JP7436823B2 - Steel plate for hot stamped parts and its manufacturing method - Google Patents

Description

The present invention relates to a steel plate for hot stamped parts and a method for manufacturing the same.

In recent years, efforts have been made to reduce the weight of automobiles by increasing the strength of the steel materials used in order to improve the fuel efficiency of automobiles. As a result, in the production of parts (hereinafter referred to as "press-formed parts") that are manufactured by cold-press forming thin steel sheets that are widely used in automobiles, it is difficult to produce press-formed parts with complex shapes. This has become difficult due to the decrease in press formability associated with the increase in strength of steel sheets.

Specifically, due to the decrease in ductility of the steel plate, there are problems such as fractures in press-formed parts at highly processed parts, and increased springback and wall warping, which reduces the dimensional accuracy of press-formed parts. is occurring frequently. In particular, it is not easy to manufacture press-formed parts made of high-strength steel plates having a tensile strength of 780 MPa or more.

By roll forming rather than cold press forming, roll formed parts made of high strength steel sheets can be easily manufactured. However, roll forming can only produce roll-formed parts having a constant cross-section in the longitudinal direction, and cannot produce press-formed parts having a complicated cross-sectional shape.

On the other hand, in the hot stamping method (also called hot press forming method) in which a heated steel plate is press-formed, the high-temperature steel plate at the time of forming is soft and highly ductile, so press-formed parts with complex shapes are used. can be molded with high dimensional accuracy without causing molding defects such as breakage, springback, or wall warping.

Furthermore, according to the hot stamping method, the steel plate is heated to a temperature in the austenite single phase range, then press-formed, and the molded product is rapidly cooled and quenched inside the mold used for press-forming. At the same time as forming, it is also possible to increase the strength of press-formed parts through martensitic transformation. As described above, the hot stamping method is an excellent technique suitable for manufacturing high-strength press-formed parts. Note that in the following description, press-molded parts manufactured by the hot stamping method will be referred to as "hot stamped parts."

BACKGROUND ART Bumper reinforcements, which have a relatively simple shape, are currently known as an example of hot-stamped parts. Hot-stamped parts with a tensile strength of 1.5 GPa class are already widely used for structural members (skeleton members) of body shells such as bumper reinforcements and B-pillar reinforcements.

In recent years, consideration has been given to manufacturing hot-stamped parts with higher strength, especially tensile strength of 1.8 GPa or higher, such as B-pillar reinforcements, which will mainly bear the impact load in the event of a collision. The use of hot-stamped parts with a tensile strength of 1.8 GPa or more is also being considered for the structural members (skeletal members) of the body shell.

However, when the tensile strength of hot-stamped parts reaches an ultra-high tensile strength of 1.8 GPa or more, the deformability of the hot-stamped parts becomes insufficient and the fracture resistance properties (e.g., bendability) of the hot-stamped parts deteriorate, resulting in There is an increased risk that the absorbed energy of the hot-stamped part will decrease or that the hot-stamped part itself will break. For this reason, the only hot-stamped parts with a tensile strength of 1.8 GPa or higher that have been put to practical use are 1.8 GPa-class bumper reinforcements. No examples of producing hot-stamped parts have been reported so far.

Therefore, in order to manufacture a hot-stamped part with a tensile strength of 1.8 GPa or more, it is necessary to further temper the hot-stamped part to increase its deformability. However, adding a tempering process to the hot stamping process significantly increases the manufacturing cost of hot stamped parts due to a decrease in work efficiency and an increase in equipment costs.

Patent Document 1 states that C: 0.25 to 0.45% (in this specification, "%" regarding chemical composition or concentration means "mass%" unless otherwise specified), Mn + Cr: 0.5 to 0.45%. 3.0% and further contains one or more of the following: Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less. Press molding is performed after holding in a temperature range of ₃ points or more (Ac ₃ points + 100 ° C.) or less for 5 minutes or less, and then the cooling rate to the Ms point is equal to or higher than the upper critical cooling rate, and the average from the Ms point to 150 ° C. By cooling at a cooling rate of 10 to 500°C/sec, the steel structure is made up of automatically tempered martensite with an average prior austenite grain size of 10 μm or less, and the tensile strength is excellent in toughness even as hardened. A hot-stamped part having a pressure of 1.8 GPa or more and a steel plate for the hot-stamped part are disclosed.

Patent Documents 2 and 3 include C: 0.26 to 0.45%, Mn+Cr: 0.5 to 3.0%, Nb: 0.02 to 1.0%, 3.42N+0.001≦Ti≦3. Ti in an amount satisfying .42N + 0.5, and one or more of the following: Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less. Press forming is performed after holding the steel plate having the chemical composition in a temperature range of Ac ₃ points or more (Ac ₃ points + 100 ° C.) or less for 5 minutes or less, and then the cooling rate to the Ms point is at least the upper critical cooling rate, By cooling at an average cooling rate of 10 to 500°C/sec from the Ms point to 150°C, it has a microstructure containing automatically tempered martensite with a prior austenite grain size of 10 μm or less, and has high toughness even as quenched. A hot stamped part having an excellent tensile strength of 1.8 GPa or more is disclosed.

Patent Document 4 describes C: 0.25 to 0.40%, Si: 0.05% or more and less than 0.5%, Mn: 1.0 to 1.7%, P: 0.020% or less, S : less than 0.0010%, Al: 0.002 to 0.06%, N: 0.006% or less, Cr: 0.02 to 0.6%, B: 0.00010 to 0.0040%, Ti: It has a chemical composition of 0.005 to 0.04%, Nb: 0.03 to 0.12%, balance: Fe and inevitable impurities, and the number density of inclusions and precipitates with a maximum length of 10 μm or more is 100. / ^mm2 or less and has a tensile strength of 1.8 to 2.5 GPa, which is obtained by hot stamping a steel sheet having a metal structure in which the prior austenite grain size after hot stamping is 10 μm or less. A hot-stamped steel plate member is disclosed that has excellent toughness.

Japanese Patent Application Publication No. 2006-152427 International Publication No. 2007/129676 Japanese Patent Application Publication No. 2012-180594 JP2017-43825A

The inventions disclosed in Patent Documents 1 to 4 certainly provide hot-stamped parts having a tensile strength of 1.8 GPa or more in the as-quenched state.

However, as mentioned above, in order to use hot-stamped parts with a tensile strength of 1.8 GPa or more for the structural members (skeleton members) of the body shell, the hot-stamped parts must have a high strength (tensile strength of 1.8 GPa or more). In addition to the above), it is necessary to further improve fracture resistance (for example, bendability) by further improving deformability. From this point of view, there is still room for improvement in the deformability and fracture resistance of the hot stamped parts disclosed in Patent Documents 1 to 4.

In particular, in hot-stamped parts having a tensile strength of 1.8 GPa or more, the plate thickness and weight can be reduced by further improving the fracture resistance by further improving the deformability.

Furthermore, when using hot-stamped parts, it is undesirable for the impact properties to differ depending on the orientation of the hot-stamped part.

The present invention has been made in view of the problems faced by the conventional technology, and has excellent deformability, fracture resistance at the time of collision, and impact properties, and anisotropy of the impact properties without performing tempering after quenching. It is an object of the present invention to provide a technology for manufacturing hot-stamped parts having a small tensile strength and a tensile strength of, for example, 1.8 GPa or more.

Generally, as the tensile strength of a hot stamped part increases, it becomes difficult to ensure bendability of the hot stamped part. The present inventors have conducted extensive research focusing on improving the fracture resistance of hot-stamped parts during collisions, and have obtained the findings A to E listed below, and have completed the present invention.

(A) When a steel plate for hot-stamped parts was subjected to three-point bending, stretched MnS was present in the cracked portion, and many dimples caused by this MnS were observed on the fracture surface. That is, the stretched MnS becomes the starting point of fracture. For this reason, it is necessary to reduce the amount of drawn MnS in steel sheets for hot-stamped parts.

(B) When hot-stamped parts were checked, it was found that although the hot-stamping method sufficiently heats the steel plate for hot-stamped parts to the austenite single-phase region, there are undissolved fine particles that cause low strength and deformability. A large amount of cementite was present. Therefore, it is necessary to promote dissolution of cementite.

(C) When Nb is added to avoid a decrease in deformability, Nb-based carbides are generated and become the starting point for void generation. In order to suppress the generation of voids, it is necessary to optimize the Nb content.

(D) In other words, in the manufacturing process of hot-stamped steel sheets and hot-stamped parts, impurities such as inclusions and carbides in the steel, which can become a starting point for the destruction of hot-stamped parts in the event of a collision, are reduced as much as possible. The fracture resistance of the stamped part can be greatly improved, thereby making it possible to provide a hot-stamped part that can also be used as a structural member (skeleton member) of a body shell, for example.

(E) On the other hand, inclusions and carbides in steel cannot be completely eliminated. Conversely, if this is used, the strength of hot-stamped parts can be increased.

The present invention is as listed below.

(1) Chemical composition: C: 0.18-0.50%, Si: 0.001-2.00%, Mn: 0.001-3.00%, P: 0.10% or less, S: 0.0015% or less, Nb: 0.010 to 0.100%, Cr: 0.001 to 0.50%, Al: 0.0001 to 0.100%, Ti: 0.001 to 0.500%, O: less than 0.0030%, B: 0.0006 to 0.0030%, N: 0.0100% or less, and the remainder: Fe and impurities, and the following formulas (i), (ii), and (iii) satisfies the formula,
The metal structure is mainly composed of ferrite and pearlite, the ferrite grain size is 30.0 μm or less, the pearlite area ratio is 15% or more, the total area ratio of ferrite and pearlite is 95% or more, and the remainder is The structure contains one or more of bainite, retained austenite, and cementite, and the area ratio of MnS having a length of 100 μm or more at a depth of 1/4 to 3/4 of the plate thickness is 0.0100% or less, A steel plate for hot-stamped parts, in which the number ratio of carbides with a grain size of 1.2 μm or more (cementite and NbTi composite carbonitride) among the carbides with a grain size of 0.5 μm or more existing in the steel is 10.0% or less .
3.0℃/sec ≦Vc90≦30℃/sec ・・・・・・・(i)
log(Vc90)=2.94-0.75A (ii)
A=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo...(iii)
However, in formula (iii), the element symbol indicates the content (mass%) of each element.

(2) The hot stamped component according to item 1, wherein the chemical composition has one or more selected from V: 2.00% or less, Ta: 0.50% or less, and W: 3.00% or less. Steel plate for use.

(3) The hot spring according to item 1 or 2, wherein the chemical composition has one or more selected from Ni: 5.00% or less, Cu: 3.00% or less, and Mo: 0.50% or less. Steel plate for stamp parts.

(4) the chemical composition has one or more selected from Mg: 0.0030% or less, Ca: 0.0030% or less, La: 0.030% or less, and Ce: 0.030% or less; The steel plate for hot stamped parts according to any one of items 1 to 3.

(5) A method for manufacturing a steel plate for hot stamped parts according to any one of items 1 to 4, comprising:
After heating the steel ingot or billet having the above chemical composition at T (°C) shown by formula (iv) for 30 minutes or more, rough rolling is completed at 1000°C to 1150°C, and the Ae is ₃ or more points. a rolling process to complete finish rolling;
a cooling step of cooling at an average cooling rate of 10 to 200°C/sec;
A winding process of winding at 450 to 600°C,
A cold rolling process performed at a reduction rate of 10 to 60%,
A method for producing a steel plate for hot-stamped parts, comprising an annealing step in a temperature range of 750° C. or higher, and cooling at a cooling rate of 3 to 40° C./sec at 750 to 550° C. after annealing.
T (℃) = [0.020 + (mass% Mn) (mass% S)] / (1.9 × 10 ^-5 )... (iv)
However, in formula (iv), mass %Mn and mass %S represent the contents (mass %) of Mn and S, respectively.

According to the present invention, hot-stamped parts with ultra-high strength (especially tensile strength of 1.8 GPa or more) and significantly improved fracture resistance can be produced without tempering, after hot-stamping and quenching. It becomes possible to manufacture. This makes it possible to use hot-stamped parts with a tensile strength of 1.8 GPa or more for structural members (skeleton members) of body shells, for example, and to reduce the manufacturing cost of hot-stamped parts with a tensile strength of 1.8 GPa or more. can prevent a rise in

The present invention will be explained.

1. Steel plate for hot stamped parts according to the present invention (1) Chemical composition First, essential elements will be explained.

(1-1) C: 0.18-0.50%
C is a very important element that enhances the hardenability of the steel plate for hot stamped parts and mainly determines the tensile strength of the hot stamped parts after hardening. In particular, in order to ensure a tensile strength of 1.8 GPa or more in the hot stamped part after quenching, the C content is 0.18% or more, preferably 0.20% or more, and more preferably 0.24% or more. . On the other hand, if the C content exceeds 0.50%, the tensile strength of the hot-stamped part after quenching becomes too high, resulting in significant deterioration of toughness. Therefore, the C content is 0.50% or less, preferably 0.40% or less, and more preferably 0.38% or less.

(1-2) Si: 0.001-2.00%
Si is effective in improving the hardenability of the steel plate for hot-stamped parts and stably achieving high strength of the hot-stamped parts after hardening. In order to obtain this effect, the Si content is 0.001% or more, preferably 0.050% or more, and more preferably 0.100% or more. On the other hand, even if the Si content exceeds 2.00%, the above effects are saturated, leading to an increase in cost. Therefore, the Si content is 2.00% or less, preferably 1.50% or less, and more preferably 1.0% or less.

(1-3) Mn: 0.001-3.00%
Mn is an element that is very effective in improving the hardenability of steel sheets for hot-stamped parts and stably obtaining high strength of hot-stamped parts after hardening. If the Mn content is less than 0.001%, this effect cannot be sufficiently obtained. Therefore, the Mn content is 0.001% or more, preferably 0.50% or more, and more preferably 1.00% or more. On the other hand, even if the Mn content exceeds 3.00%, the above effects are saturated, and on the contrary, it becomes difficult to stably secure tensile strength. Therefore, the Mn content is 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

(1-4) P: 0.10% or less Since P greatly deteriorates the toughness of hot stamped parts after quenching, the smaller the P content, the better, but a content of 0.10% is permissible. Therefore, the P content is 0.10% or less. However, reducing the P content to less than 0.001% inevitably increases steel manufacturing costs. For this reason, the P content is preferably 0.001% or more.

(1-5) S: 0.0015% or less The S content is set to 0.0015% or less because the deformability and fracture resistance improve as the S content decreases. Preferably it is less than 0.001%. However, since reducing the S content requires S removal costs, the S content is preferably 0.0001% or more. The S content is more preferably 0.0003% or more.

(1-6) Nb: 0.010-0.100%
Nb suppresses recrystallization and forms fine carbides to make austenite grains finer when steel sheets for hot-stamped parts are heated to Ac ₃ points or higher, which greatly improves the toughness of hot-stamped parts. It has the effect of In order to obtain this effect, the Nb content is 0.010% or more, preferably 0.020% or more, and more preferably 0.040% or more. On the other hand, as will be described later, when the Nb content exceeds 0.100%, Nb carbonitrides in the steel ingot or slab become coarse. Therefore, the Nb content is 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less.

(1-7) Cr: 0.001-0.50%
Cr is an element that is very effective in improving the hardenability of steel sheets for hot-stamped parts and stably obtaining high strength of hot-stamped parts after hardening. In order to obtain this effect, the Cr content is 0.001% or more, preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, even if the Cr content exceeds 0.50%, the above effects are saturated, and on the contrary, it becomes difficult to stably secure tensile strength. Therefore, the Cr content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

(1-8) Al: 0.0001-0.100%
Al is an element that is effective in improving the hardenability of the steel plate for hot-stamped parts and stably ensuring the tensile strength of the hot-stamped parts after hardening. In order to obtain this effect, the Al content is 0.0001% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, even if the Al content exceeds 0.100%, the above effects will be saturated and the cost will only increase. Therefore, the Al content is 0.100% or less, preferably 0.060% or less, and more preferably 0.040% or less.

(1-9) Ti: 0.001-0.500%
Ti has the effect of greatly improving the toughness of hot-stamped parts by suppressing recrystallization and forming fine carbides to make austenite grains finer when the steel plate for hot-stamped parts is heated to Ac ₃ points or higher. play. In order to reliably obtain this effect, the Ti content is 0.001% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, even if the Ti content exceeds 0.500%, the above effects will be saturated and the cost will only increase. Therefore, the Ti content is 0.500% or less, preferably 0.100% or less, and more preferably 0.050% or less.

(1-10) O: less than 0.0030% O exists in steel as an impurity. If O exists, it forms oxides and deteriorates the fracture resistance, so it is preferable that the O content be small, but a content of about 0.0030% is permissible. Therefore, the O content is less than 0.0030%.

(1-11) B: 0.0006-0.0030%
B is effective in increasing the hardenability of the steel plate for hot-stamped parts and increasing the effect of stably securing the tensile strength of the hot-stamped parts after quenching. Further, B is an important element in that it segregates at grain boundaries, increases grain boundary strength, and improves the toughness of hot-stamped parts. Furthermore, B has a high effect of suppressing the growth of austenite grains during heating of a steel plate for hot-stamped parts. In order to obtain this effect, the B content is 0.0006% or more, preferably 0.001% or more, and more preferably 0.0015% or more. On the other hand, even if the B content exceeds 0.0030%, the above effects will be saturated and the cost will only increase. Therefore, the B content is 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0020% or less.

(1-12) N: 0.0100% or less N exists in steel as an impurity. If N exists, it combines with B to produce BN and reduces the effect of B, so it is preferable that the N content be small, but a content of about 0.010% is permissible. Therefore, the N content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less.

Next, arbitrary elements will be explained.

(1-13) V: 2.00% or less V is an element that is effective in increasing the hardenability of steel plates for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after hardening. . However, even if the V content exceeds 2.00%, the above effects are saturated and the cost only increases. Therefore, the V content is 2.00% or less, preferably 1.50% or less, and more preferably 1.00% or less. In order to reliably obtain the above effects, the V content is 0.001% or more, preferably 0.10% or more, and more preferably 0.20% or more.

(1-14) Ta: 0.50% or less Ta is an element that is effective in increasing the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after hardening. . However, if the Ta content exceeds 0.50%, the above effects will be saturated and the cost will only increase. Therefore, the Ta content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less. In order to reliably obtain the above effects, the Ta content is 0.001% or more, preferably 0.005% or more, and more preferably 0.1% or more.

(1-15) W: 3.00% or less W is an element that is effective in increasing the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after hardening. . However, when the W content exceeds 3.00%, the above effects are saturated and the cost only increases. Therefore, the W content is 3.00% or less, preferably 2.00% or less, and more preferably 1.00% or less. In order to reliably obtain the above effects, the W content is 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more.

(1-16) Ni: 5.00% or less Ni is an element that is effective in increasing the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after hardening. . However, when the Ni content exceeds 5.00%, the above effects are saturated and the cost only increases. Therefore, the Ni content is 5.00% or less, preferably 3.00% or less, and more preferably 2.00% or less. In order to reliably obtain the above effects, the Ni content is 0.01% or more, preferably 0.10% or more, and more preferably 0.20% or more.

(1-17) Cu: 3.00% or less Cu is an element that is effective in improving the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after hardening. . However, even if the Cu content exceeds 3.00%, the above effects are saturated and the cost only increases. Therefore, the Cu content is 3.00% or less, preferably 2.00% or less, and more preferably 1.00% or less. In order to reliably obtain the above effects, the Cu content is 0.01% or more, preferably 0.20% or more, and more preferably 0.5% or more.

(1-18) Mo: 0.50% or less Mo is an element that is effective in increasing the hardenability of steel sheets for hot stamped parts and stably ensuring the tensile strength of hot stamped parts after hardening. . However, even if the Mo content exceeds 0.50%, the above effects will be saturated and the cost will only increase. Therefore, the Mo content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less. In order to reliably obtain the above effects, the Mo content is 0.005% or more, preferably 0.10% or more, and more preferably 0.20% or more.

(1-19) Mg: 0.0030% or less Mg has the effect of making inclusions in steel finer and improving the toughness of hot-stamped parts after quenching. However, when the Mg content exceeds 0.0030%, this effect is saturated and the cost increases. Therefore, the Mg content is 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0020% or less. In order to reliably obtain the above effects, the Mg content is 0.0005% or more, preferably 0.0010% or more, and more preferably 0.0015% or more.

(1-20) Ca: 0.0030% or less Ca has the effect of making inclusions in steel finer and improving the toughness of hot stamped parts after quenching. However, when the Ca content exceeds 0.0030%, this effect is saturated and the cost increases. Therefore, the Ca content is 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0020% or less. In order to reliably obtain the above effects, the Ca content is 0.0005% or more, preferably 0.0010% or more, and more preferably 0.0015% or more.

(1-21) La: 0.030% or less La has the effect of making inclusions in the steel finer and improving the toughness of hot-stamped parts after quenching. However, when the La content exceeds 0.030%, this effect is saturated and the cost increases. Therefore, the La content is 0.030% or less, preferably 0.020% or less, and more preferably 0.010% or less. In order to reliably obtain the above effects, the La content is 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more.

(1-22) Ce: 0.030% or less Ce has the effect of making inclusions in steel finer and improving the toughness of hot-stamped parts after quenching. However, when the Ce content exceeds 0.030%, this effect is saturated and the cost increases. Therefore, the Ce content is 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less. In order to reliably obtain the above effects, the Ce content is 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more.

(1-23) The following formulas (i), (ii) and (iii) are satisfied.
3.0℃/sec ≦Vc90≦30℃/sec ・・・・・・・(i)
log(Vc90)=2.94-0.75A (ii)
A=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo...(iii)
However, in formula (iii), the element symbol indicates the content (mass%) of each element.
Vc90 is the critical cooling rate at which a 90% martensitic structure is obtained, and is a parameter serving as an index of hardenability. When Vc90 is less than 3.0° C./sec, although the hardenability is excellent, cracks are likely to occur when the coil is unwound and during cold rolling, and the toughness decreases. On the other hand, when Vc90 exceeds 30° C./sec, variations in hardness at the corners and corners of the hot-stamped part become large, making it impossible to ensure collision characteristics. Therefore, Vc90 is set to 3.0 to 30°C/sec.

The remainder other than the above is Fe and impurities. Examples of impurities include those contained in raw materials such as ores and scraps, and those contained in manufacturing processes.

(2) Metal structure (2-1) Mixed structure of ferrite and pearlite The steel sheet according to the present invention is characterized by its metal The structure is mainly composed of ferrite and pearlite.

(2-2) Ferrite grain size: 30.0 μm or less When the ferrite grains become coarse, the distance between the pearlite structures that exist at the same time becomes larger, which causes variations in the distribution of carbon in the austenite phase during heating during hot stamping. This will cause the hardness to vary after quenching. Variations in hardness result in areas with higher hardness than necessary, which adversely affects impact resistance and toughness. Therefore, the ferrite grain size is preferably 30.0 μm or less in order to uniformly disperse the pearlite structure as the structure of the steel plate for hot-stamped parts so as not to cause variations in hardness during hot-stamping heating. is 25.0 μm or less.

(2-3) Pearlite area ratio: 15% or more Pearlite is a structure in which ferrite and cementite are arranged in alternating layers. Therefore, it is easily dissolved by rapid heating. If the temperature of the steel plate for hot-stamped parts can be raised in a short period of time, it will be easier to harden the steel plate, and the high strength of the hot-stamped parts can be ensured. In a steel plate for hot stamped parts that is used after rapid heating, the area ratio of pearlite is 15% or more, preferably 20% or more, and more preferably 25% or more.

(2-4) Total area ratio of ferrite and pearlite: 95% or more May contain inevitably generated structures such as bainite, retained austenite, and cementite. If the total area ratio of ferrite and pearlite is 95% or more, there is no problem in terms of characteristics when it is used as a steel plate for hot-stamped parts that is rapidly heated and used. Note that the residual structure is, for example, bainite, retained austenite, and cementite. If the total area ratio of the remaining tissue is 5% or less, there is no problem in terms of characteristics.

(2-5) Area ratio of MnS having a length of 100 μm or more at a depth of 1/4 to 3/4 of the plate thickness: 0.0100% or less There is a lot of segregation in the center of the slab manufactured by continuous casting. Become. Segregation is carried over even when a slab is rolled into a thin plate, with more segregation occurring in the center of the thin plate. Therefore, in the present invention, attention is paid to the position at the center of the plate thickness (1/4 to 3/4t).

It is known that MnS becomes a starting point for cracks. In the present invention, the generation of MnS is suppressed by reducing the S content in steel. However, if the anisotropy of the steel sheet for hot-stamped parts is large in some directions, not only will it be difficult for hot-stamped manufacturers to operate the product, but there is also a risk that the properties of the hot-stamped parts will be defective.

The present inventors investigated the cause of this anisotropy in detail and found that MnS is the cause of worsening the anisotropy of toughness. MnS is an inclusion that is easily stretched. When the length of MnS is 100 μm or more, the anisotropy associated with stretching of MnS increases. However, even if MnS of 100 μm or more is present in the steel, no particular anisotropy problem occurs if the area ratio is 0.0100% or less.

Therefore, in the present invention, the area ratio of MnS having a length of 100 μm or more is defined as 0.0100% or less. Note that the lower limit of the area ratio of MnS is not particularly defined, but considering the contents of Mn and S present in steel, MnS of 0.0001% or more exists, including MnS of 100 μm or less.

(2-6) Among the carbides with a grain size of 0.5 μm or more existing in steel, the number ratio of carbides with a grain size of 1.2 μm or more (cementite and NbTi composite carbonitride): 10.0% or less , Nb and Ti as essential elements, a composite carbonitride of Nb and Ti, NbTi(C,N), is produced. The composite carbonitride NbTi(C,N) at the slab stage is coarse, and if it remains in the steel plate after rolling, it causes a decrease in the toughness of the steel plate for hot-stamped parts or the hot-stamped parts.

Furthermore, if cementite (Fe ₃ C) remains undissolved in the steel plate for hot stamped parts, it will dissolve into austenite during heating of the steel plate for hot stamped parts, increasing the C concentration in the austenite. If the heating in the hot stamping process is insufficient, for example, if heating is done rapidly and for a short time to increase production efficiency, the cementite will not be sufficiently dissolved, leading to a decrease in the toughness of the hot stamped part. .

The presence of the composite carbonitride NbTi(C,N) and cementite (hereinafter collectively referred to as "carbide") causes a decrease in toughness. On the other hand, the fine composite carbonitride NbTi(C,N) has the effect of increasing the strength of the steel plate through precipitation strengthening due to the pinning effect. For this reason, it is necessary to allow a certain amount of the fine composite carbonitride NbTi(C,N) to exist.

The present inventors conducted a detailed investigation on the relationship between carbides and toughness, and found that regardless of the type of carbide, the presence of a large amount of some coarse carbides reduces the toughness of the steel sheet for hot-stamped parts or hot-stamped parts. I found out. In other words, when the number ratio of carbides with a grain size of 1.2 μm or more among the carbides with a grain size of 0.5 μm or more existing in the steel is 10.0% or less, the toughness of the steel plate for hot stamped parts or the hot stamped part can be secured.

Further, the composite carbonitride NbTi(C,N) having a particle size of less than 1.2 μm contributes to improving the strength of the steel sheet. On the other hand, cementite with a particle size of less than 1.2 μm can be easily dissolved in the hot stamping process and does not reduce the toughness of the hot stamped part. In other words, instead of simply reducing the carbides, if the number ratio of relatively fine carbides with a grain size of 0.5 μm or more and less than 1.2 μm existing in the steel sheet exceeds 90.0%, hot stamping parts It can achieve both strength and toughness.

Note that the reason why the number ratio of carbides with a grain size of 1.2 μm or more is defined by excluding fine carbides with a grain size of less than 0.5 μm is that fine carbides may not be observed by microstructural observation.

Note that the area ratio of ferrite and pearlite, the ferrite grain size, the area ratio of MnS, and the number ratio of carbides can be measured, for example, by the method described in Examples.

(3) Applications The steel plate according to the present invention is used for hot stamped parts. Target hot-stamped parts include bumper reinforcements and structural members of automobile body shells (e.g. A-pillar reinforcements, B-pillar reinforcements, front side members, rear side members, roof rails, various cross members, etc.) is exemplified.

2. Method for manufacturing a steel plate for hot-stamped parts according to the present invention Next, a method for manufacturing a steel plate for hot-stamped parts according to the present invention will be described.

(2-1) Rolling process A steel ingot or billet having the above-mentioned chemical composition is rolled using the formula (iv): T (°C) = [0.020 + (mass% Mn) (mass% S)] / (1.9 ×10 ^-5 ) (mass %Mn and mass % S indicate the content of Mn and S, respectively.) After heating for 30 minutes or more at a temperature T (°C) or higher, rough rolling is completed at 1000 to 1150 °C. do. The component system of the chemical composition described above is based on low S content, and MnS is reduced from a component standpoint. In addition, the temperature during heating is optimized to promote solutionization of MnS.

Limiting the heating temperature of the steel ingot or billet, specifically, promoting solution treatment by heating the steel ingot or billet to a temperature equal to or higher than the temperature T (°C) shown by equation (iv), preferably heating at 1200°C or higher for 1 hour or more. By doing so, the influence of MnS present in the steel ingot or slab can be reduced.

Furthermore, in order to prevent coarse carbides remaining in the steel plate for hot-stamped parts and to dissolve coarse cementite occurring in the slab, the temperature of cementite is set to be equal to or higher than the temperature T (° C.) shown by equation (iv).

Furthermore, the Nb-based carbonitride Nb(C,N) promotes solutionization of coarse Nb-based carbonitrides present in the steel ingot or slab. From the viewpoint of preventing coarsening of Nb carbonitrides in steel ingots or steel slabs, the upper limit of Nb is set to 0.100%, and the heating temperature of steel ingots or steel slabs is set to be equal to or higher than the temperature T (°C) shown by equation (iv). , preferably 1200° C. or higher, thereby reducing the influence of Nb.

After heating the steel ingot or billet as described above, rough rolling is started and completed at 1000 to 1150°C. If the finishing temperature of rough rolling is less than 1000°C, recrystallization will not proceed and anisotropy due to rolling will become strong. On the other hand, when the finishing temperature of rough rolling exceeds 1150° C., although recrystallization progresses, the coarsening of crystal grains progresses, leading to coarsening of crystal grains in finish rolling.

After this, finish rolling is completed at Ae of ₃ points or more. This is because if finish rolling is performed at a temperature lower than Ae ₃ , processed ferrite remains and the ductility deteriorates significantly. In the chemical composition system defined in the present invention, these problems do not occur if the completion temperature of finish rolling is Ae ₃ points or higher. On the other hand, if the finish rolling completion temperature exceeds 1050° C., surface defects such as scale encroachment may occur. Therefore, the completion temperature of finish rolling is preferably 1050°C or lower.

In addition, Ae ₃ points are calculated according to the following formula.
Ae ₃ (℃)=937-477C+56Si-20Mn-16Cu-15Ni-5Cr+38Mo+136Ti-19Nb+198Al+3315B
However, the element symbol in the above formula represents the content (mass %) of each element contained in the steel sheet, and if it is not contained, 0 shall be substituted.

(2-2) Cooling process After finish rolling, cooling is performed at an average cooling rate of 10 to 200°C/sec. Thereby, precipitation and growth of the above-mentioned precipitates after hot rolling can be suppressed. The lower limit of the average cooling rate is preferably 20°C/second, more preferably 30°C/second. On the other hand, the upper limit of the average cooling rate is preferably 150°C/sec, more preferably 100°C/sec.

(2-3) Winding process After hot rolling, it is wound into a coil at 600 to 450°C. By setting the winding temperature to 600° C. or lower, a sufficient amount of pearlite structure can be obtained, and the coarsening of the precipitates can be prevented. On the other hand, if the coiling temperature is less than 450°C, the structure will be mainly bainite, and the rolling load in cold rolling performed subsequent to hot rolling will be high, resulting in poor cold rollability. Therefore, the winding temperature is 450°C or higher, preferably 500°C or higher.

(2-4) Pickling process If necessary, the coil (steel strip) wound into a coil after hot rolling is once unwound and pickled to remove scale generated on the surface (descaling). )I do.

(2-5) Cold rolling process After coiling or pickling, cold rolling is performed at a rolling reduction ratio of 10 to 60%. Cold rolling may be performed under well-known and commonly used conditions. If the rolling reduction ratio is less than 10%, the structure after annealing tends to become mixed grains, resulting in variations in hardenability and variations in hardness. On the other hand, if the rolling reduction exceeds 60%, the load during cold rolling becomes large and it becomes difficult to carry out rolling at an industrial level in terms of cost.

(2-6) Annealing process After cold rolling, annealing is performed in a temperature range of 750°C or higher in order to utilize the austenite phase to promote cementite redissolution and to prevent Cr and Mn concentration in cementite. . By performing annealing in this temperature range, Cr and Mn concentration in cementite can be appropriately prevented. Further, in order to make the metal structure after cooling to be a mixed structure mainly composed of ferrite and pearlite, cooling is performed at a cooling rate of 3 to 40° C./sec at 750 to 550° C. after annealing.

The cold rolled steel strip may be annealed by continuous annealing performed in an uncoiled state, or by box annealing performed by winding the steel strip into a coil.

In this way, the steel plate for hot stamped parts according to the present invention is manufactured.

The present invention will be specifically described with reference to Examples.

After heating the slab having the chemical composition shown in Table 1 (the remainder is Fe and impurities other than those shown in Table 1) at the slab heating temperature and slab heating time shown in Table 2, rough rolling is started. Rough rolling was completed at the rolling end temperature, and finish rolling was completed at the finish rolling end temperature shown in Table 2 to give a plate thickness of 1.12 to 2.56 mm, and after cooling while controlling the cooling temperature, Table 2 It was wound into a coil at the winding temperature shown in . The wound coil was unwound, pickled to remove scale, cold rolled, annealed at the annealing temperature shown in Table 2, and cooled after annealing while controlling the cooling rate. Underlining in Tables 1 and 2 indicates that the results are outside the scope of the present invention.

The produced coil was subjected to hot stamping, and the structure of the hot stamped part after the hot stamping was observed, and it was evaluated whether it had the characteristics as a steel plate for hot stamped parts. Specifically, No. Steel plates for hot-stamped parts having sizes 1 to 21 and x1 to x13 were produced, and the structures shown in (1) to (3) below were observed.

(1) Metal structure After mirror-polishing the cross section of sample L, nital etching was performed, and the structure was observed using an optical microscope at a plate thickness of 1/4. The ferrite grain size, pearlite area ratio, and total area ratio of ferrite and pearlite were determined by image analysis from an optical micrograph at a magnification of 500 times.

(2) Area ratio of MnS having a length of 100 μm or more at a depth of 1/4 to 3/4 of the plate thickness A sample piece was cut out from a part of the steel plate, the cross section of sample L was mirror polished, and an optical micrograph was taken. The size of MnS was determined and its area ratio was calculated by image analysis from . Here, the measurement area is a 20 mm wide area (0.8 mm x 20 mm = 16 mm ² ) in the 1/4 to 3/4 t area of the plate thickness, that is, the 0.8 mm area at the center of the 1.6 mm thick plate. did.

(3) Number ratio of carbides with a grain size of 1.2 μm or more (cementite, NbTi composite carbonitride) among the carbides with a grain size of 0.5 μm or more existing in steel. The L cross section was mirror polished, and the number ratio of carbides (cementite and NbTi composite carbonitride) was measured using a scanning electron microscope. Here, in the case of a 1/4 to 3/4t area of the plate thickness, that is, a plate thickness of 1.6 mmt, measure a 5 mm wide area (0.8 mm x 5 mm = 4 mm ² ) in a 0.8 mm area at the center. The area is analyzed using EDX to confirm whether it is a carbide, and for carbides with a particle size of 0.5 μm or more, distinguish between carbides with a particle size of 1.2 μm or more and those with a particle size of less than 1.2 μm, and count the number of carbides.1. The number of carbides with a particle size of 0.5 μm or more was divided by the number of carbides with a particle size of 0.5 μm or more to calculate the number ratio of carbides with a particle size of 1.2 μm or more.
On the other hand, the properties of the steel sheets were evaluated using the methods shown in (4) to (6) below.

(4) Strength (TS, YS), elongation EL
A No. 5 test piece specified in JIS Z 2201 was taken from a steel plate and subjected to a tensile test in accordance with JIS Z 2241 to evaluate strength (TS, YS) and elongation EL.

(5) Limit bending R
Regarding bendability evaluation, a 30 mm x 100 mm test piece was taken from a steel plate, and a 90 degree bending test was performed using a punch with a tip R of 2.0 to 5.0 to determine the maximum R at which cracking occurs in the bent part. Ta. The maximum R at which cracks occur also depends on the plate thickness t, so divide the obtained maximum R by the plate thickness t (1.6 mm) to obtain the limit R/t. The material was evaluated as having good bendability.

(6) vTrs and vE in L direction and C direction
After hot stamping and heat treatment, the hot stamped parts were subjected to an impact test to confirm the anisotropy of the parts in terms of fracture transition temperature (vTrs) and absorbed energy (vE). A piece of steel with a thickness of 1.6 mm cut from a hot stamped part was taken so that its long side was parallel (L direction) or perpendicular (C direction) to the rolling direction, and six pieces of steel piece in the same direction were stacked. After fixing with screws, a V-notch test piece was prepared and subjected to a Charpy impact test.

Here, the fracture surface transition temperature (vTrs) parallel to the rolling direction (L direction) and perpendicular (C direction) are both -40°C or lower, and similarly, the absorbed energy (vE) at room temperature is ^both 50 J/cm2 or higher. The results were evaluated as ◯ (good).

Table 3 shows the results of tissue observation. Furthermore, Table 4 shows the results of the mechanical properties of the steel sheets. The underline in Table 3 indicates that it is outside the scope of the present invention, and the underline in Table 4 indicates that the mechanical properties are unfavorable.

No. in Table 4. 1~No. No. 21 is an example of the present invention that satisfies all the provisions of the present invention. x1 to x13 are comparative examples that do not satisfy the provisions of the present invention.

As shown in Table 4, No. 1~No. Example 21 of the present invention has a structure mainly composed of ferrite and pearlite, has a ferrite grain size of 19.9 μm or less, a pearlite area ratio of 16% or more, a total area ratio of ferrite and pearlite of 95% or more, and an MnS area ratio of 100 μm or more. is 0.0096% or less, the proportion of carbides of 1.2 μm or more is 9.7% or less, and as it is hot stamped and quenched without tempering, TS: 1803 to 2521 (MPa), YS : 1075-1615 (MPa), EL: 8.7-10.8 (%), Limit R/t: 1.25-2.19, vTrs in L direction and C direction: -40°C or less, vE: 50J / ^cm2 or more, has mechanical properties such as good hydrogen resistance, has ultra-high strength (especially tensile strength of 1.8 GPa or more), and can produce hot stamped parts with significantly improved fracture resistance. I understand.

On the other hand, No. In case of x1, pearlite could not be sufficiently produced because the coiling temperature exceeded the upper limit of the range of the present invention and the average cooling rate of 750 to 550 ° C. after annealing was also below the lower limit of the range of the present invention. . For this reason, the hardenability during hot stamping deteriorated, and sufficient tensile strength TS could not be obtained.

No. In case of x2, the average cooling rate of 750 to 550°C after annealing was also below the lower limit of the range of the present invention, so pearlite could not be sufficiently produced. For this reason, the hardenability during hot stamping deteriorated, and sufficient tensile strength TS could not be obtained.

No. In sample x3, pearlite could not be sufficiently produced because the average cooling rate after annealing exceeded the upper limit of the range of the present invention. For this reason, the hardenability during hot stamping deteriorated, and sufficient tensile strength TS could not be obtained.

No. In sample x4, pearlite could not be sufficiently produced because the coiling temperature exceeded the upper limit of the range of the present invention. For this reason, the hardenability during hot stamping deteriorated, and sufficient tensile strength TS could not be obtained.

No. x5 is a value in which the slab heating temperature is below the lower limit of the range of the present invention, the area ratio of MnS and the ratio of carbide exceed the upper limit of the range of the present invention, and the limit R/t is an unfavorable value; The fracture surface transition temperature (vTrs) and absorbed energy (vE) also had unfavorable values.

No. For x6, since the slab heating time is below the lower limit of the range of the present invention, both the area ratio of MnS and the ratio of carbide exceed the upper limit of the range of the present invention, and the limit R/t is an unfavorable value, The fracture surface transition temperature (vTrs) and absorbed energy (vE) also had unfavorable values.

No. For x7, since the average cooling rate after finish rolling was below the lower limit of the range of the present invention, the ratio of carbides exceeded the upper limit of the range of the present invention, and the limit R/t was an unfavorable value.

No. For x8, since the annealing temperature after cold rolling was below the lower limit of the range of the present invention, the ratio of the carbides exceeded the upper limit of the range of the present invention, and the limit R/t was an unfavorable value.

No. For x9, the Cr content exceeds the upper limit of the range of the present invention and the Vc90 value also exceeds the upper limit of the range of the present invention, so the hardenability during hot stamping deteriorates, the tensile strength TS decreases, and the fracture surface transition temperature ( vTrs) and absorbed energy (vE) were also unfavorable values.

No. For x10, the Vc90 value was below the lower limit of the range of the present invention, so the fracture surface transition temperature (vTrs) and absorbed energy (vE) were unfavorable values.

No. For x11, since the Vc90 value exceeds the upper limit of the range of the present invention, the hardenability during hot stamping deteriorates, the tensile strength TS decreases, and the fracture surface transition temperature (vTrs) and absorbed energy (vE) also have unfavorable values. there were.

No. In sample x12, pearlite could not be sufficiently produced because the Nb content was below the lower limit of the range of the present invention and the coiling temperature exceeded the upper limit of the range of the present invention. For this reason, the tensile strength TS became small, and the fracture surface transition temperature (vTrs) and absorbed energy (vE) had unfavorable values.

Furthermore, No. For x13, the Nb content exceeded the upper limit of the range of the present invention and the ratio of carbides of 1.2 μm or more exceeded 10%, so the limit R/t was an unfavorable value and the fracture surface transition temperature (vTrs) And the absorbed energy (vE) was also a poor value.

Claims (5)

The chemical composition is in mass%,
C: 0.18-0.50%,
Si: 0.001-2.00%,
Mn: 0.001 to 3.00%,
P: 0.10% or less,
S: 0.0015% or less,
Nb: 0.010-0.100%,
Cr: 0.001-0.50%,
Al: 0.0001-0.100%,
Ti: 0.001 to 0.500%,
O: less than 0.0030%,
B: 0.0006 to 0.0030%,
N: 0.0100% or less, and the remainder: Fe and impurities,
The following formulas (i), (ii) and (iii) are satisfied,
The metal structure is mainly composed of ferrite and pearlite, the ferrite grain size is 30.0 μm or less, the pearlite area ratio is 15% or more, the total area ratio of ferrite and pearlite is 95% or more, and the remainder is The structure contains one or more types of bainite, retained austenite, and cementite,
The area ratio of MnS having a length of 100 μm or more at a depth of 1/4 to 3/4 of the plate thickness is 0.0100% or less, and
A steel plate for hot stamped parts, wherein the number ratio of carbides with a grain size of 1.2 μm or more among carbides with a grain size of 0.5 μm or more existing in steel is 10.0% or less.
3.0℃/sec ≦Vc90≦30℃/sec ・・・・・・・(i)
log(Vc90)=2.94-0.75A (ii)
A=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo...(iii)
However, in formula (iii), the element symbol indicates the content (mass%) of each element. The chemical composition is in mass%,
V: 2.00% or less,
The steel plate for hot-stamped parts according to claim 1, comprising one or more selected from Ta: 0.50% or less, and W: 3.00% or less. The chemical composition is in mass%,
Ni: 5.00% or less,
The steel plate for hot stamped parts according to claim 1 or 2, comprising one or more selected from Cu: 3.00% or less and Mo: 0.50% or less. The chemical composition is in mass%,
Mg: 0.0030% or less,
Ca: 0.0030% or less,
The steel plate for hot stamped parts according to any one of claims 1 to 3, comprising one or more selected from La: 0.030% or less and Ce: 0.030% or less. A method for manufacturing a steel plate for hot stamped parts according to any one of claims 1 to 4, comprising:
After heating the steel ingot or billet having the above chemical composition at T (°C) shown by formula (iv) for 30 minutes or more, rough rolling is completed at 1000°C to 1150°C, and the Ae is ₃ or more points. a rolling process to complete finish rolling;
a cooling step of cooling at an average cooling rate of 10 to 200°C/sec;
A winding process of winding at 450 to 600°C,
A cold rolling process performed at a reduction rate of 10 to 60%,
A method for producing a steel plate for hot-stamped parts, comprising an annealing step in a temperature range of 750° C. or higher, and cooling at a cooling rate of 3 to 40° C./sec at 750 to 550° C. after annealing.
T (℃) = [0.020 + (mass% Mn) (mass% S)] / (1.9 × 10 ^-5 )... (iv)
However, in formula (iv), mass %Mn and mass %S represent the contents (mass %) of Mn and S, respectively.