KR101639845B1 - High strength thick sheet with cutting crack resistance and method for manufacturing the same - Google Patents

Abstract

TECHNICAL FIELD The present invention relates to a high tensile strength steel excellent in cutting crack resistance and a method of manufacturing the same.
For this purpose, in the present invention, by optimizing the composition of the steel material and the manufacturing conditions, it is possible to provide a high tensile steel having excellent cutting crack resistance from the securing of the microstructure which is advantageous for improving the hardenability of the steel.

Description

TECHNICAL FIELD [0001] The present invention relates to a high tensile strength steel having excellent cutting crack resistance and a method of manufacturing the same. BACKGROUND OF THE INVENTION [0002]

TECHNICAL FIELD The present invention relates to a high tensile strength steel excellent in cutting crack resistance and a method of manufacturing the same.

Generally, a high strength steel sheet having high hardness and abrasion resistance has high hardenability and high strength, but has a disadvantage that it is weak in toughness.

Therefore, when such a high-hardness wear-resistant steel is subjected to cutting such as gas cutting, cracks are generated on a cut portion where thermal history (thermal stress) is concentrated. Therefore, when cutting such high-hardness wear-resistant steel, it is necessary to pay close attention to preheating and post-heating at the time of machining, which leads to a problem that machining efficiency is lowered.

Therefore, many attempts have been made to improve the toughness of a wear-resistant steel having a high hardness. For example, in Patent Document 1, the hardenability is secured by adding an expensive element such as nickel (Ni) or molybdenum (Mo) There has been a plan to improve the toughness. However, such an increase in manufacturing cost is accompanied by the use of expensive elements, which is disadvantageous from the economical point of view.

Therefore, as a measure to overcome such shortcomings, Patent Document 2 attempts to improve the delayed fracture characteristics by raising the dislocation density in the martensite structure through control rolling and increasing the hydrogen trap site. However, this is intended to improve only the delayed fracture characteristics due to hydrogen, and is insufficient for improving the crack resistance.

Furthermore, when a certain level of stress or temperature is secured due to external environmental factors, the trapped hydrogen may diffuse and accumulate on the defective part to generate cracks, which limits the use range . In particular, even if a component favorable to toughness is added, if the center hardening ability is not properly controlled, there is always a risk of cracking occurring during cutting.

Therefore, there is a need for an effective method for improving the crack resistance of the steel material without adding an expensive element.

Japanese Laid-Open Patent Publication No. 1994-116637 Japanese Laid-Open Patent Publication No. 1999-229075

One aspect of the present invention is to provide a high tensile strength steel which is excellent in thermal hysteresis capacity even when a thermal history (thermal stress) is accompanied and does not cause cracks, and a method of manufacturing the same.

One aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.05 to 0.30% of carbon (C), 2 to 4% of manganese (Mn) : Not more than 0.2% (excluding 0) and niobium (Nb): not more than 0.1% (excluding 0), the balance Fe and inevitable impurities,

The microstructure of the steel surface portion (range of 0.4 t from the surface) includes a martensite single phase, and the microstructure of the steel center portion (0.4 t <center portion <0.6 t) has a bainite + ferrite composite structure of 40% To 60% or less of martensite and excellent in crack cutting resistance.

(Where t is the thickness of the steel).

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising the steps of: reheating a steel slab satisfying the above-described compositional composition to 1050 to 1250 占 폚; Hot-rolling the reheated steel slab at 800 to 950 占 폚 to produce a hot-rolled steel sheet; And cooling the hot-rolled steel sheet to 300 ° C or less, wherein the cooling step is carried out at a cooling rate satisfying the following relational expression (2).

[Relation 2]

0.7 x CR < CR = 17.2 - (3.72 x H) < 0.85 x CR

(Where CR is the cooling rate and H is the center hardening index).

According to the present invention, it is possible to provide a high tensile strength steel having excellent strength and hardness with excellent center stress holding ability by keeping the hardenability of the center of the steel at an appropriate level.

FIG. 1 is a graph showing hardness distributions according to the thicknesses of inventive steel 3 and comparative steel 2 according to an embodiment of the present invention.
FIG. 2 is a photograph of a microstructure of a center portion of an inventive steel 3 according to an embodiment of the present invention.

Generally, martensitic high hardness abrasion resistant steels have high yield strength and tensile strength and are widely used in structural materials and transportation / construction machinery. Such high-hardness wear-resistant steels generally include high-carbon and high-alloy elements, and a quenching process is indispensable to ensure a martensite structure capable of obtaining sufficient strength.

In addition, the high-hardness abrasion-resistant steel is often subjected to processing such as cutting steel to be usefully used in industrial fields such as construction and civil engineering. Since the high-hardness abrasion steel is a high strength steel having a tensile strength of 1 GPa or more Machine cutting is not easy, mainly using gas cutting.

However, it has been found that the high-hardness abrasion-resistant steel has excellent strength and hardness, but has a problem that cracks are generated in the heat affected portion of the cut portion after gas cutting due to the problem of weak toughness. Therefore, it is possible to use a cutting method that minimizes heat effects such as water jet cutting or machine cutting other than gas cutting, or by adding a process such as preheating and post heat treatment before and after the cutting process to reduce thermal stress generated by quenching after cutting, However, these methods have a problem in that the efficiency of the final product is deteriorated due to the load and the limitation of the cutting process.

The present inventors have conducted intensive studies to solve the problems in cutting the high-hardness abrasion-resistant steel, and as a result, it has been found that when a steel sheet has a soft phase at the center thereof, the stress- And completed the present invention.

Particularly, the present invention optimizes the composition of the steel material in order to optimize the microstructure of the steel material, and optimizes the conditions of the manufacturing process to ensure adequate hardenability, Is obtained.

Hereinafter, the component system of the steel of the present invention will be described in detail. (The content of each component means weight%)

C: 0.05 to 0.30%

Carbon (C) is effective in increasing strength and hardness in steels having martensite structure, and is a particularly advantageous element for improving hardenability.

In order to obtain the above-mentioned effect, it is necessary to add C at a content of 0.05% or more. If the content is too high, it is advantageous in securing strength, but there is a problem that abrasion resistance is lowered in a use environment where impact is greatly exerted, It is preferable to limit the upper limit to 0.30%.

Mn: 2 to 4%

Manganese (Mn) is an element which is very advantageous for increasing the hardenability of steel together with C, and is an element favorable for securing hardness in steel.

In order to obtain the above-mentioned effect, it is necessary to add Mn at 2% or more. If the content of Mn is less than 2%, ferrite or bainite is easily formed due to insufficient hardening ability, On the other hand, if the content exceeds 4%, there is a possibility of remarkably reducing the weldability and raising the manufacturing cost of the steel material. Therefore, in the present invention, it is preferable to limit the content of Mn to 2 to 4%. In this case, a stable martensite structure can be easily secured in the cooling step after hot rolling or solution treatment.

B: 0.01% or less (excluding 0)

Boron (B) is an element that increases the strength by effectively raising the incombustibility of the material even with a small amount of addition, and has the effect of inhibiting grain boundary fracture through grain boundary strengthening. However, if it is added in an excessively large amount, there is a problem that coarse precipitates are formed to deteriorate toughness and weldability. Therefore, the content thereof is preferably limited to 0.01% or less.

Ti: not more than 0.2% (excluding 0)

Titanium (Ti) is an element that is effective for improving the strength, and maximizes the effect of B, which is an important element for improvement of the incombustibility. When such Ti is added together with B, Ti inhibits the formation of BN by the formation of TiN, thereby increasing solubility B and maximizing improvement in entrapping property by B. Further, the precipitated TiN exhibits an effect of preventing crystal grain coarsening by pinning the austenite grains. However, if Ti is added excessively, there is a problem that the Ti precipitates are coarse and cause deterioration of the recognition, so the content thereof is preferably limited to 0.2% or less.

Nb: 0.1% or less (excluding 0)

Niobium (Nb) is an important element that is dissolved in austenite to increase the hardenability of austenite and to form Nb carbonitride, thereby inhibiting austenite grain growth with increasing strength. However, when Nb is added too much, there is a problem that the toughness is lowered due to the formation of a coarse precipitate phase. Therefore, the content thereof is preferably limited to 0.1% or less.

H value: 2.3 to 4.5

The steel material of the present invention preferably has the above-mentioned component system and satisfies the hardenability index expressed by the following relational expression 1 from 2.3 to 4.5.

The following formula 1 is derived from the contents of C and Mn which directly affect the hardenability in the component system of the present invention. If the value is less than 2.3, it becomes difficult to produce a wear-resistant steel of sufficient thickness, while 4.5 There is a problem that martensite is formed too easily and it becomes difficult to secure a target central core ductility in the present invention.

[Relation 1]

H value (center hardenability index) = 0.5C + 1.08Mn

(In the above-mentioned relational expression 1, C and Mn mean% by weight.)

It is necessary to limit the microstructure of the steel material to the steel material having the above-mentioned component system as a preferable condition for obtaining a steel material excellent in thermal stress holding capacity even in a region where thermal stress is concentrated and having excellent resistance to cracking resistance.

In the present invention, it is preferable to control the microstructure of the center portion of the steel material (0.4t < center portion < 0.6t) to be different from the surface portion (range of 0.4t from the surface) in order to improve the resistance to cracking. (Where t is the steel thickness).

That is, in order to increase the crack resistance of the steel material, it is necessary to lower the hardness of the center portion of the steel material.

Accordingly, in the present invention, it is preferable that the microstructure of the center of the steel material is contained in an area ratio of 40% or more of bainite + ferrite composite structure and 60% or less of martensite. Thus, by forming a soft microstructure at the center of the steel material, the hardness level at the center of the steel material can be relaxed. Further, when thermal stress is applied to such a steel material, the stress holding capacity at the center can be improved.

The microstructure of the steel core as described above is all produced in the step of cooling the hot-rolled steel sheet. In the present invention, it is preferable that the sum of the bainite and ferrite is 40% or more regardless of the respective fractions of the bainite and ferrite.

Since the strength and hardness are greatly influenced by the microstructure of the steel sheet surface portion, it is preferable that the microstructure of the steel sheet surface portion is included in the martensite single phase.

The microstructure of the surface portion of the steel can be produced by the above-described composition of the components. By forming the single-phase structure of martensite at the surface portion of the steel as described above, it is possible to ensure excellent hardness and strength on the surface portion.

The steel material of the present invention having the above-described composition and satisfying the microstructure condition can be provided with a surface hardness of not less than HB360 and a core hardness of not more than HB350, and can provide a steel material excellent in crack resistance.

Hereinafter, the most preferable method derived by the present inventors for producing a steel material satisfying the object of the present invention as described above will be described in detail.

The production method of the present invention roughly provides a method of heating a steel slab satisfying the above-described composition system to homogenize the steel slab, then hot rolling the steel slab to a hot rolled steel sheet, and immediately cooling the steel slab. Hereinafter, detailed conditions for each step will be described in detail.

Steel slab reheating step: 1050 ~ 1250 ℃

The reheating process of the steel slab is intended to smoothly carry out the subsequent rolling process and to fully obtain the mechanical properties of the steel material to be targeted, and the reheating process must be carried out within a suitable temperature range to suit the purpose.

If the temperature at which the steel slab is heated is lower than 1050 DEG C, the solubility of the alloy element is lowered and the Nb carbonitride is not sufficiently dissolved, thereby making it difficult to obtain the grain refinement effect. On the other hand, when the temperature exceeds 1250 DEG C, There is a problem of causing surface defects of the final product.

Hot rolling step: 800 ~ 950 ℃

The reheated slab is subjected to finish rolling at 800 to 950 DEG C to obtain a hot-rolled steel sheet.

In the present invention, the finishing rolling temperature is limited to 800 ° C. or more to complete the rolling in the austenite single phase region, and if the rolling is performed in the abnormal region below Ar 3, ferrite is formed and the strength is lowered Therefore, it is preferable to finish the rolling at a sufficiently high temperature relative to the Ar3 temperature. Further, even if the temperature is higher than the Ar3 temperature, when the rolling temperature is lowered to less than 800 deg. C, excessive pan-caked structure is formed to cause anisotropy of the plate material. On the other hand, when the temperature exceeds 950 占 폚, there is a problem that the non-recrystallized reverse rolling amount is insufficient and it becomes difficult to secure impact toughness.

After the rolling process as described above, the microstructure of the steel can be controlled according to the following cooling method.

Cooling step

The hot-rolled steel sheet produced by hot rolling as described above is cooled at a cooling rate satisfying the following relational expression 2, and cooling is stopped at 300 캜 or lower.

[Relation 2]

0.7 x CR < CR = 17.2 - (3.72 x H) < 0.85 x CR

(Where CR is the cooling rate and H is the center hardening index).

In the present invention, the reason why the cooling termination temperature is limited to 300 캜 or less is that when the cooling is terminated at a temperature higher than 300 캜, a self-tempering effect due to a double thermal phenomenon occurs, Can not do it.

In the present invention, it is preferable to control the cooling rate (CR) for securing the target microstructure, and more preferably, the cooling rate (CR) satisfies the relation (2). In the present invention, the cooling rate CR is related to the hardenability index H of the steel itself, which is determined according to the component system proposed in the present invention, and the microstructure of the steel is determined by the H and CR.

More specifically, in order to secure the intended strength and hardness, the microstructure of the steel core portion should be formed of 40% or more of bainite + ferrite composite structure, and the composite structure is a critical CR Can be formed when the value satisfies the above-described relationship (2).

Hereinafter, the present invention will be described in more detail by way of examples. It should be noted, however, that the embodiments described below are for illustrating and embodying the present invention, and not for limiting 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 )

Slabs satisfying the composition and composition ranges as shown in Table 1 below were prepared as hot rolled steel sheets through a series of reheating and hot rolling processes shown in Table 1 below. Thereafter, the above hot-rolled steel sheets were cooled under the cooling conditions shown in Table 2 below.

The microstructure and hardness of the surface portion and the center portion of each of the manufactured steels were measured and are shown in Table 2 below.

Further, each of the manufactured steel materials was cut at a speed of 300 mm / min. Then, the occurrence of cracks on the cut part was confirmed by nondestructive inspection. Further, specimens were taken and visually observed for cracks in the microstructure. In this case, when cracks were generated, the results are shown as &quot; C &quot;, and when cracks were not generated, the results are shown as &quot; O &quot;

division
Component composition (% by weight) H value Manufacturing conditions C Mn B Ti Nb Reheating
Temperature (℃) Finish rolling
Temperature (℃) Cooling rate
(° C / s) Termination temperature
(° C) Inventive Steel 1 0.06 3.8 0.002 0.025 0.05 4.134 1107 878 1.2 250 Invention river 2 0.13 3.2 0.008 0.16 0.06 3.521 1120 855 3.1 207 Invention steel 3 0.19 2.7 0.003 0.09 0.04 3.011 1160 921 4.5 220 Inventive Steel 4 0.22 2.4 0.0015 0.048 0.08 2.702 1125 819 5.2 158 Invention steel 5 0.28 2.1 0.0045 0.037 0.05 2.408 1170 804 6.3 232 Comparative River 1 0.19 2.7 0.003 0.09 0.04 3.011 1160 921 2.0 282 Comparative River 2 0.19 2.7 0.003 0.09 0.04 3.011 1160 921 8.0 175 Comparative Steel 3 0.04 2.9 0.002 0.025 0.05 3.152 1150 885 5.0 192 Comparative Steel 4 0.11 1.3 0.008 0.16 0.06 1.459 1130 822 10 218 Comparative Steel 5 0.32 4.5 0.0015 0.048 0.08 5.02 1195 932 3.0 284 Comparative Steel 6 0.19 2.7 0.003 0.09 0.04 3.011 1090 750 4.5 256 Comparative Steel 7 0.19 2.7 0.003 0.09 0.04 3.011 1160 921 2.0 358

division
Microstructure Hardness burglar
(MPa) Cracking resistance Surface portion center Surface portion
(HB) center
(HB) Inventive Steel 1 M (100%) F + B (51%) / M (49%) 390 341 1089 ○ Invention river 2 M (100%) F + B (47%) / M (53%) 423 342 1179 ○ Invention steel 3 M (100%) F + B (43%) / M (57%) 451 334 1258 ○ Inventive Steel 4 M (100%) F + B (42%) / M (58%) 464 330 1294 ○ Invention steel 5 M (100%) F + B (40%) / M (60%) 496 327 1385 ○ Comparative River 1 M (37%) F + B (52%) / M (48%) 326 312 967 ○ Comparative River 2 M (100%) M (100%) 451 442 1370 × Comparative Steel 3 M (41%) F + B (50%) / M (50%) 356 321 994 ○ Comparative Steel 4 M (45%) F + B (61%) / M (49%) 366 251 778 ○ Comparative Steel 5 M (100%) M (100%) 578 520 1613 × Comparative Steel 6 M (100%) M (100%) 453 425 1318 × Comparative Steel 7 M (39%) F + B (49%) / M (51%) 347 321 995 ○

(In Table 2, M means martensite, F means ferrite, and B means bainite.)

As shown in Tables 1 and 2, inventive steels 1 to 5 satisfying the composition and manufacturing conditions proposed in the present invention can sufficiently maintain a soft phase in the center microstructure, thereby maintaining the center hardening ability at an appropriate level, It was possible to secure excellent cutting crack resistance. This means that the central stress holding capacity is improved.

On the other hand, in the comparative steels 1 and 2, the cooling rate did not satisfy the relation (2) proposed by the present invention. In the case of the comparative steel 1, the hardness of the surface portion was insufficient and the strength was lowered. Cracks were formed in the gas cut part due to lack of hardenability due to the formation of fully martensite in the center part.

In Comparative Steel 3, the carbon content deviates from the present invention. In Comparative Steel 3, when the content of Mn and the value (H) of hardenability are deviated from the present invention, it is confirmed that the hardness in the surface portion is insufficient and the strength is lowered have.

The comparative steel 5 had carbon, manganese, and H values deviating from the present invention, and martensite was excessively formed at the center portion, causing cracking in the gas cut portion due to insufficient hardening ability.

The comparative steel 6 shows a case where the temperature during the finish rolling is too low, and it is also confirmed that the cut cracking resistance is caused by the lack of the center hardening ability.

The comparative steel 7 had a too high cooling end temperature, and the formation of the martensite phase on the surface portion was insufficient, so that the surface hardness could not be secured, and it was confirmed that the strength was lowered.

FIG. 1 is a graph showing hardness distributions according to the thicknesses of inventive steel 3 and comparative steel 2 according to an embodiment of the present invention.

As shown in Fig. 1, in the inventive steel, the hardness is lowered at the center portion of the steel (about 10 to 30 mm thick), and in the case of the comparative steel, the hardness at the thickness is similar.

In the case of the inventive steel, it is confirmed that the comparative steel has a high risk of the center crack, while the stress bearing capacity can be improved by alleviating the center hardness.

Fig. 2 shows the result of observing the microstructure of the center of Invention Steel 3 according to the embodiment of the present invention.

As shown in Fig. 2, it can be confirmed that martensite and bainite are mixed and formed in the center portion.

Claims (5)

(B): not more than 0.01% (excluding 0), titanium (Ti): not more than 0.2% (0 is not more than 0%), 0.1% or less (excluding 0) of niobium (Nb), the balance Fe and inevitable impurities, and has an H value of 2.3 to 4.5 represented by the following relational expression 1,
The microstructure of the steel surface portion (range of 0.4 t from the surface) includes a martensite single phase, and the microstructure of the steel center portion (0.4 t <center portion <0.6 t) has a bainite + ferrite composite structure of 40% High tensile strength steels with excellent crack resistance due to 60% or less of martensite.
(Where t is the thickness of the steel).
[Relation 1]
H value (center hardenability index) = 0.5C + 1.08Mn
(In the above-mentioned relational expression 1, C and Mn mean% by weight.)
delete The method according to claim 1,
Wherein the high tensile steel has a hardness of the surface portion of not less than HB360 and a hardness of the center portion of not more than HB350.
(B): not more than 0.01% (excluding 0), titanium (Ti): not more than 0.2% (0 is not more than 0%), And the balance Fe and other unavoidable impurities, and has an H value expressed by the following relational expression (1) satisfying 2.3 to 4.5 Reheating the steel slab to 1050 to 1250 占 폚;
Hot-rolling the reheated steel slab at 800 to 950 占 폚 to produce a hot-rolled steel sheet; And
And cooling the hot-rolled steel sheet to 300 DEG C or lower,
Wherein the cooling step is carried out at a cooling rate satisfying the following relational expression (2).
[Relation 2]
0.7 x CR < CR = 17.2 - (3.72 x H) < 0.85 x CR
(Where CR is the cooling rate and H is the center hardening index).

[Relation 1]
H value (center hardenability index) = 0.5C + 1.08Mn
(In the above-mentioned relational expression 1, C and Mn mean% by weight.)
delete

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