KR101695113B1 - High-tensile-strength steel plate with excellent low-temperature toughness and manufacturing process therefor - Google Patents

Abstract

(Mass%) defined by the following formula (1) is 0.345 or more and 0.428 or less, and σ is 2080 or more as defined by the following formula (2), and t / 4 : Plate thickness) microstructure is a mixed structure of ferrite and pearlite, and the mean circle equivalent diameter of the ferrite grains is 7.0 mu m or less, thereby providing a high strength steel sheet having high strength and excellent low temperature toughness.
(Cu) + [Ni] / 15 + (Mo) + [V]) / 5 + (One)
? = 2.90 x {602781.57- (1154 x CEQ-3.25) ² } ^1/2 /0.963+400x [Ni] ... (2)

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high tensile strength steel sheet excellent in low temperature toughness and a method of manufacturing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a high-strength steel sheet excellent in low-temperature toughness and a method for producing the same. The present invention relates to a technique for improving the low temperature toughness of a high tensile strength steel sheet such as those applied to an application to be used in an environment exposed to low temperature, for example, a pressure vessel, a ship or an offshore structure.

 Steel plates (high tensile steel plates) used for constructing pressure vessels, ships and offshore structures are required to have high strength, good toughness at low temperatures (low temperature toughness) and good weldability. In particular, in recent years, from the viewpoint of safety, a higher toughness at a cryogenic temperature is required.

In order to improve the toughness of the steel sheet, the addition amount of the alloying element is preferably as small as possible, but it becomes difficult to secure strength. On the other hand, if an alloy element is added to secure strength, the toughness is lowered. As described above, strength and toughness are opposite to each other, and it is extremely difficult to achieve compatibility between these properties.

One of the effective methods for improving the strength and toughness of the steel sheet is to include Ni as an alloying element. Until now, many Ni-containing steel sheets have been proposed. However, as represented by 3.5% Ni steel or 9% Ni steel, the effect can not be maximized unless Ni is contained in a large amount. On the other hand, a steel sheet containing a small amount of Ni of about 1 to 2% is proposed in, for example, Patent Document 1. However, even if high strength is satisfied, toughness at low temperature can not be satisfied and strength and low temperature toughness It is difficult.

Japanese Patent No. 3741078

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object of the present invention is to provide a high tensile strength steel sheet having high strength and low temperature toughness even at a Ni content of 2.0% or less, and a useful method for producing such high strength steel sheet.

The high-tensile steel sheet according to the present invention, which can achieve the above object, is a steel sheet having a composition of C: 0.03 to 0.09% (meaning "mass%", hereinafter the same applies to chemical components), Si: 0.05 to 0.35%, Mn: 0.9 to 1.6% P: not more than 0.01% (not including 0%), S: not more than 0.01% (not including 0%), Al: 0.01 to 0.06%, Ni: 0.2 to 2.0%, Nb: 0.007 to 0.017% (% By mass) defined by the following formula (1) is 0.345 or more, and the balance of Fe is 0.007 to 0.017%, Ca is 0.0005 to 0.003%, and N is 0.0025 to 0.0050% And a microstructure at a position of t / 4 (t: plate thickness) is a mixed structure of ferrite and pearlite, and the mean circle equivalent of the ferrite grains is 0.428 or less, And a diameter of 7.0 m or less. On the other hand, the "average circle equivalent diameter" is an average value of diameters (circle equivalent diameters) when the ferrite grains are converted into a circle having the same area.

(Cu) + [Ni] / 15 + (Mo) + [V]) / 5 + (One)

? = 2.90 x {602781.57- (1154 x CEQ-3.25) ² } ^1/2 /0.963+400x [Ni] ... (2)

(% By mass) of C, Mn, Cr, Mo, V, Cu and Ni, respectively, in terms of C, Mn, Cr, Mo, V, Cu and Ni. .

In addition to the basic components (C, Mn, Ni) of the inventive steel sheet, the above formula (1) includes elements (Cr, Mo, V, Cu and the like) The value of CEQ is calculated on the assumption that there is no such item, and when these elements are included, the value of CEQ can be calculated from the above formula (1).

In the high-tensile steel sheet of the present invention, if necessary, it is also effective to include at least one member belonging to any of the following (a) to (c), and the properties of the high-strength steel sheet are improved depending on the components to be contained.

(a) B: 0.002% or less (not including 0%)

(b) Cu: not more than 0.35% (not including 0%)

(c) Cr: not more than 0.3% (not including 0%), Mo: not more than 0.2% (not including 0%), and V: not more than 0.06% One or more species

In the production of the high-tensile steel sheet of the present invention, the steel sheet having the chemical composition as described above is subjected to a rolling reduction of not less than 30% and not more than t / 4 (t: sheet thickness) at a temperature range of 950 to 875 캜 t: plate thickness), the location is 820 ℃ less than Ar _3, with the reduction rate transformation point when the temperatures above calendar and performing the reduction by more than 30%, t / 4 (t : sheet thickness) position is less than 875 ℃ 820 ℃ than the temperature of the In the opposite phase and in the two-phase temperature range, cooling is carried out at an average cooling rate of not more than 2.0 ° C / sec after being pressed without being subjected to the reduction, and the microstructure may be a mixed structure of ferrite and pearlite.

According to the present invention, it is possible to minimize the ferrite grains in the steel sheet by setting appropriate chemical-reduction conditions in addition to the chemical composition that maximizes the Ni addition effect in the component system having the Ni content of 2.0% or less , A high tensile strength steel sheet having high strength and excellent low temperature toughness can be realized. Such a high-strength steel plate is extremely useful as a steel plate used for constructing a pressure vessel, a ship, and an offshore structure.

1 is a graph showing the relationship between the CEQ value and the tensile strength TS.
2 is a graph showing the relationship between the sigma value and the wave front transition temperature vTrs.

When an alloy element is added to secure the strength of the high-strength steel sheet, the toughness is lowered. This is because the addition of the alloying element makes it difficult to break the ductile at low temperatures. On the contrary, the addition of Ni facilitates soft fracture at low temperatures.

Under such circumstances, the present inventors have studied at various angles in order to realize a high-strength steel sheet excellent in high-temperature and low-temperature toughness. As a result, when the chemical composition is controlled so as to quantify the effect of accelerating ductile fracture by reduction of alloying elements and promoting ductile fracture by Ni and satisfying the relations of the above-mentioned formulas (1) and (2) High strength and low temperature toughness can be achieved at the same time, thereby completing the present invention.

The process for deriving the above formula (2) is as follows. In order to improve the toughness of the steel sheet at low temperature, it is necessary to suppress brittle fracture, particularly cleavage fracture. So I came up with the mechanism of cleavage destruction. First, plastic deformation occurs in some of the ferrite grains, and the dislocations in the ferrite grains migrate. The shifted dislocation stays at the grain boundary and accumulates. At this time, if the second phase represented by cementite is present in the grain boundary, the second phase is broken due to stress concentration due to accumulation of dislocations, and microscopic cracks are generated. The generated microcracks propagate to the adjacent ferrite lips and cause cleavage failure.

It is known that the stress (cleavage fracture stress)? ₀ required for causing cracks to be generated to propagate to the adjacent ferrite lips to cause cleavage fracture can be calculated by the following formula (3) (see, for example, "fracture toughness of low carbon steel , Tetsugawa Tetsuya, published in May 1994, p. 17 (doctoral dissertation, Nagoya University)]. As the stress? ₀ becomes larger, cleavage breakage becomes less likely to occur and toughness is improved.

^{_{(C / d) σ 0 2}} + τe 2 {1 + 4 / π (C / d) 1/2 × (τi / τe)} 2

= 4E? _F / {(1 -? ² ) d} (3)

However, C: a second minor axis on the (短徑), d: particle diameter of the ferrite, σ _0: cleavage fracture stress, τe: effective shear stress, τi: frictional force of the potential, E: Young's modulus γ _f: surface energy, ν: Poisson Respectively.

In the above formula (3), the first term [(C / d) σ ₀ ² ] is a term relating to a size ratio of a grain size and a carbide in a structure given as an initial condition, a second term [τe ² { 4 / π (C / d) ^1/2 × (τi / τe)} ² is a term relating to the potentials integrated in the grain boundaries. Further, the right side [4Eγ _f / {(1-ν ² ) d}] is a term relating to an unstable propagation condition of a crack known as a Griffith's equation.

Here, the Young's modulus E, the surface energy? _F, and the Poisson's ratio? Are constants. Since τi τe, τi / τe = 0 is displayed. Also, the effective shear stress τe can be expressed as a yield stress τ, and τe≈τ (yield stress).

Based on the above results, the equation (3) can be rewritten as the following equation (4). The Young's modulus E = 206,000 (MPa), the surface energy? _F = 14 (Jm ^-2 ) and the Poisson's ratio? = 0.3.

? ₀ = (d / C) ^1/2占 (4E? _f / {? (1-? ² ) d} -τ ² ) ^1/2 (4)

However, τ: yield stress

Based on the above equation (4), the present inventor further experimentally examined a parameter equation that governs low-temperature toughness. As a result, in the case of adopting a constant manufacturing condition, it can be considered that the grain size of the obtained structure, that is, the diameter of the ferrite and the minor axis of the second phase are almost constant, and the CEQ (carbon equivalent) It is found that when the value (? Value) of the above-mentioned formula (2) specified by the formula (2) is 2080 or more, good low temperature toughness can be ensured.

The value (sigma value) of sigma expressed by the above-mentioned formula (2) is a value determined by the content of each element. By defining the value of?, It is possible to clarify the chemical composition that can satisfy both strength and low-temperature toughness. Specifically, when the value of? Is smaller than 2080, the balance between Ni and the other added elements is deteriorated, the effect of Ni can not be exhibited to the maximum, and even if the high strength is satisfied, the low temperature toughness is deteriorated. The value of sigma is preferably 2150 or more, more preferably 2200 or more. Further, the preferable upper limit of the value of sigma is 2600 or less.

However, even if the value of? Satisfies 2080 or more, if the value (mass%) of CEQ specified by the above formula (1) becomes smaller than 0.345, the content of the strength improving element becomes insufficient and the strength is lowered. Even if the value of sigma is 2080 or more, if the Ni content is less than 0.2%, the effect of adding Ni is insufficient and a satisfactory low temperature toughness of the steel sheet can not be ensured. In addition, if the Ni content is excessive, the balance between the effect on the strength and toughness by Ni is broken, and the strength increasing effect is superior to the effect of suppressing ductile fracture at low temperature, and the low temperature toughness is deteriorated. For this reason, it is necessary to set the Ni content to the upper limit of 2.0%. On the other hand, the lower limit of the Ni content is preferably 0.5% or more (more preferably 0.7% or more), and the preferable upper limit is 1.8% or less (more preferably 1.5% or less).

On the other hand, if the value (% by mass) of the CEQ specified by the above-mentioned formula (1) is larger than 0.428, the balance of strength and toughness is broken and low temperature toughness is lowered. Therefore, the value (mass%) of CEQ is set to 0.428 or less so as to secure necessary toughness. On the other hand, the preferable lower limit of the CEQ value (CEQ value) is 0.350 or more (more preferably 0.355 or more), and the preferable upper limit is 0.425 or less (more preferably 0.420 or less).

In the present invention, it is intended that the microstructure at the t / 4 position be a mixed structure of ferrite and pearlite, so that the total content is 100% by area. However, the present invention does not exclude the incorporation of a small amount of ferrite structure and other structures other than pearlite structure (e.g., bainite or martensite) within a range that does not affect the intended effect of the present invention. In some cases, it is acceptable that other tissues are included up to about 10% by area. The blending ratio of ferrite to pearlite is not particularly limited, but is about 70 to 90 area% of ferrite to 10 to 30 area% of pearlite.

In order to ensure a satisfactory low-temperature toughness in a steel sheet, an average of ferrite lips (not including ferrite grains in pearlite) in a microstructure constituting a mixed structure of ferrite and pearlite (sometimes referred to as "ferrite / pearlite" It is also an important requirement to control the circle-equivalent diameter to 7.0 μm or less. By setting the average circle-equivalent diameter of the ferrite lips to not more than 7.0 mu m, it is possible to secure good low-temperature toughness (-80 DEG C or less at the wave-front transition temperature vTrs) in the high-tensile steel sheet. The preferable upper limit of the ferrite grain size is 6.7 탆 or less (more preferably 6.5 탆 or less). The lower limit of the ferrite grain size is preferably at least 0.5 탆 (more preferably at least 1.0 탆).

(C, Si, Mn, P, S, Al, Nb, Ti, Ca, and N) other than Ni need to be appropriately adjusted in order to satisfy the basic characteristics of the steel sheet of the high- The reasons for these limitations are as follows.

(C: 0.03 to 0.09%)

C is an important element in securing the strength of the steel sheet. In order to exhibit such an effect, C must be contained in an amount of 0.03% or more. However, when the content of C is excessive, toughness is lowered, so the upper limit is set to 0.09%. On the other hand, the C content is preferably 0.05% or more and 0.08% or less.

(Si: 0.05 to 0.35%)

Si acts as a deoxidizer when the steel is dissolved, and exhibits an effect of increasing the strength of the steel. In order to exhibit such an effect, Si must be contained in an amount of 0.05% or more. However, if the Si content is excessive, the toughness is lowered, so the upper limit is set at 0.35%. On the other hand, the Si content is preferably 0.07% or more (more preferably, 0.1% or more) and 0.30% or less.

(Mn: 0.9 to 1.6%)

Mn is useful as a strength increasing element of a steel sheet. In order to exhibit such an effect effectively, it is necessary to contain 0.9% or more of Mn. It is preferably at least 1.1%. However, if the Mn content is excessive, the toughness deteriorates rather than 1.6%. And preferably 1.5% or less.

(P: not more than 0.01% (not including 0%))

Since P is an element that deteriorates toughness, it is necessary to reduce it as much as possible. In the present invention, it should be suppressed to 0.01% or less.

(S: not more than 0.01% (not including 0%))

S is an element that deteriorates toughness. Therefore, it is necessary to reduce it as much as possible, and in the present invention, it is suppressed to 0.01% or less.

(Al: 0.01 to 0.06%)

Al is an element acting as a deoxidizer. In order to exhibit such an effect, the Al content should be 0.01% or more. However, if the Al content is excessive, the cleanliness of the steel sheet is impaired, so the upper limit of the Al content is set to 0.06%. On the other hand, the Al content is preferably 0.02% or more and 0.05% or less.

(Nb: 0.007 to 0.017%)

Nb is an element having an effect of refining ferrite grains through an effect of inhibiting recrystallization of austenite grains. In order to exhibit such an effect, Nb should be contained in an amount of 0.007% or more. However, if the content of Nb is excessive, the toughness is lowered, so that the upper limit of the content is set at 0.017%. On the other hand, the lower limit of the Nb content is preferably 0.010% or more, and the upper limit is preferably 0.015% or less.

(Ti: 0.007 to 0.017%)

Ti is a strong nitride-forming element and exhibits an effect of refining the crystal grains due to the fine precipitation of TiN in a very small amount. In order to exhibit such an effect effectively, Ti should be contained in an amount of 0.007% or more. It is preferably at least 0.01%. However, if Ti is contained excessively, the toughness is lowered, and therefore, it is required to be 0.017% or less, preferably 0.015% or less.

(Ca: 0.0005 to 0.003%)

Ca is an element effective for improving the toughness of a steel sheet by controlling inclusions. In order to exhibit such an effect, Ca should be contained in an amount of 0.0005% or more. However, if Ca is contained excessively, the toughness is lowered, and therefore, it is required to be 0.003% or less. On the other hand, the lower limit of the Ca content is preferably 0.001% or more, and the upper limit is preferably 0.002% or less.

(N: 0.0025 to 0.0050%)

N is an effective element for improving the toughness of the steel sheet by forming TiN together with Ti by containing an appropriate amount. In order to effectively exhibit such an effect, N should be contained in an amount of 0.0025% or more. However, when the N content is excessive, the solid solution N is increased and the toughness of the steel sheet is lowered. Therefore, it is necessary to set the upper limit to 0.0050%. On the other hand, the preferred lower limit of the N content is 0.003% or more, and the preferable upper limit is 0.0045% or less.

The contained elements defined in the present invention are as described above, and the balance is iron and unavoidable impurities. As the inevitable impurities, the incorporation of the elements that come in accordance with the conditions of the raw materials, the materials, the manufacturing facilities, and the like can be allowed. It is also effective to contain at least one member belonging to any one of the following (a) to (c), if necessary, and the properties of the high-strength steel sheet are improved depending on the components to be contained. The reason for setting the preferable range when these elements are contained is as follows.

(a) B: 0.002% or less (not including 0%)

(b) Cu: not more than 0.35% (not including 0%)

(c) Cr: not more than 0.3% (not including 0%), Mo: not more than 0.2% (not including 0%), and V: not more than 0.06% One or more species

(B: not more than 0.002% (not including 0%))

B has a function of lowering solute N which adversely affects toughness by producing BN. However, if the B content is excessively large, the precipitates of B are increased and the toughness deteriorates, so that it is preferable to suppress the B content to 0.002% or less. On the other hand, the lower limit of the B content is preferably 0.0001% or more, and when the B content is less than 0.0001%, the effect of reducing the solid solution N is insufficient. A more preferable upper limit is 0.001% or less.

(Cu: not more than 0.35% (not including 0%))

Cu is an effective element for improving the strength. If the content of Cu is too large, cracks tend to occur during hot working. Therefore, the upper limit of the Cu content is preferably 0.35% or less. On the other hand, the lower limit of the Cu content is preferably 0.001% or more, and if the Cu content is less than 0.001%, the effect thereof is not sufficient. A more preferable upper limit is 0.30% or less.

(One kind selected from the group consisting of Cr: not more than 0.3% (not including 0%), Mo: not more than 0.2% (not including 0%), and V: not more than 0.06% More than)

Cr, Mo, and V all precipitate carbonitrides and contribute to the increase in strength. However, if it is contained in an excessive amount, the toughness is lowered. Therefore, it is preferable to suppress the toughness to 0.3% or less of Cr, 0.2% or less of Mo and 0.06% or less of V. On the other hand, in order to effectively exhibit these effects, it is preferable to contain Cr of 0.01% or more, Mo of 0.01% or more, and V of 0.001% or more.

In order to produce the steel sheet of the present invention with the above-described structure, it is necessary to strictly define the production conditions thereof. That is, in producing the high-tensile steel sheet of the present invention, it is preferable that the steel sheet having the chemical composition as described above is subjected to a rolling reduction at a t / 4 (t: sheet thickness) position of 950 to 875 deg. 4 (t: plate thickness) position is less than 820 deg. C, the rolling reduction is set to 30% or more when the temperature is equal to or higher than the Ar ₃ transformation point, Is cooled to less than 875 DEG C at a temperature higher than 820 DEG C and in a two-phase temperature range, cooling is performed at an average cooling rate of 2.0 DEG C / sec or less after being pressed without being subjected to rolling, and the microstructure must be made into a ferrite pearlite structure . The reason for setting the range of each condition in this method is as follows.

The temperature at the t / 4 (t: sheet thickness) position can be obtained by subtracting the unsteady primary heat conduction equation from the slab temperature, room temperature, water temperature, slab thickness before and after rolling, (A) "Gokado Junichi," Fundamental Study on Estimation Method of Temperature Change of Material in Hot Rolling, "Firing and Processing, 1970, vol. 11, 118 , p. 816-824 ", (b) " Crude rolling model expression of hot strip mills ", Iron and Steel, 1977, Vol. 63, A29-A32 "(C)" High-Precision High-Efficiency Rolling Technology in Thin Sheet Fair Crossmills, "such as Kiyoshi Nishioka," Development of Rolling Technology and Rolling Theory and Future Birds, Section, 1994, p. 69-78, etc.].

In order to manufacture the high-strength steel sheet according to the above-described manufacturing conditions, the steel sheet having the chemical composition as described above is used as a base steel sheet. The base steel sheet is basically composed of a ferrite structure (for example, 50 area% or more). In order to make the ferrite grains in such a base steel sheet finer, the reduction rate at the recrystallization temperature and the non-recrystallization temperature is defined. On the other hand, the temperature shown below is a position where the steel sheet exhibits an average performance and is controlled at a temperature of a position of t / 4 (t: plate thickness).

First, in order to make the austenitic grains finer, it is necessary to sufficiently lower (lower the temperature after heating) in the recrystallization temperature range. It is possible to accumulate a potential in the austenitic grains by applying a reduction of 30% or more at the reduction rate in the recrystallization temperature region, and to generate new grains with this potential as the driving force. In the steel sheet having the chemical composition as described above, recrystallization occurs by basically pushing down at a high temperature region (recrystallization temperature range) of 875 DEG C or more. However, when the temperature at which the pressure is applied is excessively high, the recrystallization which is caused is also likely to grow, and becomes coarser than the austenite grains before the pressure is lowered. For this reason, it was set at 950 to 875 캜 as a reduction temperature range (recrystallization effective temperature range) effective for refining the austenitic grains. In order to effectively exhibit the above effect of pressing at this temperature range, it is necessary to set the reduction rate to 30% or more (preferably 35% or more), but it is usually 60% or less.

Next, in order to increase the deformation zone that can become the nucleus of the ferrite lips, sufficient rolling reduction is performed even in the non-recrystallized temperature region. When the pressure is applied at a lower temperature than the recrystallization temperature range, the austenite grains can not generate new grains, become a flat structure, and introduce strain balls into the grains. However, it is easy to generate a mixed grain structure to be pressed at the high temperature side in the non-recrystallized temperature region, and coarse ferrite grains are likely to be produced. Therefore, the temperature range for applying the pressing down 820 ℃ below Ar ₃ transformation point or more low-temperature side as, (but not the low temperature side of the recrystallization temperature region), and less than 875 ℃ 820 ℃ temperature range of higher than the (high-temperature side of the non-recrystallized temperature range), the reduction . In order to effectively exhibit the above-described effect of pressing at the low temperature side in the non-recrystallized temperature region, it is necessary to set the reduction rate to 30% or more (preferably 35% or more), but it is usually 80% or less. In addition, it does not mean that the pressure is lowered over the entire range of temperature in the low temperature side in the non-recrystallization temperature range. If a reduction rate of 30% or more can be ensured, the temperature within the temperature range Quot; temperature ").

On the other hand, in the two-phase temperature region, which is lower in temperature than the non-recrystallization temperature region, or in the temperature region lower than the lower temperature region, that is, in the temperature region lower than the Ar ₃ transformation point, the strength of the steel sheet is improved by increasing the stress And the toughness of the steel sheet deteriorates, so that the pressing is not performed.

After performing the above-described rolling (basically controlled rolling), the average cooling rate from the rolling finish temperature to the room temperature is set to 2.0 占 폚 / second or less, and the microstructure must be made into a ferrite pearlite structure. If the average cooling rate at this time is faster than 2.0 캜 / second, a bainite structure with a generally low toughness is generated, and the microstructure can not be made into a "ferrite-pearlite structure". The average cooling rate is preferably 1.0 占 폚 / second or less, more preferably 0.5 占 폚 / second or less.

The high strength steel sheet of the present invention can be advantageously applied as a so-called posterior steel sheet. At this time, the plate thickness is about 7 mm or more, and the upper limit is not particularly limited, but is usually about 40 mm or less.

The present invention will be described in more detail with reference to the following examples. However, the present invention is of course not limited by the following examples, and it is also possible to carry out the present invention by modifying it appropriately within a range suitable for the purposes of the preceding and latter term All of which are included in the technical scope of the present invention.

This application claims the benefit of priority based on Japanese Patent Application No. 2012-199798 filed on September 11, The entire contents of the specification of Japanese Patent Application No. 2012-199798 filed on September 11, 2012 are hereby incorporated by reference.

Example

Steel rods of various chemical composition compositions shown in the following Table 1 were subjected to controlled rolling under the production conditions shown in Table 2 below to produce a thermo-mechanical control process (TMCP) steel sheet having a thickness of 40 mm. On the other hand, Table 1 also shows the Ar ₃ transformation point of each steel ingot, and this value is obtained based on the following formula (5).

Cr ₃ +2.6 x [Si] -68.1 x [Mn] -36.1 x [Ni] -20.7 x [Cu] -24.8 x [Cr] + 29.6 x [Mo] (5)

(Mass%) of C, Si, Mn, Ni, Cu, Cr, and Mo, respectively, in terms of C, Si, Mn, Ni, Cu, .

Evaluation of microstructure (ferrite grain size, ferrite + pearlite fraction and bainite fraction) and steel sheet properties (tensile strength TS and low temperature toughness (wave-front transition temperature vTrs)) were evaluated for each steel sheet obtained as described below .

(Measurement of ferrite particle diameter, ferrite + pearlite fraction and bainite fraction)

The ferrite + pearlite fraction and the bainite fraction were measured by observing a region of 1: 600 占 퐉 800 占 퐉 at a magnification of 100 times at a position of t / 4 (t: plate thickness) of each steel sheet using an optical microscope, And the average of the five visual fields was obtained. The ferrite grain size was obtained by observing the five fields of view at 100 times at the position of t / 4 (t: plate thickness) of each steel sheet and calculating the diameter when the size of the ferrite grains was assumed to be a circle as a circle- (Average circle equivalent diameter).

(Tensile test)

The 1B test piece of JIS Z 2201 was taken from the total thickness of each steel sheet in the direction perpendicular to the rolling direction and subjected to a tensile test in accordance with JIS Z 2241 to measure the tensile strength TS. The tensile strength was 485 MPa or more.

(Evaluation of low-temperature toughness)

ASTM A370-05 (0.500-in.Round Specimen) test specimens were taken at the position of t / 4 (t: sheet thickness) of each steel sheet in the direction perpendicular to the rolling direction and, according to ASTM A370-05, (Charpy) impact test was conducted to measure the wavefront transition temperature vTrs. When the wave front transition temperature vTrs was -80 占 폚 or lower, it was evaluated that the low temperature toughness was excellent.

These results are shown in Table 3 together with the CEQ value and the sigma value.

From these results, it can be considered as follows (the following No. indicates the test No. in the table). The number satisfying the requirements stipulated in the present invention. It can be seen that the steel sheets 1 to 10 have excellent low temperature toughness and high tensile strength TS.

On the other hand, In the steel sheets 11 to 21, any one of the requirements specified in the present invention is lacking, and either of the characteristics is deteriorated. First, 11 and 16, the Ni content was less than the range specified in the present invention, and therefore, satisfactory toughness could not be satisfied. Also, since the CEQ value is also small, a predetermined strength can not be secured.

No. 12, the Ni content is within the range specified in the present invention, but since the CEQ value is small, a predetermined strength can not be secured even if the low temperature toughness is satisfied. No. 13 had an Ni content exceeding the range specified by the present invention and also had a high CEQ value, so that even if the strength was satisfied, the low temperature toughness could not be satisfied.

No. 14 and 15, the chemical composition was within the range specified in the present invention, but at least one of the CEQ value and the sigma value did not satisfy the specified value, so that even if the strength was satisfied, the low temperature toughness could not be satisfied.

No. 17 and 18 is a temperature range of 950~875 ℃ as required by the present invention (the recrystallization temperature region), and more than 820 ℃ below Ar ₃ transformation point temperature range, at least any one of the rolling reduction (low temperature side of the non-recrystallized temperature range) The ferrite grain size became large and the low temperature toughness could not be satisfied. No. 19, although the CEQ value is insufficient, the strength is satisfied by the strengthening of the work by applying the pressure at the two-phase temperature region. However, the stress concentration accompanied with the strengthening of the processing became remarkable, and the low temperature toughness could not be satisfied.

No. 20, since the average cooling rate after rolling was high, a bainite structure having generally low toughness was produced, and the low temperature toughness could not be satisfied. No. 21, the Ni content is within the range specified in the present invention. However, since a coarse seamless grain structure is formed by applying a pressure drop at a temperature range lower than 875 캜 and higher than 820 캜 (on the high temperature side in the non-recrystallized temperature range), good low temperature toughness Could not be secured. In addition, since the ferrite grain size is large and the CEQ value is also small, the strength can not be satisfied.

Based on these data, the relationship between the CEQ value and the tensile strength TS is shown in Fig. Fig. 2 shows the relationship between the sigma value and the wave front transition temperature vTrs. In FIGS. 1 and 2, a rhombus symbol "◆" filled with color represents a description example, and a triangle symbol "Δ" represents a comparative example. As is clear from the results, it can be seen that controlling the CEQ value or sigma value in an appropriate range is effective in improving the strength and low-temperature toughness of the high-strength steel sheet.

(Mass%) defined by the following formula (1) is 0.345 or more and 0.428 or less, and σ is 2080 or more as defined by the following formula (2), and t / 4 (t: plate thickness) is a mixed structure of ferrite and pearlite, and by setting the mean circle equivalent diameter of the ferrite lips to 7.0 m or less, a high tensile strength steel sheet having high strength and excellent low temperature toughness can be realized have.

(Cu) + [Ni] / 15 + (Mo) + [V]) / 5 + (One)

? = 2.90 x {602781.57- (1154 x CEQ-3.25) ² } ^1/2 /0.963+400x [Ni] ... (2)

(% By mass) of C, Mn, Cr, Mo, V, Cu and Ni, respectively, in terms of C, Mn, Cr, Mo, V, Cu and Ni. .

Claims (3)

C: 0.03 to 0.09% (meaning "mass%", the same applies hereinafter for chemical components)
Si: 0.05 to 0.35%
Mn: 0.9 to 1.6%
P: not more than 0.01% (not including 0%),
S: not more than 0.01% (not including 0%),
Al: 0.01 to 0.06%
Ni: 0.2 to 2.0%
Nb: 0.007 to 0.017%,
Ti: 0.007 to 0.017%
Ca: 0.0005 to 0.003% and
(Mass%) defined by the following formula (1) is not less than 0.345 and not more than 0.428, and the content of N: 0.0025 to 0.0050%, and the balance of iron and inevitable impurities, Wherein the microstructure at a position of t / 4 (t: plate thickness) is a mixed structure of ferrite and pearlite, and the mean circle equivalent diameter of the ferrite grains is 7.0 占 퐉 or less. As a high strength steel sheet,
Test specimens of ASTM A370-05 (0.500-in.Round Specimen; 0.500 inch, circular specimen) were taken at the position of t / 4 (t: sheet thickness) in the direction perpendicular to the rolling direction and measured according to ASTM A370-05 And the wave front transition temperature vTrs is -80 占 폚 or lower when the wave front transition temperature vTrs is measured by performing a Charpy impact test.
(Cu) + [Ni] / 15 + (Mo) + [V]) / 5 + (One)
? = 2.90 x {602781.57- (1154 x CEQ-3.25) ² } ^1/2 /0.963+400x [Ni] ... (2)
(% By mass) of C, Mn, Cr, Mo, V, Cu and Ni, respectively, in terms of C, Mn, Cr, Mo, V, Cu and Ni. . The method according to claim 1,
And further contains at least one of the following (a) to (c).
(a) B: 0.002% or less (not including 0%)
(b) Cu: not more than 0.35% (not including 0%)
(c) Cr: not more than 0.3% (not including 0%), Mo: not more than 0.2% (not including 0%), and V: not more than 0.06% One or more species A steel sheet having the chemical composition as recited in any one of claims 1 to 3, wherein the rolling reduction is 30% or more and t / 4 (t: plate thickness) when t / 4 ), with the location it is doing the reduction by the reduction ratio of 30% or more when more than 820 ℃ below Ar ₃ transformation point temperature calendar, t / 4 (t: sheet thickness), the temperature range of the position is greater than less than 875 ℃ 820 ℃, and 2 And cooling the steel sheet at a cooling rate of 2.0 占 폚 / sec or less after the steel sheet is rolled down in the case of the phase temperature region, thereby forming a microstructure of a mixed structure of ferrite and pearlite.

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