KR20190074824A - High strength steel wire rod and high strength steel with excellent delay fracture resistance and manufacturing method thereof - Google Patents

Abstract

Disclosed are a steel material and a method for manufacturing the same, which can secure excellent delay fracture resistance along with high strength through alloy composition and microstructure control. According to an embodiment of the present invention, a high strength steel material with excellent delay fracture resistance comprises: 0.4-.06 wt% of C; 0.3 wt% or less of Si; 1.5-2.5 wt% of Mn; 1.0 wt% or less of Cr; 1.0 wt% or less of Mo; 0.015 wt% or less of P; 0.010 wt% or less of S; 0.01-0.1 wt% of Al; 0.003-0.02 wt% of N; and the remainder consisting of Fe and inevitable impurities, wherein a microstructure comprises, by volume fraction, at least 90% of lower bainite and at most 10% of remaining austenite.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel material having high strength and excellent resistance to delayed fracture, and a method of manufacturing the same. BACKGROUND ART [0002]

The present invention relates to a high strength steel excellent in delayed fracture resistance and a method of manufacturing the same, and more particularly, to a steel material capable of securing excellent resistance to delayed fracture along with high strength through alloy composition and microstructure control. will be.

Recently, efforts to reduce the emission of carbon dioxide, which is considered to be the main cause of environmental pollution, have become a global issue. As a part of this, there is also a trend to regulate automobile exhaust gas, and automakers are trying to solve this problem by improving fuel efficiency. However, in order to improve fuel efficiency, the weight and high performance of automobiles are required, and hence the necessity of high strength of automobile materials or parts is increasing. With this tendency, the strength of automobile bolts is also actively increasing.

In addition, if a high-strength bolt having a much higher workability is used instead of welding the fastening portion when constructing the steel structure, it is possible to enhance the fastening force at the time of fastening the bolt and the stability of the steel structure due to the decrease in the hollow of the fastening portion, It is possible to reduce the amount of use and shorten the construction air, so that the bolts for fastening steel constructions are also actively increasing in strength.

It is well known that the resistance to delayed fracture decreases remarkably as the strength of the steel increases. Therefore, the possibility of delayed fracture is high when the high strength bolt is exposed to the external environment. Therefore, delayed fracture resistance (delayed fracture resistance) is further required for high strength steels.

Conventional techniques for improving delayed fracture resistance include: 1) inhibition of corrosion of steels; 2) minimization of hydrogen intrusion; 3) inhibition of diffusible hydrogen contributing to delayed fracture; and 4) , 5) minimization of tensile stress, and 6) relaxation of stress concentration. As a means for accomplishing this, a method of high alloying or a method of applying surface coating or plating for preventing external hydrogen intrusion is mainly used. In addition, there is a method in which a precipitate capable of trapping diffusible hydrogen is generated by adding specific elements while suppressing P and S which brittle the austenite grain boundaries to the maximum. In order to disperse and precipitate these fine precipitates, expensive alloying elements There is a problem that it is difficult to apply it to the production of a yarn, for example, it is required to keep a high post-tempering temperature.

The present invention provides a high strength steel excellent in delayed fracture resistance which can be used for fastening bolts for automobiles and structures exposed to various stress and corrosive environments by controlling microstructure through alloy composition and manufacturing method, and a method for manufacturing the same.

The high strength wire according to one embodiment of the present invention is characterized in that it contains 0.4 to 0.6% of C, 0.3% or less of Si, 1.5 to 2.5% of Mn, 1.0% or less of Cr, 1.0% 0.015% or less, S: 0.010% or less, Al: 0.01-0.1%, N: 0.003-0.02%, and the balance Fe and unavoidable impurities. The microstructure has a volume fraction of ferrite and pearlite of 30% 70% or more.

A method of manufacturing a high strength steel material excellent in delayed fracture resistance according to an embodiment of the present invention comprises: 0.4 to 0.6% of C, 0.3% or less of Si, 1.5 to 2.5% of Mn, 1.0% The wire containing 1.0% or less of Mo, 0.015% or less of P, 0.010% or less of S, 0.01 to 0.1% of Al, 0.003 to 0.02% of N and the balance of Fe and unavoidable impurities is heated to 900-1,000 ° C Heating; And rapidly heating the heated wire to a temperature in the range of 250 to 400 ° C and keeping it at a constant temperature.

Also, according to an embodiment of the present invention, the constant temperature holding time may be 30 minutes or more.

The high strength steel material excellent in the delayed fracture resistance according to one embodiment of the present invention comprises 0.4 to 0.6% of C, 0.3% or less of Si, 1.5 to 2.5% of Mn, 1.0% or less of Cr, 1.0 % Of P, less than 0.015% of P, less than 0.010% of S, 0.01 to 0.1% of Al, 0.003 to 0.02% of N and the balance of Fe and unavoidable impurities. The microstructure has a volume fraction of 90% And 10% or less of retained austenite.

Also, according to an embodiment of the present invention, the thickness of the lath of the lower bainite may be 500 nm or less.

According to an embodiment of the present invention, the misorientation angle between the bainite laths of the microstructure may be more than 90% at 50 to 70 °.

Also, according to an embodiment of the present invention, the steel material may have a delayed fracture strength of 1,200 MPa or more.

The present invention can provide a high strength steel having excellent resistance to delayed fracture required in industrial machinery and automobile materials or parts by constituting the base structure of the steel material with the lower bainite and finely controlling the grain size.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

Also, when an element is referred to as "comprising ", it means that it can include other elements as well, without departing from the other elements unless specifically stated otherwise.

The singular forms " a " include plural referents unless the context clearly dictates otherwise.

The high strength wire according to one embodiment of the present invention is characterized in that it contains 0.4 to 0.6% of C, 0.3% or less of Si, 1.5 to 2.5% of Mn, 1.0% or less of Cr, 1.0% 0.015% or less, S: 0.010% or less, Al: 0.01-0.1%, N: 0.003-0.02%, and the balance Fe and unavoidable impurities. The microstructure has a volume fraction of ferrite and pearlite of 30% 70% or more.

According to the present invention, a steel material having a high strength and excellent delayed fracture resistance is provided by using a wire material of the alloy composition according to a manufacturing method described later.

A method of manufacturing a high strength steel material excellent in delayed fracture resistance according to an embodiment of the present invention comprises: 0.4 to 0.6% of C, 0.3% or less of Si, 1.5 to 2.5% of Mn, 1.0% The wire containing 1.0% or less of Mo, 0.015% or less of P, 0.010% or less of S, 0.01 to 0.1% of Al, 0.003 to 0.02% of N and the balance of Fe and unavoidable impurities is heated to 900-1,000 ° C Heating; And rapidly heating the heated wire to a temperature in the range of 250 to 400 ° C and keeping it at a constant temperature.

Hereinafter, the reasons for limiting the numerical values of the alloy element content in the examples of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C is 0.4 to 0.6%.

C is an element added to secure the strength of the product. When the content of C is less than 0.4%, it is difficult to secure the desired strength. When the content of C is more than 0.6%, the phase transformation of ferrite and pearlite rapidly proceeds, so that the lower bainite formation It is not desirable because it can be difficult.

The content of Si is more than 0 and 0.3% or less.

Si is not only useful for deoxidation of steel, but also effective in securing strength through solid solution strengthening, but it is an element that lowers impact properties. On the other hand, when the Si content exceeds 0.3%, the cold workability is lowered, which is not preferable.

The content of Mn is 1.5 to 2.5%.

Mn is an element for improving hardenability and is an element which is solidified by forming a substitutional solid solution in the matrix, and is an extremely useful element for achieving high strength. When the content of Mn is less than 1.5%, it is difficult to secure the target strength because of the insufficient solubility strengthening effect and curing ability. When the Mn content exceeds 2.5%, the anisotropy of the structure due to Mn segregation can be increased, Can not do it.

The content of Cr is more than 0 and 1.0% or less.

Cr is effective for improving hardenability together with Mn, and contributes to improvement of corrosion resistance of steel in a corrosive environment. Further, it acts as nuclei of carbide during the heat treatment for obtaining the lower bainite structure, so that the precipitation of carbide inside the bainite can be promoted and the strength and impact properties can be improved. If the content of Cr exceeds 1.0%, coarse carbides are produced and the impact toughness is lowered. In addition, when the carbides are present in the form of a film along the grain boundaries, the delayed fracture resistance is lowered, which is not preferable.

Mo content is more than 0 and not more than 1.0%.

Mo is an effective element for improving the hardenability and inhibiting the oxidation of the grain boundary. Also, when carbide is formed during the heat treatment for forming the lower bainite, it acts as a strong hydrogen trap site and is very effective in improving the delayed fracture resistance. When the Mo content exceeds 1.0%, the delayed fracture effect due to the precipitates is saturated and the mechanical properties are lowered due to the increase of the coarse alloy carbide, which is not preferable.

The content of P is 0.015% or less.

P is segregated in the grain boundaries to decrease the toughness and decrease the delayed fracture resistance. It is desirable that P not be included as much as possible, so the upper limit is set to 0.015%.

The content of S is 0.010% or less.

S is segregated in grain boundaries as in P, and not only toughness is lowered but also an element which inhibits hot rolling by forming a low melting point emulsion is desirably not contained. Therefore, the upper limit is set to 0.010%.

The content of Al is 0.01 to 0.1%.

Al is a strong deoxidizing element which not only enhances cleanliness by removing oxygen in steel but also forms AlN by bonding with N dissolved in steel to improve not only impact toughness but also delayed fracture resistance through grain refinement. In the present invention, Al is positively added, but if the content is less than 0.01%, the effect of the addition is not expected to be expected. If the content exceeds 0.1%, a large amount of alumina inclusions may be generated and mechanical properties may be greatly reduced. Taking this point into consideration, in the present invention, the content of Al is set to 0.01 to 0.1%.

The content of N is 0.003 to 0.02%.

N is an element that forms a nitride to make crystal grains finer to improve delayed fracture resistance. If the content of N is less than 0.003%, the above effects are hardly expected. If it exceeds 0.02%, the amount of N dissolved in the steel increases to lower the cold-rolled composition.

In addition to the above composition, the balance includes Fe and unavoidable impurities. In the present invention, addition of alloying elements other than the alloy composition mentioned above is not excluded.

The hot wire is heated to a temperature in the range of 900 to 1,000 DEG C, the hot wire is quenched to a temperature in the range of 250 to 400 DEG C, kept at a constant temperature for 30 minutes or more, It is possible to manufacture a high strength steel having excellent delayed fracture resistance according to the present invention.

First, the wire material having the above-described alloy composition is heated to a temperature range of 900 to 1,000 占 폚. At this time, it can be maintained in the temperature range over a certain period of time depending on the wire diameter or the manufacturing use.

If the heating temperature is less than 900 占 폚, the carbide contained in the wire material is completely dissolved and is not reused in austenite, so that the curing ability of the steel is lowered. Therefore, it is difficult to secure the target microstructure in the subsequent constant temperature transformation heat treatment, . On the other hand, if the heating temperature exceeds 1,000 ° C, the austenite grains become coarse, which is advantageous in terms of hardenability but not in the effect of improving the delayed fracture resistance, and the fatigue crack propagation speed can be increased along the grain boundary, It is preferable to set the heating temperature range to 900 to 1,000 占 폚.

Then, the heated wire rod is quenched to a temperature range of 250 to 400 ° C exceeding the martensitic transformation start temperature (Ms), maintained at a constant temperature for 30 minutes or more, subjected to transformation heat treatment, and then air-cooled.

If the heat treatment temperature is less than 250 ° C, a large amount of martensite is generated to increase the strength, while the ductility and toughness are greatly decreased to cause brittleness. When the temperature exceeds 400 ° C, a large amount of the upper bainite is produced, It is preferable that the heat treatment transformation temperature is in the range of 250 to 400 占 폚.

The heat treatment for the constant temperature transformation is preferably carried out for 30 minutes or longer. If not maintained for more than 30 minutes, some untransformed austenite is transformed into martensite during air cooling to produce martensite in addition to the lower bainite and retained austenite in the final steel. This not only increases the strength more than necessary but also lowers softness and toughness to lower the delayed fracture resistance, which is not desirable.

The high-strength steels excellent in the delayed fracture resistance produced according to the above-described production method include 90% by volume or more of lower bainite and 10% by volume or less of retained austenite in a microstructure.

The bainite structure is a hard structure used for high strength and high toughness of a material, and a crystal of a ferrite portion (bainitic ferrite) is a microcrystal having a lath shape with a thickness of 1 탆 or less. A bainite structure is created by dividing one austenite grain into subsystem units called a packet and a block. A packet is a lattice group in which the dense crystal planes have a parallel relationship with each other, and a block is a lattice group having an equivalent crystal variant, and one packet is divided into a plurality of blocks.

The lath thickness of the lower bainite in the microstructure may be 500 nm or less. If the glass thickness of the lower bainite exceeds 500 nm, it is preferable to manage the alloy composition or the manufacturing process so that the glass thickness is 500 nm or less since it is not sufficient to greatly improve the delayed fracture resistance. More preferably, the glass thickness of the lower bainite can be 450 nm or less.

In addition, the misorientation angle between the bainites can be more than 90% at 50 to 70 °. That is, the inter-raster distribution can exhibit the highest relative frequency at 50 to 70 degrees. When the interstices distribution of 60 deg. Does not show the highest relative frequency but deviates greatly from 60 deg., The bainite having a shape other than the lower bainite coexists, which may greatly impair mechanical properties and delayed fracture resistance , It is desirable to manage such a warping angle as described above. More preferably, the angle of inclination between the bainite ridges may be more than 90% to 55 to 65 °.

Hereinafter, preferred embodiments of the present invention will be described in detail.

Example

Molten steel having the composition shown in the following Table 1 was cast into ingots, homogenized at 1,250 DEG C for 12 hours, reheated to a temperature of 1,050 DEG C by hot rolling at a final thickness of 15 mm, and then air-cooled.

division Composition (% by weight) C Si Mn Cr Mo P S Al N Inventory 1 0.52 0.2 1.6 0.5 0.3 0.009 0.007 0.028 0.010 Inventory 2 0.43 0.3 2.2 0.6 0.5 0.011 0.008 0.034 0.009 Inventory 3 0.48 0.1 1.9 0.4 0.7 0.010 0.005 0.025 0.011 Honorable 4 0.57 0.2 1.5 0.2 0.2 0.008 0.006 0.042 0.012 Comparative Example 1 0.41 0.2 2.0 0.3 0.7 0.012 0.008 0.023 0.018 Comparative Example 2 0.50 0.3 1.6 0.1 0.3 0.009 0.009 0.030 0.009 Comparative Example 3 0.44 0.1 1.7 0.4 0.6 0.010 0.005 0.045 0.008 Comparative Example 4 0.42 0.2 2.1 0.5 0.4 0.009 0.006 0.056 0.015 Comparative Example 5 0.53 0.1 1.5 0.2 0.1 0.007 0.007 0.033 0.017 Comparative Example 6 0.45 0.1 0.6 0.3 0.2 0.008 0.007 0.041 0.008

These wire rods were subjected to constant temperature heat treatment under the conditions shown in Table 2 below, and the bainite volume fraction, retained austenite volume fraction and bainite tilt distribution were measured and are shown in Table 2.

The bainite volume fraction was measured using an image analyzer, the residual austenite volume fraction was measured using a Cu-Kα X-ray diffractometer, and the bainite tilt distribution was calculated using backscattering electron diffraction (EBSD) .

The tensile strength of each steel material after the heat treatment at constant temperature was measured and shown in Table 2. The tensile strength was measured according to JIS Z 2241 using a JIS No. 4 test piece.

On the other hand, the delayed fracture strength of the steel was measured and shown together in Table 2. The delayed fracture strength was determined by a generally used constant load method. This evaluation method is a method of evaluating the delayed fracture resistance by the time required until the fracture under the additional stress or under a specific stress. In the delayed fracture test, the test stress determined the applied stress based on the notched tensile strength.

The delayed fracture tester used a constant loading type delayed fracture testing machine. The delayed fracture test specimens were prepared with a specimen diameter of 6 mm, a notch diameter of 4 mm and a notch radius of 0.1 mm, and a solution (NaCl + CH ₃ CHOOH) of a pH of 2 as a test sample atmosphere was prepared and tested at a temperature of 25 ± 5 ° C .

The critical delta fracture strength is the tensile strength at which the fracture occurs at the same stress ratio (additional stress / notch tensile strength) for not less than 150 hours. The notch strength is determined by tensile test of the notch specimen (maximum load / Respectively. The number of tests for setting the critical retardation strength was obtained based on a minimum of 15 tests.

division Constant temperature heat treatment Residue
Austenite
(vol.%) bottom
Bay knight
(vol.%) Lass
thickness
(Nm) maximum
frequency
Lass
Stiffness
(°) Seal
burglar
(MPa) delay
Destruction
burglar
(MPa) heating
Temperature
(° C) Constant temperature
maintain
Temperature
(° C) maintain
time
(min) Inventory 1 910 300 45 9 91 330 61 1,690 1,520 Inventory 2 950 350 30 10 90 440 57 1,410 1,272 Inventory 3 975 280 50 8 92 300 60 1,770 1,581 Honorable 4 930 320 40 10 90 400 63 1,550 1,373 Comparative Example 1 750 310 45 9 91 360 59 1,030 927 Comparative Example 2 1,150 380 35 9 91 480 58 1,220 990 Comparative Example 3 900 200 40 12 - - - 1,980 1,190 Comparative Example 4 990 470 55 7 - 860 9 960 725 Comparative Example 5 960 330 15 18 43 410 56 1,840 1,145 Comparative Example 6 980 390 40 9 91 470 64 1,330 1,150

As shown in Table 1 and Table 2, Examples 1 to 4 satisfying the alloy composition and the manufacturing method according to the present invention contained 90% or more of lower bainite and 10% or less of residual austenite in a volume fraction, It was found that the bainite lath thickness is 440 nm or less and the relative frequency is the highest at around 60 ° in the distribution of misorientation angle between the bainitic laths. Particularly, the inclination angle between the bainite ras was more than 90% in the range of 55 to 65 °. Thus, it was confirmed that Inventive Examples 1 to 4 exhibited excellent delayed fracture resistance with a delayed fracture strength of 1,200 Mpa or more.

On the other hand, in Comparative Example 1, since the heating temperature was low during the constant temperature heat treatment, coarse carbides which were not reused in the austenite were present even after the heat treatment, and the tensile strength and the delayed fracture resistance were weakened.

In Comparative Example 2, contrary to Comparative Example 1, since the heating temperature was too high during the constant-temperature heat treatment, the austenite grain size grew large and the density of diffusive hydrogen became large, which was not preferable for improving the delayed fracture resistance.

Comparative Example 3 is a case in which the temperature for maintaining the constant temperature during the constant-temperature heat treatment is remarkably low, and the lower bainite is hardly produced and the martensite becomes the base structure. The strength increased greatly with the formation of martensite, but the ductility and toughness were not only reduced but the delayed fracture strength was less than 1,200 MPa.

In Comparative Example 4, the temperature at which the holding temperature was constant during the constant-temperature heat treatment was remarkably high. Since the upper bainite was mainly formed, the thickness of the glass was also thick, and the maximum frequency of bainitic interception was extremely low. In addition, since there are many carbides in the boundary of the ras, it is understood that the mechanical properties are greatly lowered and the delayed fracture resistance is not sufficiently improved.

In Comparative Example 5, the duration of the constant-temperature heat treatment is too short to sufficiently form the lower bainite, and the untransformed austenite transforms into martensite upon cooling to form a composite structure. This shows that it is very effective in improving the strength but not in improving the delayed fracture resistance.

Comparative Example 6 shows that the Mn content is out of the range of the present invention, and the annealing conditions and the microstructure are satisfied, but the strength and the delayed fracture resistance are not sufficiently improved.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will recognize that other embodiments may occur to those skilled in the art without departing from the scope and spirit of the following claims. It will be understood that various changes and modifications may be made.

Claims (7)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.4-0.6% of C, 0.3% or less of Si, 1.5-2.5% of Mn, 1.0% or less of Cr, 0.01 to 0.1%, N: 0.003 to 0.02%, balance Fe and unavoidable impurities,
The microstructure has a volume fraction of 30% or less of ferrite and pearlite, and 70% or more of bainite. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.4-0.6% of C, 0.3% or less of Si, 1.5-2.5% of Mn, 1.0% or less of Cr, 0.01 to 0.1%, N: 0.003 to 0.02%, the balance Fe and unavoidable impurities to a temperature range of 900 to 1,000 占 폚; And
And rapidly cooling the heated wire to a temperature range of 250 to 400 ° C and keeping it at a constant temperature. 3. The method of claim 2,
Wherein the duration of the constant temperature holding time is 30 minutes or more. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.4-0.6% of C, 0.3% or less of Si, 1.5-2.5% of Mn, 1.0% or less of Cr, 0.01 to 0.1%, N: 0.003 to 0.02%, balance Fe and unavoidable impurities,
The microstructure is a volume fraction of high strength steel excellent in delayed fracture resistance including 90% or more of lower bainite and 10% or less of retained austenite. 5. The method of claim 4,
Wherein the lower bainite has a lath thickness of 500 nm or less and is excellent in delayed fracture resistance. 5. The method of claim 4,
A high-strength steel excellent in delayed fracture resistance having a misorientation angle of 50 to 70 degrees distributed over 90% in the bainite lath of the microstructure. 5. The method of claim 4,
The steel material is excellent in delayed fracture resistance having a delayed fracture strength of 1,200 MPa or more.