KR20230159883A - Heavy steel plate and manufacturing method thereof - Google Patents

Abstract

C: 0.04 to 0.16 mass%, Si: 0.10 to 0.50 mass%, Mn: 0.60 to 1.60 mass%, P: 0.005 to 0.030 mass%, Al: 0.015 to 0.050 mass%, Ti: 0.005 to 0.020 mass%, Ca: Contains 0.0005 to 0.0025 mass%, O: 0.0008 to 0.0025 mass%, B: 0.0005 to 0.0020 mass%, N: 0.0030 to 0.0060 mass%, the balance consists of Fe and inevitable impurities, and formula (1) and ( 2) A steel sheet that satisfies the formula, the metal structure contains a total of 90% or more of ferrite and pearlite in area ratio, the remainder is island-shaped martensite and/or bainite, and the difference between the upper and lower yield points is 30 MPa or less. 3[Si]+6.3[P]+1.4[Al]≥0.63(1), [Ca]/[O]≥0.2 (2)

Description

Heavy steel plate and manufacturing method thereof

This disclosure relates to a thick steel plate and a method for manufacturing the same, for example, to a thick steel plate that can be used in a ship's tank capable of transporting a mixture of liquefied petroleum gas (LPG) and liquid ammonia.

In response to recent environmental regulations, carbon (C)-free ammonia is attracting attention as a next-generation fuel. The use of ammonia as a fuel is limited at present, but is expected to increase in the future. On the other hand, since ships are usually used for about 20 years after construction, there is a growing demand in the shipbuilding industry to build LPG carriers with a specification capable of mixing ammonia in preparation for future increases in ammonia usage.

The IGC Code, the international rules for the structure and equipment of liquefied gas carriers, stipulates special requirements for gas carriers carrying ammonia, and for tank steel plates for tanks of general LPG carriers from the viewpoint of preventing stress corrosion cracking. There is a new upper limit of yield strength that does not exist for steel plates. For example, in the low-temperature steel KL33 standard, which is a steel plate for tanks of general LPG carriers, the upper and lower limits for tensile stress (tensile strength TS) are specified as 440 MPa ≤ tensile stress (TS) ≤ 560 MPa. However, only the lower limit of yield strength is specified as 325MPa≤yield strength (YP). On the other hand, for steel plates for tanks loaded with ammonia, the upper limit is further specified as yield strength (YP) ≤ 440 MPa.

In addition, since propane gas (PG) needs to be cooled to -42°C or lower to liquefy it, a certain level of low-temperature toughness is also required to ensure the safety of the structure of the tank storing LPG. Furthermore, the heat affected zone (HAZ) formed when welding tank steel plates is also required to have a certain level of toughness at low temperatures.

As steel plates for tanks meeting such demands, for example, thick steel plates disclosed in Patent Documents 1 and 2 are known, both of which disclose thick steel plates with a yield strength within the range of 325 to 440 MPa. The invention according to Patent Document 1 has as a task securing low-temperature toughness and resistance compound ratio, and the invention according to Patent Document 2 has also as a task securing HAZ toughness. In the thick steel plate according to Patent Documents 1 and 2, in order to realize the resistance compound ratio, the ferrite grain size is divided into a plurality of numerical ranges in the structure of a quarter of the plate thickness, and the number ratio for each numerical range is required. These challenges are being solved by becoming one of the following.

Japanese Patent Publication No. 2019-214751 Japanese Patent Publication No. 2019-214752

When manufacturing a tank by welding thick steel plates, there is a desire to apply high heat input welding with a heat input of 7 kJ/mm or more from the viewpoint of productivity. In this case, not only must the thick steel plate itself have excellent low-temperature toughness, but it is also necessary to have excellent high-heat input welding characteristics, that is, the low-temperature toughness of the heat-affected zone (HAZ) by high-heat input welding.

However, Patent Documents 1 and 2 do not mention high heat input welding, and there is a risk that excellent high heat input welding characteristics may not be obtained from the steel sheets described in them. In addition, in order to obtain both low-temperature toughness and high heat input welding characteristics by adjusting the chemical composition and manufacturing method using conventional methods, the above-described tensile stress range (440 MPa or more, There is a problem that it is difficult to satisfy both the yield strength range (560 MPa or less) and the yield strength range (325 MPa or more, 440 MPa or less).

The present disclosure has been made in consideration of such circumstances, and provides a steel sheet that satisfies both the range of tensile stress and the range of yield strength required for a steel sheet for a tank loaded with ammonia and is also excellent in low-temperature toughness and high heat input welding characteristics. The purpose is to provide a manufacturing method thereof.

Embodiment 1 of the present invention is C: 0.04 to 0.16 mass%, Si: 0.10 to 0.50 mass%, Mn: 0.60 to 1.60 mass%, P: 0.005 to 0.030 mass%, Al: 0.015 to 0.050 mass%, Ti: 0.005 Contains ∼0.020 mass%, Ca: 0.0005∼0.0025 mass%, O: 0.0008∼0.0025 mass%, B: 0.0005∼0.0020 mass%, N: 0.0030∼0.0060 mass%, with the balance consisting of Fe and inevitable impurities; , the following formulas (1) and (2) are satisfied, and the metal structure at the position t/4 in the sheet thickness direction, which is the part where the distance in the sheet thickness direction from the surface is one fourth of the sheet thickness, is expressed as an area ratio. It is a steel plate that contains a total of 90% or more of ferrite structure and pearlite structure, and the remainder consists of one or more of island-shaped martensite structure and bainite structure, and the difference between the upper and lower yield points determined by a tensile test is 30 MPa or less.

3[Si]+6.3[P]+1.4[Al]≥0.63 (1)

Here, [Si], [P], and [Al] are the contents of Si, P, and Al expressed in mass%, respectively.

[Ca]/[O]≥0.2 (2)

Here, [Ca] and [O] are the contents of Ca and O expressed in mass%, respectively.

Embodiment 2 of the present invention is V: 0.003 to 0.50 mass%, Nb: 0.003 to 0.020 mass%, Cu: 0.05 to 0.25 mass%, Ni: 0.05 to 0.25 mass%, Cr: 0.05 to 0.25 mass%, Mo: 0.05. The steel sheet according to aspect 1, further comprising at least one selected from the group consisting of ∼0.25 mass%, Zr: 0.0001-0.010 mass%, Mg: 0.0001-0.010 mass%, and REM: 0.0001-0.010 mass%.

Aspect 3 of the present invention is,

C: 0.04 to 0.16 mass%,

Si: 0.10 to 0.50 mass%,

Mn: 0.60 to 1.60 mass%,

P: 0.005 to 0.030 mass%,

Al: 0.015 to 0.050 mass%,

Ti: 0.005 to 0.020 mass%,

Ca: 0.0005 to 0.0025 mass%,

O: 0.0008 to 0.0025 mass%,

B: 0.0005 to 0.0020 mass%,

N: Contains 0.0030 to 0.0060% by mass,

The balance consists of Fe and inevitable impurities,

A process of preparing steel that satisfies the following equations (1) and (2),

After heating the steel material to 1000-1150°C, the reduction rate in the temperature range of 820°C or higher is 30% or more, the reduction rate in the temperature range of 790°C or more and less than 820°C is 10% or more, and the hot rolling end temperature is 750°C. Hot rolling is carried out at ℃ or higher, and then cooled at an average cooling rate of 0.5 to 3.0 ℃/sec to the accelerated cooling start temperature, which is lower than the Ar ₃ point and is a temperature of Ar ₃ point - 150 ℃ or higher, as shown in equation (3) below, A rolling process of cooling at an average cooling rate of 4 to 9°C/sec from the accelerated cooling start temperature to the accelerated cooling end temperature of 500°C or more and 650°C or less.

Including,

The metal structure at the position t/4 in the sheet thickness direction, which is the portion where the distance in the sheet thickness direction from the surface is one fourth of the sheet thickness, contains a total of 90% or more of ferrite structure and pearlite structure in terms of area ratio, and the remaining This is a method of manufacturing a steel plate composed of one or more of the island-like martensite structure and the bainite structure, and the difference between the upper and lower yield points obtained by a tensile test is 30 MPa or less.

3[Si]+6.3[P]+1.4[Al]≥0.63 (1)

Here, [Si], [P], and [Al] are the contents of Si, P, and Al expressed in mass%, respectively.

[Ca]/[O]≥0.2 (2)

Here, [Ca] and [O] are the contents of Ca and O expressed in mass%, respectively.

Ar ₃ (℃)=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo] (3)

Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo expressed in mass%, respectively.

In aspect 4 of the present invention, the steel material has V: 0.003 to 0.50 mass%, Nb: 0.003 to 0.020 mass%, Cu: 0.05 to 0.25 mass%, Ni: 0.05 to 0.25 mass%, and Cr: 0.05 to 0.25 mass%. , Mo: 0.05 to 0.25 mass%, Zr: 0.0001 to 0.010 mass%, Mg: 0.0001 to 0.010 mass%, and REM: 0.0001 to 0.010 mass%. This is a method of manufacturing steel plates.

According to an embodiment of the present invention, a steel sheet that satisfies both the tensile stress range and the yield strength range required for a steel sheet for a tank loaded with ammonia, and is also excellent in low-temperature toughness and high heat input welding characteristics, and a method for manufacturing the same are provided. It is possible.

Most of the thick steel plates used in ship tanks are of the upper yield point-lower yield point type, where an upper yield point and a lower yield point appear when a stress-strain diagram is obtained by performing a tensile test to evaluate the tensile properties, and the value of the upper yield point is calculated as the yield strength. It is used as. The inventors conducted the study by paying attention to the difference between the upper and lower yield points. In addition, even if the chemical composition and manufacturing conditions are optimized to improve low-temperature toughness and high-heat input welding characteristics by setting the difference between the upper and lower yield points to a small value of 30 MPa or less, the tensile strength required for steel sheets for tanks loaded with ammonia is reduced. It was found that both the stress range and the yield strength range could be satisfied.

In order to reduce the difference between the upper and lower yield points, the composition is not only optimized for the content of each element, but also controlled to satisfy a predetermined relationship for the content of Si, P, and Al, as described in detail.

Then, when hot rolling the steel material whose composition is controlled in this way by heating it to an appropriate temperature, the reduction rate is controlled for each temperature range, the hot rolling end temperature is set to an appropriate range, and additionally slowly cooled to the Ar ₃ point, and then Ar ₃ By accelerating cooling at a predetermined cooling rate between the accelerated cooling start temperature lower than the starting temperature and the predetermined accelerated cooling end temperature, the difference between the upper yield point and the lower yield point can be made very small, at 30 MPa or less.

By optimizing the composition and rolling conditions in this way, the reason why the difference between the upper and lower yield points can be reduced to 30 MPa or less is thought to be because the mobile dislocation density in ferrite can be increased. Although the technical idea of increasing the movable dislocation density in ferrite is believed to be valid based on current knowledge, it is difficult to measure the movable dislocation density only in the ferrite portion in some cases, and this is intended to limit the technical scope of the present invention. Please note that this is not the case.

In order to further improve the high-heat input welding characteristics (low-temperature toughness of the heat-affected zone of the weld by high-heat input welding) of the steel sheet (thick steel sheet) obtained through hot rolling under the above-mentioned conditions, in addition to controlling the content of individual elements, the Ca amount and It was discovered that it was necessary to manage the O amount ratio within an appropriate range, and a steel plate according to an embodiment of the present invention was obtained.

Below, details of embodiments of the present invention are given.

<1. Chemical Composition>

The steel sheet according to the embodiment of the present invention includes C: 0.04 to 0.16 mass%, Si: 0.10 to 0.50 mass%, Mn: 0.60 to 1.60 mass%, P: 0.005 to 0.030 mass%, Al: 0.015 to 0.050 mass%, Ti : 0.005 to 0.020 mass%, Ca: 0.0005 to 0.0025 mass%, O: 0.0008 to 0.0025 mass%, B: 0.0005 to 0.0020 mass%, N: 0.0030 to 0.0060 mass%.

Hereinafter, each element will be described in detail.

〔1-1. Basic ingredients]

(C: 0.04 to 0.16 mass%)

C is an element that increases the strength of steel, and its content of 0.04% by mass or more is required to ensure the desired high strength. On the other hand, if it contains more than 0.16% by mass, low-temperature toughness will decrease. For this reason, C was defined to be in the range of 0.04 to 0.16 mass%. In order to easily ensure strength, the lower limit of the preferable C content is 0.06% by mass. In addition, the upper limit of the C content is preferably 0.08% by mass in order to achieve higher toughness.

(Si: 0.10 to 0.50 mass%)

Si needs to be contained in an amount of 0.10% by mass or more in order to suppress the formation of cementite and increase the mobile dislocation in ferrite. On the other hand, if it is contained in a large amount exceeding 0.50% by mass, the high heat input welding characteristics deteriorate. For this reason, Si was specified in the range of 0.10 to 0.50 mass%. The lower limit of Si content is preferably 0.20 mass% in order to increase its contribution to introduction of movable dislocations. In addition, the upper limit of the Si content is preferably 0.30% by mass in order to further increase the joint toughness (improve the high heat input welding characteristics).

(Mn: 0.60 to 1.60 mass%)

In order to contribute to increasing strength, Mn needs to be added in an amount of 0.60% by mass or more. On the other hand, the addition of excessive Mn deteriorates the high heat input welding characteristics. For this reason, the Mn content was defined to be in the range of 0.60 to 1.60 mass%. The lower limit of the Mn content is preferably 1.35% by mass in order to further contribute to increasing strength. In addition, the upper limit of the Mn content is preferably 1.45% by mass in order to further improve the high heat input welding characteristics.

(P: 0.005 to 0.030 mass%)

P is an impurity element that is inevitably contained in steel, but is controlled to a narrower range than the range normally permitted as an impurity element for the following reasons.

If the P content is 0.005% by mass or more, cementite formation is suppressed and the mobile dislocation in ferrite increases. However, because it deteriorates low-temperature toughness, it is set to 0.030 mass% or less. The upper limit of the P content is preferably 0.01% by mass in order to further suppress deterioration of low-temperature toughness.

(Al: 0.015 to 0.050 mass%)

Al suppresses the formation of cementite, contributes to an increase in the moving potential in ferrite, and also contributes to improving cleanliness as a deoxidizer. In order to exert both effects, addition of 0.015% by mass or more is required, but if added in excess of 0.050% by mass, cleanliness decreases. For this reason, the Al content was defined to be in the range of 0.015 to 0.050 mass%.

(Ti: 0.005 to 0.020 mass%)

Ti precipitates as TiN during solidification of steel, suppresses the coarsening of austenite in the heat-affected zone of the weld during high-heat input welding, and serves as a nucleus for ferrite transformation, contributing to solidification. To ensure high heat input welding characteristics, the Ti content is set to 0.005% by mass or more. However, if the Ti content exceeds 0.020% by mass, it causes low ignition due to coarsening of TiN, so it was set at 0.005 to 0.020% by mass. The lower limit of the Ti content is preferably 0.009% by mass in order to further contribute to the improvement of high heat input welding characteristics. In addition, the upper limit of the Ti content is preferably 0.015% by mass in order to increase phosphorization.

(Ca: 0.0005∼0.0025% by mass)

Since Ca is effective in suppressing coarsening of crystal grains in the heat-affected zone of welding including high-heat input welding, its content is required to be 0.0005% by mass or more. On the other hand, if the Ca content is excessive, it causes low ignition due to a decrease in cleanliness, so it was set to 0.0025% by mass or less.

(O: 0.0008∼0.0025% by mass)

O combines with Ca in the heat-affected zone of the weld to suppress the creation of coarse TiN, and is effective in improving the low-temperature toughness (high-heat-input welding characteristics) of the heat-affected zone of the weld during high-heat input welding. In order to effectively exhibit this effect, an O content of 0.0008% by mass or more is required. On the other hand, if the O content is excessive, it causes low ignition due to a decrease in cleanliness, so it was set to 0.0025% by mass or less.

(B: 0.0005 to 0.0020 mass%)

In order to ensure toughness (low-temperature toughness) by suppressing coarsening of the grain size of the weld heat-affected zone during high-heat input welding, the B content is set to 0.0005% by mass or more. On the other hand, when the B content becomes excessive, toughness decreases. For this reason, the B content was set to 0.0005 to 0.0020 mass%. The lower limit of the B content is preferably 0.0009% by mass in order to contribute to higher ignition. In addition, the upper limit of the B content is preferably 0.0015% by mass in order to achieve higher phosphorization.

(N: 0.0030∼0.0060% by mass)

Since N reduces toughness in a solid solution state, its content needs to be 0.0060% by mass or less. On the other hand, since there is also an effect of forming AlN and refining the crystal grains, it is necessary to set it to 0.0030% by mass or more. For this reason, the N content was set to 0.0030 to 0.0060 mass%.

In addition, in order to further increase low-temperature toughness, the upper limit of the N content is preferably set to 0.0050% by mass.

(Relationship that must be satisfied by Si content, P content, and Al content)

The difference between the upper and lower yield points becomes smaller as the moving potential in the ferrite increases. In the ferrite-pearlite two-phase structure, if there are many elements that are difficult to dissolve in cementite, the pearlite transformation temperature shifts to the low temperature side, and more movable dislocations due to transformation expansion are introduced into the ferrite. The parameters shown on the left side of equation (1) below are the amounts of movable dislocations introduced by pearlite transformation, derived by considering the diffusion coefficients of each element for Si, P, and Al, which are elements that are difficult to dissolve in cementite. It is an indicator corresponding to . In order to sufficiently reduce the difference between the upper yield point and the lower yield point to 30 MPa or less, the parameter is set to 0.63 or more as shown in equation (1).

3[Si]+6.3[P]+1.4[Al]≥0.63 (1)

Here, [Si], [P], and [Al] are the contents of Si, P, and Al expressed in mass%, respectively.

On the other hand, in order to further increase the operating potential, the parameter on the left side of equation (1) is preferably 0.74 or more (i.e., 3[Si]+6.3[P]+1.4[Al]≥0.74).

(Relationship that Ca content and O content must satisfy)

Since CaO suppresses the formation of coarse TiN during solidification of steel, it contributes to improving the low-temperature toughness (large heat input welding characteristics) of the heat-affected zone of the weld during high-heat input welding.

The parameter shown on the left side of equation (2) is an index corresponding to the amount of CaO. In order to demonstrate the effect of improving high heat input welding characteristics, this parameter is set to 0.2 or more.

[Ca]/[O]≥0.2 (2)

Here, [Ca] and [O] are the contents of Ca and O expressed in mass%, respectively.

On the other hand, in order to achieve a greater effect of improving the high heat input welding characteristics, it is preferable that the parameter on the left side of equation (2) is 0.3 or more (i.e., [Ca]/[O] ≥ 0.3).

The basic ingredients are as described above, and in one preferred embodiment, the balance is iron and inevitable impurities. As unavoidable impurities, mixing of elements depending on the circumstances of raw materials, materials, manufacturing facilities, etc. is allowed. A representative unavoidable impurity includes S, and there is no problem even if it contains 0.05% by mass or less. Examples of impurity elements other than S include As, Sn, Sb, and H.

On the other hand, for example, P is often treated as an unavoidable impurity element, but there are elements whose composition range is separately specified as above. For this reason, in this specification, the term "inevitable impurities" constituting the remainder is a concept excluding elements whose composition ranges are separately specified.

〔1-2. Selective addition elements]

Additionally, in another preferred embodiment of the present invention, elements other than those described above may be added as needed, as long as they do not impair the function according to the embodiment of the present invention. Examples of the content of such selectively added elements include V: 0.003 to 0.50 mass%, Nb: 0.003 to 0.020 mass%, Cu: 0.05 to 0.25 mass%, Ni: 0.05 to 0.25 mass%, Cr: 0.05 to 0.05 mass%. One or more types selected from the group consisting of 0.25% by mass, Mo: 0.05 to 0.25% by mass, Zr: 0.0001 to 0.010% by mass, Mg: 0.0001 to 0.010% by mass, and REM: 0.0001 to 0.010% by mass.

The properties of steel are further improved depending on the elements contained in these optional elements. The effect of each optional element is shown below.

(V: 0.003 to 0.50 mass%)

V is a useful element for securing high strength by increasing hardenability. In order to exert the effect, the V content is set to 0.003% by mass or more. The V content is preferably 0.01 mass% or more. However, if it is contained in excess, the HAZ toughness during high heat input welding deteriorates, so the V content is set to 0.50% by mass or less. The V content is preferably 0.25 mass% or less.

(Nb: 0.003 to 0.020 mass%)

Nb is an element that has the effect of refining ferrite grains through the effect of suppressing recrystallization of austenite grains. In order to obtain the effect, Nb is contained in an amount of 0.003% by mass or more. The Nb content is preferably 0.008 mass% or more. On the other hand, when the Nb content is excessive, the toughness decreases, so the upper limit was set to 0.020 mass%. The Nb content is preferably 0.018 mass% or less.

(Cu: 0.05 to 0.25 mass%)

Cu is an element effective in improving strength. In order to achieve this effect, the Cu content is set to 0.05% by mass or more. The Cu content is preferably 0.10 mass% or more. If the Cu content is too large, cracks are likely to occur during hot working, so the Cu content is set to 0.25 mass% or less, preferably 0.20 mass% or less.

(Ni: 0.05 to 0.25 mass%)

Ni is a useful element for ensuring good low-temperature toughness in a steel sheet and improving both the strength and low-temperature toughness properties of the steel sheet. In order to exhibit the effect, the Ni content is set to 0.05% by mass or more. The Ni content is preferably 0.10 mass% or more, more preferably 0.15 mass% or more. On the other hand, if the Ni content is excessive, the balance between the effects of Ni on strength and toughness is lost, the effect of increasing strength is superior to the effect of suppressing ductile fracture at low temperatures, and low-temperature toughness is deteriorated. It should be less than 0.25% by mass. The Ni content is preferably 0.20 mass% or less.

(Cr: 0.05 to 0.25 mass%)

Cr is an element effective in increasing the strength of steel sheets, and its effect increases as its content increases. However, in order to effectively exhibit this effect, it is contained in an amount of 0.05% by mass or more. The Cr content is preferably 0.10 mass% or more. However, if the Cr content is excessive, the strength will increase excessively and the toughness of the HAZ including the base material and the high heat input weld zone will deteriorate, so the Cr content is suppressed to 0.25% by mass or less. The Cr content is preferably 0.20 mass% or less.

(Mo: 0.05 to 0.25 mass%)

Mo is an element effective in increasing the strength of steel sheets, and its effect increases as its content increases. However, in order to effectively exhibit this effect, it is contained in an amount of 0.05% by mass or more. The Mo content is preferably 0.10 mass% or more. However, if the Mo content is excessive, the strength will increase excessively and the toughness of the HAZ including the base material and the high heat input weld zone will deteriorate, so the Mo content is suppressed to 0.25% by mass or less. The Mo content is preferably 0.20 mass% or less.

(Zr: 0.0001∼0.010% by mass)

Zr, like Ti, forms nitride and is effective in improving high heat input welding characteristics. The lower limit of the Zr content is set to 0.0001 mass%, preferably 0.0005 mass%, and more preferably 0.0010 mass% to ensure the above effect. On the other hand, if the Zr content is excessive, cleanliness decreases. Therefore, the Zr content is set to 0.010 mass% or less, preferably 0.005 mass% or less, and more preferably 0.003 mass% or less.

(Mg: 0.0001 to 0.010 mass%)

Mg is effective in preventing coarsening of crystal grains in the HAZ by forming oxides, sulfides, oxysulfides, etc. The lower limit of the Mg content is 0.0001 mass%, preferably 0.0005 mass%, and more preferably 0.0010 mass% in order to ensure the above effect. On the other hand, when the Mg content becomes excessive, cleanliness decreases. Therefore, the Mg content is set to 0.010 mass% or less, preferably 0.005 mass% or less, and more preferably 0.003 mass% or less.

(REM: 0.0001∼0.010% by mass)

REM (rare earth elements) are effective in preventing coarsening of grains in the HAZ by forming oxides, sulfides, and oxysulfides. The lower limit of the REM content is set to 0.0001 mass%, preferably 0.0005 mass%, and more preferably 0.0010 mass% in order to ensure the above effect. On the other hand, when the REM content becomes excessive, cleanliness decreases. Therefore, the REM content is set to 0.010 mass% or less, preferably 0.005 mass% or less, and more preferably 0.003 mass% or less.

REM content means the total content of 17 elements in total, including 2 elements Sc and Y, and 15 elements from La to Lu, and containing REM means containing one or more elements selected from these 17 elements. .

<2. Metal structure>

The steel sheet according to the embodiment of the present invention has a portion where the distance in the sheet thickness direction from the surface is one fourth of the sheet thickness (hereinafter, it may simply be referred to as the “plate thickness direction t/4 position”. Here, t is The metal structure (plate thickness) contains a total of 90% or more of ferrite structure and pearlite structure in terms of area ratio. And, the remaining structure is one or both of the island-like martensite structure and the bainite structure.

The reason why the position for evaluating the metal structure was set at a quarter position from the surface in the direction of the sheet thickness is because this part is generally considered to be a representative position for showing the metal structure of a steel sheet. From the viewpoint of suppressing excessive strength increase, it is necessary to make the area ratio of the ferrite structure and pearlite structure more than 90%. On the other hand, the fraction of the ferrite structure and the pearlite structure is not particularly limited, and for example, there is no problem if the ferrite structure alone accounts for 90% or more of the entire metal structure in terms of area ratio.

<3. Difference between upper hangbok branch and lower hangbok branch>

As described above, the steel sheet according to the embodiment of the present invention has excellent low-temperature toughness and excellent high heat input welding characteristics by controlling the ratio of Ca and O within a predetermined range in addition to the composition of each element and the manufacturing method described later. is getting And, while satisfying these characteristics, it is also possible to satisfy both the above-mentioned tensile stress range (440 MPa or more, 560 MPa or less) and yield strength range (325 MPa or more, 440 MPa or less) required for steel plates for tanks loaded with ammonia. The difference between the yield point and the lower yield point is set to 30 MPa or less.

The upper yield point and lower yield point can be obtained by performing a tensile test according to JIS Z 2241, as detailed in the examples.

On the other hand, it is known that there are roughly two types of stress-strain curves obtained by tensile testing of steel materials. First, when strain is applied, the stress increases in proportion to the strain, and when the upper yield point is reached, the stress decreases, and a yield shelf appears, which is an area where the stress does not increase even if the strain increases. The stress at this time is called the lower yield point. It is a surrender-yield type. On the other hand, it is a round curve type in which strength increases continuously with increase in strain without a clear upper yield point, lower yield point, or yield shelf being observed.

In the round curve type, since there is no clear yield point, the yield strength of 0.2%, which is the stress at 0.2% deformation, is usually used as the yield strength. Since the 0.2% yield strength is set as the yield strength in this way and the clear upper and lower yield points do not appear, in the round curve type, both the upper and lower yield points are equal to the 0.2% yield strength, and therefore the upper and lower yield points are equal to the yield strength. There may also be a way of thinking that considers the difference in yield point to be zero. However, the stress continues to rise in the deformation range immediately after the 0.2% proof stress (i.e., the area where the amount of deformation slightly exceeds 0.2%), which may cause the problem of deterioration of the stress corrosion cracking resistance. For this reason, the steel plate according to the embodiment of the present invention is a steel plate in which the stress-strain curve obtained by the tensile test is of the upper yield-lower yield type and the difference between the upper yield point and the lower yield point is as small as 30 MPa or less. And, such a steel sheet can be obtained by controlling the chemical composition as described above and applying the following manufacturing method.

<4. Manufacturing method>

As described in detail below, steel for machine structures for cold working is prepared by preparing steel materials such as cast plates with a predetermined composition, heating this steel material to an appropriate temperature, and hot rolling it, resulting in a reduction rate for each temperature range. Controlling, the hot rolling end temperature is set to an appropriate range, additionally slowly cooled to the Ar ₃ point, and then cooled at a predetermined cooling rate from a predetermined accelerated cooling start temperature lower than the Ar ₃ point to a predetermined accelerated cooling end temperature. It can be manufactured by accelerated cooling.

[4-1. [Preparation of steel with predetermined chemical composition]

In order to be used for hot rolling in the following rolling process, the above-described “1. Prepare a steel material having the composition shown in “Chemical Composition.” The steel may be a steel commonly used for hot rolling of thick plates. Cast steel can be cited as such a steel material. Examples of cast steel include slabs obtained using a continuous casting method and ingots obtained by an ingot method using a mold. If necessary, these slabs and ingots may be subjected to surface treatment, heat treatment, processing, etc. to become steel materials for rolling. Additionally, it is preferable that the tundish used when performing continuous casting is a hot reusable tundish. This is because the O concentration in molten steel can be easily reduced.

[4-2. rolling]

The above-mentioned steel material is hot rolled.

(heating)

First, the steel material is heated to 1000°C to 1150°C. If the heating temperature exceeds 1150°C, the austenite grain size becomes coarse and toughness decreases, while if the heating temperature is lower than 1000°C, it becomes difficult to secure the rolling temperature described later, making it impossible to obtain the desired properties.

(Hot Rolled)

The heated steel is hot rolled. In order to refine the austenite grain size for the purpose of improving low-temperature toughness and high heat input welding characteristics, the reduction rate in the recrystallization region is 30% or more and the reduction rate in the non-recrystallization region is 10% or more for each temperature range. Control. The above-mentioned “1. In the composition system of the steel sheet according to the embodiment of the present invention described in “Chemical Component Composition”, the recrystallization zone is 820°C or higher, and the non-recrystallization zone is 790°C or higher but less than 820°C (the lower limit of 790°C is the lower limit temperature that is definitely the austenite region). . Therefore, the reduction ratio in the temperature range above 820°C is set to 30% or more, and the reduction ratio in the temperature range above 790°C and below 820°C is set to 10% or more.

These reduction rates may be achieved through one pass of rolling, or may be achieved as the total reduction amount of multiple passes of rolling. In addition, the temperature of the temperature range referred to here corresponds to the temperature of the steel material on the entrance side of the rolling roll.

Meanwhile, the recrystallization zone temperature was determined by the following method. A test piece of ϕ12 In the stress-strain curves obtained by rolling at various rolling temperatures, the temperature range where the strength increase due to work hardening does not monotonically increase was taken as the recrystallization temperature range.

In addition, the hot rolling completion temperature (temperature of the steel material at the rolling roll exit side in the final rolling pass) is set to 750°C or higher. This is to avoid excessive increase in strength and to avoid deterioration of low-temperature toughness.

On the other hand, as long as the hot rolling end temperature of 750°C or higher is satisfied, in addition to the rolling in the above-mentioned temperature range of 790°C or higher to 820°C, rolling in a temperature range of less than 790°C may or may not be performed.

(Cooling after rolling)

Immediately after completion of hot rolling, the steel material (rolled steel sheet) is cooled under the following conditions.

From the hot rolling end temperature to the accelerated cooling start temperature, cooling is performed at an average cooling rate of 0.5 to 3.0° C./sec by, for example, slow cooling means such as air cooling. This is to suppress the formation of bainite in the surface layer of the steel sheet. The accelerated cooling start temperature is lower than the Ar ₃ point and is a temperature between the Ar ₃ point and 150°C or higher (a temperature lower than the Ar ₃ point and within 150°C from the Ar ₃ point).

Subsequently, cooling (accelerated cooling) is performed between the accelerated cooling start temperature and the accelerated cooling end temperature set between 500°C and 650°C at an average cooling rate of 4 to 9°C/sec.

If the accelerated cooling start temperature is lower than the Ar ₃ point - 150°C, coarse proeutectoid ferrite is precipitated, resulting in low strength. For this reason, the accelerated cooling temperature was set to a temperature of Ar ₃ point - 150°C or higher ((Ar ₃ point - 150°C) or higher).

If the average cooling rate is less than 4°C/sec or the accelerated cooling end temperature is higher than 650°C, the density of movable dislocations in the ferrite is not sufficient and the difference between the upper and lower yield points is not sufficiently small. Additionally, if the average cooling rate is higher than 9°C/sec or the accelerated cooling end temperature is lower than 500°C, bainite is excessively generated and the strength becomes excessively high.

The accelerated cooling start temperature is preferably 50°C or more higher than the accelerated cooling end temperature. This is because if the difference between the accelerated cooling temperature and the accelerated cooling end temperature is 50°C or more, the effect of increasing strength by accelerated cooling increases.

The Ar ₃ point can be obtained using equation (3) below. In this case, if Cu, Cr, Ni, and Mo are contained only at impurity levels, their amounts may be calculated as 0.

Ar ₃ (℃)=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo] (3)

Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo expressed in mass%, respectively.

In order to further increase the density of movable dislocations in ferrite and to make the difference between the upper and lower yield points smaller, there are two means, the first means and the second means, shown below. One or both of these two means may be performed.

1st means:

After the above-described accelerated cooling, the temperature between 500°C and 200°C is cooled at an average cooling rate of 0.5°C to 4.0°C/sec. On the other hand, since the average cooling rate when air cooling is performed in this temperature range is 0.1 to 0.4°C/sec, this means cooling at a faster rate than normal air cooling.

Second method:

After the above-described accelerated cooling, the strain in the portion where the distance in the sheet thickness direction from the surface is one fourth of the sheet thickness (i.e., position t/4 in the sheet thickness direction, where t is the sheet thickness) is 0.2% or more. Plastic processing is performed to achieve this. Examples of such plastic processing include light compression using paired rolls such as upper and lower rolls, press processing, and strain introduction by tension.

This plastic working may be performed at any temperature, for example, warm or cold, as long as it is after accelerated cooling.

The first method is preferably carried out because it can be carried out relatively easily during cooling after hot rolling.

On the other hand, since the second means additionally performs plastic working, it is preferable not to use it when high productivity is to be secured. However, it is preferable to carry out this when it is important to obtain a higher mobile dislocation density in ferrite.

Meanwhile, the temperature of the steel material described during the rolling process described above may be measured, for example, with a non-contact thermometer such as a radiation thermometer, or may be measured with a contact thermometer such as a thermocouple. Additionally, it may be confirmed by simulation or the like.

Example

Hereinafter, the present invention will be described in more detail through examples. The present invention is not limited by the following examples, and can be implemented with appropriate changes within the scope consistent with the spirit described above and below, and all of them are included in the technical scope of the present invention.

1. Sample production

Steel with the component composition shown in Table 1 was melted using a converter solvent. Cast steel (slabs) were obtained through continuous casting using these molten steels.

Table 1 shows the contents of the basic components C, Si, Mn, P, Al, Ti, Ca, O, B and N, the optional components V, Nb, Cu, Ni, Cr and Mo, and the inevitable impurity S. indicated. The remainder is inevitable impurities other than Fe and S. The description “-” in the table indicates that it contains only an impurity level or less. Table 1 additionally shows the value of the left side of equation (1), the value of the left side of equation (2), and the Ar ₃ point obtained using equation (3). Additionally, values that deviate from the embodiments of the present invention are underlined.

The obtained cast steel was heated to 1000 to 1150°C and then rolled to obtain a steel sheet sample (hot rolled sheet) with a sheet thickness of 12 to 16 mm. Rolling conditions, more specifically, reduction rate in the temperature range of 820°C or higher, reduction rate in the temperature range of 790°C or higher and lower than 820°C (hot rolling in the temperature range below 790°C is not performed), hot rolling Rolling end temperature (FRT), average cooling rate from hot rolling end temperature to accelerated cooling start temperature, accelerated cooling start temperature, accelerated cooling end temperature, average cooling rate during accelerated cooling (from accelerated cooling start temperature to accelerated cooling end temperature) The average cooling rate), the cooling rate after accelerated cooling, and the presence or absence of strain introduction (if strain is introduced, the amount of strain is also listed) are shown in Table 2. In Table 2, the Ar ₃ viscosity shown in Table 1 is also listed for reference.

Meanwhile, in Table 2, conditions that deviate from the conditions of the embodiment of the present invention are underlined.

Air cooling was performed from the hot rolling end temperature to the accelerated cooling start temperature.

Steel plate sample no. In cases 6 and 7, accelerated cooling was not performed and air cooling was performed from the hot rolling end temperature to the temperature described in the “Accelerated Cooling End Temperature” column. For this reason, steel plate sample no. In 6 and 7, “accelerated cooling completion temperature” means the temperature at which “cooling after accelerated cooling” started. In addition, since air cooling was performed in this way, steel sheet sample No. 6 and 7 mean that the average cooling rate in the temperature range corresponding to accelerated cooling of other samples is significantly lower than 4.0°C/sec.

The average cooling rate after accelerated cooling is the average cooling rate between 500℃ and 200℃. Samples with an average cooling rate of "air cooling" after accelerated cooling mean that the average cooling rate after accelerated cooling was within the range of 0.1 to 0.4℃/sec. do.

In addition, in the sample (sample 3) to which strain was introduced after accelerated cooling, after cooling to room temperature, the plasticity rate in the sheet thickness direction at room temperature by upper and lower rolls was 1st pass: 85%, 2nd pass: 75%, 3rd pass. Third: Plastic working was performed so that the strain at the t/4 position in the sheet thickness direction was 1.5% by performing light reduction with the reduction setting to 65%. Meanwhile, the value of introduced strain was derived by comparing the stress-strain curves from tensile tests before and after introduction.

2. Sample evaluation

For each steel plate, structure observation, tensile test, Charpy impact test, and HAZ toughness evaluation were performed using the methods detailed below.

<Observation of metal structure>

For each steel plate sample, at the same plate thickness t/4 position as the impact test specimen collection location described later, an area with a field of view of 600 μm x 800 μm was observed using an optical microscope at a magnification of 100 times, and image analysis software was used to observe the area. , the area ratios of ferrite and pearlite were measured. In addition, for samples in which the total area ratio of ferrite + pearlite was not 100%, the structure of the remaining part, that is, other than ferrite and pearlite, was also confirmed.

<Tensile test>

From the plate thickness t/4 position of each steel sheet sample, test piece No. 4 (round bar shape) of JIS Z 2241 is taken perpendicular to the rolling direction and sheet thickness direction (collected so that the central axis of the round bar is at the plate thickness t/4 position), or A tensile test was performed according to JIS Z 2241 using the No. 1B test piece (flat plate tensile, full thickness sampling) to obtain a stress-strain curve. Then, based on this stress-strain curve, the upper and lower yield points determined in JIS Z 2241 were obtained. The difference between the upper and lower yield points obtained for each sample is shown in Table 3.

Tensile stress (tensile strength) was also determined from the obtained stress-strain curve. Additionally, the upper yield point was taken as the yield strength. Tensile stress and yield strength are also shown in Table 3. The tensile stress was 440 MPa or more and 560 MPa or less was considered acceptable, and the yield strength was 325 MPa or more and 440 MPa or less was acceptable.

<Charpy impact test>

A full-size Charpy impact test piece (V-notch test piece of JIS Z 2202) is taken from each steel sheet sample so that the central axis of the test piece is at a depth of 6 mm from the surface of the steel sheet sample and the longitudinal direction of the test piece is parallel to the rolling direction. Three copies were collected. The obtained Charpy impact test piece was subjected to a Charpy impact test at -40°C, and the absorbed energy vE _-40°C was measured. The average value (vE _-40°C (ave.)) of the Charpy impact test measurement results for each of these three samples is shown in the “vE _-40°C ” column in Table 3. Low-temperature toughness was judged to be sufficient when vE _-40℃ (ave.) exceeded 200J.

<HAZ toughness evaluation>

A test piece of 55 mm (rolling direction) (Collected to reach position 2). After holding the obtained test piece at 1460°C for 60 seconds, it was cooled by controlling the speed so that the cooling time from 800°C to 500°C was 340 seconds. This is a heat cycle simulating the case when single-sided SAW welding is performed with a large heat input (approximately 9 kJ/mm). From these test pieces, three full-size Charpy impact test pieces (V-notch test pieces of JIS Z 2202) were taken each, a Charpy impact test was performed at -20°C, and the absorbed energy vE _-20°C was measured. The average value (vE _-20°C (ave.)) of the Charpy impact test measurement results of three samples each is shown in the "HAZ toughness vE _-20°C " column in Table 3. HAZ toughness (high heat input welding characteristics) was judged to be sufficient when HAZ toughness vE _-20°C exceeded 30J.

Meanwhile, HAZ toughness evaluation was conducted using steel plate sample No. This was done only for 1, 2, 4 and 8. Steel plate sample no. 2 and steel plate sample no. 3 differs only in the presence or absence of modifications in terms of chemical composition and manufacturing conditions. That is, since the chemical composition that affects HAZ toughness (high heat input welding characteristics) and the reduction ratio during rolling are the same conditions, steel sheet sample No. The HAZ toughness of 3 is that of steel plate sample no. I think it is the same as 2.

Among the characteristics shown in Table 3, those that deviate from the embodiments of the present invention are underlined.

From Tables 1 to 3, it can be considered as follows.

Steel plate sample no. 2, 3, and 4 all satisfy all of the requirements for chemical composition and manufacturing conditions specified in the embodiment of the present invention. As a result, as shown in Table 3, the requirements for metal structure and the difference between the upper and lower yield points are satisfied, and all of the tensile stress, yield strength, low temperature toughness, and high heat input welding characteristics are excellent.

Steel plate sample no. 1, due to not satisfying the chemical composition regulations, the tensile stress and yield strength satisfy the regulations despite the large difference between the upper and lower yield points, but the high heat input welding characteristics are inferior. In addition, not satisfying equation (2) is also the cause of low high heat input welding characteristics.

Steel plate sample no. 5, the reduction ratio at temperatures above 780°C and below 820°C is insufficient, and as a result, low-temperature toughness is poor.

Steel plate sample no. 6, the hot rolling end temperature is too low, and the cooling rate during accelerated cooling is slow. As a result, the difference between the upper and lower yield points increases, resulting in excessive yield strength and poor low-temperature toughness.

Steel plate sample no. 7, the cooling rate during accelerated cooling is excessive. As a result, the difference between the upper and lower yield points increases, causing the yield strength to become excessive.

Steel plate sample no. 8 does not contain B. As a result, the high heat input welding characteristics are inferior.

Steel plate sample no. 9, the cooling rate during accelerated cooling is excessive. As a result, an excessive bainite structure is formed, and the tensile stress and yield strength become excessive. Additionally, steel plate sample no. 9 does not contain B.

This application claims priority based on Japanese Patent Application No. 2021-120011, the filing date of which is July 20, 2021. Japanese Patent Application No. 2021-120011 is incorporated herein by reference.

Claims (4)

C: 0.04 to 0.16 mass%,
Si: 0.10 to 0.50 mass%,
Mn: 0.60 to 1.60 mass%,
P: 0.005 to 0.030 mass%,
Al: 0.015 to 0.050 mass%,
Ti: 0.005 to 0.020 mass%,
Ca: 0.0005 to 0.0025 mass%,
O: 0.0008 to 0.0025 mass%,
B: 0.0005 to 0.0020 mass%,
N: Contains 0.0030 to 0.0060% by mass,
The balance consists of Fe and inevitable impurities,
Expressions (1) and (2) below are satisfied,
The metal structure at the position t/4 in the sheet thickness direction, which is the portion where the distance in the sheet thickness direction from the surface is one fourth of the sheet thickness, contains a total of 90% or more of ferrite structure and pearlite structure in terms of area ratio, and the remaining It consists of one or more of the island-like martensite structure and bainite structure,
A steel plate where the difference between the upper and lower yield points obtained by a tensile test is 30 MPa or less.
3[Si]+6.3[P]+1.4[Al]≥0.63 (1)
Here, [Si], [P], and [Al] are the contents of Si, P, and Al expressed in mass%, respectively.
[Ca]/[O]≥0.2 (2)
Here, [Ca] and [O] are the contents of Ca and O expressed in mass%, respectively. According to claim 1,
V: 0.003 to 0.50 mass%, Nb: 0.003 to 0.020 mass%, Cu: 0.05 to 0.25 mass%, Ni: 0.05 to 0.25 mass%, Cr: 0.05 to 0.25 mass%, Mo: 0.05 to 0.25 mass%, Zr: A steel plate further comprising at least one selected from the group consisting of 0.0001 to 0.010 mass%, Mg: 0.0001 to 0.010 mass%, and REM: 0.0001 to 0.010 mass%. C: 0.04 to 0.16 mass%,
Si: 0.10 to 0.50 mass%,
Mn: 0.60 to 1.60 mass%,
P: 0.005 to 0.030 mass%,
Al: 0.015 to 0.050 mass%,
Ti: 0.005 to 0.020 mass%,
Ca: 0.0005 to 0.0025 mass%,
O: 0.0008 to 0.0025 mass%,
B: 0.0005 to 0.0020 mass%,
N: Contains 0.0030 to 0.0060% by mass,
The balance consists of Fe and inevitable impurities,
A process of preparing steel that satisfies the following equations (1) and (2),
After heating the steel material to 1000-1150°C, the reduction rate in the temperature range of 820°C or higher is 30% or more, the reduction rate in the temperature range of 790°C or more and less than 820°C is 10% or more, and the hot rolling end temperature is 750°C. Hot rolling is carried out at ℃ or higher, and then cooled at an average cooling rate of 0.5 to 3.0 ℃/sec to the accelerated cooling start temperature, which is lower than the Ar ₃ point and is a temperature of Ar ₃ point - 150 ℃ or higher, as shown in equation (3) below, A rolling process of cooling at an average cooling rate of 4 to 9°C/sec from the accelerated cooling start temperature to the accelerated cooling end temperature of 500°C or more and 650°C or less.
Including,
The metal structure at the position t/4 in the sheet thickness direction, which is the portion where the distance in the sheet thickness direction from the surface is one fourth of the sheet thickness, contains a total of 90% or more of ferrite structure and pearlite structure in terms of area ratio, and the remaining A method of manufacturing a steel plate composed of one or more of an additional island-like martensite structure and a bainite structure, and where the difference between the upper and lower yield points obtained by a tensile test is 30 MPa or less.
3[Si]+6.3[P]+1.4[Al]≥0.63 (1)
Here, [Si], [P], and [Al] are the contents of Si, P, and Al expressed in mass%, respectively.
[Ca]/[O]≥0.2 (2)
Here, [Ca] and [O] are the contents of Ca and O expressed in mass%, respectively.
Ar ₃ (℃)=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo] (3)
Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo expressed in mass%, respectively. According to claim 3,
The steel materials include V: 0.003 to 0.50 mass%, Nb: 0.003 to 0.020 mass%, Cu: 0.05 to 0.25 mass%, Ni: 0.05 to 0.25 mass%, Cr: 0.05 to 0.25 mass%, Mo: 0.05 to 0.25 mass%. %, Zr: 0.0001 to 0.010 mass %, Mg: 0.0001 to 0.010 mass %, and REM: 0.0001 to 0.010 mass %.