JP2003277887A - Thin steel sheet for nitriding treatment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin steel sheet for nitriding treatment in which a high surface hardness and a sufficient hardening depth can be obtained after the nitriding treatment. <P>SOLUTION: The thin steel sheet comprises, by mass, >0.01 to 0.09% C, 0.005 to 0.5% Si, 0.01 to 3.0% Mn, 0.005 to 2.0% Al, 0.50 to 4.0% Cr, ≤0.10% P, ≤0.01% S and ≤0.010% N, and the balance Fe with inevitable impurities. Also, the grain boundary area per unit volume Sv is controlled to the range of 80 to 1,300 mm<SP>-1</SP>. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to parts that require wear resistance, fatigue strength and seizure resistance, such as tools, parts for machine structures and parts for automobiles. The present invention relates to a nitriding steel plate suitable for use as an additional component.

[0002]

2. Description of the Related Art Tools, parts for machine structures, parts for automobiles, etc. are required to have wear resistance, fatigue resistance and seizure resistance.
A typical example of such surface hardening treatment is nitriding treatment.
This is a process of invading nitrogen into steel, and has been widely used since it has excellent surface hardening ability.

Steel used for parts that are subjected to nitriding treatment is
For example, as described in JP-A-59-31850 and JP-A-59-50168, since a large amount of nitriding-promoting elements are contained, the steel sheet before nitriding treatment has high strength, but the workability is high. It was poor. Therefore, in the case of manufacturing a product having a complicated part shape, it is general that a bulk is ground into a predetermined shape and then a nitriding treatment is performed.

However, in order to finish a complicated part shape by grinding, the cost required for grinding increases, so it has been considered to manufacture parts by press forming such as deep drawing or stretch forming. That is,
If forming is performed using a steel plate with excellent press formability,
Molding is possible even with complicated parts shapes, and the time and cost required for forming parts can be greatly reduced.

However, when abrasion resistance and fatigue resistance are strongly required, sufficient surface hardness can be obtained even if a conventionally known steel plate such as low carbon steel or ultra low carbon steel is applied. There was a problem that could not be obtained.

[0006]

As described above, it is possible to obtain a compact by plastic working such as pressing or bending, even if a conventional thin steel sheet is used. The surface hardness and the distribution of the hardening depth were not sufficient, and it was not possible to satisfy the required properties such as desired wear resistance and fatigue strength. Therefore, it has been earnestly desired to develop a thin steel sheet for nitriding treatment that can be formed by a simple forming method such as pressing or bending and that can obtain sufficient surface hardness and hardening depth after nitriding treatment.

The present invention advantageously meets the above-mentioned demands, and an object of the present invention is to propose a thin steel sheet for nitriding treatment which can obtain a sufficient surface hardening ability and hardening depth by nitriding treatment.

[0008]

[Means for Solving the Problems] As a result of intensive studies on the effects of the steel composition and the steel structure on the hardness after nitriding treatment, the inventors have found that Cr, Al, V, Ti ， Nb
By including such a nitride-forming element in the range that does not impair the formability of the steel sheet, and controlling the grain boundary area per unit volume within a predetermined range, high surface hardness and sufficient hardening depth after nitriding treatment We obtained a new finding that both of them can be obtained together.

The present invention is based on the above findings. That is, the present invention, in% by mass, C: more than 0.01%, 0.
09% or less, Si: 0.005-0.5%, Mn: 0.01-3.0%, A
l: 0.005-2.0%, Cr: 0.50-4.0%, P: 0.10% or less, S: 0.01% or less and N: 0.010% or less,
The balance is a composition of Fe and unavoidable impurities, and the grain boundary area Sv per unit volume is 80 mm ^-1 or more and 1300 mm ^-1 or less, which is a thin steel sheet for nitriding treatment.

Further, in the present invention, in the steel sheet, V: 0.01 to 1.0% Ti: 0.01 to 1.0% and Nb: 0.
One kind or two kinds or more selected from 01 to 1.0% can be contained.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, the reason why the composition of the steel sheet is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, "%" display regarding components means mass%. C: more than 0.01% and 0.09% or less C is an element that affects the formability, and if the content increases, the formability decreases. Therefore, the upper limit of the amount of C is 0.09
%. On the other hand, if the C content is 0.01% or less, the strength of the steel for machine structural use is insufficient, so the lower limit of the C content is set to more than 0.01% from the viewpoint of securing the strength.

Si: 0.005 to 0.5% Si is an element having a function of increasing strength. Excessive addition not only causes deterioration of workability, but also increases surface oxidation and inhibits nitriding reaction. Therefore, the upper limit of Si is
It was set to 0.5%. On the other hand, if the amount of Si is less than 0.005%, the steelmaking cost will increase, so the lower limit was made 0.005%.

Mn: 0.01-3.0% Mn is an element having an action of enhancing the strength of steel, like Si, but excessive addition not only lowers the workability but also inhibits the nitriding reaction due to surface oxidation. Therefore, the upper limit of Mn is set to 3.0%. On the other hand, if the Mn content is less than 0.01%, the steelmaking cost will increase, so the lower limit was made 0.01%.

Al: 0.005 to 2.0% Al effectively contributes as a deoxidizing agent for molten steel, and it is necessary to add 0.005% or more in order to prevent the occurrence of defects such as blowholes. On the other hand, excessive addition exceeding 2.0% deteriorates workability, so the upper limit was made 2.0%. Note that Al has a strong affinity with nitrogen and also has an effect of increasing the hardness of the surface compound layer formed after the nitriding treatment, but in order to effectively exhibit this effect, it is preferable to add 0.10% or more.

Cr: 0.50 to 4.0% Cr is an important element for nitriding hardening, and if the content is less than 0.50%, sufficient hardness increase and hardening depth cannot be obtained after nitriding treatment. On the other hand, if the content is more than 4.0%, the moldability is lowered. Therefore, the Cr content is limited to the range of 0.50 to 4.0%.

P: 0.10% or less P is an element that improves strength without lowering workability, but if the content exceeds 0.10%, it is not preferable from the viewpoint of formability and secondary work embrittlement of the steel sheet. Therefore, P
Was contained at 0.10% or less. In addition, P is 0.0
If it is less than 01%, steelmaking cost will increase dramatically,
Since it is economically disadvantageous, it is preferable to contain P in an amount of 0.001% or more, but 0.001 if there is no problem in steelmaking cost.
It may be less than%.

S: 0.01% or less If the S content exceeds 0.01%, the occurrence of surface flaws and the decrease in ductility will occur, so the S content should be suppressed to 0.01% or less.

N: 0.010% or less N is preferably as small as possible in order to secure deep drawability.
If it exceeds 010%, the deep drawability deteriorates. Therefore, N is
The limit is 0.010% or less.

Although the essential components have been described above, other elements described below can be appropriately contained in the present invention. V: 0.01 to 1.0% V is a nitride-forming element and has an effect of increasing the amount of hardening after the nitriding treatment. Further, V has a function of promoting diffusion of nitrogen, and is also a useful element in increasing the hardening depth after the nitriding treatment. In order to exert these effects, it is preferable to contain 0.01% or more. However, if the V content exceeds 1.0%, the formability deteriorates, so the upper limit is 1.0%.
And Further, since V is also a carbide-forming element, when it is contained in an excessive amount with respect to the amount of C, the amount of C that forms a solid solution in the grain boundaries is reduced, resulting in a decrease in grain boundary strength, and slab cracking during hot rolling and The toughness of the nitriding product may be deteriorated. Therefore, V is represented by the following formula V <51 / 12C. However, V and C are more preferably contained within the range of the content (mass%) of each element.

Ti: 0.01 to 1.0% Ti is also a nitride-forming element, and has the effect of increasing the amount of hardening after nitriding treatment. If the steel contains Ti, the surface hardness is shortened by nitriding treatment for a short time. The height can be increased. To obtain this effect, it is preferable to add 0.01% or more.
If the amount exceeds 1.0%, the formability deteriorates, so Ti is 0.01
It is preferable that the content is in the range of 1.0%. Also, Ti
Is a carbide-forming element similar to V, and when contained in excess with respect to the amount of C, it reduces the amount of C that forms a solid solution in the grain boundaries and causes a decrease in grain boundary strength. It may cause cracking or decrease in toughness of the nitrided product. Therefore, Ti
Is the following formula Ti <48 / 12C. However, Ti and C are more preferably contained in the range of the content (mass%) of each element.

Nb: 0.01 to 1.0% Nb is also a nitride-forming element and has the effect of increasing the amount of hardening after nitriding treatment. If Nb is contained in the steel, the surface hardness is shortened by nitriding treatment for a short time. The height can be increased. To obtain this effect, it is preferable to add 0.01% or more, but Nb
If the amount exceeds 1.0%, the formability deteriorates, so Nb is 0.01
It is preferable that the content is in the range of 1.0%. Also, Nb
Also, like V and Ti, it is a carbide-forming element, and if it is contained in an excessive amount with respect to the amount of C, the amount of C that forms a solid solution in the grain boundaries is reduced, leading to a decrease in grain boundary strength, and during hot rolling. There is a risk of causing slab cracking and deterioration of the toughness of the nitrided product. Therefore,
Nb is represented by the following formula Nb <93 / 12C. However, it is more preferable that Nb and C be contained within the range of the content (mass%) of each element.

Other than the above-described elements, Fe and unavoidable impurities may be added. In addition to these, elements having the ability to increase the strength such as Cu, Ni and Mo may be added within a range that does not impair the workability. .

Next, the grain boundary area per unit volume Sv (mm
^-1 ) The reason for limitation is described. Now, as a result of intensive studies on the conditions under which the desired surface hardness and hardening depth can be obtained after the nitriding treatment, the inventors have found that it is important to control the grain boundary area per unit volume. It was FIG. 1 is a diagram showing the influence of the grain boundary area per unit volume on the surface hardness and the hardening depth after the nitriding treatment. The experimental results shown in FIG. 1 show that C: 0.055%, Si: 0.02%, Mn:
0.3%, P: 0.02%, S: 0.004%, N: 0.0035%, A
l: 0.13% and Cr: 1.3%, with the balance being Fe and unavoidable impurities in the steel ingot, hot rolling or hot rolling followed by cold rolling followed by recrystallization By carrying out the above, a thin steel plate with a plate thickness of 1.6 mm having various grain boundary areas per unit volume is manufactured, and this thin steel plate is subjected to an atmosphere gas containing RX and NH ₃ gas.
This is based on the results of examining the surface hardness and the hardening depth of a sample that was oil-cooled after being subjected to gas nitriding treatment at 70 ° C for 3 hours.

Here, the surface hardness means a hardness (Hv) obtained by measuring a depth position of 0.030 mm from the outermost layer in the plate thickness direction using a micro Vickers hardness meter. The hardening depth is defined as the distance (mm) from the outermost layer at the position where the Vickers hardness value at the center of the plate thickness is +50 Hv. Grain boundary area per unit volume Sv (mm
^-1 ) was changed by adjusting the reduction ratio and rolling temperature during hot rolling, the reduction ratio during cold rolling and the recrystallization annealing temperature during the production of the thin steel sheet.

The grain boundary area Sv per unit volume was measured by determining the average grain section length L (mm) from the metal structure observed by an optical microscope by the cutting method. This method is a method of drawing a straight line of a known length on an optical micrograph and calculating the number of ferrite grains intersecting the straight line. And the grain boundary area Sv (m
m ^-1 ) can be calculated from the average intercept length L (mm) by the following equation Sv = 2 / L. This formula is well known as a formula for calculating the grain boundary area per unit volume, and it is also known that it is not affected by the shape and distribution of crystal grains at all.

From FIG. 1, the grain boundary area Sv per unit volume is
It can be seen that the surface hardness tends to increase and the curing depth tends to decrease as the value increases. Further, it is understood that the grain boundary area Sv per unit volume needs to be 80 mm ^-1 or more and 1300 mm ^-1 or less in order to achieve both surface hardness and hardening depth. Grain boundary area Sv per unit volume is 80mm
^When it is less than ^-1 , although the hardening depth is deep, the surface hardness is low. On the other hand, when the grain boundary area Sv per unit volume exceeds 1300 mm ^-1 , the surface hardness is extremely high, but the hardening depth becomes shallow.

Diffusion of N and C in the plate thickness direction at the time of gas nitrocarburizing occurs mainly along the crystal grain boundaries, and the fine carbonitrides precipitated along the crystal grain boundaries have different hardness. It is considered that as the grain boundary area per unit volume increases, the number of fine carbonitrides precipitates and the surface hardness increases as the grain boundary area contributes to the increase. This means that, on the other hand, the diffusion of N and C toward the central portion in the plate thickness direction becomes slower, and it is considered that the hardening depth becomes shallower as the grain boundary area per unit volume increases.

Next, FIG. 2 shows a result of observing a crystal grain boundary at a depth position of 0.030 mm in the plate thickness direction from the outermost layer after nitriding treatment with a transmission electron microscope (TEM). Many precipitates were found near the grain boundaries, and these precipitates were
It was confirmed from the analysis results by EELS and EDX that it was a nitride of Al or Al, a composite nitride, a carbonitride, or a composite carbonitride. From the above observation results, it can be seen that the diffusion of nitrogen and carbon in the plate thickness direction during the nitriding treatment mainly occurs along the crystal grain boundaries. In our investigation, if these precipitates were fine, the hardness increased, especially at 300 nm.
It was confirmed that the hardness was sufficiently increased when the grain size was reduced to the following.

The grain boundary area per unit volume can be adjusted to 80 mm by adjusting the rolling reduction rate and rolling temperature during hot rolling, the rolling reduction rate during cold rolling and the recrystallization annealing temperature during the production of the thin steel sheet. Adjustable from ^-1 or more to 1300mm ^-1 .

[0030]

Example 1 Steels having the chemical compositions shown in Table 1 were melted, continuously cast into slabs, hot-rolled and pickled to obtain hot-rolled steel sheets. At this time, the slab heating temperature was 1180 ° C, the hot rolling finishing temperature was 870 ° C or higher, and the coiling temperature was 590 ° C. After performing plastic working by pressing on the obtained hot rolled steel sheet,
Nitriding was performed by the gas soft nitriding method. The nitriding treatment was performed by performing treatment at 580 ° C. for 3 hours in a mixed gas atmosphere containing RX and NH ₃ and then cooling with oil. After the nitriding treatment, the Vickers hardness (test force: 0.9807N) at the position in the plate thickness direction was measured to investigate the surface hardness and the hardening depth. The surface hardness is evaluated by the Vickers hardness at a depth of 0.030 mm from the outermost layer in the plate thickness direction, while the hardening depth is 50 Hv higher than the hardness at the center of the plate thickness. The position was evaluated by the distance from the surface layer.

Regarding the hot-rolled steel sheet before nitriding, the grain boundary area Sv (mm ^-1 ) per unit volume and the yield point YP (MP
a), tensile strength TS (MPa) and elongation El (%) were determined. The grain boundary area Sv (mm ^-1 ) per unit volume was determined by the method described above from the results of observation with an optical microscope. Yield point YP (MP
a), tensile strength TS (MPa) and elongation El (%), JIS No. 5 test piece is taken from the hot rolled steel sheet, and the tensile test is performed according to the metal material tensile test method described in JIS Z 2241. It was calculated from the results. Table 2 shows the investigation results of the grain boundary area Sv, the yield point YP, the tensile strength TS, and the elongation El of the hot rolled steel sheet before the nitriding treatment, and the measurement results of the surface hardness and the hardening depth after the nitriding treatment.

[0032]

[Table 1]

[0033]

[Table 2]

As shown in Table 2, the invention examples No. 1 to
7 is the composition, grain boundary area S per unit volume
Both v were within the range of the present invention, the surface hardness showed a high hardness of 650 Hv or more, and the hardening depth of 0.30 mm or more could be secured. On the other hand, No. 8 has a small amount of C, so T
Not only does S show a low value, but the grain boundary area Sv per unit volume is less than 80 mm ^-1 , and therefore the surface hardness is low. In addition, No. 9 has an excessively large amount of Ti, so the hardening depth is shallow. It is considered that N is diffused while TiN is being formed during the nitriding treatment, but it takes time to form TiN and that the precipitated TiN hinders the diffusion so that the hardening depth becomes shallow. Further, No. 10 has a small Cr content, so that the surface hardness is small, and the hardening depth is shallow, and the hardening degree by nitriding treatment is small.

Example 2 Steels having the chemical compositions shown in Table 3 were melted and made into slabs by continuous casting, and then hot rolling was performed at a slab heating temperature of 1200 ° C and a finishing rolling temperature of 870 ° C or higher. Plate thickness: 3.5
mm hot rolled steel sheet. After pickling this hot-rolled steel sheet, the reduction rate:
After cold rolling at 60%, recrystallization annealing was performed at a temperature range of 800 to 950 ° C to obtain cold rolled steel sheets in which the grain boundary area Sv per unit volume was variously changed. The cold-rolled steel sheet thus obtained was subjected to plastic working by pressing and then subjected to nitriding treatment by a gas soft nitriding treatment method. The nitriding treatment is RX
Treatment was carried out at 570 ° C. for 4 hours in a mixed gas atmosphere containing NH ₃ and NH ₃ and then oil cooling was performed. After nitriding, Vickers hardness in the thickness direction (test force: 0.9807
N) was measured and surface hardness and hardening depth were investigated. The surface hardness and the curing depth were determined by the same method as described in Example 1 above. Further, the grain boundary area Sv (mm ⁻¹ ) per unit volume of the cold rolled steel sheet before nitriding treatment was determined in the same manner as in the case of Example 1 described above. Table 4 shows the measurement results of the annealing temperature during the production of the cold rolled steel sheet, the grain boundary area Sv of the cold rolled steel sheet before the nitriding treatment, the surface hardness after the nitriding treatment, and the hardening depth.

[0036]

[Table 3]

[0037]

[Table 4]

As shown in Table 4, Nos. 11, 12, 15, 16
In all cases, since the annealing temperature is appropriate, the grain boundary area per unit volume is within the range of the present invention, and both the surface hardness and the hardening depth show satisfactory values. On the other hand, in No. 13, the annealing temperature was too high and the crystal grains were coarsened, and the grain boundary area per unit volume was small. Therefore, the surface hardness after nitriding is small. No.14 is
Since the annealing temperature was low, the grain boundary area per unit volume was large, resulting in a shallow hardening depth.

[0039]

As described above, according to the present invention, it is possible to easily process the shape of a component by press molding, and it is possible to obtain a high surface hardness and a sufficient hardening depth by the subsequent nitriding treatment. A thin steel plate for processing can be stably provided. Therefore, according to the present invention, it is possible to manufacture high-strength general structural parts, automobile parts, and the like having excellent dimensional accuracy, strength, and durability at low cost.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of grain boundary area Sv per unit volume on surface hardness and hardening depth.

[Fig. 2] 0.030 mm in the plate thickness direction from the outermost layer after nitriding
Transmission electron microscope (TE) of grain boundaries at the depth of
M) It is a photograph.

Claims (2)

[Claims] 1. By mass%, C: more than 0.01%, 0.09% or less, Si: 0.005-0.5%, Mn: 0.01-3.0%, Al: 0.005-2.0%, Cr: 0.50-4.0%, P: 0.10. % Or less, S: 0.01% or less and N: 0.010% or less, the balance is Fe and inevitable impurities, and the grain boundary area Sv per unit volume is 80 mm ^-1 or more, 1300
A thin steel sheet for nitriding treatment, characterized in that it is less than mm ^-1 . 2. The steel sheet according to claim 1, further comprising, in mass%, one or more selected from V: 0.01 to 1.0% Ti: 0.01 to 1.0% and Nb: 0.01 to 1.0%. A thin steel sheet for nitriding treatment, which has the following composition.