JP6771429B2 - Thick steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a thick steel sheet and a method for producing the same.

For large structures such as ships, buildings, bridges and construction machinery, the size of the structures is increasing, but due to the magnitude of damage in the event of damage, the structural members are required to be even more reliable. Has been done. Along with this, high strength is required for the steel plates constituting the structure. On the other hand, as the strength of a steel sheet increases, the workability represented by uniform elongation tends to decrease, and a steel sheet having both of them is required.

For example, Patent Document 1 examines a technique for producing a steel having good uniform elongation and strength by obtaining a fine metallographic structure mainly composed of α phase and further strengthening by precipitation.
As a method for obtaining the above metal structure, after heating the steel piece to the Ac3 transformation point to 1050 ° C., hot rolling is performed at a start temperature of 850 ° C. or lower, an end temperature of 750 ° C. or higher, and a cumulative reduction rate of 50 to 95%. After that, it is disclosed that accelerated cooling with a cooling rate of 5 to 100 ° C./s starts at 750 ° C. or lower and stops at 600 ° C. or higher.

In Patent Document 2, a technique for enhancing uniform elongation without deterioration of toughness by obtaining a metal structure having a fine hard phase and retained austenite is studied.
As a method for obtaining the above metal structure, heating is performed at a temperature of Ac3 transformation point or higher and 1300 ° C. or lower, and unrecrystallized rolling of austenite having a cumulative reduction rate of 30% or higher in a range of at least 950 ° C. to Ar3 transformation point or higher is performed. After performing hot rolling including, accelerated cooling at 3 to 100 ° C./s is performed from a temperature above the Ar3 transformation point to a temperature at which the austenite fraction becomes 20 to 70%, and after the accelerated cooling is stopped, the temperature is raised and maintained. One or a combination of two or more types of cooling at a cooling rate of 0.5 ° C./s or less was performed to maintain the steel temperature within ± 100 ° C. of the accelerated cooling stop temperature for 10s to 100s after the acceleration cooling stop. Later, cooling is disclosed.

Patent Document 3 states that the space factor of ferrite in the entire structure is more than 90%, the average ferrite grain size is 3 to 12 μm, the maximum ferrite grain size is 40 μm or less, and the average circle equivalent diameter of the second phase is 0.8 μm. A technique for producing a steel having excellent impact absorption (that is, uniform elongation) and base metal toughness by satisfying the following conditions and having a tensile strength of 490 MPa or more is being studied.
As a method for obtaining the above metal structure, the finish rolling temperature is set to 700 to 850 ° C., the temperature range of 700 to 500 ° C. is cooled at 3 ° C./s or higher, reheated at a predetermined temperature, and after reheating, 600 to 500 ° C. It is disclosed that the temperature range of ° C. is further cooled at 2 ° C./s.

JP-A-2002-105534 Japanese Unexamined Patent Publication No. 2003-253331 JP-A-2007-162101

In both Patent Documents 1 and 2, a fine structure is realized by precisely controlling the cooling rate by using accelerated cooling after rolling, but in the actual manufacturing process, from the tip to the tail of a long thick steel sheet. Strict control is difficult, and the characteristics vary depending on the position of the steel sheet, which may reduce productivity.

Further, in the production method of Patent Document 3, the above-mentioned reheating step is required in order to leave the retained austenite contributing to uniform elongation at room temperature, and there is a problem that the number of production steps is large and the productivity is lowered. ..

As described above, in Patent Documents 1 to 3, various studies have been made from the viewpoints of uniform elongation, strength, toughness, etc., but especially when focusing on uniform elongation, problems still remain from the viewpoint of productivity. The current situation is that it has to be said that it has been done.

The present invention has been made in view of the above circumstances, and a main object thereof is to provide a thick steel sheet having excellent uniform elongation and a method for producing the same.

The thick steel plate according to the present invention has C: 0.04 to 0.06% by mass, Si: 0.35 to 0.45% by mass, Mn: 1.49 to 1.59% by mass, P: more than 0% by mass. , 0.01% by mass or less, S: more than 0% by mass, 0.002% by mass or less, Cu: 0.23 to 0.33% by mass, Al: 0.02 to 0.06% by mass, Ni: 0. 24-0.34% by mass, Nb: 0.015-0.021% by mass, Ti: 0.012-0.018% by mass, B: 0.0007-0.0013% by mass, Ca: 0.0010- The second phase contains 0.0030% by mass and N: 0.0040 to 0.0060% by mass, the balance is composed of iron and unavoidable impurities, and the metal structure is the first phase and the hard phase harder than the first phase. The hard phase includes a phase, and the hard phase is a phase made of pearlite, and the hardness of the second phase is 260 HV (Vickers hardness) or less.

The method for producing a thick steel sheet according to the present invention includes (a) a heating step of heating a steel piece having the chemical composition to 900 to 1250 ° C., and (b) after the step (a) of 680 to 800 ° C. It includes a step of finish rolling at a finish rolling temperature and a step of (c) after the step (b) of cooling to room temperature at a cooling rate A satisfying the following formula (1).
736.02 × [C] +8.5 × A + 208.53 ≦ 260 (1)
Here, [C] is the content of C (mass%), and A is the cooling rate (° C./s) after finish rolling.

INDUSTRIAL APPLICABILITY The present invention provides a thick steel sheet having excellent uniform elongation and a method for producing the same.

FIG. 1 is a graph showing the relationship between the hardness of the second phase and the value on the left side of the equation (1). FIG. 2 is a graph showing the relationship between uniform elongation and the hardness of the second phase.

As a result of diligent studies to solve the above problems, the present inventors have appropriately controlled the chemical composition of the thick steel sheet, and the metal structure is a hard phase harder than the first phase and the first phase. By controlling the hard phase to be a phase composed of pearlite, including the phase, and further controlling the hardness of the second phase to 260 HV or less, a thick steel sheet having excellent uniform elongation can be obtained. I found.
Further, the correlation between the C content (% by mass) and the cooling rate after finish rolling and the hardness of the second phase formed after rolling was found, and the C content was satisfied so as to satisfy the following equation (1). It was found that the hardness of the second phase can be reduced to 260 HV or less and excellent uniform elongation can be obtained by controlling the cooling rate after finish rolling and cooling to room temperature.
736.02 × [C] +8.5 × A + 208.53 ≦ 260 (1)
Here, [C] is the content of C (mass%), and A is the cooling rate (° C./s) after finish rolling.

Hereinafter, the thick steel plate of the present invention and a method for producing the same will be described in detail.

<1. Thick steel plate ＞
[1-1. Metallic structure]
The thick steel sheet according to the present invention (hereinafter, may be referred to as “steel”) has a metal structure of the first phase and the second phase (hereinafter, “second phase”, which is harder than the first phase). Includes (sometimes referred to as "hard second phase"). By controlling the hardness of the hard second phase to 260 HV or less, a desired uniform elongation can be obtained.
In a thick steel plate having a plate thickness of t, for example, the hardness of the second phase at a portion t / 4 from the surface of the steel plate may be controlled as described above.
Hereinafter, each configuration will be described in detail.

(Hard second phase)
In the thick steel sheet according to the present invention, the hard phase to be the hard second phase is made of pearlite. The thick steel sheet according to the present invention may contain martensite as a third phase other than the first phase and the second phase, but does not contain bainite. From the viewpoint of obtaining high uniform elongation, the area ratio of the hard second phase is preferably 10% or less, more preferably 5% or less.

(Hardness of second phase: 260 HV or less)
If the second phase is too hard, it becomes a very brittle phase and the toughness is lowered, and the uniform elongation is insufficient. Therefore, it is necessary to make the hardness of the second phase 260 HV or less, preferably 255 HV. Below, it is more preferably 250 HV or less.

The first phase of the thick steel sheet according to the present invention is not particularly limited, and examples thereof include a soft phase made of ferrite. When the first phase is made of ferrite, if the average particle size of ferrite is too large, the toughness deteriorates and the uniform elongation becomes insufficient. Therefore, it is preferable that the average particle size of ferrite is 30 μm or less. On the other hand, if the average particle size of ferrite is too small, the restrictions on production conditions become large. Therefore, it is preferable that the average particle size of ferrite is 5 μm or more. The average particle size of ferrite may be measured by a line segment method, for example, by photographing a metal structure using a scanning electron microscope (SEM).

The thickness of the thick steel plate according to the present invention is not particularly limited, but is preferably 10 mm or more and 50 mm or less.

[1-2. Chemical composition]
The thick steel plate according to the present invention has C: 0.04 to 0.06% by mass, Si: 0.35 to 0.45% by mass, Mn: 1.49 to 1.59% by mass, P: more than 0% by mass. , 0.01% by mass or less, S: more than 0% by mass, 0.002% by mass or less, Cu: 0.23 to 0.33% by mass, Al: 0.02 to 0.06% by mass, Ni: 0. 24-0.34% by mass, Nb: 0.015-0.021% by mass, Ti: 0.012-0.018% by mass, B: 0.0007-0.0013% by mass, Ca: 0.0010- It contains 0.0030% by weight and N: 0.0040 to 0.0060% by weight, with the balance consisting of iron and unavoidable impurities.
By controlling the chemical composition as described above, a thick steel sheet having excellent uniform elongation can be obtained.
Hereinafter, each element will be described in detail.

(C: 0.04 to 0.06% by mass)
C has the effect of increasing the strength of the steel sheet, but is also an element that increases the hard phase and deteriorates ductility. If the C content is less than 0.04% by mass, it becomes difficult to secure the required strength.
Therefore, the C content is set to 0.04% by mass or more. The C content is preferably 0.042% by mass or more, more preferably 0.045% by mass or more. On the other hand, when the C content exceeds 0.06% by mass, the strength can be easily secured, but the hard phase is increased, leading to deterioration of ductility. Therefore, the C content is set to 0.06% by mass or less. The C content is preferably 0.058% by mass or less, more preferably 0.055% by mass or less.

(Si: 0.35 to 0.45% by mass)
Si is an element necessary for ensuring high strength because it is possible to obtain a first phase that does not hinder elongation by utilizing solid solution strengthening by suppressing precipitation. In order to effectively exert this action, the amount of Si needs to be 0.35% by mass or more. The amount of Si is preferably 0.36% by mass or more, more preferably 0.37% by mass or more. However, if the amount of Si is excessive, a martensite-austenite mixed phase is likely to be formed, which may reduce other properties such as toughness. Therefore, the amount of Si needs to be 0.45% by mass or less. The amount of Si is preferably 0.44% by mass or less, more preferably 0.43% by mass or less.

(Cu: 0.23 to 0.33% by mass)
Cu is an element necessary for ensuring the strength by strengthening the solid solution, and the amount of Cu needs to be 0.23% by mass or more in order to effectively exert this action. The amount of Cu is preferably 0.24% by mass or more, more preferably 0.25% by mass or more. However, if the amount of Cu is excessive, not only the ductility is lowered by precipitation, but also the hardenability is excessive and cracks are likely to occur during hot working, so the amount of Cu needs to be 0.33% by mass or less. The amount of Cu is preferably 0.32% by mass or less, more preferably 0.31% by mass or less.

(Mn: 1.49 to 1.59% by mass)
Mn is an effective element for improving hardenability and ensuring strength and toughness. In order to exert such an effect, it is necessary to contain Mn in an amount of 1.49% by mass or more. The Mn content is preferably 1.50% by mass or more, more preferably 1.51% by mass or more. However, if Mn is excessively contained, the weldability and the like deteriorate, so the upper limit is set to 1.59% by mass. The Mn content is preferably 1.58% by mass or less, more preferably 1.57% by mass or less.

(Al: 0.02 to 0.06% by mass or less)
Al is an element necessary for deoxidation, and also has an effect of fixing N in steel and preventing deterioration of base metal toughness due to solid solution N. In order to exert such an effect, it is necessary to contain Al in an amount of 0.02% by mass or more. The Al content is preferably 0.025% by mass or more, more preferably 0.030% by mass or more. On the other hand, if Al is excessively contained, coarse alumina-based inclusions are formed and the toughness of the base metal is lowered. Therefore, the Al content needs to be 0.06% by mass or less. The Al content is preferably 0.055% by mass or less, more preferably 0.050% by mass or less.

(Ni: 0.24 to 0.34% by mass)
Ni has the effect of improving hardenability and making the structure finer, and at the same time, has the effect of suppressing cracking during hot working, which tends to occur due to the addition of Cu. In order to exert such an effect, it is necessary to contain 0.24% by mass or more of Ni. The Ni content is preferably 0.25% by mass or more, more preferably 0.26% by mass or more. However, if Ni is excessively contained, the hardenability becomes excessive, and the desired uniform elongation cannot be obtained. Therefore, the amount of Ni needs to be 0.34% by mass or less. The Ni content is preferably 0.33% by mass or less, more preferably 0.32% by mass or less.

(Nb: 0.015-0.021% by mass)
Nb is an element effective for forming carbides and carbonitrides to improve the strength. In order to obtain such an effect, it is necessary to contain Nb in an amount of 0.015% by mass or more. The Nb content is preferably 0.016% by mass or more, more preferably 0.017% by mass or more. On the other hand, if Nb is excessively contained, the amount of precipitated carbides and carbonitrides becomes excessive, the precipitation strengthening ability becomes excessive, and the yield ratio increases. Therefore, the Nb content needs to be 0.021% by mass or less. The Nb content is preferably 0.020% by mass or less, more preferably 0.019% by mass or less.

(Ti: 0.012 to 0.018% by mass)
Ti is an element that combines with N to form TiN, prevents coarsening of austenite grains, that is, γ grains, during heating before hot rolling, and contributes to improvement of base metal toughness. It also has the effect of fixing N in the steel to prevent deterioration of the toughness of the base metal due to the solid solution N. In order to exert these effects, it is necessary to contain Ti in 0.012% by mass or more. The Ti content is preferably 0.013% by mass or more, more preferably 0.014% by mass or more. On the other hand, if the Ti content becomes excessive, TiN becomes coarse and the toughness of the base metal deteriorates, so that the content needs to be 0.018% by mass or less. The Ti content is preferably 0.017% by mass or less, more preferably 0.016% by mass or less.

(B: 0.0007 to 0.0013% by mass)
B makes it easy to suppress the formation of a coarse ferrite structure. In order to exert such an effect, it is necessary to contain B in an amount of 0.0007% by mass or more. The B content is preferably 0.0008% by mass or more, more preferably 0.0009% by mass or more. However, if the amount of B is excessive, the hardenability becomes excessive and the desired uniform elongation cannot be obtained. Therefore, the amount must be 0.0013% by mass or less. The B content is preferably 0.0012% by mass or less, more preferably 0.0011% by mass or less.

(Ca: 0.0010% to 0.0030% by mass)
Ca is an element that contributes to the spheroidization of MnS and is effective in improving the toughness of the base metal and the ductility in the plate thickness direction. In order to exert such an effect, the Ca content is preferably 0.0010% by mass or more. The Ca content is preferably 0.0012% by mass or more, more preferably 0.0015% by mass or more. However, when the Ca content exceeds 0.0030% by mass and becomes excessive, inclusions become coarse and the toughness of the base metal deteriorates. Therefore, the Ca content is set to 0.0030% by mass or less. The Ca content is preferably 0.0028% by mass or less, more preferably 0.0025% by mass or less.

(N: 0.0040 to 0.0060% by mass)
N is an element that produces TiN and AlN, prevents coarsening of γ grains during heating before hot rolling and during welding, and is effective in improving base metal toughness and HAZ toughness. If the content of N is less than 0.0040% by mass, the TiN and the like are insufficient, the γ grains become coarse, and the toughness of the base metal deteriorates. Therefore, the N content needs to be 0.0040% by mass or more. The N content is preferably 0.0042% by mass or more, and more preferably 0.0044% by mass or more. On the other hand, if the N content exceeds 0.0060% by mass and becomes excessive, the solid solution N increases, which adversely affects the base metal toughness and HAZ toughness. Therefore, the N content is set to 0.0060% by mass or less. The N content is preferably 0.0058% by mass or less, more preferably 0.0056% by mass or less.

(P: More than 0% by mass, 0.010% by mass or less)
P is an unavoidable impurity that adversely affects the toughness of the base metal and the weld. In order not to cause such inconvenience, it is necessary to suppress the content to 0.010% by mass or less. The P content is preferably 0.009% by mass or less, more preferably 0.008% by mass or less. It is industrially difficult to set it to 0%, and the lower limit is about 0.002% by mass.

(S: More than 0% by mass, 0.002% by mass or less)
S is an unavoidable impurity that adversely affects the toughness and ductility of the steel sheet in the plate thickness direction, and is preferably small. From this point of view, the S content needs to be suppressed to 0.002% by mass or less. The S content is more preferably 0.001% by mass or less, still more preferably 0.0005% by mass or less.

The basic components of the thick steel sheet according to the present invention are as described above, and the balance is substantially iron.
However, it is naturally permissible for steel to contain unavoidable impurities other than P and S brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like.
In addition, the unavoidable impurities may include Cr, Mo and / or V as other impurities mixed by the use of scrap or the like or other factors.

If Cr is excessively contained, the hardenability becomes excessive and the desired uniform elongation cannot be obtained. Therefore, when Cr is contained as an impurity, the Cr content is preferably 0.1% by mass or less. The Cr content is more preferably 0.09% by mass or less, still more preferably 0.08% by mass or less.

If Mo is contained in excess, the hardenability becomes excessive, and as a result, the weld cracking resistance deteriorates. Therefore, when Mo is contained as an impurity, the Mo content is preferably 0.05% by mass or less. The Mo content is more preferably 0.04% by mass or less, still more preferably 0.03% by mass or less.

If V is excessively contained, the amount of precipitated carbide or carbonitride becomes excessive, the precipitation strengthening ability becomes excessive, and the yield ratio increases. Therefore, when V is contained as an impurity, the V content is preferably 0.005% by mass or less. The V content is more preferably 0.003% by mass or less, still more preferably 0.001% by mass or less.

The thick steel sheet according to the present invention having such a structure is excellent in uniform elongation and may be preferably used as a structural material for ships, buildings, bridges, construction machinery and the like.

<2. Manufacturing method of thick steel plate ＞
In order to produce the thick steel sheet according to the present invention, a steel piece containing the above-mentioned chemical composition, for example, a slab, is used, and the heating temperature, finish rolling temperature, and subsequent cooling rate of the steel piece are appropriately adjusted.
Specifically, (a) a heating step of heating the steel piece having the chemical composition to 900 to 1250 ° C., and (b) after the step (a), finish rolling at a finish rolling temperature of 680 to 800 ° C. This step includes (c) and after the step (b), a step of cooling to room temperature at a cooling rate A satisfying the following formula (1).
736.02 × [C] +8.5 × A + 208.53 ≦ 260 (1)
Here, [C] is the content of C (mass%), and A is the cooling rate (° C./s) after finish rolling.

Hereinafter, each step will be described in detail. The "temperature" specified in this specification is the temperature of the material.

[(A) Heating step of heating a steel piece having the chemical composition to 900 to 1250 ° C.]
Steel pieces containing the above-mentioned chemical composition, for example, slabs, are heated to 900 to 1250 ° C. where hot rolling is possible. The heating temperature is preferably 1000 ° C. or higher, more preferably 1050 ° C. or higher, preferably 1200 ° C. or lower, and more preferably 1150 ° C. or lower.

[(B) Step of finish rolling at a finish rolling temperature of 680 to 800 ° C. after the step (a)] After the step (a), the finish rolling temperature is controlled to 680 to 800 ° C. in order to secure strength and elongation. And finish rolling. The finish rolling temperature is preferably 690 ° C. or higher, more preferably 700 ° C. or higher, preferably 790 ° C. or lower, and more preferably 780 ° C. or lower.

[(C) After the step (b), a step of cooling to room temperature at a cooling rate A satisfying the formula (1)]
After the step (b), the mixture is cooled to room temperature at a cooling rate A satisfying the following equation (1).
736.02 × [C] +8.5 × A + 208.53 ≦ 260 (1)
Here, [C] is the content of C (mass%), and A is the cooling rate (° C./s) after finish rolling.
The technical significance of the above equation (1) will be described below.

FIG. 1 is a graph showing the relationship between the hardness of the second phase and the value on the left side of the above equation (1). Further, FIG. 2 is a graph showing the relationship between the hardness of the second phase and the uniform elongation.
In FIGS. 1 and 2, the plot represented by “□” shows the thick steel plate of the example of the present invention manufactured by cooling at a cooling rate satisfying the above equation (1). On the other hand, the plot represented by "○" shows a thick steel plate of a comparative example manufactured at a cooling rate that does not satisfy the above equation (1).

As shown in FIG. 1, as the value on the left side of the above equation (1) increases, the hardness of the second phase increases. Further, as shown in FIG. 2, as the hardness of the second phase increases, the uniform elongation decreases. From the results of FIGS. 1 and 2, by controlling the value on the left side of the above equation (1), that is, by controlling the content of C in the thick steel sheet and the cooling rate after finish rolling, uniform elongation and hardness are obtained. It can be seen that both the hardness of the two phases can be controlled.

The present inventors perform the above steps (a) and (b) on a steel piece having the chemical composition specified in the present application, and cool the steel piece after finish rolling according to the C content [C] in the steel piece. By controlling the velocity A so as to satisfy the above equation (1), the hardness of the second phase can be reduced to 260 HV or less, and excellent uniform elongation of 17.5% or more can be achieved. We found that we could do it, and defined the above equation (1) in the present application.
From the viewpoint of obtaining a thick steel sheet having excellent uniform elongation, the value on the left side of the above equation (1) is preferably 200 or more, more preferably 210 or more, and more preferably 255 or less. Is 250 or less.

The cumulative reduction rate in the total hot rolling step is preferably 60% or more. More preferably, it is 65% or more. In order to produce α grains, it is necessary to apply sufficient reduction in the unrecrystallized temperature range. The amount of reduction in the unrecrystallized temperature range is preferably 20% or more, more preferably 25% or more, still more preferably 30% or more.

Although the method for manufacturing a thick steel sheet according to the present invention has been described above, a person skilled in the art who understands the desired characteristics of the thick steel sheet according to the present invention will carry out trial and error to carry out trial and error, and the thick steel sheet having the desired characteristics according to the present invention. There is a possibility of finding a method other than the above-mentioned manufacturing method.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following Examples as well as the present invention, and appropriate modifications are made to the extent that it can be adapted to the above or the purpose described below. Of course, it is possible to carry out, and all of them are included in the technical scope of the present invention.

Steel pieces having the chemical composition of steel types A to X shown in Table 1 are melted and cast according to a normal melting method, and then the steel pieces are heated, finished rolled and cooled under the manufacturing conditions shown in Table 2 to obtain a thickness. Test No. 12 to 50 mm. 1 to 24 thick steel plates were manufactured.
Underlined items in Tables 1 and 2 mean that they are outside the scope of the present invention.

For each thick steel sheet, observe the metallographic structure according to the following procedure, and measure the average grain size of ferrite, the hardness of the second phase, and the tensile properties (uniform elongation: U.El, tensile strength: TS). went.

[1. Observation of metallographic structure]
The metallographic structure was observed according to the following procedure.
(1) A sample was taken from the above steel sheet so that the thickness cross section including the front and back surfaces of the steel sheet, which was parallel to the rolling direction and perpendicular to the surface of the steel sheet, could be observed.
(2) The observation surface was mirror-finished by polishing with wet emery abrasive paper (# 150 to # 1000) or by polishing with a polishing agent such as diamond slurry as polishing having an equivalent function. ..
(3) The polished sample was corroded with a 3% nital solution to reveal grain boundaries.
(4) At the plate thickness t / 4 site, observe the exposed structure at a magnification of 400 times using an optical microscope, and if the structure has ferrite, use a hard second phase other than ferrite and ferrite. Was the first phase. That is, the hard second phase is harder than the first phase. When the structure did not have ferrite and had bainite and martensite, bainite was used as the first phase and martensite, which was harder than bainite, was used as the second phase.

[2. Average grain size of ferrite]
For the above sample corroded with a 3% nital solution, the first phase was observed at a magnification of 100 times at a plate thickness of t / 4 site using an optical microscope, and a photograph of 10 fields of view was taken. The particle size of ferrite was determined from the micrograph by a comparative method (JIS G0551), and the average value was taken as the average particle size of ferrite.

[3. Method of measuring the hardness of the second phase]
For the above sample corroded with a 3% nital solution, the hardness of the second phase was measured at a site having a plate thickness of t / 4 using a Micro Vickers hardness tester at a measurement load of 0.05 N. The hardness was measured at 10 or more points in the second phase, and the average value was taken as the hardness of the second phase. When the structure was martensite only, that is, martensite single phase, the hardness of the phase was measured as the hardness of the second phase.

[4. Tensile test]
A full-thick plate-shaped test piece (No. 5) is collected so that the longitudinal direction of the test piece is perpendicular to the rolling direction, and a tensile test is performed in the manner of JIS Z2241: 2015. Tensile strength (TS) and uniform elongation (U. El) was measured.
U.S. A thick steel sheet having an El of 17.5% or more was judged to be at a practical level.

Table 3 shows the metallographic structure, the average particle size of ferrite, the hardness of the second phase, and the tensile properties (uniform elongation: U.El, tensile strength: TS). Those underlined in Table 3 mean that they are out of the provisions of the present invention.

From the results in Table 3, it can be considered as follows. Test No. 1 to 4 and 19 to 24 are examples that satisfy all of the requirements specified in the present invention, and are excellent in uniform elongation.

On the other hand, Test No. 5 to 18 are examples that do not meet any of the requirements specified in the present invention.

Test No. Reference numeral 5 denotes an example of a thick steel sheet manufactured by using a steel type E having an excess of Si, Cu and Ni and cooling at a high cooling rate that does not satisfy the equation (1). The excess Cu reduces ductility and is also the first. The hardness of the two phases exceeded 260 HV and the desired uniform elongation was not achieved.

Test No. 6 and 7 were produced by using steel grades F and G having an excess of C, performing finish rolling at a temperature higher than the finish rolling temperature specified in the present application, and further cooling at a high cooling rate that does not satisfy the formula (1). In this example of a thick steel sheet, the hardness of the second phase exceeded 260 HV, and the desired uniform elongation was not achieved.

Test No. 8 to 11 are examples of thick steel sheets manufactured by cooling at a high cooling rate that does not satisfy the equation (1), and the desired uniform elongation was not achieved.

Test No. Reference numerals 12 and 13 are examples of thick steel sheets produced by performing finish rolling at a temperature higher than the finish rolling temperature specified in the present application and further cooling at a fast cooling rate that does not satisfy the equation (1), and are examples of the second phase hard steel sheet. The temperature exceeded 260 HV specified in the present application, and the desired uniform elongation was not achieved.

Test No. Reference numeral 14 denotes an example of a thick steel sheet manufactured by using a steel type N having an excess of Si, Cu and Ni and further cooling at a high cooling rate that does not satisfy the equation (1), and the hardness of the second phase is defined in the present application. It was above 260 HV and the desired uniform elongation was not achieved.

Test No. Each of 15 to 18 was produced by using steel grades O to R having an excess of C, performing finish rolling at a temperature higher than the finish rolling temperature specified in the present application, and further cooling at a high cooling rate that does not satisfy equation (1). In this example of a thick steel sheet, the hardness of the second phase exceeded 260 HV, and the desired uniform elongation was not achieved.

Claims (2)

C: 0.04 to 0.06% by mass,
Si: 0.35 to 0.45% by mass,
Mn: 1.49 to 1.59% by mass,
P: More than 0% by mass, 0.01% by mass or less,
S: More than 0% by mass, 0.002% by mass or less,
Cu: 0.23 to 0.33% by mass,
Al: 0.02 to 0.06% by mass,
Ni: 0.24 to 0.34% by mass,
Nb: 0.015-0.021% by mass,
Ti: 0.012 to 0.018% by mass,
B: 0.0007 to 0.0013% by mass,
It contains Ca: 0.0010 to 0.0030% by mass and N: 0.0040 to 0.0060% by mass.
The rest consists of iron and unavoidable impurities,
The metal structure includes a first phase and a second phase, which is a hard phase harder than the first phase, and the hard phase is a phase composed of pearlite.
The first phase is a phase made of ferrite, and the average particle size of the ferrite is 30 μm or less.
A thick steel plate having a hardness of 260 HV or less in the second phase. The method for manufacturing a thick steel sheet according to claim 1.
(A) A heating step of heating a steel piece having the chemical composition to 900 to 1250 ° C., and (b) a step of finish rolling at a finish rolling temperature of 680 to 800 ° C. after the step (a).
(C) A method for producing a thick steel sheet, which comprises, after the step (b), a step of cooling to room temperature at a cooling rate A satisfying the following formula (1).

736.02 × [C] +8.5 × A + 208.53 ≦ 260 (1)
Here, [C] is the content of C (mass%), and A is the cooling rate (° C./s) after finish rolling.

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