WO2017149999A1

WO2017149999A1 - Steel sheet for hardening, hardened member, and method for manufacturing steel sheet for hardening

Info

Publication number: WO2017149999A1
Application number: PCT/JP2017/002186
Authority: WO
Inventors: 紗江濱本; 達也浅井; 裕之大森
Original assignee: 株式会社神戸製鋼所
Priority date: 2016-02-29
Filing date: 2017-01-23
Publication date: 2017-09-08

Abstract

An aspect of the present invention is a steel sheet for hardening characterized in satisfying a prescribed component composition and in the Mn concentration satisfying expression (1). S1+S2 < −10 × [Mn] + 44 (1) [Mn]: Mn concentration (mass%) of the steel sheet analyzed by inductively coupled plasma emission spectroscopy S1: In the structure of the steel sheet at the ¼ sheet thickness position, the area% of regions in which the Mn concentration, as analyzed by an electron microprobe analyzer, is at least 2 times [Mn] S2: In the structure of the steel sheet at the ¼ sheet thickness position, the area% of regions in which the Mn concentration, as analyzed by an electron microprobe analyzer, is not more than 0.5 times [Mn]

Description

Quenching steel plate, quenching member, and quenching steel plate manufacturing method

The present invention relates to a steel plate for quenching, a quenching member, and a method for producing a steel plate for quenching. More specifically, a steel sheet for quenching useful as a material for providing a quenching member having excellent bending workability in T-direction bending in which the bending ridge line is parallel to the rolling direction in a region where the hardness after quenching is 515 HV or more. And a manufacturing method thereof.

In order to reduce fuel consumption of automobiles and transport aircraft, it is desired to reduce the weight of automobiles and transport aircraft. In order to reduce the weight, for example, it is effective to use a high-strength steel plate to reduce the plate thickness. However, when cold working is performed on a high-strength steel sheet having a tensile strength exceeding 980 MPa, there are problems such as an increase in press forming load and a significant deterioration in dimensional accuracy.

As a method for solving the above problem, a hot press forming technique is adopted in which press forming is performed with a mold in a state where the strength is lowered by heating to a temperature at which the austenite single phase is formed, and the forming is facilitated. However, when the tensile strength of the hot press-formed product is increased, breakage tends to occur at the time of collision. In order to suppress the occurrence of breakage, the bendability of the hot press-formed product needs to be excellent.

Examples of the steel material for hot press working used for hot press-molded products include steel materials described in Patent Document 1 and Patent Document 2.

Patent Document 1 describes a steel material for hot press working that has a specific chemical composition and has a steel structure in which the spheroidization rate of carbides in steel is 0.60 to 0.90.

Further, Patent Document 2 has a specific chemical composition, a steel structure having a prior austenite average particle size of 10 μm or less, and mechanical properties having a tensile strength of 1.8 GPa or more and 2.0 GPa or less. A hot pressed steel sheet member is described.

JP 2011-195958 A JP 2014-15638 A

The present invention provides a quenched member excellent in T-direction bendability, a quenched steel plate capable of producing the quenched member, and a method for producing the same, even in a high-strength region having a hardness after quenching of 515 HV or higher. With the goal.

In one aspect of the present invention, the component composition is mass%, C: more than 0.2% and 0.4% or less, Si: 0.8% or more and 1.4% or less, Mn: 1% or more and 3% or less, P: more than 0% and 0.02% or less, S: more than 0% and 0.002% or less, sol. Al: 0.02% to 0.06%, N: more than 0% to 0.01%, O: more than 0% to 0.01%, B: 0.0005% to 0.005%, and Ti : A steel sheet for quenching characterized by satisfying 0.005% or more and 0.1% or less, the balance being iron and inevitable impurities, and Mn concentration satisfying the following formula (1).

S1 + S2 <−10 × [Mn] +44 (1)
[Mn]: Mn concentration (% by mass) of the steel sheet analyzed by inductively coupled plasma emission spectroscopy
S1: Area% of the region where the Mn concentration analyzed by the electron microprobe analyzer is more than twice the above [Mn] in the structure of the steel plate at the thickness 1/4 position.
S2: Area% of a region in which the Mn concentration analyzed by an electron beam microprobe analyzer is 0.5 times or less of the above [Mn] in the structure of the steel plate with a thickness of 1/4.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings.

FIG. 1 is a diagram showing the relationship between the hardness of the quenched member and the T-direction bending angle.

Generally, strength and bendability tend to conflict. That is, the bendability tends to decrease as the strength increases. In particular, the higher the strength, the lower the bending workability of T-direction bending with the bending ridge line parallel to the rolling direction.

According to the study by the present inventors, the steel sheets described in Patent Document 1 and Patent Document 2 have excellent bending workability in L-direction bending in which the bending ridge line is perpendicular to the rolling direction, but the bending ridge line It has been found that the bending workability of T-direction bending (hereinafter referred to as T-direction bending property) that is parallel to the rolling direction is insufficient.

In the above description, the hot press forming technique has been described as an example. However, the above problem that it is difficult to achieve both high strength and bendability (particularly high strength and bendability in the T direction) is a hot press. It is not limited to molded products but can be found in all hardened members.

The present inventors provide a steel sheet for quenching that can be a quenching member excellent in T-direction bendability even when the tensile strength after quenching is about 1600 MPa or higher, that is, the hardness after quenching is high strength of 515 HV or higher. For this reason, we have been studying earnestly.

As a result, on the premise that the component composition is appropriately controlled, if a steel sheet for quenching in which the Mn concentration distribution is appropriately controlled is used, even if the hardness after quenching is a high strength region of 515 HV or more, The inventors found that a quenched member having improved T-direction bendability was obtained, and reached the present invention.

In this specification, “quenching” means not only a mode in which a press working is performed in a softened state by heating to about 900 ° C. as in a hot press, and at the same time quenching is performed by a cooling effect accompanying contact with a mold. It is also intended to include a mode in which quenching is performed after press working such as warm press and cold press other than hot press.

Hereinafter, embodiments according to the present invention will be described, but the present invention is not limited thereto.

First, the component composition of the steel sheet for quenching according to one embodiment of the present invention will be described.

[C: more than 0.2% and 0.4% or less]
Since the hardness of the quenched member is roughly determined by the C content, C is a necessary element. In order to increase the hardness of the quenched member, the C content is more than 0.2%, preferably 0.22% or more, more preferably 0.24% or more. However, if the C content is excessive, the strength after hot rolling increases, cracks occur during cold rolling, and the weldability of the steel sheet decreases, so the C content is preferably 0.4% or less, preferably Is 0.38% or less, more preferably 0.36% or less.

[Si: 0.8% to 1.4%]
Si is one of the important elements in the present invention. Si can improve the adhesion of the scale after quenching and prevent scale peeling. Moreover, since hardenability improves by containing Si, the hardness of a quenching member can be improved. In order to effectively exhibit such an action, the Si content is 0.8% or more, preferably 0.9% or more, more preferably 1% or more. However, when the Si content is excessive, retained austenite is likely to be generated, which promotes the diffusion of Mn into the retained austenite, and as a result, the Mn concentration in the steel sheet tends to be non-uniform. Therefore, the Si content is 1.4% or less, preferably 1.35% or less, more preferably 1.3% or less.

[Mn: 1% to 3%]
Mn is an element that contributes to increasing the hardness of the quenched member. In order to effectively exhibit such an action, the Mn content is 1% or more, preferably 1.1% or more, more preferably 1.2% or more. However, when the Mn content is excessive, the strength after hot rolling is increased, causing cracks during cold rolling or degrading the weldability of the steel sheet. In addition, excessive addition of Mn causes segregation of Mn and deterioration of workability. Therefore, the Mn content is 3% or less, preferably 2.8% or less, more preferably 2.6% or less.

[P: more than 0% and 0.02% or less]
P is an element inevitably contained, and is an element that deteriorates the weldability of the steel sheet. Therefore, the P content is 0.02% or less, preferably 0.018% or less, more preferably 0.017% or less. In addition, since it is better that the P content is as small as possible, the P content may be over 0%, but industrially it is 0.0005% or more.

[S: more than 0% and 0.002% or less]
S, like P, is an element that is inevitably contained, and is an element that degrades the weldability of the steel sheet. Further, when S is contained, MnS is generated in the steel sheet. As a result, the homogeneity of the concentration distribution of Mn is lowered, and Mn is segregated. Therefore, the S content is 0.002% or less, preferably 0.0018% or less, more preferably 0.0015% or less. Since the S content should be as low as possible, it should be over 0%, but industrially it is 0.0001% or more.

[Sol. Al: 0.02% to 0.06%]
sol. Al is an element that acts as a deoxidizer. In order to effectively exhibit such an action, sol. The Al content is 0.02% or more, more preferably 0.025% or more. However, sol. When the Al content is excessive, the hardness of the quenched member is lowered. The Al content is 0.06% or less, preferably 0.055% or less, more preferably 0.05% or less.

[N: more than 0% and 0.01% or less]
N is an element inevitably contained, and when the N content is excessive, boride is generated and the B content is reduced, so that the hardenability of the steel sheet may be lowered. Therefore, the N content is 0.01% or less, preferably 0.008% or less, more preferably 0.005% or less. In addition, since it is better that the N content is as small as possible, the N content may be over 0%, but industrially it is 0.0001% or more.

[O: more than 0% and 0.01% or less]
O is an element inevitably contained, and an element that causes a decrease in the T-direction bendability of the quenched member when included in excess. Therefore, the O content is 0.01% or less, preferably 0.005% or less, more preferably 0.003% or less. In addition, since it is better that the O content is as small as possible, it may be over 0%, but industrially it is 0.0001% or more.

[B: 0.0005% to 0.005%]
B is an element that improves the hardenability of the steel sheet. In order to effectively exert such effects, the B content is 0.0005% or more, preferably 0.001% or more, more preferably 0.0012% or more, and further preferably 0.0015% or more. However, if B is contained excessively, coarse iron nitride is generated and toughness is deteriorated, so that the B content is 0.005% or less, preferably 0.004% or less, more preferably 0.0035. % Or less.

[Ti: 0.005% to 0.1%]
Ti can suppress the decrease in the B content by generating TiN, and can improve the hardenability of the steel sheet by B. Therefore, the Ti content is 0.005% or more, preferably 0.01% or more, more preferably 0.015% or more. However, when it contains excessively, a carbide will precipitate in a grain boundary and the hardenability of a steel plate will deteriorate. Therefore, the Ti content is 0.1% or less, preferably 0.08% or less, more preferably 0.06% or less.

[Other ingredients]
The steel sheet for quenching satisfies the above component composition, and the balance is iron and inevitable impurities. In the steel sheet for quenching, as an unavoidable impurity, for example, not only P, S, N, and O which may be brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc., but Pb , Bi, Sb, and Sn may be included. The inevitable impurities here are impurities other than the above P, S, N, and O, and examples thereof include Trump elements such as Pb, Bi, Sb, and Sn.

Moreover, as long as the effect of the present invention is not adversely affected, as another element, at least selected from the group consisting of (A) Cr: more than 0% and 3% or less and Mo: more than 0% and 3% or less One type, (B) Nb: more than 0% and 0.1% or less, and V: at least one selected from the group consisting of more than 0% and 0.1% or less may be contained. These elements (A) and (B) can be contained alone or in combination with the element described in (A) and the element described in (B). The reason for setting this range is as follows.

[(A) Cr: at least one selected from the group consisting of more than 0% and 3% or less and Mo: more than 0% and 3% or less]
Cr and Mo are both effective elements for improving the hardenability and improving the strength of the quenched member, and can be used alone or in combination. In order to effectively exhibit these actions, the Cr and Mo contents are each more than 0%, preferably 0.1% or more, more preferably 0.3% or more. However, since excessive strength increases the strength after hot rolling, the cold rolling property deteriorates and the cost increases. Therefore, the contents of Cr and Mo are preferable when each is contained alone. Is 3% or less, more preferably 2.5% or less, and still more preferably 2% or less.

[(B) At least one selected from the group consisting of Nb: more than 0% and 0.1% or less and V: more than 0% and 0.1% or less]
Nb and V are both effective elements for forming carbides in the steel sheet and improving the strength of the quenched member, and can be used alone or in combination. In order to effectively exert such actions, the contents of Nb and V are each over 0%, preferably 0.005% or more, more preferably 0.008% or more. However, when it contains excessively, a carbide will precipitate in a grain boundary and the hardenability of a steel plate will deteriorate. Therefore, the contents of Nb and V are each preferably 0.1% or less, more preferably 0.08% or less, and still more preferably 0.06% or less.

[Mn concentration distribution]
In the steel sheet for quenching satisfying the above component composition, the present inventors appropriately control the Mn concentration distribution to be within the range of the following formula (1), that is, quenching by suppressing segregation of Mn. It was derived that a quenched member excellent in T-direction bendability can be obtained even in a high strength region where the later hardness is 515 HV or higher. That is, quenching is performed by setting the steel sheet thickness 1/4 position (the position of 1/4 t portion of the steel sheet for quenching with thickness t, the same applies hereinafter) to a steel sheet for quenching that satisfies the following formula (1). It has been clarified that despite the high hardness of the member, it exhibits good T-direction bendability. In addition, the evaluation method of T direction bendability is mentioned later.

S1 + S2 <−10 × [Mn] +44 (1)
[Mn]: Mn concentration (% by mass) of the steel sheet analyzed by inductively coupled plasma emission spectroscopy
S1: Area% of the region where the Mn concentration analyzed by the electron microprobe analyzer is more than twice the above [Mn] in the structure of the steel plate at the thickness 1/4 position.
S2: Area% of a region in which the Mn concentration analyzed by an electron beam microprobe analyzer is 0.5 times or less of the above [Mn] in the structure of the steel plate with a thickness of 1/4.

In this specification, “Mn segregation” means the area% (S1) of the region that is twice or more the Mn concentration of the base material (quenched steel plate) and 0.5 times or less the Mn concentration of the base material. It means that the sum (S1 + S2 (area%)) with the area% (S2) of the region is large. A method for obtaining S1 + S2 (area%) will be described later.

The Mn concentration [Mn] of the base material is calculated by chemically analyzing the quenching steel plate by inductively coupled plasma emission spectroscopy. That is, [Mn] is an average value of Mn concentration in the whole steel plate.

In addition, as shown in the above formula (1), the present inventors must further suppress the segregation of Mn when the Mn concentration of the base material is higher than when the Mn concentration of the base material is low. Also derived. For example, when [Mn] is 1.3 mass%, S1 + S2 may be less than 31 area%, but when [Mn] is 2.3 mass%, S1 + S2 must be less than 21 area%. .

The value of S1 + S2 only needs to be smaller than the value of −10 × [Mn] +44, but since the lower limit of the Mn content is 1%, it is preferably less than 34 area%, and is 31 area% or less. Is more preferably 25% by area or less, and particularly preferably 21% by area or less. The lower limit is not particularly limited and may be 0 area%, but is industrially 5 area% or more, and practically 10 area% or more.

The value of S1 + S2 only needs to be smaller than the value of −10 × [Mn] +44. That is, −10 × [Mn] + 44− (S1 + S2) only needs to be larger than zero. Further, −10 × [Mn] + 44− (S1 + S2) is preferably 1.0 or more, and more preferably 2.0 or more. Further, −10 × [Mn] + 44− (S1 + S2) is preferably 10 or less, and more preferably 5.0 or less.

[Structure of steel plate for quenching]
Next, the structure of the steel sheet for quenching according to the present embodiment will be described.

Since Mn hardly dissolves in ferrite, Mn tends to segregate when a large amount of ferrite is formed. Therefore, the area ratio of ferrite with respect to the entire structure is preferably 50% or less. The area ratio of ferrite with respect to the entire structure is more preferably less than 50%, further preferably 45% or less, and still more preferably 30% or less. Further, the amount of ferrite is preferably small, and may be 0%. The area ratio of the ferrite is measured by observing the position of the steel sheet with a thickness of 1/4 with an optical microscope or a scanning electron microscope (SEM). In some cases, precipitation of carbides may be observed in the ferrite grains. In that case, the ferrite area ratio may be measured including the carbides, assuming that there are no carbides. That is, the ferrite area ratio does not change due to precipitation of carbide.

In the steel sheet for quenching, the main structure is preferably a structure other than ferrite. For example, it is preferable that the main structure is pearlite, bainite, and martensite (including autotemper martensite). In addition, since the said steel plate for hardening is manufactured without performing tempering so that it may mention later, it is preferable that a tempered martensite is 0%.

[Method of manufacturing steel sheet for quenching]
Next, the manufacturing method of the steel plate for hardening which concerns on this embodiment is demonstrated.

First, hot rolling is performed using steel having the above-described component composition. In hot rolling, after finish rolling is performed in the austenite region, cooling is performed at an average cooling rate [R] (° C./s) from the finish rolling temperature to the winding temperature, and winding is performed at the winding temperature [T] (° C.). take. After winding, hold at a temperature from the winding temperature to “winding temperature−50 ° C.” for [t] time. Here, the above [R], [t], and [T] need to satisfy the following formula (2).

6.0 <2 × 10 ⁴ × (ln [R] +10) / ((ln [t] +70) × [T]) (2)

Mn becomes more segregated as the coiling temperature [T] increases. Further, as the holding time [t] at the temperature from the coiling temperature to the coiling temperature −50 ° C. becomes longer, Mn is more easily segregated. And Mn becomes easy to segregate, so that average cooling rate [R] from finish rolling temperature to coiling temperature is slow. The relationship between the parameters [R], [t] and [T] described above and segregation of Mn, and “Tatsuhiro Tsuchiyama, interpretation of physical meaning of tempering parameters and application to continuous heating / cooling heat treatment process, heat treatment , 42, No. 3, P163 ”, the above formula (2) was derived with reference to the tempering parameters during continuous heating obtained by the method described in the paper. Hereinafter, each parameter of the formula (2) will be described in detail.

<Average cooling rate from finish rolling temperature to winding temperature [R] (° C./s)>
When the cooling rate is slow, ferrite is generated during cooling, and Mn that is difficult to dissolve in ferrite diffuses into untransformed austenite, so that Mn tends to segregate. Therefore, the average cooling rate [R] is preferably 10 ° C./s or more, more preferably 15 ° C./s or more. The upper limit of the average cooling rate [R] is not particularly limited, but is industrially preferably 200 ° C./s or less, more preferably 100 ° C./s or less, and 50 ° C./s or less. Is more preferable.

The finish rolling temperature is not particularly limited as long as it is an austenite region, but it is preferably at least the Ar ₃ transformation point from the viewpoint of suppressing an increase in hot deformation resistance. Moreover, it is preferable that it is 950 degrees C or less from a viewpoint of suppressing scale generation according to a conventional method.

<Winding temperature [T] (° C)>
When the coiling temperature is high, untransformed austenite is likely to be generated, which promotes the diffusion of Mn into the untransformed austenite, and as a result, the Mn concentration in the steel sheet may become uneven. On the other hand, when the coiling temperature is low, the strength of the steel sheet is increased and the cold rolling property is impaired. Therefore, winding temperature [T] becomes like this. Preferably it is 320 degreeC or more and 650 degrees C or less, More preferably, it is 350 degreeC or more and 600 degrees C or less.

<Holding time [t] (hours) at a temperature from the winding temperature to "winding temperature-50 ° C">
Although depending on the coiling temperature, the holding time [t] in the above temperature range is preferably 15 hours or less, more preferably 10 hours or less. If the holding time at the temperature from the coiling temperature to the coiling temperature −50 ° C. is too long, Mn tends to segregate. Moreover, the lower limit of the holding time [t] is not particularly limited, but is preferably 0.25 hours or more industrially.

The “holding” does not necessarily have to be held at the same temperature, and may be varied as long as it is within the above temperature range. For example, the temperature may be kept constant within the above temperature range, or changes within this range, that is, temperature increase due to temperature decrease or heating, temperature increase due to recuperation due to transformation, and the like may be included.

In the manufacturing method according to the present embodiment, the temperature is kept at the above temperature range for a predetermined time, and then cooled to room temperature, but the cooling rate at that time is not particularly limited, and may be air cooling, for example.

[Pickling, cold rolling]
After the hot rolling, pickling may be performed as necessary, and cold rolling with a cold rolling rate of about 30 to 80% may be performed.

[Plating]
After the hot rolling, plating may be performed if the temperature of the steel sheet in the production process is 300 ° C. or less.

[Hardened material]
By using the steel sheet for quenching according to this embodiment in which segregation of Mn is suppressed, a hardened member having a high strength of 515 HV or more and excellent in T-direction bendability can be obtained. . Specifically, by quenching the steel sheet for quenching according to the present embodiment in which segregation of Mn is suppressed, the quenching member has high strength after quenching of 515 HV or more and excellent in T-direction bendability. Can be obtained. The hardness of the quenched member is preferably 525 HV or more, and more preferably 535 HV or more. The upper limit of the hardness of the quenched member is not particularly limited, but is, for example, 680 HV or less, preferably 650 HV or less, more preferably 600 HV or less, and even more preferably 570 HV or less.

The quenched member has a relationship between a bending angle obtained by converting a displacement at the maximum load obtained by a bending test in a T-direction bending according to a VDA standard (VDA238-100) defined by the German Automobile Manufacturers Association and the hardness. The value of the following formula (5) showing is preferably larger than 0, more preferably 5 or more. The value of the following formula (5) being greater than 0 indicates that both the hardness and the bending angle are large.

Bending angle-(-0.6 x hardness + 376) (5)
In general, strength and bendability tend to conflict with each other, and the higher the strength, the lower the bendability, but the quenched member has high strength and bendability in this way.

Next, a method for manufacturing a quenched member will be described.

For example, the manufacturing method of the quenching member in the case where the steel plate for quenching according to the present embodiment is applied to hot press forming is not particularly limited, and a known method such as a die quench method can be used. Specifically, there is a method in which the steel sheet for quenching is heated to a temperature at which it becomes an austenite single phase to reduce the strength and press forming with a mold in a state where forming is facilitated. More specifically, the steel sheet for quenching according to the present embodiment is heated to a temperature equal to or higher than the Ac ₃ point defined by the following formula (3), and then press forming of the steel sheet is started by a mold. After the start of the above, there is a method of cooling up to the range of the Ms point defined by the following formula (4) while securing an average cooling rate of 20 to 300 ° C./s in the mold.

Ac ₃ (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] − 11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (3)
Ms (° C.) = 550-361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al] (4)

In addition, the manufacturing method of the quenching member in the case of applying to hot press molding is not particularly limited as long as it satisfies the hardness of 515 HV or higher. After performing, cooling such as air cooling may be performed.

Alternatively, the steel sheet for quenching according to the present embodiment can be applied to press forming other than hot pressing, and then quenched to form a quenched member. For example, when the steel sheet for quenching according to the present embodiment is applied to warm press forming, after performing warm press by heating to a temperature of approximately 200 to 700 ° C., only a portion requiring hardness is quenched with high frequency or the like. And a hardened member can be manufactured. Moreover, when applying the steel plate for hardening which concerns on this embodiment to cold press molding, after performing cold press, only the part which requires hardness can be hardened with a high frequency etc., and a hardened member can be manufactured.

This specification discloses various modes of technology as described above, and the main technologies are summarized below.

It is preferable that the quenching steel sheet has a ferrite area ratio of 0% or more and 50% or less at a ¼ thickness position of the steel sheet.

The quenching steel plate preferably has a composition of mass% and satisfies B: 0.001% or more and 0.005% or less.

The quenching steel plate further contains, as other elements, mass%, Cr: more than 0% and 3% or less, Mo: more than 0% and 3% or less, Nb: more than 0% and 0.1% or less, and V: 0%. It is preferable to contain at least one selected from the group consisting of more than 0.1%.

Another aspect of the present invention is a quenched member manufactured using the steel sheet for quenching, which has a hardness of 515 HV or more and has excellent T-direction bendability. .

Another aspect of the present invention is a method for producing the steel sheet for quenching, which includes a step of satisfying the following formula (2) after performing finish rolling in the austenite region. It is a manufacturing method of a steel plate.

6.0 <2 × 10 ⁴ × (ln [R] +10) / ((ln [t] +70) × [T]) (2)
[R]: Average cooling rate from “finish rolling temperature” to “coiling temperature” (° C./s)
[T]: Time (h) held at a temperature from “winding temperature” to “winding temperature −50 ° C.”
[T]: “Taking-up temperature” (° C.)

In the method for manufacturing a steel sheet for quenching, the average cooling rate R is preferably 10 ° C./s or more and 200 ° C./s or less.

In the method for manufacturing a steel sheet for quenching, it is preferable that the held time t is 0.25 hours or more and 15 hours or less.

In the method for manufacturing a steel sheet for quenching, the winding temperature T is preferably 320 ° C. or higher and 650 ° C. or lower.

According to the present invention, by using the quenching steel plate, it is possible to provide a quenching member having excellent T-direction bendability even in a high strength region where the hardness after quenching is 515 HV or higher.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the purpose described above and below. Of course, it is also possible to implement them, and they are all included in the technical scope of the present invention.

[Experiment No. 1]
Steel with the component composition shown in Table 1 below (the balance is iron and inevitable impurities, and the blank in Table 1 means that no element is added) is melted and subjected to hot rolling as described below, A rolled steel sheet was obtained. Then, the surface was ground and the steel plate for hardening of thickness 1.4mm was obtained.

[Hot rolling]
The slab was heated to 1250 ° C. and hot-rolled at a reduction rate of 90% to a plate thickness of 2.3 mm so as to have a “finish rolling temperature (° C.)” shown in Table 2. After cooling from this temperature to the “winding temperature (° C.)” shown in Table 2 at the “average cooling rate (° C./s)” shown in Table 2, the “holding time (h ) ”Was held at a temperature of“ winding temperature−50 (° C.) ”or more and“ winding temperature (° C.) ”or less. Subsequently, the hot-rolled steel sheet was manufactured by air cooling to room temperature.

[Experiment No. 6, 11, 17, 22]
Except for changing the component composition, finish rolling temperature, average cooling rate, winding temperature, and holding time to the conditions shown in Tables 1 and 2, Experiment No. A steel plate for quenching was obtained by the same production method as in No. 1.

[Experiment No. 2-5, 7-10, 12-16, 18-21, 23-26]
Except that the hot rolled steel sheet was manufactured by changing the component composition, finish rolling temperature, average cooling rate, coiling temperature, and holding time to the conditions shown in Tables 1 and 2, Experiment No. A hot-rolled steel sheet was manufactured by the same manufacturing method as in No. 1. Thereafter, the obtained hot-rolled steel sheet was pickled to remove scale on the surface, and then cold-rolled to produce a cold-rolled steel sheet having a thickness of 1.4 mm to obtain a quenched steel sheet.

Experiment No. With respect to the steel plates for quenching 1 to 26, the metal structure and Mn concentration were measured as described in detail below. Furthermore, Experiment No. As shown in detail below, various mechanical properties of the quenched members obtained by quenching steel plates for quenching 1 to 26 obtained by the quenching test described below were evaluated and are shown in Table 2.

[Ferrite area ratio]
After polishing the cross section in the L direction (parallel to the rolling direction) of the steel sheet for quenching, it was corroded with nital. Then, using an optical microscope, three fields of view (100 μm × 100 μm size / field of view) were observed at a 1/4 position of the plate thickness at a magnification of 1000 times, and the area of the ferrite was determined by a point calculation method with a lattice spacing of 5 μm and a lattice number of 20 × 20. The rate was measured, and the average value of 3 fields of view was calculated.

[Mn concentration distribution]
Using the following formula (1), the concentration distribution of Mn was evaluated according to the following criteria, and A evaluation was passed and B evaluation was rejected. In addition, the measurement method of [Mn] and the calculation method of S1 + S2 are as follows.

(Evaluation criteria)
A: satisfies the formula (1) (the value of S1 + S2 is smaller than the value of −10 × [Mn] +44)
B: Formula (1) is not satisfied (the value of S1 + S2 is a value of −10 × [Mn] +44 or more)

(Measuring method of [Mn])
A sample having a size of 30 mm × 100 mm was cut out from the center in the width direction of the steel plate for quenching. The cut sample is made into a powder, this powder is dissolved in a mixed acid solution of hydrochloric acid and nitric acid, and this solution is then bonded using an inductively coupled plasma emission spectrometer (ICPV-1017 manufactured by Shimadzu Corporation). [Mn] was obtained by chemical analysis by plasma emission spectroscopy.

(Calculation method of S1 + S2)
The steel plate for quenching was cut along a cross section in the L direction, embedded in resin, and the cross section was polished. Thereafter, at a position of 1/4 of the plate thickness of the steel sheet, a range of about 120 μm × 95 μm was measured using an electron probe microprobe analyzer (Electron Probe Micro Analyzer: EPMA, JXA-8100 series manufactured by JEOL Ltd.) and a beam diameter of about 5 μm. The Mn concentration was measured under the following conditions. The specific setting in the EPMA apparatus was as follows.

Measurement area X: 300 points Y: 240 points Feed: 0.4 μm
Beam diameter setting: Zero Acquisition time: 20 msec / point Electron beam acceleration voltage: 15 kV
Irradiation current: 1 × 10 ⁻⁶ A (1 μA)
Next, the Mn concentration at each point measured under the above conditions is divided by [Mn], the number of points at which the Mn concentration is 2 times or more of [Mn], and the Mn concentration is 0.5 times or less of [Mn]. A certain number of points was obtained. Furthermore, the total value of the number of points where the Mn concentration is 2 times or more of [Mn] and the number of points where the Mn concentration is 0.5 times or less of [Mn] is the total number of measurement points (300 × 240 points). Then, S1 + S2 (area%) was calculated.

[Quenching test]
The quenching test was performed under the following conditions using a die quench method simulating a mold.

Plate temperature of steel plate for quenching: 900 ° C
Heating time: 100 seconds Cooling time: about 15 seconds Die quench start temperature: 700 ° C
Die quench load: 2000kgf
Molded bottom dead center retention time: 30 seconds

[Evaluation of scale adhesion]
The molded product (quenched member) after the quenching test was naturally allowed to cool in a state in which the mold was retracted, cooled to room temperature, and the surface of the quenched member was visually observed to examine the presence or absence of scale peeling. In this example, the ratio of the scale peeling portion relative to the surface area of the quenched member was calculated and evaluated according to the following criteria.

(Evaluation criteria)
Pass (◯): Scale peeling part is less than 15% of the surface area of the quenching member Fail (x): Scale peeling part is 15% or more of the surface area of the quenching member

[hardness]
The Vickers hardness (HV) of the quenched member was measured by the method described in JIS Z 2244.

[Evaluation of bendability in T direction considering hardness]
The T-direction bendability of the quenched member was evaluated under the following measurement conditions based on the VDA standard (VDA238-100) defined by the German Automobile Manufacturers Association. In this example, the displacement at the maximum load obtained in the bending test was converted into an angle based on the VDA, and the bending angle was obtained. Further, since the correlation is generally such that the higher the hardness of the quenched member, the lower the bending angle, the T-direction bendability was evaluated based on the magnitude of the bending angle with respect to the hardness of the quenched member. Specifically, the T-direction bendability was evaluated according to the following criteria based on the value of the following formula (5), and A evaluation was determined to be acceptable (◯), and B evaluation was determined to be unacceptable (x). Moreover, the relationship between the hardness [hardness after quenching (HV)] and the bending angle [bending angle after quenching (°)] of each quenched member is shown in FIG.

Bending angle-(-0.6 x hardness + 376) (5)

(Evaluation criteria)
A: The value of the formula (5) is larger than 0 (the value of the bending angle is larger than the value of −0.6 × hardness + 376).
B: The value of the formula (5) is 0 or less (the value of the bending angle is -0.6 × hardness + 376 or less)

(Measurement condition)
Test method: roll support, punch push-in roll diameter: φ30mm
Punch shape: Tip R = 0.4mm
Distance between rolls: 3.5mm
Pushing speed: 20mm / min
Specimen size: 60mm x 60mm
Bending direction: rolling perpendicular direction Testing machine: AUTOGRAPH 20kN made by Shimadzu Corporation

From Table 1 and Table 2, it can be considered as follows.

Each experiment in Table 2 (Experiment Nos. 1, 4 and 6) produced under the production conditions satisfying the above formula (2) using the steel types A1 to A7 and B1 to B4 in Table 1 satisfying the composition of the present invention. , 8, 9, 11, 14, 15, 17, 18, and 23 to 26) satisfy the above formula (1), and the quenched member has a high strength of 515 HV or more and has a bendability in the T direction. Excellent, and also excellent in scale adhesion.

On the other hand, as described in detail below, the steel sheets other than the above did not satisfy the component composition or manufacturing conditions defined in the present invention, and the desired characteristics could not be obtained.

Experiment No. manufactured under manufacturing conditions not satisfying the above formula (2). 2, 3, 5, 7, 10, 12, 13, and 16 did not satisfy the above formula (1), and the T-direction bendability of the quenched member was poor.

Since the steel types A8 and A9 in Table 1 are below the lower limit (0.8%) defined in the present invention, the test No. In Nos. 19 to 22, the hardness of the quenched member was insufficient, and the T-direction bendability and scale adhesion of the quenched member were also poor.

This application is based on Japanese Patent Application Japanese Patent Application No. 2016-037635 filed on February 29, 2016 and Japanese Patent Application Japanese Patent Application No. 2016-207673 filed on October 24, 2016. The contents thereof are included in the present application.

In order to express the present invention, the present invention has been properly and fully described through the embodiments with reference to the drawings. However, those skilled in the art can easily change and / or improve the above-described embodiments. It should be recognized that this is possible. Therefore, unless the modifications or improvements implemented by those skilled in the art are at a level that departs from the scope of the claims recited in the claims, the modifications or improvements are not limited to the scope of the claims. To be construed as inclusive.

According to the present invention, by using the steel plate for quenching, a quenching member having excellent T-direction bendability is provided even in a high strength region where the hardness after quenching is 515 HV or higher.

Claims

Ingredient composition is mass%,
C: more than 0.2% and 0.4% or less,
Si: 0.8% or more and 1.4% or less,
Mn: 1% or more and 3% or less,
P: more than 0% and 0.02% or less,
S: more than 0% and 0.002% or less,
sol. Al: 0.02% to 0.06%,
N: more than 0% and 0.01% or less,
O: more than 0% and 0.01% or less,
B: 0.0005% or more and 0.005% or less, and Ti: 0.005% or more and 0.1% or less, the balance is made of iron and inevitable impurities, and the Mn concentration is expressed by the following formula (1) A steel sheet for quenching characterized by filling.
S1 + S2 <−10 × [Mn] +44 (1)
[Mn]: Mn concentration (% by mass) of the steel sheet analyzed by inductively coupled plasma emission spectroscopy
S1: Area% of the region where the Mn concentration analyzed by the electron microprobe analyzer is more than twice the above [Mn] in the structure of the steel plate at the thickness 1/4 position.
S2: Area% of a region in which the Mn concentration analyzed by an electron beam microprobe analyzer is 0.5 times or less of the above [Mn] in the structure of the steel plate with a thickness of 1/4.
The steel sheet for quenching according to claim 1, wherein the area ratio of ferrite at a position of 1/4 of the steel sheet thickness is 0% or more and 50% or less.
The component composition is mass%,
B: The steel plate for quenching according to claim 1, which satisfies 0.001% or more and 0.005% or less.
The component composition is, as another element, in mass%,
Cr: more than 0% and 3% or less,
Mo: more than 0% and 3% or less,
The steel sheet for quenching according to claim 1, comprising at least one selected from the group consisting of Nb: more than 0% and 0.1% or less and V: more than 0% and 0.1% or less.
A quenched member manufactured using the steel sheet for quenching according to any one of claims 1 to 4,
A hardened member excellent in T-direction bendability, characterized by having a hardness of 515 HV or higher.
A method for producing the steel sheet for quenching according to any one of claims 1 to 4,
A method for producing a steel sheet for quenching, comprising a step of satisfying the following formula (2) after finish rolling in an austenite region.
6.0 <2 × 10 4 × (ln [R] +10) / ((ln [t] +70) × [T]) (2)
[R]: Average cooling rate from “finish rolling temperature” to “coiling temperature” (° C./s)
[T]: Time (h) held at a temperature from “winding temperature” to “winding temperature −50 ° C.”
[T]: “Taking-up temperature” (° C.)
The method for producing a steel sheet for quenching according to claim 6, wherein the average cooling rate is 10 ° C / s or more and 200 ° C / s or less.
The method for producing a steel sheet for quenching according to claim 6, wherein the held time is 0.25 hours or more and 15 hours or less.
The method for producing a steel sheet for quenching according to claim 6, wherein the coiling temperature is 320 ° C or higher and 650 ° C or lower.