JP2012237041A - Sheared component and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sheared component improved in a hydrogen cracking property at an end surface of a sheared edge by paying attention to a structure of an outermost layer of the end surface of the sheared edge of a high strength steel sheet, and making the structure hard to cause hydrogen cracking, and a method for manufacturing the same.SOLUTION: The sheared component is manufactured by applying a shearing work to the steel sheet, and includes at least one sheared portion consisting of a structure in which a region within the range of at least 30 μm in a depth direction from the end surface of the sheared edge contains 50% or more of polygonal ferrite having an average particle diameter of 5.0 μm or less and an average aspect ratio of 1 to 3.

Description

  The present invention relates to a part subjected to shearing among parts requiring strength, such as structural members and reinforcing members such as automobiles and construction machines, and in particular, a sheared part with improved hydrogen cracking property on a shearing end face. And its manufacturing method.

  In order to satisfy both the weight reduction of automobiles and the improvement of collision safety originating from global environmental problems, it is necessary to increase the strength of steel plates used as much as possible. For this reason, steel sheets with increased strength by controlling additive elements and steel structure have been developed. However, when shearing such as piercing / trimming is required, a large plastic strain is applied to the end surface, so high strength steel is originally post-processed, such as hydrogen or paint, contained in the steel. There is a problem that cracks are likely to occur due to hydrogen that has entered from the time and the environment, and the emergence of this solution has been eagerly desired.

  On the other hand, a technology has been developed to obtain a high-strength molded part by heating to Ac3 point or higher at the time of molding and then transforming it into a hard structure in the cooling process. Is disclosed.

  For example, Patent Document 1 discloses a technique of reducing the residual stress by reducing the cooling rate of a portion to be subjected to shearing and making quenching insufficient, thereby reducing the strength of the portion.

Patent Document 2 discloses a technique for controlling the atmosphere in the heating furnace to reduce the amount of hydrogen in the steel and performing shearing in the vicinity of the bottom dead center having a relatively small deformation resistance.
Further, in Patent Document 3, the atmosphere in the heating furnace is controlled to reduce the amount of hydrogen in the steel, and by reducing the cooling rate of the site to be sheared to make quenching insufficient, A technique for reducing the residual stress by reducing the strength of the steel is disclosed.

Furthermore, Patent Document 4 discloses a technique for reducing the amount of hydrogen in steel by controlling the atmosphere in the heating furnace and reducing the strength by reheating the sheared portion.
In addition, Patent Document 5 discloses a technique for reducing the residual stress by using a punch or die having a shear angle while controlling the atmosphere in the heating furnace to reduce the amount of hydrogen in the steel. .

  Patent Document 6 discloses a technique for controlling the atmosphere in the heating furnace to reduce the amount of hydrogen in the steel and compressing the end face subjected to the shearing process.

  Furthermore, Patent Document 7 discloses a technique for controlling the atmosphere in the heating furnace to reduce the amount of hydrogen in the steel and to reduce the ratio of the droop length to the plate thickness.

  In addition, Patent Document 8 discloses a technique for improving wear resistance by performing rapid heating and quenching after performing a shearing process to form a final base plate shape and then pressing.

  Patent Documents 9 to 11 disclose a technique for improving hot punchability by dispersing TiN in a range from the steel sheet surface to a depth of 100 μm.

JP 2003-328031 A JP 2006-104526 A JP 2006-104527 A JP 2006-83419 A JP 2008-266721 A JP 2006-82099 A JP 2008-284610 A JP-A-8-333628 JP 2010-174276 A JP 2010-174278 A JP 2010-174279 A

  The present invention provides a sheared part that reliably and stably improves the hydrogen cracking property of the end face of the sheared surface of a high-strength steel sheet and a method for manufacturing the same. In particular, focusing on the surface structure of the end face of the sheared surface of high-strength steel sheet, and making the structure less susceptible to hydrogen cracking, the sheared part has improved the hydrogen cracking property of the end face of the sheared surface It is an object to provide a manufacturing method thereof.

  The inventor examined in detail the condition of the end face during shearing of the high-strength steel sheet and the conditions for causing hydrogen cracking. High-strength steel sheets generally contain a large amount of a metal structure containing a large amount of carbon, such as martensite, bainite, and pearlite. When this high-strength steel sheet is subjected to shearing, its end face is composed of a shear plane and a fracture surface, but both have extremely large plastic strains, and the metal structure becomes a significantly elongated structure. It has been found that metal structures containing a large amount of carbon such as martensite, bainite, and pearlite are more susceptible to hydrogen cracking than polygonal ferrite, and the danger increases when plastic strain is applied.

  Further, as shown in FIG. 1, the steel plate 1 is heated to a temperature of Ac3 or higher to 1400 ° C. using a heating furnace 2 or the like, and placed in the mold 3 in a state of an austenite structure, and then sheared. Even in this case, it has been found that the end surface after shearing usually has a high strength structure containing a large amount of martensite, bainite, pearlite, and the like, and a metal structure that is more susceptible to hydrogen cracking than polygonal ferrite is generated. It was also found that even a polygonal ferrite structure has a high crystal grain size and a high plastic strain, hydrogen cracking sensitivity is higher than that of a fine grain structure with little plastic strain.

  Based on such knowledge, the present inventors have conducted intensive studies, and as a result, the surface layer of the shearing end face of the high-strength steel sheet, particularly the region within the range of at least 30 μm in the depth direction from the surface of the end face of the shearing process, It was found that if a polygonal ferrite structure with little plastic strain is formed, the hydrogen cracking property of the sheared end face can be improved. Further, by heating a steel plate having a specific chemical composition between Ac3 point to 1400 ° C. and shearing it under specific processing conditions, as shown in FIG. 2, the surface layer region of the shearing end face 4 of the high-strength steel plate 1 It was found that a polygonal ferrite structure having a small grain size and a small plastic strain can be formed. This is because a large plastic strain is applied to the austenite structure by shearing, causing polygonal ferrite transformation only in the surface layer region of the shearing end face, and the other parts to a high-strength structure such as martensite, bainite, pearlite, and carbide. It can be achieved by transformation. Since the surface layer region of the end face of the shearing process is a structure that has undergone transformation induced by the plastic strain due to the shearing process, a polygonal ferrite structure with very little plastic strain can be obtained. The surface layer region of the shearing end face composed of a polygonal ferrite structure with little plastic strain is not prone to hydrogen cracking, so there is very little risk of hydrogen cracking regardless of the amount of residual stress and the amount of hydrogen. A high-strength shearing part can be obtained.

  The present invention has been made on the basis of the above-described novel findings. Basically, a fine-grained polygonal ferrite structure is formed in the surface layer region of the end face of the sheared surface of the high-strength steel sheet. Thus, the hydrogen cracking property of the end face is improved.

  Of the techniques disclosed in the above-mentioned prior art documents, Patent Document 1 is a technique for changing the cooling rate of a part subjected to shearing to reduce the residual stress with the cooling rate of other molding parts. In this case, since it becomes easier to transform into a bainite structure when the cooling rate of the quenched structure is slower than this, it is a technique different from the technique of the present invention in which a fine-grained polygonal ferrite structure is formed in a range of at least 30 μm from the surface of the shearing end face. . In addition, the technique of Patent Document 1 is complicated and the cost is increased, and hydrogen cracking is not mentioned at all, and it is unclear whether it is effective for hydrogen cracking.

  Further, Patent Document 2 controls the amount of hydrogen in the heating atmosphere and the dew point to reduce the amount of hydrogen in the steel, and performs shearing near the bottom dead center in order to reduce the residual stress and maintain the shape and position accuracy. It is a technology, and if it is sheared near the bottom dead center of a press machine, it becomes easy to transform into a martensite structure or a bainite structure before applying the shearing process. This is a technique different from the technique of the present invention having a null ferrite structure.

  Furthermore, Patent Document 3 is a technique for reducing the amount of hydrogen in the steel by controlling the amount of hydrogen and the dew point in the heating atmosphere, and lowering the cooling rate at the site where shearing is performed for the purpose of reducing residual stress, When the cooling rate is lowered, the bainite structure is easily transformed. Therefore, this technique is different from the technique of the present invention in which the surface layer region within the range of at least 30 μm from the surface of the shearing end face is a fine-grained polygonal ferrite structure.

  In addition, Patent Document 4 is a technique in which the amount of hydrogen in steel is reduced by controlling the amount of hydrogen and dew point in the heating atmosphere, and then the portion to be subjected to shearing is reheated to 400 to Ac3 and then sheared. Even if the quenched structure is reheated to Ac3 point or less, the quenched structure remains, so that a polygonal ferrite structure cannot be obtained on the shearing end face. Therefore, this is a technique different from the technique of the present invention in which the surface layer region within the range of at least 30 μm from the surface of the shearing end face is a fine-grained polygonal ferrite structure.

  Patent Document 5 is a technique for reducing the amount of hydrogen in steel by controlling the amount of hydrogen and dew point in a heated atmosphere, and further performing shearing using a punch or die having a shear angle. Even if the corners are provided, a polygonal ferrite structure cannot be obtained on the sheared end face. Therefore, this is a technique different from the technique of the present invention in which the surface layer region within the range of at least 30 μm from the surface of the shearing end face is a fine-grained polygonal ferrite structure.

  Furthermore, Patent Document 6 is a technique for reducing the amount of hydrogen in steel by controlling the amount of hydrogen and the dew point in the heating atmosphere, and further compressing the shearing end face, and there is no change in the structure even if compression processing is performed. A polygonal ferrite structure cannot be obtained on the end face. Therefore, this is a technique different from the technique of the present invention in which the surface layer region within the range of at least 30 μm from the surface of the shearing end face is a fine-grained polygonal ferrite structure.

  Further, Patent Document 7 is a technique for reducing the tensile residual stress of the cut surface by reducing the ratio of the shear length of the shearing end face to the plate thickness, and there is no change in the structure even if the shear length is reduced. A polygonal ferrite structure cannot be obtained on the end face. Therefore, this is a technique different from the technique of the present invention in which the surface layer region within the range of at least 30 μm from the surface of the shearing end face is a fine-grained polygonal ferrite structure.

  In addition, Patent Document 8 is a technique in which the surface is hardened by rapid heating and quenching of a sheared and pressed part and the wear resistance is improved, and since a hardened structure is obtained, a polygonal ferrite structure is obtained on the end face. Absent. Therefore, this is a technique different from the technique of the present invention in which the surface layer region within the range of at least 30 μm from the surface of the shearing end face is a fine-grained polygonal ferrite structure.

  Patent Documents 9 to 11 are all techniques for reducing burrs on the shear end face by dispersing TiN in a range of 100 μm in depth from the steel sheet surface in the die quench method, and the relationship between TiN and the structure is not mentioned. However, since the austenite is coarsened under the heating conditions for shearing (900 ° C. × 15 minutes) and fine polygonal ferrite cannot be obtained on the end face of the shearing process, this technique is different from the technique of the present invention.

  As described above, in the prior arts of Patent Documents 1 to 11, consideration is not given to improving the hydrogen cracking property of the end face of the sheared surface of the high-strength steel plate, or even if consideration is paid, The structure of the surface region of the end face effective for improving hydrogen cracking has not been sufficiently studied, and in particular, the fine grain in the region within 30 μm in the depth direction (surface layer region) from the end face of the sheared surface. It was not thought that the hydrogen cracking property of the end face could be improved by forming the polygonal ferrite structure.

Therefore, the gist of the present invention is as follows.
(1) A sheared part obtained by subjecting a steel sheet to shearing, wherein an area within the range of at least 30 μm in the depth direction from the surface of the end face of the shearing process has an average particle size of 5.0 μm or less, an average aspect ratio of 1 to A shearing part having at least one shearing part composed of a structure containing 50% or more of the polygonal ferrite of 3;

(2) The shear according to (1) above, wherein the region other than the region within the range of at least 30 μm in the depth direction from the surface of the end face is composed of a structure containing one or more of martensite, pearlite, and bainite. Processed parts,

(3) The steel sheet is mass%,
C: 0.05 to 0.55%,
Si: 2% or less,
Mn: 0.1 to 3%,
P: 0.1% or less,
S: 0.03% or less,
Al: 0.1% or less,
O: 0.015% or less,
N: 0.01% or less,
A shear-processed part according to (1) or (2) above, wherein the balance consists of Fe and inevitable impurities,

(4) The steel sheet is in% by mass,
B: 0.0002 to 0.0050%,
V: 2.0% or less,
W: 3.0% or less,
Containing one or more selected from among the sheared parts according to (3) above,

(5) The said steel plate is the mass%, and also Cr: 0.01-1.0%,
Mo: 0.02 to 3.0%,
Ti: 0.01 to 0.70%,
The shear-processed part according to (4) or (5) above, which contains one or more selected from

(6) In mass%,
C: 0.05 to 0.55%,
Si: 2% or less,
Mn: 0.1 to 3%,
P: 0.1% or less,
S: 0.03% or less,
Al: 0.1% or less,
O: 0.015% or less,
N: 0.01% or less,
A steel plate containing Fe and the inevitable impurities in the balance, heating the steel plate to Ac 3 to 1400 ° C., holding it for 30 to 300 seconds, and then shearing at a temperature of 400 to 900 ° C. A method of manufacturing a sheared component,

(7) The steel sheet is in% by mass, and
B: 0.0002 to 0.0050%,
V: 2.0% or less,
W: 3.0% or less,
The method for producing a sheared part according to (6) above, which comprises one or more selected from

(8) The steel sheet is in% by mass,
Cr: 0.01 to 1.0%
Mo: 0.02 to 3.0%,
Ti: 0.01 to 0.70%,
The method for producing a sheared part according to (6) or (7) above, which contains one or more selected from

(9) The method for producing a sheared part according to (6) to (8) above, wherein the shearing speed is 30 spm or more and the clearance is 5 to 20% of the plate thickness,

(10) The method for producing a sheared part according to the above (6) to (9), wherein a molding process is accompanied during the shearing process,
It is in.

    According to the present invention, by forming a fine-grained polygonal ferrite structure in the surface layer of the end face of the sheared surface of the high-strength steel sheet, particularly in the region within the range of at least 30 μm in the depth direction from the end face, The hydrogen cracking property of the processed end face can be improved reliably and stably. Therefore, according to the present invention, since it is possible to reliably and stably produce a sheared part having excellent hydrogen cracking property at the end face, its industrial significance in fields where high-strength steel sheet sheared members are required, such as automobiles and construction machinery. Is big.

It is a schematic diagram which shows the shaping | molding and shear process in hot. (a) is a schematic diagram showing a state of a steel plate and a mold before being subjected to shearing, and (b) is a schematic diagram showing a shearing end face and a surface layer region after the shearing.

    Next, the present invention will be described in detail.

First, the conditions relating to the metal structure of the sheared end face and the reasons for limitation will be described.
In the present invention, the structure of the region within the range of at least 30 μm in the depth direction from the surface of the end surface subjected to the shearing process (hereinafter referred to as the surface layer region) has a mean particle size of 5.0 μm or less and an average aspect ratio of 1 to 3 It has been found that hydrogen cracking is greatly improved in the case of a structure containing 50% or more of nalferrite in area percent. Here, area% means area% when observed in a cross section perpendicular to the shear plane and along the thickness direction. The range of the surface layer region of the polygonal ferrite structure of the shearing end face is set to at least 30 μm from the surface in the depth direction. If the width is narrower than 30 μm, the martens present inside the surface layer region of the polygonal ferrite structure is present. This is because hydrogen cracking may occur in hard structures such as sight, bainite and pearlite. The upper limit of the range from the surface of the polygonal ferrite structure is not particularly specified, but if it is larger than 300 μm, the strength near the end face of the shearing process is lowered overall, for example, when the shearing process is a piercing hole process. It is preferably 300 μm or less in order to cause a problem such as a large deformation when contacting the bolt.

  The reason why the average grain size of the polygonal ferrite structure is 5.0 μm or less is that hydrogen cracking is liable to occur in the polygonal ferrite structure when the average particle diameter is larger than 5.0 μm. The lower limit of the average particle diameter is not particularly specified, but the lower limit is determined depending on the magnitude of plastic strain by shearing, and is preferably 0.3 μm or more from the viewpoint of ensuring the homogeneity of strength characteristics. Here, the grain size means the diameter of a circle when the area of each crystal grain of polygonal ferrite when observed in a cross section perpendicular to the shear plane and along the thickness direction is replaced with a circle of the same area. That is, it means a circle equivalent diameter (circle equivalent diameter).

  Furthermore, the reason why the average aspect ratio of the polygonal ferrite structure is set to 1 to 3 is that when the average aspect ratio is 3 or more, there is a high possibility that plastic strain is largely introduced, and hydrogen cracking easily occurs. Here, the average aspect ratio is an average value of the ratio of the major axis to the minor axis when the crystal grains are assumed to be elliptical, and is a value greater than 1 or 1.

  In addition, the ratio of polygonal ferrite in the structure of the surface layer region of the sheared end face is 50% or more in area%. When it is less than 50%, a hard structure other than polygonal ferrite, such as martensite, pearlite, bainite. This is because the ratio of the hard structure such as is increased and hydrogen cracking is likely to occur. The upper limit is not particularly defined, and depends on the chemical composition of the steel material. For example, in a composition containing a large amount of C, pearlite and carbide are inevitably generated together with the formation of polygonal ferrite.

  Next, the structure of the region other than the surface layer region within the range of at least 30 μm from the surface of the shearing end surface, that is, the structure of the region inside the surface layer region includes any one or more of martensite, pearlite, and bainite. This is because the sheared parts maintain high strength. When high strength properties are to be obtained with a mold or a structure quenched with water after being heated to a high temperature, the tensile strength must be 1200 MPa or more. In this case, martensite It is preferable that the composition has a high ratio.

  Basically, the sheared part of the present invention only needs to have a surface layer region in the range of at least 30 μm of the end face of the shearing process that satisfies the above-described structure condition. The steel type and the component composition are not particularly limited, but it is desirable to use a steel plate having a component composition as defined in claims 3 to 8, and the components of the steel plate defined in claims 3 to 8 are described below. The reason for limiting the composition will be described.

C: 0.05-0.55%
C is an element added to secure the material with the cooled structure as martensite. To ensure a strength of 1000 MPa or more, 0.05% or more, preferably 0.1% or more may be added. desirable. However, if the addition amount is too large, it becomes difficult to ensure the strength during impact deformation, so the upper limit is made 0.55%. Therefore, the range was made 0.05 to 0.55%.

Si: 2% or less Si is an alloy element of a solid solution strengthening type, but if it exceeds 2%, a problem of surface scale occurs, so it is limited to 2% or less. In addition, when plating is performed on the surface of the steel sheet, if the amount of Si added is large, the plateability deteriorates, so the upper limit is desirably set to 1.1%.

Mn: 0.1 to 3%
Mn is an element that improves strength and hardenability. If it is less than 0.1%, sufficient strength at the time of quenching cannot be obtained, and even if added over 3%, the effect is saturated. It is limited to a range of 0.1 to 3%.

P: 0.1% or less Since P is an element that adversely affects weld cracking and toughness, P is limited to 0.1% or less. In addition, Preferably it is 0.02% or less. Further, it is more preferably 0.015% or less.

S: 0.03% or less S affects non-metallic inclusions in the steel and degrades workability and causes toughness deterioration / anisotropy and reheat cracking sensitivity. For this reason, S is limited to 0.03% or less. In addition, More preferably, it is 0.01% or less. Moreover, by restricting S to 0.005% or less, impact characteristics are dramatically improved.

Al: 0.1% or less Al is a necessary element to be used as a deoxidizer for molten steel and is unavoidably present. However, if it exceeds 0.1%, non-metallic inclusions increase and surface flaws appear in the product. Since it is likely to occur, the range is made 0.1% or less.

O: 0.015% or less O is contained in an amount of 0.015% or less because excessive addition causes generation of an oxide that adversely affects toughness and generates an oxide that becomes a starting point of fatigue fracture. To do.

N: 0.01% or less When N exceeds 0.01%, the toughness tends to deteriorate due to coarsening of nitrides and age hardening due to solute N. For this reason, N is made 0.01% or less.

  In addition, the following elements may be added as necessary.

B: 0.0002 to 0.0050%
B is added in order to improve the hardenability during hot press molding or cooling after press molding, but 0.0002% or more must be added in order to exhibit this effect. However, if the amount of addition increases excessively, there is a concern of hot cracking and the effect is saturated, so the upper limit is made 0.0050%.

V: 2.0% or less V is an element that improves the strength and is effective for improving the strength even with a small amount. However, even if added in excess of 2.0%, the effect is saturated and the toughness is deteriorated. 0.0% or less.

W: 3.0% or less W is an element that improves the strength and is effective in improving the strength even in a small amount. However, even if added over 3.0%, the effect is saturated and the toughness is deteriorated. 0.0% or less.

Cr: 0.01 to 1.0%
Cr is an element that improves hardenability, and has the effect of precipitating M23C6 type carbide in the matrix, and has the effect of increasing the strength and miniaturizing the carbide. However, if less than 0.01%, these effects cannot be expected sufficiently, and if it exceeds 1%, the yield strength tends to increase excessively, so Cr is in the range of 0.01 to 1.0%. . More desirably, it is 0.05 to 1%.

Mo: 0.02-3.0%
Mo is an element that improves the strength. To increase the strength, 0.02% or more is necessary. However, if added over 3.0%, the effect is saturated and the toughness is deteriorated. 0.02 to 3.0%.

Ti: 0.01 to 0.70%
Ti is an element that improves the strength, and addition of 0.01% or more is necessary to improve the strength. However, if added over 0.7%, the effect is saturated and the toughness is deteriorated. 0.01 to 0.70%.

  As other elements, elements such as Ni, Cu and Sn which are considered to be mixed from scrap may be contained. Furthermore, Ca, Mg, Y, As, Sb, and REM may be added from the viewpoint of inclusion morphology control. Further, Nb and Zr may be added for the purpose of improving the strength. However, if these elements increase excessively, the amount of C not bonded to these elements decreases and sufficient strength cannot be obtained after cooling. . In addition, there is no problem even if impurities inevitably included.

  In producing the sheared parts of the present invention, the ingot that is preferably adjusted to the above-described component composition and cast according to a conventional method is hot-rolled, and further pickled and cold-rolled as necessary. -A steel plate obtained by annealing may be subjected to shearing hot. The process conditions for the steel plate manufacturing process may be any conventional method.

  Here, although the steel plate obtained as described above may be used as it is for a sheared part, it may be subjected to aluminum plating, aluminum-zinc plating, or galvanization in order to impart and improve corrosion resistance. As for the production method, pickling and cold rolling may be performed by a conventional method, and thereafter, the aluminum plating step, the aluminum-zinc plating step, and the galvanizing may be performed by a conventional method. That is, 5 to 12% of the Si concentration in the bath is suitable for aluminum plating, and 40 to 50% of the Zn concentration in the bath is suitable for aluminum-zinc plating. Even if Mg or Zn is mixed in the aluminum plating layer or Mg is mixed in the aluminum-zinc plating layer, a steel plate having the same characteristics can be manufactured without any particular problem.

  As for the atmosphere in the plating process, it is possible to perform plating under normal conditions in either a continuous plating facility with a non-oxidizing furnace or a continuous plating facility without a non-oxidizing furnace, and only this steel plate needs special control. It does not hinder productivity. Moreover, as long as it is a galvanization method, what kind of methods, such as hot dip galvanization, electrogalvanization, and alloying hot dip galvanization, may be taken. Under the above manufacturing conditions, metal pre-plating is not performed on the steel plate surface before plating, but there is no particular problem even if Ni pre-plating, Fe pre-plating, or other metal pre-plating that improves plating properties is performed. Further, there is no particular problem even if different metal plating or a coating of inorganic or organic compound is applied to the surface of the plating layer.

  In carrying out a shearing process on the above steel plates (including plated steel plates), the steel plates are heated to a temperature within the range of Ac3 to 1400 ° C and held for 30 seconds to 300 seconds, and then 400 to 900 ° C. Shearing is performed at a temperature within the range. Specifically, for example, as shown in FIG. 1, the steel plate 1 is heated and held in the above temperature range by an appropriate heating means such as a heating furnace 2, and then placed in a die 3 for drawing with shearing. Then, after forming or simultaneously with forming, the steel sheet 1 is sheared by the punch 3a and the die 3b as shown in FIG. In this shearing process, the shearing speed is set to 30 spm or more, and the shearing clearance S (the gap between the punch 3a and the die 3b in FIG. 2) is within a range of 5 to 20% of the plate thickness T. It is desirable. The reasons for limiting these heating conditions and shearing conditions will be described below.

The reason why the heating temperature of the steel sheet is defined as Ac3 or higher and 1400 ° C. or lower is to keep the structure of the steel sheet to be austenite for strengthening by quenching after forming. Moreover, it is because the conveyance after a heating becomes impossible when heating temperature is 1400 degreeC or more.
The holding time of the heating temperature was set to 30 seconds to 300 seconds. If it is shorter than 30 seconds, the temperature of the steel sheet may not be uniform, and if it is longer than 300 seconds, the austenite grain size becomes remarkably large. This is because the grain size of polygonal ferrite generated after the shearing process also increases and the effect of improving hydrogen cracking decreases. Productivity is also significantly reduced.

  The reason why the shearing temperature is defined as 400 to 900 ° C. is that when the temperature is less than 400 ° C., any of pearlite, bainite, and martensite transformation may occur before the start of the shearing process. This is because there is a shortage. Further, when the temperature is higher than 900 ° C., the deformation resistance of the steel material is very small, and the shearing end face is remarkably deformed, and a treatment such as care in a subsequent process is required.

  In addition, in order to hold | maintain an austenite structure | tissue after heating and hold | maintaining a steel plate to the temperature range of Ac3 or more and 1400 degrees C or less, and performing a shearing process in the temperature range of 400-900 degreeC (for example, 10 degreeC). Cooling at a cooling rate of / sec or less), and after shearing, it is desirable to rapidly cool to near room temperature (for example, cooling at a cooling rate of 50 ° C./sec or more) in order to obtain a high-strength quenched structure. . This rapid cooling can be realized by bringing the steel plate into contact with a mold during forming or by spraying cooling water on the steel plate. However, when forming such as drawing or bending before shearing, the steel sheet is heated to a temperature range of Ac3 or higher and 1400 ° C or lower, and immediately after forming, a temperature range of 400 to 900 ° C is formed. It is desirable that the material is rapidly cooled (for example, cooled at a cooling rate of 50 ° C./sec or more), and then sheared.

  The reason why the shearing speed is defined as 30 spm or more is that when the shearing speed is less than 30 spm, the plastic strain application rate becomes low, and the recovery of the plastic strain proceeds at the same time, making it difficult to induce the polygonal ferrite transformation. In order to obtain polygonal ferrite stably, it is desirable that it is 45 spm or more. The upper limit of the shearing speed is not particularly specified, and a higher one is preferable within the range of the press machine capability. In addition, when performing a shearing process by heating a steel plate, a waiting time often exists in the shearing process because the heating time of the steel plate is longer than the shearing process. Since it is not appropriate to indicate the shearing speed in such a case as the number of times of shearing within a certain period of time that is normally used, it is displayed at a speed that the time required for one shearing process is repeated within a certain period of time. In the present invention, such a calculation is performed. The shearing speed is indicated by the number of shearing per minute (Shot Per Minute). Here, “time required for one shearing process” means a period of time from when the shearing tool starts an operation for shearing from the initial position to when the shearing tool ends and returns to the initial position. .

  Further, the clearance S of the shearing process is defined as 20% or less of the plate thickness T of the steel sheet. When the clearance exceeds 20%, shear deformation and plastic strain are dispersed in the gap between the punch and the die, inducing polygonal ferrite transformation. It is because it becomes difficult to do. The lower limit of the clearance is not particularly specified, but if the clearance is too narrow, damage to the mold becomes significant, so it is preferably 5% or more.

  As described above, the steel sheet is heated to Ac3 to 1400 ° C. and held for 30 to 300 seconds to austenitize the structure of the steel sheet, and then at a temperature within the range of 400 to 900 ° C. under appropriate conditions. By applying a shearing process, a polygonal ferrite transformation is induced in the surface layer region of the sheared end face, and the surface layer region has an average particle size of 5.0 μm or less and an average aspect ratio of 1 as already described. A structure containing 50% or more of ~ 3 polygonal ferrite can be generated.

  As mentioned above, although the method of shearing after heating a steel plate and the sheared component were described, this invention is not limited to the component which performs only a shearing process. For example, as shown in Fig. 1, when a steel sheet is heated and then subjected to shearing, it is also applied to a method of obtaining a high-strength processed part by simultaneously performing forming such as drawing or bending, and then cooling. On the other hand, a high-strength processed part can be obtained by performing the forming process and then performing the shearing process of the present invention, or by performing the forming process after performing the shearing process. Furthermore, it is of course applicable to a sheared part having two or more sheared parts. In that case, the surface layer regions of the sheared end faces of the two or more sheared parts all satisfy the conditions specified in the present invention. It is desirable to be satisfied.

    Hereinafter, the present invention will be specifically described with reference to examples. The following examples are for showing specific effects according to the present invention, and it goes without saying that the conditions described in the examples do not limit the technical scope of the present invention.

  Slabs having chemical components shown in Tables 1 to 3 were cast. These slabs were heated to 1050 to 1350 ° C. and hot rolled to form hot rolled steel sheets having a finishing temperature of 800 to 900 ° C. and a winding temperature of 450 to 680 ° C. and a thickness of 4 mm. Then, after pickling, it was set as the cold steel plate of 1.6 mm thickness by cold rolling. Among the cold-rolled plates, A1 to A10 are kept cold-rolled, B1 to B3 are hot-dip aluminum plated, C1 to C13 are hot-dip aluminum-galvanized, and C3-1 to C3-5 are alloyed hot-dip galvanized. C3-5 to C3-10 were hot dip galvanized. Thereafter, the steel sheets were heated and held by furnace heating under various conditions shown in Tables 4 to 14, and then subjected to hot shearing. The atmosphere of the heating furnace changed the amount of hydrogen and the dew point. The shearing process was a piercing process using a punch with a diameter of 20 mm. The blank size was 1.6 mm thick × 300 mm × 500 mm. Tables 1 to 3 also show the Ac3 point of the test steel.

  For each sheared part obtained by hot shearing by piercing as described above, observe the metal structure near the surface of the shearing end face, and examine the formation status of polygonal ferrite structure. The average grain size, average aspect ratio, ratio of polygonal ferrite structure, and depth (range) from the surface of the shearing end face in the region where the polygonal ferrite structure was generated were obtained. 4 to Table 14 Here, the observation of the structure and the measurement of each condition of the polygonal ferrite structure were specifically performed as follows.

  From a sheared part, a surface that is perpendicular to the shear plane and parallel to the thickness direction is embedded in resin, mirror-polished and etched to produce an optical microscope sample, and a metal structure from the shear plane to 30 μm is lasered at a magnification of 1000 to 3000 times Observed with a microscope. The crystal grains of the polygonal ferrite structure were easily distinguished from other metal structures because almost no precipitate was observed. The average grain size, average aspect ratio, and area ratio of the crystal grains of the polygonal ferrite structure were examined by image analysis in each 10 fields of view, and the average value was obtained.

  Further, the hydrogen cracking property of the shearing end face portion was evaluated and judged as follows. That is, one week after the piercing process, the entire pierced hole was observed and the presence or absence of cracks was visually determined. Visual observation was performed with a 20-fold magnifier. The determination results are shown in Tables 4-14.

Tables 4 and 5 show various examples of the present invention related to claim 1 and the comparative example corresponding to claim 1 in which the chemical composition of the steel sheet is not particularly defined, that is, the condition of the surface layer region of the shearing end face (polygonal ferrite structure condition). An example is shown.
Among these, the experiment numbers 1, 6, 15, 16, 26, 31, 40, 41, in which the range from the surface of the end face of the region having the structure containing polygonal ferrite is outside the scope of the invention defined in claim 1 Experiment Nos. 5, 20, 21, 30, 45, and 46 in which the polygonal ferrite particle size is outside the scope of the invention defined in claim 1 have the worst evaluation of hydrogen cracking. In other experiments of the present invention, The evaluation of hydrogen cracking is better than the comparative example with the above experiment numbers. In Experiment Nos. 25 and 50, deformation at the time of conveyance was large, and hydrogen cracking could not be evaluated. The experiment numbers 1, 16, 26, and 41 are not limited to the range from the surface of the end face of the region having the structure including polygonal ferrite, but the fraction of the polygonal ferrite structure is defined in claim 1. The experiment numbers 6, 15, 31, and 40 are out of the range, and the particle size and average aspect ratio are further defined in the experiment numbers 21, and 46, and the particle size and average aspect ratio are defined in claim 1 in the experiment numbers 21 and 46, respectively. Outside. From these facts, it has been found that the invention defined in claim 1 is effective in improving hydrogen cracking in the sheared portion.

Next, Tables 6 to 9 show examples of the present invention relating to claims 4 and 7 that define the chemical composition of the steel sheet and comparative examples corresponding thereto.
Among these, the experiment numbers 101-105, 126-130, 151-155, 166-170 looked at the influence of shear start temperature, and shear processing temperature is in the range of the invention prescribed | regulated by Claim 6. 102-104, 127-129, 152-154, and 167-169 have improved evaluation of hydrogen cracking over 101, 105, 126, 130, 151, 155, 166, and 170 which are examples outside the range. Furthermore, 102-104, 127-129, 152-154, and 167-169 have improved evaluation of hydrogen cracking more than 2-4 and 27-29 in Tables 4 and 5 which are the same shearing conditions. 101, 126, 151, and 166 are the ranges from the end face of the polygonal ferrite structure and the fraction of the polygonal ferrite structure is outside the scope of the invention defined in claim 1, and 105, 130, 155, and 170 are crystals. The particle size is outside the scope of the invention defined in claim 1.

  Experiment numbers 106 to 110, 131 to 135, 156 to 160, and 170 show the influence of the shearing speed, and the shearing speed is within the scope of the invention defined in claim 9. ˜135, 157 to 160, and 172 to 175 have improved hydrogen cracking evaluations over 106, 131, 156, and 171 which are examples outside the range. Furthermore, 107 to 110, 132 to 135, 157 to 160, and 172 to 175 have improved evaluation of hydrogen cracking more than 7 to 10 and 32 to 35 in Tables 4 and 5 which are the same shearing conditions. . 106, 131, 156, and 171 are outside the scope of the invention defined in claim 1 in the range from the end face of the polygonal ferrite structure, the fraction of the polygonal ferrite structure, the particle size, and the average aspect ratio.

  Furthermore, the experiment numbers 111 to 115, 136 to 140, 161 to 165, and 176 to 180 show the influence of the clearance at the time of shearing, and the clearance is within the scope of the invention defined in claim 9. 114, 136 to 139, 161 to 164, and 176 to 179 have improved hydrogen cracking evaluations over 115, 140, 165, and 180, which are examples outside the range. Furthermore, the evaluation of hydrogen cracking is further improved in 111-114, 136-139, 161-164, 176-179, compared to 11-14 and 36-39 in Tables 4 and 5 which are the same shearing conditions. . In addition, 115, 140, 165, and 180 are outside the scope of the invention defined in claim 1 in the range from the end face of the polygonal ferrite structure, the fraction of the polygonal ferrite structure, the particle size, and the average aspect ratio.

  In addition, the experiment numbers 116 to 120 and 141 to 145 show the influence of the holding time, and the holding times 117 to 119 and 142 to 144 that are within the scope of the invention defined in claim 6 are out of the range. The evaluation of hydrogen cracking is improved over the examples 116, 120, 141, and 145. Furthermore, 117 to 119 and 142 to 144 are further improved in the evaluation of hydrogen cracking than 17 to 19 and 42 to 44 in Tables 4 and 5 which are the same shearing conditions. 116 and 141 are the ranges from the end face of the polygonal ferrite structure and the fraction of the polygonal ferrite structure is outside the scope of the invention defined in claim 1. 120 and 145 are the crystal grain size and average aspect ratio. It is outside the scope of the invention defined in Item 1.

  Experiment numbers 121 to 125 and 146 to 150 show the influence of the heating temperature, and 122 to 124 and 147 to 149 whose heating temperature is within the scope of the invention defined in claim 6 are examples outside the range. The evaluation of hydrogen cracking is improved over 121 and 146. Furthermore, the evaluation of hydrogen cracking is further improved in 122 to 124 and 147 to 149 than 22 to 24 and 47 to 49 in Tables 4 and 5 which are the same shearing conditions. 121 and 146 are outside the scope of the invention defined by claim 1 in terms of the grain diameter of the polygonal ferrite structure and the average aspect ratio. In Experiment Nos. 125 and 150, deformation during transportation was large and hydrogen cracking could not be evaluated.

  Furthermore, experiment numbers 107-110, 111-114, 117-119, 127-129, 132-135, 136-139, 147-149, 152-154, 157-160, 167-169, 172-175, 176 ˜179, the additive elements such as B, V, and W are within the scope of the invention defined in claims 4 and 7, and these are out-of-range examples 102 to 104, 122 to 124, 142 to 144 and 161 to 164 have improved hydrogen cracking.

  From these experimental results, it was confirmed that the present invention is effective in improving hydrogen cracking in the sheared portion.

  Next, Tables 10 to 13 are examples relating to Claims 5 and 8 that define the chemical composition of the steel sheet.

  Among these, the experiment numbers 201-205, 226-230, 251-255 looked at the influence of the shearing temperature, and the shearing temperature is within the scope of the invention defined in claim 6. 227 to 229 and 252 to 154 have improved hydrogen cracking evaluations over the examples 201, 205, 226, 230, 251 and 255 which are out of range. Furthermore, 202-204, 227-229, and 252 to 154 are further improved in evaluation of hydrogen cracking than 2 to 4 and 27 to 29 in Tables 4 and 5 which are the same shearing conditions. In addition, 201, 226, and 251 are outside the scope of the invention defined in claim 1 in terms of the range from the end face of the polygonal ferrite structure and the fraction of the polygonal ferrite structure, and 205, 230, and 255 are claimed in crystal grain size. It is outside the scope of the invention defined in Item 1.

  The experiment numbers 206 to 210, 231 to 235, and 256 to 260 show the influence of the shearing speed, and the shearing speed is within the scope of the invention defined in claim 9 207 to 210, 232 to 235. In 257 to 260, evaluation of hydrogen cracking is improved over 206, 231 and 256 which are examples outside the range. Furthermore, 207 to 210, 232 to 235, and 257 to 260 have further improved evaluation of hydrogen cracking than 7 to 10 and 32 to 35 in Tables 4 and 5 which are the same shearing conditions. 206, 231, 256 are outside the scope of the invention defined in claim 1 in terms of the range from the end face of the polygonal ferrite structure, the fraction of the polygonal ferrite structure, the particle size and the average aspect ratio.

  Furthermore, experiment numbers 211 to 215, 236 to 140, and 261 to 265 show the influence of the clearance during shearing, and the clearance is within the scope of the invention defined in claim 9. 239 and 261 to 264 have improved hydrogen cracking evaluations over 215, 240, and 265, which are examples outside the range. Furthermore, 211 to 214, 236 to 239, and 261 to 264 are further improved in the evaluation of hydrogen cracking compared to 11 to 14 and 36 to 39 in Tables 4 and 5 which are the same shearing conditions. In addition, 215, 240 and 265 are outside the scope of the invention defined in claim 1 in the range from the end face of the polygonal ferrite structure, the fraction of the polygonal ferrite structure, the particle size and the average aspect ratio.

  In addition, the experiment numbers 216 to 220 and 241 to 245 show the influence of the holding time, and the holding times 217 to 219 and 242 to 244 that are within the scope of the invention defined in claim 6 are out of the range. The evaluation of hydrogen cracking is improved over the examples 216, 220, 241, and 245. Furthermore, the evaluation of hydrogen cracking in 217 to 219 and 242 to 244 is further improved than 17 to 19 and 42 to 44 in Tables 4 and 5 which are the same shearing conditions. Note that 216 and 241 are outside the scope of the invention defined by claim 1 in terms of the range from the end face of the polygonal ferrite structure and the fraction of the polygonal ferrite structure, and 220 and 245 are claimed in terms of crystal grain size and average aspect ratio. It is outside the scope of the invention defined in Item 1.

  The experiment numbers 221 to 225 and 246 to 250 show the influence of the heating temperature, and the heating temperatures within the scope of the invention defined in claim 6 are examples out of the range. The evaluation of hydrogen cracking is improved compared to 221 and 246. Furthermore, the evaluation of hydrogen cracking is further improved in 222 to 224 and 247 to 249 as compared with 22 to 24 and 47 to 49 in Tables 4 and 5 which are the same shearing conditions. Incidentally, 221 and 246 are outside the scope of the invention defined in claim 1 in terms of the grain size and average aspect ratio of the polygonal ferrite structure. In Experiment Nos. 225 and 250, deformation during transportation was large, and hydrogen cracking could not be evaluated.

  Furthermore, experiment numbers 207 to 210, 211 to 214, 217 to 219, 227 to 229, 232 to 235, 236 to 239, 247 to 249, 252 to 254, and 257 to 260 are additions of Cr, Mo, Ti, and the like. The elements are within the scope of the invention defined in claims 5 and 8, and hydrogen cracking is further improved than those examples 202 to 204, 222 to 224, 242 to 244, and 261 to 264, which are examples outside the scope. Has been.

  Therefore, it was confirmed from these experimental results that the present invention is effective in improving hydrogen cracking in the sheared portion.

Next, Table 14 shows another embodiment relating to the invention defined in claims 5 and 8 in which the chemical composition of the steel sheet is defined.
Among them, the experiment numbers 301 to 305 and 326 to 330 look at the influence of the shearing temperature, and 302 to 304 and 327 to 329 whose shearing temperature is within the scope of the invention defined in claim 6 are: The evaluation of hydrogen cracking is improved over 301, 305, 326, and 330, which are examples outside the range. Furthermore, 302 to 304, 327 to 329 are the same shearing conditions as shown in Table 4, Tables 2 to 4, 27 to 29, Tables 6 to 8, 102 to 104, 127 to 129, 152 to 154, 167. Evaluation of hydrogen cracking is further improved from 202 to 204, 227 to 229, and 252 to 254 in ˜169 and Tables 9 to 11. Note that 301 and 326 are outside the scope of the invention defined by claim 1 in terms of the range from the end face of the polygonal ferrite structure and the fraction of the polygonal ferrite structure, and 305 and 330 have crystal grain sizes defined in claim 1. It is out of the scope of the invention.

  Experiment numbers 306 to 310 and 331 to 335 show the influence of the shearing speed, and 307 to 310 and 332 to 335 where the shearing speed is within the scope of the invention defined in claim 9 are out of range. The evaluation of hydrogen cracking is improved compared to 306 and 331. Furthermore, 307 to 310 and 332 to 335 are the same shearing conditions as shown in Tables 4 and 5, 7 to 10, 32 to 35, and Tables 6 to 8, 107 to 110, 132 to 135, 157 to 160, 172. Evaluation of hydrogen cracking is further improved than 207 to 210, 232 to 235, and 257 to 260 in ˜175 and Tables 9 to 11. In addition, the range from the end face of the polygonal ferrite structure, the fraction of the polygonal ferrite structure, the particle size, and the average aspect ratio of 306 and 331 are outside the scope of the invention defined in claim 1.

  Furthermore, experiment numbers 311 to 315 and 336 to 340 show the influence of the clearance, and 311 to 314 and 336 to 339 where the clearance is within the scope of the invention defined in claim 9 are examples outside the range. The evaluation of hydrogen cracking is improved over some 315,340. Further, 311 to 314 and 336 to 339 are the same shearing conditions as shown in Tables 4 and 11, 11 to 14 and 36 to 39, and Tables 6 to 8, 111 to 114, 136 to 139, 161 to 164, 176. The evaluation of hydrogen cracking is further improved than 211 to 214, 236 to 239, and 261 to 264 in ˜179 and Tables 9 to 11. 315 and 340 are outside the scope of the invention defined by claim 1 in the range from the end face of the polygonal ferrite structure, the fraction of the polygonal ferrite structure, the particle size and the average aspect ratio.

  In addition, the experiment numbers 316 to 320 and 341 to 345 looked at the influence of the retention time, and the retention times 317 to 319 and 342 to 344 that are within the scope of the invention defined in claim 6 are out of the range. The evaluation of hydrogen cracking is improved over the examples 316, 320, 341, and 345. Further, 317 to 319 and 342 to 344 are the same shearing conditions, Table 4, Tables 17 to 19, 42 to 44, Tables 6 to 8, 117 to 119, 142 to 144, Tables 9 to 11 Evaluation of hydrogen cracking is further improved than those of Nos. 217 to 219 and 242 to 244. In addition, 316 is outside the scope of the invention defined in claim 1 in terms of the range from the end face of the polygonal ferrite structure and the fraction of the polygonal ferrite structure, and 320 is defined in claim 1 in terms of the crystal grain size and average aspect ratio. It is out of the scope of the invention.

  The experiment numbers 321 to 325 and 346 to 350 show the influence of the heating temperature, and the heating temperatures within the scope of the invention defined in claim 6 are examples 322 to 324 and 347 to 349 which are out of the range. The evaluation of hydrogen cracking is improved compared to 321, 325, 346 and 350. Furthermore, 322 to 324 and 347 to 349 are the same shearing conditions as shown in Tables 4 and 5, 22 to 24 and 47 to 49, Tables 6 to 8 122 to 124, 147 to 149, Tables 9 to 11 The evaluation of hydrogen cracking is further improved than those of Nos. 222-224, 247-249. Incidentally, the particle sizes and average aspect ratios of polygonal ferrite structures 321 and 346 are outside the scope of the invention defined in claim 1. In Experiment Nos. 325 and 350, deformation during transportation was large, and hydrogen cracking could not be evaluated.

  Therefore, it is clear from the above experimental results that the present invention is effective in improving hydrogen cracking in the sheared portion.

    INDUSTRIAL APPLICABILITY The present invention relates to a part subjected to shearing among parts requiring strength, such as structural members and reinforcing members such as automobiles and construction machines, and can be applied to sheared parts having excellent hydrogen cracking properties.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Heating furnace 3 Die 3a Punch 3b Die 4 Shearing end face 5 Surface layer area of shearing end face

Claims (10)

  A sheared part obtained by subjecting a steel plate to a shearing process, wherein a region within a range of at least 30 μm in the depth direction from the surface of the end face of the shearing process has an average particle size of 5.0 μm or less and an average aspect ratio of 1 to 3 A shearing part having at least one shearing part composed of a structure containing 50% or more of null ferrite.   2. The shear-processed part according to claim 1, wherein the region other than the region within the range of at least 30 μm in the depth direction from the surface of the end face is composed of a structure containing one or more of martensite, pearlite, and bainite. . The steel sheet is in mass%,
C: 0.05 to 0.55%,
Si: 2% or less,
Mn: 0.1 to 3%,
P: 0.1% or less,
S: 0.03% or less,
Al: 0.1% or less,
O: 0.015% or less,
N: 0.01% or less,
The shear processed component according to claim 1, wherein the balance is made of Fe and unavoidable impurities. The steel sheet is in mass%, and
B: 0.0002 to 0.0050%,
V: 2.0% or less,
W: 3.0% or less,
The shear-processed part according to claim 3, comprising at least one selected from the group consisting of: The steel sheet is in mass%, and Cr: 0.01 to 1.0%,
Mo: 0.02 to 3.0%,
Ti: 0.01 to 0.70%,
The shear-processed part according to claim 3, wherein the shear-processed part contains at least one selected from the group consisting of: % By mass
C: 0.05 to 0.55%,
Si: 2% or less,
Mn: 0.1 to 3%,
P: 0.1% or less,
S: 0.03% or less Al: 0.1% or less,
O: 0.015% or less,
N: 0.01% or less,
Using a steel sheet containing Fe and the inevitable impurities in the balance, heating the steel sheet to Ac3 to 1400 ° C, holding it for 30 to 300 seconds, and then shearing at a temperature of 400 to 900 ° C A method for manufacturing sheared parts. The steel sheet is in mass%, and
B: 0.0002 to 0.0050%,
V: 2.0% or less,
W: 3.0% or less,
The method for producing a sheared part according to claim 6, comprising at least one selected from the group consisting of: The steel sheet is in mass%, and
Cr: 0.01 to 1.0%
Mo: 0.02 to 3.0%,
Ti: 0.01 to 0.70%,
It contains 1 or more types chosen from among these, The manufacturing method of the shearing process component in any one of Claim 6 and Claim 7 characterized by the above-mentioned.   The method for manufacturing a sheared part according to any one of claims 6 to 8, wherein the shearing speed is 30 spm or more and the clearance is 5 to 20% of the plate thickness.   The method for manufacturing a sheared part according to any one of claims 6 to 9, wherein a molding process is accompanied during the shearing process.

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