KR100489021B1 - A method for manufacturing high tensile high toughness steel having superior internal quality - Google Patents

Abstract

The present invention relates to the production of high strength high toughness steel used in bulletproof steels, etc. The purpose of the present invention is to provide a method of manufacturing steel having high strength and high toughness while having excellent internal quality by appropriately controlling steelmaking conditions and continuous casting conditions. .

The present invention for achieving the above object in the method for producing high strength high toughness steel, in weight%, C: 0.13 ~ 0.18%, Si: 0.20 ~ 0.40%, Mn: 0.50 ~ 0.80%, P: 0.05% or less, S: 0.005% or less, Soluble Al: 0.025% or less, Ni: 3.5-4.5%, Cr: 0.8-1.2%, Mo: 0.4-0.6%, Nb: 0.03-0.06%, Ti: 0.005-0.020%, and the balance Fe After refining, the superheat of the refined molten steel in the tundish is maintained in the range of 18 ~ 23 ℃, while continuously casting the molten steel at a casting speed of 0.95 ~ 1.0m / min in the secondary cooling zone The technical gist of the present invention relates to a method for producing a high strength, high toughness steel having excellent internal quality, including continuous casting with a yield in the range of 0.4 to 0.45 l / Kg, and casting at a constant speed at the end of the continuous casting.

Description

Manufacturing method of high strength high toughness steel with excellent internal quality {A METHOD FOR MANUFACTURING HIGH TENSILE HIGH TOUGHNESS STEEL HAVING SUPERIOR INTERNAL QUALITY}

The present invention relates to the production of high strength, high toughness steel used for bulletproof steel, and more particularly, to a method of manufacturing steel having high strength and high toughness while having excellent internal quality by appropriately controlling steelmaking conditions and continuous casting conditions.

Normally, high strength steel sheets used for bulletproof materials such as electronic shell plates and self-propelled shell plates are required to have high strength and high toughness. Specifically, based on MIL-A-12560H and MIL-A-12560D presented in the US Navy standard, at least tensile strength 90-110kgf / mm2, low temperature toughness (-40 ℃) 3.9Kg When m or more was satisfied, it was evaluated to have desirable properties.

Looking at a typical conventional technology as a steel that satisfies these conditions, according to the Republic of Korea Patent Publication No. 98-45927 has proposed a method for producing a 100Kg class high-tension thick plate having excellent internal quality by the continuous casting method. That is, according to the present disclosure, in weight percent, C: 0.10 to 0.19%, Si: 0.15 to 0.35%, Mn: 0.10 to 0.40%, P: 0.012% or less, S: 0.005% or less, Ni: 2.75 ~ 3.50%, Cr: 1.30 ~ 1.80%, Mo: 0.35 ~ 0.45%, V: 0.010 ~ 0.025%, When continuously casting molten steel consisting of balance Fe, the specific quantity of secondary cooling zone is 0.32 ~ 0.40ℓ / Kg. After the slab is manufactured by continuous casting under the conditions, the slab is multi-stacked and air-cooled to room temperature, and after cooling by varying the cooling rate according to the thickness after the usual rolling, the thick plate material in the temperature range of 900 ~ 920 ℃ It is thoughtful. According to the published patent, an ultra high tensile plate for bulletproof having excellent internal quality is provided. However, the steel is a steel added with V in addition to Ni, Cr, Mo components, but the strength of the material is increased due to the V content, but the internal quality is not very excellent, especially the process control is very difficult for continuous casting have.

Therefore, the present invention has been proposed to solve the above-mentioned conventional problems, and its object is to add Ti, Nb instead of V in the steelmaking process, while maintaining a low Ti and Al component, such as Ni, Cr, Mo, Nb, etc. By degassing the element-containing steel sufficiently and in the continuous casting process, low-speed casting and intermediate cooling, high strength with excellent surface and internal quality of the cast steel without problems such as nozzle clogging and bending of the casting during continuous casting To provide toughness steel.

The present invention for achieving the above object in the method of manufacturing high strength high toughness steel,

By weight%, C: 0.13 to 0.18%, Si: 0.20 to 0.40%, Mn: 0.50 to 0.80%, P: 0.05% or less, S: 0.005% or less, Soluble Al: 0.025% or less, Ni: 3.5 to 4.5% , Cr: 0.8 ~ 1.2%, Mo: 0.4 ~ 0.6%, Nb: 0.03 ~ 0.06%, Ti: 0.005 ~ 0.020%, and after refining to make up the balance Fe, the superheat degree of the refined molten steel in tundish In the range of 18 ~ 23 ℃, continuously casting the molten steel at a casting speed of 0.95 ~ 1.0m / min in the secondary cooling zone in a continuous casting in the range of 0.4 ~ 0.45L / Kg, At the end of continuous casting, the present invention relates to a method for producing high strength high toughness steel having excellent internal quality, including casting at a constant speed.

Hereinafter, the present invention will be described in detail.

Carbon contained in the steel of the present invention is a component that increases the hardness and strength of the steel by increasing the hardenability during the heat treatment of the steel. If the carbon content is more than 0.18%, the toughness and weldability of the steel may be impaired. If the carbon content is less than 0.13%, the hardenability may be difficult to secure desired hardness.

Si is an element that forms carbide to form a solid solution in Fe to increase the elastic limit tensile force. However, if the Si content is less than 0.20%, the effect is insufficient. If the Si content is more than 0.4%, the ferrite structure of the steel is lowered and toughness is formed by forming a non-metallic inclusion such as SiO ₂ .

The Mn is an element that improves the hardenability, and is an effective component capable of improving the hardness during heat treatment of the final steel. However, when the Mn content is less than 0.50%, the hardenability improvement effect is insufficient, and when the content of Mn is more than 0.80%, the strength improvement effect is great, but the weldability is not good, which is not preferable.

The P component is the largest impurity that inhibits low-temperature impact toughness of the steel, and in particular, deteriorates the internal quality of the cast steel, so the content thereof is preferably limited to 0.015% or less.

S is not only an element deteriorating the internal quality of the cast steel, but also a harmful element like the P component. In particular, the S component is not only a cause of deterioration of low-temperature impact toughness in a thick plate product, but also impairs weldability. Therefore, it is preferable to refine the S component to 0.005% or less.

The Al is mainly added to refine the particles by deoxidizer or AlN, but in Ti-added steel, it is preferable to refine it to 0.025% or less since it is an element causing nozzle clogging.

Ni is an alloying element that increases low-temperature toughness and improves hardenability. However, addition of Ni to less than 3.5% is not preferable because the effect is small. However, it is necessary to keep the Ni content higher than 4.5% because Ni itself is expensive and disadvantageous economically.

Cr is an element detrimental to quenchability and is widely used to increase hardness and strength. However, when a large amount of Cr is added, it is detrimental to weldability, and when a small amount of Cr is added, it is difficult to secure the hardness of the steel. Therefore, it is preferable to manage the content in the range of 0.8 to 1.2%.

The Mo contributes to the improvement of creep rupture strength of the austenite state in the steel structure and improves the quenching property to increase the strength, but when the addition of a small amount is insufficient, the effect of causing a brittle brittleness when a large amount is added. And it is desirable to manage the content in the range of 0.4 ~ 0.6% because it is harmful to the deterioration of the weld toughness.

The Ti is a component that precipitates in the structure in the form of TiN in general, thereby improving toughness and contributing to increase in strength by particle refinement and precipitation strengthening. However, when the Ti component is added, it is a component that is the main cause of nozzle clogging during continuous casting operation. That is, Ti has the characteristic of growing with Al ₂ O ₃ as a nucleus, and at this time, Al ₂ O ₃ is present inside TiN, and the melting point of molten steel decreases as the Al decreases. If this increases, nozzle clogging is frequent. Therefore, in order to prevent this, the Ti and Al should be reduced as much as possible or the casting temperature should be increased. In the case of the Ti component, the present invention is preferably managed in the range of 0.005% to 0.020% as much as possible.

The Nb is an element that precipitates in the structure in the form of Nb (C, N) to contribute to the increase in strength, for this purpose, it is preferable to manage in the range of 0.03 to 0.06%.

In the refining process, if the hydrogen content is high in the molten steel, it will cause cracks in the cast steel or steel products over time, it is preferable to sufficiently degassing at low vacuum. Preferably, degassing of molten steel is performed for about 20 minutes or less in 2 torr or less.

In the present invention, when continuously casting using the refined molten steel, the superheat degree of the tundish, the speed of the continuous casting machine and the cooling conditions of the cast steel are very important.

First, superheat management of molten steel during continuous casting is very important to secure castability and improve the internal quality of cast steel. To this end, in the case of the present invention, the superheat degree of molten steel needs to be in the range of 18 to 23 ° C. However, if the superheat of molten steel is too high, it is likely that central segregation is generated in the cast steel due to the development of columnar tissue and a decrease in equiaxed tissue. On the contrary, if too low, nozzle clogging may occur during casting or insufficiency of inclusion separation may deteriorate the cleanliness of the steel.

In addition, in the present invention, it is suitable that the continuous casting speed is in the range of 0.95 to 1.0 m / min. This casting speed is a low-speed casting that does not cause the bending of the cast, the uniformity of molten steel flow and casting cooling within the mold, and the sufficient solidification shell can be secured to reduce the bulging phenomenon in the lower part of the mold Internal quality can be made favorable.

In addition, the cast steel that is continuously cast at the above-described casting speed to the bottom of the mold to the secondary cooling zone is required to cool the secondary water by spraying the cooling water in the range of 0.4 ~ 0.45ℓ / Kg. The high strength high toughness steel according to the present invention is a steel to which Nb, Ti, and Ni are added, and in order to prevent surface cracks caused by high temperature embrittlement by Nb and Ti precipitates, the cooling pattern in the secondary cooling zone is preferably weakly cooled. However, when too cold, the internal quality deteriorates and it is difficult to prevent the surface cracks of the high Ni-containing steel due to the miniaturization of the austenitic particles of the final steel.

On the other hand, continuous casting is called the end of continuous casting from the time when the ladle collector nozzle gate is closed to the time when the tundish sliding gate is closed. After that, it is accelerated to finish casting. However, since the surface and the internal quality deteriorate with the change in the casting speed, it is preferable that the casting is performed in the casting speed range even at the end of the continuous casting in the present invention to perform the constant casting without changing the casting speed.

As described above, the present invention provides continuous degassing treatment of alloy element-containing steels such as Ni, Cr, Mo, and Nb while maintaining low Ti and Al components in the steelmaking process, and low-speed casting and intermediate cooling in the continuous casting process. When casting, it is possible to obtain high strength and high toughness steel with excellent surface and internal quality of the cast steel without any operational problems such as nozzle clogging and cast bending.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

After the molten steel was blown in the converter to have the composition as shown in Table 1, after degassing for 20 minutes or more in a low vacuum of 2torr or less, the refined molten steel was maintained in the tundish in the range of 18 ~ 23 ℃. Continuous casting. At this time, the casting speed during the continuous casting and the specific quantity in the secondary cooling zone was performed as shown in Table 2. In this way, a cast piece having a thickness of 200 mm and a width of 1600 mm was obtained.

In Table 2 of this example, the surface crack index is classified by classifying from 0 to 3 according to the degree of cracking, and then calculated by dividing the total defect index by 4 by checking each surface defect index on 4 sides of the cast steel. It was. In addition, the central segregation index was determined by classifying 0 to 4 grades.

division Chemical composition (% by weight) C Si Mn P S s. Al Ni Cr Mo V Nb Ti Inventive Steel a 0.15 0.29 0.59 0.008 0.002 0.009 4.25 0.98 0.49 0.045 0.012 Inventive Steel b 0.16 0.30 0.57 0.010 0.003 0.012 4.01 1.0 0.50 0.046 0.014 Invention steel c 0.15 0.28 0.60 0.011 0.002 0.024 3.58 0.99 0.48 0.048 0.020 Comparative Steel A 0.17 0.25 0.25 0.010 0.002 0.035 3.00 1.50 0.40 0.020 Comparative Steel B 0.15 0.25 0.60 0.008 0.002 0.030 4.4 1.0 0.50 0.045 0.025 Comparative Steel C 0.16 0.25 0.60 0.009 0.001 0.015 3.95 0.99 0.48 0.050 0.010 Comparative Steel D 0.16 0.27 0.59 0.009 0.003 0.024 4.14 0.98 0.48 0.047 0.020 Comparative Steel E 0.15 0.25 0.57 0.012 0.002 0.032 3.78 0.99 0.50 0.049 0.020 Comparative Steel F 0.16 0.27 0.57 0.011 0.002 0.027 3.99 1.0 0.47 0.046 0.024

division Steel grade Superheat degree (℃) Casting speed (m / min) Specific water quantity (ℓ / Kg) Mechanical properties Cast bending Nozzle clogging during casting Cast quality index Tensile Strength (kg / ㎡) Impact value (J) Surface Defect Index Internal Defect Index Inventive Example 1 a 23 0.95 0.38 133 59 Good Good 0.12 0.5 Inventive Example 2 b 18 1.0 0.43 131 58 Good Good 0.20 0.5 Inventive Example 3 c 21 0.98 0.40 130 57 Good Good 0.10 0.5 Comparative Example 1 A 15 0.95 0.40 118 45 Occur Good 0.43 0.5 Comparative Example 2 B 17 1.05 0.32 128 47 Good Occur 0.41 1.5 Comparative Example 3 C 17 0.95 0.36 127 45 Good Good 0.40 1.0 Comparative Example 4 D 18 0.90 0.36 132 49 Occur Good 0.87 0.5 Comparative Example 5 E 22 0.95 0.38 127 46 Good Occur 0.51 1.0 Comparative Example 6 F 19 0.1 0.43 129 47 Good Occur 0.91 1.0

As shown in Table 2, in Comparative Example (1) and (4) which were subjected to low temperature or low speed casting, castability was inferior due to the occurrence of cast bending, and in Comparative Example (2) and (6) where Ti and Al contents were high. Nozzle clogging occurred. The stronger the cold, the better the internal quality as in Comparative Example (6), but the surface quality tended to be insufficient. In addition, in the case of Comparative Example (2) in which the casting speed was increased while being weakly cooled, the opposite phenomenon appeared. In the case of Comparative Example (3), the surface defects were somewhat insufficient due to casting with low temperature and low temperature.

On the other hand, in the case of Inventive Example (1-3) that satisfies the conditions of the present invention, it was found that the surface quality and the internal quality were very good without the nozzle clogging as well as the bending phenomenon during the continuous casting.

As described above, according to the present invention, in the continuous casting of high-strength high toughness steel containing various alloying elements such as Ni and Ti, by appropriately controlling the refining process and continuous casting conditions, high alloy steel thick plate material having excellent surface quality as well as internal quality There is a very useful effect that can supply a stable.

Claims (1)

In the manufacturing method of high strength high toughness steel, By weight%, C: 0.13 to 0.18%, Si: 0.20 to 0.40%, Mn: 0.50 to 0.80%, P: 0.05% or less, S: 0.005% or less, Soluble Al: 0.025% or less, Ni: 3.5 to 4.5% , Cr: 0.8 ~ 1.2%, Mo: 0.4 ~ 0.6%, Nb: 0.03 ~ 0.06%, Ti: 0.005 ~ 0.020%, and after refining to make up the balance Fe, the superheat degree of the refined molten steel in tundish In the range of 18 ~ 23 ℃, continuously casting the molten steel at a casting speed of 0.95 ~ 1.0m / min in the secondary cooling zone to a continuous casting in the range of 0.4 ~ 0.45ℓ / Kg, High strength with excellent internal quality, characterized in that it comprises casting at constant speed at the end of continuous casting, from the time when the collector nozzle gate is closed to the time when the tundish sliding gate is closed. Manufacturing method of high toughness steel