KR101696094B1 - Steel sheet having superior hardness and method for manufacturing the same - Google Patents

Abstract

The purpose of an aspect of the present invention is to provide a hard steel sheet with at least 500 HB by forming a composition of the steel sheet based on a minimum carbon content interaction formula to obtain at least 500 HB; and a manufacturing method thereof. The aspect of the present invention relates to the hard steel sheet with at least 500 HB manufactured by performing a process of cooling a hot-rolled steel sheet; and the manufacturing method thereof. The minimum content of carbon (C) satisfies the below interaction formula (1). [Interaction formula 1] (The minimum content of carbon (C)) >= 0.481 - 0.104Mn - 0.035Si - 0.088Cr - 0.054Ni - 0.035Mo - 0.0003C.R. [Here, Mn, Si, Cr, Ni, and Mo are weight percentages for each element content; C.R. are a cooling speed in case of cooling the hot-rolled steel sheet; and a unit thereof is C/sec.]. The hard steel sheet includes a fine structure containing at least 95 vol% of martensitic phases.

Description

TECHNICAL FIELD [0001] The present invention relates to a high hardness steel sheet and a method of manufacturing the same. BACKGROUND ART [0002]

The present invention relates to a high hardness steel sheet for use in various fields and a manufacturing method thereof.

The steel sheet with high hardness is excellent in abrasion resistance and load supporting ability and can guarantee long service life and durability and is used in various parts.

Particularly, in the case of abrasion resistant steel, its grade is specified on the basis of Brinell hardness, and it is generally manufactured at various hardness levels from HB (brinell hardness) of 350 to HB 600 of high.

In addition, the steel sheet having high hardness has a high strength at the same time, and can be used in a sector that utilizes a high-strength structure such as a collision member and a stiffener, and has economic value in terms of weight reduction and efficiency.

Such a high hardness steel sheet usually transforms the steel sheet into a martensite or bainite structure during the cooling from the austenite temperature region to the room temperature, and utilizes the high hardness and strength of the low temperature transformation structure.

The prior art, however, has attempted to use various components and process control schemes to achieve the required hardness according to the component, but fails to provide a standard for obtaining unified hardness.

One aspect of the present invention is to provide a high hardness steel sheet having a Brinell hardness of 500 HB or more and having a steel composition set by using a minimum carbon content relation for obtaining a Brinell hardness of 500 HB or more.

Another aspect of the present invention is to provide a method of manufacturing a high hardness steel sheet having a Brinell hardness of 500HB or more by setting a steel composition according to a minimum carbon content relation for obtaining a Brinell hardness of 500HB or more.

According to a preferred aspect of the present invention, there is provided a steel sheet comprising a step of cooling a hot-rolled steel sheet,

(C): 0.05 to 0.3% by weight, Si: 0.5% or less (excluding 0%), manganese (Mn): not more than 2.5% Not more than 1.5% (excluding 0%), molybdenum (Mo): not more than 1.0% (excluding 0%), nickel (Ni): not more than 1.0% (excluding 0%) and niobium (Excluding 0%), titanium (Ti): not more than 0.1% (excluding 0%), vanadium (V): not more than 0.1% (Al): not more than 0.1% (excluding 0%), the balance iron (Fe) and other unavoidable impurities;

The minimum content of carbon (C) satisfies the following relational expression (1);

[Relation 1]

C (minimum content of carbon (C))? 0.481-0.104Mn-0.035Si-0.088Cr-0.054Ni-0.035Mo-0.0003C.R.

Wherein the contents of Mn, Si, Cr, Ni, and Mo are expressed in weight%, C.R. is a value indicating a cooling rate during cooling of the hot-rolled steel sheet, and the unit is ° C / sec.

At least 95 vol.% Of a martensite phase; And

Hardness steel having a Brinell hardness of 500HB or more.

Another preferred aspect of the present invention is a steel sheet comprising 0.05 to 0.3% by weight of carbon (C), 0.5% or less (excluding 0%) of manganese (Mn) 1.0% or less (excluding 0%), nickel (Ni): not more than 1.0% (excluding 0%), chromium (Cr) 0.1% or less (excluding 0%), titanium (Ti): 0.1% or less (excluding 0%), vanadium (V) : Steel slabs composed of not more than 0.01% (excluding 0%), aluminum (Al): not more than 0.1% (excluding 0%), the balance iron (Fe) and other unavoidable impurities are hot rolled into hot rolled steel sheets, A method for producing a steel sheet having a microstructure comprising 95 vol% or more of martensite phase and a Brinell hardness of 500HB or more, wherein a minimum content of carbon (C) satisfies the following relational expression (1) .

[Relation 1]

C (minimum content of carbon (C))? 0.481-0.104Mn-0.035Si-0.088Cr-0.054Ni-0.035Mo-0.0003C.R.

Wherein the content of Mn, Si, Cr, Ni and Mo is expressed in weight%, CR is a value indicating the cooling rate during cooling of the hot-rolled steel sheet, and the unit is ° C / sec.

According to an aspect of the present invention, it is possible to design a more economical and unified steel sheet component for producing a microstructure containing 95 vol% or more of martensite phase and a steel sheet having a Brinell hardness of 500 HB or more.

The prior art relating to high hardness steel sheets has proposed various components and process control methods to obtain the required hardness according to the parts, but fails to provide the standard of composition hardness acquisition.

Accordingly, the present inventors conducted research and experiments on the conditions of component design for securing the necessary hardness level when forming the microstructure of the steel sheet into a martensite structure of 95 vol% or more in order to secure high hardness and strength, Based on the results, the present invention has been completed.

That is, one of the main technical ideas of the present invention is to provide a condition of component design for securing the necessary hardness level when forming the microstructure of the steel sheet into the martensite structure of 95 vol% or more in order to secure high hardness and strength A steel sheet having a microstructure containing 95 vol% or more of martensite phase and a Brinell hardness of 500 HB or more can be produced more economically and a unified hardness can be obtained.

Hereinafter, a steel sheet according to a preferred aspect of the present invention will be described.

Carbon (C): 0.05 to 0.3% by weight (hereinafter referred to as "%"),

The content of carbon (C) may be 0.05 to 0.3%.

If the content of carbon is less than 0.05%, it may be difficult to cause martensite transformation upon cooling from the austenite region. If the content of carbon exceeds 0.3%, the vulnerability of the steel increases, have.

Silicon (Si): 0.5% or less (excluding 0%)

The content of silicon (Si) may be 0.5% or less (excluding 0%).

Silicon is the preferred alloying element in applications that use hardness because it increases the abrasion resistance of steels. However, if too much Si is added, the surface properties and plating properties of the steel become poor, and a complete austenitization may not be achieved during reheating.

Manganese (Mn): 2.5% or less (excluding 0%) and chromium (Cr): 1.5% or less (excluding 0%)

Both manganese (Mn) and chromium (Cr) are elements that greatly lower the transformation point of martensite. Manganese and chromium are usually added to the steel after the carbon, and the effect of lowering the transformation point is great. It is an element.

The upper limit of the manganese content is preferably limited to 2.5%, and the upper limit of the chromium content is preferably limited to 1.5%.

If the content of manganese and chromium is too high, the austenite will remain at room temperature, and there is a possibility that a desired martensite structure of 95 vol% or more can not be obtained.

Molybdenum (Mo): not more than 1.0% (excluding 0%) and nickel (Ni): not more than 1.0% (excluding 0%)

Molybdenum (Mo) and nickel (Ni) are elements that lower the martensitic transformation starting temperature

However, the degree of lowering the martensitic transformation starting temperature is lower than that of Mn and Cr, and the upper limit of the addition amount of these elements as the expensive elements is preferably limited to 1.0%.

Niobium (Nb): 0.1% or less (excluding 0%) and titanium (Ti): 0.1% or less (excluding 0%),

Niobium (Nb) and titanium (Ti) can be added at a level of 0.1% or less (excluding 0%), respectively, and the effect of improving the impact characteristics of the steel sheet through microfine austenite grains is effective. However, excessive addition of Nb and Ti causes coarsening of the Nb carbonitride that fixes the grain boundary, thereby losing grain refinement effect, and therefore the upper limit is preferably limited to 0.1% each.

On the other hand, Ti is often added to protect B from N when B is added, and titanium (Ti) reacts first with carbon or nitrogen in the steel to form TiC or TiN, so that addition of boron (B) Increase effect. In this case, it is sufficient that the content of titanium (Ti) satisfies the following relational expression 2 depending on the stoichiometry with respect to the nitrogen content in the steel.

[Relation 2]

Ti (wt%) > N (wt%) x 3.42

Vanadium (V): 0.1% or less (excluding 0%)

The vanadium (V) can be added at a level of 0.1% or less (excluding 0%), and serves to prevent precipitation hardening and deterioration of the properties of the welded part through formation of fine V carbide.

If the addition amount is too large, the effect is reduced due to the coarsening of the carbide, so that the upper limit of the content is preferably limited to 0.1%.

Boron (B): 0.01% or less (excluding 0%)

Boron (B) can be added at a level of 0.01% or less (excluding 0%), and B is an element which significantly inhibits the nucleation of ferrite and pearlite and greatly improves the hardenability of the steel. The utilization is very high.

In the present invention, the final microstructure is obtained with at least 95 vol.% Of martensite. There is no particular restriction on the production process thereof, and thus B can be added for securing curing ability if necessary. However, when the content of B is excessively added, it functions as a nucleus generation site on the ferrite or pearlite, thereby deteriorating the hardenability. Therefore, the upper limit of the content is preferably limited to 0.01%.

Aluminum (Al): 0.1% or less (excluding 0%)

Aluminum (Al) is added for deoxidation and grain refinement, and its content is preferably limited to 0.1% or less (excluding 0%).

The remainder excluding the above-mentioned elements include iron (Fe) and other unavoidable impurities.

In the present invention, the minimum content of carbon (C) satisfies the following relational expression (1).

[Relation 1]

C (minimum content of carbon (C))? 0.481-0.104Mn-0.035Si-0.088Cr-0.054Ni-0.035Mo-0.0003C.R.

Wherein the content of Mn, Si, Cr, Ni and Mo is expressed in weight%, CR is a value indicating the cooling rate during cooling of the hot-rolled steel sheet, and the unit is ° C / sec.

The relational expression (1) is defined as the minimum carbon (C) for obtaining a Brinell hardness of 500 HB or more from the composition of the silicon (Si), manganese (Mn), chromium (Cr), molybdenum (Mo), nickel (Ni) ) Content.

Even if the carbon (C) content satisfies 0.05 to 0.3 wt%, the Brinell hardness of 500 HB or more can not be obtained unless the above relation (1) is satisfied.

The above relational expression (1) can be designed using, for example, the following relational expression (3).

[Relation 3]

HB = Brinell Hardness = 100.4 + 830.5 * C + 86.5 * Mn + 28.8 * Si + 73.4 * Cr +

44.5 * Ni + 28.8 * Mo + 0.252 * C.R.

Wherein CR, Mn, Si, Cr, Ni and Mo are values indicating the content of each element in weight%, CR is a value indicating cooling rate during cooling of the hot-rolled steel sheet,

From the above relationship (3), a relational expression (1) for the minimum carbon content for HB ≥ 500 can be derived.

In addition, by using the relationship (3) within the range of the steel sheet composition of the present invention, it is possible to derive a suitable alloy component designing condition for obtaining any required hardness level of HB 350 or more.

The microstructure of the steel sheet of the present invention contains 95 vol% or more of martensite phase.

When the fraction of the martensite phase is less than 95 vol.%, It may be difficult to secure the desired strength and hardness.

The microstructure of the steel sheet of the present invention may include one or two kinds of ferrite and bainite having a second phase structure other than martensite and less than 5.0 vol%.

The steel sheet of the present invention has a Brinell hardness of 500HB or more.

Hereinafter, a method for manufacturing a steel sheet according to another preferred embodiment of the present invention will be described.

In a steel sheet manufacturing method according to another preferred aspect of the present invention, 0.05 to 0.3% by weight of carbon (C), 0.5% or less of silicon (excluding 0%), manganese (Mn) Mo content: not more than 1.0% (excluding 0%), nickel (Ni): not more than 1.0% (excluding 0%), 0.1% or less (excluding 0%), titanium (Ti): 0.1% or less (excluding 0%), vanadium (V): 0.1% or less ), Boron (B): not more than 0.01% (excluding 0%), aluminum (Al): not more than 0.1% (excluding 0%), iron (Fe) and other unavoidable impurities is hot rolled Rolled and then cooled to prepare a steel sheet having a microstructure containing 95 vol% or more of martensite phase and having a Brinell hardness of 500 HB or more.

The minimum content of carbon (C) in the steel slab satisfies the following relational expression (1).

[Relation 1]

C (minimum content of carbon (C))? 0.481-0.104Mn-0.035Si-0.088Cr-0.054Ni-0.035Mo-0.0003C.R.

Wherein the content of Mn, Si, Cr, Ni and Mo is expressed in weight%, CR is a value indicating the cooling rate during cooling of the hot-rolled steel sheet, and the unit is ° C / sec.

The steel slab can be reheated before hot rolling the steel slab to the hot-rolled steel sheet.

The conditions for reheating the slab are not particularly limited, and homogenization is sufficient.

The slab reheating temperature is preferably 1100 to 1300 ° C.

The hot rolling condition is not particularly limited, and the hot finish rolling temperature may be a temperature at which austenitization is sufficiently performed.

The hot finish rolling temperature may be, for example, 870 to 930 캜, and the whole hot rolling may be performed in a temperature range of 1150 캜 to a hot finish rolling temperature after extraction by heating.

The cooling rate during cooling of the hot-rolled steel sheet is not particularly limited as long as it is a cooling rate at which a martensite phase of 95 vol% or more can be obtained. For example, it is 20 ° C / sec or more, preferably 20 to 150 ° C / sec.

The cooling end temperature at the time of cooling of the hot-rolled steel sheet is not particularly limited as long as it is not higher than the Ms point (martensitic transformation starting temperature) and can obtain a martensite phase of 95 vol% or more.

Hereinafter, the present invention will be described more specifically by way of examples. These embodiments are only for illustrating the present invention, and the present invention is not limited thereto.

(Example)

Experiments were conducted using 17 kinds of steels A to Q having the composition shown in Table 1 below

The compositions of the steels shown in Table 1 below all satisfy the composition ranges of the present invention.

A steel sheet having a thickness of 30 mm and a width of 200 mm having a steel composition shown in Table 1 below was prepared and reheated at 1200 ° C for 180 minutes. Next, the reheated steel sheet was hot rolled in a hot finish temperature range of 900 占 폚 to produce a hot rolled steel sheet having a thickness of 3.0 mm, and then cooled to 200 占 폚 at the cooling rate shown in Table 2 below.

The Brinell hardness (HB) and microstructure of the hot-rolled steel sheet thus prepared were measured, and the results are shown in Table 2 below.

Table 2 below shows a second phase structure excluding martensite, and the second phase is martensite.

In the following second phase structure, F represents ferrite, B represents bainite, and M represents martensite.

Table 2 also shows the difference between the required carbon content, the actual carbon content, and the actual carbon content and the required carbon content determined by the relational expression (1).

Steel grade C Si Mn Cr Mo Ni Al Ti Nb V B
A 0.081 0.298 1.85 0.498 0.101 0.008 0.03 0.006 0.032 0.006 0.0002 B 0.121 0.351 2.11 0.313 0.798 0.012 0.032 0.025 0.023 0.005 0.0017 C 0.195 0.354 2.01 0.297 0.006 0.812 0.031 0.029 0.025 0.003 0.0016 D 0.152 0.248 1.49 0.296 0.008 0.011 0.033 0.03 0.056 0.005 0.003 E 0.242 0.432 1.72 0.411 0.312 0.013 0.036 0.03 0.003 0.006 0.0033 F 0.148 0.243 1.48 0.607 0.012 0.005 0.034 0.029 0.004 0.004 0.0032 G 0.148 0.24 1.48 0.3 0.007 0.007 0.035 0.098 0.005 0.005 0.0033 H 0.297 0.253 1.51 0.3 0.211 0.006 0.035 0.03 0.007 0.002 0.0016 I 0.212 0.25 1.49 1.1 0.203 0.008 0.035 0.03 0.022 0.098 0.0029 J 0.2 0.249 1.47 0.3 0.011 0.021 0.03 0.029 0.005 0.003 0.0029 K 0.252 0.254 2.31 0.125 0.012 0.015 0.033 0.03 0.032 0.005 0.0028 L 0.198 0.243 1.49 0.297 0.015 0.023 0.034 0.03 0.008 0.004 0.0031 M 0.199 0.254 1.47 1.12 0.012 0.015 0.033 0.03 0.032 0.005 0.0028 N 0.2 0.207 1.47 0.3 0.011 0.014 0.034 0.098 0.045 0.002 0.0025 O 0.26 0.297 2.11 0.02 0.101 0.005 0.027 0.007 0.022 0.011 0.0003 P 0.27 0.212 1.51 0.52 0.112 0.012 0.021 0.005 0.023 0.012 0.0020 Q 0.232 0.491 1.78 0.298 0.005 0.003 0.026 0.021 0.015 0.055 0.0018

division Steel grade Ms
(° C) Cooling rate
(° C / sec) Required carbon content
(wt.%, relational expression 1)
① Actual carbon
Content (wt.%)
② ②-① Brinell hardness
(HB) 2nd phase organization Comparative Example 1 A 432 100 0.200 0.081 -0.119 395 F8%, B11% Comparative Example 2 B 401 50 0.178 0.121 -0.057 445 F2%, B3% Inventory 1 C 381 50 0.174 0.195 0.021 519 B3% Comparative Example 3 D 433 50 0.275 0.152 -0.123 404 F1%. B4% Inventory 2 E 387 35 0.229 0.242 0.013 505 F1%, B3% Inventory 3 E 379 70 0.218 0.242 0.024 523 100% M Comparative Example 4 F 425 50 0.249 0.148 -0.101 405 B4% Comparative Example 5 G 434 20 0.286 0.148 -0.138 364 F6%, B7% Honorable 4 H 380 50 0.266 0.297 0.031 531 B3% Inventory 5 I 379 35 0.202 0.212 0.010 511 100% M Comparative Example 6 J 411 35 0.281 0.2 -0.081 437 F2%, B2% Inventory 6 K 372 100 0.190 0.252 0.062 551 100% M Comparative Example 7 L 417 35 0.279 0.198 -0.081 440 F2%, B2% Comparative Example 8 M 394 20 0.213 0.199 -0.014 491 F1%, B3% Comparative Example 9 N 417 70 0.272 0.2 -0.072 448 B4% Honorable 7 O 377 80 0.222 0.26 0.038 527 B3% Honors 8 P 386 50 0.251 0.27 0.019 510 B2% Proposition 9 Q 396 100 0.222 0.232 0.010 502 B3%

As shown in Table 2, according to the present invention, in the case of Examples 1 to 9 in which the actual carbon content is larger than the necessary carbon content, the Brinell hardness (HB) value is more than 500 HB.

On the other hand, in the case of Comparative Examples 1 to 9 in which the actual carbon content is smaller than the necessary carbon content, the Brinell hardness value is less than 500 HB.

It will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the following claims.

Claims (6)

A steel sheet produced by including a step of cooling a hot-rolled steel sheet subjected to hot rolling,
(C): 0.05 to 0.3% by weight, Si: 0.5% or less (excluding 0%), manganese (Mn): not more than 2.5% Not more than 1.5% (excluding 0%), molybdenum (Mo): not more than 1.0% (excluding 0%), nickel (Ni): not more than 1.0% (excluding 0%) and niobium (Excluding 0%), titanium (Ti): not more than 0.1% (excluding 0%), vanadium (V): not more than 0.1% (Al): not more than 0.1% (excluding 0%), the balance iron (Fe) and other unavoidable impurities;
The minimum content of carbon (C) satisfies the following relational expression (1);
[Relation 1]
C (minimum content of carbon (C))? 0.481-0.104Mn-0.035Si-0.088Cr-0.054Ni-0.035Mo-0.0003CR
Wherein the content of Mn, Si, Cr, Ni and Mo is expressed in weight%, CR is a value indicating the cooling rate during cooling of the hot-rolled steel sheet, and the unit is ° C / sec.
At least 95 vol.% Of a martensite phase; And
High hardness steel sheet having Brinell hardness of 500HB or more.
The high hardness steel sheet according to claim 1, wherein the microstructure comprises a second phase structure other than martensite and contains at least one of ferrite and bainite of less than 5.0 vol%.
The method according to claim 1,
Wherein the relational expression (1) is derived from the following relational expression (3).
[Relation 3]
HB = Brinell Hardness = 100.4 + 830.5 * C + 86.5 * Mn + 28.8 * Si + 73.4 * Cr +
44.5 * Ni + 28.8 * Mo + 0.252 * CR
Wherein CR, Mn, Si, Cr, Ni and Mo are values indicating the content of each element in weight%, CR is a value indicating cooling rate during cooling of the hot-rolled steel sheet,
(C): 0.05 to 0.3% by weight, Si: 0.5% or less (excluding 0%), manganese (Mn): not more than 2.5% Not more than 1.5% (excluding 0%), molybdenum (Mo): not more than 1.0% (excluding 0%), nickel (Ni): not more than 1.0% (excluding 0%) and niobium (Excluding 0%), titanium (Ti): not more than 0.1% (excluding 0%), vanadium (V): not more than 0.1% Steel slabs composed of iron (Fe) and other unavoidable impurities are hot-rolled into hot-rolled steel sheets, and then cooled to obtain 95 vol% or more of martensite phase And a Brinell hardness of 500 HB or more, wherein the minimum content of carbon (C) satisfies the following relational expression (1): " (1) "
[Relation 1]
C (minimum content of carbon (C))? 0.481-0.104Mn-0.035Si-0.088Cr-0.054Ni-0.035Mo-0.0003CR
Wherein the content of Mn, Si, Cr, Ni and Mo is expressed in weight%, CR is a value indicating the cooling rate during cooling of the hot-rolled steel sheet, and the unit is ° C / sec.
The method of manufacturing a high-hardness steel plate according to claim 4, wherein the cooling rate of the hot-rolled steel sheet during cooling is 20 to 150 ° C / sec.
The method for producing a high hardness steel sheet according to claim 4 or 5, wherein the cooling end temperature during cooling of the hot-rolled steel sheet is equal to or lower than the Ms point (martensitic transformation start temperature).