KR101467029B1 - Steel - Google Patents

Abstract

A steel material capable of reducing the hydrolytic defects while having high hardness through adjustment of alloy components such as calcium, and a method for producing the same.
The steel according to the present invention contains, by weight%, 0.44 to 0.46% of carbon (C), 0.25 to 0.30% of silicon (Si), 0.8 to 1.2% of manganese (Mn) (Si): 0.01-0.03%, Cu: 0.05-0.15%, Ni: 0.04-0.07%, Cr: 0.1-0.2%, Mo: 0.01-0.05%, aluminum 0.005 to 0.01% of vanadium (V), 0.004 to 0.005% of calcium (Ca), 0.008 to 0.015% of nitrogen (N), 0.005% or less of oxygen (O) (Fe) and inevitable impurities.

Description

Steel {STEEL}

The present invention relates to a steel material manufacturing technique, and more particularly, to a steel material capable of decreasing hydraulic defects while having high hardness by controlling an alloy component such as calcium and a manufacturing method thereof.

The hydrogen solubility of steels decreases with decreasing temperature.

In addition, as the temperature decreases, the hydrogen diffusing ability also decreases, and the external diffusion of the internal hydrogen due to the cooling of the surface layer of the steel is inhibited. As a result, the hydrogen remains in the steel.

This residual hydrogen causes reduction of ductility at room temperature, reduction of fracture stress, and decrease of load capacity, which are factors of internal cracks and delayed cracks.

Therefore, there is a demand for a technique capable of reducing the crack caused by such hydrogen.

As a background art related to the present invention, there is a high strength hot-rolled steel sheet for a line pipe excellent in resistance to cracking by hydrogen organic cracking disclosed in Korean Patent Laid-Open Publication No. 10-2012-0044118 (published on May 21, 2012) and a method for manufacturing the same.

An object of the present invention is to provide a steel material capable of reducing the hydrothermal defects while exhibiting high hardness through adjustment of alloy components such as calcium (Ca), and a method for producing the same.

In order to achieve the above object, steel according to an embodiment of the present invention comprises 0.44 to 0.46% of carbon, 0.25 to 0.30% of silicon, 0.8 to 1.2% of manganese (Mn) (P): 0.02% or less, S: 0.01-0.03%, Cu: 0.05-0.15%, Ni: 0.04-0.07%, Cr: 0.1-0.2%, molybdenum (N): 0.008 to 0.015%, and oxygen (O) in a range of 0.01 to 0.05% Mo, 0.005 to 0.01% aluminum, 0.05 to 0.1% vanadium (V), 0.004 to 0.005% O): 0.005% or less and the balance of iron (Fe) and unavoidable impurities.

At this time, the steel may have a Brinell hardness (HB) of 250 or more.

The steel material may have an average length of MnS inclusions of 2.2 mu m or less and an aspect ratio of 1.8 or less.

In order to achieve the above object, a method of manufacturing a steel material according to an embodiment of the present invention includes: 0.44 to 0.46% of carbon; 0.25 to 0.30% of silicon; 0.8 to 1.2% of manganese (Mn) (P): 0.02% or less, S: 0.01-0.03%, Cu: 0.05-0.15%, Ni: 0.04-0.07%, Cr: 0.1-0.2% And the molybdenum content is preferably in the range of 0.01 to 0.05% of molybdenum (Mo), 0.005 to 0.01% of aluminum (Al), 0.05 to 0.1% of vanadium (V), 0.004 to 0.005% of calcium (Ca) Providing a semi-finished steel material consisting of 0.005% or less of oxygen (O) and the balance of iron (Fe) and unavoidable impurities; Hot-rolling the semi-finished steel material at a finishing rolling temperature condition of Ar3 or higher; And cooling the hot-rolled steel material.

According to the method for manufacturing a steel material according to the present invention, it is possible to maintain the high hardness through controlling the alloy component such as calcium, and to control the shape of the MnS inclusion according to the addition of a proper amount of calcium, So that it is possible to inhibit the hydrogenation defects.

Fig. 1 shows the shape of MnS inclusions of the specimens according to Comparative Examples 1 to 3 and Example 1. Fig.
Fig. 2 shows a size distribution diagram of MnS inclusions of the specimens according to Comparative Examples 1 to 3 and Example 1. Fig.
Fig. 3 shows the distribution of aspect ratios of MnS inclusions of the specimens according to Comparative Examples 1 to 3 and Example 1. Fig.
Fig. 4 compares the inclusion shapes of the specimens according to Comparative Example 1 and Example 1. Fig.
5 shows results of impact anisotropy evaluation of the specimens according to Comparative Examples 1 to 3 and Example 1. Fig.
6 shows the HIC evaluation results of the specimens according to Comparative Examples 1 to 3 and Example 1. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a steel material according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Steel

The steel according to the present invention contains, by weight%, 0.44 to 0.46% of carbon (C), 0.25 to 0.30% of silicon (Si), 0.8 to 1.2% of manganese (Mn) (Si): 0.01-0.03%, Cu: 0.05-0.15%, Ni: 0.04-0.07%, Cr: 0.1-0.2%, Mo: 0.01-0.05%, aluminum , 0.005 to 0.01% of aluminum (Al), 0.05 to 0.1% of vanadium (V), 0.004 to 0.005% of calcium (Ca), 0.008 to 0.015% of nitrogen (N) .

The rest of the above components are composed of iron (Fe) and unavoidable impurities.

Hereinafter, the role and content of each component contained in the steel according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added to secure the hardness of the steel.

The carbon is preferably added in an amount of 0.44 to 0.46% by weight based on the total weight of the steel material. When the addition amount of carbon is less than 0.44 wt%, it is difficult to secure the desired hardness. On the contrary, when the addition amount of carbon exceeds 0.46% by weight, the workability is deteriorated.

Silicon (Si)

In the present invention, silicon (Si) is added as a deoxidizer to remove oxygen in the steel, and also serves to improve the solid solution strengthening effect.

The silicon is preferably added in an amount of 0.25 to 0.30% by weight based on the total weight of the steel material. When the addition amount of silicon is less than 0.25% by weight, the deoxidation effect and the solid solution strengthening effect by the addition of silicon are insufficient. On the contrary, when the addition amount of silicon exceeds 0.30 wt%, there is a problem that the workability of a steel material to be produced is lowered.

Manganese (Mn)

In the present invention, manganese (Mn) is a very effective element as a solid solution strengthening element and is an effective element for securing the strength of a steel material to be produced.

The manganese is preferably added in an amount of 0.8 to 1.2% by weight based on the total weight of the steel material. When the addition amount of manganese is less than 0.8% by weight, the solid solution strengthening effect and the strength securing effect due to the addition of manganese are insufficient. On the contrary, when the addition amount of manganese exceeds 1.2% by weight, the machinability is deteriorated.

In (P)

Phosphorus (P) is a segregating element, and if it is contained in excess, it impairs the impact characteristics of the steel and causes cracks in the process.

In the present invention, the content of phosphorus is limited to 0.02% by weight or less based on the total weight of the steel material.

Sulfur (S)

Sulfur (S) is added to improve the machinability or workability of the steel material according to the present invention.

The sulfur is preferably added in an amount of 0.01 to 0.03% by weight based on the total weight of the steel material. When the addition amount of sulfur is less than 0.01% by weight, MnS inclusion formation is insufficient and does not contribute to improvement of processability. On the other hand, when the content of sulfur (S) exceeds 0.03% by weight, tearing of the steel is caused, and surface defects are caused by the formation of large inclusions.

Copper (Cu)

Copper (Cu) promotes fine precipitates and contributes to the increase in strength.

The copper is preferably added in an amount of 0.05 to 0.15% by weight based on the total weight of the steel material. When the addition amount of copper is less than 0.05% by weight, the effect of increasing the strength is insufficient. On the other hand, when the addition amount of copper exceeds 0.15% by weight, the workability of the steel is lowered and surface defects are caused.

Nickel (Ni)

Nickel (Ni) increases the hardenability and contributes to the strength improvement of steel.

The nickel is preferably added in an amount of 0.04 to 0.07% by weight based on the total weight of the steel material. When the addition amount of nickel is less than 0.04% by weight, the effect of addition is insufficient. On the other hand, when the addition amount of nickel exceeds 0.07% by weight, the manufacturing cost of steel material increases greatly, and the workability may be lowered.

Chromium (Cr)

Chromium (Cr) is added as an element for improving hardenability and contributes to the improvement of strength.

The chromium is preferably added in an amount of 0.1 to 0.2% by weight based on the total weight of the steel material. If the addition amount of chromium is less than 0.1 wt%, the effect of compensating the strength is insignificant. On the contrary, when the addition amount of chromium exceeds 0.2% by weight, the workability is deteriorated.

Molybdenum (Mo)

Molybdenum (Mo) is added as an ingot element and contributes to strength improvement.

The molybdenum is preferably added in an amount of 0.01 to 0.05% by weight based on the total weight of the steel material. If the addition amount of molybdenum is less than 0.01% by weight, the effect of the addition is insufficient. On the other hand, when the addition amount of molybdenum exceeds 0.05% by weight, the workability is lowered and the manufacturing cost of steel material can be greatly increased.

Aluminum (Al)

Aluminum (Al) provides an excellent deoxidizing effect and also contributes to particle refinement by bonding with nitrogen (N).

The aluminum is preferably added in an amount of 0.005 to 0.01% by weight based on the total weight of the steel material. When the addition amount of aluminum is less than 0.005% by weight, the effect of addition is insufficient. On the contrary, when the added amount of aluminum exceeds 0.01% by weight, Al ₂ O ₃ can be excessively produced.

Vanadium (V)

Vanadium (V) plays a role in improving the strength by precipitation strengthening.

The vanadium is preferably added in an amount of 0.05 to 0.1% by weight based on the total weight of the steel material. If the addition amount of vanadium is less than 0.05% by weight, the precipitation strengthening effect is insufficient. On the contrary, when the addition amount of vanadium exceeds 0.1 wt%, the workability may be greatly lowered.

Calcium (Ca)

Calcium (Ca) functions as a nucleation site of MnS inclusions, and suppresses the elongation of MnS inclusions that inhibit hydrogen diffusion, thereby suppressing the occurrence of hydrogenation defects in the steel.

The calcium is preferably added in an amount of 0.004 to 0.005% by weight (40 to 50 ppm) of the total weight of the steel material. When the addition amount of calcium is less than 0.004% by weight, the effect is insufficient. On the other hand, when the addition amount of calcium exceeds 0.005% by weight, a large amount of hard CaO can be produced and the workability of the steel can be inhibited.

Nitrogen (N)

Nitrogen (N) combines with aluminum or the like to form a nitride, thereby contributing to improvement of mechanical characteristics due to miniaturization of austenite grains. However, excessive nitrogen content inhibits hot stripping.

The nitrogen is preferably contained in an amount of 0.008-0.015 wt% (80-150 ppm) of the total weight of the steel material. When the content of nitrogen is less than 0.008% by weight, the nitride forming effect is insufficient. On the other hand, if the content of nitrogen exceeds 0.015 wt%, the hot stripping can be inhibited.

Oxygen (N)

Oxygen (O) binds calcium to form hard CaO, thereby inhibiting the stretching inhibition of the intended MnS inclusion in the present invention.

For this reason, in the present invention, the oxygen content is limited to 0.005 wt% (50 ppm) or less of the total weight of the steel material.

The steel material according to the present invention may exhibit a hardness of Brinell hardness (HB): 250 or more. In addition, the steel material according to the present invention may have an average length of MnS inclusions of 2.2 탆 or less and an aspect ratio of 1.8 or less.

Steel manufacturing method

The method for manufacturing a steel material according to the present invention includes a semi-finished steel material preparing step, a hot rolling step and a cooling step.

In the semi-finished steel product preparing step, semi-finished steel having the above-mentioned composition, that is, slab, ingot, billet and the like are provided.

Next, the hot rolling step hot-rolls the semi-finished steel. At this time, the hot rolling can be carried out at a finish rolling temperature of Ar3 or higher, more preferably 900 to 1000 占 폚. Under the finish rolling temperature condition of Ar3 or higher, the structure of the steel sheet before cooling after hot rolling may become a stable austenite phase, and rolling at a lower temperature may cause problems such as generation of coarse grain structure. However, if the finishing rolling temperature is excessively high exceeding 1000 캜, the austenite grains may become coarse and the strength may be hardly secured.

Next, in the cooling step, the hot-rolled steel is cooled. Cooling can be both natural cooling and forced cooling, but natural cooling to room temperature is more preferable in terms of steel production cost.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Manufacture of steel specimens

Using the compositions shown in Table 1-1 and Table 1-2, steel specimens were prepared. For the preparation of the steel specimens according to Specimens 1 to 9, ingots of 50 kg each having the compositions shown in Tables 1 and 2 were prepared by using a vacuum induction melting furnace. Thereafter, each of the ingots was hot-rolled at a finishing rolling temperature of 950 캜 and then air-cooled to room temperature.

[Table 1-1] (Unit:% by weight)

[Table 1-2] (Unit:% by weight)

2. Evaluation of hardness and MnS inclusions

(1) Hardness evaluation

The hardness of each steel specimen was measured five times using a Brinell hardness tester, and the average value of the intermediate values excluding the upper limit value and the lower limit value was shown, and the results are shown in Table 1-2.

Referring to Table 1-2, the steel specimen according to Comparative Example 1 in which calcium was not added and the steel specimen according to Example 1 including 0.0048 wt% of calcium exhibited a Brinell hardness (HB) of 250 or more, In the case of the steel specimens according to Comparative Examples 2 to 3 in which relatively little addition was made, the Brinell hardness (HB) was relatively low.

(2) Evaluation of inclusions

Table 2 shows the results of evaluating MnS inclusions by the ASTM E45 method.

[Table 2]

The results are shown in Table 2. As shown in Table 2, in the case of Comparative Examples 2 to 3 in which calcium was added and the specimen according to Example 1 in comparison with the specimen according to Comparative Example 1 in which calcium was not added, the elongation properties of inclusions were suppressed, , It can be seen that the extensibility is most suppressed. In the case of the specimen according to Example 1, the average length is 2.2 탆 or less and the aspect ratio (A / R) is 1.8 or less.

Figs. 1 to 3 show the shapes of MnS inclusions, the size distribution of MnS inclusions, and the aspect ratio distribution of MnS inclusions of the specimens according to Comparative Examples 1 to 3 and Example 1. Fig.

Referring to FIGS. 1 to 3, when calcium is added in a relatively large amount, the elongation of the MnS inclusions is relatively more suppressed, the number of inclusions having a shorter length is increased, have.

Fig. 4 compares the inclusion shapes of the specimens according to Comparative Example 1 and Example 1. Fig.

4, in the case of the steel specimen according to Comparative Example 1 in which calcium was not added, although the MnS inclusions were elongated in length, in the case of the steel specimen according to Example 1 in which calcium was added in an amount of 48 ppm, the MnS inclusions were close to spherical It can be seen that it is in the form.

5 shows results of impact anisotropy evaluation of the specimens according to Comparative Examples 1 to 3 and Example 1. Fig.

Referring to FIG. 5, the higher the calcium content in the rolling direction (0 ㅀ) impact toughness than the 45 ㅀ and 90 ㅀ, the higher the characteristics were.

6 shows the results of HIC (Hydrogen Induced Cracking) evaluation of the specimens according to Comparative Examples 1 to 3 and Example 1. Fig.

Referring to FIG. 6, it can be seen that hydrogen organic cracking occurs when calcium is not added or when calcium calcium is added in a small amount. However, when calcium is added in a large amount, hydrogen organic cracking does not occur.

Therefore, in the case of the present invention, the addition of Ca or the like suppresses the elongation of the MnS inclusions, thereby achieving the effect of reducing the hydrolytic defects while having high hardness.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

Claims (5)

delete (P): 0.02% or less, sulfur (S): 0.01 to 0.5% by weight, carbon (C): 0.44 to 0.46%, silicon (Si): 0.25 to 0.30%, manganese (Ni): 0.04 to 0.07%, chromium (Cr): 0.1 to 0.2%, molybdenum (Mo): 0.01 to 0.05%, aluminum (Al): 0.005 to 0.03% (Fe) and unavoidable impurities (Fe) in an amount of 0.001 to 0.01%, vanadium (V) in an amount of 0.05 to 0.1%, calcium (Ca) in an amount of 0.004 to 0.005%, nitrogen (N) in an amount of 0.008 to 0.015% Lt; / RTI >
And a Brinell hardness (HB): 250 or more.
3. The method of claim 2,
The steel
An MnS inclusion having an average length of 2.2 탆 or less and an aspect ratio of 1.8 or less. delete delete

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