KR101439656B1 - A steel containing phosphorous with excellent impact toughness - Google Patents

Abstract

The present invention relates to a phosphorus-containing steel having excellent impact toughness, and more particularly, to a phosphorus-containing steel of a new concept overcoming the conventional concept of causing brittleness when contained in a large amount in the steel.
The phosphorus-containing steel according to one aspect of the present invention contains 0.05 to 0.3% of C, 0.5 to 3.0% of Mn, 0.05 to 0.3% of P, 0.0005 to 0.01% of B, 0.05 to 0.5% of Mo, 0.05 to 0.5% of La, 0.01% or less of N, the balance of Fe and unavoidable impurities, and the sum of the area fraction of ferrite and pearlite is 80% or more.

Description

A STEEL CONTAINING PHOSPHOROUS WITH EXCELLENT IMPACT TOUGHNESS,

The present invention relates to a phosphorus-containing steel having excellent impact toughness, and more particularly, to a phosphorus-containing steel of a new concept overcoming the conventional concept of causing brittleness when contained in a large amount in the steel.

The iron ore taken from the earth contains a large amount of phosphorus although it is slightly different depending on the mine. The phosphorus component is not removed from the ore during the ore mining process after mining, and most of the raw phosphorus component is contained in the raw materials of the form such as sinter, minerals, and briquettes. In addition, the phosphorus component is hardly removed even in the so-called steel making process in which iron ore is treated to produce molten iron, and a considerable amount of molten iron remains in the molten iron.

This phenomenon is due to the unique chemical reaction characteristics of phosphorus. In other words, in order to remove phosphorus from iron in steelmaking process, it is necessary to convert phosphorus to iron-separated material. One method is to convert phosphorus dissolved in iron into vapor phosphorus (vaporized tallin) And then reacting it with oxygen to convert it into an oxide of phosphorus (for example, P ₂ O ₅ or a similar compound). However, since the former method requires extreme reduction conditions, it is not practical in an industrial way. Therefore, a commonly used method is the latter oxidation process. In order to produce molten iron from iron ore, it is necessary to use a reducing agent having a strong reducing power such as coke, so that the molten iron has a reducing property (however, Of reductive) atmosphere. It is not practically possible to remove the phosphorus component in the manufacturing process.

Therefore, in the currently commercialized steel making process, the removal of phosphorus is concentrated in the steelmaking process (including the pre-treatment process). A typical phosphorus removal process is a primary refining process represented by a converter. The conversion process is primarily a process for removing carbon to convert molten iron to molten steel, characterized by the injection of pure oxygen during the process to remove carbon. At this time, oxygen-enriched elements including phosphorus are oxidized together with the injection of pure oxygen, and the talline in the converter uses this phenomenon.

However, in the secondary refining step after the conversion, the reducing agent is introduced again to maintain the reducing atmosphere, so that it is difficult to expect additional talline, so that it is preferable that the whole talline is conducted in the converter as much as possible. Therefore, when the content of initial phosphorus in the molten iron is high or the content of the final phosphorous as the target is low, not only the phosphorus removal load in the converter is increased but also the amount of oxygen Which inevitably leads to an increase in the amount of dissolved oxygen having a thermodynamic relationship closely related to carbon. When the amount of oxygen is increased, the amount of the reducing agent including the elements such as aluminum, silicon, and manganese added during the excrement or the secondary refining is inevitably increased. As a result, the reducing agent and the oxygen generated by the reduction reaction of the dissolved oxygen The deoxidation product is increased in the molten steel and the quality of the molten steel is deteriorated.

In addition, an additional problem caused by an increase in the amount of talline in the converter is numerous, such as an increase in the molten steel temperature due to an increase in the amount of oxygen injected and deterioration of the life of the furnace body.

In order to reduce the amount of talline in the converter and to alleviate the above-mentioned problem, a method has been proposed in which talline is added by adding an oxidizing agent to the molten iron in the pre-converter process. This method is also referred to as a preliminary ironing process because it is performed in a pre-transfer process called primary refining. The amount of phosphorus in the charcoal charged into the converter by the charcoal pretreatment is reduced, so that the talline load on the converter can be alleviated. However, the above-described process requires an additional process and is not a method for solving the problem fundamentally, such as causing the temperature of the charcoal to decrease.

Therefore, in the case of avoiding problems such as brittleness caused by conventional phosphorus while maintaining the phosphorus content in the steel at a high level, the load of the steelmaking process can be reduced and the quality of the steel can be improved. Has not yet been proposed.

One aspect of the present invention provides a novel high-strength phosphorous steel capable of reducing the burden in the steelmaking process by preventing brittleness from being generated even at a relatively high phosphorus content.

Another aspect of the present invention provides a high strength, containing steel capable of having high strength by phosphorus.

The object of the present invention is not limited to the above description. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.

The phosphorus-containing steel according to one aspect of the present invention contains 0.05 to 0.3% of C, 0.5 to 3.0% of Mn, 0.05 to 0.3% of P, 0.0005 to 0.01% of B, 0.05 to 0.5% of Mo, 0.05 to 0.5% of La, 0.01% or less of N, the balance of Fe and unavoidable impurities, and the sum of the area fraction of ferrite and pearlite is 80% or more.

It may further include at least one selected from the group consisting of Ti of 0.1% or less, Si of 1.0% or less, Cr of 2.0% or less, and Ni of 3.0% or less.

The P may be 0.1 to 0.2%.

The average particle size of the ferrite may be 5 to 50 탆.

In the phosphorus-containing steel of the present invention, the cleavage wave front face ratio of the cross section of the shock wave at -20 占 폚 may be 80% or more based on the area basis.

According to the present invention, the addition of B, Mo and La into the steel can prevent the embrittlement caused by phosphorus and ensure sufficient toughness even at a relatively high phosphorous content, thereby providing a steel containing high toughness. Accordingly, the burden on the steelmaking process can be reduced, and the overall process can be controlled reasonably.

In addition, when phosphorus is present in a crystal grain base, high strength can be obtained by addition of a relatively high content of phosphorus as an element that gives employment strengthening effect.

FIG. 1 is a micrograph of the microstructure of the inventive steel 3, and FIG.
Fig. 2 is a microscope photograph showing the fracture surface of the sample 3 subjected to the impact test at -100 deg.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have found that phosphorus contained in the steel is segregated at the crystal grain boundary as a result of studies on a mechanism in which brittleness is generated when a large amount of phosphorus is contained, leading to the present invention.

That is, in the present invention, it can be achieved by suppressing the phenomenon of segregation at the grain boundary interface of phosphor as much as possible. For this purpose, in the present invention, the steel component is controlled in an appropriate range and one element is added to the crystal grain boundary to precipitate phosphorus first or to compete with phosphorus.

Particularly, in the present invention, B is added as an element which competes with P in the grain boundary of steel. That is, when B is added, the added B segregates preferentially in grain boundaries, thereby reducing the segregation of P into segregation. As a result, the amount of P dissolved in the crystal grains is increased more than the amount of crystal segregation in the grain boundaries, so that the brittleness (due to P segregated in grain boundaries) decreases and the strength (due to P solubilized in the grain) increases. In addition, B, which is bound to grain boundaries instead of P, does not significantly cause embrittlement problems such as P, and therefore toughness of the steel increases.

Further, in the present invention, Mo and La, which form phosphorus precipitates or clusters, are added to reduce the amount of phosphorus segregated at the grain boundary interface. That is, when Mo or La is added, the added Mo forms precipitates or aggregates together with phosphorus, and La also forms precipitates with P, so that the amount of P segregated in the grain boundaries is relatively reduced. In addition, Mo and La are not only elements that favor the strengthening of the steel, but also precipitation strengthening phenomenon of the precipitates of Mo / La and P can be expected, which is also advantageous in improving the strength.

Further, the strength and toughness of the steel can be further improved by appropriately controlling additional components in addition to the above-mentioned addition components.

More specifically, the phosphorus-containing steel of the present invention contains 0.05 to 0.3% of C, 0.5 to 3.0% of Mn, 0.05 to 0.3% of P, 0.0005 to 0.01% of B, 0.05 to 0.5% of Mo, 0.5%, N: 0.01% or less, the balance Fe and unavoidable impurities, and the sum of the area fraction of ferrite and pearlite is 80% or more.

Hereinafter, the components of the present invention will be described. In the present invention, it should be noted that the content of each element to be added means the weight% unless otherwise specified.

C: 0.05 to 0.3%

C is an element necessary for securing strength of steel in the carbon steel component system. Therefore, it is preferable that the content is 0.05% or more. However, if it is added excessively, it reacts with B to form a precipitate, which deteriorates the grain boundary segregation effect of B atoms and adversely affects securing impact toughness, so that the upper limit of C content is preferably set to 0.3%.

Mn: 0.5 to 3.0%

Mn is an element which is effective for strengthening solubility and hardening ability, but segregation such as center segregation or micro segregation when a large amount is added, adversely affects the quality of the product, so that the lower limit and the upper limit are set to 0.5% and 3.0% Limit.

P: 0.05 to 0.3%

As already explained, P segregates in the grain boundaries and causes embrittlement of steel. Therefore, conventionally, the content range of P is generally limited to 0.03% or less. In the present invention, however, P is positively added to 0.05% or more by providing a means for minimizing crystal grain segregation of P. In this case, besides the effect of reducing the cost of scouring and removing P from steelmaking, it is possible to reduce the cost, and it is advantageous that the employment reinforcement effect of P can be obtained. However, if the content is too high, there is a problem that the impact characteristics are disadvantageously lowered, and the lower limit and the upper limit are limited to 0.05% and 0.3%, respectively. The content of P that can obtain the effects of the present invention is more preferably 0.1 to 0.2%.

B: 0.0005 to 0.01%

B is an element that plays a role in reducing the segregation of P. As described above, B plays a role of reducing P segregation by preferentially segregating grain boundaries. Therefore, even if a relatively large amount of P is added, deterioration of impact toughness due to P can be prevented. In order to achieve the above-mentioned effects, the B content is preferably 0.0005% or more, more preferably 0.001% or more, and still more preferably 0.005% or more. However, if it is added in excess, the toughness deterioration due to the B precipitates precipitated in the grain boundaries may be a problem, so the content thereof is preferably 0.01% or less.

Mo: 0.05 to 0.5%

Mo is another element that plays a role in reducing P segregation. As described above, Mo plays a role of forming precipitates or aggregates with P and reducing P segregation. That is, Mo has an affinity for P in a state in which it is not segregated, thereby preventing P from being segregated at grain boundaries. In order to achieve the above-described effects, the Mo content is preferably 0.05% or more, more preferably 0.1% or more, and still more preferably 0.25% or more. However, if it is added excessively, toughness deterioration due to precipitates of Mo and P may be a problem, so it is preferable to set the content to 0.5% or less.

La: 0.05 to 0.5%

La is another element that plays a role in reducing P segregation. As described above, La plays a role of reducing P segregation by forming precipitates with P. That is, La has an affinity for P in a state in which it is not segregated, so that P can be prevented from being segregated at grain boundaries. In order to achieve the above-described effects, the La is preferably added in an amount of 0.05% or more, more preferably 0.1% or more, and still more preferably 0.25% or more. However, if it is added in excess, the toughness deterioration due to the precipitates of La and P may be a problem, so it is preferable to set the content to 0.5% or less.

N: not more than 0.01%

N is an impurity contained in the steel. If the content is high, the B-bond is formed to form BN, which interferes with the grain boundary precipitation of B. Therefore, the content of N is preferably 0.01% or less. However, since N is an impurity and its content is low, it is advantageous to be low, so that the lower limit is not separately defined. And more preferably 0.005% or less.

Accordingly, the phosphorus-containing steel of the present invention contains 0.05 to 0.3% of C, 0.5 to 3.0% of Mn, 0.05 to 0.3% of P, 0.0005 to 0.01% of B, 0.05 to 0.5% of Mo, 0.05 to 0.5% of La, , And N: 0.01% or less. It should also be noted that the phosphorus-containing steel of the present invention does not exclude that it contains an appropriate amount of known elements capable of improving the steel function in addition to the above-mentioned components.

Examples of known elements capable of improving the steel function include Ti: not more than 0.1%, Si: not more than 1.0%, Cr: not more than 2.0%, and Ni: not more than 3.0% . These components may be contained only in one kind, and two or more components selected therefrom may be added in combination. Hereinafter, reasons for addition of the additional components will be described in detail. However, since these components are not necessarily added as optional components, they are not specifically defined.

Ti: 0.1% or less

By precipitating N, Ti can inhibit BN precipitation and maximize the B effect. However, when a large amount of TiN is added, the upper limit is limited to 0.1% because the impact characteristics may be deteriorated due to the precipitation of TiN.

Si: 1.0% or less

Si is an element having an effect of improving the strength by solid solution strengthening. However, when the Si is added in a large amount, the upper limit is limited to 1.0%, because the increase in scale defects causes a decrease in surface quality.

Cr: 2.0% or less

Cr is known as an element improving the ingotability of steel. However, in the case where a large amount of Cr is added, the upper limit of the Cr content is limited to 2.0% or less, because the adverse effect of center segregation is expected.

Ni: 3.0% or less

Ni is known as an element to improve the impact properties of steel. However, Ni is limited to 3.0% or lower because it may cause a large burden on cost increase when added in large amounts.

In addition, the steel material of the present invention may include other impurities which can be normally included in the industrial production process of steel. These impurities can be known to anyone with ordinary knowledge in the art to which the present invention belongs, so that the kind and content of the impurities are not specifically limited in the present invention.

In order to ensure sufficient toughness, the steel material of the present invention is required to have an area fraction of ferrite and pearlite (hereinafter referred to as " pearlite " And the sum is 80% or more. If the sum of these tissues is not sufficient, a large amount of hard tissues are formed inside, which makes it difficult to secure sufficient toughness. It is also preferable that the area fraction of the ferrite in the whole structure is 30% or more. The structure other than the ferrite and the pearlite is not particularly limited as long as it is a structure formed in the steel material, and for example, a bainite structure including a part of martensite can be mentioned.

Further, the average grain size of the ferrite in the structure is more preferably 50 탆 or less. That is, when the particle size of the ferrite is small, the amount of P segregation increases, and therefore it is possible to prevent the P content in the segregated place from becoming excessively high. Therefore, the average particle size of the ferrite is preferably 50 탆 or less. The pearlite formed together is not limited because it is influenced by the particle size of the ferrite. Theoretically, the smaller the particle size of ferrite is, the more advantageous it is. However, since it is not easy to industrially control it to less than 5 mu m, the lower limit of the particle size of the ferrite is set to 5 mu m. In the present invention, B is added to segregate in grain boundaries to improve impact toughness, so that a phosphorus-containing steel having improved toughness can be obtained even if the grain size is in the above range. As a result, the steel material of the present invention can have a cleavage wave fracture rate of a cross section of 80% or more on an area basis even at -20 ° C.

The method of controlling the structure of the steel as described above can be achieved by a method known in the technical field of the present invention, and the method is not particularly limited in the present invention. For example, the rolling temperature, the reduction rate, the cooling rate, the cooling section, and the like can be mentioned. If the person skilled in the art is familiar with the present invention, there is no particular difficulty in obtaining the organization of the present invention by changing these conditions will be.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate and specify the present invention and not to limit the scope of the present invention. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

(Example)

The steel ingots having the compositions shown in Tables 1 and 2, which were obtained by casting, were cast by vacuum induction melting to a thickness of 60 mm and a width of 175 mm. The obtained ingot was reheated at 1,200 DEG C for one hour, and then hot rolled to a hot rolled thickness of 7 mm. The hot-rolled steel sheet was austenitized at 900 ° C for 30 minutes in an atmospheric furnace, and then ferritic and pearlite structures were secured through a heat treatment at 650 ° C for 3 hours to form a final 5mm subsize Charpy impact specimen And the impact toughness test was carried out. The strength and impact properties of each specimen are also shown in Tables 1 and 2. In the table, strength means tensile strength (MPa), toughness means impact toughness (J / cm ² , -20 ° C, Charpy impact absorption energy). In addition, the fraction (%) of each tissue is measured on an area basis, and the unit of granularity is micrometer (占 퐉).

division Chemical composition (% by weight) group Properties C Mn P B Mo La N Other ingredients Ferrite fraction Perlite fraction Ferrite particle size burglar tenacity Comparative River 1 0.11 0.69 0.21 0.0002 0.03 0.02 0.0065 44 43 21 421 38 Comparative River 2 0.11 0.72 0.18 0.0004 0.05 0.06 0.0076 45 43 24 401 41 Comparative Steel 3 0.10 0.70 0.24 0.0004 0.06 0.04 0.0078 54 34 24 442 44 Comparative Steel 4 0.11 0.69 0.20 0.0006 0.04 0.04 0.0058 47 36 25 413 42 Inventive Steel 1 0.11 0.70 0.23 0.0006 0.07 0.06 0.0032 45 40 19 441 53 Invention river 2 0.11 0.71 0.20 0.0035 0.25 0.21 0.0076 45 46 24 435 71 Invention steel 3 0.11 0.70 0.19 0.0068 0.37 0.36 0.0086 44 46 22 445 68 Inventive Steel 4 0.10 0.71 0.20 0.0098 0.48 0.45 0.0035 49 33 23 463 56 Comparative Steel 5 0.12 0.72 0.18 0.011 0.49 0.47 0.0066 41 42 24 454 46 Comparative Steel 6 0.10 0.69 0.20 0.0097 0.54 0.51 0.0087 52 34 16 466 39 Comparative Steel 7 0.10 0.70 0.23 0.013 0.58 0.57 0.0062 53 28 19 499 31

division Chemical composition (% by weight) group Properties C Mn P B Mo La N Other ingredients Ferrite fraction Perlite fraction Ferrite particle size burglar tenacity Comparative Steel 8 0.11 0.69 0.03 0.0018 0.17 0.18 0.0035 46 42 16 292 94 Comparative Steel 9 0.09 0.7 0.04 0.002 0.22 0.2 0.0060 57 30 24 297 88 Invention steel 5 0.08 0.7 0.06 0.0021 0.23 0.2 0.0027 63 26 20 311 83 Invention steel 6 0.1 0.7 0.12 0.0018 0.18 0.21 0.0086 52 31 17 363 72 Invention steel 7 0.11 0.72 0.23 0.0018 0.17 0.2 0.0059 46 40 19 454 64 Inventive Steel 8 0.11 0.72 0.29 0.002 0.2 0.22 0.0084 46 42 18 504 51 Invention river 9 0.12 0.68 0.14 0.0018 0.19 0.18 0.0041 Ti: 0.06 42 47 18 384 61 Invented Steel 10 0.11 0.70 0.13 0.0018 0.22 0.20 0.0023 Si: 0.45 45 43 19 376 62 Invention steel 11 0.11 0.72 0.12 0.0017 0.21 0.21 0.0026 Cr: 1.13 48 41 18 369 63 Invention steel 12 0.12 0.72 0.12 0.0017 0.21 0.21 0.0041 Ni: 1.84 41 49 20 374 65 Invention steel 13 0.10 0.72 0.12 0.0020 0.21 0.21 0.0012 Ti: 0.06
Si: 0.81 53 37 18 368 67 Invented Steel 14 0.11 0.70 0.11 0.0023 0.19 0.20 0.0038 Cr: 0.82
Ni: 1.69 46 38 23 364 73 Invented Steel 15 0.11 0.68 0.12 0.0020 0.17 0.19 0.0073 Ti: 0.07
Cr: 0.91
Ni: 1.51 47 46 16 371 69 Invented Steel 16 0.09 0.72 0.13 0.0023 0.21 0.20 0.0088 Ti: 0.07
Si: 0.64
Cr: 1.12
Ni: 1.69 56 32 22 380 70 Comparative Steel 10 0.1 0.69 0.31 0.0019 0.2 0.21 0.0019 52 39 21 516 43 Comparative Steel 11 0.1 0.7 0.35 0.0019 0.21 0.23 0.0043 52 36 19 550 35

As can be seen from the above Table 1, the effect of increasing the tensile strength according to the increase of the content of the component B can be confirmed. This is because the amount of segregated P in the grain boundaries decreases as the B content increases, resulting in an increase in the solid solution strengthening effect by P and also an improvement in strength by B. In addition, when the content of B is insufficient, it is impossible to prevent segregation of grain boundaries of P under the condition of high P content, so that it is difficult to expect an effect of improving the impact toughness. Thus, it has a shock absorption energy of less than 48 J / cm ² at -20 캜. When B is excessively added, the strength may increase, but the steel is brittle due to grain boundary precipitates such as BN and the impact absorption energy is less than expected. Therefore, it was confirmed that the B content in the preferred composition range of the present invention is preferably 0.0005 to 0.01%.

This phenomenon is likewise applied when Mo and La are added. If Mo and La are insufficient, the P segregation of grain boundaries can not be completely prevented. On the other hand, if these contents are excessive, the impact properties are deteriorated due to precipitates with P.

Further, as shown in Table 2, when the other composition satisfies the composition of the present invention and the P content is low, the impact property is good, but the strength is less than 300 MPa, which is not sufficient. As the P content increases, the tensile strength tends to increase. However, if the P content is excessive, the strength improvement effect can be confirmed, but when it exceeds the range specified in the present invention, the impact absorption energy is less than 48 J / cm ² and it is confirmed that the toughness is not good.

Fig. 1 shows the microstructure of invention steel 2. As shown in the figure, it can be seen that the microstructure of Invention Steel 2 is composed of 45% of ferrite, 46% of pearlite and an extra gravel phase. Fig. 2 shows a fractured section of the sample 2 subjected to impact test at -100 ° C. As can be seen from the drawing, it was confirmed that cleavage fracture appears not in the grain boundary fracture appearing in normal high P steel (satisfying the preferable range of cleavage wave surface area fraction of 80% or more). Normally, The shock wave section at -20 ° C has a cleavage wavefront of at least 80% area fraction). This implies that B prevents the grain boundaries from becoming fragile by preventing B from segregating first in the grain boundaries and causing P to segregate.

Claims (5)

0.05 to 0.3% of Mn, 0.5 to 3.0% of Mn, 0.05 to 0.3% of P, 0.0005 to 0.01% of B, 0.05 to 0.5% of Mo, 0.05 to 0.5% of La, 0.01 to 0.01% of N, Hereinafter, with the composition including the remainder Fe and unavoidable impurities,
The total area ratio of ferrite and pearlite is 80% or more,
Charpy phosphorus-containing steel whose impact absorption energy is greater than or equal to 48 J / cm ² at -20 ° C.
The phosphorus-containing steel according to claim 1, further comprising at least one selected from the group consisting of Ti: not more than 0.1%, Si: not more than 1.0%, Cr: not more than 2.0%, and Ni: not more than 3.0%.
The phosphorus containing steel according to any one of claims 1 to 3, wherein P is 0.1 to 0.2%.
The phosphorus-containing steel according to any one of claims 1 to 5, wherein the ferrite has an average particle size of 5 to 50 μm.
The phosphorus-containing steel according to claim 1 or 2, wherein the cleavage wave fracture rate of the cross section of the shock wave at -20 캜 is 80% or more based on the area.

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