WO2007088985A1

WO2007088985A1 - Steel sheet with excellent suitability for fine blanking and process for producing the same

Info

Publication number: WO2007088985A1
Application number: PCT/JP2007/051835
Authority: WO
Inventors: Kazuhiro Seto; Takeshi Yokota; Nobuyuki Nakamura; Nobusuke Kariya
Original assignee: Jfe Steel Corporation
Priority date: 2006-01-31
Filing date: 2007-01-29
Publication date: 2007-08-09
Also published as: US20090173415A1; EP1980635A1; KR101023633B1; KR20080077254A; EP1980635A4; EP1980635B1; CN101379208A; CN101379208B

Abstract

A steel sheet which has excellent suitability for FB and has excellent formability after FB. The steel sheet has: a composition which contains, in terms of mass%, 0.1-0.5% C and 0.2-1.5% Si and Mn and in which Si, P, and S are regulated so as to be in respective proper ranges; ferrite having an average particle diameter of 1-10 µm; and a structure which has a degree of spheroidization of 80% or higher and in which the ferrite grain boundary carbide amount defined by Sgb (%) = {Son/(Son+Sin)} × 100 (wherein Son is the total area occupied by carbide present at the grain boundaries, of the existing carbide, per unit area and Sin is the total area occupied by carbide present in the grains, of the existing carbide, per unit area) is 40% or larger. This steel sheet is excellent in suitability for FB, die life, and formability after FB.

Description

Specification Steel plate opioid manufacturing method with excellent fine blanking workability Technical Field

The present invention relates to a steel plate suitable for use in automobile parts and the like, and is particularly suitable for fine blanking workability that is suitable for being subjected to precision punching (hereinafter also referred to as fine blanking: E or FB processing). It relates to an excellent steel sheet. Background art

It is known that fine-blanking is a very difficult processing method compared to cutting from the viewpoint of improving dimensional accuracy, manufacturing process defects, etc. . .

In normal punching, the clearance between tools is 5 to 10% ¾! ¾ of the thickness of the metal plate that is the material to be punched. However, fine blanking is different from normal punching. The clearance between them is almost zero (actually, 2% or less of Kff of the metal plate to be punched; 1¾ ^) and punching is performed by applying compressive stress to the material near the tool cutting edge. This is a processing method. And the fine blanking process

(1) Suppressing the cracks from the tool cutting edge, the fracture surface seen in normal punching is almost zero, and the machined surface (punched. End face) has a nearly 100% shear surface and a smooth machined surface. can get,

( ² Gamma dimensional accuracy is good, ·

(3)

I. However, in the fine blanking process, the degree of processing that the material (metal plate) receives is extremely severe. Also, in fine blanking, the clearance between tools is set to almost zero, so there is a problem that the load on the mold becomes excessive and the mold is shortened.

For this reason, materials to which fine blanking is applied have been required to have excellent fine planing workability and to prevent a decrease in gold »^.

... to such a demand, for example, Patent Document 1, '0';: 0. ~ 0. 90 weight ^_0/0, Si: 0.4 wt ^_0/0 less, Mn: 0. from 3 to 1.0 Thread loss containing ⁰ weight, spheroidization rate 80% or more, average particle size 0. -1.0 / zm A high-carbon steel sheet that has a woven structure in which carbonized carbon is dispersed in a ferrite matrix and has a notch drawing of 20 or more and a precision punching workability of 1 is proposed. According to the technique described in Patent Document 1, the precision punchability is improved and the mold is also improved. '

However, the carbon steel sheet described in Patent Document 1 has a problem of poor formability after fine blanking. .

In addition, Patent Document 2 includes: C: 0.08 to 0.19%, Si, Mn, A1 having proper S ^, Cr: 0.05 to 0-80%,

Β: Steel sheets for precision punching have been proposed in which steel sheets containing 0.0005 to 0.005% are appropriately hot rolled into steel sheets. The steel sheet described in Patent Document 2 has a low yield strength, is excellent in fine blanking workability, has a low distortion range, has a high η value, and is excellent in composite forming workability. It is said that the steel plate is excellent. However, Permissible Document 2 does not show any specific details about fine planking processability. In addition, the steel described in Patent: ¾;

¾ has a problem that the molding processability after the iso-blanking process is inferior.

Patent Document 3 includes C: 0 to 0.45%, and the contents of Si, Mn, P, S, Al, and N are within appropriate ranges.

It has a composition adjusted to the surrounding, and perlite + cementite

Strong blowjob

• High carbon steel sheets with a structure with an average grain size of 10-20μπι and excellent formability in rolling and fine blanking have been proposed. The high-carbon steel sheet described in Patent Document 3 is excellent in fine blanking workability, and it is said that mold wrinkles in fine blanking work are also improved. However, the high charcoal steel plate described in Patent Document 3 has a problem that it has poor F-fre-low characteristics after fine blanking.

Furthermore, all of the steel sheets described in Patent Document 1,... Patent Document 2, and Patent Document 3 have a fine blanking workability that is satisfactory in the fine blanking process under severe conditions. In addition, the gold plate has not been sufficiently improved, and the problem remains that the formability after the fine blanking process is inferior! /

Initially, the poor blanking process was performed after fine blanking even for gear parts.

-Has been applied to parts that are not processed. But in the auto parts (reclining

'Final blanking tends to be widely applied to parts, etc.)

Application to parts that require flaring after kin processing is being considered. As an automotive part, it has excellent fine blanking workability and fine

.. Stretch flange processing and overhanging after blanking and angling: Steel plate with excellent formability such as ΐί

I'm eagerly aspired. '

No. 2 There have been many keys to improve r / f flange processability. For example, Patent Document 4 includes C: 0.20 to 0.33%, Si: Mn, P, S, 'sol. Al, N content is adjusted to an appropriate range, and Cr: 0.15. A steel plate having a composition containing ˜0.7% and having a ferrite-bainite mixed yarn male which may contain pearlite and excellent in elongation larangability has been proposed. In the hot-rolled steel sheet described in Patent Document 4, by using the above-mentioned fiber, the hole expansion rate is increased and the stretch flangeability is improved. Further, Patent Document 5 includes ferrite particles containing C: 0.2 to 0.7%, an average particle size of carbide of 0.1 μm or more and less than 1.2 μm, and ferrite particles containing no carbide. A high-carbon steel sheet having a structure with a volume fraction of 15% or less and excellent in stretch flangeability has been proposed. The high charcoal Kashioka plate described in Patent Document 5 suppresses the generation of voids at the end face during punching, and can slow the growth of cracks in hole expansion processing, and has a stretch flangeability. It is supposed to improve.

Patent Document 6 includes C: containing 0.2% or more, mainly composed of ferrite and carbide, having a carbide particle size of 0 or less, a celite particle size of 0.5 to 1 and 0 fibers. A high charcoal ¾ίoka board with excellent punching and firing properties has been proposed. As a result, both the punching ability determined by the height of ¾ and gold and the ¾Λ ^ τ property are improved.

Patent Document 1 Japanese Unexamined Patent Publication No. 2000-265240

Patent Document 2 JP 59-76861 A

Patent Document 3 Japanese Patent Laid-Open No. 2001-140037

Patent Document 4 JP ⁹ - ⁴⁹ No. 0Deruta5 ^ -

Patent Document 5 Japanese Patent Laid-Open No. 2001-214234

Patent Document 6 Japanese Patent Application Laid-Open No. 9-316595 Disclosure of Invention However, the techniques described in Patent Document 4 and Patent Document 5 are all based on the premise that conventional punching is performed, and the clearance is Fine blanking is almost zero. It does not consider the application of processing. Therefore, it is difficult to ensure the same stretch flangeability after severe fine-buffing, and there is a problem that even if it can be confirmed, gold »^ is shortened. -'One- ―,: Further, 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 μηι, and it is necessary to stably and industrially manufacture a steel sheet having such ferrite i ^. Is difficult, and it has been given the title “L” when it leads to a decrease in product yield.

The present invention has been made in view of the above-mentioned conventional problems, and provides a method for producing a steel sheet opioid having excellent fine blanking workability and excellent forming workability after fine blanking. »Aim to do.

In order to achieve the above-mentioned purpose, the present inventors have been aware of the influence of metal and «on fine blanking workability (hereinafter abbreviated as FB workability), particularly the influence of the morphology and distribution of ferrite and carbide. And studied earnestly.

As a result, we found that FB workability and the carbide opipherite grain size in the ferrite grains are closely related. Then, a steel material having a predetermined range of ¾¾ is made into an m¾i slab with nearly 100% pearlite weave, with hot rolling finish rolling conditions and subsequent cooling as appropriate conditions, and further subjected to hot plate annealing under appropriate conditions. If the average ferrite particle size is 10 zm or less, the spheroidization rate of the carbide is above, and the area of the carbide that forms a strong ferrite grain boundary is 40% or more in proportion to the oxide area. It has been found that FB workability and gold S life are remarkably improved by using ferrite + spheroidized cementite (spherical charcoal), which has a limited amount of carbide in the ferrite grains. It was also found that by limiting the amount of carbide in the ferrite grains, the formability after FB processing is significantly improved.

In FB processing, the material strength is reduced with zero clearance and compressive stress. For this reason, the material generates cracking force 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 grain size of carbide are important for preventing cracks. However, in FB processing, even if the Tsuruta charcoal is 100% spheroidized, they are in the ferrite grains. For this reason, the present inventors thought that after the FB processing, the stretch flange processing is further performed: ^, the microcracks generated during the FB processing are connected to each other, resulting in a decrease in stretch flangeability. . In addition, regarding the gold, the present inventors have inferred that when a large amount of carbide is added to the ferrite granule, the knee of the tool cutting edge is promoted and the gold is lowered.

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

To the high-coal Mineoka slab (equivalent to S35C) containing 0.34% C—0.2% Si—0.8% Mn in mass%

After heating to 115Q ° C, hot rolling consisting of 5 passes ¾E rolled and 7 passes ± ^ rolled, ¾J ¥ 4.2imi The steel plate. Incidentally, finish rolling of hot rolling was set to 860 ° C,卷取is 600 ° C, and the cooling after the above rolling cooled (5 ° C / s) ~250 with / ₃ to be changed!] And specifications. In addition, the cooling stop dredging that performed cooling other than air cooling (forced cooling) was set to 650. Next, these boat steel plates were pickled and then subjected to batch annealing (720 ° CX 5 to 40 hours) as plate annealing. These steel sheets that were annealed with β sheets were observed for metal paper weave and evaluated for FB workability.

For metal yarn observation, specimens were collected from the obtained steel plate, ¾ ^ after polishing a ffi-section in the rolling direction of the specimen and undergoing nital corrosion, about ¾J ¥ 1/4 position, a scanning electron microscope The metal and «were observed with SEM (SEM), and the ferrite grain size 'and the spheroidization rate of the carbide were measured.

For each ferrite grain, the area of each ferrite grain was measured, the equivalent circle diameter was determined from the obtained area, and each grain size was determined. The obtained ferrite grain sizes were arithmetically averaged, and the value was defined as the average ferrite wrinkle of the steel sheet. The measured ferrite grains were 5000 pieces each. In addition, the maximum length a and the minimum length b of each carbide are calculated using the image W device in each field of view of the paper weave observation (magnification: 3000 times), the ratio a / b is calculated, and a / b The number of carbide particles with a particle size of 3 or less was expressed as a percentage (%) of the number of ^^ compounds measured and used as the spheroidization rate (%) of the carbide. The measured number of carbide grains was 9000 each.

Also, in the field of yarn and weave observation, we know the carbides on the ferrite grain boundaries and the carbides that become fficient within the ferrite grains. the area occupied by the carbides that «on the grain boundaries S _∞, and the occupied area S _to the carbides to the ferrite grains were measured, and the following formula '

(%) = {S (S _m + S _to )} X100

The amount of ferrite grain boundary carbide defined by The area of the carbide grains was measured in 30 fields of view (magnification: 3000 times).

In addition, a test plate (size: 100X80nm) was taken from the obtained steel plate and a fine blanking test (FB test) was performed. In the FB test, using a 110 t hydraulic press machine, a sample of size: 60nrnX40im (corner part R: 10mm) from the test piece, clearance: 1.5% of 0.060mm), Karoeka: 8.5ton 、 Lubrication: I struck under the condition. The surface roughness (ten-point average roughness Rz) of the end face (punched surface) of the punched sample was measured to improve FB workability. Note that the test piece has a clearance; in order to eliminate the influence of the W deviation, both sides were ground in equal amounts in advance to obtain a plate thickness of 4.0 ± 0.010 m. The surface roughness is measured at four end faces excluding the R section, and at each end face, as shown in Fig. 4, from the punch side surface 0.5; Scan the surface of TO (X direction) 10m on the surface with a stylus type surface meter repeatedly 35 times in the book direction (t direction) at 100 ιη pitch, and according to JIS B 0601-1994. Rz was measured. Furthermore, the surface roughness Rz of the measurement surface was calculated by adding the Rz values of each running spring to the average value. Measure the four end faces in the same way as above and

Rz ave = (Rz 1+ Rz 2+ Rz 3+ Rz 4) / 4

(Where Rz 1, Rz 2, Rz 3, Rz 4: each side)

The average surface roughness defined by: Rz ave (; z _m ) was calculated.

In general, the force that makes the appearance of a fractured surface at the punched end face 10% or less “Excellent FB workability” In the present invention, the smaller the average surface roughness: Rz ave becomes 10 m or less, FB processing It is supposed to be excellent in properties. Note that the surface roughness of the test piece of ¾ 異なる different from the above is measured. ¾ ^ is the same as the above, from the surface 0, 5 ™ to the plate thickness direction, and on the strong surface in the range of (mm) -0. Iran) ¾g Then, the 10-nm region is repeatedly scanned in the thickness direction at 100 m pitch to obtain Rz of each surface, and Rzave can be obtained from Rz of each surface.

The results obtained are shown in Figs.

From the relationship between the average surface roughness: Rz ave and the spheroidization rate of carbide shown in Fig. 2, when the spheroidization rate exceeds 80%, ave becomes 10 m or less, and the FB workability increases drastically. I understand. The data shown in FIG. 2 is ¾ ^ when the average ferrite grain size is 3 8 πι @ ^. Furthermore, we found that when the spheroidization rate is 80% or more and the amount of grain boundary carbide increases, Rz ave becomes smaller and FB workability is remarkably improved. From the relationship between the surface roughness (average surface roughness: Rz ave) and the ferrite grain boundary carbide content shown in Fig. 1, the ferrite grain boundary carbide content is 40% or more, and the grain boundary carbides account for the carbide. As the ratio increases, the Rz ave becomes the following, and it can be seen that the FB workability is drastically improved.

The present invention has been completed based on the above findings and further research. That is, the gist of Honjo is as follows.

(1) At mass ⁰ / o, C: 0.1 0.5%, Si: 0.5% or less, Mn: 0.2 1.5%, Ρ: 0.03% or less, S: 0.02% or less And the remainder consisting of Fe and unavoidable impurities, and yarns mainly composed of ferrite and carbide, and the average particle diameter of self-ferrite is 110 / ζ ηι, and the spheroidization rate of S carbide Is the amount of carbides present at the grain boundaries of the ferrite among the above-mentioned carbides.

(%) = (s. _n + sj} 100 …… (1)

(Where _{w w} is the total occupied area of carbide on the ferrite grain boundary among carbides # ¾ per unit area, and is the total carbide area within the ferrite grains among carbides per unit area Occupied area)

Fine Plan King Carr® † Steel with excellent grain boundary carbide content defined by

(2) In (1), the steel that has an average grain size of 5 m or less in carbides that crystallize the grain boundaries of tiflB ferrite.

(3) A steel sheet having one bed as defined in (1) or (2), wherein the slaughtering further comprises a mass% of Α1: 0 · 1% or less.

In any one of (4) (1) to (3), in addition to FfJlEl ^, mass ^_0/0,

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% selected from one or more yarns! Steel with ^

(5) In the steel plate manufacturing method, the filBl 罔 material is applied to the steel material by hot rolling the steel material and rolling the steel material to make a steel plate, and annealing the boat plate β plate; In mass%, C:

0.1 to 0.5%, Si: 0.5% or less, n: 0.2 to 1.5%, P: 0.03% or less, S: 0.02% or less, and ¾ steel material with yarn destruction consisting of Fe and inevitable impurities, tfflS heat During rolling, the end of finishing ± ff is set to 800 to 950 ° C, and after the finish rolling is finished, an average ί ^ ί of 50 ° C / s or more is obtained, and the range of 500 to 700 ° C is set. The method for producing a steel plate excellent in fine blanking processing, wherein the cooling is stopped at 450 to 60 (the treatment is to remove at C).

(6) In (5), in addition further mass ^_0/0, A1: manufactured ^^ method of the steel sheet to a flame to be set formed containing 0.1% or less.

(7) In (5) or (6), in addition to mass, in terms of mass%, Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01—0.1% and B: A method of manufacturing a steel sheet using 1 + or more of yarns selected from 0.0005 to 0.005% as 1!

(8) A method for producing a steel sheet according to any one of (5) to (7), wherein the annealing of the iron and iron β plate is performed at a temperature of 600 to 750 ° C. According to the present invention, it is possible to easily and inexpensively manufacture a steel sheet having excellent FB workability and excellent formability after FB processing, and has a remarkable industrial effect. In addition, according to the present invention, the steel sheet is excellent in FB workability, and it is not necessary to perform end face processing after FB processing, which enables ^ I during the manufacturing period, improves productivity, and reduces manufacturing costs. [There is also an effect that 1 reduction is possible. Brief Description of Drawings

Figure 1 is a graph showing the relationship between FB workability (surface roughness of the punched surface) and ferrite grain boundary carbide content.

Figure 2 is a graph showing the relationship between FB workability (surface roughness of the punched surface) and carbide spheroidization rate.

Figure 3 is a graph showing the relationship between FB workability (surface roughness of the punched surface) and average ferrite crystal grain size. '

FIG. 4 is an explanatory view for schematically explaining the surface roughness measurement region of the punched surface after FB processing. BEST MODE FOR CARRYING OUT THE INVENTION

First, the thread limitation of the steel sheet of the present invention will be described. Unless otherwise specified, mass% in is simply written as%.

C: 0.1 ~ 0.5%

C is an element that affects the hardness after annealing and in the present invention, and in the present invention, it needs 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 it is impossible to secure an industrially sufficient mold life. For this reason, C is limited to the range of 0.1 to 0.5%.

Si: 0.5% or less

Si acts as a deoxidizing agent, and when it is contained in a large amount exceeding '0.5%, which is an element that increases strength (hardness) by solid solution strengthening, the ferrite phase becomes hard and FB workability decreases. Let If the Si content exceeds 0.5%, a surface defect called red scale occurs in the thermal stage. For this reason, Si was limited to 0.5% or less. In addition, Preferably it is 0.35% or less.

Mn: 0.2 to 1.5%

Mn is an element that effectively increases the strength of the steel and improves the i¾AL properties by increasing the solid solution strength. In order to obtain such effects, it is desirable to contain 0.2% or more, but more than 1.5% If it is contained excessively, the solid solution strength becomes too strong and the ferrite strength becomes harder, and the FB processability decreases. For this reason, Mn was limited to the range of 0.2 to 1.5%. Preferably, it is 0.6 to 0.9% P: 0.03% or less

P segregates at grain boundaries and lowers the workability, so it is desirable to observe as much as possible in the present invention, but it is acceptable up to 0.03%. Therefore, P is limited to 0.03% or less. Preferably, it is 0.02% or less.

S: 0.02% or less

S is an element that forms sulfides such as MnS in steel as inclusions and lowers the FB workability. It is desirable to reduce it as much as possible, but it is acceptable up to 0.02%. For these reasons, S was limited to 0.02% or less. In addition, Preferably it is 0.01% or less.

In the present invention, in addition to the above-described »yarn destruction, one or two types selected from Al, and / or Cr, Mo, Ni, Ti and B are used in the present invention. The above can be contained.

A1: 0.1% or less

A1 is an element that acts as a deoxidizing agent and combines with N to form A1N, thereby contributing to prevention of the formation of λλ of austenite grains. In addition to the soot, the soot is fixed, so that the soot becomes soot and has the effect of preventing the amount of soot that is effective for improving feAL properties. Such an effect becomes remarkable when the content is 0.02% or more, but the content exceeding 0.1% lowers the cleanliness of the steel. For this reason, it is preferable to limit A1 to 0.1% or less. In addition, A1 as an inevitable impurity is Ο.Ό1% or less.

Cr, Mo, Ni, Ti, and B are all elements that contribute to improving the properties or further improving the temper softening resistance, and can be selected and contained as necessary.

Cr: 3.5% or less

Cr is an element effective in improving the ^ Ab property. In order to obtain such an effect, it is preferable to contain 0.1% or more. However, if it exceeds 3.5%, FB workability Decreases, and temper softening resistance increases excessively. For this reason, when Cr is contained, it is preferably limited to 3.5% or less. More preferably, it is 0.2 to: L. 5%.

Mo: 0.7% or less

Mo is an element that effectively acts to improve i¾AL properties. In order to obtain such an effect, it is preferably contained in an amount of 0.05% or more. However, if it exceeds 0.7%, the steel is hardened. FB workability descend. For this reason, Mo is contained: ^ is preferably limited to 0.7% or less. More preferably, the content is 0.1 to 0.3%.

Ni: 3.5% or less,

Ni! ^ A element that improves AU growth. To achieve this effect, it is preferable to contain 0.1% or more. However, if it exceeds 3.5%, it will cause the steel to become harder and the FB workability will be reduced. descend. For this reason, it is preferable to limit the Ni content to 3.5% or less. More preferably, the content is 0.1 to 2.0%.

Ti: 0.01 to 1%

Ti is an element that easily binds to N to form TiN and effectively acts to prevent γ grains from becoming ft during firing A. In addition, since # ^ contained with B is added with 1 ^ forming BN, it also has the effect of reducing the additive amount of B necessary for improving ¾A b property. In order to obtain such an effect, a content of 0.01% or more is required. On the other hand, if the content exceeds 0.1%, the precipitation of TiC will strengthen the ferrite by precipitation strengthening, resulting in gold! Incurs a drop in ^. For this reason, when it is contained, Ti is preferably limited to a range of 0.01 to 0.1%. 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 ^ Ai properties in a small amount, and is particularly effective when combined with Ti. In order to improve the burnability, it is necessary to contain 0.0005% or more. 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, it is preferable to limit the content of B to 0.0005 to 0.005%. More preferably, it is 0.0008-0.004%.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include, for example, N: 0.01% or less, 0: 0.01% or less, and Cu: 0.1% or less.

Next, the reason for limiting the yarn thread of the steel sheet of the present invention will be described. .

The steel sheet of the present invention has «, mainly composed of ferrite and carbide. Yarn mainly composed of ferrite and carbide means a weave in which ferrite and carbide are 95% or more in terms of ΦΦ¾ ratio.

In the present invention, the ferrite grain size is 1 to 10 / im in terms of the average crystal grain size. If the average ferrite crystal grain size is less than 1 μπι, the steel sheet hardens significantly, and the amount of carbide in the ferrite grains increases, so that FB workability, gold ¾ ^, and formability such as hole expandability after RB processing, etc. Decreases. on the other hand, 超える If it exceeds 質, it softens and gold increases, but as shown in Fig. 3, FB workability decreases. Therefore, the average ferrite crystal is limited to the range of 1 to 10 μπι. The preferred range is 1 to 5 111.

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 become hard, and the deformability will be small and the FB processability will deteriorate. As shown in Fig. 2, when the spheroidization rate is less than 80%, it becomes larger than Rz ave カ μ πι, and the wrinkle workability decreases rapidly. For this reason, in the present invention, the spheroidization rate of the carbide is limited to 80% or more in order to ensure sufficient FB processability. In order to increase the spheroidization rate, 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 is 40% or more. The amount of ferrite grain boundary carbide is the ratio of the carbide area occupied on the ferrite grain boundary to the area occupied by the carbide, and the following equation (1)

(Where S _{∞ is} the total occupation of carbides on the ferrite grain boundary among the carbides per unit area, S. _n , among the carbides per unit area, the carbides in the ferrite grains Total occupancy

It is a value defined by. When the amount of ferrite grain boundary carbide is less than 40%, the amount of charcoal in the ferrite grain increases, and as shown in Fig. 1, Rz ave increases beyond 10 m, and 1¾ workability decreases rapidly. . This is thought to be because even if fine and spheroidized carbides are present in the ferrite grains, fine cracks are generated around the carbides during EB processing, and the FB workability deteriorates due to their connection. It is thought that fine cracks are generated and remain around the carbide during FB processing, and they are connected in the subsequent molding process, and the moldability is reduced. In addition, if carbide is covered in the ferrite grains, the ferrite grains themselves become hard, leading to a decrease in gold. For this reason, in the present invention, the amount of ferrite grain boundary carbide is limited to 40% or more. In addition, Preferably it is 50% or more.

In the steel sheet of the present invention, the carbides on the ferrite grain boundaries are preferably 5 / m or less in average grain size. This is because when the amount of carbide at the ferrite grain boundary is 40% or more, the carbide on the ferrite grain boundary contributes to the improvement of EB workability and further to the improvement of gold metal # as the grain size becomes smaller. This is due to the new finding that this is a big deal. In addition, the smaller the particle size of the carbide, the easier it is to dissolve the carbide in the austenite even when heating for a short time in the high-frequency iftAi, and it becomes easy to secure the desired ¾A OI. This For this reason, it is preferable to limit the average grain size of carbides on the crystal grain boundaries of ferrite to 5 m or less.

Next, preferred and manufacturing methods of the steel sheet of the present invention will be described.

It is preferable that the molten steel having the yarn thread described above is made difficult by a conventional melting method such as a converter and used as a steel material (slab) by a conventional separation method such as a continuous ^ method.

Next, the obtained steel material is hot-rolled by heating and rolling the steel material to form a plate. In hot rolling, finish rolling is finished at 800-950, and after finishing rolling, it is cooled with an average cooling of 50 ° C / s or more and stopped at ¾S in the range of 500-700 ° C. It is preferable that the scouring process is performed at 450 to 600 ° C. In the hot rolling according to the present invention, the end of finishing is adjusted and the subsequent cooling conditions are adjusted. As a result, a hot-rolled sheet having almost 100% pearlite is obtained.

Finish rolling finished ¾: 800 ~ 950 ° C

The finish of the finish is preferably ¾ within the range of 800 to 950 ° C, which is the finish area of normal finish rolling. When finish rolling finish ¾g is higher than 950 ° C, the generated scale becomes thicker and pickling property is deteriorated, and a! Layer may be formed on the surface layer of the steel sheet. On the other hand, if the finishing temperature of finish rolling is less than 800 ° C, the rolling load increases significantly, and an excessive load on the rolling mill becomes a problem. For this reason, it is preferable to finish the finish rolling within the range of 800-950 ° C V ,. Average cooling after finish rolling ¾S: 50 ° C / s or more

After finishing rolling, the average cooling rate is 50 ° C / s or more. If the average cooling rate is less than 50 ° C / s, the ferrite containing no carbide in the ί ^ ίΡ is the average cooling rate from the end of finish rolling to the P (forced stop). The resulting paper weave becomes non-uniform ferrite and pearlite yarns and wrinkles, making it impossible to secure a uniform paper weave consisting of almost 100% pearlite f 诞; With II weaving, no matter how the subsequent plate annealing is done, the amount of carbides in the grains increases, and the amount of carbides that fall into the grain boundaries decreases, thus reducing FB processability. For this reason, it is preferable to limit the average cooling rate after finishing the rolling to 50 ° C / s or higher, but to prevent the formation of benite, it should be 120 ° C / s or lower. More preferable: Cooling stop: 500 to 700 The cooling (forced cooling) is preferably stopped at 500 to 700 ° C. If the cooling stop ¾ is less than 500, hard beanite will cause martensite and the plate annealing will take a long time, and there will be other problems such as cracking during cutting. On the other hand, when the cooling stop exceeds 700 and the temperature becomes high, the ferrite transformation nose is close to 700. Therefore, flare is generated during the cooling after the cooling stop, and it becomes impossible to secure uniform yarns composed of almost 100% pearlite. . For this reason, the cooling stop temperature is preferably limited to a temperature within the range of 500 to 700 ° C. The temperature is more preferably 500 to 650 ° C, and further preferably 500 to 600 ° C.

After cooling is stopped, the plate is immediately coiled. The wrinkling is 450 to 600, more preferably 500 to 600 ° C.

If the scraping is less than 450 ° C, cracks occur in the steel plate when scraping, which causes a top problem. On the other hand, if the scraping exceeds 600 ° C, there is a problem that ferrite is generated during scraping.

The plate (heated steel plate) thus obtained is subjected to a difficult male after removing the oxide scale on the surface by pickling or shot plast. Appropriate hot-plate annealing is applied to hot-rolled sheets with almost 100% pearlite yarns to promote spheroidization of carbides and suppress ferrite grain growth. You will be able to move up.

In sheet annealing, annealing is performed in the range of 600 to 750 ° C. However, when the temperature is less than 600 ° C, sufficient spheroidization of carbide cannot be achieved. On the other hand, when the temperature is higher than 750 ° C, pearlite is regenerated inside, and the fine blanking workability and other strengths are reduced. The TO time of the hot plate 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 & λ converted. Example

The steel material (slab) of the thread band shown in Table 1 was subjected to the rolling shown in Table 2 and a hot-rolled sheet male, resulting in a steel plate (4.3 mm).

Regarding the obtained heat plate 1, the structure, FB workability, and stretch flangeability after FB processing were investigated. The adjustment method is as follows.

(1) Fiber From the obtained steel plate, a sample for observation of wrinkles j † was collected. Then, after polishing the TO section in the rolling direction of the test piece and subjecting it to nital corrosion, the scanning electronic caulking (SEM) (double, ferrite: 1000 times, carbide: 3000 times) (Observation »: 30 locations) Ferrite and carbide fineness, ferrite grain size, carbide spheroidization rate, ferrite grain boundary carbide amount, and average grain size of carbide on ferrite grain boundary The diameter was measured.

The volume ratio of ferrite and carbide was determined by observing the metal yarn with a SEM (magnification: 3000 times) (number of fields: 30) and dividing the total area of ferrite and carbide by the total field of view. The ratio was determined and this was judged as the ratio of ferrite and carbide.

The ferrite grain size was determined by measuring the area of each ferrite grain and calculating 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 of the Taoka plate.

The spheroidization rate of carbide is determined by the maximum length a and the minimum length b of each carbide using the surface image orientation in each field of view (viewing point: 30 places) of metal thread and weave observation (magnification: 3000 times). The ratio a / b is calculated and the number of carbide grains with a / b of 3 or less is displayed as a percentage (%) of the number of carbides measured. The ratio of carbide spheroidization (%) ■

The amount of ferrite grain boundary carbides is determined by observing the metal thread habit (magnification: 3000 times) in the field (view: 30 locations). Using the screen, on the ferrite grain boundary per unit area! ½ carbide occupying area S _∞ and carbide occupying area S _to 1 in ferrite grains

(%) = {s _∞ / (S _m + Sj} X 100 …… (1)

Calculated using

In addition, for each carbide on the ferrite grain boundary, the diameter passing through the center of gravity of the two points on the outer circumference of the carbide and the equivalent ellipse of the carbide (the ellipse having the same area as the carbide, the same primary and secondary moments) is 2 °. The circle-equivalent diameter was determined by measurement, and this was defined as each carbide; ^, and the average value of the obtained carbide particle size was defined as the average of the carbide on the ferrite grain boundary.

(2) FB processability

From the obtained steel plate, a test plate (size: 100X8 was sampled and subjected to FB test. The FB test was a sample of size: 60mmX40nm (corner: lOmn) from a test piece using a 110 t hydraulic press. , Clearance between tools: 0.05% of 0.060nm), processing force: 8.5ton, Lu: Punched under certain conditions. The surface roughness (10-point average roughness Rz) of the end face (punched surface) of the punched sample was measured in the same way as tiifB, and the FB workability was increased. In addition, the test piece was subjected to the clearance; in order to eliminate the influence of the deviation, both sides were ground in advance by equal amounts to obtain a plate thickness of 4.0 ± 0. OlOmn.

In other words, the surface roughness is measured at four end faces excluding the R section, and at each end face (¾J ¥ plane), as shown in Fig. 4, from the punch side surface 0.5 mm to 3.9 mm in the Ki ¥ direction. In the range of, scan the area of 10mm on the surface (X direction) with a touch surface meter 35 times at 100 / m pitch in the ¾i ¥ direction (t direction), and β in JIS B 0601-199 Then, the surface roughness Rz was measured on each run. Furthermore, the surface roughness Rz of the measurement surface was the average value of the total Rz of each run ». Measure the four end faces in the same way as above and

Rz ave = (Rz 1+ Rz 2+ Rz 3+ Rz 4) / 4 ( where, Rz 1, Rz 2, Rz 3, Rz 4: each side of) is defined by the average surface roughness: R _Z ave (m) was calculated.

In addition, # ^ of the tool (die) used was evaluated. The surface roughness (10-point average roughness Rz) of the sample end face (punched surface) when the number of punches in FB processing reached 30000 times was measured, and gold was evaluated. The method for measuring the surface roughness was the same as that described above. When the surface roughness (+ point average roughness Rz) of the sample end face was 10 / zm or less, the evaluation was evaluated as 0, 10111 to 16111 or less, and X exceeding 16 / zm as X.

(3) Stretch flangeability after 1¾ processing

Specimens (size: lOOX lOOmn) 'were punched out from the obtained hot oka board by FB processing, and the stretch flangeability was investigated. FB machining was performed under the conditions of clearance between tools: 1.5% of 0.06Qmm), machining force: 8.5 tons, and lubrication: with.

The stretch flangeability was determined by fHffi to obtain the hole expansion ratio λ by adding the hole expansion: m test. In the hole expansion test, after punching a punch hole of ΙΟππ φ (d ₀ ) into the test piece, the punch hole is pushed out with a jig, line V, and when a ¾J through crack is generated at the punch hole edge Measure the hole diameter d at

λ (%) = (d-d ₀ ) / d ₀ X 100

The hole expansion rate (%) defined in (1) was obtained.

The results obtained are also shown in Table 2.

In all of the examples of the present invention, the surface roughness of the punched surface is Rz: 10 m or less, EB addition: excellent L †, and the surface of the punched surface at the time of punching: 30000 times is also smooth (Evaluation: 〇 ) Deterioration of mold life

'Is also not allowed. In addition, the inventive example is excellent in stretch flangeability after FB processing. Before Although the work rates of ferrite and carbide were improved by the method described above, the total ratio of ferrite and carbide was 95% or more, and it was β that it was mainly composed of ferrite and carbide. In addition, although the grain size of carbides at the ferrite grain boundaries was set by t & fB, the deviation was 5 μm or less in average grain size.

On the other hand, in the comparative example that is outside the scope of the present invention, the surface roughness of the punched surface exceeds Rz: 10 / im, and the FB workability decreases, and the decrease in gold »^ is also observed. Flangeability has deteriorated. Steel plate No. 15 did not undergo any treatment after plate annealing because cracking occurred during cutting.

1 & table 1

Table 2

Claims

The scope of the claims

1. In the mass ^_0/0,

C: 0.1 to 0.5%, Si: 0.5% or less,

Mn: 0.2 to 1.5%, 、: 0.03% or less,

S: 0.02% or less

¾¾ Fe and an inevitable impurity yarn! ^ And a paper weave mainly composed of ferrite and carbide, t & IE ferrite has an average particle diameter of 1 to 10 μm, and tillH carbide spheroidization rate Is the amount of carbide at the grain boundary of ferrite among the Ml self-carbides.

(1) A steel plate with excellent fin blanking workability that has a ferrite grain boundary carbide content defined by the formula S ^ S of ₄₀ % or more.

Record

S (%) = {S ノ (S _∞ + SJ} X 100 …… (1)

Where s _∞ : the total occupied area of carbides on the ferrite grain boundary among the carbides per unit area,

s _to : Total carbide occupies in ferrite grains out of carbides per unit area ®¾ '

2. Steel according to claim 1, wherein the carbides at the grain boundaries of f & IB ferrite have an average grain size of 5 m or less.

3. In addition to ItJfBli ^, the steel fe according to claim 1 or 2, wherein a thread containing, by mass%, A1: 0.1% or less!

4. In addition to the ItifS a 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 steel according to any one of claims 1 to 3, wherein a paste containing one or two or more selected from 0.0005-0.005% is used as a paste.

5. A method for producing a copper plate, in which a steel material is heated and rolled into a β plate by subjecting the steel material to rolling, and a ¾ is applied to the β plate, and then a sheet annealing is performed! In a hurry 岡 ίΙΒ! Oka material in mass%

C: 0.1 to 0.5%, Si: 0.5% or less,

n: 0.2 ~ 1.5%, P: 0.03% or less,

S: 0.02% or less

A steel material having a yarn basket composed of the balance Fe and unavoidable impurities,

t & lB Hot rolling is performed at an end of the rolling ± ff of 800 to 950, and after the end of the rolling, it is cooled with an average cooling of 50 ° C / s or more, and the rolling is performed at ¾ g in the range of soo yoc C. A method for producing a steel plate with excellent fine blanking workability, which is characterized by stopping cooling and scraping at 450 to 60 cm.

6. The method for producing a steel plate according to claim 5, wherein in addition to jfB destruction, the fiber further comprises A1: 0.1% or less by mass%.

7-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 method for producing a steel sheet according to claim 5 or 6, wherein the steel sheet contains at least one selected from 0.0005 to 0.005%.

8. The method for producing a steel sheet according to any one of claims 5 to 7, wherein the t & | E «sheet annealing is set to a treatment of 600 to 750 ° C.