KR102557520B1 - Steel plate and method of producing same - Google Patents

Abstract

A thick steel sheet having both excellent fatigue crack propagation resistance and overall elongation and excellent fatigue crack propagation resistance in all of the sheet thickness direction, rolling direction, and width direction is provided. In mass%, C: 0.01 to 0.16%, Si: 1.00% or less, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.020% or less, Al: 0.06% or less, and N: 0.0060% or less , the remainder being Fe and unavoidable impurities, and containing 75 to 97% of bainite and 3 to 25% of pearlite in area fraction, and the crystal grain size of bainite is 18 as an average equivalent circle diameter. A thick steel sheet having a microstructure in which the crystal grain size of pearlite is 10 µm or less as an average equivalent circle diameter.

Description

Steel plate and its manufacturing method {STEEL PLATE AND METHOD OF PRODUCING SAME}

The present invention relates to a thick steel sheet, and more particularly, to a thick steel sheet excellent in both overall elongation and fatigue crack propagation resistance. The steel plate of the present invention can be suitably used for welded structures requiring strong structural safety, such as ships, offshore structures, bridges, buildings, and tanks. Further, the present invention relates to a method for manufacturing the thick steel plate.

BACKGROUND OF THE INVENTION Thick steel plates are widely used in structures such as ships, offshore structures, bridges, buildings, and tanks. In addition to excellent mechanical properties such as strength and toughness and weldability, the thick steel sheet is required to have excellent fatigue properties.

That is, when a structure as described above is used, a repeated load such as wind or vibration caused by an earthquake is applied to the structure. For this reason, the thick steel plate is required to have fatigue characteristics capable of ensuring the safety of the structure even when such repeated loads are applied. In particular, in order to prevent eventual failure such as fracture of members, it is effective to improve the fatigue crack propagation resistance of the thick steel sheet.

Therefore, various studies have been conducted to improve the fatigue crack propagation resistance of steel sheets.

For example, Patent Literature 1 proposes a steel sheet for tankers having excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment. The steel sheet has a mixed structure composed of ferrite as a first phase and bainite and/or pearlite as a second phase. In addition, in the above steel sheet, the average grain size of ferrite is 20 μm or less.

Further, Patent Literature 2 also proposes a steel sheet having excellent fatigue crack propagation resistance. The steel sheet is characterized in that it has a microstructure composed of a hard portion and a soft portion, and a difference in hardness between the hard portion and the soft portion is 150 or more in terms of Vickers hardness.

Patent Literature 3 proposes a two-phase steel having a microstructure composed of bainite and ferrite with an area ratio of 38 to 52%. In the technique proposed in Patent Literature 3, the fatigue crack propagation resistance is improved by controlling the Vickers hardness of the ferrite phase portion and the density of the boundary between the ferrite phase and the bainite phase.

Japanese Unexamined Patent Publication No. 06-322477 Japanese Unexamined Patent Publication No. 07-242992 Japanese Unexamined Patent Publication No. Hei 08-225882

However, it has been found that the conventional techniques described in Patent Literatures 1 to 3 have the following problems (1) to (3).

(1) In steel materials used for structures such as ships, marine structures, bridges, buildings, and tanks, it is common that the total elongation value is specified in the standard. Therefore, even if the steel sheet has excellent fatigue crack propagation resistance, it is required that the total elongation satisfy the standard value.

However, since fatigue crack propagation resistance and total elongation are opposite properties, it was not possible to achieve both excellent fatigue crack propagation resistance and total elongation in the conventional techniques described in Patent Literatures 1 to 3.

That is, in the techniques proposed in Patent Literatures 1 to 3, total stretching is not considered. In fact, all of the steel plates proposed in Patent Literatures 1 to 3 have a microstructure composed of ferrite as a soft phase and bainite or martensite as a hard phase. In the steel sheet, the fatigue crack propagation resistance is improved by widening the hardness difference between the soft phase and the hard phase. However, when the difference in hardness between the soft phase and the hard phase is large, the structure becomes heterogeneous, and as a result, the total elongation of the steel sheet decreases.

(2) Further, from the viewpoint of ensuring structural safety, thick steel plates are required to have excellent fatigue crack propagation resistance not only in one direction, but also in the thickness direction, rolling direction, and width direction.

That is, in a general structure, welding is performed freely from various directions with respect to a steel plate. Therefore, the directions in which fatigue cracks are generated and propagated vary. In addition, at a welding construction site having an included corner corner, fatigue cracks are unavoidable due to their structural characteristics, and the generated fatigue cracks tend to first propagate in the sheet thickness direction. However, in order to prevent collapse of the structure due to fatigue cracking, it is important to suppress fatigue crack propagation in the sheet width direction and rolling direction even after the fatigue crack penetrates in the thickness direction of the steel sheet.

However, in the conventional techniques described in Patent Literatures 1 to 3, the directional dependence of the fatigue crack propagation resistance was not considered.

(3) In addition, it is difficult to control the production conditions of the conventional steel sheet having the microstructure. That is, when the steel sheet is manufactured by an online process, in order to obtain a desired structure, in the cooling step after hot rolling, it is necessary to start accelerated cooling from the ideal region of ferrite and austenite and to lower the cooling stop temperature. there is. At that time, the area fraction of the soft phase and the hard phase in the finally obtained microstructure fluctuates greatly depending on the temperature at the start of cooling. Therefore, in the manufacture of the conventional steel sheet, it is necessary to strictly control the cooling conditions in order to obtain a desired microstructure.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thick steel sheet having the excellent characteristics of (1) to (3) below.

(1) It has excellent fatigue crack propagation resistance and total elongation.

(2) Excellent fatigue crack propagation resistance in all of the sheet thickness direction, rolling direction, and width direction.

(3) It can be manufactured without requiring a high degree of cooling control in the ideal region.

Another object of the present invention is to provide a method for manufacturing the thick steel plate.

The present inventors obtained the following knowledge, as a result of examining in order to solve the said subject.

(a) Sufficient fatigue crack propagation resistance can be obtained even if the hardness difference between the soft phase and the hard phase in the microstructure is not as large as in Patent Literatures 1 to 3.

(b) By using bainite as the first phase, the fatigue crack propagation resistance can be improved more than before.

(c) Excellent fatigue crack propagation resistance and overall It is possible to obtain a steel sheet after drawing.

(d) The thick steel sheet having the microstructure can be manufactured by controlling manufacturing conditions, in particular, hot rolling and subsequent accelerated cooling conditions. Since the above-mentioned thick steel sheet has bainite as the first phase, it is more suitable for manufacturing by an online process than conventional steel sheets.

This invention was made based on the knowledge mentioned above, and makes the following a summary.

1. In mass %,

C: 0.01 to 0.16%,

Si: 1.00% or less;

Mn: 0.50 to 2.00%,

P: 0.030% or less;

S: 0.020% or less;

Al: 0.06% or less, and

N: contains 0.0060% or less,

The balance has a component composition consisting of Fe and unavoidable impurities,

As an area fraction,

75 to 97% bainite, and

3 to 25% perlite,

The crystal grain size of bainite is 18 μm or less as an average equivalent circle diameter;

A thick steel sheet having a microstructure in which the crystal grain size of pearlite is 10 μm or less as an average equivalent circle diameter.

2. The above component composition, further in mass%,

Cr: 0.01 to 1.00%,

Cu: 0.01 to 1.00%,

Ni: 0.01 to 1.00%,

Mo: 0.01 to 1.00%,

Nb: 0.005 to 0.050%,

V: 0.005 to 0.050%,

Ti: 0.005 to 0.050%,

B: 0.0001 to 0.0050%,

Ca: 0.0001 to 0.020%,

Mg: 0.0001 to 0.020%, and

REM: 0.0001% to 0.020% The thick steel sheet according to 1 above, containing one or two or more selected from the group consisting of 0.0001% to 0.020%.

3. The thick steel sheet according to 1 or 2 above, wherein the difference between the Vickers hardness at a position at a depth of 1 mm from the surface of the thick steel sheet and the Vickers hardness at the mid-thickness portion of the thick steel sheet is 40 HV or less.

4. The steel material having the component composition described in 1 or 2 above is heated at a heating temperature of 1000 ° C. or higher and 1250 ° C. or lower,

Hot-rolling the heated steel material to obtain a hot-rolled steel sheet,

The hot-rolled steel sheet is cooled under the conditions of a cooling start temperature: Ar3 point or higher, a cooling stop temperature: 450 to 700 ° C., and an average cooling rate on the steel sheet surface from the start of cooling to the stop of cooling: 20 to 60 ° C./s. , as a method for manufacturing a thick steel plate,

The method for producing a thick steel sheet, wherein the cumulative reduction ratio in the temperature range of 950 ° C. or higher in the hot rolling is 80% or more, and the cumulative reduction ratio in the temperature range of less than 950 ° C. or higher is 50% or more.

5. The method for manufacturing a thick steel sheet according to 4 above, wherein the average cooling rate in the accelerated cooling is 20 to 50°C/s.

The thick steel sheet of the present invention has both excellent fatigue crack propagation resistance and overall elongation, and is also excellent in fatigue crack propagation resistance in all of the sheet thickness direction, rolling direction, and width direction. In addition, the thick steel sheet of the present invention can be stably manufactured without requiring a high level of cooling control in the abnormal region. Therefore, the thick steel plate of the present invention greatly contributes to improving the reliability of steel structures and reducing life cycle costs.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing the shape and dimensions of a one-sided notched simple tensile type fatigue test specimen used for evaluation of fatigue crack propagation characteristics in the plate thickness direction.

Hereinafter, the present invention will be described in detail. In addition, this invention is not limited to this embodiment.

[Ingredient Composition]

First, the component composition of the thick steel sheet of the present invention will be described. Unless otherwise specified, "%" representing the content of each component means "% by mass".

C: 0.01 to 0.16%

C is an element having an effect of improving strength. In addition, C has an effect of accelerating the formation of a pearlite phase that is advantageous for fatigue resistance. If the C content is less than 0.01%, desired strength and fatigue crack propagation resistance cannot be obtained. Therefore, the C content is made 0.01% or more. On the other hand, when the C content exceeds 0.16%, total elongation and weldability deteriorate. Therefore, the C content is set to 0.16% or less, preferably 0.15% or less, and more preferably 0.10% or less.

Si: 1.00% or less

Si is an element that has a deoxidizing effect and also has an effect of further improving the strength. Moreover, Si also has the effect of suppressing excessive cementite formation. However, when the Si content exceeds 1.00%, in addition to deterioration of weldability and toughness, formation of a pearlite phase advantageous to fatigue resistance is suppressed. Therefore, the Si content is 1.00% or less, preferably 0.50% or less. On the other hand, the lower limit of the Si content is not particularly limited, but from the viewpoint of enhancing the effect of adding Si, the Si content is preferably 0.01% or more, and preferably 0.10% or more.

Mn: 0.50 to 2.00%

Mn is an element having an effect of improving hardenability and, as a result, improving strength and toughness of a thick steel sheet. In order to obtain the above effects, the Mn content is set to 0.50% or more, preferably 0.80% or more. On the other hand, when the Mn content exceeds 2.00%, the quenching property becomes excessively high, and as a result, the formation of a pearlite phase advantageous to the fatigue resistance property is suppressed. Moreover, when Mn content exceeds 2.00 %, total elongation and toughness will fall. Therefore, the Mn content is 2.00% or less, preferably 1.65% or less.

P: 0.030% or less

P is an element contained in a thick steel sheet as an impurity and deteriorates toughness and overall elongation. Therefore, the P content is made 0.030% or less. On the other hand, the lower the P content, the better. Therefore, the lower limit of the P content is not particularly limited, and the P content may be 0% or more or more than 0%. However, excessive reduction increases manufacturing cost, so from the viewpoint of manufacturing cost, the P content is preferably 0.0005% or more, more preferably 0.001% or more.

S: 0.020% or less

S is an element contained in a thick steel sheet as an impurity and deteriorates toughness. Therefore, the S content is 0.020% or less, preferably 0.010% or less. On the other hand, the lower the S content, the better. Therefore, the lower limit of the S content is not particularly limited, and the S content may be 0% or more or more than 0%. However, since excessive reduction increases manufacturing cost, from the viewpoint of manufacturing cost, the S content is preferably 0.0005% or more, more preferably 0.001% or more.

Al: 0.06% or less

Al is an element that acts as a deoxidizing agent and is generally used in a molten steel deoxidizing process. Moreover, Al fixes N in steel as AlN, and contributes to the improvement of the toughness of the base material. However, when the Al content exceeds 0.06%, the toughness and total elongation of the base material (thick steel sheet) decrease, and Al is mixed in the weld metal portion during welding, deteriorating the toughness of the weld portion. Therefore, the Al content is 0.06% or less, preferably 0.05% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of enhancing the effect of adding Al, it is preferable to set the Al content to 0.01% or more.

N: 0.0060% or less

N is an element that combines with Al in steel to form AlN and contributes to strength improvement through refinement of crystal grains during hot rolling. However, when N content exceeds 0.0060%, toughness will fall. Therefore, the N content is 0.0060% or less, preferably 0.0050% or less. On the other hand, the lower limit of the N content is not particularly limited, but from the viewpoint of enhancing the effect of adding N, the N content is preferably 0.0020% or more.

The thick steel sheet in one embodiment of the present invention may have a component composition including the above elements, the balance being Fe and unavoidable impurities.

In addition, the component composition of the thick steel sheet in another embodiment of the present invention may further optionally contain at least one of the elements exemplified below. By adding these optional additional elements, characteristics such as strength, toughness, weldability and weather resistance of the thick steel sheet can be further improved.

Cr: 0.01 to 1.00%

Cr is an element having an effect of further improving strength and weather resistance. In addition, Cr is an element that promotes the formation of cementite, and promotes the formation of a pearlite phase that is advantageous for fatigue resistance properties. When adding Cr, the Cr content is set to 0.01% or more, preferably 0.10% or more, in order to obtain the above effect. On the other hand, when the Cr content exceeds 1.00%, weldability and toughness are impaired. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

Cu: 0.01 to 1.00%

Cu is an element having an effect of further increasing strength by solid solution and improving weather resistance. When adding Cu, in order to acquire the said effect, Cu content is made into 0.01 % or more. On the other hand, when the Cu content exceeds 1.00%, weldability is impaired, and scratches easily occur during production of a thick steel sheet. Therefore, the Cu content is 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.

Ni: 0.01 to 1.00%

Ni is an element having an effect of improving low-temperature toughness and weather resistance. Moreover, Ni improves hot brittleness when Cu is added. When adding Ni, the Ni content is made 0.01% or more in order to obtain the above effects. On the other hand, when Ni content exceeds 1.00 %, weldability will be impaired and steel materials cost will rise. Therefore, the Ni content is 1.00% or less, preferably 0.70% or less, more preferably 0.40% or less.

Mo: 0.01 to 1.00%

Mo is an element having an effect of further improving strength. When adding Mo, Mo content is made 0.01% or more in order to acquire the said effect. On the other hand, when the Mo content exceeds 1.00%, weldability and toughness are impaired. Therefore, the Mo content is 1.00% or less, preferably 0.70% or less, more preferably 0.40% or less.

Nb: 0.005 to 0.050%

Nb is an element having an effect of suppressing recrystallization of austenite during hot rolling and refining the finally obtained crystal grains. In addition, Nb precipitates during air cooling after accelerated cooling and further improves the strength. In the case of adding Nb, the Nb content is made 0.005% or more in order to obtain the above effects. On the other hand, when the Nb content exceeds 0.050%, the quenching property becomes excessive and martensite is generated, so that a desired structure cannot be obtained and the toughness decreases. Therefore, the Nb content is 0.050% or less, preferably 0.040% or less.

V: 0.005 to 0.050%

V is an element that is precipitated during air cooling after accelerated cooling and has an effect of further improving strength. In the case of adding V, the V content is set to 0.005% or more in order to obtain the above effects. On the other hand, when the V content exceeds 0.050%, weldability and toughness deteriorate. Therefore, the V content is 0.050% or less, preferably 0.030% or less.

Ti: 0.005 to 0.050%

Ti is an element that has an effect of further increasing strength and improving weld toughness. When adding Ti, Ti content is made 0.005% or more in order to acquire the said effect. On the other hand, when Ti content exceeds 0.050 %, the raise of cost becomes remarkable. Therefore, the Ti content is 0.050% or less, preferably 0.030% or less, more preferably 0.020% or less.

B: 0.0001 to 0.0050%

B is an element that has an effect of enhancing hardenability and, as a result, further improving strength. When B is added, the B content is made 0.0001% or more in order to obtain the above effects. On the other hand, when the B content exceeds 0.0050%, the quenchability becomes excessive, martensite is generated, and a desired structure is no longer obtained, and weldability deteriorates. Therefore, the B content is 0.0050% or less, preferably 0.0030% or less.

Ca: 0.0001 to 0.020%

Ca is an element having an effect of controlling the form of sulfide and, as a result, further improving toughness. When adding Ca, in order to acquire the said effect, Ca content is made into 0.0001% or more. On the other hand, when the Ca content exceeds 0.020%, the effect is saturated. Therefore, the Ca content is made 0.020% or less.

Mg: 0.0001 to 0.020%

Mg is an element having an effect of improving toughness through refinement of crystal grains. When adding Mg, the Mg content is made 0.0001% or more in order to obtain the above effect. On the other hand, when the Mg content exceeds 0.02%, the effect is saturated. Therefore, the Mg content is made 0.020% or less.

REM: 0.0001 to 0.020%

REM (rare earth metal) is an element having an effect of improving toughness. When adding REM, in order to acquire the said effect, REM content is made into 0.0001% or more. On the other hand, when the REM content exceeds 0.020%, the effect is saturated. Therefore, the REM content is made 0.020% or less.

[micro organization]

Next, the microstructure of the thick steel plate will be described. The thick steel sheet in one embodiment of the present invention contains 75 to 97% of bainite and 3 to 25% of pearlite in area fraction, and the crystal grain size of bainite is 18 μm or less as an average equivalent circle diameter , has a microstructure in which the crystal grain size of pearlite is 10 μm or less as an average equivalent circle diameter. In addition, the microstructure in the present invention refers to the microstructure at the 1/4 position (1/4t position) of the sheet thickness t of the thick steel plate. The area fraction and grain size of each structure can be measured by subjecting a cross section parallel to the rolling direction at a depth of 1/4 from the surface of the thick steel sheet to nickel corrosion and observing it. More specifically, the area fraction and crystal grain size can be obtained by the methods described in Examples.

Area fraction of bainite: 75 to 97%

In the present invention, bainite is the first phase in the microstructure and functions as a soft phase. Ferrite is typical as a soft phase contained in steel materials, but bainite has a higher crack propagation suppression effect than ferrite. Therefore, propagation of fatigue cracks can be suppressed by setting the area fraction of bainite to 75% or more. If the area fraction of bainite is less than 75%, desired fatigue crack propagation resistance cannot be obtained. The area fraction of bainite is preferably 80% or more. On the other hand, when the area fraction of bainite exceeds 97%, pearlite becomes insufficient, and as a result, propagation of fatigue cracks cannot be suppressed. Therefore, the area fraction of bainite is 97% or less.

Crystal grain size of bainite: 18 μm or less

The crystal grain size of bainite is set to 18 μm or less as an average equivalent circular diameter. By refining bainite, desired toughness and overall elongation properties can be obtained. When the crystal grain size of bainite exceeds 18 μm in average equivalent circular diameter, desired toughness and total elongation cannot be obtained. On the other hand, the lower limit of the crystal grain size of bainite is not particularly limited, but since excessive miniaturization makes production difficult, it is preferable to set the grain size of bainite to 5 µm or more in actual production.

In addition, the bainite in the present invention shall contain upper bainite, ascular ferrite, and granular bainite.

Area fraction of pearlite: 3 to 25%

In the present invention, pearlite is the second phase in the microstructure and functions as a hard phase. When a fatigue crack propagating in bainite reaches pearlite, which is a hard phase, the crack is rectified or bent at the interface between bainite and pearlite. And as a result, crack propagation is suppressed. In order to obtain the above effect, the area fraction of pearlite is set to 3% or more, preferably 5% or more. On the other hand, when the area fraction of pearlite exceeds 25%, total elongation decreases. Therefore, the area fraction of pearlite is 25% or less, preferably 20% or less.

Crystal grain size of pearlite: 10 μm or less

The crystal grain size of pearlite is 10 μm or less as an average equivalent circle diameter. By miniaturizing pearlite, desired toughness and overall elongation properties can be obtained. If the crystal grain size of pearlite exceeds 10 μm in average equivalent circular diameter, desired toughness and total elongation cannot be obtained. On the other hand, the lower limit of the crystal grain size of pearlite is not particularly limited, but may be 1 μm or more and may be 2 μm or more.

In addition, the pearlite in this invention shall contain pearlite and pseudo pearlite.

(another organization)

The thick steel sheet in one embodiment of the present invention may have a microstructure composed of bainite and pearlite. However, the microstructure may further optionally contain other structures. The other structure may be either or both of martensite and ferrite, for example. Here, the martensite includes island martensite, lath martensite, and lenticular martensite.

When other tissues are present, the area fraction (total area fraction) of the other tissues is not particularly limited. However, if martensite is excessively present, a region of high hardness is locally formed, and although the strength is increased, overall elongation may deteriorate and toughness may decrease. In addition, when ferrite is present in excess, the fatigue crack propagation speed deteriorates, and a soft region is formed locally, and the overall elongation may deteriorate due to the expansion of the hardness difference. Therefore, the lower the area fraction of other tissues, the better, but the influence can be ignored if it is 5% or less. Therefore, it is preferable to set the total area fraction of structures other than bainite and pearlite to 5% or less.

In other words, the thick steel plate in one embodiment of the present invention,

75 to 97% bainite,

3 to 25% perlite, and

It may have a microstructure composed of structures other than 0 to 5% of bainite and pearlite.

(plate thickness)

In the present invention, according to the usual definition, a steel plate having a thickness of 6 mm or more is defined as a "thick steel plate". On the other hand, the upper limit of the plate thickness is not particularly limited, but the present invention is particularly preferably applied to relatively thin thick steel plates. Therefore, the thickness of the thick steel plate in the present invention is preferably 25 mm or less, and more preferably less than 20 mm.

(tensile strength)

The thick steel sheet of the present invention can have excellent tensile strength (TS) as a result of having the above component composition and microstructure. The value of TS is not particularly limited, but is preferably 500 MPa or more, more preferably 530 MPa or more, and still more preferably 550 MPa or more. On the other hand, the upper limit of TS is not limited either, but may be, for example, 720 MPa or less, 700 MPa or less, 640 MPa or less, or 620 MPa or less.

(yield stress)

The yield stress (YS) of the thick steel sheet of the present invention is not particularly limited, but may be 420 MPa or more, 430 MPa or more, or 440 MPa or more. Moreover, YS may be 560 MPa or less, 530 MPa or less, or 520 MPa or less may be sufficient.

(tenacity)

The thick steel sheet of the present invention has excellent toughness as a result of having the above component composition and microstructure. The toughness of the thick steel sheet of the present invention is not particularly limited, but the Charpy absorbed energy vE ₀ at 0°C, which is one of the indicators of toughness, is preferably 100 J or more, more preferably 130 J or more, It is more preferably 150 J or more, and most preferably 200 J or more. On the other hand, although it is not limited also about the upper limit of _vE0 , it may be 400 J or less, 300 J or less, or 270 J or less, for example. In addition, vE ₀ can be measured by the method described in the Example.

(full stretching)

The thick steel sheet of the present invention has excellent total elongation (EL) as a result of having the above component composition and microstructure. The value of EL is not particularly limited, but is preferably 15% or more, more preferably 16% or more, still more preferably 17% or more, and most preferably 20% or more. The upper limit of EL is not particularly limited either, but may be 30% or less. In addition, EL can be measured by the method described in the Example.

(hardness difference)

It is preferable that the difference between the Vickers hardness at a position at a depth of 1 mm from the surface of the thick steel plate and the Vickers hardness at the center of the thickness of the thick steel plate (hereinafter referred to as "difference in hardness") is 40 HV or less. . By setting the hardness difference to 40 HV or less, the overall stretching characteristics can be further improved. From the viewpoint of improving the overall stretching characteristics, the lower the hardness difference, the better, so the lower limit of the hardness difference may be 0 HV. However, since it is difficult to set the hardness difference to 0 HV in actual production, the hardness difference may be, for example, 10 HV or more. The said hardness difference can be measured by the method described in the Example.

(Fatigue crack propagation resistance)

As a result of having the above component composition and microstructure, the thick steel sheet of the present invention can have excellent fatigue crack propagation resistance in all of the sheet thickness direction, rolling direction, and width direction. As an index of fatigue crack propagation resistance, fatigue crack propagation speed (da/dN) can be used. The value of the fatigue crack propagation speed is not particularly limited.

In addition, the fatigue crack propagation speed in the sheet thickness direction (Z direction) preferably satisfies the following conditions (a) and (b).

(a) stress intensity range ΔK: fatigue crack propagation speed under the condition of 15 MPa/m ^1/2 is 8.75 × 10 ^-9 (m/cycle) or less,

(b) Stress magnification coefficient range ΔK: Fatigue crack propagation speed under the condition of 25 MPa/m ^1/2 is 4.25 × 10 ^-8 (m/cycle) or less

In addition, it is preferable that either of the fatigue crack propagation speed in the rolling direction (L direction) and the fatigue crack propagation speed in the width direction (C direction) satisfy the following conditions (c) and (d). And it is more preferable that both satisfy the conditions of (c) and (d).

(c) Stress magnification coefficient range ΔK: Fatigue crack propagation speed under the condition of 15 MPa/m ^1/2 is 1.75 × 10 ^-8 (m/cycle) or less,

(d) Stress magnification coefficient range ΔK: Fatigue crack propagation speed under the condition of 25 MPa/m ^1/2 is 8.50 × 10 ^-8 (m/cycle) or less

[Manufacturing conditions]

Next, the manufacturing method of the thick steel plate of this invention is demonstrated. The thick steel sheet in one embodiment of the present invention can be manufactured by sequentially performing the following steps (1) to (3) on a steel material having the above component composition.

(1) heating

(2) hot rolling

(3) accelerated cooling

Hereinafter, the conditions in each process are demonstrated. In addition, unless otherwise specified, temperature refers to the surface temperature of a to-be-processed object (steel raw material or hot-rolled steel sheet). In addition, the cooling rate is taken as the cooling rate in the average temperature of the thickness direction of a steel plate.

(River material)

Any steel material can be used as long as it has the above-mentioned component composition. The component composition of the finally obtained thick steel sheet is the same as that of the used steel raw material. As the steel material, for example, a steel slab can be used.

(1) heating

Heating temperature: 1000 ~ 1250 ℃

First, the steel material is heated to a heating temperature of 1000 ° C. or more and 1250 ° C. or less. If the heating temperature is less than 1000°C, the temperature necessary for the next hot rolling cannot be secured. On the other hand, when the said heating temperature exceeds 1250 degreeC, the crystal grains of steel coarsen and toughness deteriorates.

(2) hot rolling

Next, the heated steel material is hot-rolled to obtain a hot-rolled steel sheet. At that time, in order to manufacture a thick steel sheet that satisfies the conditions of the present invention, the cumulative reduction ratio in the hot rolling process needs to satisfy the following conditions.

Cumulative reduction rate in the temperature range of 950 ° C or higher: 80% or more

By setting the cumulative reduction ratio in a temperature range of 950°C or higher to 80% or higher, the austenite grains are refined. As a result, bainite formed by transformation during accelerated cooling and pearlite formed from non-transformed austenite are refined. If the cumulative reduction ratio is less than 80%, the miniaturization of bainite and pearlite becomes insufficient, and overall elongation deteriorates due to a decrease in toughness. On the other hand, the upper limit of the cumulative reduction ratio in the temperature range of 950°C or higher is not particularly limited, but may be, for example, 90% or less.

Cumulative reduction ratio in the temperature range of less than 950 ° C and above the Ar3 point: 50% or more

By setting the cumulative reduction ratio in the temperature range below 950 ° C. and above the Ar3 point to 50% or more, austenite grains are refined, and bainite generated by transformation during accelerated cooling or pearlite generated from untransformed austenite is refined. . When the reduction ratio at or above the Ar3 point is less than 50%, the refinement of bainite and pearlite becomes insufficient, and toughness and total elongation deteriorate. On the other hand, the upper limit of the cumulative reduction ratio in the temperature range below 950°C and above the Ar3 point is not particularly limited, but may be, for example, 80% or less, or 75% or less.

Here, the Ar3 point can be obtained by the following equation.

Ar3 (℃) ＝ 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo

However, the element symbol in the above formula indicates the content (mass%) of the element in the steel material, and is set to zero when the element is not contained in the steel material.

(3) accelerated cooling

Subsequently, the hot-rolled steel sheet obtained in the hot rolling process is subjected to accelerated cooling. The conditions for the accelerated cooling need to be as follows.

Cooling start temperature: above Ar3 point

When the cooling start temperature in the accelerated cooling is less than the Ar3 point, ferrite and coarse pearlite precipitate excessively, and strength and fatigue crack propagation resistance decrease. Therefore, the cooling start temperature is set to the Ar3 point or higher. On the other hand, the upper limit of the cooling start temperature is not particularly limited, but is preferably set to 870°C or less from the viewpoint of securing the cumulative reduction ratio in the temperature range of Ar3 point or higher.

In addition, that the cooling start temperature is above the Ar3 point necessarily means that the rolling end temperature is above the Ar3 point. When the rolling end temperature is less than the Ar3 point, ideal range rolling occurs and total elongation deteriorates. However, when the rolling end temperature is above the Ar3 point, rolling is performed in the austenite single phase region, so deterioration in total elongation can be prevented. .

Cooling stop temperature: 450 ~ 700 ℃

In order to transform untransformed austenite into a hard phase (pearlite), the cooling stop temperature in the accelerated cooling is set to 700°C or lower, preferably 650°C or lower. When the cooling stop temperature exceeds 700°C, the production of pearlite becomes insufficient, and the desired fatigue crack propagation resistance is not obtained. On the other hand, when the cooling stop temperature is less than 450°C, the amount of martensite produced increases, and as a result, a desired microstructure cannot be obtained, and toughness and total elongation decrease. In addition, since the formation of pearlite becomes insufficient, desired fatigue crack propagation resistance cannot be obtained. Therefore, the cooling stop temperature is set to 450°C or higher, preferably 500°C or higher, and more preferably higher than 550°C.

Average cooling rate: 20 to 60 °C/s

The average cooling rate in the accelerated cooling is set to 20°C/s or more. If the average cooling rate is lower than 20°C/s, ferrite is formed and the desired microstructure is not obtained, so the fatigue crack propagation resistance is lowered. In addition, since the toughness is lowered, desired total elongation cannot be obtained. On the other hand, when the average cooling rate exceeds 60°C/s, residual stress due to cooling deformation and excessive martensite are generated, resulting in deterioration of overall elongation. For this reason, the upper limit of the cooling rate is made into 60 degreeC/s. Also from the viewpoint of reducing the difference between the Vickers hardness at a position 1 mm deep from the surface of the thick steel plate and the Vickers hardness at the mid-thickness portion of the thick steel plate, it is preferable to lower the average cooling rate. Specifically, the hardness difference can be made 40 HV or less by setting the average cooling rate to 50° C./s or less. In addition, the said average cooling rate shall refer to the average cooling rate in the steel plate surface from the start of accelerated cooling to the stop of accelerated cooling.

A method of performing the accelerated cooling is not particularly limited, and any method may be used. For example, the accelerated cooling may be performed by intermittent cooling in which water cooling and air cooling are alternately repeated. After water cooling for a certain period of time from the start of cooling, water cooling is stopped and then air-cooled. The heat retained in the center portion of the steel sheet, which has not yet cooled sufficiently, generates recuperation on the surface side of the steel sheet, and the temperature distribution in the thickness direction is uniformed. Then, from the reheated temperature range, accelerated cooling by water cooling is performed again. By repeating this water cooling and air cooling at least once or more, the average cooling rate near the surface can be controlled within a predetermined range, and formation of a hard phase can be suppressed.

The treatment after completion of the accelerated cooling is not particularly limited. For example, the thick steel sheet after completion of the accelerated cooling may be left to cool in the atmosphere. In the said air cooling, it can cool to room temperature, for example. Further, after the end of the accelerated cooling, the warpage of the thick steel sheet may optionally be corrected by a hot leveler.

Also, after hot rolling, the steel sheet temperature is immediately lowered. Therefore, it is preferable to manufacture the thick steel sheet of the present invention by an online process using equipment in which a rolling device and an accelerated cooling device are provided on a conveyance line.

Example

Hereinafter, the actions and effects of the present invention will be described using examples. In addition, this invention is not limited to the following example.

A thick steel plate was manufactured in the following procedure.

First, a steel slab (steel material) having the component composition shown in Table 1 was manufactured by the converter-continuous casting method. The thickness of the steel slab was as shown in Table 2.

Next, the steel slab was heated at the heating temperature shown in Table 2, and then hot-rolled at the cumulative reduction ratio shown in Table 2 to obtain a hot-rolled steel sheet. The rolling end temperature in the hot rolling and the sheet thickness (final sheet thickness) of the obtained hot-rolled steel sheet are listed together in Table 2. Thereafter, the hot-rolled steel sheet was subjected to accelerated cooling under the conditions shown in Table 2 to obtain a thick steel sheet. The plate thickness of the obtained thick steel plate is the same as the final plate thickness described above.

For each of the obtained thick steel sheets, the microstructure, mechanical properties, and fatigue crack propagation properties were evaluated. The evaluation method is explained below. Moreover, the evaluation result is shown in Table 3.

(micro organization)

First, samples for microstructure observation were taken from the position of 1/4t in the thickness direction of the thick steel plate so that the cross section in the longitudinal direction became the observation surface. Here, the cross section in the longitudinal direction refers to a cross section perpendicular to the width direction of the thick steel plate. Subsequently, the surface of the sample was subjected to nital corrosion, and the tissue was photographed under a 400x optical microscope and a 2000x scanning electron microscope (SEM). Using the photographed image, the existing tissue was identified, and the image was analyzed to determine the area fraction of bainite, the area fraction of pearlite, and the total area fraction of other tissues. In addition, SEM images were used to identify the pearlite structure, and optical microscope images were used to measure the area fraction of each structure.

(Crystal grain size of bainite)

In addition, the crystal grain size of bainite was measured using the microstructure observation sample. In the measurement, first, the surface of the sample was mirror-polished, and the crystal orientation was measured from an electron beam backscattering diffraction image using an Electron Back-Scattering Pattern (EBSP) device accompanying the SEM. The area surrounded by 200 μm in all directions was measured at intervals of 0.3 μm, and the area surrounded by grain boundaries having a crystal orientation difference of 15 ° or more with neighboring crystal grains was defined as a crystal grain, and the average equivalent circle diameter of the crystal grain was obtained. The obtained average equivalent circle diameter is regarded as the crystal grain size of bainite.

(Crystal grain size of pearlite)

When the observation surface of the sample for observing the microstructure after nital corrosion was observed with an optical microscope image, the area showing black was observed by SEM, and it was identified that it was pearlite having a lamellar structure. Thereafter, using image analysis software (Image-J), the area was obtained from the number of pixels in the black region in the optical microscope image, and the area was converted to the average equivalent circle diameter of pearlite. The obtained average equivalent circle diameter is regarded as the crystal grain size of pearlite.

(mechanical properties)

A full-thickness tensile test piece was taken from the sheet width direction (C direction) of the thick steel sheet. A tensile test was conducted in accordance with JIS Z 2241 using the above full-thickness tensile test piece, and yield strength (YS), tensile strength (TS), and total elongation (EL) were measured. In addition, in accordance with the provisions of JIS Z 2241, as the full thickness tensile test piece, a JIS No. 1A test piece was used for a thick steel plate with a C content of less than 0.16%, and a JIS No. 5 test piece was used for a thick steel plate with a C content of 0.16% or more. each was used.

In addition, a Charpy impact test piece was taken from the thickness center of the thick steel plate parallel to the rolling direction (L direction), and a Charpy impact test was conducted at 0°C in accordance with JIS Z 2202, and absorbed energy vE ₀ was measured. .

(hardness difference)

In the following procedure, the difference between the Vickers hardness at a position 1 mm deep from the surface of the thick steel plate and the Vickers hardness at the mid-thickness portion of the thick steel plate (difference in hardness) was measured. After mirror polishing the cross section of the sample used for the observation of the microstructure, Vickers hardness was measured in accordance with JIS Z 2244. The above measurement was carried out at 3 points each at both a position at a depth of 1 mm from the surface of the thick steel plate and at the center of the plate thickness, and an average value was obtained. The load at the time of measurement was 10 kgf. Using the obtained average value, a difference (difference in hardness) between the Vickers hardness at a position 1 mm deep from the surface of the thick steel plate and the Vickers hardness at the mid-thickness portion of the thick steel plate was calculated.

(Fatigue crack propagation resistance)

As an index of fatigue crack propagation resistance, the fatigue crack propagation speed (da/dN) in the plate thickness direction (Z direction), rolling direction (L direction), and width direction (direction perpendicular to the rolling direction, C direction) , Stress magnification coefficient range ΔK: measured under two conditions of 15 MPa/m ^1/2 and 25 MPa/m ^1/2 respectively.

·Rolling direction and width direction

The fatigue crack propagation speed in the rolling direction (L direction) was measured using a test piece taken from a thick steel plate so that the loading direction was the rolling direction. Similarly, the fatigue crack propagation speed in the width direction (C direction) was measured using a test piece taken from a thick steel plate so that the load bearing direction was the width direction. The test piece was a compact tension test piece conforming to ASTM E647. In the above measurement, a fatigue crack propagation test was conducted based on the crack gauge method, and the fatigue crack propagation speed was determined.

·Plate thickness direction

On the other hand, in the measurement of the fatigue crack propagation speed in the plate thickness direction (Z direction), a single-notch simple tensile type fatigue test piece shown in FIG. 1 was used. The test piece was taken from the thick steel plate, and the fatigue crack propagation speed when the crack propagated in the plate thickness direction was measured.

As can be seen from the results shown in Table 3, the thick steel sheet satisfying the conditions of the present invention had very excellent characteristics that satisfied all of the following conditions. In particular, it had both excellent fatigue crack propagation resistance and total elongation, and was also excellent in fatigue crack propagation resistance in all of the sheet thickness direction, rolling direction, and width direction. On the other hand, the thick steel plates of the comparative examples that did not satisfy the conditions of the present invention did not satisfy at least one of the following conditions.

TS: 500 MPa or more

EL: 15% or more (when using JIS No. 1A test piece),

EL: 19% or more (when using JIS No. 5 test piece)

vE ₀ : 100 J or more

Fatigue crack propagation speed in L and C directions:

ΔK: 1.75 × 10 ^-8 (m/cycle) or less under the condition of 15 MPa/m ^1/2 ;

ΔK: 8.50 × 10 ^-8 (m/cycle) or less under the condition of 25 MPa/m ^1/2

Fatigue crack propagation speed in Z direction:

ΔK: 8.75 × 10 ^-9 (m/cycle) or less under the condition of 15 MPa/m ^1/2 ;

ΔK: 4.25 × 10 ^-8 (m/cycle) or less under the condition of 25 MPa/m ^1/2

Claims (5)

in mass %,
C: 0.01 to 0.16%,
Si: 1.00% or less;
Mn: 0.50 to 2.00%,
P: 0.030% or less;
S: 0.020% or less;
Al: 0.06% or less, and
N: contains 0.0060% or less,
The balance has a component composition consisting of Fe and unavoidable impurities,
As an area fraction,
75 to 97% bainite, and
3 to 25% perlite,
The total area fraction of structures other than bainite and pearlite is 5% or less,
The crystal grain size of bainite is 18 μm or less as an average equivalent circle diameter;
A thick steel sheet having a microstructure at a position of 1/4 of the sheet thickness, in which the crystal grain size of pearlite is 10 μm or less in average equivalent circle diameter. According to claim 1,
The above component composition, further in mass%,
Cr: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ni: 0.01 to 1.00%,
Mo: 0.01 to 1.00%,
Nb: 0.005 to 0.050%,
V: 0.005 to 0.050%,
Ti: 0.005 to 0.050%,
B: 0.0001 to 0.0050%,
Ca: 0.0001 to 0.020%;
Mg: 0.0001 to 0.020%, and
REM: A thick steel plate containing one or two or more selected from the group consisting of 0.0001% to 0.020%. According to claim 1 or 2,
A difference between the Vickers hardness at a position 1 mm deep from the surface of the thick steel plate and the Vickers hardness at a mid-thickness portion of the thick steel plate is 40 HV or less. Heating a steel material having the component composition according to claim 1 or 2 at a heating temperature of 1000 ° C. or higher and 1250 ° C. or lower,
Hot-rolling the heated steel material to obtain a hot-rolled steel sheet,
The hot-rolled steel sheet is cooled under the conditions of a cooling start temperature: Ar3 point or higher, a cooling stop temperature: 450 to 700 ° C., and an average cooling rate on the steel sheet surface from the start of cooling to the stop of cooling: 20 to 60 ° C./s. , as a method for manufacturing a thick steel plate,
In the hot rolling, the cumulative reduction ratio in the temperature range of 950 ° C. or higher is 80% or more, and the cumulative reduction ratio in the temperature range of less than 950 ° C. or higher is 50% or more,
The thick steel plate, in area fraction,
75 to 97% bainite, and
3 to 25% perlite,
The total area fraction of structures other than bainite and pearlite is 5% or less,
The crystal grain size of bainite is 18 μm or less as an average equivalent circle diameter;
A method for producing a thick steel sheet having a microstructure at a position of 1/4 of the sheet thickness, wherein the crystal grain size of pearlite is 10 μm or less in average equivalent circle diameter. According to claim 4,
The method for producing a thick steel sheet, wherein the average cooling rate in the accelerated cooling is 20 to 50°C/s.

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