JP7077801B2 - Low yield specific thickness steel sheet - Google Patents

Description

The present invention relates to a low yield specific thickness steel sheet, for example, to a tank steel sheet used for a tank capable of carrying liquefied petroleum gas (LPG) and liquid ammonia in a mixed manner.

With the expansion of energy demand in recent years, it is required to increase the capacity of the tank of the energy transport ship, and the demand for increasing the strength and thickness of the steel plate used for the tank is increasing. In addition, the steel plate used as a material for tanks such as LPG and ammonia has excellent low-temperature toughness for transporting liquefied gas, and has a low yield ratio (YR: yield) to suppress stress corrosion cracking caused by ammonia. Strength (YS) / tensile strength (TS)) is also required. In general, low temperature toughness and stress corrosion cracking resistance deteriorate as the strength and yield ratio of the steel sheet increase, so it is difficult to achieve both high strength and high strength. Further, these steel sheets are welded to become a structure, and as the steel sheets become stronger and thicker, the toughness of the heat-affected zone (hereinafter referred to as "HAZ") due to welding due to welding (hereinafter referred to as "HAZ") is toughness (hereinafter referred to as "HAZ"). It is also difficult to secure "HAZ toughness").

So far, techniques for obtaining excellent low temperature toughness while having a low YR have been studied. For example, in Patent Document 1, after heating a slab having a predetermined chemical composition, hot rolling is completed at ₃ points or more of Ar, the steel sheet is allowed to cool to 620 to 720 ° C., and then accelerated cooling is performed to 350 to 450. Stop cooling at ° C. Further, in Patent Document 2, the slab is heated, hot rolled with a cumulative reduction amount of 30% or more in the unrecrystallized temperature range, stopped at 800 ° C. or higher, and then (Ar ₃ temperature -50 ° C.) or higher. Accelerated cooling is performed at 5 to 50 ° C./sec from the temperature of.

Japanese Unexamined Patent Publication No. 2008-261000 Japanese Unexamined Patent Publication No. 11-80832.

In both of the inventions of Patent Documents 1 and 2, in order to generate initial ferrite, the steel sheet is allowed to cool to a predetermined temperature after the completion of hot rolling, and then water cooling is started. Since the initial ferrite generated while the steel sheet is allowed to cool undergoes transformation mainly from the triple points of the grain boundaries, it is difficult to obtain a fine metal structure uniformly, and the required low temperature toughness of -60 ° C is achieved. It is difficult to secure. In addition, since it is considered that the cooling is carried out on the production line, the productivity is lowered if the cooling time is long. Further, in these patent documents, ensuring the HAZ toughness, which is a problem in welding work, is not examined at all.

The present inventors have studied to obtain a low yield specific thickness steel sheet having high strength, excellent stress corrosion cracking resistance and low temperature toughness, and also excellent HAZ toughness, which are used for large tanks such as LPG and liquid ammonia. Specifically, in order to solve the above technical problems, the present inventor has a composition range in which the tensile strength is 490 MPa or more, the yield ratio is low, the low temperature toughness is excellent, and the welded HAZ toughness is also excellent. And the manufacturing method was examined.

As a method of lowering YR, the metal structure of the steel plate is made into a mixed structure of a soft phase such as ferrite and a hard phase such as bainite and martensite, so that the soft phase is plastically deformed with low stress when a tensile load is applied. It is known that a microstructure design in which the ratio of yield strength to breaking strength is increased by increasing the breaking strength of the hard phase is effective. On the other hand, the hard phase becomes the starting point of fracture during the impact test, and it becomes difficult to secure low temperature toughness. Although grain refinement of ferrite is effective as a method for improving toughness even if a hard phase is contained, it is difficult to secure a low YR because the grain refinement causes an increase in yield stress. .. Therefore, in order to achieve both low YR and low temperature toughness, it is effective to disperse the hard phase while maintaining a fine ferrite structure. Generally, in order to obtain these fine ferrite structures, a means is used in which Nb is added and rolling is performed in the unrecrystallized region of austenite to increase the number of nucleation sites and make the structure finer. However, when the heat-affected zone is transformed into austenite by welding, the effect of rolling is lost, the structure becomes coarse, and the toughness decreases. In addition, the Nb that has been solid-solved due to the influence of welding heat increases the hardenability, thereby hardening the HAZ portion and lowering the toughness.

An object of the present invention is to provide a low yield specific thickness steel sheet having a tensile strength of 490 MPa or more, low YR and low temperature toughness, and excellent HAZ toughness by welding.

The present inventors have conducted repeated studies to solve the above technical problems, and in order to satisfy the HAZ toughness while ensuring the low YR and low temperature toughness of the steel sheet, the Nb content is reduced and the Ti is contained. By keeping the amount within a predetermined range and containing a predetermined amount of N, the Ti precipitate is utilized for suppressing the coarsening of γ grains by heating, and then the ferrite area ratio and the ferrite grain size in the steel sheet and their ferrite grain size are controlled by the rolling process control. It was newly found that it is effective to optimize the metal structure so that the number ratio is within a predetermined range.

First, in order to secure the desired tensile strength in the rolling process of the present invention and to obtain a range in which the low temperature toughness does not deteriorate, the hardenability defined by the International Welding Society (IIW) represented by the following (Equation 1) is obtained. As a result of examining the carbon equivalent (Ceq) which is an index, it is necessary to set Ceq to 0.30 to 0.40.
Ceq = [C%] + [Mn%] / 6 + ([Cu%] + [Ni%]) / 15 + ([Cr%] + [Mo%] + [V%]) / 5 ... (Equation 1)
In (Equation 1), the element symbol with [] represents the content (mass%) of each element.

Therefore, as a result of examining the composition and metal structure capable of ensuring low YR, low temperature toughness and HAZ toughness in this range, the following was clarified.

Nb is work-induced and precipitates during rolling to form inclusions, but these inclusions are melted during welding to become a solid solution Nb. The solid solution Nb hardens the metal structure in order to significantly improve the hardenability. On the other hand, during hot rolling, Nb expands the unrecrystallized region of austenite, and by rolling in that region, it has the effect of increasing nucleation sites, refining the structure, and improving the low temperature toughness of the steel sheet. From the viewpoint of ensuring HAZ toughness, in order to make the structure finer during rolling without using the effect of Nb, the rolling process control of the present invention is performed with Ti precipitates dispersed to disperse an appropriate ferrite grain size. By obtaining it, both low YR and low temperature toughness can be achieved.

In order to obtain the required dispersion of ferrite grain size without containing Nb, the slab is heated at a low temperature to suppress the formation of coarse γ grains and rolled, and a sufficient cumulative reduction rate is secured at 900 ° C or lower. Then, immediately after finish rolling, primary cooling is performed to increase the transformation driving force to increase the frequency of nucleation, and after adjusting the ferrite area ratio and particle size distribution between the primary cooling and the secondary cooling, 2 It was found that the desired metal structure can be obtained by performing the next cooling and stopping in the auto-temper temperature range. The present invention is as follows.

[1] The composition of the steel sheet is mass%,
C: 0.04 to 0.08%,
Si: 0.05-0.25%,
Mn: 0.8-1.6%,
P: 0.015% or less,
S: 0.005% or less,
Al: 0.015 to 0.05%,
Nb: less than 0.005%,
Ti: 0.008-0.020%,
N: 0.0045 to 0.0070%,
O: 0.0040% or less,
Cu: 0 to 0.40%,
Ni: 0 to 0.80%,
Cr: 0 to 0.20%,
Mo: 0 to 0.10%,
V: 0 to 0.08%,
B: 0 to 0.002%,
Ca: 0 to 0.005%,
The balance has a chemical composition consisting of Fe and impurities and has a chemical composition.
Ceq defined by the following equation 1 is 0.30 to 0.40.
The NBT defined by the following equation 2 is 0.010 to 0.020.
In the metal structure at the position of 1/4 of the plate thickness,
The ferrite area ratio is 40 to 70%,
In the ferrite particle size distribution,
The number ratio of ferrites less than 10 μm is 55 to 80%.
The number ratio of ferrites of 10 to 20 μm is 20 to 45%, and
The number ratio of ferrites exceeding 20 μm is 2% or less,
Ti precipitates present in the ferrite structure,
HAZ Low yield ratio thick steel sheet with excellent toughness.
Ceq = [C%] + [Mn%] / 6 + ([Cu%] + [Ni%]) / 15 + ([Cr%] + [Mo%] + [V%]) / 5 ... (Equation 1)
NBT = 2 × [Nb%] + [Ti%] ... (Equation 2)
In (Equation 1) and (Equation 2), the element symbol with [] represents the content (mass%) of each element.

If the requirements of the composition and the metal structure specified in the present invention are satisfied, the characteristics of the present invention can be obtained in the range of, for example, 16 to 40 mm in the plate thickness of the thick steel plate, but in order to secure the shape and the characteristics in the present invention. The preferred thickness range of the thick steel plate is 20 to 32 mm.

According to the present invention, the tensile strength is high at 490 MPa or more, the low temperature toughness at −60 ° C. is excellent, and the yield ratio can be low, so that the resistance to stress corrosion cracking is high. In addition, since the HAZ toughness of welding is excellent, welding conditions with high welding efficiency can be applied. Therefore, since it is possible to provide a steel material suitable for use in tanks such as LPG and ammonia, which are concerned about stress corrosion cracking, the industrial effect is extremely large.

It is a figure which shows the groove shape welded in order to evaluate the HAZ toughness in an Example. It is a figure which shows the test piece collecting procedure for evaluating the HAZ toughness in an Example.

Hereinafter, the present invention will be described in detail.
First, the reason for limiting the composition of the thick steel sheet according to the present invention as described above will be described in detail.

C: 0.04 to 0.08%
C is the most important element that determines the strength. Further, since it affects the hardness of the hard phase when the hard phase is dispersed, it is oriented toward low C. If the C content is less than 0.04%, it will be difficult to obtain the required strength. On the other hand, if it exceeds 0.08%, the hard phase is excessively hardened and the toughness is deteriorated, so this is set as the upper limit. In order to suppress deterioration due to the hard phase and stably secure low temperature toughness, C is preferably 0.04 to 0.07%.

Si: 0.05-0.25%
Si is an element effective for preliminary deoxidation of molten steel, and has an effect of improving strength without deteriorating toughness. Therefore, the Si content is set to 0.05% or more. On the other hand, if it is contained in excess of 0.25%, MA is generated in the HAZ portion and the toughness is deteriorated, so this is set as the upper limit. In order to suppress the formation of MA during welding and stably obtain HAZ toughness, it is preferable to set Si to 0.05 to 0.19%.

Mn: 0.8-1.6%
Mn improves the strength by enhancing the hardenability, and reduces the adverse effect of S by forming MnS, thereby improving the elongation and toughness of the tensile test. In order to obtain these effects, Mn needs to be contained in an amount of 0.8% or more. On the other hand, if it is contained in excess of 1.6%, segregation due to solidification becomes strong and a band structure of pearlite is likely to be formed, so that the low temperature toughness deteriorates, so this is the upper limit. As the Mn content increases, MnS is formed, so that the elongation becomes high, and it is preferable to set Mn to 1.2 to 1.6% in order to stably secure the elongation.

P: 0.015% or less P exists as an impurity and segregates at the grain boundaries to deteriorate the toughness and also causes tempering embrittlement. Therefore, it is desirable that the P content is as low as possible. If the P content exceeds 0.015%, the deterioration is remarkable, so the P content is limited to 0.015% or less.

S: 0.005% or less S exists as an impurity and deteriorates the ductility and toughness of the steel sheet, so its content is desirable to be as low as possible. If the content exceeds 0.005%, the adverse effect becomes remarkable, so the S content is limited to 0.005% or less.

Al: 0.015 to 0.05%
Al is an element added to obtain the cleanliness of molten steel. In order to obtain the effect, it is necessary to contain 0.015% or more of Al. On the other hand, if the Al content exceeds 0.05%, coarse alumina is generated and the toughness is deteriorated, and MA is generated in the welded HAZ portion and the HAZ toughness is deteriorated. In order to further suppress the deterioration of HAZ toughness due to MA formation, it is preferable to set Al to 0.015 to 0.03%.

Nb: Less than 0.005% Generally, Nb is an element effective for expanding the unrecrystallized region of austenite and contributes to microstructure miniaturization by rolling, so that toughness can be improved. Under the above process conditions, YP improvement due to microstructure miniaturization becomes remarkable, and it becomes difficult to reduce YR. Further, at the time of welding, the Nb precipitate is solid-dissolved to become a solid-dissolved Nb, which hardens the structure and deteriorates the HAZ toughness. When Nb is less than 0.005%, the dispersion of the hard phase is appropriately obtained, low YR is obtained, and there is no adverse effect on HAZ toughness. Therefore, Nb is set to less than 0.005%.

Ti: 0.008-0.020%
Ti forms a nitride and has an effect of suppressing coarsening of austenite crystal grains due to slab heating and welding heat, which contributes to improvement of toughness and HAZ toughness. In order to obtain the effect, it is necessary to contain 0.008% or more of Ti. On the other hand, if it is contained in excess of 0.020%, Ti becomes excessive and Ti that does not form a nitride becomes solid-melted Ti, and the toughness deteriorates to significantly improve hardenability, and solid-melted Ti also in the welded portion. Hardens the tissue and deteriorates the HAZ toughness, so this is the upper limit.

N: 0.0045 to 0.0070%
By forming TiN, N suppresses the coarsening of the structure during heating and contributes to the improvement of toughness. In order to disperse and precipitate Ti nitride in an appropriate amount, it is necessary to contain N in an amount of 0.0045% or more. On the other hand, if the content exceeds 0.0070%, the TiN becomes coarse and the dispersity decreases, and the effect of suppressing the tissue coarsening decreases. Therefore, this is set as the upper limit.

O: 0.0040% or less O exists as an impurity and forms an oxide in the steel sheet. If a large amount of O is present, the number of oxides increases and the toughness deteriorates. Therefore, the content of O is set to 0.0040% or less.

Cu: 0 to 0.40%
Since Cu can improve the strength by improving the hardenability, it may be contained as needed. In order to obtain the above effects, it is preferably contained in an amount of 0.05% or more. On the other hand, if it exceeds 0.40%, the toughness deteriorates, so this is the upper limit.

Ni: 0 to 0.80%
Since Ni is a useful element that can not only improve hardenability and obtain strength but also improve low temperature toughness at the same time, it may be contained as necessary. In order to obtain the above effects, it is preferably contained in an amount of 0.05% or more. On the other hand, Ni is an expensive alloying element, and its content exceeding 0.80% does not suit economic rationality, so this is the upper limit.

Cr: 0 to 0.20%
Since Cr can improve the strength by improving the hardenability, it may be contained as needed. In order to obtain the above effects, the content is preferably 0.05% or more. On the other hand, if it exceeds 0.20%, it becomes difficult to obtain a low YR, so this is set as the upper limit.

Mo: 0 to 0.10%
Mo can be contained as necessary because the strength can be improved by improving the hardenability and forming carbides. In order to obtain the above effects, the content is preferably 0.02% or more. On the other hand, if it exceeds 0.10%, it becomes difficult to obtain a low YR, so this is set as the upper limit.

V: 0 to 0.08%
V can generally improve hardenability and strength by forming carbides, and may be contained as necessary. In order to obtain the above effects, the content is preferably 0.02% or more. On the other hand, if it exceeds 0.08%, the toughness deteriorates, so this is the upper limit.

B: 0 to 0.002%
Since B is effective in improving hardenability and strength in a small amount, it may be contained as necessary. On the other hand, if it exceeds 0.002%, the toughness deteriorates, so this is the upper limit. In order to obtain the above effects, the content is preferably 0.0004% or more.

Ca: 0 to 0.005%
Since Ca reduces the adverse effect of S by forming a sulfide and is effective in improving toughness, it may be contained as necessary. On the other hand, if it exceeds 0.005%, coarse oxides will be formed, which will adversely affect the toughness, so this is the upper limit. In order to obtain the above effects, the content is preferably 0.0008% or more.

The balance in the chemical composition of the low yield specific thickness steel sheet of the present invention is Fe and impurities. Impurities mean components that are mixed in by raw materials such as ores and scraps and other factors when steel materials are industrially manufactured.

In order to secure the required tensile strength in the rolling process, the carbon equivalent (Ceq), which is an index of quenching hardness defined by the Japan Welding Society (IIW) as shown below (Equation 1), is 0.30 to It is set to 0.40.
Ceq = [C%] + [Mn%] / 6 + ([Cu%] + [Ni%]) / 15 + ([Cr%] + [Mo%] + [V%]) / 5 ... (Equation 1)
In (Equation 1), the element symbol with [] represents the content (mass%) of each element.

Ceq defined by the above formula (Equation 1) is an index showing the hardenability of the steel sheet. In order to secure the tensile strength, the content of C, Mn, Cu, Ni, Cr, Mo, and V contained in the steel sheet needs to be 0.30 or more in Ceq defined by the above (Equation 1). There is. If Ceq is less than 0.30, sufficient tensile strength cannot be obtained due to insufficient hardenability. The larger the Ceq, the higher the tensile strength, but if it exceeds 0.40, the tensile strength becomes excessive, and the absorbed energy of Charpy is significantly reduced accordingly. Therefore, the Ceq defined in the above (Equation 1) is set to 0.30 to 0.40.

Further, in the present invention, the NBT defined by the following (Equation 2) is set to 0.010 to 0.020.
NBT = 2 × [Nb%] + [Ti%] ... (Equation 2)
In (Equation 2), the element symbol with [] represents the content (mass%) of each element.

In general, Nb is an element effective for expanding the unrecrystallized region of austenite, and contributes to microstructure miniaturization by rolling, so that toughness can be improved. Further, Ti forms TiN, and the coarsening of austenite can be suppressed by the pinning effect at the time of heating the slab or welding, and the structure can be miniaturized by rolling. On the other hand, when the effects of Nb and Ti are used in combination in the rolling process of the present invention, the ferrite grains become excessively fine and the yield strength becomes high, so that it becomes difficult to obtain a low YR. In order to obtain a soft phase having an appropriate particle size and secure a low YR, it is necessary to set the NBT to 0.010 to 0.020%. If the NBT is less than 0.010%, austenite becomes coarse when the slab is heated and fine ferrite cannot be obtained. On the other hand, if the NBT exceeds 0.020%, the ferrite becomes excessively fine and low YR is ensured. Is difficult, so this is the upper limit.

Further, in the present invention, it is necessary to control the metal structure in the rolling process in order to achieve low YR and improvement of toughness of the steel material, and the metal structure at the position of 1/4 of the plate thickness of the steel sheet is defined. The metal structure is basically ferrite, but the main structure other than ferrite is bainite, and martensite, pearlite, and MA may be present in addition to this.

Ferrite area ratio of metal structure: 40-70%
Ferrite has a small amount of C solid solution, and C diffuses into the surrounding austenite during transformation to form a soft phase. The larger the area ratio of ferrite, the lower the yield strength and tensile strength. Therefore, it is necessary to control the area ratio of ferrite to 70% or less in order to obtain a predetermined strength and low YR. On the other hand, when the ferrite area ratio is less than 40%, the role of the soft phase becomes insufficient and the yield strength becomes high, which makes it difficult to reduce the YR. Therefore, the ferrite area ratio is set to 40 to 70%.

In the ferrite structure at the position of 1/4 of the plate thickness, the particle size distribution of ferrite is as follows. Here, the particle size in the ferrite particle size is a so-called circle-equivalent diameter (diameter of a circle when the projected area is regarded as a circle having the same area), and can be easily measured by using an image analysis device. In measurements using an actual image analyzer, ferrite with a very small particle size may also be observed. When all the numbers of such ferrites are counted and the number ratio for each particle size is calculated, the number ratio of ferrites having a ferrite particle size of less than 10 μm may be prominently increased. Since the total area of ferrite having such a small particle size is very small, it is not necessary to actually count them as a number. Specifically, the number of ferrites may be calculated by counting the number of ferrites having an equivalent diameter of 2 μm or more.

Percentage of ferrites with a ferrite grain size of less than 10 μm: 55-80%
The smaller the particle size and the larger the ratio of the number of ferrites, the better the low temperature toughness. In order to secure the required low temperature toughness, the number ratio of ferrites having a particle size of less than 10 μm needs to be 55% or more. On the other hand, when the ratio of the number of ferrites less than 10 μm exceeds 80%, the number of coarse ferrite grains having a particle size of more than 10 μm decreases, and the number of ferrites that yield early under a tensile load becomes insufficient, so that the yield strength becomes high and low. YR conversion becomes difficult. Therefore, the ratio of the number of ferrites having a ferrite particle size of less than 10 μm is 55 to 80%.

Percentage of ferrites with a ferrite grain size of 10 to 20 μm: 20 to 45%
When the ratio of coarse ferrites having a particle size of 10 μm or more increases, yielding occurs at an early stage under a tensile load, which is effective in ensuring low YR. On the other hand, coarse ferrite has a problem of deteriorating low temperature toughness. A ferrite having a particle size of 10 to 20 μm can achieve both low YR and low temperature toughness. If the number ratio of ferrite is less than 20%, the yield strength becomes high, while if it exceeds 45%, the low temperature toughness deteriorates. The ratio of the number of ferrites having a diameter of 10 to 20 μm is 20 to 45%.

Ferrite number ratio exceeding 20 μm: 2% or less Ferrite with a particle size exceeding 20 μm significantly deteriorates low temperature toughness, so it is necessary to minimize it, but if the number ratio can be controlled to 2% or less, it is necessary. Since low temperature toughness can be obtained, the ratio of the number of ferrites having a ferrite particle size exceeding 20 μm is 2% or less.

The grain size of ferrite at the 1/4 position of the plate thickness is specified in order to obtain the ferrite grain size at the average position of the steel sheet. In addition, when determining the ferrite particle size distribution at the 1/4 position of the plate thickness, observe the microstructure in the region of ± 2 mm centered on the 1/4 position of the plate thickness, and based on the observation results, for each ferrite particle size. The number ratio may be measured.

In order to obtain low YR as described above, it is necessary to secure the required ferrite in the metallographic structure of the steel. On the other hand, Ti precipitates, more specifically TiN, are dispersed in the metal structure during casting. By dispersing TiN, the growth of γ grains can be suppressed during welding, and the HAZ toughness does not decrease. As a method for securing the ferrite ratio, a method is used in which the steel sheet is allowed to cool for a certain period of time after finish rolling, the steel sheet is cooled to a predetermined temperature, subjected to ferrite transformation, and then water-cooled.

However, in the conventional method, it is necessary to allow the steel sheet to cool for a certain period of time on the production line, which is not preferable from the viewpoint of production efficiency, and the ferrite that transforms from a high temperature during cooling tends to be coarsened to 10 μm or less. The ferrite number ratio becomes smaller.

Therefore, as a result of repeated studies by the present inventors, the problem of manufacturing efficiency is solved by performing preliminary cooling (primary cooling) such as water cooling before accelerated cooling (secondary cooling) such as water cooling. It was found that the ferrite transformation can be performed finely in a short time by increasing the transformation driving force of the ferrite by the primary cooling, so that the ferrite number ratio of less than 10 μm can be efficiently secured.

The method for producing a low yield specific thickness steel sheet according to the present invention is not particularly limited as long as it is possible to produce a steel sheet having the above-mentioned chemical composition and metal structure, but for example, it shall be produced by the method shown below. Can be done.

First, a slab having the above-mentioned chemical composition is heated in a heating furnace to a temperature range of 1000 to 1120 ° C., then extracted from the heating furnace and hot-rolled to produce a steel sheet. At that time, the cumulative rolling reduction rate in the temperature range of 900 ° C. or lower is 30% or more, and the rolling end temperature _TFR (° C.) is the steel sheet surface temperature under the condition that Ar is ₃ points or more defined in the following (Equation 3). Roll. After rolling, primary cooling and secondary cooling are performed.
Ar ₃ (° C.) = 910-310 x [C%] -80 x [Mn%] -20 x [Cu%] -15 x [Cr%] -55 x [Ni%] -80 x [Mo%] +0 .35 x ([plate thickness (mm)] -8) ... (Equation 3)
In (Equation 3), the element symbol with [] represents the content (mass%) of each element.

In the above-mentioned primary cooling and secondary cooling, the cooling treatment is performed under the conditions shown in the following (a) to (e).
(A) The first cooling is started in the range where the surface temperature of the steel sheet is T _FR to T _FR -30 ° C and Ar is ₃ points or more, and is stopped in the range of 600 to 700 ° C.
(B) The average cooling rate of the first cooling is 10 ° C./sec or more.
(C) The time from the first cooling to the second cooling is 10 to 50 sec.
(D) The second cooling is stopped in the range of 350 ° C to 480 ° C.
(E) The average cooling rate in the second cooling is set to 15 ° C./sec or more.
As described above, the first cooling and the second cooling may be performed by using one cooling device or by continuously moving the steel sheet by using two cooling devices.

Each process will be described in detail below. The temperature shown below is the surface temperature of the steel sheet unless otherwise specified.

Heating temperature of slab before hot rolling: 1000-1120 ° C
When the heating temperature is less than 1000 ° C., the temperature of the slab is low, so that the rolling resistance becomes large. Therefore, the number of rolling passes until the plate thickness reaches a predetermined value increases too much, and the manufacturing efficiency deteriorates. On the other hand, if the heating temperature exceeds 1120 ° C., the crystal grains of austenite are coarsened, so that the low temperature toughness may decrease.

Cumulative reduction rate in the temperature range of 900 ° C or less: 30% or more The cumulative reduction rate in the temperature range of 900 ° C or less is the decrease in the sheet thickness rolled to the plate thickness after finish rolling based on the plate thickness at 900 ° C. The rate. If the cumulative reduction rate is less than 30%, fine crystal grains cannot be obtained after transformation, so that the low temperature toughness may decrease.

Rolling end temperature T _FR : Ar ₃ points or more If the rolling end temperature T _FR is less than Ar ₃ points, proeutectoid ferrite may be generated before cooling. Therefore, the rolling end temperature _TFR is set to Ar ₃ points or more at the surface temperature of the steel sheet. The Ar ₃ points are as shown in the above (Equation 3), and the Ar ₃ points in the cooling step shown below are also the same.

Next, the cooling process will be described in detail below.
(A) The first cooling is started in the range where the surface temperature of the steel sheet is T _FR to T _FR -30 ° C and Ar is ₃ points or more, and is stopped in the range of 600 to 700 ° C.
The start temperature of the first cooling is equal to or lower than the rolling end temperature T _FR because the rolling end temperature is T _FR . If the temperature drops to less than _TFR -30 ° C by the start of cooling of the first cooling, the dislocations introduced by rolling are recovered, and the driving force of transformation is reduced. Therefore, the ratio of the number of ferrites having a coarse particle size may increase. On the other hand, if the cooling start temperature is less than Ar ₃ points, proeutectoid ferrite may be formed before cooling. Therefore, the first cooling is started in the range where the surface temperature of the steel sheet is T _FR to T _FR -30 ° C and Ar is ₃ points or more.

Further, if the cooling stop temperature of the first cooling is higher than 700 ° C., the transformation driving force due to cooling cannot be sufficiently obtained, and the toughness is lowered by increasing the ratio of the number of ferrites having a coarse particle size. On the other hand, when the temperature is lower than 600 ° C., the transformation driving force becomes large, but the transformation nucleation frequency becomes small, so that ferrite transformation is unlikely to occur between the first cooling and the second cooling, so that the ferrite area ratio becomes small. This makes it difficult to obtain low YR. Therefore, the first cooling is stopped in the range of 600 to 700.

(B) The average cooling rate in the first cooling is 10 ° C./sec or more.
If the average cooling rate of the first cooling is less than 10 ° C./sec, the ferrite transformation in the middle of cooling starts, so that the ratio of the number of ferrites having a coarse particle size increases, and the toughness may decrease. Therefore, the average cooling rate of the first cooling is set to 10 ° C./sec or more. Although the upper limit of the average cooling rate of the first cooling is not specified, the average cooling rate of the first cooling is usually 40 ° C./sec or less from the viewpoint of the performance of the water cooling device.

(C) The time from the first cooling to the second cooling is 10 to 50 sec.
The time from the primary cooling to the secondary cooling is the cooling time from the primary cooling stop, but if this is less than 10 sec, the ferrite transformation after the primary cooling stop is insufficient and the ferrite area ratio is insufficient. .. On the other hand, if it exceeds 50 sec, the ferrite crystal grains become coarse and the toughness may deteriorate. Therefore, the time from the first cooling to the second cooling is set to 10 to 50 sec. At this time, the surface temperature of the steel sheet at the start of the second cooling becomes 580 ° C. or higher due to the reheat of the steel sheet.

(D) The second cooling is stopped in the range of 350 ° C to 480 ° C.
If the cooling shutdown temperature is higher than 480 ° C., untransformed austenite may be transformed into upper bainite having low toughness, resulting in deterioration of toughness. On the other hand, if the cooling stop temperature is excessively rapidly cooled to less than 350 ° C., the steel sheet may warp and the flatness may deteriorate. Therefore, the cooling shutdown temperature in the second cooling is in the range of 350 to 500 ° C.

(E) The average cooling rate of the second cooling is set to 15 ° C./sec or more.
If the average cooling rate of the second cooling is slow, upper bainite transformation with low toughness may occur during cooling and the toughness may deteriorate. Therefore, the average cooling rate of the second cooling is set to 15 ° C./sec or more. Although the upper limit of the average cooling rate of the second cooling is not specified, the average cooling rate of the second cooling is usually 40 ° C./sec or less from the viewpoint of the performance of the water cooling device.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Steels having the chemical components shown in Tables 1 and 2 were melted and slabs were prepared by a continuous casting machine. The obtained slab was hot-rolled under the conditions shown in Table 2 and then subjected to first cooling and second cooling to obtain a steel sheet.

The microstructure and the following various physical properties of the obtained thick steel sheet were investigated.

<Mechanical characteristics>
After stress relief annealing, a round bar tensile test piece having a parallel portion length of 8.5 mm and a gauge point distance of 42.5 mm was prepared from each of the obtained steel plates. At this time, the test piece was cut out so that the length direction of the round bar tensile test piece was perpendicular to the rolling direction (plate width direction). A tensile test was carried out at room temperature and atmospheric pressure using a round bar tensile test piece, and the yield strength YS (MPa), tensile strength TS (MPa), yield ratio YR (= YS / TS × 100, unit is%). And, the total elongation EL (%) was obtained.

<Low temperature toughness>
For the evaluation of low temperature toughness, a Charpy test piece (2 mm V notch test piece) conforming to JIS Z2242-2016 was taken from a position of 1/4 of the plate thickness. The notch position was in the plate thickness direction. Three tests were conducted under the condition of -60 ° C, and the lowest value was defined as absorption energy (vE-60).

HAZ toughness is obtained by abutting steel plates with an X-shaped groove as shown in FIG. Both sides were subjected to 1-pass welding as shown in FIG. 2 at a welding heat input of 40 to 60 kJ / cm using Nippon Steel & Sumikin Welding & Co., Ltd.). After that, it is cut perpendicular to the longitudinal direction of the weld, and as shown in Fig. 2, a Charpy test piece is taken with the 1 / 2t melted part as the notch position, and a Charpy impact test is performed at -60 ° C, and the lowest value is taken as the absorbed energy. did.

These results are summarized in Table 3. In the present invention, when TS is 490 MPa or more, YS is 355 MPa or more, YR is 80% or less, and EL is 21% or more, it is evaluated to have the characteristics of low YR steel, and the steel sheet and HAZ are also evaluated. The vE-60 of the part was evaluated to be excellent in low temperature toughness when it was 47 J or more, and was passed in the comprehensive judgment.

<Micro organization>
For the microstructure, after polishing the cross section cut out from the center of the Sharpy test piece used for the test from the center of the test piece, the surface was corroded with nital and the region ± 2 mm from the center was observed with an optical microscope. , The ferrite structure was identified and each fraction (number ratio) was determined for each particle size. Although not shown in Tables 5 and 6, when a TEM sample was taken from the same position where the microstructure was observed and confirmed by TEM, minute Ti was found in the ferrite structure (inside the ferrite grain and at the ferrite grain boundary). It was confirmed that inclusions (TiN) were present.

With reference to Tables 1 to 6, test symbols A1 to A40, which are examples of the present invention satisfying all of the chemical composition, metallographic structure and production conditions specified in the present invention, have a low YR and are composed of the base material and HAZ. The results were excellent in low temperature toughness in both cases.

On the other hand, Test Nos. B1 to B31, which are comparative examples, did not satisfy any one or more of the chemical composition, the metallographic structure, and the production conditions, and as a result, the desired characteristics could not be obtained.

According to the present invention, it is possible to provide a low yield specific thickness steel sheet having excellent low temperature toughness and HAZ toughness suitable for use as a tank material such as LPG or ammonia.

1 Steel plate 2 Welded bead 3 2mm V notch Charpy test piece

Claims (1)

The composition of the steel sheet is mass%,
C: 0.04 to 0.08%,
Si: 0.05-0.25%,
Mn: 0.8-1.6%,
P: 0.015% or less,
S: 0.005% or less,
Al: 0.015 to 0.05%,
Nb: less than 0.005%,
Ti: 0.008-0.020%,
N: 0.0045 to 0.0070%,
O: 0.0040% or less,
Cu: 0 to 0.40%,
Ni: 0 to 0.80%,
Cr: 0 to 0.20%,
Mo: 0 to 0.10%,
V: 0 to 0.08%,
B: 0 to 0.002%,
Ca: 0 to 0.005%,
The balance has a chemical composition consisting of Fe and impurities and has a chemical composition.
Ceq defined by the following equation 1 is 0.30 to 0.40.
The NBT defined by the following equation 2 is 0.010 to 0.020.
In the metal structure at the position of 1/4 of the plate thickness,
The ferrite area ratio is 40 to 70%,
When calculating the number of ferrites by counting the number of ferrites with a diameter equivalent to a circle of 2 μm or more,
In the ferrite particle size distribution,
The number ratio of ferrites less than 10 μm is 55 to 80%.
The number ratio of ferrites of 10 to 20 μm is 20 to 45%, and
The number ratio of ferrites exceeding 20 μm is 2% or less,
Ti precipitates present in the ferrite structure,
HAZ Low yield ratio thick steel sheet with excellent toughness.
Ceq = [C%] + [Mn%] / 6 + ([Cu%] + [Ni%]) / 15 + ([Cr%] + [Mo%] + [V%]) / 5 ... (Equation 1)
NBT = 2 × [Nb%] + [Ti%] ... (Equation 2)
In (Equation 1) and (Equation 2), the element symbol with [] represents the content (mass%) of each element.

Images