JP2010174275A - Steel sheet for die quenching having excellent hot punchability and method for producing member by die quenching process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet having excellent punchability in which the height of a bur generated upon hot punching in a die quench processing is low, and to provide a member produced by a die quenching process. <P>SOLUTION: The steel sheet for die quenching having excellent hot punchability is a steel sheet having a composition comprising, by mass, 0.05 to <0.10% C, 0.05 to 0.5% Si, 1.0 to 1.8% Mn, ≤0.03% P, 0.002 to <0.020% S, 0.015 to 0.07% Al, 0.0025 to 0.010% N and 0.02 to 0.06% Ti, and further comprising one or more selected from, by mass, 0.15 to 1% Cr and 0.1 to 0.5% Mo, and the balance iron with inevitable impurities, the steel sheet for die quenching having excellent hot punchability is characterized in that after the heating of the steel sheet to an austenitic single phase region, in a range at least from the outermost surface to a depth of 100 μm of the steel sheet, TiN is scattered, and the average of the length of the side longest in TiN is 0.1 to 1 μm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a die quench steel plate excellent in punchability, which is suitable when hot pressing is performed using a die quench method, and a member manufacturing method using the die quench method using the steel plate. In addition, the tensile strength of the steel plate after die quenching is 980 MPa class (900 MPa to 1.1 GPa vicinity).

  In reducing the weight of automobiles, the use of high-strength steel sheets and the use of higher-strength steel sheets are being studied. However, when pressing steel sheets into complex shapes and cold pressing high-strength steel sheets exceeding 980 MPa, various problems such as shape freezing and press cracking are likely to occur, and the parts that you want to increase are not always strong. The current situation is not done. One method for solving this situation is the die quench method. In the die quench method, the steel sheet to be processed is heated to the austenite single-phase region and softened, and then molded with a water-cooled normal temperature mold, then rapidly cooled, and the steel sheet is baked to ensure the strength of the molded product. It is a construction method. In this die quench method, the strength of the steel plate after quenching (quenching) exceeds 980 MPa, so it becomes difficult to make holes for attaching parts to the molded product. Moreover, even if it can be opened, since the residual stress is large, it may cause delayed fracture. Therefore, generally, in the die quench method, holes are drilled with a laser after quenching. However, recently, as the die-quenching method has become widespread, a method for drilling holes without relying on expensive laser equipment has begun to be studied. When making a hole in a part by the die quench method, after heating a steel plate (blank material), the hole is punched out before quenching, and the method of die quenching the blank material which made the hole can be considered. At this time, since the hot-softened steel plate is punched, burrs are greatly formed on the punched end surface. Since the burrs are hardened and remain after quenching, the shape of the parts is deteriorated, and the burr is easily caught during transportation, which not only interferes with transportation but also damages the press die in some cases.

In Patent Document 1, any one of Mg oxide, sulfide, composite crystallized product, and composite precipitate containing V of 0.002 to 0.5% and Mg of 0.0002 to 0.01% and having an average particle size of 0.01 to 5 μm is 1 mm 2. A high-strength steel sheet for hot pressing having excellent punching characteristics, characterized by containing 10 ² to 10 ⁷ hits, is disclosed. In this technique, Mg oxide is dispersed in nuclei to suppress the generation of coarse cracks at the time of punching and to suppress delayed fracture by using hydrogen trap sites. In other words, the punchability in this technique refers to the likelihood of delayed fracture of the punched end face of a material having increased strength after quenching, and does not have a problem with the shape after punching. In addition, this technique cannot be used to improve the punchability of austenite at a high temperature, which is different from martensite and crystal system.

Although it does not mention punchability, Patent Document 2 discloses a technique for increasing fatigue strength by including 0.002 to 0.02% of Ce and making the number of Ce oxides of 2 μm or less 4 pieces / mm ² or more. Has been. This technique improves the fatigue properties after die quenching by attaching MnS to Ce-Ti-O fine particles. In this technique, Ce-Ti-O fine particles with MnS adhered do not affect the austenite punchability at high temperatures. In addition, even if such fine particles are present before die quenching, they are dissolved and dispersed during the heating of die quench, so that the effect is not recognized when using the die quench method.

  In Patent Document 3, a large amount of Si is added to increase the strength of the welded portion, a decrease in hot workability due to the addition of Si is compensated for by B addition, and Ti is added in a larger amount at an atomic concentration than N. In addition, a technique for ensuring hardenability by suppressing the generation of BN is disclosed. With this technology, it is possible to give hot forming to steel by adding Si to Si, but this causes a problem that burrs are generated at the time of punching at the high temperature, and the punchability is reduced. There is.

JP 2006-9116 A JP 2007-247001 A JP 2007-169679 A

  As described above, in the above prior art, when a hole is made in a part by the die quench method, when a steel plate is heated and then a hole is punched before quenching and die quenching is performed, there is a problem that large burrs are generated on the punched end face. It is difficult to solve.

  The present invention has been made under such circumstances, and the die quench steel sheet having excellent punchability and the member of the die quench technique having a low height of burrs generated during the punching performed hot in the die quench technique. An object is to provide a manufacturing method.

  In order to achieve the above object, it is considered necessary to find precipitates that are stable at high temperatures and have good austenite punchability. Since the TiN grows to a certain size at a high temperature, if the optimum size, amount, and dispersion state when using TiN can be realized by punching with the die quench method, the punching property is good. The idea was that a die quench steel sheet could be made. Unlike normal steel plate processing, die quenching heats a blank material to be processed. Therefore, N in the air is more likely to enter the steel sheet surface than a normal steel sheet. Since these TiNs usually cause surface scratches, conventionally, the atmosphere and the blank are shielded by controlling the heating atmosphere or using an antioxidant or plating. However, the present inventors did not prevent the formation of TiN, but conversely, as a result of intensive research on whether or not TiN could be used, the punchability was improved by arranging coarse TiN on the surface layer. Knowing that the problem of burrs can be solved, the present invention has been completed.

The features of the present invention that have solved the above problems are as follows.
(1) By mass%, C: 0.05% or more, less than 0.10%, Si: 0.05-0.5%, Mn: 1.0-1.8%, P: 0.03% or less, S: 0.002% or more, less than 0.020%, Al: 0.015 ~ 0.07%, N: 0.0025 ~ 0.010%, Ti: 0.02 ~ 0.06% included, and more than 1% by mass, Cr: 0.15 ~ 1%, Mo: 0.1 ~ 0.5%, the balance being iron and inevitable A steel plate having a composition comprising impurities, and after heating the steel plate to an austenite single phase region, TiN is scattered in a range from at least the outermost surface of the steel plate to a depth of 100 μm, the longest side of the TiN. A die quench steel sheet excellent in hot punching, characterized in that the average length is 0.1 to 1 μm.
(2) The hot punching according to (1), wherein the steel sheet further contains one or two elements of B: 0.0008 to 0.0030% and W: 0.05 to 1% in mass%. Steel plate for die quenching with excellent properties.
(3) In the cross section in the thickness direction of the steel sheet after the heating, the number of TiNs in the surface layer portion within the range of 5 mm in length from the outermost surface to a depth of 100 μm and parallel to the plate surface is 10 or more. (1) or (2) characterized in that the steel plate for die quenching excellent in hot punching property.
(4) The die quench steel sheet excellent in hot punching according to any one of (1) to (3), wherein the density of TiN in the steel sheet is 30 pieces / mm ² or more.
(5) In a method for producing a member by a die quench method having a step of heating a steel plate to at least an austenite single phase region, then performing hot punching and then quenching by molding with a mold, in mass%, C: 0.05 %: Less than 0.10%, Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, P: 0.03% or less, S: 0.002% or more, less than 0.020%, Al: 0.015 to 0.07%, N: 0.0025 to 0.01% , Ti: 0.02 to 0.06%, further mass%, Cr: 0.15 to 1%, Mo: 0.1 to 0.5% of one or more steel plate having a composition composed of iron and inevitable impurities, When hot stamping, TiN is scattered at least in the range from the steel plate surface to a depth of 100 μm, and the average length of the longest side of TiN is 0.1 to 1 μm. The manufacturing method of the member by the die quench method.
(6) According to the die quench method according to (5), the steel sheet further contains one or two elements of B: 0.0008 to 0.0030% and W: 0.05 to 1% in mass%. Manufacturing method of member.
(7) In the cross section in the thickness direction of the steel sheet after heating, the number of TiNs in the surface layer portion within the range of 5 mm in length from the outermost surface to a depth of 100 μm and parallel to the plate surface is 10 or more. (5) Or the manufacturing method of the member by the die quench method as described in (6) characterized by the above-mentioned.
(8) The method for producing a member by the die quench method according to any one of (5) to (7), wherein the density of TiN in the steel sheet is 30 pieces / mm ² or more.

  According to the present invention, by controlling the size and dispersion state of TiN, in the die quench method, a steel plate excellent in punchability is obtained, in which the height of burrs generated when punching is performed immediately before die quench is low. Thus, a punching member having a good shape can be obtained.

  As a result, drilling can be performed without using expensive laser equipment, and the cost of hot pressing using the die quench method can be reduced.

  Since the high temperature austenite dissolves the precipitate, unless it is a precipitate that does not dissolve in the high temperature austenite, the punching improvement effect in the die quench process cannot be obtained. Therefore, the present inventors have focused on nitrides that are stable even at high temperatures. However, when focusing on nitrides in conventional steel sheets, the aim was to reduce the amount of N by focusing on the formability of the cold press, to reduce TiN, or to use TiN in ferrite. For this reason, the state of TiN that improves punchability with austenite was newly studied. In austenite, there are fewer slip systems that are active during plastic deformation than ferrite, so the size of TiN in the cuboid has a large effect on plastic deformation, and by controlling the size of TiN, hot punching is good. I found out.

  Each component of the present invention will be described below.

  First, chemical components of the steel sheet used in the present invention will be described. In the following description, the content% of component elements means mass%.

C: 0.05% or more, less than 0.10%
C is the most important element for increasing the strength after quenching. If it is less than 0.05%, the tensile strength becomes too low to satisfy the 980 MPa class. Therefore, the lower limit was made 0.05%. Further, when the content is 0.10% or more, the strength after quenching is too high than the target, so the upper limit of the C amount is set to less than 0.10%.

Si: 0.05-0.5%
Si is an element that increases the temper softening resistance, and prevents carbides from precipitating in the martensite immediately after die quenching due to residual heat after leaving the mold, resulting in a decrease in strength. If it is less than 0.05%, the resistance to softening by tempering is low, the martensite is easily tempered, and the resulting coarse carbide makes the martensite brittle. Therefore, the lower limit of Si addition is set to 0.05%. Further, if added over 0.5%, ferrite starts to precipitate during cooling and it becomes difficult to cause firing, so the upper limit was made 0.5%.

Mn: 1.0-1.8%
Mn is an element effective for improving the hardenability of steel. If it is less than 1.0%, the hardenability is poor, pearlite is generated during quenching, and the tensile strength is significantly reduced. Therefore, the lower limit of addition is set to 1.0%. On the other hand, if it exceeds 1.8%, segregation is likely to occur, and the steel sheet is separated into two at the segregated central part, resulting in press defects. Therefore, the upper limit of Mn is set to 1.8%.

P: 0.03% or less
P segregates at the austenite grain boundary and promotes the growth of cracks at the punched end face. Therefore, the upper limit of the addition amount of P is set to 0.03%.

S: 0.002% or more, less than 0.020%
S forms MnS and promotes the generation of voids by punching, and makes the effect of TiN of the present invention remarkable. For this reason, since the burr height is reduced in the punching in which the breakage progresses by connecting the voids, the lower limit of the S addition amount is set to 0.002%. Also, when S is 0.020% or more, the hot ductility deteriorates and cracks occur on the surface around the punched part, the descalability deteriorates, fatigue strength, and impact absorption energy decreases, The upper limit of S was made less than 0.020%. Preferably it is 0.019% or less.

Al: 0.015-0.07%
Al is used as a deoxidizer. Since addition of 0.015% or more is necessary to obtain the deoxidation action, the lower limit of the Al content is set to 0.015%. On the other hand, if the content exceeds 0.07%, a lot of fine AlN precipitates and the hardenability deteriorates. For this reason, the upper limit of the Al content is set to 0.07%.

N: 0.0025 to 0.01%
N forms TiN and has a function of improving punchability by cracking from the surroundings. When the content is less than 0.0025%, the punching property deteriorates due to dissolution or agglomeration in austenite, so the lower limit of the addition amount is set to 0.0025%. On the other hand, if it exceeds 0.01%, cracks on the fracture surface become too large and induce cracks, so the upper limit of the addition amount was set to 0.01%.

Ti: 0.02 to 0.06%
When the Ti content is less than 0.02%, TiN does not grow to a sufficient size, so improvement in punchability cannot be expected. Therefore, the lower limit of the Ti content is set to 0.02%. On the other hand, if it exceeds 0.06%, TiN aggregates and becomes coarse and the density becomes diffuse, so the cracks on the fracture surface become too large and induce cracks, so the upper limit of the addition amount was set to 0.06%.

  In addition to the above, the present invention further contains one or more selected from Cr and Mo.

Cr: 0.15 to 1%
Cr delays ferrite transformation during cooling from high temperatures and improves hardenability. For this reason, it is preferable to add 0.15% or more of Cr. When adding over 1%, Cr carbide precipitated during heating in the die quench method will not dissolve, so the amount of Cr added is set to 1% or less.

Mo: 0.1-0.5%
Mo is preferably added because it is an element that improves hardenability. In order to improve the hardenability by adding Mo, the lower limit of the addition amount is 0.1%. On the other hand, if it exceeds 0.5%, Mo carbide that is difficult to dissolve precipitates and the hardenability decreases, so the upper limit of the amount added is set to 0.5% or less.

  In the present invention, one or two elements of B and W can be added.

B: 0.0008-0.0030%
Since B is an element that can delay the ferrite transformation dramatically and ensure hardenability, it is preferably added according to the step of the die quench method. If it is less than 0.0008%, the hardenability becomes insufficient, so 0.0008% was made the lower limit. On the other hand, if it exceeds 0.0030%, Fe ₂₃ (CB) _{6 which} is difficult to dissolve during heating in the die quench method is likely to precipitate, so the upper limit of the B addition amount is set to 0.0030%.

W: 0.05 to 1%
W can be added because it is an element that improves hardenability. In order to improve the hardenability by adding W, the lower limit of the addition amount is 0.05%. On the other hand, if it exceeds 1%, W carbide that is difficult to dissolve precipitates and the hardenability decreases, so the upper limit of the addition amount is preferably 1%.

  Components other than the above consist of iron and inevitable impurities. Even if Nb and V are mixed up to about 0.3% in total, the effect of the present invention does not change. Further, the elements mixed from scrap raw materials such as Cu, Ni, and Sn do not affect the effects of the present invention as long as the total is up to about 0.2% or less.

The member of this invention is manufactured by the die quench method using the steel plate for die quenches of the said invention. The steel sheet of the present invention is subjected to heat treatment in the die quench method, thereby forming a predetermined TiN distribution, and has an effect on punching such as suppression of burrs as described above.
That is, when the steel sheet of the present invention is heated to the austenite single-phase region, which is a heat treatment in the die quench method, the TiN is scattered as follows. The heating to the austenite single phase region may be performed in the air or in an atmosphere containing nitrogen, for example, at 900 ° C. for about 15 minutes and may be performed in the air.

Average length of the longest side of TiN on the steel sheet surface: 0.1-1 μm
TiN precipitates in a generally rectangular parallelepiped shape. The length of the longest side of the rectangular parallelepiped shape is measured. In addition, when performing observation of a precipitate in a steel plate cross section, it may be observed in a polygonal shape that is not a rectangular parallelepiped. In this case, the length of the longest side of TiN that is observed is defined as the length of the longest side of TiN. When the average length of the longest side of TiN is less than 0.1 μm, voids cannot be expected during punching, and the effect of improving punchability is small. On the other hand, if the average length of the longest side of TiN exceeds 1 μm, the density becomes diffuse, so the crack on the fracture surface becomes too large and induces cracking, so the average length of the longest side of TiN Was 1 μm or less. Further, it is effective to disperse 0.1 to 1 μm of TiN at least in the range from the outermost surface to a depth of 100 μm on the steel sheet surface. TiN may be scattered in a portion other than the above range, and if it is scattered throughout the steel plate, the punchability is further improved.

The number of TiNs in the range of 5 mm in length from the outermost surface to a depth of 100 μm and parallel to the plate surface is preferably 10 or more. In addition to the above-mentioned surface TiN size, the number is preferably large. In the punchability based on the burr height, the origin of voids existing on the steel sheet surface plays an important role. In particular, since the burr height is lowered by the distribution of TiN up to a depth of 100 μm, the number of TiNs on the surface layer in the range of 100 μm × 5 mm is preferably 10 or more. The number is measured in the cross-sectional direction of the steel plate as described below.

TiN density in steel sheet: 30 pieces / mm ² or more TiN in steel sheet serves as a starting point for voids and improves the punchability of the steel sheet itself. However, the punchability is not improved only by a few scattered TiN. In the present invention, when the density of TiN in the steel plate as the raw material is less than 30 pieces / mm ² , the punchability is lowered. Therefore, the density of TiN in the steel plate is preferably 30 pieces / mm ² or more. The number of TiN is measured by observing the cross section of the steel sheet and evaluated by the number per 1 mm ² cross section.

  In this invention, the steel plate which has said composition is processed by the die quench method, and the member which has a hot punching hole is manufactured. In the die quench method, the steel plate is heated to the austenite single-phase region, punched holes are punched out, die quenching (processing and quenching) is performed, and the steel plate during hot stamping after heating in the austenite single-phase region is as described above. Scatter TiN precipitates. In addition, TiN which exists as mentioned above in a heating state exists in the same state also in the member manufactured by the die quench method.

  To bring TiN into the precipitation state as described above, the steel sheet composition is within the above range, the Ti amount and N amount of the steel plate before heat treatment are adjusted, and the heating atmosphere (nitrogen amount) before quenching in the die quench method is set. By adjusting, it can be in a desired state.

  After melting the steel plate of the material shown in Table 1, it was manufactured by hot rolling at a heating temperature of 1200 ° C, a finish rolling temperature of 850 ° C, and a winding temperature of 600 ° C. The obtained hot-rolled sheet was pickled and cold-rolled to produce a material with a sheet thickness of 1.4 mm as cold-rolled.

  These steel Nos. 1 to 23 were heated at 900 ° C. in an air atmosphere for 15 minutes, and then punched hot with a punch having a diameter of 10 mm. The punching temperature was controlled between 750 ° C and 500 ° C. The punching clearance was 12% of the plate thickness. After punching, it was allowed to cool, and the burr height was measured. The height of the burr was set to the height of the highest vertex of the burr based on the surface at a position 2 mm away from the hole.

  Moreover, the cross section of the material after punching was ground, and TiN was observed with an electron microscope (SEM). The length of TiN in the range of 100 μm depth from the surface was observed at 10 magnifications at a magnification of 1000 to 5000, the TiN length (maximum length) was measured, and the average value was obtained. Here, the length of TiN is the length of the longest side of the observed polygonal shape. Further, the number of TiNs on the surface layer was observed by observing a range of depth of 100 μm × length 5 mm from the surface by SEM at a magnification of 5000 times, and the number of TiNs observed was counted. Regarding the density of TiN in the steel plate, the cross section of the steel plate before heating is polished, and a total of 10 fields of view are observed with a SEM at a magnification of 1000 times at 1/4 to 3/4 positions of the plate thickness. The TiN density in the steel sheet was measured.

  Further, the amount of N in the surface layer 100 μm portion of the steel plate after punching was also measured. The measurement results are also shown in Table 1. Here, in measuring the N amount of the steel sheet surface layer, the surface layer of 100 μm was shaved, and the shaved portion was analyzed.

  Steel No. 1-4 is an Example when the amount of S changes. Steel No. S having a S content lower than the range of the present invention. 1 has a high burr height. On the other hand, the S content is within the range of the present invention. In Nos. 2 to 4, the burr height is as low as 40 μm or less, indicating good punchability.

  Steel No. 5-9 is the Example which changed N content in steel. Steel No. In No. 5, the length of TiN is short and fine, the number of surface layers is small, the density is low, and the punchability is inferior. Steel No. No. 9 has a large N content, a long TiN length, and cracks are produced in the punched portion, so that the effect of the present invention is not obtained. Steel No. with chemical composition within the scope of the present invention. Nos. 6 to 8 have a low burr height and exhibit good punchability.

Steel No. Nos. 10 to 12 are those in which the atmosphere (N ₂ amount) was changed by heating before quenching for steel sheets having substantially the same composition, and the N amount after heat treatment was particularly changed. Steel No. No. 10 is Ar 90 volume% + nitrogen 10 volume%. No. 11 is Ar 50% by volume + Nitrogen 50% by volume. 12 is Ar 15% by volume + nitrogen 85% by volume. It can be seen that as the amount of nitrogen in the heating atmosphere increases, the number of TiN in the surface layer increases and the burr height decreases. In any case, the TiN length on the surface layer is within the scope of the present invention and is an example of the present invention.

  Steel No. Examples 13 to 19 are examples when the Ti content is changed. Steel No. with low Ti content In Nos. 13 and 14, the length of TiN is short and fine, the TiN density is low, and the burr height is high. Steel No. No. 19 is long and coarse in TiN, has a high burr height, and has cracks in the punched portion. Steel No. In Nos. 15 to 18, the chemical components were within the scope of the present invention, the length of TiN was within the scope of the present invention, and good punchability was exhibited.

  Steel No. 20-23 is an Example at the time of changing Cr content. In both cases, the chemical components were within the scope of the present invention, and the length of TiN was within the scope of the present invention, indicating good punchability.

  In addition, the die quench method was applied separately and it was confirmed that the tensile strength of the steel plate of the present invention after die quench satisfies the 980 MPa class.

Claims (8)

  In mass%, C: 0.05% or more, less than 0.10%, Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, P: 0.03% or less, S: 0.002% or more, less than 0.020%, Al: 0.015 to 0.07% , N: 0.0025 to 0.010%, Ti: 0.02 to 0.06%, further mass%, Cr: 0.15 to 1%, Mo: 0.1 to 0.5%, with the balance being iron and inevitable impurities A steel plate having a composition, after heating the steel plate to an austenite single phase region, TiN is scattered in a range from at least the outermost surface of the steel plate to a depth of 100 μm, the length of the longest side of the TiN. A die quench steel sheet excellent in hot punching, characterized in that the average is 0.1 to 1 μm.   The steel sheet further comprises one or two elements of B: 0.0008 to 0.0030% and W: 0.05 to 1% in mass%, and is excellent in hot punchability according to claim 1 Steel plate for die quenching.   In the cross section in the thickness direction of the steel sheet after the heating, the number of TiNs in the surface layer part in the range of the length of 5 mm parallel to the plate surface from the outermost surface to a depth of 100 μm is 10 or more. The steel plate for die quenching excellent in hot punching property according to claim 1 or 2. The die quench steel sheet having excellent hot punching properties according to any one of claims 1 to 3, wherein the density of TiN in the steel sheet is 30 pieces / mm ² or more.   In a method for producing a member by a die quench method having a step of quenching by heating a steel plate to at least an austenite single phase region, then performing hot punching, and then forming with a die, C: 0.05% or more, Less than 0.10%, Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, P: 0.03% or less, S: 0.002% or more, less than 0.020%, Al: 0.015 to 0.07%, N: 0.0025 to 0.01%, Ti: Hot stamping using steel sheet containing 0.02 to 0.06%, further mass%, Cr: 0.15 to 1%, Mo: 0.1 to 0.5%, and the balance consisting of iron and inevitable impurities By applying a die quench method characterized in that TiN is scattered at least in the range from the steel plate surface to a depth of 100 μm, and the average length of the longest side of TiN is 0.1 to 1 μm. Manufacturing method of member.   The steel plate further contains one or two elements of B: 0.0008 to 0.0030% and W: 0.05 to 1% in mass%, and manufacturing the member by the die quench method according to claim 5 Method.   In the cross section in the thickness direction of the steel sheet after the heating, the number of TiNs in the surface layer part in the range of the length of 5 mm parallel to the plate surface from the outermost surface to a depth of 100 μm is 10 or more. The manufacturing method of the member by the die-quenching method of Claim 5 or Claim 6 to do. The method for producing a member by die quenching according to any one of claims 5 to 7, wherein the density of TiN in the steel sheet is 30 pieces / mm ² or more.