JP7381842B2 - thick steel plate - Google Patents

Description

The present invention relates to a method for producing a thick steel plate and a steel plate, and particularly relates to a method for producing a low yield ratio high strength steel plate having a tensile strength of 440 MPa or more, and such a low yield ratio high strength steel plate. be.

As steel structures become larger, there is a need to develop stronger steel. In addition to this, in recent years there has been a strong demand for steel materials with a low yield ratio (yield ratio = yield strength / tensile strength). Examples include earthquake-resistant materials that can absorb earthquake energy and steel for liquid ammonia storage tanks.

In order to achieve a low yield ratio in thick steel plates used in such steel structures, Patent Documents 1 and 2 disclose a technique for making thick steel plates containing ferrite and martensite in a certain proportion. There is.

Patent Document 1 discloses that after heating the steel material to 950°C or higher and 1250°C or lower, rolling is started, and after finishing the rolling at 820°C or higher, the steel material is cooled to 600 to 700°C at a cooling rate of 20°C/s or higher. , After holding the temperature in the temperature range for 10 to 200 seconds and/or cooling slowly, cool it to 300 ° C or less at a cooling rate of 5 ° C / s or more, and the space factor with respect to the whole structure is ferrite: 70 to 90%, By setting martensite or a mixed phase of martensite and austenite: 3 to 15%, and the remainder: bainite (including cases of 0%), it is possible to create a steel plate with a tensile strength of 490 N/mm2 or ^more and a yield ratio of 70% or less. A technique for doing so has been disclosed.

Patent Document 2 discloses that a steel material is heated to 1050 to 1200°C or less, hot rolled, and after finishing the hot rolling at 880 to 720°C, the steel material is heated to 600 to 700°C at a cooling rate of 10°C/second or more. Cool to 550 to 350 degrees Celsius, stop cooling midway, air cool for 30 seconds or more, and then cool from the temperature range to 550 to 350 degrees Celsius at a cooling rate of 10 degrees Celsius or more, until the ferrite fraction in the total structure is 45 to 350 degrees Celsius. Disclosed is a steel plate in which the ferrite has a ferrite grain size of 85 area %, the remainder consists of a bainitic structure and/or a martensitic structure, and the average crystal grain size of the ferrite is 19 μm or less.

The techniques disclosed in Patent Documents 1 and 2 allow for the inclusion of some bainite, but by mixing a ferrite structure with a low yield stress and a martensitic structure with a high yield stress, the yield stress is low as a whole, On the other hand, it is a thick steel plate with high tensile strength. With this technology, the initial yield phenomenon of a heterogeneous body largely depends on the structure with low yield stress, while the tensile strength depends on the structure with high tensile strength. Basically, the technology that makes use of this mechanism makes use of this kind of mechanism.

In the technologies disclosed in Patent Documents 1 and 2, it is necessary to mix a certain amount of ferrite phase into the steel material in order to achieve low yield stress, and there is a problem that there is a limit to the tensile strength. Ta. So far, the lack of strength has been compensated for by adding alloys such as V. However, the addition of alloys such as V increases the cost of the alloy and increases the yield stress due to the addition of the alloy, and there is a limit to the amount of the alloy added.

Japanese Patent Application Publication No. 2007-056294 Japanese Patent Application Publication No. 2008-240004

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength thick steel plate that can achieve a low yield ratio without the inclusion of a ferrite phase.

Therefore, the present inventors have conducted extensive studies on methods to solve the above problems. As a result, they discovered that a low yield ratio could be achieved using a completely different idea. That is, even when the microstructure of a thick steel plate is 100% bainite, it is possible to produce a thick steel plate that achieves a low yield ratio by freezing the internal stress introduced by transformation.

The present invention has been made based on the above findings, and its gist is as follows.
(1) After heating the steel slab to the A3 point or higher to turn it into an austenite phase, it is rolled to the product thickness at a cumulative reduction rate of 50% or more in rough rolling and finish rolling, and after rolling, as the first stage of cooling, ℃ to a temperature of 650 ℃ or higher at an average cooling rate of 15 ℃ to 50 ℃ / s, and then, as a second stage cooling, at an average cooling rate of 15 ℃ / s or less, Air cooling or The temperature is maintained, and the third stage of cooling includes cooling to 300°C or less at an average cooling rate of 50°C/s or more, and the tensile strength is 440 MPa or more, and the bainite A method for producing a thick steel plate, characterized in that it has a microstructure in which the area ratio of bainite is 50% or more, the average grain size of bainite is 20 μm or less, and the average value of local equivalent strain in the bainite is 0.050 or more.
(2) The tensile strength is 440 MPa or more, the microstructure has a bainite area ratio of 50% or more, an average grain size of bainite of 20 μm or less, and an average local equivalent strain in bainite of 0.050 or more. A thick steel plate characterized by:
(3) mass%,
C: 0.05-0.50%,
Si: 0.01-2.00%,
Mn: 0.50-3.00%
P: 0.030% or less,
S: 0.020% or less,
Al: 0.05% or less, and
The thick steel plate according to (2) above, which contains N: 0.050% or less, with the remainder consisting of Fe and impurities.
(4) Furthermore, in mass%,
Ti: 0.50% or less,
Nb: 0.50% or less,
B: 0.010% or less,
Cr: 1.50% or less,
Cu: 1.00% or less, and
The thick steel plate described in (3) above, containing one or more types of Ni: 10.00% or less.
(5) Furthermore, in mass%,
Mo: 1.00% or less,
W: 0.50% or less, and
The thick steel plate described in (3) or (4) above, containing one or more types of V: 0.50% or less.

According to the present invention, it is possible to provide a high-strength thick steel plate that can achieve a low yield ratio, specifically a yield ratio of 70% or less, even without the inclusion of a ferrite phase. Furthermore, according to the present invention, as described above, a low yield ratio can be achieved without the inclusion of a ferrite phase, so the ratio of the hard phase can be increased compared to conventional low yield ratio thick steel plates that require ferrite. Therefore, it is possible to realize a thick steel plate with higher strength.

It is a figure which shows the continuous cooling transformation diagram of a test piece. It is a figure which shows the result of the tensile test of a test piece.

<Production method of thick steel plate>
Hereinafter, the method for manufacturing a thick steel plate of the present invention (hereinafter referred to as the "manufacturing method of the present invention") will be explained.

The manufacturing method of the present invention involves heating a steel billet to the A3 point or higher to turn it into an austenite phase, and then rolling it to the product thickness at a cumulative reduction rate of 50% or more in rough rolling and finish rolling.After rolling, the first step is cooling. , the average cooling rate up to 650°C is cooled to a temperature above the Ms point and below 650°C at an average cooling rate of 15°C to 50°C/s, and then as the second stage cooling, the average cooling rate is 15°C/s. Any of the following points: (When the bainite area ratio is 50% or more and the austenite area ratio is 10%) ~ (When 100 seconds have passed since the bainite area ratio is 50% or more and the austenite area ratio is 10%) The third stage of cooling includes cooling to 300°C or less at an average cooling rate of 50°C/s or more, and the tensile strength is 440 MPa or more. The purpose is to manufacture a thick steel plate having a microstructure in which the area ratio of bainite is 50% or more, the average grain size of bainite is 20 μm or less, and the average value of local equivalent strain in bainite is 0.050 or more. . Here, in the present invention, the cumulative rolling reduction rate is a value that is the sum of the nominal rolling reduction rate (rolling amount/inlet side plate thickness) for each pass when rolling is performed in multiple passes.

In conventional technology, in order to create steel plates with low yield ratio thickness, a certain amount of ferrite phase is mixed into the steel material, which limits the tensile strength.In contrast, adding alloys compensates for the lack of strength. It's here. However, since adding an alloy increases both the tensile strength and the yield stress, there is a limit to the amount of the alloy to be added.

Therefore, in order to reduce the mixing of ferrite phase and realize a low yield ratio thick steel plate that does not depend on excessive alloy addition, the present inventors conducted the following study and discovered the thick steel plate of the present invention. .

Many steel materials undergo phase transformation from austenite to ferrite, cementite, martensitic, etc. during cooling, but it is known that phase transformation generally occurs heterogeneously within the material. ing. That is, nucleation occurs from specific positions such as grain boundaries and dislocations, and the growth rate of the new phase is not constant. At this time, volume expansion caused by phase transformation causes strain mismatch between the new phase and the old phase, together with the above-mentioned heterogeneity. To accommodate this strain mismatch, elastic and sometimes plastic deformation occurs. At this time, a stress distribution occurs inside the material.

The inventors performed theoretical calculations and experiments on materials in which stress distribution occurs internally due to phase transformation, and found that such materials behave as if they were heterogeneous as a whole. . In other words, when a tensile test is assumed, the parts where tensile internal stress originally existed yield at a relatively early stage, so they behave like a soft material. On the other hand, the parts where compressive internal stress originally existed behave as if their tensile strength has increased at first glance, so the material as a whole is different from the conventional one, which has a certain proportion of ferrite and martensite. As in the case of phase steels, it was found that the result is a low yield ratio steel with low yield stress and high tensile strength.

However, in general, except in the case of martensitic transformation, the transformation temperature is higher than the recovery or recrystallization temperature, so stress relaxation occurs during slow cooling after the transformation, and the internal stress introduced by the phase transformation disappears. For example, in the rolling of thick steel plates, usually air cooling, that is, slow cooling, is performed on a cooling bed after hot rolling, or after hot rolling, an accelerated cooling device is used to cool down to the target temperature, and then slow cooling is performed on a cooling bed. It is something to do. In addition, in bainite transformation, cooling is performed using an accelerated cooling device to a phase transformation temperature of about 500 to 600° C., and then gradual cooling is performed on a cooling bed. During slow cooling in such a cooling bed, most of the internal stress introduced by bainite transformation disappears, and a low yield ratio steel with low yield stress and high tensile strength cannot be obtained.

The present inventors investigated ways to prevent the disappearance of the stress distribution inside the material caused by phase transformation, and found that by performing accelerated cooling immediately after the completion of phase transformation, stress remains inside the material even at room temperature, reducing the stress distribution. We thought that this would result in a thick steel plate that exhibits a yield ratio. Therefore, we conducted the following tests to confirm that by accelerating the cooling of the material immediately after the completion of phase transformation to create a steel plate with residual stress inside the material at room temperature, a steel plate that exhibits a low yield ratio can be obtained. Ta.

First, it contains C: 0.4%, Si: 1.0%, and Mn: 2.2% in mass %, P: 0.02%, S: 0.02%, Al: 0.05 % and N: A test piece of steel with a plate thickness of 8 mm and limited to 0.005% was prepared. A continuous cooling transformation diagram of this test piece was obtained in advance by a Formaster test. Figure 1 shows the continuous cooling transformation diagram of the test piece.

This test piece was heated to 950°C with a high-frequency heating device and held for 5 minutes to undergo bainite transformation at a cooling rate of 30°C/s to form a structure of 90% bainite and 10% austenite. , Cooling was performed to room temperature at a cooling rate of 50° C./s to obtain a test piece 1. In addition, the same process as above was performed until bainite was 90% and austenite was 10%, and then, after holding at 500°C for 4 hours, cooling was performed at a cooling rate of 50°C/s to room temperature, and test piece 2 was obtained. Obtained.

A tensile test was conducted on the obtained test pieces 1 and 2. FIG. 2 shows the results of the tensile test of the test piece. In Figure 2, the solid line shows the tensile test results of specimen 1, which was rapidly cooled immediately after changing from 90% bainite to 10% austenite, and the tensile test result of specimen 2, which was heated immediately after changing from 90% bainite to 10% austenite. The tensile test results are shown by dotted lines.

As shown in FIG. 2, in test specimen 1, the stress/strain curve shifts to the low stress side, indicating that the material softens. Further, although the maximum stress value of test piece 1 is lower than that of test piece 2, the tensile strength is 440 MPa or more, indicating that the decrease in tensile strength is small.

Furthermore, using the test piece before the tensile test, local strain in bainite and average particle size of bainite were measured. Local strain in bainite was measured using the fine lattice marker method.
The average grain size of bainite can be determined by creating an embedded sample from around 1/4 part of the thickness of the test piece obtained above, corroding the grain boundaries using picric acid, and measuring the grain size using the line segment method. It was done by doing. The line segment method is performed by randomly drawing about 20 to 30 lines on the observed structure and counting the bainite grain boundaries that intersect with the lines. The average particle size calculated for all the lines was taken as the average particle size of bainite. When the average particle size of bainite was measured using this method, it was 5 μm.

The fine grid marker method was performed as follows. First, a steel plate for measuring local strain is taken, the surface is chemically polished, and then corroded with nital solution to reveal the microstructure. Next, a positive photoresist (photosensitive material) is applied. As the photoresist, for example, ZEP520A manufactured by Zeon Corporation can be used. A photoresist film formed on a steel plate is scanned with an electron beam to expose it to light in the form of a square lattice. The width of the grid lines is 100 nm, and the distance between the centers of the widths of the grid lines is targeted to be 500 nm. The grid is created in an area of 500 μm×500 μm.
The photoresist that has been irradiated with an electron beam is developed and the exposed portions, that is, the portions corresponding to the sides of the grid, are removed. For development, ZED-N50 manufactured by Nippon Zeon Co., Ltd. or the like can be used. Next, gold is deposited on the developed surface. The gold is deposited directly on the surface of the steel plate in the areas that are removed by development (the sides of the grid), while it is deposited on the resist in the areas that are not exposed to light and have not been removed. Then, when the photoresist is dissolved in an organic solvent, the gold on the resist is removed together with the resist, leaving only the gold deposited directly on the steel plate surface, forming a gold lattice (marker) on the steel plate surface. .

The steel plate on which the grid markers are formed is held at 300° C. for 2 hours. This releases local strain, which can be observed from the surface as deformation of the lattice. Note that this heat treatment is preferably performed in a non-oxidizing atmosphere (for example, an Ar gas atmosphere).
Next, a lattice in which all four of the lattice points are in bainite is observed by SEM, and the distance between the lattice points (length of the side) for each lattice is measured. Then, the maximum absolute value of the difference between those measured values and 500 nm is set as Δmax (unit: nm), and the value obtained by dividing Δmax by 500 nm (Δmax/500) is calculated before the above heat treatment is applied to that lattice part. It is defined as the strain that existed in Then, the strain is determined for at least 500 lattices, and the average value thereof is calculated and used as the average value of the local equivalent strain in bainite.

Using this fine lattice marker method, the local strain in 500 lattices per test piece was measured for test pieces 1 and 2, and the average value of the local equivalent strain was calculated. The average value of the local equivalent strain of specimen 1, which was subjected to accelerated cooling immediately after the completion of transformation, was 0.08, and the average value of the local equivalent strain of specimen 2, which was kept warm immediately after the completion of transformation, was 0.001. Met. Based on the measurement results of these average values of local equivalent strain and the results of the above-mentioned tensile test, it has been found that steel plates with high average values of local equivalent strain have less decrease in tensile strength, that is, steel plates with a low yield ratio. I got it.

Next, we investigated the structure of the microstructure and the lower limit of the average value of the local equivalent strain in bainite, which will result in a thick steel plate with a tensile strength of 440 MPa or more and a low yield ratio. First, a plurality of steel specimens having the same chemical composition as above were prepared, heated to 950° C. using a high-frequency heating device, and held for 5 minutes. By changing the cooling rate after holding, the temperature at which isothermal transformation occurs, and the time until accelerated cooling again, we can create microstructures with various configurations and obtain the average value of local equivalent strain in various bainites. A plurality of test specimens were prepared.

These specimens were subjected to tensile tests, observation of the microstructure, and measurement of the average value of local equivalent strain in bainite. If the average value of the local equivalent strain is 0.050 or more, the tensile strength is 440 MPa or more without the inclusion of ferrite phase or the addition of special alloying elements as in the conventional technology. It has been found that a thick steel plate with a low yield ratio, specifically a thick steel plate with a yield ratio of 70% or less, can be obtained.

The present invention was achieved through the above study process and by performing specific stepwise cooling after hot rolling. I will explain them one by one.

(hot rolling)
In hot rolling, the steel slab is heated to the A3 point or higher to turn it into an austenite phase, and then rolled at a cumulative reduction rate of 50% or higher using a rough rolling mill and a finishing rolling mill to obtain the product thickness. If the cumulative reduction rate is low, it is difficult to control the grain size of the steel material, small voids called porosity remain in the product, which adversely affects the performance of the steel material, and the average grain size of bainite is set to 20 μm or less. In addition, it is necessary to roll at a cumulative reduction rate of 50% or more using rough rolling and finishing rolling mills. The cumulative reduction rate may be, for example, 100% or more or 150% or more, and/or 400% or less or 300% or less. Note that other conditions for hot rolling may be according to conventional methods and are not particularly limited.

(1st stage cooling)
After hot rolling, the first stage of cooling is performed by cooling to a temperature above the Ms point and below 650°C at an average cooling rate of 15°C to 50°C/s. The reason for cooling in this manner is to obtain a bainite-based structure without producing martensite. If the cooling rate is less than 15°C/s, ferrite is likely to be formed, and if it is more than 50°C/s, it is difficult to stop cooling at the target temperature and a martensitic structure is likely to occur, resulting in the desired bainite-based structure. This becomes difficult. The above average cooling rate may be, for example, 20° C. to 50° C./s or 30° C. to 50° C./s.

(Second stage cooling)
Subsequently, as the second stage of cooling, at an average cooling rate of 15° C./s or less, (when the bainite area ratio is 50% or more and the austenite area ratio is 10%) to (when the bainite area ratio is 50% or more and the austenite area ratio is 10%). Air cooling or temperature maintenance is performed until 100 seconds have elapsed since the temperature reached 10%.
In this cooling, if the tensile strength is 440 MPa or more and 50% or more of bainite structure or residual structure exists, the residual structure is one or more of ferrite, pearlite, martensite, and austenite. In order to obtain a thick steel plate, the transformation is allowed to proceed until the austenite phase becomes 10% or less.
The fastest point at which cooling ends is defined as "the time when the bainite area ratio is 50% or more and the austenite area ratio is 10%" when the bainite area ratio is less than 50% or more than 10% austenite remains, This is because a large amount of martensite will be generated when cooling is started, and the latest point at which cooling ends is ``the time when 100 seconds have passed since the time when the area ratio of bainite is 50% or more and the area ratio of austenite is 10%.'' The reason why the period is within 100 seconds from the upper limit is that if it exceeds 100 seconds, the microscopic residual stress introduced by bainite transformation will be released, and the desired effect will not be obtained. Preferably, it is within 60 seconds, more preferably within 10 seconds. Furthermore, if the material is cooled at an average cooling rate of more than 15° C./s, the austenite phase will transform into martensite, making it difficult to obtain a bainite-based structure. There is no need to particularly specify the lower limit of the average cooling rate, and the desired structure can be obtained even if the temperature is maintained at an isothermal state.
The progress of transformation can be grasped by applying the temperature transition of cooling from the Ar3 point or higher to a continuous cooling transformation (CCT) curve predetermined according to the chemical composition, so the CCT curve By creating a database for each chemical composition, it becomes possible to determine the cooling water volume density after rolling in advance from the processing/temperature history in the actual process production line and obtain the desired texture fraction.

(3rd stage cooling)
Subsequently, in the third stage of cooling, the average cooling rate up to 300°C is set to 50°C/s or more, and cooling is performed to 300°C or less. If the average cooling rate is less than 50° C./s, the desired effect cannot be obtained because the microscopic residual stress introduced by bainite transformation is relaxed. The average cooling rate may be, for example, 60° C./s or more or 80° C./s or more. Although there is no particular upper limit to the average cooling rate, if the average cooling rate is increased more than necessary, equipment costs will increase, so the average cooling rate is preferably less than 1000° C./s. Furthermore, if the cooling stop temperature exceeds 300° C., microscopic residual stress will similarly be relaxed, making it impossible to obtain the effects of the present invention.

By satisfying the above conditions, the tensile strength is 440 MPa or more, the area ratio of bainite is 50% or more, the average grain size of bainite is 20 μm or less, and the average value of local equivalent strain in bainite is 0.050. A thick steel plate with a low yield ratio and a microstructure as described above can be obtained.

<Thick steel plate>
The thick steel plate of the present invention has a tensile strength of 440 MPa or more, a microstructure in which the area ratio of bainite is 50% or more, the average grain size of bainite is 20 μm or less, and the average value of local equivalent strain in bainite is It is a thick steel plate with a low yield ratio of 0.050 or more. Note that the thickness of the thick steel plate is not particularly limited, but may be 6 mm or more and 100 mm or less.
First, a preferable chemical composition of the thick steel plate of the present invention will be explained. The following chemical composition is intended to be merely an example of a preferred embodiment, and is not intended to limit the steel plate of the present invention to a steel plate having such a specific chemical composition.

(C: 0.05-0.50%)
C is an essential element to obtain a high-strength steel plate. In order to obtain a steel plate with a tensile strength of 440 MPa or more, it is preferable to contain 0.05% or more. The C content may be 0.10% or more or 0.20% or more. On the other hand, since it is necessary to ensure weldability and the toughness of the welded part, the upper limit is set at 0.50%. The C content may be 0.40% or less or 0.30% or less.

(Si: 0.01-2.00%)
Si is an effective element for increasing the strength of steel materials through solid solution strengthening. It also has the effect of suppressing the formation of cementite, and is an element that has the advantage of less deterioration in workability as compared to the effect of increasing strength due to addition. However, if it is contained in excess, it will adversely affect surface treatment properties such as pickling properties and chemical conversion treatments. Therefore, it is set to 2.00% or less. Preferably, it is 1.50% or less or 1.00% or less. On the other hand, if the content is reduced to less than 0.01%, the strength will decrease, so 0.01% is set as the lower limit. The Si content may be 0.05% or more or 0.10% or more.

(Mn: 0.50-3.00%)
In addition to solid solution strengthening, Mn has high hardenability, so it is an extremely important element for increasing the strength of steel sheets through strengthening the transformation structure. Therefore, the lower limit is set at 0.50%, which provides a clear effect. The Mn content may be 0.70% or more or 1.00% or more. On the other hand, if the content exceeds 3.00%, mechanical properties may deteriorate due to solidification segregation, so the upper limit is set at 3.00%. The Mn content may be 2.50% or less or 2.00% or less.

(P: 0.030% or less)
P is an element contained as an impurity in steel. Further, since it has the effect of reducing hot workability, it is limited to 0.030% or less. Preferably it is 0.010% or less. On the other hand, the lower limit is not particularly limited, but reducing it more than necessary places a great burden on the steel manufacturing process, so 0.001% is the practical lower limit for practical steels.

(S: 0.020% or less)
S is an element contained as an impurity in steel. Furthermore, since it has an adverse effect on hot workability and mechanical properties such as ductility and toughness, it is limited to 0.020% or less. Preferably it is 0.010% or less. On the other hand, the lower limit is not particularly limited, but reducing the content more than necessary places a great burden on the steel manufacturing process, so 0.0001% is the practical lower limit for practical steels.

(Al: 0.05% or less)
Al is an element necessary for deoxidation and has the effect of increasing the cleanliness in steel materials, but if it exceeds 0.05%, it forms nitrides and oxides and impairs toughness. % or less. Preferably it is 0.04% or less or 0.03% or less. On the other hand, the lower limit is not particularly limited, but if it is less than 0.005%, the effect of addition may not be sufficiently expressed, so it is preferably 0.005% or more.

(N: 0.050% or less)
N is an element contained as an impurity in steel. In addition, B forms nitrides with B, reducing the contribution of B to hardenability, and forms nitrides with Ti, degrading mechanical properties, so it is desirable to reduce it as much as possible. However, it is permissible if it is 0.050% or less. Preferably it is 0.010% or less. On the other hand, the lower limit is not particularly limited, but reducing it more than necessary places a heavy load on the steel manufacturing process, so 0.0010% is a practical lower limit for practical steel.

The above are the basic components of the thick steel plate targeted by the present invention. Furthermore, the following components can be contained as necessary for the purpose of improving various properties.

(Ti: 0.50% or less)
Ti combines with N and suppresses N from reducing the contribution of B to hardenability. Therefore, when incorporating B and utilizing its hardenability, Ti should be added in an amount of 0.01% or more. It is desirable to include it as a guideline. The Ti content may be 0.02% or more or 0.03% or more. On the other hand, TiN deteriorates mechanical properties, and excessive addition may inhibit recrystallization after cold rolling, leading to a decrease in productivity. Since there is a possibility that C combines with C to reduce effective C and deteriorate hardenability, the content is set to 0.50% or less. Preferably, it is 0.30% or less or 0.10% or less.

(Nb: 0.50% or less)
Nb, like Ti, binds to N and suppresses N from reducing the contribution of B to hardenability. % or more is desirable as a guideline. The Nb content may be 0.02% or more or 0.03% or more. On the other hand, if the content exceeds 0.50%, the effect will be saturated, and furthermore, recrystallization after cold rolling may be suppressed, leading to a decrease in productivity. Since there is a possibility that it combines with C and reduces hardenability, the upper limit is set at 0.50%. Preferably, it is 0.30% or less or 0.10% or less.

(B: 0.010% or less)
Since B has the effect of increasing hardenability when added in an amount of 0.0001% or more, it can be added as necessary. On the other hand, excessive addition leads to deterioration of hot workability and reduction of ductility, so the content should be 0.010% or less. Preferably, it is 0.005% or less or 0.003% or less.

(Cr: 1.50% or less)
Since Cr is an element that has hardenability, it is preferably contained in an amount of 0.01% or more. However, even if it is added in an amount exceeding 1.50%, the effect will be saturated and the manufacturing cost will increase, so the content should be 1.50% or less. Preferably, it is 1.00% or less.

(Cu: 1.00% or less)
Cu is an element that has the effect of increasing strength, and can be added as necessary. In order to obtain such an effect, it is preferable to contain 0.01% or more. However, if it exceeds 1.00%, the surface quality of the hot rolled steel sheet will be degraded, so the content should be 1.00% or less. Preferably, it is 0.50% or less or 0.10% or less.

(Ni: 10.00% or less)
Since Ni is an element that improves hardenability, it is preferably contained in an amount of 0.01% or more. On the other hand, since it is an expensive element, the upper limit is set at 10.00% at which the effect of addition is saturated. Preferably, it is 1.00% or less or 0.50% or less. Further, since Ni has the effect of suppressing deterioration of the surface quality of hot rolled steel sheets due to Cu, it is desirable to include Ni at the same time as Cu.

(Mo: 1.00% or less)
(W: 0.50% or less)
(V: 0.50% or less)
All of these elements are elements that improve hardenability, and can be added as necessary. In order to obtain the effect of addition, it is preferable to contain 0.01% or more of each. On the other hand, since these elements are expensive, the upper limit is set at the point where the effect of addition is saturated. The upper limit of Mo is 1.00%, and the upper limit of W and V is 0.50%. The upper limit of these elements may be 0.10% or less.

In the thick steel plate of the present invention, the remainder other than the above elements consists of Fe and impurities. Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, and are not intended for the steel plate of the present invention. This includes components that are not intentionally added (so-called unavoidable impurities). In addition, impurities include elements other than the components explained above, which are included in the steel plate of the present invention at a level where the effects specific to the element do not affect the properties of the steel plate of the present invention. It is.

Next, the tensile strength of the thick steel plate of the present invention will be explained. The thick steel plate targeted by the present invention is a high-strength thick steel plate, and its required tensile strength is 440 MPa or more. The upper limit of the tensile strength is not particularly limited, but is generally 1000 MPa or less, and may be 900 MPa or less.

Next, the microstructure of the thick steel plate of the present invention will be explained.

In the thick steel plate of the present invention, the bainite structure occupies the maximum, and there may be cases where the entire structure is bainite (bainite area ratio is 100%). When a residual structure exists, the residual structure is composed of one or more of ferrite, pearlite, martensite, and austenite.

Since the bainite structure can provide a steel plate with a tensile strength of 440 MPa or more, it is contained in an area ratio of 50% or more. Preferably it is 60% or more, more preferably 70% or more.
The average particle size of bainite is 20 μm or less, and may be 15 μm or less in order to obtain the desired strength and toughness. It is known that when the particle size is extremely large, strength and toughness decrease. The lower limit is not particularly defined, but in order to make the average grain size of bainite smaller than necessary, it is necessary to increase the amount of processing, so the average grain size of bainite is preferably 1 μm or more.

In order to improve ductility, the ferrite structure may be contained in an area ratio of 5% or more. However, if the content exceeds 30%, the tensile strength decreases, which is not preferable.
In order to increase the strength, the pearlite structure is preferably contained in an area ratio of 2% or more. However, if the content exceeds 10%, the strength increases excessively and the ductility decreases, which is not preferable.
Since the martensitic structure excessively reduces ductility, it is preferable that the area ratio is 10% or less.
In order to increase ductility, the austenite structure is preferably contained in an area ratio of 1% or more. However, if it is attempted to contain more than 20%, the amount of alloying elements added increases, resulting in an increase in manufacturing costs.

The proportion of the microstructure can be determined from the area ratio of the structure as follows. The microstructure was determined by polishing the 1/4th section of the steel plate thickness parallel to the rolling direction, corroding it with 3% nital, and observing it with a scanning electron microscope (SEM) at 2000x magnification over 10 fields of view. is analyzed by image analysis processing, and the area ratio of each tissue is determined.

Next, local strain in bainite of the thick steel plate of the present invention will be explained.

In the thick steel plate of the present invention, the average value of local equivalent strain in bainite is 0.050 or more. When it is 0.050 or more, it becomes a low yield ratio steel with low yield stress and high tensile strength. The average value of local equivalent strain in bainite may be 0.100 or more or 0.150 or more. Although the upper limit is not particularly limited, it is preferably 1.000 or less in order to ensure total elongation.

Next, an example of the present invention will be described. The conditions in the example are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is based on this example of conditions. It is not limited. The present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.

(Example 1)
The influence of the chemical composition of the steel billet, the cooling rate, and the start and end temperatures in each of the first to third cooling stages was investigated.
A steel piece with the chemical composition shown in Table 1 was used as a test piece, heated with a high frequency heating device to 1100°C, which corresponds to the temperature of point A3 or higher, and held for 3 hours. Cooling to the end temperature (temperature at the start of transformation), then cooling at the average cooling rate of the second stage to perform transformation, and from the time when the bainite area ratio becomes 50% or more and the austenite area ratio becomes 10%, the same table After the time shown in , cooling was performed to room temperature at the third stage of re-accelerated average cooling rate.
In Table 3, the test piece of steel type B was subjected to temperature history No. The processing process will be explained using the case of processing in step 6 as an example. After heating the test piece to 1100°C, it is held for 3 hours, and then cooled to 500°C at an average cooling rate of 50°C/s. Bainite transformation begins during cooling, and the proportion of austenite decreases as the transformation progresses. As described above, the time when the austenite area ratio reaches 10% is defined as the end of bainite transformation in the present invention. After 100 seconds have passed after the completion of the transformation, it is cooled to 300°C or less at a rate of 50°C/second. The time required for the austenite area ratio to reach 10% after the first quenching can be determined from a CCT curve determined in advance based on the chemical composition of steel type b. In addition, in all the examples, the cumulative rolling reduction ratio after heating and before cooling exceeded 50%.

The obtained test piece was subjected to a cold tensile test, observation of the structure, and measurement of local strain in bainite. The results of these measurements are shown in Tables 3 and 4 (continued from Table 3). The tensile test was conducted using a test method based on JIS Z2241:2011, and tensile strength, yield strength, and yield ratio were measured. If the upper yield point is not clear, 0.2% proof stress is taken as the yield stress. A yield ratio of 70% or less was evaluated as a low yield ratio. The average local equivalent strain in bainite was measured using the lattice marker method.

As shown in Table 3, the local strain of the steel type a test piece is low regardless of the temperature history (excluding No. 13). This is because the C content was low at 0.01%, so ferrite transformation progressed early, and a structure containing 50% or more of bainite phase and 10% or less of austenite phase could not be obtained. In addition, temperature history No. In No. 13, the average cooling rate in the first stage was too fast, so the cooling stop accuracy was poor, and the temperature fell below the Ms point, resulting in a martensitic phase, and no decrease in yield stress was observed.

Similarly, regarding the test piece of steel type B, temperature history No. 1, 2, 5, 6, 10, 14, 17, 18, 20, 21, and 23, the holding time in the second stage cooling is within 100 seconds, the residual stress is frozen, and the average value of the local equivalent strain is Is high. Temperature history No. In Nos. 3, 4, 7, 8, 16, 19, and 22, the holding time was longer than 100 seconds, and the residual stress was released after the bainite content was 50% or more and the austenite phase was 10%, resulting in hardening. Ta. Temperature history No. In samples 1 to 4, 9, 10, and 14 to 16, the cooling rate in the first stage was less than 15 °C/s, the proportion of ferrite phase was high, and a structure with bainite of 50% or more was not obtained, so the tensile strength did not reach 440 MPa. Temperature history No. In Nos. 9, 11, and 12, the cooling rate in the third stage was less than 50° C./s, and after the bainite content was 50% or more and the austenite phase was 10%, the residual stress was released and hardening occurred. Temperature history No. In No. 13, the average cooling rate in the first stage was too fast, so the cooling stop accuracy was poor, and the temperature fell below the Ms point, resulting in a martensitic phase, and no decrease in yield stress was observed. Temperature history No. In No. 23, the average cooling rate in the second stage was fast and the proportion of bainite phase did not reach 50% because martensite phase was formed. As a result, the test piece of steel type B has a temperature history No. that satisfies all the cooling conditions. Steels satisfying the conditions of the present invention were obtained in Examples 5, 6, 20, and 21.

As shown in Table 4, the same results as steel type b were obtained regarding the difference in temperature history for the specimens of steel type c, but because the C content was higher than that of steel type b, the stress was slightly higher. There is. The phenomenon in which the material softens due to freezing of residual stress has been reproduced.

Similarly, regarding the test piece of steel type d, since the C content was high at 0.60% or more, a large amount of pearlite structure containing cementite was generated, and the desired reduction in yield stress could not be obtained.

Under the above conditions, the rolling reduction ratio after heating and before cooling exceeded 50% cumulatively, but the temperature history No. of steel type c. In the case of No. 6, when the average particle size of bainite was measured when the cumulative reduction rate was 40%, it was 25 μm. The tensile strength at this time is determined by steel type c and temperature history No. in Table 4. Compared to 676 MPa, which was the result in case No. 6, it was about 60%, and it was not possible to obtain the desired material quality.

Furthermore, regarding the test pieces of steel types e, f, and g, temperature history No. When cooling was performed in Step 5, a steel satisfying the present invention was obtained.

(Example 2)
Examples will be described in which steel code b in Table 1 (Ar 3 point temperature: 761° C.) was manufactured under various manufacturing conditions on an actual thick plate rolling line. Table 5 shows examples of manufacturing conditions and measurement results performed under the conditions of the present invention and as a comparative example.
The steel plate after hot rolling was cooled in cooling zones 1 to 3 (corresponding to stages 1 to 3, respectively) using accelerated cooling equipment (ACC) installed at the rear of the rolling mill.
Here, the temperature and cooling rate of the steel material are based on the cross-sectional average temperature, and show values calculated within the process computer. Further, the ferrite (indicated by F) in the structure in Table 5 also includes pearlite.

Production number 1 in Table 5 has cooling conditions within the range of the present invention, has a microstructure containing 50% or more of bainite, has a tensile strength of 510 MPa, has a yield ratio of 62%, and produces a thick steel plate with a low yield ratio. We were able to. However, under manufacturing condition 2, the finished thickness of the product was large and the cumulative rolling reduction ratio was 45%, which is outside the range of the present invention, so the grain size could not be finely controlled and the tensile strength was insufficient. In addition, under manufacturing condition 3, a thick steel plate with a low yield ratio was achieved because it was within the scope of the present invention, and under manufacturing condition 4, a low yield ratio was achieved due to the slow initial cooling rate resulting in a ferrite-based structure. However, under manufacturing condition 5, the cooling rate after the austenite fraction reached 10% was slow, so the strain during bainite transformation was released, and a low yield ratio could not be achieved. Furthermore, in manufacturing condition 6, the initial cooling rate was too fast, resulting in a martensite-based structure, which was outside the scope of the present invention where the bainite area ratio was 50% or more, making it impossible to achieve a low yield ratio. There wasn't.

According to the present invention, a thick steel plate with a low yield ratio and high strength can be provided at low cost without the inclusion of a ferrite phase or the addition of special alloying elements. Therefore, the present invention has high industrial applicability.

Claims (1)

In mass%,
C: 0.05-0.50%,
Si: 0.01-2.00%,
Mn: 0.50-3.00%,
P: 0.030% or less,
S: 0.020% or less,
Al: 0.05% or less, and
N: Contains 0.050% or less, the remainder consists of Fe and impurities,
The tensile strength is 440 MPa or more, the microstructure has a bainite area ratio of 50% or more, an average grain size of bainite of 20 μm or less, and an average value of local equivalent strain in bainite of 0.050 or more, A thick steel plate having a thickness of 6 mm or more and 100 mm or less .

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