JPH0925513A - Method for manufacturing nitriding steel sheet with excellent formability - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To provide a method for producing a nitriding steel plate having good press formability for obtaining a tool, a machine structural part, and an automobile part having wear resistance, fatigue resistance and seizure resistance. To do. SOLUTION: Press molding that requires high molding is performed,
A method for producing a nitriding steel sheet having excellent formability, which is used for a component that is subjected to a nitriding treatment to obtain a hard surface layer, comprising:
The content is limited to 0.010% to 0.080%, the Cr content is limited to more than 0.15% to 5.00%, and as a nitride hardening element group, Ti: 0.010% or more and less than 4 C [%]. V;
Steel containing 0.010 to 1.00% of 1 type or 2 types, the balance of which is iron and unavoidable impurities, is melted, hot-rolled, and wound at 500 ° C. or higher to obtain a hot-rolled steel sheet. Or a cold rolling at a rolling reduction of 50% or more, followed by recrystallization annealing to obtain a cold rolled steel sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to nitriding which is excellent in formability and is used for parts requiring wear resistance, fatigue strength and seizure resistance such as tools, parts for machine structures and parts for automobiles. The present invention relates to a method for manufacturing a steel plate.

[0002]

2. Description of the Related Art Tools, parts for mechanical structures, parts for automobiles, and the like are required to have wear resistance, fatigue resistance, and seizure resistance. For this reason, a treatment method called nitriding has been used in which nitrogen is infiltrated into steel to produce parts having high surface hardness and internal hardness (excluding thin steel sheet molded products). Steels used for these parts (for example, JP-A-59-31850 and JP-A-59-50158) are high in strength and difficult to work since they contain a large amount of nitriding-promoting elements. A shape is formed by grinding, and after that, nitriding is performed to increase hardness.

[0003]

In such a conventional technique, it takes time and cost to grind to form a shape. Even if a steel such as free-cutting steel that is easy to grind is used, the forming method in which a shape is formed by grinding from a steel bar requires much labor and cost. So press work, bending work, etc.
If a forming method often used for steel sheets, particularly thin steel sheets, can be used, the cost for forming parts can be significantly reduced and the production efficiency can be greatly increased. Therefore, there has been a strong demand for a steel sheet that can be formed into a shape by an inexpensive forming method such as press working or bending and that has excellent nitriding property, that is, an increase in hardness due to nitriding. The present invention is intended to solve the above problems, and an object of the present invention is to provide a nitriding steel sheet having excellent formability, which can be used in a forming method such as press working or bending, and a manufacturing method.

[0004]

The features of the present invention are as follows: (1) C: 0.01 to less than 0.08% by weight, Si: 0.005 to 1.00%, Mn: 0 .010
~ 3.00%, P: 0.001 to 0.150%, N
: 0.0002 to 0.0100%, Cr: more than 0.15 to 5.00%, Al: more than 0.060 to 2.00%, and Ti: 0.010 as a nitride hardening element group.
% Or more, and less than 4 C [%], V: more than 0.10.
50% after hot rolling, containing 1.00% of 1 type or 2 types, the balance being steel consisting of iron and unavoidable impurities.
Winding at 0 ℃ or higher to form hot rolled steel sheet, or
After that, cold rolling is performed at a reduction rate of 50% or more, and after that, recrystallization annealing is performed to form a cold rolled steel sheet, which is a method for producing a nitriding steel sheet having excellent formability.

Hereinafter, the present invention will be described in detail. First,
The reasons for limiting the components of steel will be described below. C is
It is an element that affects the formability of steel, and if the content increases, the formability deteriorates. Further, a large content promotes deterioration of formability when other elements are added. Therefore 0.0
It is less than 8%. If it is less than 0.01%, the strength for mechanical structure is insufficient, so 0.01% is made the lower limit. As the nitriding promoting element group for enhancing the nitriding property, C is used.
There are r, Al, V, and Ti. If the added amount is too small, the nitriding property cannot be improved, so the lower limit is specified. If the added amount is too large, the steel cannot be practically used in terms of formability. Therefore, the upper limit is specified depending on the steel composition.

[0006] Cr is a very important element for nitriding hardening, and if the content is less than 0.15%, the amount of increase in hardness due to nitriding is small.
%, The moldability deteriorates, so the upper limit is 5.00%. Al and V are very important as hardening elements by nitriding together with Cr. Al is usually added as a deoxidizing component, and it is necessary to add 0.005% or more in order to prevent the occurrence of defects such as blowholes. Al,
When used as a deoxidizing component, the lower limit is 0.005%. Generally, since Al is contained up to about 0.060%, even if Al is not used for nitriding promotion,
It can contain up to 0.060%. Al has a strong affinity with nitrogen and is an element that makes the surface layer of the nitride layer extremely hard. When added to enhance the nitriding property, Al is
When the content is 0.060% or less, the amount of hardness increase due to nitriding is small, so the content exceeds 0.060%. Preferably 0.08
0% or more. Further, if it exceeds 2.00%, the formability deteriorates, so 2.00% is made the upper limit.

[0007] When Ti and V are added together with predetermined Cr and Al, the nitriding treatment significantly increases the hardness. T
i is a stronger nitride forming element than Cr and Al,
It is an element that strongly promotes nitriding even if the nitriding treatment time is short, and a surface-hardened layer can be obtained in a short treatment time.
If it is less than 0.010%, the amount of increase in hardness due to nitriding is small, so 0.010% is made the lower limit. Further, Ti is a strong carbide-forming element, and if it is 4 times or more of the C content (C [%]), all the carbon in the steel becomes coarse precipitates, weakening the adhesive force between crystal grains, and during casting. And slab cracking during hot rolling becomes very easy to occur. Therefore, the upper limit is less than 4 C [%].

V accelerates the diffusion of nitrogen and allows nitrogen to penetrate into the inside of the steel, so that a thick nitride layer can be obtained on the surface of the steel. If it is less than 0.010%, the hardness increase due to nitriding is small, so 0.010% is the lower limit, and 1.0
If it exceeds 0%, the formability will deteriorate, so 1.00%
Is the upper limit. V is a carbide-forming element, and carbon in the steel is used as a precipitate to weaken the adhesive force between crystal grains,
Less prone to slab cracking. Therefore, it is desirable to add C in an amount of 5.65 times (5.65 C [%]) or less.

The above are the basic important components for improving the nitriding property of the steel material used in the present invention, but in order to secure the formability as a steel sheet, elements can be contained within the following ranges. If Si is less than 0.005%, the manufacturing cost will increase dramatically and it will be uneconomical. Therefore, the lower limit is 0.005%, and if it exceeds 1.00%, high formability cannot be obtained. % Is the upper limit. Mn is 0.0
If it is less than 10%, the manufacturing cost will increase dramatically and it will be uneconomical. Therefore, the lower limit is 0.010%, and if it exceeds 3.00%, high moldability cannot be obtained.
Is the upper limit.

P is an element capable of increasing the strength without impairing the formability, and is added according to the strength level.
If it is less than 01%, the manufacturing cost will increase dramatically and it will become uneconomical.
If it exceeds 0%, a problem of secondary working brittleness occurs, so 0.150% is made the upper limit. The N content is preferably as small as possible in order to secure the moldability, but if it is less than 0.0002%, the manufacturing cost will increase dramatically and it will become uneconomical.
The lower limit is 0.0002%, and if it exceeds 0.0100%, the formability deteriorates, so 0.0100% is the upper limit.

Next, the manufacturing method will be described. Steel having the above chemical composition is melted and hot rolled. The heating and rolling conditions at this time are not specified, and the effects of the present invention can be sufficiently obtained even if the heating before rolling is not performed, provided that the conditions after the winding are within the scope of the present invention. If the coiling temperature after hot rolling is less than 500 ° C., precipitation of carbides, nitrides or carbonitrides is not sufficient, and sufficient workability cannot be obtained in the final product, so the temperature is set to 500 ° C. or higher. When severe workability is required, it is necessary to control the morphology of carbides, nitrides or carbonitrides, and for that purpose, it is more desirable to set the coiling temperature to 600 ° C or higher.

When further cold rolling is performed,
By applying a reduction rate of 50% or more and then performing recrystallization annealing, workability can be improved, and when nitriding treatment is performed, fine precipitation that can serve as nuclei for forming nitrides or carbonitrides can be obtained. Since a large number of substances are present in the grain boundaries and in the grains, the steel sheet can be sufficiently hardened by the nitriding treatment, and thus desired wear resistance, fatigue strength, and seizure resistance can be obtained. If the cold rolling rate is increased to 70% or more, the cause is not clear, but the form of the nitriding hardened layer in the cold rolled steel sheet is achieved in a short time, and the workability is further improved without being impaired, which is more desirable.

In order to form a cold rolled steel sheet, it is sufficient to carry out recrystallization annealing in a usual manner after cold rolling. The method can be either box annealing or continuous annealing. The annealing condition is not particularly specified, but it is preferably performed at a recrystallization temperature or higher and 900 ° C. or lower at which coarse grains are not formed. It may be pickled after winding and made into a hot rolled steel sheet. Further, in order to improve the workability and improve the appearance after processing, the effect of the present invention is lost even if temper rolling, coating oil, application of solid lubricating oil, etc. is performed according to desired characteristics. Not a thing.

[0014]

【Example】

Example 1 The present invention will be specifically described below based on examples.
Steels having the components shown in Table 1 were melted and continuously cast into slabs according to a conventional method. Then, it was heated to 1200 ° C. in a heating furnace, hot-rolled at a finishing temperature of 910 ° C. or higher, wound at a winding temperature shown in Table 1, and then pickled to obtain a hot-rolled steel sheet. After cold rolling at the rolling reduction shown in Table 1, 8
Recrystallization annealing was performed at 00 ° C for 60 seconds to obtain a cold rolled steel sheet. The obtained hot-rolled steel sheet and cold-rolled steel sheet were cut out into a 60Φ disc (blank), punches of various diameters and dies were combined to form cup parts. The moldability was evaluated by the ratio of the blank diameter to the inner diameter of the cup bottom, which is the limit at which breakage occurs during molding (limit drawing ratio, LDR). Separately, a test piece was prepared and degreased, followed by nitriding treatment in a mixed atmosphere gas of NH ₃ gas and an endothermic gas at 570 ° C. for 4 hours and oil cooling. And using a micro Vickers hardness tester, 3 from the surface
The nitriding property was evaluated by the hardness (Hv) at the position of 0 μm. The results are shown in Table 1. As is clear from Table 1, when the comparative example having the same nitriding hardness and the inventive example are compared, it is understood that the inventive example has a larger limiting drawing ratio and is excellent in formability.

[0015]

[Table 1]

Example 2 Steels having the components shown in Table 2 were melted and continuously cast into a slab according to a conventional method. Then, it is heated to 1200 ° C. in a heating furnace, and hot-rolled at a finishing temperature of 910 ° C. or higher.
After winding at 00 ° C., pickling, cold rolling at a reduction rate of 80%, recrystallization annealing at 800 ° C. for 60 seconds was performed to obtain a cold rolled steel sheet. Using this cold-rolled steel sheet, a test of the easiness of forming a surface-hardened layer depending on the nitriding treatment time (rapidity of nitriding) was conducted. After the test piece was prepared, it was subjected to nitriding treatment in a mixed atmosphere gas of NH ₃ gas and an endothermic gas at 570 ° C. while changing the time, and cooled with oil. And the hardness (Hv) of the surface hardened layer was measured using the micro Vickers hardness meter. The nitriding treatment time required to obtain the hardness of the surface-hardened layer of Hv400 was determined, and the rapidity of nitriding was evaluated based on the ratio to the treatment time when Ti = 0%. The above results are summarized in FIG. As is clear from FIG. 1, in the steel containing Ti in an amount of 0.01% or more and less than 4C [%], a short time treatment for obtaining a surface hardened layer having the same hardness is superior in rapid nitriding. I understand.

[0017]

[Table 2]

Example 3 Steels having the components shown in Table 3 were melted and continuously cast into a slab according to a conventional method. Then, it is heated to 1200 ° C. in a heating furnace, and hot-rolled at a finishing temperature of 910 ° C. or higher.
After winding at 00 ° C., pickling, cold rolling at a reduction rate of 80%, recrystallization annealing at 800 ° C. for 60 seconds was performed to obtain a cold rolled steel sheet. Using this cold-rolled steel sheet, a test of the hardening depth of the surface hardening layer by the nitriding treatment was conducted. After the test piece was prepared, it was nitrided in a mixed atmosphere gas of NH ₃ gas and an endothermic gas at 570 ° C. for 4 hours and cooled with oil. Then, using a micro Vickers hardness meter, the hardness (H
v) was measured to determine the depth range from the surface where Hv400 was obtained. The depth (μm) was used as an index of the surface hardening depth. The above results are summarized in FIG. As is clear from FIG. 2, a deeper surface hardened layer was obtained in the steel to which V was added in an amount of 0.01% or more, and it is understood that the hardening depth characteristics of nitriding are excellent.

[0019]

[Table 3]

Example 4 Steels having the components shown in Table 4 were melted and continuously cast into a slab according to a conventional method. Then, it is heated to 1200 ° C. in a heating furnace, and hot-rolled at a finishing temperature of 910 ° C. or higher.
It was wound at 50 ° C. and then pickled to obtain a 1.6 mm thick hot rolled steel sheet. Separately, a slab having the same composition is heated to 1200 ° C. in a heating furnace, hot-rolled at a finishing temperature of 910 ° C. or higher, wound at 650 ° C., and then,
After pickling and cold rolling at a reduction rate of 80%, 8
Recrystallization annealing was performed at 00 ° C for 60 seconds to obtain a cold rolled steel sheet having a thickness of 1.0 mm. The obtained hot rolled steel sheet and cold rolled steel sheet are 60Φ.
Cut out into a disk (blank) and draw ratio 1.9, 2.0
The cup parts were press molded with. This part was subjected to a nitriding treatment at 570 ° C. × 4 hours in a mixed atmosphere gas of NH ₃ gas and endothermic gas, followed by oil cooling. Then, using a micro Vickers hardness meter, the nitriding property was evaluated by the hardness (Hv) at a position of 30 μm from the surface. Table 4 summarizes the above results. As is clear from Table 4, when the comparative example and the present invention example are compared, it is understood that the deep-drawn molded product of the present invention has excellent formability, a hard surface nitride layer is obtained, and nitriding property is excellent.

[0021]

[Table 4]

Example 5 Steels having the components shown in Table 5 were melted and continuously cast into slabs according to a conventional method. Then, it is heated to 1200 ° C. in a heating furnace and hot-rolled at a finishing temperature of 910 ° C. or higher.
After winding at 00 ° C., pickling, cold rolling at a reduction rate of 80%, recrystallization annealing at 800 ° C. for 60 seconds was performed to obtain a cold rolled steel sheet with a thickness of 1.2 mm. did. The obtained cold-rolled steel sheet was cut into a 60Φ disc (blank), and a cup-shaped deep-drawing formed body was press-formed at a drawing ratio of 2.0.
On the other hand, by cutting a steel slab from the same slab and grinding it,
A cup component of the same type was molded to prepare a comparative molded body. These compacts were subjected to nitriding treatment at 570 ° C. for 4 hours in a mixed atmosphere gas of NH ₃ gas and endothermic gas, and then cooled with oil. Then, using a micro Vickers hardness meter, the nitriding property was evaluated by the hardness (Hv) at a position of 30 μm from the surface. Table 5 summarizes the above results. As is clear from Table 5, when the comparative example and the example of the present invention are compared, it can be seen that the press-formed product of the present invention has a harder surface nitride layer and is superior in nitriding property.

[0023]

[Table 5]

Example 6 Steels having the components shown in Table 6 were melted and continuously cast into a slab according to a conventional method. Then, it is heated to 1250 ° C. in a heating furnace and hot-rolled at a finishing temperature of 910 ° C. or higher.
After winding at 30 ° C., pickling, cold rolling at a rolling reduction of 75%, recrystallization annealing at 780 ° C. × 40 seconds was performed to obtain a cold rolled steel sheet with a thickness of 1.8 mm. did. The obtained cold-rolled steel sheet was cut into a disk (blank) of 80Φ, and a cup-shaped deep-drawing formed body was press-formed with a drawing ratio of 2.0.
This part was subjected to a nitriding treatment at 570 ° C. × 4 hours in a mixed atmosphere gas of NH ₃ gas and endothermic gas, followed by oil cooling. Then, a 10 × 10 mm test piece was cut out from the bottom portion. Thus, a test piece having a hard nitride layer on both surfaces was prepared.
Further, during the nitriding treatment, the opening of some cup-shaped parts was sealed, and the inner surface was not exposed to a mixed atmosphere gas of NH ₃ gas and endothermic gas, and a hard nitride layer was formed only on the outer surface of the cup-shaped parts. . Thus, a test piece having a hard nitride layer on only one side was prepared. A rotary polishing plate was pressed against these test pieces with a constant load to apply rotary wear. The abrasion resistance was evaluated by the total number of revolutions of the polishing plate until the plate thickness of the test piece decreased by 0.1 mm at the maximum. Table 6 summarizes the above results. As is clear from Table 6, a comparison between the comparative example and the present invention example shows that the press-formed body having the hard nitride layer according to the present invention is more excellent in wear resistance.

[0025]

[Table 6]

[0026]

EFFECTS OF THE INVENTION According to the present invention, a steel sheet having high nitriding property and excellent formability can be produced, which is excellent in economical efficiency and productivity, and has wear resistance, fatigue strength, and resistance to fatigue. It is possible to obtain a press-molded article used for tools, machine structural parts, automobile parts, etc., which have both seizure resistance and corrosion resistance.

[Brief description of drawings]

FIG. 1 is a relationship diagram between Ti concentration and nitriding speed,

FIG. 2 is a relationship diagram of V concentration and hardening depth characteristics of nitriding.

Claims (1)

[Claims] 1. C: 0.01 to less than 0.08% by weight, Si: 0.005 to 1.00%, Mn: 0.010 to 3.00%, P: 0.001 to 0. 150%, N: 0.0002 to 0.0100%, Cr: more than 0.15 to 5.00%, Al: more than 0.060 to 2.00%, and as a nitride hardening element group, Ti: : 0.010% or more, and less than 4 C [%], V: more than 0.10 to 1.00%, containing 1 type or 2 types, and the balance being iron and inevitable impurities. After hot rolling, take up at 500 ℃ or more to make hot rolled steel sheet, or 50% thereafter
A method for producing a nitriding steel sheet having excellent formability, which comprises performing cold rolling at the above reduction rate, and then performing recrystallization annealing to obtain a cold rolled steel sheet.