KR100940038B1 - Non quenched and tempered steel for hot forging with excellent impact toughness and a method for manufacturing the same and the chassis parts for automobile using the same - Google Patents

Abstract

The present invention relates to a hot forging non-ferrous forged steel excellent in impact toughness, C: 0.29 ~ 0.33 wt%, Si: 0.15 ~ 0.35 wt%, Mn: 1.30 ~ 1.60 wt%, P: 0.030 wt% or less, S: 0.015 ~ 0.030% by weight, Ni: 0.05-0.20% by weight, Cr: 0.05-0.20% by weight, V: 0.04-0.08% by weight, Al: 0.010-0.030% by weight, B: 0.0003-0.0020% by weight, Ti: 0.010-0.020 Wt%, and N: 0.0050-0.013 wt%, with the balance being made of Fe and unavoidable impurities. Since the non-alloyed steel according to the present invention has excellent balance between strength and toughness, compared to conventional tempered heat-treated steel, there is an effect that can be applied to parts for automobile chassis such as spindles, knuckles, upper arms, etc., which require high impact toughness. .

High Toughness, High Strength, Non-Steel Steel, Chassis Parts

Description

Non-quenched and tempered steel for hot forging with excellent impact toughness and a method for manufacturing the same and the chassis parts for automobile using the same}

The present invention relates to hot-forged non-ferrous steel, a method for manufacturing the same, and a component for an automobile chassis manufactured by the same, and more particularly, an impact that can be applied to an automobile chassis component requiring high toughness such as a spindle, a knuckle, and an upper arm. The present invention relates to a hot forging non-ferrous forged steel, a method for manufacturing the same, and a component for an automobile chassis manufactured by the same.

Conventional methods for manufacturing parts for automobile chassis such as spindles, knuckles, upper arms, etc., are heated at 1200 ° C to 1250 ° C using carbon steel containing 0.38 to 0.48% by weight of C. Air cooled after forging. Thereafter, the forged product is reheated in the range of 820 ° C. to 870 ° C., followed by quenching by water cooling or oil cooling, and then subjected to heat treatment at 550 ° C. to 650 ° C. to give heat and toughness.

Conventionally, while reflecting the poor scale of the heat treatment company, in order to suppress the deformation generated during the heat treatment is quenched by oil cooling rather than water cooling. However, due to incomplete quenching by oil cooling, the hardness is low and the toughness is excellent, but it is difficult to meet the requirements for yield strength. As a result, there was a limit to the application of parts requiring durability under severe conditions such as being subjected to strong bending stress and torsional stress during operation.

On the other hand, Patent No. 10-0716345 is a high toughness invented to solve the problems caused by the increase in manufacturing costs due to the addition of expensive alloying elements and tempered (annealed-concerned) heat treatment in the manufacturing method of the spindle manufactured using conventional carbon steel and alloy steel Amorphous steel, 0.15 to 0.25% by weight C, 1.90 to 2.30% by weight Cr, 0.40 to 0.80% by weight Si, 0.30 to 0.50% by weight Mo, 0.50 to 0.70% by weight Mn and Al, Nb, etc. It is a method of giving toughness by strict controlled cooling after forging. In addition, in order to compensate for the strength due to low carbonization, the production cost is increased by the addition of alloying elements such as Cr, Mo, and Si, and strict controlled cooling is accompanied, and there is a limit to the advantages of replacing the conventional carbon tempered steel heat treatment material.

The present invention is to solve the problems as described above, to reduce the carbon content in order to improve the impact toughness, and to improve the toughness of the hot forging composition added by adding Ti, V, etc. to prevent grain coarsening at high temperature heating To develop vaginal steel.

In addition, an object of the present invention is to provide a method for producing a high toughness hot forged steel for application of parts for automobile chassis that can secure strength and impact toughness not only by control cooling but also by air cooling after forging.

In addition, the object of the present invention is to replace the carbon steel that is applied by securing the strength and toughness by hardening-concern, excellent impact toughness that can effectively contribute to cost reduction, shortening the manufacturing process and environmental pollution by eliminating the heat treatment process To provide chassis parts.

In order to achieve the above object, the hot forging non-ferrous forged steel with excellent impact toughness according to the present invention is C: 0.29 ~ 0.33% by weight, Si: 0.15 ~ 0.35% by weight, Mn: 1.30 ~ 1.60% by weight, P: 0.030% by weight % Or less, S: 0.015-0.030 weight%, Ni: 0.05-0.20 weight%, Cr: 0.05-0.20 weight%, V: 0.04-0.08 weight%, Al: 0.010-0.030 weight%, B: 0.0003-0.0020 weight% , Ti: 0.010% to 0.020% by weight and N: 0.0050% to 0.013% by weight, and the balance consists of Fe and unavoidable impurities.

In addition, an object according to the present invention is achieved by a method for producing non-alloyed steel, characterized in that it comprises the step of hot forging the non-alloyed steel having the above compositions at a temperature of 1150 ℃ 1250 ℃.

In addition, it is preferable to further include the step of air cooling by maintaining the hot forged non-coated steel at a temperature of 400 ℃ to 700 ℃.

In addition, it is preferable to further include the step of rolling the non-elastic steel before the step of hot forging in order to give the size and properties of the hot forging of the non-rough steel.

In addition, the rolling may include rolling the non-coarse steel through the first rolling step and the first rolling step at a temperature of 1100 ° C. to 1140 ° C., primarily at the temperature of 1180 ° C. to 1220 ° C. It is preferred to include a second rolling step.

In addition, the annealing forming ratio of the alloy steel for non-alloyed steel that passed through the first and second rolling steps is preferably 15S or more.

The object of the present invention is also achieved by an automobile chassis component manufactured according to any of the above methods.

According to the present invention as described above, in the case of the present invention steel, even though it contains less carbon, because the balance of strength and toughness is superior to that of conventional tempered heat-treated steel, spindle, knuckle, upper arm, etc. requiring high impact toughness It is possible to replace automobile chassis parts.

In addition, the present invention can reduce the production cost of the conventional heat treatment, that is, the elimination of the hardening and soaking process, as well as to suppress the generation of carbon dioxide and soot during heat treatment, thereby preventing environmental pollution and shortening the delivery time due to the shortening of the manufacturing process, productivity It can also contribute greatly to improvement.

The present invention is a hot forging non-steel for excellent impact toughness, C: 0.29 to 0.33% by weight, Si: 0.15 to 0.35% by weight, Mn: 1.30 to 1.60% by weight, P: 0.030% by weight or less, S: 0.015 to 0.030 Weight%, Ni: 0.05-0.20 weight%, Cr: 0.05-0.20 weight%, V: 0.04-0.08 weight%, Al: 0.010-0.030 weight%, B: 0.0003-0.0020 weight%, Ti: 0.010-0.020 weight% And N: 0.0050 to 0.013% by weight, and the balance consists of Fe and unavoidable impurities.

Hereinafter, the reason for the addition of the alloying component and the limit of the component range of the present invention will be described.

C: 0.29 wt% ~ 0.33 wt%

C is an austenite stabilizing element and is an important element which is dissolved in a matrix at the time of hardening to increase strength and hardness. In the present invention, if the C content is low, it is difficult to secure the hardness by high frequency quenching after hot forging-machining, so it is necessary to contain the C content range of 0.29% by weight or more. However, addition of more than 0.33% by weight is not preferable because the toughness is lowered. Therefore, the content of C is limited to 0.29% by weight to 0.33% by weight.

Si: 0.15 wt% ~ 0.35 wt%

Si is used as an effective deoxidizer in steelmaking and is an element that is added to the base to secure strength by strengthening ferrite. If the content is lower than 0.15% by weight, the strength may be lowered. Since toughness falls, it is not preferable. Therefore, the content of Si is limited to 0.15% by weight to 0.35% by weight.

Mn: 1.30 wt% ~ 1.60 wt%

Mn is a deoxidizer, which improves hardenability and strength, and is added to prevent the harmfulness of S present in the steel to form MnS, thereby preventing red light brittleness and improving machinability. When added more than 1.30% by weight, there is an advantage of increasing the strength by lowering the transformation temperature of austenite to ferrite, but by adding more than 1.60% by weight, the toughness is lowered, so to obtain the required strength without lowering the toughness. It is preferable to limit the Mn content to 1.30% by weight to 1.60% by weight.

V: 0.04 wt% ~ 0.08 wt%

V refines grains by fine carbonitride formation to improve strength and toughness. If the added amount is less than 0.04% by weight, the effect of increasing strength is small. If the addition amount is more than 0.08% by weight, the strength is increased, but toughness is not only lowered, but it is not preferable because there is no economic effect due to the increase in manufacturing cost. Therefore, the V content is limited to 0.04% by weight to 0.08% by weight.

B: 0.0003% to 0.0020% by weight

B is selectively added to improve the hardenability, and is limited to 0.0003% by weight to 0.0020% by weight in consideration of the boron effect in molten steel.

Ti: 0.010 wt% ~ 0.020 wt%

Ti combines with nitrogen (N) in steel to form TiN. The formed TiN increases the impact toughness by the austenite grain growth inhibiting effect. When the added amount is less than 0.010% by weight, the absolute amount of TiN is small, so that the effect of inhibiting grain growth is small, and when the added amount is 0.020% by weight or more, the effect is saturated, so that expensive alloy elements do not need to be added. Therefore, in consideration of the minimum content and saturation amount, the appropriate addition amount is limited to the range of 0.010% to 0.020% by weight.

Al: 0.010 wt% ~ 0.030 wt%

Al acts as a strong deoxidizer and combines with N to refine grains, but adding less than 0.01% by weight decreases deoxidation or grain refining, which is undesirable. Increasing the amount of nonmetallic inclusions, such as ₂ O ₃ , can have a rather detrimental effect. Therefore, the appropriate content range of Al is limited to 0.010 to 0.030% by weight.

N: 0.005% by weight to 0.013% by weight

N combines with V, Ti, Al and the like in the non-steel to form nitrides, thereby contributing to the improvement of strength and toughness due to austenite grain refining. If the added amount is less than 0.005% by weight, sufficient nitride is not formed and if the added amount is more than 0.013% by weight, the effect is saturated, so the amount is limited to 0.005% by weight to 0.013% by weight.

Ni: 0.05% to 0.20% by weight

Ni is an element that increases the hardenability, but less than 0.05% by weight, the hardening effect is not sufficient. If it is more than 0.20% by weight, Ni improves toughness, but it is limited to 0.05% by weight to 0.20% by weight because it is not economical by increasing the manufacturing cost of parts. do.

Cr: 0.05% to 0.20% by weight

Cr is an element that increases the quenchability, and the quenching effect is not sufficient at less than 0.05% by weight, and if it is more than 0.20% by weight, Cr improves toughness, but it is not economical by increasing the manufacturing cost of parts, so it is 0.05% by weight to 0.20% by weight. It is limited.

P: 0.030% by weight or less

If the amount added is more than 0.030% by weight, it will segregate in the austenite grain boundary and lower the toughness, so it is limited to 0.030% by weight or less.

S: 0.015 wt% ~ 0.030 wt%

S combines with Mn in steel to form MnS. It is preferable to add at least 0.015% by weight by forming MnS to improve machinability. If the amount is more than 0.030% by weight, the amount of addition is 0.015% by weight since it causes surface defects and paths due to large inclusions during hot working such as rolling and forging. It is limited to%-0.030 weight%.

(First embodiment)

Hereinafter, the physical properties and processing method of the non-coated steel of the present invention will be described through Examples.

Table 1 shows the chemical components of the inventive steel and the conventional steel having the composition of the present invention.

division C (% by weight) Si (% by weight) Mn (wt%) P (% by weight) S (% by weight) Ni (% by weight) Cr (% by weight) Al (% by weight) V (% by weight) B (% by weight) Ti (% by weight) N (% by weight) Foot river A 0.31 0.26 1.37 0.014 0.022 0.08 0.15 0.019 0.061 0.0007 0.014 0.009 B 0.32 0.25 1.46 0.018 0.018 0.08 0.15 0.019 0.062 0.0014 0.015 0.008 Zhonglai River C 0.40 0.20 0.69 0.026 0.009 0.05 0.11 0.011 0.003 0.0001 0.002 0.008 D 0.38 0.21 0.80 0.009 0.017 0.03 0.05 0.016 0.001 0.0001 0.001 0.004

Heat treatment condition Yield strength YS (kgf / mm ² ) Tensile Strength TS (kgf / mm ² ) Elongation EL (%) Sectional shrinkage ROA (%) Lamination value IV (kgf · m / cm ² ) Hardness HD (HB) Invention River A - 55.5 81.7 21.0 45.9 12.52 223.0 - 53.2 80.7 20.3 45.6 13.45 229.0 Invention River B - 55.5 82.3 20.5 45.9 8.67 239.0 - 56.6 83.5 20.8 45.4 11.10 238.0 Zhonglai River C Water cooling Considering 610 ℃ 61.7 81.7 26.4 61.4 12.15 241.0 Considering 650 ℃ 54.3 75.0 28.8 65.7 13.70 241.0 Oil-cooled 600 ℃ 56.3 83.6 21.8 46.7 7.60 248.0 Considering 630 ℃ 54.2 80.6 24.6 50.7 8.14 239.0 Considering 650 ℃ 52.2 79.0 26.5 51.0 9.04 235.0

Table 2 is a high temperature soak (normalizing) after heating the invention steels A and B having the components shown in Table 1 at 1140 ° C, sampling the material rolled to φ85mm, processing it into a specimen of φ25mm, and then air-cooling after holding at 1200 ° C for 30 minutes. The characteristics and comparative evaluation results are shown with the conventional heat-treated material, which is quenched by water cooling or oil cooling after holding the conventional steel C at 850 ° C. and treated at each temperature.

The evaluation results of the inventive steels A and B in Table 2 show data values normalized at a high temperature at 1200 ° C., and heat treatment was performed under the same conditions. The high temperature heat treatment of 1200 ℃ test material of high temperature steel is selected in consideration of the actual temperature of forging parts.In the case of conventional steel, hardening after heating should be done by water cooling, but in order to minimize deformation occurring during hardening, it is mass produced by oil cooling. Due to incomplete quenching, the microstructure is not uniform, resulting in low strength and impact toughness. In contrast, in the case of the inventive steel according to the present invention, although the carbon content is reduced and the hardening and annealing are omitted, the inventive steel exhibits the same strength and impact toughness as the conventional steel.

Test Items Yield strength YS (kgf / mm ² ) Tensile Strength TS (kgf / mm ² ) Elongation EL (%) Sectional shrinkage ROA (%) Impact value IV (kgfm / cm ² Hardness HD (HB) Foot river Ⅰ (air cooling) 51.4 80.4 20.4 55.6 10.51 225.4 Ⅱ (blowing) 48.6 77.6 19.4 63.4 10.85 220.5 Ⅲ (air cooling) 55.4 83.1 18.5 53.5 10.29 235.0 Ⅲ (controlled cooling) 52.4 79.8 19.0 57.5 12.03 217.0 Ⅳ (slow cooling) 48.0 72.4 23.2 64.7 12.96 197.0 Zhonglai River Ⅰ (QT) 46.8 73.5 23.7 60.9 12.70 207.0 Ⅱ (QT) 52.2 78.5 21.6 54.1 8.26 235.0 Ⅲ (QT) 51.1 78.6 22.0 56.2 9.02 235.0 Ⅳ (QT) 43.4 69.4 25.2 63.5 13.51 207.0

Table 3 shows the results of the forging evaluations for the test items of FIG. 3 manufactured using steel materials having the components of Invented Steel A and Conventional Steel C of Table 1. FIG. In the case of invented steel, blowing, air cooling, slow cooling, and controlled cooling were performed after forging. Controlled cooling was carried out for the purpose of imparting the inhibitory effect of bainite phase and toughness by increasing the ferrite fraction. In controlled cooling, a heating zone of 520 ° C. ± 20 ° C. (500 ° C. to 540 ° C.) installed on the conveyor was used, and the cooling rate at this time was 40 ° C./min. On the other hand, the air was cooled at a rate of 50 ° C./min.

According to Table 3, in the case of the invention steel it can be confirmed that the balance between strength and toughness is superior to the tempered heat treatment material of the conventional steel. This is because conventional steel is hardened by oil cooling, so it is incompletely hardened so that the strength is controlled by the soaking temperature. Therefore, when the strength is high, impact toughness is lowered, resulting in a decrease in balance between strength and toughness.

Second Embodiment

Hereinafter, with reference to the accompanying drawings and examples will be described the manufacturing method of the non-coated steel of the present invention and parts for automobile chassis manufactured thereby.

1 is a view showing a manufacturing process of the hot forged steel produced in accordance with an embodiment of the present invention, Figure 2 is a view showing a forging process according to an embodiment of the present invention, Figure 3 of the present invention 4 is a view showing the external appearance of the parts for automobile chassis to which the non-coarse steel is applied according to an embodiment, Figure 4 is a microstructure of the non-coarse steel produced according to the manufacturing method of the present invention and the crude steel produced according to the conventional manufacturing method This is a diagram showing the comparison.

Referring to FIG. 1, a method of manufacturing non-steel bars and forgings using an amorphous steel for hot forging having excellent impact toughness having the above compositions will be described.

First, after preparing the cast steel of the non-rough steel having the composition as described above, the cast steel is rolled in order to give the size and physical properties of the hot forging (S1, S2).

Rolling is divided into primary rolling, which produces a small billet for billet and product by heating 510 × 390mm, and secondary rolling, which produces a product by heating a rolled billet or performance billet. 1st rolling (S1) is for compact rolling by Cogging Mill after heating □ 510 × 390mm boulder produced by electric melting and continuous casting at 1200 ℃ ± 20 ℃ (1180 ℃ -1212 ℃) Manufacture billets.

After that, the semi-finished rolling billet or performance billet (□ 180 × 180 mm) is reheated at 1120 ° C. ± 20 ° C. (1100 ° C. to 1140 ° C.), and then secondarily rolled into an amorphous bar steel having an annealing forming ratio of 15 S or more (S2). The reason why the reheating temperature is lowered to 1100 ° C. to 1140 ° C. during the secondary rolling is that when the reheating temperature exceeds 1140 ° C., the austenite grains grow excessively and the toughness decreases, and the forging temperature decreases below 1100 ° C. This is because the life of the forging is significantly reduced. Therefore, the reheating temperature in the secondary rolling before forging is preferably performed at 1100 ° C to 1140 ° C.

Thereafter, the secondary rolled steel is cut to an appropriate length (S3), and then subjected to induction heating (Induction heating) for 7 to 10 minutes in a temperature range of 1150 ° C to 1250 ° C (S4). Here, if the heating temperature is less than 1150 ℃ heating temperature is too low, hot forging must be performed after reheating, if the heating temperature exceeds 1250 ℃ austenite particles are grown excessively to reduce the toughness of the non-coated steel Since it can not be bonded, it is carried out in a temperature range of 1150 ℃ ~ 1250 ℃. However, induction heating in the temperature range of 1200 ° C to 1250 ° C is more preferable.

 After the hot forging (S5) and trimming (S6) in the shape of the parts for the automobile chassis. The forged product according to the present invention, which has undergone hot forging and trimming, does not require strict controlled cooling and hardening heat treatment, and is then air cooled after charging the conveyor (S7). By air cooling, a forged product excellent in impact toughness is produced. Here, the forging product manufacturing process was manufactured through an upsetting process, a blocker process, and a final process, as shown in FIG.

Further, at air cooling, at a temperature of 400 ° C. to 700 ° C., a cooling rate of 10 ° C./min to 50 ° C./min, preferably 40 ° C./min to 50 ° C./min at a temperature of 500 ° C. to 540 ° C. By carrying out a controlled cooling step at a speed, this results in a forging with excellent balance of strength and toughness and a uniform microstructure, i.e. parts for automobile chassis (spindles, knuckles, upper arms, etc.) as shown in FIG. Are manufactured.

Figure 3 shows the raw material dimensions and appearance shape for the test item to which the non-coarse steel manufactured according to the present invention is applied, that is, parts for automobile chassis. Here, in the case of the present invention steel after the hot forging, the conveyor charging temperature is 950 ℃ ~ 1000 ℃, extraction temperature was 480 ℃ ~ 510 ℃ and the transfer time was 10 to 15 minutes.

On the other hand, Figure 4 is a view showing a comparison between the microstructure of the crude steel produced according to the conventional manufacturing method and the non-crude steel produced according to the manufacturing method of the present invention, it shows a microstructure of III of the test items of FIG. In the case of Test Item III, after the high temperature heating forging (hot forging) of 1200 ° C. or more, the microstructure according to the cooling method showed a uniform ferrite-pearlite structure, and the conventional steel exhibited an incompletely quenched structure in which ferrite was present.

As described above, the steel of the present invention is excellent in balance between strength and toughness as compared to the tempered heat treatment material of the conventional steel, and thus it can be confirmed that an alternative application of parts for automobile chassis applied to spindles, knuckles, uppers, and the like is possible.

1 is a view showing a manufacturing process of the hot forgings forged steel produced according to an embodiment of the present invention.

2 is a view showing a forging process according to an embodiment of the present invention.

3 is a view showing the external appearance of the parts for automobile chassis to which non-coarse steel is applied according to an embodiment of the present invention.

Figure 4 is a view showing a comparison between the microstructure of the crude steel produced according to the conventional manufacturing method and non-crude steel produced according to the production method of the present invention.

Claims (7)

As forged steel for hot forging with excellent impact toughness, C: 0.29 to 0.33% by weight, Si: 0.15 to 0.35% by weight, Mn: 1.30 to 1.60% by weight, P: 0.030% by weight or less, S: 0.015 to 0.030% by weight, Ni: 0.05 to 0.20% by weight, Cr: 0.05 -0.20 wt%, V: 0.04-0.08 wt%, Al: 0.010-0.030 wt%, B: 0.0003-0.0020 wt%, Ti: 0.010-0.020 wt%, and N: 0.0050-0.013 wt% Hot forged steel with high impact toughness made of Fe and unavoidable impurities. A method for producing non-alloyed steel, comprising hot forging the non-alloyed steel having the compositions according to claim 1 at a temperature of 1150 ° C to 1250 ° C. The method of claim 2, And maintaining the hot forged non-alloyed steel at a temperature of 400 ° C to 700 ° C for air cooling. The method of claim 2, In order to give the size and physical properties of the hot forging of the non-steel, further comprising the step of rolling the non- crude steel before the step of hot forging. The method of claim 4, wherein The rolling may include a first rolling step of primarily rolling the non-alloyed steel at a temperature of 1180 ° C. to 1220 ° C. and a second rolling of the non-alloyed steel that has been subjected to the first rolling step at a temperature of 1100 ° C. to 1140 ° C. A method for producing non-alloyed steel, comprising the step of rolling. The method of claim 5, The tempered forming ratio of the alloy steel for non-alloyed steel that passed through the first and second rolling steps is 15S or more. Vehicle chassis parts manufactured according to any one of claims 2 to 6.

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