KR101795401B1 - Carburizing steel and mrthod for manufacturing the same - Google Patents

Abstract

One embodiment of the present invention is to provide a carburizing steel. According to one embodiment of the present invention, based on 100 wt% of a total composition, the carburizing steel comprises: 0.6-0.9 wt% of C; 0.3 wt% or less of Si (excluding 0 wt%); 0.4-1.0 wt% of Mn; 1.3-2.5 wt% of Cr; 0.1-1.5 wt% of Mo; 0.3 wt% or less of Cu (excluding 0 wt%); 0.01-0.06 wt% of Nb; 0.05-0.3 wt% of Al; 0.001-0.005 wt% of B; and the remaining consisting of Fe and other inevitable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to a carburizing steel and a carburizing steel,

Carburizing steel and carburizing steel.

Generally, the manufacturing process of an automobile gear is manufactured through such processes as material → forging → normalizing or annealing → shaving & Hobbing → carburizing heat treatment → submerged heat treatment.

Cr alloy steel, Cr-Mo alloy steel, and Ni-Cr-Mo alloy steel are widely used as conventional alloy steel for gears. The transmission gear of an automobile has a rough shape by forging and has a precise shape by machining, so that it is required to have excellent mono-composition and easy machinability.

The chromium alloy steel and the chromium-molybdenum alloy steel are inexpensive but are not used for fatigue characteristics and impact properties, and are therefore often used for gears which are not subject to large load.

On the other hand, the nickel-chromium-molybdenum alloy steel has a disadvantage of being expensive and difficult to process due to the content of nickel, but is excellent in fatigue characteristics and impact characteristics and is mainly used for gears subjected to large load.

In the case of general gears, the gears are engaged with each other and are subjected to contact stress. As a result, fittings and the like are generated on the surface of the gear portion, which causes a reduction in durability and noise. Particularly, as the load on the gear increases due to the downsizing and downsizing of the powertrain and the effect of high output, these problems are increasing.

Therefore, development of gear steels improved in fatigue strength and fatigue life is required.

One embodiment of the present invention is to provide a carburized steel.

Yet another embodiment of the present invention is to provide a method of manufacturing a carburized steel.

The carburized steel according to one embodiment of the present invention comprises 0.6 to 0.9% by weight of C, 0.3% by weight or less of Si (excluding 0% by weight), 0.4 to 1.0% by weight of Mn, Cu: 0.3 wt% or less (excluding 0 wt%), Nb: 0.01 to 0.06 wt%, Al: 0.05 to 0.3 wt%, and B: 0.1 to 1.5 wt% 0.001 to 0.005% by weight, the remainder including Fe and other inevitably incorporated impurities.

P: 0.02 wt% or less (excluding 0 wt%), and S: 0.03 wt% or less (excluding 0 wt%).

Mn: 0.4 to 0.7% by weight.

And 1.3 to 2.0% by weight of Cr.

Mo: 0.1 to 0.7% by weight.

The value of the following formula (1) may be 10.5 or more.

(10-11.5 * [C] -4.25 * [Si] -0.5 * [Mn] + 1.35 * [Cr] + 1.85 * [Mo]

(C), [Si], [Mn], [Cr] and [Mo] refer to the addition amount (% by weight) of C, Si, Mn, Cr and Mo,

A method for producing a carburized steel according to an embodiment of the present invention comprises: 0.15 to 0.23% by weight of C, 0.3% by weight or less of Si (excluding 0% by weight), 0.4 to 0.4% by weight of Mn, (Excluding 0% by weight), Nb: 0.01 to 0.06% by weight, Al: 0.05 to 0.3% by weight, And B: 0.001 to 0.005% by weight, the balance comprising Fe and other inevitably incorporated impurities; Preheating the steel material; Carburizing the preheated steel; Diffusing carbon in the carburized steel; Cooling and cracking the carbon-diffused steel; And cooling the cold and cracked steels.

The temperature in the step of carburizing the preheated steel may be 900 to 950 캜.

The carbon potential in the step of carburizing the preheated steel may be 0.8 to 1.2%.

The temperature at the step of diffusing carbon in the carburized steel may be 900 to 950 占 폚.

The carbon potential in the step of diffusing carbon in the carburized steel may be 0.75 to 1.0%.

The temperature at the step of cooling and cracking the carbon-diffused steel may be 820 to 870 캜.

The carbon potential at the step of cooling and cracking the carbon-diffused steel material may be 0.7 to 0.9%.

The step of cooling down and cooling the cracked steel may be cooled to 120 to 250 ° C.

According to one embodiment of the present invention, a carburized steel having improved fatigue strength and fatigue life can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims. Like reference numerals refer to like elements throughout the specification.

Thus, in some embodiments, well-known techniques are not specifically described to avoid an undesirable interpretation of the present invention. Unless defined otherwise, all terms (including technical and scientific terms) used herein may be used in a sense commonly understood by one of ordinary skill in the art to which this invention belongs. Whenever a component is referred to as "including" an element throughout the specification, it is to be understood that the element may include other elements, not the exclusion of any other element, unless the context clearly dictates otherwise. Also, singular forms include plural forms unless the context clearly dictates otherwise.

The carburized steel according to one embodiment of the present invention comprises 0.6 to 0.9% by weight of C, 0.3% by weight or less of Si (excluding 0% by weight), 0.4 to 1.0% by weight of Mn, Cu: 0.3 wt% or less (excluding 0 wt%), Nb: 0.01 to 0.06 wt%, Al: 0.05 to 0.3 wt%, and B: 0.1 to 1.5 wt% 0.001 to 0.005% by weight, the remainder including Fe and other inevitably incorporated impurities.

The reason for the composition limitation will be described in detail below.

Carbon (C) increases the strength and hardness of the material, and the element is an essential element for the carbide precipitation. C may be contained in an amount of 0.6 to 0.9% by weight based on 100% by weight of the whole composition. When too little carbon is added, the tensile strength may decrease. When too much carbon is added, impact toughness may be reduced. The carburized steel is carbon added during the carburizing process in the manufacturing process, so that the initial steel contains less carbon than the final carburized steel. Specifically, the initial steel may contain 0.15 to 0.23% by weight of carbon.

Silicon (Si) is an element that increases hardness and strength. Si may be contained in an amount of 0.3% by weight or less based on 100% by weight of the whole composition. When Si is contained in a large amount during the tempering, the carbide is prevented from being decomposed and the tempering resistance is high. Therefore, it is an element that is very vulnerable to toughness. In addition, since it is an element generating oxidation in the grain boundary upon carburization, it hinders carburization characteristics.

The manganese (Mn) may contain 0.4 to 1.0 wt% based on 100 wt% of the whole composition. If Mn is added too little, incombustibility and strength reinforcement may be insufficient. If too much Mn is added, the workability is lowered and the impact toughness may be lowered. More specifically, Mn may be contained in an amount of 0.4 to 0.7% by weight.

Chromium (Cr) is a major element that increases strength when carburized. Cr may contain 1.3 to 2.5% by weight based on 100% by weight of the whole composition. If too little Cr is included, the strength improvement may be insufficient. When Cr is excessively added, carbide precipitates, which may deteriorate impact toughness. More specifically, Cr may be contained in an amount of 1.3 to 2.0% by weight.

Molybdenum (Mo) has an effect of improving the hardenability and prevents tempering brittleness and imparts resistance to tempering brittleness. Mo may contain 0.1 to 1.5% by weight based on 100% by weight of the total composition. If Mo is included too little, the strength and toughness improvement may be insufficient. More specifically, Mo may be contained in an amount of 0.1 to 0.7% by weight.

Copper (Cu) may be contained in an amount of 0.3% by weight or less based on 100% by weight of the total composition. If too much Cu is contained, the hot workability becomes a problem and it may cause red brittleness.

Niobium (Nb) is a strong grain refinement element, which increases the grain coarsening temperature, thereby improving strength and toughness. Nb may contain 0.01 to 0.06% by weight based on 100% by weight of the total composition. If Nb is included too little, the strength and toughness improvement may be insufficient.

Aluminum (Al) is a powerful deoxidizer and acts to remove oxygen during steelmaking. Therefore, it helps to improve toughness. In addition, since AlN is formed to inhibit growth of particle size, it is effective to improve strength and toughness. Al may be contained in an amount of 0.05 to 0.3% by weight based on 100% by weight of the whole composition. If too little Al is included, the improvement in strength and toughness may be insufficient. If Al is contained too much, Al inclusions may be formed and adversely affected.

Boron (B) is a very effective element for strengthening the weak grain boundary. Therefore, it is a very effective element for improving impact toughness. B may be contained in an amount of 0.001 to 0.005% by weight based on 100% by weight of the total composition. If B is included too little, impact toughness improvement may be insufficient. Even if B is no longer included, impact toughness improvement is limited.

Phosphorus (P) to form the compound of Fe ₃ P. This compound is extremely fragile and segregated and does not become homogeneous even after annealing treatment, and is elongated at the time of forging rolling. Thus, the impact resistance is lowered and the tempering brittleness is promoted. Therefore, when P is further included, it is limited to 0.02 wt% or less.

Sulfur (S) is recognized as an inclusion and its content is preferably minimized. MnS combined with Mn improves the machinability but if the content thereof is increased, the strength is lowered. S, the content thereof is limited to 0.03% by weight or less.

The carburized steel according to one embodiment of the present invention may have a value of the following formula (1) of 10.5 or more.

(10-11.5 * [C] -4.25 * [Si] -0.5 * [Mn] + 1.35 * [Cr] + 1.85 * [Mo]

(C), [Si], [Mn], [Cr] and [Mo] refer to the addition amount (% by weight) of C, Si, Mn, Cr and Mo,

When the value of the formula (1) is 10.5 or more, the impact value is further improved.

The carburized steel according to one embodiment of the present invention may be a carburizing steel for automobile gears.

Hereinafter, a method for manufacturing carburized steel according to an embodiment of the present invention will be described.

0.1 to 0.23 wt% of C, 0.3 wt% or less of Si (excluding 0 wt%), 0.4 to 1.0 wt% of Mn, 1.3 to 2.5 wt% of Cr, 0.1 to 1.5 wt% of Mo, 0.3 wt% or less of Cu (excluding 0 wt%), 0.01 to 0.06 wt% of Nb, 0.05 to 0.3 wt% of Al, and 0.001 to 0.005 wt% of B, The balance prepares a steel material containing Fe and other inevitably incorporated impurities.

The description of each composition of the steel is the same as that of the above-described carburized steel, and a duplicate description will be omitted.

The steel may be manufactured in a predetermined gear shape through at least one of forging, normalizing, annealing, and working.

More specifically, a material having the same composition as the steel material is subjected to hot forging or cold forging, followed by normalizing or annealing, followed by shaving & Hobbing to produce a predetermined gear shape .

Next, preheat the steel.

Next, the preheated steel material is carburized.

The temperature at the carburizing treatment may be 900 to 950 캜. If the temperature is too low, the carbide may precipitate. If the temperature is too high, coarse grains can be formed. Therefore, the temperature can be controlled within the above-mentioned range.

Carbon potential can be controlled to 0.8 to 1.2% at carburizing treatment. If the carbon potential is too low, carburization may not be performed properly. If the carbon potential is too high, surface carbides may occur. Therefore, the carbon potential can be controlled within the above-mentioned range.

The term "carbon potential" is a term indicating the carburizing ability of the atmosphere for heating the steel. Basic Definition of Carbon Potential "It is the carbon concentration of the surface of the steel when the steel is carburized at a certain temperature in a certain gas atmosphere and equilibrium is reached with the gas atmosphere ".

Next, carbon is diffused in the carburized steel.

The temperature at the step of diffusing carbon in the carburized steel may be 900 to 950 占 폚. If the temperature is too low, the carbide may precipitate. If the temperature is too high, coarse grains can be formed. Therefore, the temperature can be controlled within the above-mentioned range.

The carbon potential in the step of diffusing carbon in the carburized steel may be 0.75 to 1.0%. In the case of the diffusion step, the carbon potential can be controlled to be lower than the carburization step since the carbon that permeates at the carburizing stage must diffuse into the steel.

Next, the carbon-diffused steel material is cooled down and cracked.

The temperature at the step of cooling and cracking the carbon-diffused steel material may be 820 to 870 캜. The temperature can be adjusted to the above-mentioned range to reduce thermal deformation before cooling. If the temperature is too low, the carbide may precipitate. If the temperature is too high, severe thermal deformation may occur.

The carbon potential at the step of cooling and cracking the carbon-diffused steel can be 0.7 to 0.9%.

Next, the cooled and cooled steel is cooled.

At this time, it can be cooled to 120 to 250 ° C. If the temperature is cooled too low, excessive thermal deformation and cracks may occur. If the temperature is not sufficiently cooled, a stable high-carbon martensite structure may not be properly formed.

Hereinafter, the present invention will be described in detail with reference to experimental examples. EXAMPLES The following experimental examples are illustrative of the present invention, but the contents of the present invention are not limited by the following experimental examples.

Example

A steel material containing the components listed in Table 1, the balance being Fe and unavoidable impurities, was prepared. Cu was included at the impurity level.

The steel material was preheated to 930 ℃ at room temperature and carburized at 1.0% carbon potential at 930 ℃. Thereafter, it was diffused at a carbon potential of 930 占 폚 of 0.9%, cooled and cooled at 850 占 폚 and 0.8% of carbon potential, and then cooled to 200 占 폚.

Tensile strength, tensile strength, yield strength, elongation, load at an infinite lifetime of 10 million times, fatigue failure at 1 million cycles, and contact fatigue life were measured and are summarized in Table 2 below.

(wt%) C Si Mn Cr Mo Al B Nb The value of equation (1) Inventive Steel 1 0.18 0.1 0.5 1.7 0.4 0.1 0.003 0.035 11.319 Comparative River 1 0.25 0.1 0.5 1.7 0.4 0.1 0.003 0.035 10.4335 Comparative River 2 0.18 0.5 0.5 1.7 0.4 0.1 0.003 0.035 9.449 Invention river 2 0.18 0.1 0.9 1.7 0.4 0.1 0.003 0.035 11.099 Invention steel 3 0.18 0.1 0.5 2.3 0.4 0.1 0.003 0.035 12.21 Inventive Steel 4 0.18 0.1 0.5 1.7 One 0.1 0.003 0.035 12.54 Comparative Steel 3 0.18 0.1 0.5 1.7 0.4 - - - 11.319 Comparative Steel 4 0.13 0.05 0.3 One 0 0.1 0.003 0.035 10.44175

Impact toughness
(J) The tensile strength
(MPa) Elongation
(%) Load at infinite lifetime of 10 million times
(MPa) Load in case of fatigue failure of 1 million cycles
(MPa) Contact fatigue life
(3.51 GPa, times) Inventive Steel 1 42 1461 2.4 825 975 498 million Comparative River 1 36 1498 1.8 810 960 522 million Comparative River 2 25 1452 1.6 805 970 62.1 million Invention river 2 40 1477 2.3 845 965 453 million Invention steel 3 50 1486 2.65 855 980 681 million Inventive Steel 4 53 1522 2.7 860 990 723 million Comparative Steel 3 38 1405 2.2 790 920 265 million Comparative Steel 4 36 1393 2.1 755 825 187 million

As shown in Tables 1 and 2, the inventive steel of the present invention exhibits excellent impact strength, tensile strength, yield strength, elongation, load at lifetime of 10 million times, load at 1 million cycles of fatigue failure and contact fatigue life, .

Specifically, the comparative steel 1 and the comparative steel 2 have a value of less than 10.5, which means that the impact toughness is considerably poorer than that of the invention steel, and that the load is poor when the elongation and the lifetime of an infinite life of 10 million times.

In comparison steel 3, the value of formula (1) is not less than 10.5 but does not include Al, B, and Nb, and the impact toughness is poorer than that of the invention steel, and the load is poor at a lifetime of 10 million times. Particularly, Which is quite poor.

It can be seen that the comparative steel 4 has a poor impact toughness as compared with the invention steel and a poor load at a lifetime of 10 million times, particularly in terms of contact fatigue life, because the value of Equation (1) is less than 10.5.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims (14)

(Excluding 0% by weight), Mn: 0.4 to 1.0% by weight, Cr: 1.3 to 2.5% by weight, Mo: 0.1 to 1.5 wt% Cu, 0.3 wt% or less Cu (excluding 0 wt%), Nb: 0.01 to 0.06 wt%, Al: 0.05 to 0.3 wt%, and B: 0.001 to 0.005 wt% Fe and other inevitably incorporated impurities,
A carburized steel having a value of the following formula (1) of 10.5 or more.
(10-11.5 * [C] -4.25 * [Si] -0.5 * [Mn] + 1.35 * [Cr] + 1.85 * [Mo]
(C), [Si], [Mn], [Cr] and [Mo] refer to the addition amount (% by weight) of C, Si, Mn, Cr and Mo, The method according to claim 1,
P: 0.02 wt% or less (excluding 0 wt%) and S: 0.03 wt% or less (excluding 0 wt%). The method according to claim 1,
Mn: 0.4 to 0.7% by weight. The method according to claim 1,
Carburizing steel containing 1.3 to 2.0 wt% of Cr. The method according to claim 1,
Mo: 0.1 to 0.7% by weight. delete 0.1 to 0.23 wt% of C, 0.3 wt% or less of Si (excluding 0 wt%), 0.4 to 1.0 wt% of Mn, 1.3 to 2.5 wt% of Cr, 0.1 to 1.5 wt% Cu, 0.3 wt% or less Cu (excluding 0 wt%), Nb: 0.01 to 0.06 wt%, Al: 0.05 to 0.3 wt%, and B: 0.001 to 0.005 wt% Fe and other inevitably incorporated impurities and having a value of the following formula (1) of not less than 10.5;
Preheating the steel material;
Carburizing the preheated steel;
Diffusing carbon in the carburized steel;
Cooling and cracking the carbon-diffused steel; And
Cooling the cold and cracked steels;
Wherein the method comprises the steps of:
(10-11.5 * [C] -4.25 * [Si] -0.5 * [Mn] + 1.35 * [Cr] + 1.85 * [Mo]
(C), [Si], [Mn], [Cr] and [Mo] refer to the addition amount (% by weight) of C, Si, Mn, Cr and Mo, 8. The method of claim 7,
Wherein the temperature at the step of carburizing the preheated steel material is 900 to 950 占 폚. 8. The method of claim 7,
And the carbon potential in the step of carburizing the preheated steel material is 0.8 to 1.2%. 8. The method of claim 7,
Wherein the temperature at the step of diffusing carbon in the carburized steel is 900 to 950 캜. 8. The method of claim 7,
Wherein the carbon potential in the step of diffusing carbon in the carburized steel is 0.75 to 1.0%. 8. The method of claim 7,
Wherein the temperature at the step of cooling and cracking the carbon-diffused steel material is 820 to 870 ° C. 8. The method of claim 7,
And the carbon potential in the step of cooling and cracking the carbon-diffused steel material is 0.7 to 0.9%. 8. The method of claim 7,
Wherein the step of cooling the cold-rolled steel and the cold-rolled steel is cooled to 120 to 250 ° C.