JP2000248338A - Steel sheet for induction hardening excellent in toughness in hardened part, induction hardening strengthened member and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel sheet for induction hardening excellent in hardenability, in which the hardened part is provided with toughness and which is excellent in impact absorbing characteristics, to produce an induction hardening strengthened member and to provide methods for producing them. SOLUTION: This steel sheet for induction hardening contains, by mass, 0.05 to 0.20% C, 0.3 to 2.5% Mn, <=0.02% P, <=0.02% S, <=0.06% Al, <=0.015% Ti, <=0.010% N, 0.0005 to 0.0040% B, and the balance Fe with inevitable impurities. In addition to the fundamental components, as elements for moreover improving the characteristics of the steel sheet, one or more kinds among Si, Cr, Mo, V, W, Cu and Ni can be incorporated respectively by <=1.0%. The steel sheet for induction hardening is formed to a prescribed shape, and, after that, the part to be imparted with improved strength is subjected to induction hardening, by which an induction hardening strengthened member can be obtd. The old austenite grain size in the hardened part is preferably controlled to <=20 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material steel plate for working of a member for automobiles and the like, and in particular, after working into a required shape, a required portion is induction hardened to increase the strength of the member. The present invention relates to a steel plate, an induction hardening member and a method for manufacturing the same.

[0002]

2. Description of the Related Art In some cases, a molded member for a vehicle made of a thin steel plate is required to have a characteristic of absorbing the impact energy at the time of collision by deforming the member without completely breaking it at the time of a vehicle collision. is there. Members requiring such characteristics are provided with a reinforcing material for absorbing impact energy, and are designed to satisfy the required characteristics. For example,
Since the center pillar, which is an important member on the side of the automobile, undergoes a shocking deformation due to three-point bending at the time of collision, a reinforcing material is used in a portion where bending deformation is expected.

On the other hand, in order to reduce the weight of an automobile, it is desirable to omit a reinforcing material. For this purpose, it is conceivable to use a high-strength steel sheet as the material steel sheet. However, there is a problem that high-strength steel sheets are inferior in formability. Therefore, in recent years, instead of using a reinforcing material or a high-strength steel plate, a relatively low-strength steel plate has been formed into a predetermined shape, and a portion requiring strength has been subjected to induction hardening after forming and quenching. Reinforcement technology is being applied.

[0004] When such a technique is applied, it is necessary to improve the hardenability of the steel sheet. B is often used as a means for improving the hardenability of steel. At that time, it is free B in the steel that contributes to the hardenability, but since B is an element that is very easily bonded to N in the steel, B is bonded to N to form boron nitride (BN). When formed, the quenching effect of B disappears. For this reason, Ti, which is easier to bond with N than B, is added to N equivalent or more, N in steel is fixed as TiN, bonding between N and B is prevented, and free B is secured. .

[0005]

[0003] The impact absorption characteristics at the time of collision are important for automobile pillars such as center pillars and bumper reinforces. The quenched portion is not merely required to have a high hardness for strengthening, but it is important that no crack occurs at the time of collision deformation and the impact absorption energy is large.

However, it has been found that, when N is fixed by Ti and B is added, cracks are easily generated in the quenched portion, and the shock absorption characteristics are not necessarily improved. When the cause was investigated, coarse TiN was present on the fractured surface, and it was presumed that this TiN caused cracking.
Since TiN is generated at the time of melting steel, it is coarsened and dispersed in the material. Furthermore, it does not decompose by heating during quenching, and remains as it is after quenching. Therefore, it is effective to reduce the amount of Ti as much as possible in order to suppress cracks in the quenched portion and improve the shock absorption characteristics. However, the effect of improving the hardenability of B cannot be expected as the amount of Ti decreases.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a steel sheet for induction hardening, which is excellent in hardenability and has a toughness in a hardened portion, and which is excellent in shock absorption properties, and a member for strengthening induction hardening. It provides a method.

[0008]

The steel sheet for induction hardening according to the present invention described in claim 1 has a mass% of C: 0.05 to
0.20%, Mn: 0.3-2.5%, P: 0.02%
Hereinafter, S: 0.02% or less, Al: 0.06% or less, T
i: 0.015% or less, N: 0.010% or less, B:
0.0005 to 0.0040%, the balance being Fe and unavoidable impurities. According to this steel sheet, since Ti, N, and B are regulated to predetermined amounts, the effect of improving the quenchability by B, the effect of suppressing the growth of austenite grains before quenching, and the effect of strengthening the crystal grain boundaries are combined to achieve the effect of high-frequency heating. Even with a short heating time and a relatively low quenching temperature,
The quenching effect can be sufficiently exerted, and the shock absorption characteristics can be improved by improving the toughness of the quenched portion.

[0009] In addition to the above basic components, the steel sheet of the present invention may further comprise Si, C
Any one or more of r, Mo, V, W, Cu, and Ni can be contained in an amount of 1.0% or less, and it is considered that the impact absorption characteristics can be further improved by the inclusion of these elements.

[0010] The induction hardening member of the present invention described in claim 3 is formed of the steel sheet for induction hardening, and is subjected to induction hardening in a part for improving strength, and is capable of absorbing shock. Excellent characteristics.

Further, the induction hardening member according to the present invention described in claim 4 is formed of a hot-dip galvanized steel sheet having the steel sheet for induction hardening as a base plate. The coating layer is left in the quenched part and has excellent shock absorption characteristics,
Also excellent in corrosion resistance. The hot-dip galvanized steel sheet includes a galvannealed steel sheet.

[0012] In these induction hardening strengthening members, as described in claim 5, observed at the quenched portion,
By setting the prior austenite particle size before quenching to 20 μm or less, the static-dynamic ratio (the deformation rate in the tensile test is 2.0 mm /
The maximum stress in the case of low-speed deformation of about sec is σA, and the maximum stress in the case of high-speed deformation of about 10 m / sec is σB.
, The static-dynamic ratio = σB / σA) can be improved, and excellent shock absorption characteristics can be obtained.

According to a sixth aspect of the present invention, there is provided a method for manufacturing an induction hardening member according to the present invention, wherein the steel sheet for induction hardening according to the first or second aspect is formed into a predetermined shape to improve the strength. Is subjected to induction hardening at a quenching temperature of not less than ₃ points and not more than 1000 ° C. Further, the method of manufacturing the induction hardening strengthening member according to the present invention described in claim 7 is based on claim 1.
Alternatively, a steel sheet for induction hardening described in 2 is formed into a predetermined shape, and a portion where the strength is to be improved is Ar at least ₃ points at 1000 ° C.
Induction quenching is performed at the following quenching temperature, and the heat cycle time from the start of heating at the time of quenching to the quenching temperature until cooling to 350 ° C. is 60 seconds or less. In these inventions, the steel sheet for induction hardening described in claim 1 or 2 or a galvanized steel sheet (including a galvannealed steel sheet) using the steel sheet for induction hardening as a base plate is used. Therefore, molding is easy, and quenching at a relatively low temperature of quenching temperature of 1000 ° C. or less becomes possible, and the prior austenite particle size before quenching observed in the quenched part can be reduced to 20 μm or less. Excellent shock absorption characteristics. Moreover, deformation after quenching can be reduced by low-temperature quenching. Furthermore, in the case of a hot-dip galvanized steel sheet, the quenching temperature is as low as 1000 ° C. or less, so that it is possible to prevent the galvanized layer from disappearing due to evaporation during quenching. Can be prevented from deteriorating the paintability due to the generation of slag.
In addition, the heat cycle time (see FIG. 11) is 60 seconds.
Since it is c or less, it is possible to prevent excessive alloying during quenching of the hot-dip galvanized layer, that is, deterioration of corrosion resistance due to excessive diffusion of Fe atoms into the hot-dip galvanized layer.

[0014]

DETAILED DESCRIPTION OF THE INVENTION The present inventors have paid attention to the fact that BN is decomposed even at the heating temperature during induction hardening. However, in quenching by high-frequency heating, since the heating time is shorter than that of normal heat treatment, it is a problem whether BN is sufficiently decomposed and sufficient free B contributing to improvement in hardenability can be secured. Therefore, as a result of investigating the hardenability and impact absorption energy of steel sheets manufactured by melting various steels of Ti, N and B amounts,
Under a specific amount of Ti, N, and B, the quenching effect of B can be sufficiently exhibited even in a short-time heating such as high-frequency heating, and no crack occurs at the time of high-speed deformation such as a collision.
Knowing that high values of shock absorption energy can be obtained,
The present invention has been completed.

That is, the steel sheet for induction hardening of the present invention comprises:
mass%, C: 0.05 to 0.20%, Mn: 0.3 to
2.5%, P: 0.02% or less, S: 0.02% or less,
Al: 0.06% or less, Ti: 0.015% or less, N:
0.010% or less, B: 0.0005 to 0.0040%
And the balance consists of Fe and inevitable impurities.

Here, the reasons for limiting the components of the steel sheet of the present invention will be described. C: 0.05 to 2.0% C is an important element that determines quenching hardness.
%, The required hardness (Vickers hardness (load 1kgf)
Over 300 Hv). For this reason, the lower limit of the C content is set to 0.05%, preferably 0.10%. on the other hand,
If it exceeds 2.0%, delayed fracture is likely to occur in the quenched portion, so the upper limit is made 2.0%, preferably 0.18%.

Mn: 0.3 to 2.5% Mn is an element for improving hardenability, and if it is less than 0.3%, the effect of improving hardenability is too small to obtain a necessary hardenability. It becomes difficult. Therefore, the lower limit of the amount of Mn is 0.3
%, Preferably 0.5%, more preferably 1.0%. On the other hand, Mn tends to micro-segregate during casting, and this segregation is not eliminated even after quenching, and the upper limit is set to 2.5%, preferably 2.0%, in order to promote a decrease in toughness and delayed fracture.

P: not more than 0.02% P is also an element that segregates microscopically, like Mn, and the smaller the better, the better. If it exceeds 0.02%, significant center segregation (segregation at the center of the sheet thickness) occurs and delayed fracture occurs. Is promoted, the upper limit is made 0.02%, preferably 0.015%.

S: not more than 0.02% S combines with Mn to form MnS, deteriorating the workability of the steel sheet, and also serves as a starting point of delayed fracture, so the smaller the better, the better.
The upper limit is made 0.02%, preferably 0.015%.

Al: not more than 0.06% Al is added as a deoxidizing material. If it exceeds 0.06%, alumina-based inclusions increase, and surface defects such as scabs and slivers increase rapidly. The upper limit is made 0.06%, preferably 0.05%.

Ti: 0.015% or less Ti binds preferentially to N and has an effect of suppressing the binding of B to N, but if it exceeds 0.015%, coarse TiN
This TiN is not decomposed even by high-frequency heating, and is present in the structure. Therefore, as will be apparent from the examples described later, cracks occur during high-speed deformation. For this reason, 0.015% or less, preferably 0.01%
2% or less, more preferably 0.010% or less.

N: 0.010% or less N should be small in order to combine with B to reduce the amount of free B in steel, but excessive reduction is accompanied by difficulty in steel making.
Preferably, the lower limit is 0.0
010% is preferable. On the other hand, if it exceeds 0.010%, the effect of improving the hardenability by B cannot be exhibited, so the upper limit is made 0.010%, preferably 0.008%.

B: 0.0005 to 0.0040% B is an important element that improves the hardenability and enables a sufficient hardened structure to be obtained even at a low temperature. Also, when heated to the quenching temperature, that is, the austenitizing temperature, B precipitates at the austenite crystal grain boundaries, and in combination with the fact that low-temperature quenching is possible, there is an effect of suppressing grain growth, and the quenching structure is refined. This is an element that can improve the static-dynamic ratio. Furthermore, the precipitation at the grain boundary is an element that can improve the toughness of the low-temperature transformation structure in order to improve the strength of the grain boundary. If the amount of B is less than 0.0005%, an effective amount of B cannot be secured during quenching, and the above-mentioned effect becomes too small. Therefore, the lower limit is 0.0005%, preferably 0.0010%, more preferably 0.1%. 0025%. On the other hand, 0.0040%
Is exceeded, Fe ₂ B (iron nitride) is generated, which serves as a starting point for cracking during high-speed deformation, and rather reduces the absorbed energy during impact bending deformation. Therefore, the upper limit is 0.0040%, preferably 0.0035%.
And

The steel sheet of the present invention comprises the above basic components, the balance of Fe and unavoidable impurities, but does not hinder the content of other elements as long as the effects of the basic components are not impaired. Further, an element for further improving the properties of the steel sheet can be contained. As such an element, any one or more of Si, Cr, Mo, V, W, Cu, and Ni can be contained in an amount of 1.0% or less.

These elements convert the microstructure of the quenched part into bainite to improve the ductility, contribute to the prevention of cracking, and can secure the necessary quench strength. It also contributes to the improvement of delayed fracture characteristics. In order to exhibit this effect effectively, 0.0
A content of 5% or more is preferred. On the other hand, if too much is added, S
For i, Cr, Mo, V, and W, the chemical conversion property deteriorates,
Since u and Ni cause hot cracking and surface flaws due to scale, the upper limits are each set to 1.0%, preferably 0.60%. The microstructure does not necessarily need to be a single phase of bainite, and may include ferrite, carbide, and the like. Further, these elements cannot be used as basic elements for improving hardenability. The reason is that if the hardenability is improved with these elements, the chemical conversion property deteriorates or quenching occurs during steel sheet manufacturing, making it difficult to ensure the workability of the material steel sheet before induction hardening strengthening. It is.

The steel sheet for induction hardening is produced by smelting steel of a predetermined component and subjecting the steel to hot rolling or cold rolling in a conventional manner. The hot-rolled or cold-rolled steel sheet or the hot-dip galvanized steel sheet after cold-rolling has a ferrite and pearlite structure, and the tensile strength before induction hardening is about 500 MPa or less, so that press forming is easy. Thus, it can be easily formed into a predetermined member shape. After the molding, induction hardening is performed on a portion (including the entire region of the member) where the strength is to be improved, whereby the induction hardened member of the present invention can be obtained. In addition, as a cooling method after quenching, an appropriate method such as water cooling, mist cooling, air / water cooling, air cooling (including forced air cooling), contact with a cooling mold, or the like can be adopted according to the plate thickness.

In a carbon steel sheet having a C content specified in the present invention, usually, in order to obtain martensite by quenching, the austenite grain size before quenching is increased, thereby improving the hardenability. Therefore, the quenching temperature is set to be higher than 1000 ° C., but in the present invention, the quenching temperature can be set to a relatively low temperature by the effect of improving the hardenability of B. It can be carried out at a relatively low temperature of preferably 950 ° C. or lower, more preferably 900 ° C. or lower. By setting the quenching temperature to such a temperature, the old austenite grain size observed after quenching can be reduced to 20 μm or less, together with the effect of suppressing the growth of austenite grains due to the precipitation of B at the grain boundary. By setting the prior austenite particle size to 20 μm or less, preferably 15 μm or less, the static-dynamic ratio of the quenched portion can be improved,
As a result, the shock absorption characteristics of the induction hardening member can be improved.

FIG. 1 is a stress-strain diagram for explaining the relationship between the static-dynamic ratio and the shock absorption energy. In FIG. 1, A indicates the stress-strain curve in the case of low-speed tension at a tensile speed of about 2 mm / sec. And B is a stress-strain line of high-speed tension assuming a collision at a tensile speed of about 10 m / sec. The static-dynamic ratio is represented by the ratio σB / σA of the maximum stress σB of B to the maximum stress σA of A. On the other hand, the region surrounded by the stress-strain line (the region indicated by the hatched portion with respect to the stress-strain line A) indicates the shock absorption energy at the time of deformation. As is clear from the figure, the larger the static-dynamic ratio, the larger the impact absorption energy during high-speed deformation. In high-tensile steel sheets such as high-tensile steels, the static-dynamic ratio tends to approach 1 as the strength increases, and it cannot be said that merely increasing the steel sheet strength does not necessarily mean that the shock absorbing properties are advantageous. As is clear from the examples, while the strength is improved by the induction hardening, the static-dynamic ratio is also improved, and excellent shock absorbing characteristics are provided.

The steel sheet of the present invention may be subjected to a hot-dip galvanizing treatment after cold rolling to obtain a hot-dip galvanized steel sheet. Of course, an alloying heat treatment may be performed after galvanizing to form an alloyed galvanized steel sheet.

The induction hardening strengthening member of the present invention can also be obtained by press-molding such a hot-dip galvanized steel sheet into a predetermined shape and performing induction hardening on a portion to be strengthened. In this case, if the quenching temperature is too high, zinc evaporates during quenching, the galvanized layer disappears, and an oxide film may be formed on the surface of the steel sheet. When the galvanized layer disappears and an oxide film is formed, when coating the surface of the member, the phosphate film, which is the base material of the coating, becomes difficult to adhere to, and the adhesion of the film deteriorates. In the present invention, the quenching temperature can be lowered by the action of B,
A quenching temperature of 1000 ° C. or less, preferably 950 ° C. or less,
More preferably, by setting the temperature to 900 ° C. or lower, the disappearance of the galvanized layer can be prevented, and good coating adhesion can be secured. Further, the heat cycle time (see FIG. 11) from the start of heating at the time of quenching to the quenching temperature to cooling to 350 ° C. is 60 seconds or less, preferably 30 seconds or less, more preferably 10 seconds.
By setting it as follows, excessive alloying of the hot-dip galvanized layer can be suppressed, so that deterioration of the corrosion resistance of the hot-dip galvanized layer can be prevented. Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not construed as being limited to such Examples.

[0031]

[Example A] Based on the following steels A and B, steels with various proportions of Ti added to the base steel are melted, and the slab is hot-rolled by a conventional method (finish temperature). 8
70 ° C, winding temperature 650 ° C), cold rolling (cold rolling rate 55
%, A recrystallization annealing temperature of 720 ° C.) to produce a cold-rolled steel sheet having a thickness of 1.6 mm, produce an impact three-point bending test member shown in FIG. 2, and measured the impact absorption energy by an impact three-point bending test. . -Steel A (mass%, balance substantially Fe) C: 0.12%, Mn: 1.49%, P: 0.013
%, S: 0.005%, Al: 0.043%, N: 0.
041%, B: 0.0029% B steel (mass%, substantially Fe) C, Mn, P, S, Al are the same as A steel. N: 0.033%, B: 0.0055%

As shown in FIG. 2, the test member 1 has a flat plate 3 attached to an opening of a molded member 2 having a hat-shaped cross section, and the flange portion of the molded member 2 is spotted at a pitch of 40 mm as shown in the figure. Welded. In the figure, the dimensional unit is mm, and hatched portions provided at upper corners (two places) of the hat-shaped member 2 indicate hardened portions formed by induction hardening after assembling the test members. The quenching conditions were a heat cycle time of about 5 seconds, high-frequency heating to 900 ° C., and water cooling, and the quenched portion had a martensite structure.

In the impact three-point bending test, as shown in FIG. 3, the test member 1 is placed with the hat-formed member 2 down, and both ends of the test member 1 (load interval 500 mm).
With a load P of 100 kg applied to each symmetrical position and horizontally held, the center of the test member 1 in the longitudinal direction was caused to collide with a bending jig at 10.6 m / sec. It measures energy. The impact absorbing energy is calculated by bending jig 10 (upper radius = 150 mm)
Laser displacement meter 11 provided 200 mm away
As a result, the displacement at the moment when the test member 1 comes into contact with the bending jig 10 (the state indicated by the two-dot chain line in the figure) is set to 0, the test member 1 is bent and deformed, and the displacement measured by the laser displacement meter 11 is changed. Was determined by measuring the load applied to the bending jig 10 until the value became 70 mm. FIG. 4 is a diagram schematically showing the relationship between the displacement and the load, and the area of the hatched portion in the figure indicates the absorbed energy value. The load applied to the jig 10 was measured by a load cell 12 to which the bending jig 10 was attached.

FIG. 5 shows the results of the impact three-point bending test. From the figure, in the high content region where the Ti content is more than 0.015%, coarse TiN is generated in both the steel sheets using steel A and steel B, and the absorbed energy value in impact three-point bending is low. In the case of using steel B exceeding the range of the present invention, cracks were observed in the quenched portion. On the other hand, particularly in the low content region where the Ti content is 0.010% or less, the steel sheet using the steel A in the component range of the present invention has a much larger absorbed energy value than the steel sheet using the steel B outside the invention range. It can be seen that excellent impact resistance was obtained.

[Example B] The steels shown in Table 1 below were melted, and the slabs were prepared according to the production conditions shown in the same table by cold-rolled steel sheets (sheet thickness 1.6 mm) and alloyed hot-dip galvanized steel sheets (sheet thickness). 1.6 mm) and a hot-rolled steel sheet (sheet thickness 2.0 mm) were manufactured, and the mechanical properties were measured. The results are shown in Table 1.

[0036]

[Table 1]

Further, a test piece was sampled from a sample steel plate, a predetermined region was heated at a high frequency at 900 ° C., immediately after the temperature reached, the heating was stopped, and quenching was performed by cooling under the cooling conditions shown in Table 2. The hardness distribution around the quenched part was examined to determine the average hardness of the quenched part and the microstructure was examined. Further, similarly to Example A, an impact three-point bending test member was manufactured.
After quenching the same portion under the quenching conditions shown in Table 2, the impact three-point bending test was performed to measure the impact absorption energy, and the occurrence of cracks in the bent portion after the test was observed. Table 2 shows the results. The microstructure in the table is 50% in area ratio
The structure occupying the above is shown, with the balance being ferrite and / or retained austenite. In addition, sample Nos.
FIG. 6 is a graph in which the effects of N and B on hardenability are arranged for 13 and 16. The number in the figure is “Sample No./
Average hardness (Hv) of the quenched part ". 7 (Sample No. 13) and FIG. 8 (Sample No. 7) show examples of the hardness distribution measurement results around the quenched portion. Note that the quenched portion in FIGS. 7 and 8 corresponds to the dotted line position at the center of the quenched region (hatched region) in FIG.

[0038]

[Table 2]

As is clear from Table 2 and FIG. 6, in Sample Nos. 11 and 12 in which N is more than 0.010%, the hardness of the quenched portion is lower than 300 Hv, and sufficient quenching hardness is obtained. It can be seen that the reinforcement was insufficient. These samples also have low absorption energy. This is presumed to be due to insufficient BN decomposition during high-frequency heating due to an excessive amount of N.

Further, from Table 2 and FIG. 6, B is 0.00
Sample No. 16 with more than 4%, sample with B less than 0.0005%
In No. 13, the hardness of the quenched portion was good, but the absorbed energy was low, and cracks occurred in the bent portion. This is the sample
In No. 16, since the amount of B was excessive, Fe ₂ B
On the other hand, in Sample No. 13, the B content was too small to obtain sufficient hardenability. Incidentally, looking at the hardness distribution of Sample No. 13, as is clear from FIG. 7, the average hardness is 344 Hv, which is good, but the hardness of the quenched portion is uneven, and the strength is uneven when pulled. Therefore, it is presumed that cracks occurred due to the concentration of deformation in the low-strength portion. FIG. 8 shows Sample No. 7 of the invention example.
In this example, the average hardness of the quenched portion is 3
69Hv, and the hardness in the quenched part is also uniform.

From Table 2, it can be seen that Sample Nos. 17 to 20 (inventive examples) in which a characteristic improving element was added in addition to the basic components,
Since the microstructure is mainly composed of bainite, a further improvement in the absorbed energy is recognized.

Example C The steels shown in Table 3 below were melted, and the slabs were hot-rolled (finishing temperature 860 ° C., winding temperature 550 ° C.) and cold-rolled (cold rolling rate 60%, A cold-rolled steel sheet (sheet thickness 1.6 mm) was manufactured at a crystal annealing temperature of 700 ° C. Using the test steel sheet collected from this cold-rolled steel sheet, FIG.
As shown in Table 4, a test steel plate W is fed from a steel plate guide 21 between a high-frequency coil 22 and cooling nozzles 23, 23 arranged opposite to each other, and induction hardening is performed on the entire surface of the test steel plate under the hardening conditions shown in Table 4. Was given. The heat cycle time was about 3 seconds, and the sample was cooled immediately after reaching the quenching temperature.

[0043]

[Table 3]

A tensile test piece was sampled from the obtained induction hardened steel sheet, the maximum stress (static TS) was determined under low-speed tension (tensile speed 2 mm / sec), and the high-speed tensile (tensile speed 10 m / sec) was obtained. ), The maximum stress (dynamic TS) was determined, and the static-dynamic ratio was determined. The stress was calculated from the average load measured by attaching strain gauges on both sides of the test piece. In addition, a microstructure observation piece was collected from the induction hardened steel sheet, and traces of the prior austenite crystal grain boundaries before quenching were observed with a microscope, and the average crystal grain size was measured. The results of these investigations are shown in Table 4. FIG. 10 shows a graph in which the relationship between the static-dynamic ratio and the prior austenite grain size is arranged. Table 4 also shows the tensile strength before quenching.

[0045]

[Table 4]

As shown in Table 4 and FIG. 10, Sample Nos. 31 to 37 using the invention steel had a prior austenite grain size of 20 μm.
It was as follows, and the static-dynamic ratio was a higher value than the comparative example, and it was inferred that the shock-absorbing property was excellent.

Further, using steel types A to C (inventive steels) shown in Table 3, after cold rolling to a sheet thickness of 1.6 mm, hot-dip galvanizing of 45 g / m ^{2 was performed on} both sides in a continuous hot-dip galvanizing line. And a three-point impact test member 1 in the same manner as in Example A.
Was manufactured, and induction hardening was performed on the same portion under the conditions shown in Table 5 below. The heat cycle time was about 3 seconds, and the sample was cooled immediately after reaching the quenching temperature.

After induction hardening, the presence or absence of a galvanized layer in the hardened portion of the test member was observed. Further, a coating film peeling test was performed in the following manner, and the presence or absence of peeling of the coating film formed on the quenched portion was examined. The test member is degreased, rinsed and dried, immersed in a phosphating solution at 40 ° C for 2 minutes to form a phosphate film on the quenched part, rinsed and dried, and then electrocoated with a film having a thickness of about 20 μm. Formed. After drying, a square of 1 mm pitch was put into a 10 × 10 mm ² test area with a cutter knife,
After immersion in pure water for 240 hrs and drying, an adhesive tape was stuck to the test area and peeled off, and the coating film was peeled off at 50% or more in a square of 1 mm square, and there was peeling even at one place (× ).

[0049]

[Table 5]

As shown in Table 5, when the invention steel was used and the quenching temperature was 10
Induction hardened at a temperature of 00 ° C or less (Sample No. 5
In Examples 1 to 55, 57, and 58), it was confirmed that the hot-dip galvanized layer remained and the adhesion of the coating film was excellent.

Example D A steel having the following composition was melted, and the continuously cast slab was hot-rolled to a thickness of 4.0 mm (finishing temperature 87).
0 ° C, coiling temperature 660 ° C), cold-rolled to a thickness of 2.0 mm (cold rolling rate 50%), and subjected to recrystallization annealing at 720 ° C in a continuous annealing hot-dip galvanizing line. Then, hot dip galvanizing (amount of plating: 45 g / m ^{2 on} both sides) was performed at ℃, followed by alloying treatment at 690 ° C. × 7 sec. A test steel sheet (2.0 t × 40 w × 300 L in mm) was sampled from the obtained galvannealed steel sheet, and quenched using the induction hardening apparatus shown in FIG. -Steel plate component (mass%, balance substantially Fe) C: 0.13%, Mn: 1.98%, P: 0.013
%, S: 0.012%, Al: 0.041%, Ti <
0.01%, N: 0.004%, B: 0.0037%

The feed rate of the steel sheet was adjusted so that the heat cycle time was about 3 seconds. After quenching at various quenching temperatures, the water-cooled test steel sheet was examined for the presence or absence of the plating layer and the presence of the plating layer. The Fe content was examined. FIG. 12 is a graph showing the relationship between the quenching temperature and the Fe content. From FIG. 12, it was confirmed that the plating layer remained when the quenching temperature was 1000 ° C. or lower. In addition, F in the plating layer
It was also confirmed that the amount of e was about 25% or less.

Next, the quenching temperature was set to 700 ° C., 800 ° C., 9
Induction quenching was performed under various heat cycle times by adjusting the cooling rate after heating to 00 ° C and 1000 ° C. FIG. 13 is a graph in which the Fe content in the plating layer of the obtained steel sheet was examined, and the relationship between the heat cycle time and the Fe content was arranged. FIG. 13 shows that when the heat cycle time is 60 seconds or less, the Fe in the plating layer is 35%.
% Or less was confirmed.

Further, corrosion test pieces (2.0 t × 70 w × 150 L in mm) were sampled from the test steel sheets subjected to induction hardening at various heat cycle times and subjected to a corrosion test. The corrosion test was carried out by measuring the maximum drilling depth after 170 cycles with the following process as one cycle according to the JASO automotive material corrosion method.
The result is shown in FIG. -One cycle process Salt water (35 ° C, 5% concentration) spray: 8 hr Drying (60 ° C, relative humidity 30%): 4 hr Wet (50 ° C, relative humidity 90%) Exposure: 2 hr From FIG. If the amount is 35% or less, the maximum drilling depth is about 500 μm or less,
% Or less, it stayed at about 200 μm, and it was confirmed that it had corrosion resistance that was not problematic in practical use.

According to the present invention, since Ti, N, and B are restricted to specific ranges, hardenability is improved by free B generated by decomposition of BN during high-frequency heating, and hardenability is improved. Prevents the growth of austenite grains and strengthens the grain boundaries at the same time, so the mechanical properties of the quenched part are uniform, not only the strength but also the toughness, the static-dynamic ratio is improved, and excellent impact resistance is obtained. Can be

[Brief description of the drawings]

FIG. 1 shows stress-strain diagrams at a time of low-speed deformation A and at a time of high-speed deformation B.

FIG. 2 is a perspective view of a test member used for an impact three-point bending test.

FIG. 3 is an explanatory diagram of a procedure of a three-point impact bending test.

FIG. 4 is a displacement-load diagram schematically showing the results of an impact three-point bending test.

FIG. 5 is a graph showing the relationship between the amount of Ti and impact three-point bending absorption energy according to Example A.

FIG. 6 is a graph showing the influence of the amounts of N and B on the hardness of a quenched portion according to Example B.

FIG. 7 is a graph showing a hardness distribution near a quenched portion of Sample No. 13 of Example B.

FIG. 8 is a graph showing a hardness distribution near a quenched portion of Sample No. 7 of Example B.

FIG. 9 is a conceptual diagram showing a method of quenching a steel sheet in Example C.

FIG. 10 is a graph showing a relationship between a static-dynamic ratio and a prior-austenite grain size in Example C.

FIG. 11 is an explanatory diagram of a heat cycle time in induction hardening.

FIG. 12 shows the quenching temperature and F in the plating layer in Example D.
It is a graph which shows the relationship with e content.

FIG. 13 is a graph showing a relationship between a heat cycle time and an Fe content in a plating layer in Example D.

FIG. 14 is a graph showing the relationship between the Fe content in a plating layer and the maximum hole depth in a corrosion test in Example D.

Claims (7)

[Claims] 1. mass%, C: 0.05 to 0.20%,
Mn: 0.3-2.5%, P: 0.02% or less, S:
0.02% or less, Al: 0.06% or less, Ti: 0.0
15% or less, N: 0.010% or less, B: 0.0005
A steel sheet for induction hardening which contains about 0.0040% and is excellent in toughness in a hardened part consisting of the balance of Fe and unavoidable impurities. 2. The method according to claim 1, further comprising 1.0% or less of at least one of Si, Cr, Mo, V, W, Cu, and Ni, in addition to the components described in claim 1. Induction hardened steel sheet with excellent toughness in the quenched part. 3. An induction hardening strengthening member formed of the steel sheet for induction hardening according to claim 1 or 2, wherein a part for improving strength is subjected to induction hardening. 4. A hot-dip galvanized steel sheet comprising the steel sheet for induction hardening according to claim 1 or 2 as a base plate, a part for improving strength is subjected to induction hardening, and a plated layer is formed on the hardened part. The induction hardening strengthening member which remains. 5. The induction hardening member according to claim 3, wherein the prior austenite grain size before quenching observed in the quenched portion is 20 μm or less. 6. The induction hardening steel sheet according to claim 1 or 2 is formed into a predetermined shape, and induction hardening is performed at a quenching temperature of not less than ₃ points and not more than 1000 ° C. at a portion where strength is to be improved. A method for manufacturing an induction hardened reinforcing member. 7. A hot-dip galvanized steel sheet using the steel sheet for induction hardening according to claim 1 or 2 as a base plate is formed into a predetermined shape, and a portion where the strength is to be improved is at least _three points, and
Induction quenching at a quenching temperature of 0 ° C or less, and a heat cycle time of 60 seconds or less from the start of heating during quenching to the quenching temperature until cooling to 350 ° C. A method for manufacturing a reinforcing member.