KR20200043599A - High toughness steel alloy - Google Patents

Abstract

The present invention relates to high toughness alloy steel which improves tensile strength and toughness while minimizing expensive alloy components. The high toughness alloy steel according to one embodiment of the present invention comprises: 0.52 to 0.58 wt% of carbon (C), 0.35 to 0.40 wt% of silicon (Si), 1.50 to 1.60 wt% of manganese (Mn), 0.25 to 0.35 wt% of copper (Cu), 0.02 to 0.04 wt% of niobium (Nb), 0.01 to 0.03 wt% of titanium (Ti), 0.03 wt% or less (excluding 0 wt%) of phosphorus (P), 0.03 wt% or less (excluding 0 wt%) of sulfur (S), and the remaining amount of iron (Fe) and other inevitable impurities.

Description

High toughness alloy steel {HIGH TOUGHNESS STEEL ALLOY}

The present invention relates to a high toughness alloy steel, and more particularly, to a high toughness alloy steel with improved tensile strength and toughness while minimizing expensive alloy components.

In the suspension, steering, and braking systems of a vehicle, a component called a knuckle, which serves as a connecting hub for each component, is used.

For example, the knuckle transmits the steering force input from the steering device to the wheel to enable direction change, and performs a function of transmitting the shock absorbed from the road surface to the suspension device.

Accordingly, the knuckle is a component that requires strength and toughness, and is generally manufactured by hot forging using an alloy steel made of a steel material.

The alloy steel used to manufacture the knuckle uses Q / T (quench and temper) heat treatment, in which expensive vanadium (V) is added to a certain content for high strength and toughness, but the non-alloy steel is omitted. As it was added, there was a problem that the price competitiveness deteriorated.

Therefore, recently, alloy steels for knuckle production having high stiffness and high toughness without using expensive alloy components are required.

The above descriptions as background arts are only for improving understanding of the background of the present invention, and should not be taken as an admission that they correspond to the prior arts already known to those skilled in the art.

The present invention provides a high toughness alloy steel with enhanced tensile strength and toughness and improved corrosion resistance through content control of copper (Cu), niobium (Nb), and titanium (Ti).

High toughness alloy steel according to an embodiment of the present invention is by weight, carbon (C): 0.52 ~ 0.58%, silicon (Si): 0.35 ~ 0.40%, manganese (Mn): 1.50 ~ 1.60%, copper (Cu) : 0.25 to 0.35%, niobium (Nb): 0.02 to 0.04%, titanium (Ti): 0.01 to 0.03%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.03% or less (0 %), Balance iron (Fe) and other unavoidable impurities.

The alloy steel is characterized in that the average grain size (grain size) is less than 16㎛.

The alloy steel is formed of ferrite and pearlite, and is characterized in that the ferrite fraction is 10 to 15%.

The alloy steel is characterized in that the tensile strength is 800MPa or more.

The alloy steel is characterized in that the impact toughness is more than 90J / ㎠.

The alloy steel is characterized in that the depth of corrosion cracking measured after salt spraying of a 5% NaCl aqueous solution at a temperature of 35 ° C. for 360 hours using a salt spray tester is 30 μm or less.

According to an embodiment of the present invention, the tensile strength and toughness are improved by actively controlling the contents of copper (Cu), niobium (Nb), and titanium (Ti) without adding an expensive alloy component, which is required in the knuckle. Light weight can be expected while satisfying the properties for tensile strength and toughness.

In addition, there is an effect that durability is improved due to improvement of corrosion resistance.

1 is a graph showing the tensile strength according to the carbon (C) content control in the present invention,
2 is a graph showing the impact toughness according to the carbon (C) content control in the present invention,
3 is a graph showing the tensile strength according to the silicon (Si) content control in the present invention,
4 is a graph showing the impact toughness according to the silicon (Si) content control in the present invention,
5 is a graph showing the degree of surface decarburization according to the silicon (Si) content control in the present invention,
6 is a graph showing the tensile strength according to the manganese (Mn) content control in the present invention,
7 is a graph showing the impact toughness according to the manganese (Mn) content control in the present invention
8 is a graph showing the depth of corrosion in accordance with the copper (Cu) content control in the present invention,
9 is a graph showing the impact toughness according to the copper (Cu) content control in the present invention,
10 is a graph showing the average of the grain size according to the niobium (Nb) content control in the present invention,
11 is a graph showing the impact toughness according to the niobium (Nb) content control in the present invention,
12 is a graph showing the average grain size according to the titanium (Ti) content control in the present invention,
13 is a graph showing the impact toughness according to the titanium (Ti) content control in the present invention,
14 is a graph showing the average of the grain size according to the content control of the main alloy component in the present invention,
15 is a graph showing the tensile strength according to the content control of the main alloy component in the present invention,
16 is a graph showing the impact toughness according to the content control of the main alloy component in the present invention,
17 is a graph showing the depth of corrosion damage according to the content control of the main alloy component in the present invention,
18 is a photograph showing the tissue according to the cooling rate control in the present invention.

Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various different forms, and only the present embodiments allow the disclosure of the present invention to be complete, and the scope of the invention to those skilled in the art is completely It is provided to inform you.

High toughness alloy steel according to an embodiment of the present invention is an alloy steel used to manufacture a knuckle that serves as a connecting hub for each component in a vehicle suspension, steering, and braking device, without using expensive alloy components. It is an alloy steel with improved physical properties such as tensile strength, toughness and corrosion resistance by optimizing the content of the alloy components.

Specifically, carbon (C): 0.52 to 0.58%, silicon (Si): 0.35 to 0.40%, manganese (Mn): 1.50 to 1.60%, copper (Cu): 0.25 to 0.35%, niobium (Nb): 0.02 ~ 0.04%, titanium (Ti): 0.01 ~ 0.03%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), balance iron (Fe) and other inevitable The alloy steel for knuckles containing impurities is targeted.

The reasons for limiting the alloy component and its composition range in the present invention are as follows. Hereinafter, unless stated otherwise,% described in units of the composition range means weight percent.

Next, in the high-durability coil spring steel according to an embodiment of the present invention, the reason for limiting the component conditions of the steel will be described in detail.

Carbon (C): 0.52 ~ 0.58%

Carbon (C) is the most effective and important element for increasing the strength of the alloy steel, and improves the hardness as the amount of carbon increases, but the toughness decreases. In particular, carbon (C) improves the strength and hardness of the alloy steel by forming carbides by combining with alloying elements such as iron (Fe) and chromium (Cr).

When less than 0.52% is added, tensile strength and fatigue strength decrease. On the other hand, when it exceeds 0.58%, impact toughness is reduced. Therefore, the content of carbon (C) was limited to 0.52 to 0.58%.

Silicon (Si): 0.35 ~ 0.40%

Silicon (Si)

Silicon (Si) is an element that improves the hardness and strength of steel and strengthens the pearlite phase, but lowers elongation and impact. It is oxygen-friendly.

When less than 0.35% is added, tensile strength and fatigue strength decrease. On the other hand, when it exceeds 0.40%, the fatigue strength due to decarburization is reduced. Therefore, the content of silicon (Si) was limited to 0.35 to 0.40%.

Manganese (Mn): 1.50 ~ 1.60%

Manganese (Mn) is an element that improves the strength, but when it is contained in a large amount, causes quenching cracking, thermal deformation, and toughness reduction. Reaction with sulfur (S) forms an inclusion called MnS.

When less than 1.50% is added, tensile strength and fatigue strength decrease. On the other hand, when it exceeds 1.60%, impact toughness and workability are deteriorated. Therefore, the content of manganese (Mn) was limited to the range of 1.50 to 1.60%.

Copper (Cu): 0.25 to 0.35%

Copper (Cu) is an element that improves the density of corrosion oxides on the steel surface and prevents corrosion from developing inside. However, when a large amount is contained, fine cracks are generated in the steel depending on the cause of embrittlement at high temperatures (red heat embrittlement).

When less than 0.25% is added, permanent deformation (deflection) occurs due to a decrease in strength. On the other hand, when it is added in excess of 0.35%, there is a problem of crack generation and cost increase due to the increase in hardness and decrease in toughness. Therefore, the content of copper (Cu) was limited to the range of 0.25 to 0.35%.

Niobium (Nb): 0.02 ~ 0.04%

Niobium (Nb) refines the structure of the steel and is well employed in austenite to strengthen the base. In addition, it exhibits excellent curing ability, and in particular, it is expected to have an effect of improving corrosion properties as a corrosion resistance improving element.

When added to less than 0.02%, fatigue life due to corrosion resistance decreases. On the other hand, when it is added more than 0.04%, there is a problem of raw material cracking and cost increase due to red brittleness. Therefore, the content of niobium (Nb) was limited to 0.02 to 0.04%.

Titanium (Ti): 0.01 ~ 0.03%

Titanium (Ti) can play a role in minimizing the structure to secure impact toughness, but when added in large quantities, the fatigue strength decreases due to the coarse and coarse precipitate as a starting point.

When less than 0.01% is added, the strength decreases with coarse grain size. On the other hand, when it exceeds 0.03%, fatigue life and toughness are reduced due to the formation of coarse inclusions, and there is a problem of cost increase. Therefore, the content of titanium (Ti) was limited to 0.01 to 0.03%.

Phosphorus (P): 0.03% or less (excluding 0%)

The content of phosphorus (P) is preferably 0.03% or less. When the amount of phosphorus (P) exceeds 0.03 wt%, there is a problem in that workability is deteriorated due to an increase in impurities in the material.

Sulfur (S): 0.03% or less (excluding 0%)

The content of sulfur (S) is preferably 0.03% or less. When the amount of sulfur (S) exceeds 0.03 wt%, there is a problem that corrosion resistance and workability deteriorate.

On the other hand, the remainder other than the above-mentioned components is iron (Fe) and inevitably contained impurities.

Hereinafter, the present invention will be described using comparative examples and examples.

According to the production conditions of the alloy steel for the production of commercially produced knuckles, experiments were performed to produce alloy steels according to Examples and Comparative Examples and to measure physical properties.

The tensile strength was measured using a standard tensile test piece (KS B 0801), and the impact toughness was measured using a standard impact test piece (KS D ISO148-1).

In addition, the depth of corrosion of the alloy steel was measured by using a salt spray tester to measure the corrosion depth after salt spraying of a 5% NaCl aqueous solution at a temperature of 35 ° C for 360 hours. In the case of the corrosion defect depth (㎛), the shallower the corrosion depth, the better the corrosion characteristics.

First, looking at the effect of controlling the content of carbon (C) in detail, it can be confirmed through Table 1 and Figures 1 and 2 below.

1 is a graph showing the tensile strength according to the carbon (C) content control in the present invention, Figure 2 is a graph showing the impact toughness according to the carbon (C) content control in the present invention.

division C Si Mn Cu Nb Ti P S Sample 1 0.48 0.38 1.55 0.3 0.03 0.02 0.018 0.01 Sample 2 0.5 0.37 1.54 0.31 0.03 0.02 0.017 0.017 Sample 3 0.52 0.38 1.55 0.29 0.03 0.02 0.019 0.01 Sample 4 0.58 0.38 1.55 0.3 0.02 0.01 0.018 0.015 Sample 5 0.6 0.39 1.56 0.3 0.03 0.02 0.017 0.012

In Table 1, samples 1, 2, and 5 are comparative examples, and samples 3 and 4 are examples, and the comparative examples and examples are other elements except carbon (C), which is a limited range of the high toughness alloy steel according to the present invention. Equal levels were controlled within the system, and only carbon (C) was used as a control variable.

Since the content of carbon (C) was limited to the range of 0.52 to 0.58%, the content of carbon (C) was less than 0.52% for sample 1 and sample 2, and the content of carbon (C) was greater than 0.58% for sample 5 do.

As can be seen in Figure 1 it was confirmed that the tensile strength increases as the carbon (C) content increases. In particular, it was confirmed that the increase in tensile strength was increased in the section where the content of carbon (C) was 0.52% or more.

However, as can be seen in Figure 2, the carbon (C) content in the range of 0.58% or less, although the impact toughness change was small as the increase of carbon (C), it was confirmed that the impact toughness is rapidly decreased while exceeding 0.58% there was.

Therefore, it is reasonable to limit the content of carbon (C) to a range of 0.52 to 0.58% in order to secure a tensile strength value of 800 MPa or more and an impact toughness value of 90 J / cm 2 or more.

Next, looking at the effect of controlling the content of silicon (Si) in detail, it can be confirmed through Table 2 and FIGS. 3 to 5 below.

3 is a graph showing the tensile strength according to the silicon (Si) content control in the present invention, Figure 4 is a graph showing the impact toughness according to the silicon (Si) content control in the present invention, Figure 5 is a silicon (Si) in the present invention Si) It is a graph showing the degree of surface decarburization according to the content control.

division C Si Mn Cu Nb Ti P S Sample 6 0.55 0.3 1.55 0.31 0.02 0.02 0.018 0.01 Sample 7 0.55 0.33 1.54 0.31 0.03 0.03 0.017 0.012 Sample 8 0.55 0.35 1.55 0.3 0.03 0.02 0.019 0.015 Sample 9 0.56 0.4 1.55 0.3 0.04 0.01 0.02 0.011 Sample 10 0.55 0.42 1.56 0.3 0.03 0.03 0.017 0.013

In Table 2, samples 6, 7 and 10 are comparative examples, and samples 8 and 9 are examples, and the comparative examples and examples are other elements except silicon (Si). It was controlled to the same level within, and only silicon (Si) was used as a control variable.

Since the content of silicon (Si) was limited to the range of 0.35 to 0.40%, the content of silicon (Si) was less than 0.35% for samples 6 and 7, and the content of silicon (Si) was greater than 0.40% for samples 10 do.

As can be seen in Figure 3, it was confirmed that the tensile strength increased as the silicon (Si) content increased. In particular, it was confirmed that the increase in tensile strength was increased in the section where the content of silicon (Si) was 0.35% or more.

However, as can be seen in Figure 4, the silicon (Si) content in the range of 0.40% or less, although the impact toughness was not significantly changed according to the increase of the silicon (Si), it was confirmed that the impact toughness is rapidly reduced while exceeding 0.40% there was.

In addition, as can be seen in FIG. 5, although decarburization did not occur on the surface in a silicon (Si) content of 0.40% or less, the fatigue life is expected to decrease as surface decarburization occurs while exceeding 0.40%.

Accordingly, in order to secure a tensile strength value of 800 MPa or more and an impact toughness value of 90 J / cm 2 or more, it is reasonable to limit the content of silicon (Si) to a range of 0.35 to 0.40%.

Next, looking specifically at the effect of controlling the content of manganese (Mn), it can be confirmed through Table 3 and Figures 6 and 7 below.

6 is a graph showing tensile strength according to manganese (Mn) content control in the present invention, and FIG. 7 is a graph showing impact toughness according to manganese (Mn) content control in the present invention.

division C Si Mn Cu Nb Ti P S Sample 11 0.56 0.38 1.45 0.3 0.03 0.01 0.017 0.01 Sample 12 0.54 0.38 1.48 0.3 0.03 0.02 0.018 0.017 Sample 13 0.55 0.38 1.5 0.31 0.02 0.02 0.017 0.01 Sample 14 0.56 0.38 1.6 0.31 0.03 0.03 0.019 0.015 Sample 15 0.54 0.39 1.62 0.3 0.02 0.02 0.02 0.012

In Table 3, samples 11, 12 and 15 are comparative examples, and samples 13 and 14 are examples, and the comparative examples and examples are other elements except for manganese (Mn). Equal level within the control and only manganese (Mn) was used as the control variable.

Since the content of manganese (Mn) was limited to the range of 1.50 to 1.60%, the content of manganese (Mn) was less than 1.50% for sample 11 and sample 12, and the content of manganese (Mn) for sample 15 was greater than 1.60%. do.

As can be seen in Figure 6, as the manganese (Mn) content increases, it was confirmed that the tensile strength gradually increases.

However, as can be seen in Figure 7, the manganese (Mn) content of 1.60% or less in the section, as the content of manganese (Mn) increases, the impact toughness increases, but it can be confirmed that the impact toughness rapidly decreases while exceeding 1.60%. . In particular, it was confirmed that the increase in impact toughness increased in the section where the manganese (Mn) content was 1.50% or more.

Therefore, it is reasonable to limit the content of manganese (Mn) to a range of 1.50 to 1.60% in order to secure a tensile strength value of 800 MPa or more and a shock toughness value of 90 J / cm 2 or more.

Next, looking specifically at the effect of controlling the content of copper (Cu), it can be confirmed through Table 4 and FIGS. 8 and 9 below.

8 is a graph showing the depth of corrosion in accordance with the copper (Cu) content control in the present invention, Figure 9 is a graph showing the impact toughness in accordance with the copper (Cu) content control in the present invention.

division C Si Mn Cu Nb Ti P S Sample 16 0.54 0.37 1.55 0.2 0.02 0.01 0.019 0.01 Sample 17 0.56 0.38 1.56 0.22 0.03 0.03 0.02 0.017 Sample 18 0.55 0.38 1.54 0.25 0.03 0.01 0.017 0.01 Sample 19 0.56 0.38 1.55 0.35 0.02 0.02 0.017 0.015 Sample 20 0.55 0.37 1.56 0.38 0.03 0.02 0.018 0.012

In Table 4, samples 16, 17, and 20 are comparative examples, and samples 18 and 19 are examples, and the comparative examples and examples are other elements except copper (Cu). The level was controlled to the same level within, and only copper (Cu) was used as the control variable.

Since the content of copper (Cu) was limited to the range of 0.25 to 0.35%, the content of copper (Cu) was less than 0.25% for sample 16 and sample 17, and the content of copper (Cu) was greater than 0.40% for sample 20 do.

As can be seen in Figure 8, in the section where the content of copper (Cu) was less than 0.25%, it was confirmed that the depth of the corrosion defect was deeper than 30 μm, and the depth of the corrosion defect was maintained below 30 μm while exceeding 0.25%. I could confirm that.

And, as can be seen in Figure 9, the copper (Cu) content of 0.35% or less in the section, but the impact toughness was maintained at 90J / ㎠ or more, it was confirmed that the impact toughness is rapidly decreased while exceeding 0.35%. In particular, it was confirmed that the copper (Cu) content exceeds 0.35%, while the precipitation of copper (Cu) alone occurs, the impact toughness decreases.

Therefore, it is reasonable to limit the content of copper (Cu) to the range of 0.25 to 0.35% in order to secure the impact toughness value of 90 J / cm 2 or more while maintaining the depth of corrosion defects of 30 µm or less.

Next, looking specifically at the effect of controlling the content of niobium (Nb), it can be confirmed through Table 5 and Figures 10 and 11 below.

10 is a graph showing the average grain size according to the niobium (Nb) content control in the present invention, Figure 11 is a graph showing the impact toughness according to the niobium (Nb) content control in the present invention.

division C Si Mn Cu Nb Ti P S Sample 21 0.55 0.39 1.55 0.3 - 0.02 0.019 0.01 Sample 22 0.56 0.37 1.56 0.3 0.01 0.03 0.02 0.015 Sample 23 0.54 0.38 1.55 0.3 0.02 0.02 0.017 0.012 Sample 24 0.54 0.38 1.54 0.3 0.04 0.01 0.017 0.01 Sample 25 0.56 0.38 1.55 0.31 0.06 0.03 0.018 0.017

In Table 5, samples 21, 22, and 25 are comparative examples, and samples 23 and 24 are examples, and the comparative examples and examples are other elements except niobium (Nb). The level was controlled to the same level within, and only niobium (Nb) was used as a control variable.

Since the content of niobium (Nb) was limited to the range of 0.02 to 0.04%, the content of niobium (Nb) was less than 0.02% for samples 21 and 22, and the content of niobium (Nb) was greater than 0.04% for samples 25 do.

As can be seen in Figure 10, in the section where the content of niobium (Nb) is less than 0.02%, it was confirmed that the average of the grain size (grain size) exceeds 16㎛, grain size (grain size) in the section over 0.02% It was confirmed that the average of was maintained at 16 µm or less. Toughness can be expected to be improved as the average of the grain sizes is kept below 16 µm.

And, as can be seen in Figure 11, the niobium (Nb) content is less than 0.02% in the section, as the content of niobium (Nb) increases, the impact toughness increases, but it can be confirmed that the impact toughness rapidly decreases while exceeding 0.04%. .

Therefore, it is reasonable to limit the content of niobium (Nb) to 0.02 to 0.04% in order to secure an impact toughness value of 90 J / cm 2 or more while maintaining an average of grain sizes of 16 µm or less.

Next, looking specifically at the effect of controlling the content of titanium (Ti), it can be confirmed through Table 6 and Figures 12 and 13 below.

12 is a graph showing the average grain size according to the titanium (Ti) content control in the present invention, Figure 13 is a graph showing the impact toughness according to the titanium (Ti) content control in the present invention.

division C Si Mn Cu Nb Ti P S Sample 26 0.56 0.38 1.56 0.3 0.02 0.005 0.017 0.01 Sample 27 0.55 0.38 1.54 0.3 0.02 0.01 0.018 0.017 Sample 28 0.56 0.39 1.55 0.3 0.03 0.03 0.019 0.01 Sample 29 0.55 0.37 1.56 0.3 0.04 0.05 0.02 0.015

In Table 6, samples 26 and 29 are comparative examples, and samples 27 and 28 are examples, and the comparative examples and examples are other elements except titanium (Ti) within the limits of the high toughness alloy steel according to the present invention. It was controlled at an equivalent level and only titanium (Ti) was used as a control variable.

Since the content of titanium (Ti) was limited to the range of 0.01 to 0.03%, the content of titanium (Ti) was less than 0.01% for sample 26 and the content of titanium (Ti) was greater than 0.03% for sample 29.

As can be seen in Figure 12, in the section where the content of titanium (Ti) is less than 0.01%, it was confirmed that the average of the grain size (grain size) exceeds 16㎛, the grain size (grain size) in 0.01% or more section It was confirmed that the average of) is maintained at 16 µm or less. Toughness can be expected to be improved as the average of the grain sizes is kept below 16 µm. Particularly, when the content of titanium (Ti) maintains an appropriate range, fine titanium (Ti) -based precipitates are formed in the grain boundaries of the metal structure to prevent the grains from growing, but the content of titanium (Ti) is 0.02%. It was confirmed that the impact toughness and fatigue life were reduced by forming coarse titanium (Ti) precipitates in excess.

And, as can be seen in Figure 13, the titanium (Ti) content in the section less than 0.01%, as the content of titanium (Ti) increases, the impact toughness increases, but it can be confirmed that the impact toughness rapidly decreases while exceeding 0.02%. .

Therefore, it is reasonable to limit the content of titanium (Ti) to a range of 0.01 to 0.03% in order to secure an impact toughness value of 90 J / cm 2 or more while maintaining an average of grain sizes of 16 µm or less.

Next, looking at the effect of controlling the content of the main alloy component in detail, it can be confirmed through Table 7 and FIGS. 14 to 17 below.

14 is a graph showing the average of the grain size according to the content control of the main alloy component in the present invention, Figure 15 is a graph showing the tensile strength according to the content control of the main alloy component in the present invention, Figure 16 is in the present invention It is a graph showing the impact toughness according to the content control of the main alloy component, and FIG. 17 is a graph showing the depth of corrosion cracking according to the content control of the main alloy component in the present invention.

division C Si Mn Cu Nb Ti P S Sample 30 0.5 0.32 1.48 0.22 0.01 0.005 0.018 0.018 Sample 31 0.52 0.35 1.5 0.25 0.02 0.01 0.016 0.018 Sample 32 0.55 0.38 1.55 0.3 0.03 0.02 0.012 0.015 Sample 33 0.58 0.4 1.6 0.35 0.04 0.03 0.015 0.015 Sample 34 0.6 0.42 1.63 0.38 0.06 0.05 0.013 0.013

In Table 7, samples 30 and 34 are comparative examples, and samples 32, 33 and 34 are examples, and the comparative examples and examples are titanium (Ti), silicon (Si), and manganese (Mn), which are main alloy components. The contents of copper (Cu), niobium (Nb) and titanium (Ti) were used as control variables.

In the case of Sample 30, the content of the main alloy component is less than the content of the main alloy component suggested in the present invention, and in the case of Sample 34, the content of the main alloy component exceeds the content of the main alloy component suggested in the present invention.

As can be seen in Figure 14, the sample 30 in which the content of the main alloy component is less than the content of the main alloy component presented in the present invention was confirmed that the average of the grain size (grain size) exceeds 16㎛, the main Samples 31, 32 and 33 in which the content of the alloy component satisfies the content of the main alloy component suggested in the present invention and sample 34 in which the content of the main alloy component exceeds the content of the main alloy component suggested in the present invention include grain size ( It was confirmed that the average of the grain size) was kept as small as 16 µm or less.

As can be seen in FIG. 15, it was confirmed that the sample 30 in which the content of the main alloy component was less than the content of the main alloy component suggested in the present invention was confirmed to be less than the tensile strength of 800 MPa, and the content of the main alloy component was the present invention. It was confirmed that the samples 31, 32 and 33 satisfying the content of the main alloying components suggested in and the samples 34 in which the content of the main alloying components exceeded the content of the main alloying components suggested in the present invention maintain a tensile strength of 800 MPa or more. Could.

As can be seen in FIG. 16, the sample 30 in which the content of the main alloy component is less than the content of the main alloy component suggested in the present invention and the sample in which the content of the main alloy component exceeds the content of the main alloy component suggested in the present invention 34 was confirmed that the impact toughness is less than 90J / ㎠, samples 31, 32 and 33, the content of the main alloy component satisfies the content of the main alloy component suggested in the present invention, impact toughness of 90 J / cm 2 or more It was confirmed that it was maintained.

As can be seen in FIG. 17, the sample 30 in which the content of the main alloy component was less than the content of the main alloy component suggested in the present invention was confirmed to have a depth of corrosion defects greater than 30 μm, and the content of the main alloy component was Samples 31, 32 and 33 satisfying the content of the main alloying components suggested in the present invention and samples 34 in which the content of the main alloying components exceed the content of the main alloying components suggested in the present invention have a depth of corrosion of 30 µm or less It was confirmed that it is maintained.

Next, when looking specifically at the effect of controlling the cooling rate during heat treatment, it can be confirmed through Table 8 below.

To find out the effect of cooling rate control during heat treatment, the alloy with the alloy component of Sample 32 was hot forged at 1200 ° C and cooled at the cooling rate shown in Table 8 below, and the microstructure, tensile strength, and impact toughness were cooled. Confirmed.

18 is a photograph showing a tissue according to the cooling rate control in the present invention.

division Cooling rate (℃ / min) Ferrite fraction (%) Tensile strength (MPa) Impact strength (J / ㎠) Sample 35 20 20 765 110 Sample 36 30 17 780 100 Sample 37 40 15 835 97 Sample 38 50 12 875 95 Sample 39 60 10 900 92 Sample 40 70 8 915 80 Sample 41 80 7 920 65

As can be seen in Figure 18, as a result of confirming the microstructure of samples 35 to 41, it was confirmed that ferrite and pearlite were formed in each sample. However, as can be seen in Table 8, samples 37, 38, and 39 having a cooling rate of 40 to 60 ° C / min were able to confirm that the ferrite fraction was 10 to 15%, and samples 35 with a cooling rate of less than 40 ° C / min. It was confirmed that the fraction of ferrite exceeded 15%, and the samples of samples 40 and 41 having a cooling rate exceeding 60 ° C./min had a ferrite fraction of less than 10%.

Further, it was confirmed that samples 37 to 41 having a cooling rate of 40 ° C./min or more maintained a tensile strength of 800 MPa or more, and samples 35 and 36 having a cooling rate of less than 40 ° C./min were less than a tensile strength of 800 MPa. Could.

In addition, it was confirmed that samples 35 to 39 having a cooling rate of 60 ° C./min or less maintained an impact toughness of 90 J / cm 2 or more, and samples 40 and 41 having a cooling rate of more than 60 ° C./min had an impact toughness of 90 J / cm 2 It was confirmed that it was not enough.

Therefore, when the fraction of ferrite was maintained at 10-15%, the tensile strength value was maintained at 800 MPa or more, and the impact toughness value was secured at 90 J / cm 2 or more, and the fraction of ferrite was maintained at 10-15%. In order to confirm that it is desirable to maintain the cooling rate at 40 ~ 60 ℃ / min.

Although the present invention has been described with reference to the accompanying drawings and the preferred embodiments described above, the present invention is not limited thereto, and is limited by the claims below. Therefore, those of ordinary skill in the art can variously modify and modify the present invention without departing from the technical spirit of the claims to be described later.

Claims (6)

In weight percent, carbon (C): 0.52 to 0.58%, silicon (Si): 0.35 to 0.40%, manganese (Mn): 1.50 to 1.60%, copper (Cu): 0.25 to 0.35%, niobium (Nb): 0.02 ~ 0.04%, titanium (Ti): 0.01 ~ 0.03%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), residual iron (Fe) and other inevitable High toughness alloy steel containing impurities.
The method according to claim 1,
The alloy steel is a high toughness alloy steel, characterized in that the average grain size (grain size) of 16㎛ or less.
The method according to claim 1,
The alloy steel is formed of ferrite and pearlite, a high toughness alloy steel characterized in that the ferrite fraction is 10 to 15%.
The method according to claim 1,
The alloy steel is a high toughness alloy steel, characterized in that the tensile strength is 800MPa or more.
The method according to claim 1,
The alloy steel is a high toughness alloy steel, characterized in that the impact toughness is more than 90J / ㎠.
The method according to claim 1,
The alloy steel is a high toughness alloy steel characterized in that the depth of corrosion cracking measured after salt spraying of a 5% NaCl aqueous solution at a temperature of 35 ° C. for 360 hours using a salt spray tester is 30 μm or less.

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