KR101304852B1 - Steel sheet having excellent weldability, hardness uniformity in the thickness direction and low temperature toughness, and method for manufacturing the same - Google Patents

Abstract

The present invention aims to provide a steel sheet having excellent weldability and excellent overall hardness value, small hardness difference in thickness direction, and excellent low temperature toughness. The weight ratio is C: 0.05 to 0.18% and Si: 0.15 to 0.3%, Mn: 0.5-1.5%, Cr: 0.1-1.5%, Mo: 0.1-0.5%, Ni: 0.1-0.5%, B: 0.0005-0.0050%, Ti: 0.01-0.03%, Nb: 0.01-0.05 %, Al: 0.005 ~ 0.1%, P: 0.015% or less, S: 0.010% or less, the rest contains Fe and unavoidable impurities, weldability of Ceq value of 0.50 or less by Equation 1 below, thickness direction material variation characteristic and low temperature Provided is a steel sheet excellent in toughness and a method of manufacturing the same.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15

Weldability, hardness, low temperature toughness, carbon equivalent (Ceq)

Description

STEEL SHEET HAVING EXCELLENT WELDABILITY, HARDNESS UNIFORMITY IN THE THICKNESS DIRECTION AND LOW TEMPERATURE TOUGHNESS, AND METHOD FOR MANUFACTURING THE SAME}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wear resistant steel sheet used in fork cranes, bulldozers, excavators, and the like, and more particularly, to a steel sheet having excellent weldability and excellent material variation in the thickness direction and low temperature toughness, and a method of manufacturing the same.

In the case of heavy equipment used in many industries such as construction, civil engineering, mining, and cement industry, abrasion wear is seriously required, and thus, materials that exhibit wear resistance are required. Wear resistant steels require many properties such as strength, hardness, toughness, and wear resistance. In particular, as the heavy equipment is manufactured through welding, excellent weldability is required for the applied material.

In addition, since wear resistance is generally improved as hardness increases, hardness values of 360HB or more are required for Brinell hardness standards for materials applied to forklifts, bulldozers, excavators and rock drills. In order to obtain such high hardness, after rolling, reheating to a temperature of Ac3 or higher and then quenching is commonly used.

Conventional techniques in this regard include Japanese Patent Laid-Open Nos. 2-179842, 8-41535, and 61-166954. In the above patents, a method of increasing the surface hardness by adding a high C content and a large amount of hardenability improving elements such as Cr and Mo is proposed. However, in the above case, as the amount of C and the hardenable alloy is added in order to secure the hardness, the manufacturing cost increases and the weldability and the toughness decrease.

On the other hand, Japanese Unexamined Patent Application Publication Nos. 2002-20837, 2004-10996, and 2006-328512, and Korean Patent Publication Nos. 2000-0038156, 2001-01060644, and the like after rolling without using a reheating hardening method A method of increasing the hardness by securing the martensite structure using a direct quenching and treating method has been proposed.

The method according to the patents can be obtained by using the excellent hardening ability of the direct quenching method, the reduction of alloying elements and the improvement of weldability accordingly, but coarsening of austenite grains occurs compared to the reheating quenching method There is a problem that the reduction occurs, the production conditions are difficult, there is a restriction in the productivity and the shape control of the steel sheet is difficult due to rapid cooling from a high temperature.

In addition, in the case of the patents to achieve abrasion resistance is limited to improving the surface hardness of the steel sheet, there is a problem that the wear resistance of the steel sheet is significantly reduced when abrasion of the high hardness surface area during use significantly reduces the life of the equipment.

One aspect of the present invention is to provide a steel sheet having excellent weldability and excellent total hardness value, small hardness difference in thickness direction, and excellent low temperature toughness.

In the present invention, C: 0.05 to 0.18%, Si: 0.15 to 0.3%, Mn: 0.5 to 1.5%, Cr: 0.1 to 1.5%, Mo: 0.1 to 0.5%, Ni: 0.1 to 0.5%, B: 0.0005 to 0.0050%, Ti: 0.01 to 0.03%, Nb: 0.01 to 0.05%, Al: 0.005 to 0.1%, P: 0.015% or less, S: 0.010% or less, the remainder includes Fe and unavoidable impurities, and the following formula It provides a steel sheet excellent in weldability, thickness direction material variation characteristic and low temperature toughness of Ceq value of 0.50 or less.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15

In addition, the present invention comprises the steps of reheating the steel slab satisfies the composition and Ceq value according to the formula 1 0.50 or less to 1100 ℃ ~ 1250 ℃;

Performing a step-down rolling of at least 70% of the cumulative reduction rate at a reduction ratio of 10% or more per pass in a region above the recrystallization temperature and terminating the finishing hot rolling at an Ar3 temperature or higher; And

Weldability including the step of cooling at a cooling rate that satisfies the H value of Equation 2 below 2.0 after reheating at a temperature of 850 ° C. to 950 ° C. for 1.6 t + (10 to 30 minutes) (where t is the thickness of the steel). The present invention provides a method for manufacturing a steel sheet having excellent thickness variation material variation and low temperature toughness.

Equation 2. H = Ceq × (QCR) ^1/2 (QCR is Quenching Cooling Rate)

The steel sheet of the present invention has excellent weldability by minimizing alloying elements and at the same time, the hardness value of the total thickness is 360HB or more, the hardness difference in the thickness direction is 50HB or less, and the low temperature toughness is excellent by optimizing the cooling rate during reheating and quenching after rolling. Has an effect.

Hereinafter, the present invention will be described in detail.

Hereinafter, the composition range of the present invention will be described in detail (hereinafter,% by weight).

C: 0.05% ~ 0.18%

The C is an element effective in increasing the strength and hardness in the steel having a martensitic structure, but when the content thereof is high, the weldability and toughness are decreased, so that C is limited to 0.05% to 0.18% to secure the hardness required by the present invention. It is preferable.

Si: 0.15%-0.3%

The Si is an element exhibiting strength increase due to deoxidation and solid solution strengthening, but when the content thereof is high, it is preferable to limit it to 0.15% to 0.3% because it decreases the toughness of the base metal as well as decreases the weldability and the weld toughness.

Al: 0.005%-0.1%

The Al is a strong deoxidizer to lower the oxygen content in the molten steel, an element effective for the production of clean steel, but the production cost increases when added in excess of 0.1%, it is preferable to limit the content to 0.005% to 0.1%.

Mn: 0.5% ~ 1.5%

The Mn is an element that effectively increases the hardenability by suppressing the formation of ferrite and lowers the Ar3 temperature, and increases the strength of the material, but it is preferable to limit the Mn to 0.5% to 1.5% because the carbon equivalent decreases the weldability of the material.

Cr: 0.1%-1.5%

The Cr is an element that increases the hardenability to increase the strength of the material, but when excessively added, the Cr decreases the weldability, so it is preferably limited to 0.1% to 1.5%.

Mo: 0.1% ~ 0.5%

The Mo is an element that is very effective to increase the strength by increasing the hardenability of the material, such as Cr, but it is preferable to limit to 0.1% to 0.5% because the manufacturing cost increases and the weldability decreases when a large amount is added as an expensive element.

Ni: 0.1%-0.5%

Ni is a very effective element for improving low-temperature toughness and increasing strength, but since it is an expensive element, it is preferable to limit it to 0.1% to 0.5% because the production cost increases and the weldability decreases when a large amount is added.

B: 0.0005%-0.0050%

The B is effective to increase the hardenability of the material even with the addition of a small amount, and increase the strength, and the effect is very high in the complex addition with Mo, V, Ti, but toughness and weldability when excessive addition is 0.0005% It is desirable to limit to 0.0050%.

Ti: 0.01%-0.03%

Ti is an element maximizing the effect of B, which is an important element for improving the hardenability, and Ti is inhibited by the formation of TiN, thereby increasing the solid solution B to maximize the hardenability improvement by B, and precipitated TiN is austenite. Pinning of nitrite grains has the effect of suppressing coarsening of grains, but due to coarsening of Ti precipitates when excessively added, there are problems of deterioration of toughness and segregation and oxide formation during steelmaking. It is preferable to limit it to%-0.03%.

Nb: 0.01% to 0.05%,

The Nb is dissolved in austenite to increase the hardenability of the austenite, and is an important element that increases strength by precipitating carbonitrides such as Nb (C, N), but is brittle due to the formation of coarse precipitated phase when added in large quantities. It is preferable to limit the content to 0.01% to 0.05% because it reduces the toughness as a starting point of fracture.

P: not more than 0.015%

The P is an element that lowers the low temperature toughness, but the content thereof is controlled to be low, but the process for removal is difficult, so excessive cost is managed in the range of 0.015% or less.

S: not more than 0.01%

S is an element that reduces low-temperature toughness, such as P, so that MnS inclusions in steel lower the physical properties of the steel. Therefore, S is managed to be lower than 0.01% because excessive removal costs are required because the removal process is difficult.

The remainder contains Fe and unavoidable impurities.

In the present invention, it is preferable that the Ceq value of the following formula 1 is 0.5 or less. The Ceq refers to a carbon equivalent, when the value exceeds 0.5, the weldability is lowered, the working conditions are difficult during welding, there is a problem that the productivity is lowered.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15

In the present invention, it is preferable that the H value defined by Equation 2 below for the relationship between the Ceq and the quenching cooling rate (QCR) is 2.0 or more. When the H value is less than 2.0, since the martensite structure of 90% or more is not possible in the total thickness of the steel sheet after the hardening treatment, the H value is preferably 2.0 or more in order to have a uniform hardness in the steel plate thickness direction.

Equation 2. H = Ceq × (QCR) ^1/2 (QCR is Quenching Cooling Rate)

In the steel sheet of the present invention, the microstructure of the martensite phase at a full thickness satisfies 90% or more by volume fraction, and the rest is composed of a bainite phase or a ferrite phase. In the case of alloy design with low Ceq or low quenching cooling rate to improve weldability, the difference in hardenability occurs in the thickness direction of steel sheet, and the volume fraction of martensite phase is less than 90% toward the center of steel sheet. To form. As such, when the fraction of the martensite phase in the center portion decreases, and the soft phase such as the bainite phase or the ferrite phase increases, the hardness value in the thickness direction is greatly reduced, resulting in a large material difference in the thickness direction. There is a problem of shortening the life of the equipment. In particular, when the residual structure is increased in the bainite phase, the low temperature toughness is reduced due to the formation of coarse carbides in the bainite phase, so that the volume fraction of the martensite phase is preferably 90% or more.

In the steel sheet of the present invention, when the hardness value of the entire thickness is measured by a Brinell hardness meter, the hardness difference between the surface layer portion and the central portion in the thickness direction is 50 HB or less.

In order to have the structure and the hardness value, an appropriate rolling and heat treatment process must be performed. Hereinafter, the manufacturing method of the present invention will be described in detail.

 In this case, the steel slab whose composition and Ceq value of Formula 1 satisfy 0.5 or less is reheated at 1100-1250 degreeC. This is because when the reheating temperature is lower than 1100 ° C., solute atoms such as Nb are hardly dissolved, and when the reheating temperature is higher than 1250 ° C., coarsening of austenite grains is difficult to be suppressed.

After reheating, hot rolling is performed. Hot rolling is carried out by rolling down the cumulative reduction rate of 70% or more at a rolling reduction rate of 10% or more per pass in a region above the recrystallization temperature, and cooling after finishing finishing rolling above the Ar3 temperature.

If the cumulative reduction ratio is less than 70% at a temperature above the recrystallization temperature, it is not possible to obtain a uniform rolled structure up to the center of the thick steel sheet, because it is not possible to produce a thick steel sheet with a small material deviation in the thickness direction. When the finish rolling temperature is less than Ar3, ferrite is formed and the target martensite structure cannot be obtained.

Cooling rate at which the H value of Equation 2 satisfies 2.0 or more after reheating the hot rolled and cooled steel sheet at a temperature of 850 to 950 ° C. for 1.6 t + (10 to 30 minutes), where t is the thickness of the steel. Quench processing with. If the reheating temperature is less than 850 ℃ difficult to re-employment of the employment element is difficult to secure strength, if it exceeds 950 ℃ grain growth occurs to cause toughness.

In addition, the time limit for the heat treatment time is less than 1.6t + 10 minutes, it is difficult to homogenize the tissue, if it exceeds 1.6t + 30 minutes, productivity is inhibited.

Hereinafter, embodiments of the present invention will be described in detail. However, the following examples do not limit the present invention.

(Example)

The steel slab that satisfies the composition range and Ceq of Table 1 was heated at a temperature range of 1100 ° C to 1250 ° C, and subjected to rolling reduction of 70% or more at a cumulative reduction ratio of 10% or more per pass in a region above the recrystallization temperature. After the finish rolling over the Ar3 temperature and cooled down, it is reheated at 1.6 to + (10 to 30 minutes) (where t is the thickness of the steel) at a temperature of 850 ° C. to 950 ° C., and at various cooling rates shown in Table 2. The surface hardness and the hardness of the core were measured by the quenching treatment, and the results are shown in Table 2.

Steel grade C Mn Si Cr P S Al Ti Nb Mo Ni B Ceq A 0.08 1.0 0.30 0.30 0.012 0.010 0.035 0.015 0 0.2 0 0.0015 0.35 B 0.10 1.2 0.30 0.30 0.010 0.010 0.030 0.015 0.010 0.2 0.30 0.0010 0.42 C 0.14 1.2 0.25 0.40 0.010 0.005 0.030 0.015 0.015 0.2 0.30 0.0010 0.48 D 0.18 1.1 0.30 0.50 0.012 0.006 0.025 0.015 0 0.4 0.35 0.0020 0.57

The hardness measurement was measured by a Brinell hardness meter in accordance with the ISO-6506 standard, the surface layer was measured after removal of 2mm from the steel plate surface, the center portion was measured after grinding by cutting 1 / 2t of the steel plate thickness. The measurement was evaluated five times at random and averaged.

On the other hand, in order to specify the low temperature toughness, 10 V-notch specimens were taken from 1 / 4t points of each steel sheet according to JIS Z 2242, and the Charpy impact test was performed. The average value was CVN [-40 ° C] Was obtained. In this test, when CVN [-40 degreeC] was 50 J or more, it was set as the pass.

In addition, in order to evaluate the weldability, welding was performed at a current of 280 to 320 A and a welding speed of 22 to 30 cm / min using a GMAW (CO ² gas) manual welding method. Cracks were generated, and three Charpy impact test specimens were taken from the HAZ part, and the CVN [-20 ° C.] low temperature toughness value was measured, and the average value was taken as the low temperature toughness value of the HAZ part. The weldability was judged to be good when there were no defects or cracks in the ultrasonic flaw detection and the CVN [-20 ° C] value of the HAZ part was 50J or more.

Steel grade
thickness
(t)
QCR
(° C / s)
H
Surface layer
Hardness
(HB)
center
Hardness
(HB)
Hardness difference
(HB)
CVN [-40 ℃]
(1 / 4t, J)
Weldability Remarks
ultrasonic wave
inspection HAZ
vE [-20 ℃] judgment A




13 30 1.899 395 346 49 32 radish 55 Good Comparative example 25 30 1.899 389 321 68 24 radish 53 Good Comparative example 50 20 1.550 384 250 134 44 radish 62 Good Comparative example 50 15 1.343 382 211 171 34 radish 67 Good Comparative example 50 10 1.096 376 195 181 32 radish 54 Good Comparative example 50 5 0.775 371 180 191 25 radish 52 Good Comparative example B




13 30 2.300 405 398 7 65 radish 74 Good Honor 25 30 2.300 408 386 22 64 radish 62 Good Honor 50 20 1.878 397 241 156 37 radish 55 Good Comparative example 50 15 1.627 395 228 167 34 radish 58 Good Comparative example 50 10 1.328 395 212 183 35 radish 66 Good Comparative example 50 5 0.939 392 196 196 24 radish 78 Good Comparative example C




13 30 2.629 420 415 5 54 radish 62 Good Honor 25 30 2.629 422 412 10 56 radish 67 Good Honor 50 20 2.147 418 385 33 64 radish 73 Good Honor 50 15 1.859 410 341 69 42 radish 64 Good Comparative example 50 10 1.518 406 323 83 32 radish 66 Good Comparative example 50 5 1.073 405 259 146 27 radish 52 Good Comparative example D

13 30 3.104 438 435 3 34 Occur 15 Bad Comparative example 25 30 3.104 435 431 4 32 Occur 14 Bad Comparative example 50 20 2.534 433 425 8 37 Occur 21 Bad Comparative example

In the case of the invention example satisfying the composition and conditions of the present invention in Table 2, it can be seen that the hardness value of the surface layer portion and the central portion is 360HB or more, the difference in hardness value is 50HB or less, and has excellent weldability.

However, in the case of comparative examples among A, B, and C grades, the H value is 2.0 or less, so the hardness value of 360HB or more cannot be secured at all thicknesses, and the hardness value of the surface and the central part shows a big difference, so it is expected to secure uniform material values. Can't.

In addition, the relationship between the H value and hardness value difference in Table 2 is shown in FIG. As shown in Figure 1, the difference in hardness between the surface portion and the central portion is 50HB or more when the value of H is less than 2.0 it can be confirmed that the material deviation in the thickness direction is severe.

As a result of observing weldability, it can be seen that A, B, and C steels have good weldability irrespective of H value, but D steels have H value of 2.0 or more, but as shown in Table 1, Ceq) is 0.5 or more and hardness value is too high, and it turns out that weldability falls.

Figure 1 shows the hardness difference between the center of the steel sheet and the surface layer according to the H value.

Claims (4)

By weight% C: 0.05 ~ 0.18%, Si: 0.15 ~ 0.3%, Mn: 0.5 ~ 1.5%, Cr: 0.1 ~ 1.5%, Mo: 0.1 ~ 0.5%, Ni: 0.1 ~ 0.5%, B: 0.0005 ~ 0.0050 %, Ti: 0.01% to 0.03%, Nb: 0.01% to 0.05%, Al: 0.005% to 0.1%, P: 0.015% or less, S: 0.010% or less, the remainder includes Fe and unavoidable impurities, A steel sheet having a Ceq value of 0.50 or less, a total thickness hardness value of 360 HB or more, and a hardness difference between the surface portion and the center portion of 50 HB or less, excellent weldability, thickness direction material variation, and low temperature toughness. Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, wherein C, Mn, Cr, Mo, V, Ni, Cu are alloy elements of the steel sheet In weight percent) delete The method according to claim 1, The microstructure of the steel sheet is excellent in weldability, thickness direction material variation characteristics and low temperature toughness of the martensite phase in volume fraction of more than 90% at full thickness. By weight% C: 0.05 ~ 0.18%, Si: 0.15 ~ 0.3%, Mn: 0.5 ~ 1.5%, Cr: 0.1 ~ 1.5%, Mo: 0.1 ~ 0.5%, Ni: 0.1 ~ 0.5%, B: 0.0005 ~ 0.0050 %, Ti: 0.01% to 0.03%, Nb: 0.01% to 0.05%, Al: 0.005% to 0.1%, P: 0.015% or less, S: 0.010% or less, the remainder includes Fe and unavoidable impurities, Reheating the steel slab having a Ceq value of 0.50 or less to 1100 ° C to 1250 ° C; Performing a step-down rolling of at least 70% of the cumulative reduction rate at a reduction ratio of 10% or more per pass in a region above the recrystallization temperature and terminating the finishing hot rolling at an Ar3 temperature or higher; And After reheating the hot rolled hot rolled steel sheet at a temperature of 850 ° C. to 950 ° C. for 1.6t + (10 to 30 minutes) (where t is the thickness of steel), the H value of Equation 2 below satisfies 2.0 or more. Cooling down at speed The manufacturing method of the steel sheet excellent in weldability, thickness direction material deviation characteristics and low temperature toughness comprising a. Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, wherein C, Mn, Cr, Mo, V, Ni, Cu are alloy elements of the steel sheet In weight percent) Equation 2. H = Ceq × (QCR) ^1/2 (QCR is Quenching Cooling Rate)

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