JPH09310165A - Thin steel sheet for working excellent in fatigue characteristic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the formability and durability of a steel sheet at machine parts by specifying its componental comps. SOLUTION: The components in the internal layer of this steel are composed of, by weight, 0.0002 to 0.060% C, 0.001 to 0.50% Si, 0.01 to 0.50% Mn, 0.001 to 0.100% P, 0.001 to 0.500% S, 0.001 to 0.100% Al and 0.0002 to 0.0050% N. Furthermore, as for the surface layer components in the part of 0.010 to 0.20mm per side from the surface and back faces of the steel sheet, in the case the content of C in the above internal layer components is defined as C* and the content of N as N*, in addition to the internal layer components, one or two kinds of C; (C*+0.010) to 0.80% and N; (N*+0.010) to 2.4% are contained. In this way, the surface layer of the steel sheet is concentrated by C and N to improve its fatigue characteristics, and the structure of the internal layer is formed into the soft one good in workability, by which the workability of the whole body of the steel sheet is secured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet for working which is used for parts such as automobiles and machines and has excellent fatigue characteristics, and a method for producing the steel sheet.

[0002]

2. Description of the Related Art In recent years, parts such as automobiles and mechanical structures have become complicated, and there is an increasing demand for steel sheets which can be formed into various shapes in a small number of steps. On the other hand, the quality requirement level for parts such as automobiles and mechanical structures is improving year by year.
Its durability is also required. The durability as a part depends largely on the shape of the part and the environment in which it is used, but also depends on the material properties of the part, and a steel material with excellent durability is required. However, improvement of workability and improvement of fatigue characteristics are contradictory technical issues, and it is not easy to improve fatigue characteristics while providing workability. Various techniques have been proposed as techniques for improving the fatigue strength of steel while maintaining workability, from the standpoint of suppressing the occurrence of fatigue cracks and preventing the propagation of fatigue cracks.

From the standpoint of suppressing the occurrence of fatigue cracks, there are methods such as giving a residual stress to the surface by a method such as shot peening, or making a steel sheet into multiple layers so that a hard layer exists on the surface. For example, in JP-A-3-147840 and JP-A-7-60408, the alloy composition of the surface layer and the inner layer is changed at the stage of casting the slab, and only the surface layer has a hard layer. However, this method requires enormous capital investment, raises the manufacturing cost and is not economically viable, and it is not easy to control the surface layer thickness which is excellent in fatigue characteristics. Further, from the standpoint of preventing the propagation of fatigue cracks, there is, for example, JP-A-7-90479. This softens the surface of the steel sheet and prevents crack propagation. This method is effective for materials that have high strength and are susceptible to fatigue fracture, but it does not increase the fatigue strength of soft steel sheets with good workability.

[0004]

DISCLOSURE OF THE INVENTION In order to solve the above problems, the inventors of the present invention have not deteriorated the workability of machine parts and the like, and have excellent durability as parts, and a manufacturing method thereof. I conducted a diligent research.

[0005]

[Means for Solving the Problems] As a result of earnest research on the mechanism of fatigue fracture of thin steel sheets, the present researchers
Many fatigue fractures in thin steel sheets cause strain to accumulate on the surface due to bending stress repeatedly applied to the steel sheet, resulting in cracks,
It has become clear that the progress will result. further,
As a result of repeated studies on prevention of surface cracks, it was found that a multilayer steel sheet in which the surface layer of the steel sheet is harder than the ultra-low carbon steel of the inner layer is less likely to cause bending strain and less likely to be damaged by fatigue. Furthermore, as a method for producing a multilayer steel sheet in which the surface layer is harder than the inner layer, as a result of repeated ingenuity and examination, utilizing the thermal history during continuous annealing of the steel sheet, carburization,
It has been found that nitriding can produce a multi-layer steel sheet having excellent fatigue properties without deteriorating workability. Furthermore, it has been found that by controlling the cooling history after continuous annealing and performing annealing, it is possible to manufacture a multi-layer steel sheet having more excellent fatigue properties. Further, since this method can be manufactured by using the conventional annealing equipment, it does not require a large capital investment and is economical.

The gist of the present invention is as follows: (1) In a multi-layer steel sheet having different components in the surface layer and the inner layer, C: 0.0002 to 0.060%, Si: 0.001 to 0. 50%, Mn: 0.01 to 0.50%, P: 0.001 to 0.100%, S: 0.001 to 0.050%, Al: 0.001 to 0.100%, N: 0 0.0002 to 0.0050%, balance: Fe and unavoidable impurities, and 0.010 per side from the front and back surfaces of the multilayer steel sheet.
Assuming that the surface layer component of the portion of ~ 0.20 mm is C * and N amount of the inner layer component, in addition to the inner layer component, C: (C * + 0.010) to 0.80% N: (N * + 0.010) to 2.4% of 1 type or 2 types of containing, and the thin steel plate for processing excellent in fatigue characteristics.

(2) In a multi-layer steel sheet having different components in the surface layer and the inner layer, C: 0.0002 to 0.0070%, Si: 0.001 to 0.50%, Mn: 0 as an inner layer component in% by weight. 0.01 to 0.50%, P: 0.001 to 0.100%, S: 0.001 to 0.050%, Al: 0.001 to 0.100%, N: 0.0002 to 0.0050 %, And further contains one or more of Ti: 0.005 to 0.100%, Nb: 0.005 to 0.050%, B: 0.0003 to 0.0050%, and the balance : Fe and unavoidable impurities, 0.010 per side from the front and back of the multilayer steel sheet
Assuming that the surface layer component of the portion of ~ 0.20 mm is C * and N amount of the inner layer component, in addition to the inner layer component, C: (C * + 0.010) to 0.80% N: (N * + 0.010) to 2.4% of 1 type or 2 types of containing, and the thin steel plate for processing excellent in fatigue characteristics.

(3) A steel sheet having an inner layer composition as described in (1) or (2) above is subjected to hot rolling, descaling treatment, cold rolling, and then carburizing and / or nitriding in annealing. From the front and back sides of 0.010 to 0.20 per side
A method for producing a thin steel sheet for working having excellent fatigue characteristics, characterized in that the surface layer component in the mm portion contains the surface layer component described in (1) or (2). (4) Hot rolling, descaling treatment, and cold rolling are performed on the steel having the inner layer component described in (1) or (2) above, and then when annealed, carburizing and / or carburizing in a ferrite region above the recrystallization temperature. Nitriding is started, and the surface layer of 0.010 to 0.20 mm per surface from the front and back surfaces of the steel sheet is made to contain the surface layer component described in (1) or (2) to form only the surface layer as austenite. A method for producing a thin steel sheet for working having excellent fatigue characteristics, characterized by cooling at 50 ° C./sec or more after carburizing and / or nitriding to form a cold rolled steel sheet. A feature of the present invention is that a carburizing or nitriding is performed during continuous annealing, and the surface layer of the steel sheet is a multi-layer steel sheet in which C and N are concentrated,
Austenite only the surface layer during annealing, strengthening the structure only in the surface layer during cooling after annealing to improve fatigue properties, and the inner layer has a structure with good soft workability, and to secure the workability of the entire steel sheet. .

The reasons for limiting the components of the inner layer will be described below.
If C is less than 0.0002%, the cost for decarburization becomes high and it is not economical, so 0.0002% is made the lower limit. Further, if it exceeds 0.060%, the workability deteriorates, so 0.060% is made the upper limit. When stricter workability is required, the amount of carbides increases and workability deteriorates even if a carbide forming element is added if it exceeds 0.0070%, so 0.0070% is made the upper limit.

If Si is reduced to less than 0.001%, the manufacturing cost is increased and the economic efficiency is impaired.
The lower limit is 1%, and if it exceeds 0.50%, the workability deteriorates, so 0.50% is the upper limit. If Mn is reduced to less than 0.01%, the manufacturing cost rises and the economy is impaired. Therefore, the lower limit is 0.01%, and if it exceeds 0.50%, the workability deteriorates. The upper limit. P is
Reducing the amount to less than 0.001% raises the manufacturing cost and impairs the economical efficiency.
If it exceeds 030%, the workability deteriorates, so 0.030%
Is the upper limit.

If S is reduced to less than 0.001%, the manufacturing cost rises and the economic efficiency is impaired.
% Is the lower limit, and if it exceeds 0.050%, the workability deteriorates, so 0.050% is the upper limit. Al is 0.001
If it is less than 0.1%, deoxidation will be insufficient and blowholes will occur in the steel, impairing the cleanliness of the steel sheet, cracking during pressing, and surface defects, so 0.001% is the lower limit, and If it exceeds 0.100%, workability deteriorates, so 0.100% is made the upper limit. Although N is preferably as small as possible, if it is less than 0.0002%, the manufacturing cost increases, so 0.0002% is the lower limit, and if it exceeds 0.0050%, the workability deteriorates. , 0.0050% is the upper limit.

Further, as an element for improving deep drawability, Ti:
0.005% or more and 0.100% or less, Nb: 0.005
% To 0.050%, B: 0.0003% to 0.
It is possible to select a component system containing one or two or more of 50% or less. This component system is added in order to form fine carbonitrides in steel and prevent the deterioration of deep drawability due to the presence of solute C and N. If Ti is less than 0.005%, the effect of precipitating and fixing C and N is small.
The lower limit is 005%, and if it exceeds 0.100%, the deep drawability deteriorates, so 0.100% is the upper limit. Nb
Is less than 0.005%, the effect of precipitating and fixing C and N is small, so 0.005% is the lower limit, and 0.050%
If it exceeds the limit, the deep drawability will deteriorate, so 0.050%
Is the upper limit.

If B is less than 0.0002%, the effect of precipitating and fixing N is small, so 0.005% is made the lower limit,
If it exceeds 0.0050%, the deep drawability deteriorates, so 0.0050% is made the upper limit. Although the components are adjusted as described above, when severe deep drawability is required, C
In order to precipitate and fix N and N to improve deep drawability, one or more of Ti or Nb is used as Ti * = Ti-1.5S.
4C + 3.42N ≦ Ti * + 0.52Nb
It is desirable to add in the range of ≦ 20C + 17.1N.

Next, the reasons for limiting the C and N contents of the surface layer component added to the inner layer component as a fatigue property improving element will be described.
C is an element effective for transformation strengthening steel, but must be austenite for transformation strengthening during annealing. If the amount of C in the inner layer is C *, C is (C ++
If it is less than 0.010)%, it is difficult to control the temperature in which the inner layer is ferrite and only the surface layer is austenite, and the transformation strengthening effect is small. Therefore, the lower limit is (C * + 0.010)% or more, and more than 0.80%. However, the transformation strengthening effect is saturated and the workability of the entire steel sheet deteriorates, so the upper limit is 0.80%.

Like C, N is an element effective for transformation strengthening steel, but it must be austenite in order to transform strengthen during annealing. The amount of N in the inner layer is N *
Then, if N is less than (N * + 0.010)%, it is difficult to control the temperature in which the inner layer is ferrite and only the surface layer is austenite, and the transformation strengthening effect is small.
The lower limit is 0.010)% or more, and even if it exceeds 2.40%, the transformation strengthening effect is saturated and the workability of the entire steel sheet deteriorates. Therefore, the upper limit is 2.40%.

Next, the manufacturing method will be described. In the present invention, it is preferable to adjust the surface layer component in the continuous annealing step so that the surface layer component is obtained. Steel having the chemical composition of the inner layer is melted to form a slab. First, the slab is cast or directly or once cooled to an appropriate temperature and then heated in a heating furnace. The heating is preferably performed at about 1000 ° C to 1300 ° C at which hot rolling can be performed. Depending on the use of the steel sheet, the heating can be omitted. Thereafter, the hot-rolled at A ₃ transformation point or above the temperature. It may be cast below the A ₃ transformation point heat, but preferably a sufficient workability can not be obtained and A ₃ transformation point or more. After hot rolling, it is wound at an appropriate temperature to obtain a hot rolled steel sheet.

Further, when deep drawability is strictly required, it is desirable to wind at 600 ° C. or higher in order to precipitate and fix C and N so as to obtain a precipitation form having good workability. Further, if rolled up at a temperature higher than 800 ° C., the steel sheet is softened and a flaw easily occurs, and the quality of the steel sheet surface deteriorates.
It is desirable to carry out winding at a temperature of ℃ or below. After that, descaling treatment such as pickling is performed, and then preferably 50% or more cold rolling is performed. Cold rolling of 50% or more is effective in providing high deep drawability, but more preferably 70%
That is all.

Then, annealing is carried out at a temperature not lower than the recrystallization temperature, and during or after annealing, carburization is carried out so that the surface layer component in the portion of 0.010 to 0.20 mm per side from the front and back surfaces of the steel sheet becomes the above surface layer component. , And / or nitriding treatment to obtain a multilayer cold-rolled steel sheet having excellent fatigue properties. Carburization or nitriding treatment may be performed before annealing, but since C and N diffuse during heating and annealing and it becomes difficult to control the thickness of the surface C and N concentrated layer, carburizing during or after annealing. It is desirable to nitride. The amount of carburizing or nitriding and the temperature are controlled during annealing, the inner layer is recrystallized in the ferrite region, and the surface layer is recrystallized in the austenite region.

At this time, if the inner layer is annealed in the austenite region, the structure advantageous for deep drawability tends to be lost, so annealing in the ferrite region is desirable. The method of carburizing and nitriding treatment is not particularly specified, and any of solid, liquid, gas carburizing and nitriding treatment may be used.
A ₃ transformation start temperature (austenite forming temperature), for example Fe-C, Fe-N, may be determined based on the Fe-C-N phase diagram.

Further, by utilizing the cooling history after annealing, it is possible to more effectively transform and strengthen the surface layer. After carburizing and nitriding, cooling is performed at 50 ° C./sec or more to transform and strengthen the surface layer. Particularly, it can be transformed into a hard structure more effectively than when the carburizing amount and the nitriding amount are small. In batch annealing, since it is difficult to obtain a cooling rate advantageous for transformation, it is desirable to anneal and cool during continuous annealing.

A product is obtained by carburizing, nitriding, and then skin-pass rolling, or on the cold-rolled steel sheet obtained by the method of the present invention, for the purpose of improving corrosion resistance, paintability, and weldability, one side or both sides. It is also possible to carry out hot dip and / or electroplating on, without departing from the invention. Further, it is of course possible to add various treatments to the steel sheet of the present invention, for example, chromate treatment, phosphate treatment, treatment for improving phosphate treatment, lubricity improving treatment,
Even if the weldability improving treatment and the resin coating treatment are performed, it does not depart from the scope of the present invention, and various treatments can be additionally performed depending on the required characteristics.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) Hereinafter, the present invention will be specifically described based on embodiments. Steels having the components shown in Table 1 were melted and continuously cast into slabs according to a conventional method. Then, in a heating furnace, 1200
It was heated to 0 ° C., hot rolled at a finishing temperature of 910 ° C. or higher, wound at 650 ° C., and then pickled to obtain a hot rolled steel sheet. After cold rolling at a reduction rate of 80%, 88
Recrystallization annealing was performed at 0 ° C for 60 seconds, and carburizing and nitriding treatments shown in Table 2 were performed to obtain a cold rolled steel sheet. The temperature at the time of carburizing and nitriding treatment is {910− (910−590) × surface N concentration / in order to set the inner layer to the ferrite region and the surface layer to the austenite region.
2.35- (910-727) x surface layer C concentration / 0.8}
° C or higher {910- (910-590) x inner layer N concentration /
2.35- (910-727) x inner layer C concentration / 0.8}
It was performed at a temperature of ℃ or less. A 80 Φ disk was punched out from the obtained cold-rolled steel sheet to form a cylindrical cup with a drawing ratio of 2.0 and 2.2, and the shapeability at that time was used as a measure of the deep drawability. In addition, a test piece was prepared separately, and a bending fatigue test was performed by repeatedly bending at a rotation speed of 1800 times / min with a double swing Schenk type fatigue tester. The load stress that does not break even after bending 10 ⁷ times was set as the fatigue limit and used as an index of fatigue resistance.
The above results are also shown in Table 2. As is clear from Table 2, when the comparative example and the inventive example are compared, the inventive example is
It can be seen that it has excellent deep drawability and fatigue properties.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

According to the present invention, a steel sheet having excellent fatigue resistance can be obtained without impairing the deep drawability. According to the present invention, the surface layer of a steel sheet contains a large amount of C and N that prevent intergranular cracks and improves fatigue properties, and the inside of the steel sheet is an extremely low carbon steel with a particularly good deep drawability. A steel sheet having excellent fatigue properties can be provided. Further, according to the method of the present invention, since a surface layer having excellent fatigue properties can be provided without losing a metal structure having good deep drawing properties, it is possible to produce a multilayer steel sheet having excellent deep drawing properties and fatigue properties. .

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical indication C22C 38/14 C22C 38/14

Claims (4)

[Claims] 1. A multi-layer steel sheet having different components in the surface layer and the inner layer, wherein C: 0.0002 to 0.060%, Si: 0.001 to 0.50%, Mn: 0. 01 to 0.50%, P: 0.001 to 0.100%, S: 0.001 to 0.050%, Al: 0.001 to 0.100%, N: 0.0002 to 0.0050% , Balance: Fe and unavoidable impurities, 0.010 per side from the front and back of the multilayer steel sheet
Assuming that the surface layer component of the portion of ~ 0.20 mm is C * and N amount of the inner layer component, in addition to the inner layer component, C: (C * + 0.010) to 0.80% N: (N * + 0.010) to 2.4% of 1 type or 2 types of containing, and the thin steel plate for processing excellent in fatigue characteristics. 2. In a multi-layer steel sheet having different components in the surface layer and the inner layer, C: 0.0002 to 0.0070%, Si: 0.001 to 0.50%, Mn: 0. 01 to 0.50%, P: 0.001 to 0.100%, S: 0.001 to 0.050%, Al: 0.001 to 0.100%, N: 0.0002 to 0.0050% , And further contains one or more of Ti: 0.005 to 0.100%, Nb: 0.005 to 0.050%, B: 0.0003 to 0.0050%, and the balance: Contains Fe and unavoidable impurities, and is 0.010 per side from the front and back of the multilayer steel sheet.
Assuming that the surface layer component of the portion of ~ 0.20 mm is C * and N amount of the inner layer component, in addition to the inner layer component, C: (C * + 0.010) to 0.80% N: (N * + 0.010) to 2.4% of 1 type or 2 types of containing, and the thin steel plate for processing excellent in fatigue characteristics. 3. The steel having the inner layer component according to claim 1 or 2 is subjected to hot rolling, descaling treatment, cold rolling, followed by carburizing and / or nitriding when annealing, and A method for producing a thin steel sheet for working having excellent fatigue properties, characterized in that the surface layer component of a portion of 0.010 to 0.20 mm per one surface contains the surface layer component according to claim 1 or 2. 4. When the steel having the inner layer component according to claim 1 or 2 is subjected to hot rolling, descaling treatment, cold rolling, and then annealing, carburizing and / or nitriding in a ferrite region above a recrystallization temperature. Starting from the front and back surfaces of the steel sheet, the surface layer in the portion of 0.010 to 0.20 mm per surface is made to contain the surface layer component according to claim 1 or 2, and only the surface layer is made austenite, and further carburization or nitriding is completed. Later 50 ℃ /
A method for manufacturing a thin steel sheet for working, which is excellent in fatigue characteristics, characterized by cooling for not less than sec.