in the rolling direction was measured.

  Further, a test plate (size: 100 × 80 mm) was collected from the obtained steel plate, and a fine blanking test (FB test) was performed. In the FB test, a 110t hydraulic press was used to sample a sample of size: 60mm x 40mm (corner radius R: 10mm), clearance: 0.060mm (1.5% of the plate thickness), processing force: 8.5 Ton, lubrication: punching was performed up to 30000 times under the condition of existence. In addition, when deterioration was visually recognized on the punched surface (processed surface), the punching test was stopped at that time.

The surface roughness (ten-point average roughness Rz) of the punched sample end face (punched surface) was measured to evaluate the FB workability. In addition, in order to remove the influence of the plate thickness deviation on the clearance, the test pieces were ground in equal amounts on both sides in advance to make the plate thickness 4.0 ± 0.010 mm.
The surface roughness is measured on four end faces excluding the R part. As shown in FIG. 3, each end face has a range from 0.5 mm on the punch side surface to 3.9 mm in the plate thickness direction and parallel to the surface (X direction). ) The 10mm area is repeatedly scanned 35 times at 100μm pitch in the plate thickness direction (t direction) with a stylus type surface roughness meter, and the surface roughness Rz at each scanning line is determined according to JIS B 0601-1994. It was measured. Furthermore, the surface roughness Rz of the measurement surface was obtained by adding the Rz of each scanning line to the average value. Measure the four end faces in the same way as above,
Rz ave = (Rz 1+ Rz 2+ Rz 3+ Rz 4) / 4
(Here, Rz 1, Rz 2, Rz 3, Rz 4: Rz of each surface)
The average surface roughness defined by: Rz ave (μm) was calculated.

In general, when the appearance of a fractured surface at the punched end face is 10% or less, it is considered “excellent in FB workability”, but in the present invention, FB workability decreases as the average surface roughness: Rz ave becomes 15 μm or less. It is excellent in.
The obtained results are shown in FIG. 1 in relation to Rz ave of the punched surface after FB processing and the major axis of inclusions elongated in the rolling direction.

From Fig. 1, when the major axis of inclusions is 50 μm or less, even after 30000 punches, Rz ave: 15 μm or less, the punched surface has good properties, no deterioration of the mold, and excellent FB workability. Have. On the other hand, when the major axis of the inclusion exceeds 50 μm, the properties of the punched surface deteriorate after 5000 or 500 times of punching, and it is necessary to care for the die or replace the die.
The relationship between the major axis of inclusions elongated in the rolling direction and the atomic concentration ratio of S and Ca (S / 32) / (Ca / 40); where S and Ca are the contents of S and Ca (mass%), respectively. As shown in FIG.

  From FIG. 2, in order to make the major axis of inclusions elongated in the rolling direction to 50 μm or less, S is low-sulfided to 0.0011 mass%, and (S / 32) / (Ca / 40) is 1 or less, that is, ( It can be seen that S / 32) ≦ (Ca / 40). Moreover, when S is as high as 0.0045%, it is found that even if (S / 32) / (Ca / 40) is 1 or less, it is difficult to make the major axis of inclusions 50 μm or less.

The present invention has been completed based on the above findings and further research. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.1 to 0.5%, Si: 0.5% or less, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.002% or less, and Ca: 0.0005 to 0.005%, REM: When one or two kinds selected from 0.001 to 0.02% contain one kind of Ca, the following formula (1) (S / 32) ≦ (Ca / 40) ≦ 3 (S / 32 ) (1)
In the case of containing one kind of REM, the following formula (2) (S / 32) ≦ (REM / A) ≦ 3 (S / 32) (2)
In the case where Ca and REM are contained, the following formula (3) (S / 32) ≦ {(Ca / 40) + (REM / A)} ≦ 3 (S / 32) (3)
(Wherein S, Ca: S, Ca content (mass%), (REM / A): Σ (REM _i / A _i ), REM _i : content of each rare earth element contained (mass% ), A _i : atomic weight of each rare earth element)
And a composition composed of the remaining Fe and inevitable impurities, and a structure mainly composed of ferrite and carbide, the average particle diameter of the ferrite is 1 to 10 μm, the spheroidization rate of the carbide Is 80% or more, and among the carbides, the amount of carbides present at the crystal grain boundaries of ferrite is expressed by the following formula (4)
S _gb (%) = {S _on / (S _on + S _in )} × 100 (4)
(Where S _{on is} the total occupied area of carbides present on the ferrite grain boundary among the carbides present per unit area, and S _in is the carbide existing within the ferrite grains among the carbides present per unit area. A steel sheet excellent in fine blanking workability, characterized in that the ferrite grain boundary carbide amount S _gb defined by (total occupied area) is 40% or more.

(2) The steel plate according to (1), wherein the carbides present in the ferrite grain boundaries have an average particle size of 5 μm or less.
(3) In (1) or (2), in addition to the said composition, it is set as the composition containing Al: 0.1% or less by the mass% further.
(4) In any one of (1) to (3), in addition to the above composition, in addition to mass, Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01 to 0.1% And B: A steel sheet characterized by having a composition containing one or more selected from 0.0005 to 0.005%.

(5) In the method for producing a steel sheet, in which the steel material is successively subjected to hot rolling to heat and roll the steel material to form a hot-rolled sheet, and hot-rolled sheet annealing to anneal the hot-rolled sheet, Steel material includes, in mass%, C: 0.1 to 0.5%, Si: 0.5% or less, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.002% or less, and Ca: 0.0005 to 0.005% , REM: When one or two kinds selected from 0.001 to 0.02% contain one kind of Ca, the following formula (1) (S / 32) ≦ (Ca / 40) ≦ 3 (S / 32) ...... (1)
In the case of containing one kind of REM, the following formula (2) (S / 32) ≦ (REM / A) ≦ 3 (S / 32) (2)
When Ca and REM are contained, the following formula (3): (S / 32) ≦ {(Ca / 40) + (REM / A)} ≦ 3 (S / 32) (3)
(Wherein S, Ca: S, Ca content (mass%), (REM / A): Σ (REM _i / A _i ), REM _i : content of each rare earth element contained (mass% ), A _i : atomic weight of each rare earth element)
Is a steel material having a composition comprising the balance Fe and unavoidable impurities, and the hot rolling, the finishing temperature of finish rolling is set to 800 to 950 ° C, and after the finish rolling, Excellent fine blanking processability, characterized by cooling at an average cooling rate of 50 ° C / s or more, stopping the cooling at a temperature in the range of 500-700 ° C, and winding up at 450-600 ° C Steel plate manufacturing method.

(6) In (5), in addition to the said composition, it is set as the composition containing Al: 0.1% or less by the mass% further, The manufacturing method of the steel plate characterized by the above-mentioned.
(7) In (5) or (6), in addition to the above composition, in mass%, Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01 to 0.1% and B: The manufacturing method of the steel plate characterized by setting it as the composition containing 1 type, or 2 or more types chosen from 0.0005-0.005%.

  (8) In any one of (5) to (7), the method for producing a steel sheet is characterized in that the hot-rolled sheet annealing is performed at a temperature of 600 to 750 ° C.

  According to the present invention, it is possible to easily and inexpensively produce a steel sheet having excellent FB workability and excellent formability after FB processing, which can be continuously punched on a commercial scale without deteriorating the mold life. It can be manufactured and has a remarkable industrial effect. In addition, according to the present invention, it is not necessary to perform end face processing after FB processing, the manufacturing period can be shortened, the productivity can be improved, and the manufacturing cost can be reduced.

First, the reasons for limiting the composition of the steel sheet of the present invention will be described. The mass% in the composition is simply expressed as% unless otherwise specified.
C: 0.1-0.5%
C is an element that affects the hardness after hot rolling annealing and after quenching, and in the present invention, it is necessary to contain 0.1% or more. If C is less than 0.1%, the hardness required for automobile parts cannot be obtained. On the other hand, if the content exceeds 0.5%, the steel sheet becomes hard, so that an industrially sufficient mold life cannot be secured. For this reason, C was limited to the range of 0.1 to 0.5%.

Si: 0.5% or less
Si is an element that acts as a deoxidizer and increases the strength (hardness) by solid solution strengthening. In order to obtain such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 0.5%, the ferrite phase becomes hard and the FB workability is lowered. If Si is contained in excess of 0.5%, a surface defect called red scale occurs at the hot rolling stage. For this reason, Si was limited to 0.5% or less. In addition, Preferably it is 0.35% or less.

Mn: 0.2-1.5%
Mn is an element that effectively increases the strength of the steel by solid solution strengthening and effectively improves the hardenability. In order to obtain such an effect, it is desirable to contain 0.2% or more. However, if it exceeds 1.5%, the solid solution strengthening becomes too strong, the ferrite becomes hard, and the FB workability decreases. For this reason, Mn was limited to the range of 0.2 to 1.5%. In addition, Preferably, it is 0.4 to 1.5%, More preferably, it is 0.6 to 1.2%.

P: 0.03% or less P is segregated at grain boundaries and reduces workability. Therefore, in the present invention, it is desirable to reduce it as much as possible, but 0.03% is acceptable. For these reasons, P is limited to 0.03% or less. In addition, Preferably it is 0.02% or less.
S: 0.002% or less S is an element that forms sulfides such as MnS in steel and exists as inclusions, and lowers FB workability, and it is desirable to reduce it as much as possible. When the S content exceeds 0.002%, it is difficult to adjust the major axis of inclusions to 50 μm or less. For these reasons, S is limited to 0.002% or less. In addition, Preferably it is 0.0015% or less.

In addition to the components described above, the present invention contains one or two selected from Ca: 0.0005 to 0.005% and REM: 0.001 to 0.02%. Both Ca and REM are elements that have the effect of suppressing the formation of MnS, which is an inclusion that tends to extend in the rolling direction, producing inclusions in a nearly spherical form, and controlling the form of inclusions. 1 type or 2 types are contained.
Ca has an effect of controlling the form of inclusions, and such an effect becomes remarkable when the content is 0.005% or more, but the content exceeding 0.005% decreases the corrosion resistance. For this reason, Ca was limited to 0.0005 to 0.005% of range.

REM: 0.001 to 0.02%
REM has the effect | action which controls the form of an inclusion like Ca, Such an effect becomes remarkable by content of 0.001% or more, but the content exceeding 0.02% leads to a rise in material cost. For this reason, REM was limited to the range of 0.001 to 0.02%. REM (rare earth element) includes various rare earth elements such as La, Ca, and Y. Here, REM refers to the total amount of various rare earth elements contained.

In the present invention, the contents of Ca and REM are adjusted within the above-described range and according to the S content.
When one kind of Ca is contained, Ca is within the above-mentioned range and the following formula (1)
(S / 32) ≦ (Ca / 40) ≦ 3 (S / 32) (1)
(Where S, Ca: content of each element (mass%))
Is contained so as to satisfy. If (Ca / 40) is less than (S / 32), Ca is insufficient with respect to S, so that inclusions elongated in the rolling direction are generated, and FB workability is reduced. On the other hand, when (Ca / 40) exceeds 3 times (S / 32), Ca becomes excessive with respect to S, so that the corrosion resistance decreases. For this reason, it limited so that S and Ca might satisfy (1). Preferably, (Ca / 40) is in the range of 1.5 (S / 32) to 2.5 (S / 32).

When one type of REM is contained, REM is within the above range, and the following formula (2)
(S / 32) ≦ (REM / A) ≦ 3 (S / 32) (2)
(Where S: S content (mass%), (REM / A): Σ (REM _i / A _i ), REM _i : content of each rare earth element contained (mass%), A _i : Atomic weight of each rare earth element)
Is contained so as to satisfy. Σ (REM _i / A _i ) is the sum of the ratio of the content (mass%) of each rare earth element and the atomic weight of the rare earth element, that is, (REM / A) = (REM ₁ / A ₁ ) + (REM ₂ / A ₂ ) +... + (REM _i / A _i ) When (REM / A) is less than (S / 32), REM is insufficient with respect to S, so that inclusions elongated in the rolling direction are generated, and FB workability is reduced. On the other hand, when (REM / A) exceeds 3 (S / 32), REM becomes excessive with respect to S, so that the corrosion resistance is lowered and the material cost is increased. For this reason, S and REM were limited so as to satisfy (2). Preferably, (REM / A) is 1.5 (S / 32) to 2.5 (S / 32). The rare earth element (REM) is preferably La, Ce, or Y alone or in combination.

When compounding two types of Ca and REM, the following formula (3) (S / 32) ≦ {(Ca / 40) + (REM / A)} ≦ 3 (S / 32) (3)
(Wherein S, Ca: S, Ca content (mass%), (REM / A): Σ (REM _i / A _i ), REM _i : content of each rare earth element contained (mass% ), A _i : atomic weight of each rare earth element)
Is contained so as to satisfy. When {(Ca / 40) + (REM / A)} is less than (S / 32), Ca and REM are insufficient with respect to S, so that inclusions elongated in the rolling direction are generated, and FB workability is increased. Decreases. On the other hand, when {(Ca / 40) + (REM / A)} exceeds 3 (S / 32), Ca and REM are excessive with respect to S, so that the corrosion resistance is lowered and the material cost is increased. To do. For this reason, S, Ca, and REM were limited so as to satisfy (3). Preferably, {(Ca / 40) + (REM / A)} is in the range of 1.5 (S / 32) to 2.5 (S / 32).

In the present invention, in addition to the above basic composition, one or more selected from Al, and / or Cr, Mo, Ni, Ti and B is contained in the present invention. it can.
Al: 0.1% or less
Al is an element that acts as a deoxidizer and combines with N to form AlN, thereby contributing to prevention of austenite grain coarsening. Since such an effect becomes remarkable when the content is 0.02% or more, when added, it is preferable to contain 0.02% or more. However, the content exceeding 0.1% lowers the cleanliness of the steel. For this reason, when it contains, it is preferable to limit Al to 0.1% or less. In addition, Al as an inevitable impurity is 0.01% or less.

Cr, Mo, Ni, Ti, and B are all elements that contribute to improving hardenability or further improving temper softening resistance, and can be selected and contained as necessary.
Cr: 3.5% or less
Cr is an element effective for improving hardenability. To obtain such an effect, it is preferable to contain 0.1% or more. However, if it exceeds 3.5%, FB workability deteriorates and temper softening occurs. This causes an excessive increase in resistance. For this reason, when Cr is contained, it is preferably limited to 3.5% or less. More preferably, it is 0.2 to 1.5%.

Mo: 0.7% or less
Mo is an element that effectively works to improve hardenability. In order to obtain such an effect, it is preferable to contain 0.05% or more. However, if it exceeds 0.7%, the steel is hardened and FB processing is performed. Sexuality decreases. For this reason, when it contains Mo, it is preferable to limit to 0.7% or less. More preferably, it is 0.1 to 0.3%.

Ni: 3.5% or less,
Ni is an element that improves hardenability. In order to obtain such an effect, Ni is preferably contained in an amount of 0.1% or more. However, if it exceeds 3.5%, the steel becomes hard and the FB workability decreases. . For this reason, when it contains Ni, it is preferable to limit to 3.5% or less. In addition, More preferably, it is 0.1 to 2.0%.

Ti: 0.01-0.1%
Ti is an element that easily binds to N to form TiN and effectively acts to prevent coarsening of γ grains during quenching. Further, when it is contained together with B, since N forming BN is reduced, there is an effect that the amount of B necessary for improving the hardenability can be reduced. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if the content exceeds 0.1%, ferrite is precipitated and strengthened by precipitation of TiC or the like, resulting in a hardened mold life. For this reason, when it contains, it is preferable to limit Ti to the range of 0.01 to 0.1%. In addition, More preferably, it is 0.015 to 0.08%.

B: 0.0005-0.005%
B is an element that segregates at the austenite grain boundaries and improves the hardenability in a small amount, and is particularly effective when combined with Ti. In order to improve hardenability, a content of 0.0005% or more is required. On the other hand, even if the content exceeds 0.005%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, when it contains, it is preferable to limit B to 0.0005 to 0.005% of range. In addition, More preferably, it is 0.0008 to 0.004%.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include N: 0.01% or less, O: 0.01% or less, and Cu: 0.1% or less, for example.
Next, the reason for limiting the structure of the steel sheet of the present invention will be described.
The steel sheet of the present invention has a structure mainly composed of ferrite and carbide. “A structure mainly composed of ferrite and carbide” refers to a structure in which the volume ratio of ferrite and carbide is 95% or more.

  In the present invention, the ferrite has a mean grain size of 1 to 10 μm. If the average ferrite crystal grain size is less than 1 μm, the steel sheet hardens significantly, the amount of carbide in the ferrite grains increases, and FB workability, mold life, and moldability such as hole expandability after FB processing are improved. descend. On the other hand, when the thickness exceeds 10 μm, the FB processability is lowered although the mold life is improved and the mold life is improved. For this reason, the average ferrite crystal grain size is limited to a range of 1 to 10 μm. In addition, Preferably it is 1-5 micrometers.

In the steel sheet of the present invention, the spheroidization rate of carbide is 80% or more. If the spheroidization rate is less than 80%, it will be hard, and the deformability will be small and the FB processability will deteriorate. For this reason, in this invention, in order to ensure sufficient FB workability, the spheroidization rate of the carbide was limited to 80% or more. In order to increase the spheroidization ratio, annealing for a long time is required, so 80 to 85% is preferable.
In the steel sheet of the present invention, the ferrite grain boundary carbide content S _{gb is set} to 40% or more. The ferrite grain boundary carbide amount S _gb is the ratio of the carbide occupied area on the ferrite crystal grain boundary to the total carbide occupied area.
S _gb (%) = {S _on / (S _on + S _in )} × 100 (4)
(Where S _on : the total occupied area of carbides present on the ferrite grain boundary among the carbides present per unit area, S _on , S _in : within the carbide grains among the carbides present per unit area Total area occupied by carbides present in
It is a value defined by. If the ferrite grain boundary carbide amount S _gb is less than 40%, the amount of carbide present in the ferrite grains increases, and the FB workability decreases rapidly. This is thought to be because even if fine and spheroidized carbides are present in ferrite grains, fine cracks are generated around the carbides during FB processing, and the FB workability is reduced due to their connection. It is considered that fine cracks are generated and remain around the carbide during the FB processing, and they are connected in the subsequent molding process and the molding processability is lowered. In addition, if carbides are present in the ferrite grains, the ferrite grains themselves are hardened, leading to a reduction in mold life. For this reason, in the present invention, the ferrite grain boundary carbide amount S _gb is limited to 40% or more. In addition, Preferably it is 50% or more.

In the steel sheet of the present invention, the carbides present on the ferrite grain boundaries are preferably 5 μm or less in average grain size. This is because when the ferrite grain boundary carbide amount _Sgb is 40% or more, the carbide present on the ferrite grain boundary improves the FB workability and further improves the die life as the grain size decreases. This is due to a new finding that contribution is significant. In addition, the smaller the carbide particle size, the easier it is to dissolve the carbide in the austenite even during the short-time heating in the induction hardening, and it becomes easy to ensure the desired quenching hardness. For this reason, it is preferable to limit the average grain size of carbides present on the ferrite grain boundaries to 5 μm or less.

Below, the preferable manufacturing method of this invention steel plate is demonstrated.
It is preferable to melt the molten steel having the above-described composition by a conventional melting method such as a converter and to obtain a steel material (slab) by a conventional casting method such as a continuous casting method.
Next, the obtained steel material is hot-rolled to obtain a hot-rolled sheet. Heating for hot rolling is preferably 1000 to 1250 ° C. When the heating temperature is less than 1000 ° C., the rolling load increases. When the heating temperature exceeds 1250 ° C., scale formation becomes significant and crystal grains become too coarse.

  In hot rolling, the finishing temperature of finish rolling is set to 750 to 950 ° C, and after finishing rolling, cooling is performed at an average cooling rate of 50 ° C / s or more, and cooling is stopped at a temperature in the range of 450 to 700 ° C. The treatment is preferably performed at 450 to 600 ° C. The hot rolling in the present invention is characterized in that the rolling finishing temperature of finish rolling and the subsequent cooling conditions are adjusted. As a result, a hot-rolled sheet having almost 100% pearlite structure can be obtained.

Finishing temperature of finish rolling: 750-950 ° C
The rolling end temperature of finish rolling is preferably set to a temperature within the range of 750 to 950 ° C., which is the end temperature range of normal finish rolling. When the finishing temperature of finish rolling exceeds 950 ° C., the generated scale becomes thick and the pickling property decreases, and a decarburized layer may be formed on the steel sheet surface layer. On the other hand, if the finishing temperature of finish rolling is less than 750 ° C., the rolling load increases remarkably, and an excessive load on the rolling mill becomes a problem. For this reason, it is preferable to make the finish temperature of finish rolling into the temperature within the range of 750-950 degreeC.

Average cooling rate after finishing rolling: 50 ° C./s or more After finishing rolling, cooling is performed at an average cooling rate of 50 ° C./s or more. The average cooling rate is an average cooling rate from the finish temperature of finish rolling to the stop temperature of the cooling (forced cooling). When the average cooling rate is less than 50 ° C / s, ferrite that does not contain carbides is generated during cooling, and the structure after cooling becomes a non-uniform structure of ferrite + pearlite, ensuring a uniform structure consisting of almost 100% pearlite. Disappear. If the hot-rolled sheet structure is an uneven structure of ferrite and pearlite, no matter how the subsequent hot-rolled sheet annealing is devised, the amount of carbides present in the grains increases, and the amount of carbides present at the grain boundaries decreases. For this reason, FB processability falls. Therefore, it is preferable to limit the average cooling rate after finishing rolling to 50 ° C./s or more. In addition, Preferably it is 60 degrees C / s or more.

Cooling stop temperature: 450-700 ° C
The temperature at which the cooling (forced cooling) is stopped is preferably 450 to 700 ° C. When the cooling stop temperature is less than 450 ° C., problems such as hard bainite and martensite are generated, and hot-rolled sheet annealing takes a long time, and operational problems such as cracks occur during winding. On the other hand, when the cooling stop temperature exceeds 700 ° C, the ferrite transformation nose is around 700 ° C, so ferrite is generated during cooling after the cooling stop, ensuring a uniform structure consisting of almost 100% pearlite. become unable. For this reason, the cooling stop temperature is preferably limited to a temperature within the range of 450 to 700 ° C. In addition, More preferably, it is 450-650 degreeC, More preferably, it is 500-600 degreeC.

After the cooling is stopped, the hot rolled plate is immediately wound into a coil.
Winding temperature: 450-600 ° C
The coiling temperature is 450 to 600 ° C, more preferably 500 to 600 ° C. When the coiling temperature is less than 450 ° C., cracks occur in the steel sheet during coiling, which causes operational problems. On the other hand, when the winding temperature exceeds 600 ° C., there is a problem that ferrite is generated during winding.

  The hot-rolled sheet (hot-rolled steel sheet) thus obtained is subjected to hot-rolled sheet annealing after removing the oxide scale on the surface by pickling or shot blasting. Appropriate hot-rolled sheet annealing is performed on hot-rolled sheets with a pearlite structure of almost 100% to promote carbide spheroidization and to suppress ferrite grain growth. To be able to exist.

  In hot-rolled sheet annealing, the annealing temperature is preferably set to a temperature in the range of 600 to 750 ° C. When the annealing temperature is less than 600 ° C., sufficient carbide spheroidization cannot be achieved. On the other hand, when the temperature is higher than 750 ° C., pearlite is regenerated during cooling, and fine blanking workability and other workability tend to be deteriorated. The holding time for hot-rolled sheet annealing is not particularly limited, but is preferably 8 hours or longer in order to sufficiently spheroidize the carbide. Further, if it exceeds 80 hours, the ferrite grains may be excessively coarsened.

The steel material (slab) having the composition shown in Table 1 is subjected to hot rolling as shown in Table 2 to form a hot-rolled steel sheet (sheet thickness: 5.0 mm), and the hot-rolled steel sheet is subjected to hot-rolled sheet annealing (720 ° C. × 40 h). ) And then pickling treatment.
The obtained hot-rolled steel sheet was examined for structure, FB workability, and stretch flangeability after FB processing. The survey method is as follows.
(1) Structure A specimen for structure observation was collected from the obtained steel sheet. Then, after polishing the cross section parallel to the rolling direction of the test piece and corroding the nital, metal with a scanning electron microscope (SEM) (magnification, ferrite: 1000 times, carbide: 3000 times) at the 1/4 thickness position The structure was observed (number of fields of view: 30 locations), and the ferrite grain size, the spheroidization rate of carbide, and the amount of ferrite grain boundary carbide were measured.

The ferrite grain size was determined by measuring the area of each ferrite grain and determining the equivalent circle diameter from the obtained area. The obtained ferrite grain sizes were arithmetically averaged, and the value was defined as the average ferrite grain size of the steel sheet.
The spheroidization rate of carbide is obtained by calculating the maximum length a and the minimum length b of each carbide by using an image analyzer in each field of view (number of fields: 30) of metal structure observation (magnification: 3000 times). The ratio a / b was calculated, and the number of carbide grains having a / b of 3 or less was expressed as a ratio (%) to the total number of measured carbides, and was defined as the spheroidization rate (%) of the carbide.

The ferrite grain boundary carbide amount S _gb identifies the carbides present on the ferrite grain boundaries and the carbides present in the ferrite grains in each field of view (number of fields: 30) in the metal structure observation (magnification: 3000 times), using an image analysis apparatus, per unit area, the area occupied by S _on the cementite present on the ferrite grain boundary, and the occupied area S _in the cementite present in ferrite grains is measured, the following equation (4)
S _gb (%) = {S _on / (S _on + S _in )} × 100 (4)
It calculated using.

Moreover, the form of the inclusion was investigated using the tissue observation specimen.
The thickness cross section parallel to the rolling direction of the specimen for texture observation is buffed, and the form of inclusions is observed with an optical microscope in the region of the thickness 1/4 to 3/4 (magnification: 400 times) and rolled. The length (major axis) of inclusions extending in the direction was measured.
(2) FB workability A test plate (size: 100 × 80 mm) was collected from the obtained steel plate and subjected to FB test. The FB test uses a 110-ton hydraulic press to machine a sample of size: 60 mm x 40 mm (corner radius R: 10 mm) from the test piece, clearance between tools: 0.060 mm (1.5% of the plate thickness), Punched under the conditions of force: 8.5 tons and lubrication. About the end surface (punched surface) of the punched sample, the surface roughness (10-point average roughness Rz) was measured in the same manner as described above to evaluate the FB workability. In addition, in order to remove the influence of the plate thickness deviation on the clearance, the test piece was ground in equal amounts on both sides in advance to make the plate thickness 4.0 ± 0.010 mm.

That is, the surface roughness is measured at four end faces excluding the R portion, and at each end face (plate thickness surface), as shown in FIG. 3, the punch side surface is in the range from 0.5 mm to 3.9 mm in the plate thickness direction. In parallel with the surface (X direction), the area of 10mm is scanned 35 times at 100μm pitch in the plate thickness direction (t direction) with a stylus type surface roughness meter, and each scan according to JIS B 0601-1994 The surface roughness Rz at the line was measured. Furthermore, the surface roughness Rz of the measurement surface was obtained by adding the Rz of each scanning line to the average value. Measure the four end faces in the same way as above,
Rz ave = (Rz 1+ Rz 2+ Rz 3+ Rz 4) / 4
(Here, Rz 1, Rz 2, Rz 3, Rz 4: Rz of each surface)
Average surface roughness defined by: R z ave (μm) was calculated.

Moreover, the lifetime of the used tool (mold) was evaluated.
A test piece (size: 100 x 80 mm) is collected from the obtained steel plate, and a sample of size: 60 mm x 40 mm (corner radius R: 10 mm) is cleared from the test plate using a 110-ton hydraulic press. : 0.060mm (1.5% of the plate thickness), processing force: 8.5ton, lubrication: Continuous punching test was performed up to 30000 times under FB processing conditions. In addition, when deterioration was visually recognized on the punched surface, the continuous punching test was stopped at that time.

About the end surface (punched surface) of the punched sample, the surface roughness (10-point average roughness Rz ave) was measured in the same manner as described above to evaluate the die life.
(3) Stretch flangeability after FB processing A test piece (size: 130 × 130 mm) was sampled from the obtained hot-rolled steel sheet and examined for stretch flangeability.

Stretch flangeability was evaluated by conducting a hole expansion test to obtain a hole expansion ratio λf. In the hole expansion test, a 10mmφ (d ₀ ) punched hole (punch hole) is punched in the center of the test piece by FB processing (clearance: 0.03mm), and then the punched hole is turned into a cylindrical flat bottom punch (50mmφ, 5R). Measure the hole diameter d when a crack that penetrates the thickness of the punched hole occurs at the edge of the punched hole.
λf (%) = (d−d ₀ ) / d ₀ × 100
The hole expansion rate λf (%) defined by

  The obtained results are shown in Table 3.

  In all of the examples of the present invention, the surface roughness of the punched surface at the time of continuous punching: 30000 times is Rz: 15 μm or less, the surface of the punched surface is smooth, the FB workability is excellent, and the die life is also reduced. I can't. In addition, the example of the present invention is extremely excellent in stretch flangeability after FB processing. On the other hand, in the comparative example that is out of the scope of the present invention, the surface roughness of the punched surface exceeds Rz: 15 μm, the FB workability is lowered, the mold life is reduced, and the elongation after FB processing is also observed. Flangeability has deteriorated.

4 is a graph showing the relationship between Rz ave of the punched surface after FB processing and the major axis of inclusions elongated in the rolling direction. It is a graph which shows the relationship between the long diameter of the inclusion extended in the rolling direction, and S / Ca. It is explanatory drawing which illustrates typically the surface roughness measurement area | region of the punching surface after FB process.

Claims (8)

% By mass
C: 0.1 to 0.5%, Si: 0.5% or less,
Mn: 0.2 to 1.5%, P: 0.03% or less,
In the case where one or two selected from S: 0.002% or less, Ca: 0.0005 to 0.005%, and REM: 0.001 to 0.02% are contained, and the following formula (1) In the case of containing one REM, the following formula (2) is satisfied, and in the case of containing two types of Ca and REM, the following formula (3) is contained so as to satisfy the balance Fe and inevitable impurities. And an average grain size of the ferrite is 1 to 10 μm, the spheroidization rate of the carbide is 80% or more, and among the carbides, ferrite crystal grains A steel plate excellent in fine blanking workability, characterized in that the amount of carbide _{Sgb of} ferrite grain boundary defined by the following formula (4), which is the amount of carbide present in the boundary, is 40% or more.
Record
(S / 32) ≦ (Ca / 40) ≦ 3 (S / 32) (1)
(S / 32) ≦ (REM / A) ≦ 3 (S / 32) (2)
(S / 32) ≦ {(Ca / 40) + (REM / A)} ≦ 3 (S / 32) (3)
Here, S, Ca: S, Ca content (mass%) of each element,
(REM / A): Σ (REM _i / A _i ),
REM _i : Content of each rare earth element contained (mass%),
A _i : Atomic weight of each rare earth element
S _gb (%) = {S _on / (S _on + S _in )} × 100 (4)
Here, S _on : Of the carbides present per unit area, the total occupied area of carbides present on the ferrite grain boundaries,
S _in : Of the carbides present per unit area, the total occupied area of carbides present in the ferrite grains   The steel plate according to claim 1, wherein the carbides present at the ferrite grain boundaries have an average grain size of 5 μm or less.   The steel sheet according to claim 1 or 2, wherein in addition to the composition, the composition further contains Al: 0.1% or less by mass%.   In addition to the above-mentioned composition, one kind selected from Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01 to 0.1% and B: 0.0005 to 0.005% by mass% Or it is set as the composition containing 2 or more types, The steel plate in any one of Claim 1 thru | or 3 characterized by the above-mentioned. In the method of manufacturing a steel sheet, in which the steel material is subjected to hot rolling to heat and roll the steel material to form a hot rolled sheet, and hot rolled sheet annealing to anneal the hot rolled sheet,
The steel material in mass%,
C: 0.1 to 0.5%, Si: 0.5% or less,
Mn: 0.2 to 1.5%, P: 0.03% or less,
In the case where one or two selected from S: 0.002% or less, Ca: 0.0005 to 0.005%, and REM: 0.001 to 0.02% are contained, and the following formula (1) In the case of containing one REM, the following formula (2) is satisfied, and in the case of containing two types of Ca and REM, the following formula (3) is contained so as to satisfy the balance Fe and inevitable impurities. A steel material having the composition
In the hot rolling, the finishing temperature of finish rolling is set to 750 to 950 ° C., and after the finish rolling is finished, cooling is performed at an average cooling rate of 50 ° C./s or more, and the cooling is performed at a temperature in the range of 450 to 700 ° C. And a method of manufacturing a steel sheet excellent in fine blanking workability, characterized in that it is treated at 450 to 600 ° C.
Record
(S / 32) ≦ (Ca / 40) ≦ 3 (S / 32) (1)
(S / 32) ≦ (REM / A) ≦ 3 (S / 32) (2)
(S / 32) ≦ {(Ca / 40) + (REM / A)} ≦ 3 (S / 32) (3)
Here, S, Ca: S, Ca content (mass%) of each element,
(REM / A): Σ (REM _i / A _i ),
REM _i : Content of each rare earth element contained (mass%),
A _i : Atomic weight of each rare earth element   The method for producing a steel sheet according to claim 5, wherein in addition to the composition, the composition further contains, by mass%, Al: 0.1% or less.   In addition to the above-mentioned composition, one kind selected from Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01 to 0.1%, and B: 0.0005 to 0.005% by mass% Or it is set as the composition containing 2 or more types, The manufacturing method of the steel plate of Claim 5 or 6 characterized by the above-mentioned.   The method for producing a steel sheet according to any one of claims 5 to 7, wherein the hot-rolled sheet annealing is performed at a temperature of 600 to 750 ° C.

Images