JP5747398B2 - High strength steel - Google Patents

Description

The present invention is mainly used for steel structures such as ships, bridges, buildings and construction machines, and contains C, Si, Mn, P, S, Al and N, and the balance is made of Fe and inevitable impurities. The present invention relates to a high-strength steel having a strength (TS) of 500 MPa or more and an N content of 0.0040 to 0.0300 mass %.

  As shown in Patent Documents 1 to 4, this type of high-strength steel has been required to reduce its nitrogen content to 0.0022% by mass or less. Therefore, a high-content nitrogen material such as recycled iron is required. Is used, denitrification treatment is required, and even if such treatment is performed, if the nitrogen content cannot be sufficiently reduced, especially if the treatment cost is too high, it is not used, or Cr, Mo or The problem of high nitrogen content was solved by using rare elements such as rare earth elements, but there was a contradiction that the improvement of resource efficiency due to reuse of scrap iron was lost by the use of rare elements.

Further, as shown in Patent Document 5, attempts have been made to solve this problem by containing Ti, but as a result, it is only lacking in practicality for the following reasons. It was.
The purpose of adding Ti was to form TiN precipitates with Ti and N in the material in order to improve the impact properties of the HAZ, and to reduce N in the solid solution state as much as possible. However, on the contrary, by including Ti, Ti (N, C) precipitates are coarsened, and the effect of refinement of ferrite crystal grains due to Ti precipitated particles is lost, and the cost of Ti is added. As a result, the material cost is only increased.

Patent Document 6 exemplifies high-strength steel with high nitrogen content exceeding the nitrogen content, but does not show any properties that can be used for the above-mentioned applications.
The purpose of this patent document is mainly used for steel structures such as ships, bridges, buildings, construction machines, etc., containing C, Si, Mn, P, S, Al and N, the remainder from Fe and inevitable impurities The high strength steel is provided without adding an expensive additive element having a tensile strength (TS) of 500 MPa or more and an N content of 0.0040 to 0.0300 mass %. On the other hand, the purpose of Patent Document 6 relates to a soft nitrided steel excellent in cold forging and bending fatigue strength, and is completely different from this patent.

  In view of such circumstances, an object of the present invention is to provide a high-strength steel containing high nitrogen that can be used in the above-described applications without using a rare element.

Invention 1 has a chemical component composition of mass%,
C: 0.10 to 0.20%,
Si: 0.10 to 0.50%,
Mn: 0.60 to 1.60%,
P: 0.025% or less,
S: 0.015% or less,
Al: 0.010 to 0.060%
N: 0.0040 to 0.0300%
And the balance is high strength steel consisting of Fe and inevitable impurities,
The tensile strength (TS) is 500 MPa or more, and the separation index (SI value) in the V-notch Charpy impact test is 0.10 or less.
Invention 2 is characterized in that in the high-strength steel of Invention 1, the ductile-brittle transition temperature is −50 ° C. or lower.

The present invention not only has the necessary characteristics for the application, but also can significantly improve the nitrogen content to the extent that rare elements are unnecessary and denitrification treatment is unnecessary even in the use of recycled iron. Realized high strength steel.
In addition, it does not require special equipment or inclusions in its production, and has a practical advantage in that it can be realized by combining conventional equipment and methods.
As part of an increase in the recycling rate of steel materials in recent years, electric furnace melting steel using recycled iron as a main raw material has attracted attention. However, in the electric furnace melting process, it absorbs nitrogen gas and becomes molten steel containing high nitrogen. In addition, it is not necessary to perform degassing refining treatment for denitrification on high-concentration nitrogen molten steel, which is not possible with molten steel melted in a converter, and prevents deterioration of steel properties due to high nitrogen content Therefore, it is not necessary to add an expensive rare alloy element for the purpose, so that the intended steel material can be produced at a low cost.
For rolled steel plates, it is important to use rolling methods that avoid the need for large-scale investment in rolling equipment by avoiding low-temperature, large-strain processing, especially when used in ships mainly composed of welded structures. Among steel material properties, in particular, a high-strength steel plate in which separation parallel to the rolled surface of the steel plate, which causes anisotropy of Charpy impact properties, is difficult to occur was obtained.

It is an illustration photograph of the crack (separation) which generate | occur | produced in parallel with the rolling surface observed on the test piece fracture surface after the Charpy impact test of the steel plate obtained in Comparative Example 4. It is an example of the SEM image structure which indicated the residual martensite recognized by the steel plate obtained by the comparative example 4 (the tempering process is not performed) with the arrow. It is an example of the SEM image structure | tissue in which the residual martensite was erase | eliminated by the tempering process in the steel plate obtained in Example 2.

The embodiment of the method for producing a steel sheet according to the present invention is as follows.
<From melting and refining process in electric furnace to slab preparation process>
First, as a melting method of molten steel, melting and refining are performed using an electric furnace and recycled iron as a main raw material, and molten steel having a chemical component composition under the ladle (notation in mass%) within the following range is melted.
C: 0.10 to 0.20%,
Si: 0.10 to 0.50%,
Mn: 0.60 to 1.60%,
P: 0.025% or less,
S: 0.015% or less,
Al: 0.010 to 0.060%
N: 0.0040 to 0.030%
Then, the components are adjusted so that the balance is Fe and inevitable impurities.

In the electric furnace melting process, the nitrogen content inevitably increases due to the melting of nitrogen in the atmosphere.
The amount of increase in nitrogen content varies depending on various conditions such as dissolution conditions, but is about 0.01 to 0.02% by mass.
In high-strength thick steel plates, impact characteristics are reduced due to an increase in the nitrogen content. However, in the present invention, degassing refining for denitrification is not performed in the refining process, and expensive Nb, V, Zr, etc. No rare alloy elements are added. As described above, the reason for not performing denitrification refining is that if the steel sheet manufacturing method of the present invention is carried out, an equiaxed fine grain structure having an average ferrite grain size of 6 μm or less can be obtained. Since a high-tensile thick steel plate exceeding 500 MPa can be obtained and the residual martensite can be eliminated, a thick steel plate having a separation index SI value, which is an index of anisotropy of Charpy impact properties, of 0.10 or less can be obtained. is there. The upper shelf energy is 120 J / cm ² or more, and there is no problem with the level of shock absorption energy. In the above, the equiaxed is defined as an aspect ratio of crystal grains of 3.0 or less.

  After the obtained molten steel is cast into a steel ingot or slab, it is roughly rolled into a slab having a shape and size suitable for steel plate rolling. The conditions for rough rolling from the steel ingot or slab to the slab are not particularly specified, but may be within the range in which the steel plate as the final product can ensure the desired dimensions and shape.

<First rolling process>
The first object is to perform rolling in the range of 900 to 1050 ° C. to obtain a sufficient γ phase so that a coarse cast structure does not remain, and subsequent γ / α phase by cooling. The transformation is to obtain α particles as fine as possible.
The slab is heated and the next first stage rolling is performed within the range of 900 to 1050 ° C. That is, one-pass rolling is performed within a range where the reduction rate per pass is 10 to 50%, or the reduction rate per pass is within a range of 10 to 50% and the time between passes is within 30 seconds. Multiple pass rolling with a cumulative rolling reduction of 70% or less is performed.
Either one-pass rolling or multiple-pass rolling may be performed, and as long as the above conditions are satisfied, the number of passes is not limited, but should be performed in consideration of the shape and dimensions of the thick steel plate of the final product. After this rolling, a hot rolled material is prepared by cooling to a temperature below the Ar ₁ transformation point at a cooling rate of 3 ° C./second or more.
The process from slab heating to rolling and cooling will be referred to as a “first rolling process”. For example, see the first rolling process column in the items described in Table 2.
After the first rolling, the steel sheet is in the state of a steel sheet having a sheet thickness of 40 mm and having physical properties of a TS400 MPa level with a normal average grain size of 20 μm.

(Regarding heating in the first rolling step and rolling conditions)
The reason for rolling the slab within a range of 900 to 1050 ° C. is that a coarse cast structure does not remain by obtaining a sufficient γ phase. The reason why the rolling temperature is set to 1050 ° C. or lower is to prevent the γ phase particle size from becoming coarse due to grain growth in the heating process of the slab.

In the first rolling process, among the reasons why the rolling reduction per pass is in the range of 10 to 50%, the reason for setting it to 10% or more is to sufficiently secure the α-phase nucleation site and promote fine graining. It is to do. On the other hand, the reason why the rolling reduction per pass is 50% or less is to prevent an excessive load on the rolling equipment.
The case where the rolling of the first rolling process is a multi-pass rolling occurs based on the size / shape of the steel ingot or slab, the size / shape of the final steel plate, and the specifications of the rough rolling and steel plate rolling equipment specifications, etc. By setting appropriate operating conditions. In this case, the reason for setting the time between passes within 30 seconds is that the strain accumulated in the γ phase is prevented from recovering under a single pressure, and the α phase nucleation site is sufficiently secured to make the particles fine. Because it is appropriate to promote. And the reason why the cumulative reduction ratio in the multi-pass rolling is set to 70% or less is that the cumulative reduction ratio in the second stage rolling is secured 60% or more as will be described later. The rolling reduction in the relatively low temperature rolling to be performed can be performed without imposing an excessive load on the rolling equipment by defining the upper limit, and therefore, the upper limit of the cumulative rolling reduction is set to 70%. Is appropriate.
In the present invention, the thickness of the thick steel plate is not particularly specified, but the current steel ingot or slab dimensions and shape, specifications of the heating and rolling equipment, etc., and rolling in the first rolling step and the second rolling step described below. 5 to 150 mm is an appropriate range in consideration of the regulation of each rolling reduction in.

(About cooling conditions of the first rolling process)
In the final stage of the first rolling step, the steel is cooled to a temperature not higher than the Ar ₁ transformation point at a cooling rate of 3 ° C./second or more. This is to prevent the structure of the material from becoming coarse due to grain growth. On the other hand, the reason why the cooling rate is set to 50 ° C./second or less is to prevent an extreme difference in the thermal history of the entire plate thickness depending on the portion of the thick steel plate when the plate thickness is relatively thin.

Further, the reason why the cooling ultimate temperature is made not more than the Ar1 transformation point is that by completing the α phase transformation from the old γ phase, the old γ phase that does not contribute to the refinement of ferrite does not remain. For this reason, the lower limit value of the cooling reached temperature after rolling in the first rolling step is not particularly defined, but when cooling to an excessively low temperature, the energy consumed for heating before rolling in the next first rolling step is excessive. It becomes. Therefore, it is preferable that the cooling ultimate temperature does not become about 500 ° C. or less. Incidentally, Ar ₁ transformation point depending on the chemical composition of the steel may be calculated by a known method. Thereafter, in the present invention, other transformation point temperatures are obtained by the same method.

<Second rolling process>
The purpose of this second rolling is to obtain as fine a ferrite structure as possible by strongly reducing in the γ + α2 phase region, and to secure a structure having a high strength of over 500 MPa.
The hot-rolled material obtained in the first rolling step is heated at a heating rate of 0.5 ° C./second or more, and from Ac ₁ transformation point or higher to {Ac ₁ transformation point + 0.5 (Ac ₃ transformation point-Ac ₁ transformation point)} In the following temperature range, a multi-pass rolling is performed in which the rolling reduction per pass is 35% or more and the time between passes is within 20 seconds. The cumulative rolling reduction in this multi-pass rolling is set to 60% or more. This rolling is performed at a strain rate of 0.1 to 200 / sec.
The process from the heating of the hot-rolled material to the end of the rolling is referred to as a “second rolling process”. For example, refer to the second rolling process column in the items described in Table 2.
The physical property of the steel sheet after the second rolling is TS> 500 MPa as strength, but because of the γ + α2 phase rolling, the texture develops and the Charpy impact property is toughness when cracks parallel to the plate surface develop. It is characterized by having an inferior organization. The thickness is about 15 mm.

(Regarding heating in the second rolling step and rolling conditions)
The reason for setting the heating rate to 0.5 ° C./second or more is to prevent the α phase from coarsening due to grain growth. On the other hand, the upper limit of the heating rate is not defined from the viewpoint of preventing the α-phase grain growth. Note that the upper limit may be set at 50 ° C./second based on the capacity of the current heating equipment.

The reason why the heating temperature at the start of rolling in the second rolling step is set to the Ac ₁ transformation point or higher is to reduce the deformation resistance due to rolling and at the same time to ensure a sufficient α-phase to which strain is applied. The upper limit of the temperature is {Ac ₁ transformation point +0.5 (Ac ₃ transformation point-Ac ₁ transformation point)}} because the γ phase necessary for promoting the introduction of working strain into the α phase where strain is applied by rolling is sufficient. This is to promote the finer ferrite.

  In the rolling of the second rolling step, the rolling reduction per pass is set to 35% or more in order to secure sufficient α-phase recrystallization sites and γ-phase precipitation sites and promote ferrite refinement. It is. Here, although the upper limit value of the rolling reduction per pass is not particularly defined, it is preferably 50% or less in consideration of the current rolling equipment capacity. Furthermore, the reason for setting the time between passes within 20 seconds is to prevent recovery of the strain accumulated in the α phase under reduction, and to make the cumulative reduction amount 60% or more by multiple pass rolling. This is because the recrystallization site of the α phase and the precipitation site of the γ phase are sufficiently secured to further promote the refinement of ferrite.

  Furthermore, setting the strain rate of rolling in the second rolling step to 0.1 / second or more is that the time required for rolling is too long, and dislocations that should have been added during rolling are recovered, and a large number of dislocations are generated. This is because it cannot be introduced, and even if dynamic recrystallization occurs in the α phase, fine crystal grains cannot be obtained. Moreover, the reason why the strain rate is set to 200 / second or less is to ensure the generation of dynamic recrystallization of the α phase and promote the refinement of ferrite.

<Reheating / holding and cooling immediately after the second rolling step>
Immediately after rolling in the second rolling step, the external heating means reheats to a predetermined temperature range at a predetermined heating rate, and holds for a predetermined time.
The reheating and holding process and the α + γ two-phase region rolling in the rolling of the second rolling process described above promote the introduction of processing strain into the α phase by the γ phase, and the recrystallization of the α phase and the γ phase. This is because a steel material having an equiaxed fine grain structure, here, a steel sheet can be obtained by utilizing the suppression of grain growth by the mutual pinning effect due to competition with precipitation due to reverse transformation. In particular, the rolling flattenes the grain shape and simultaneously introduces strain into the α phase, and then reheating causes the flat grain shape to be equiaxed by recrystallization of the α phase and precipitation of the γ phase.

(About reheating and holding conditions)
By setting the heating rate in the reheating to 0.5 ° C./second or more, the α phase and the γ phase were prevented from becoming coarse due to grain growth. By setting the holding temperature to (Ac ₁ transformation point + 0.8 (Ac ₃ transformation point−Ac ₁ transformation point)) or less, the γ phase becomes dominant and the γ phase is prevented from becoming coarse due to grain growth. It is. In order to improve production efficiency in the manufacturing process, the reheating / holding temperature is more preferably in the range of 800 to 850 ° C.
The reheating is carried out so that the holding time is within 90 seconds because the γ phase is prevented from coarsening due to grain growth. On the other hand, the reheating / holding time is set to 5 seconds or more because the reheating holding effect is ensured.

  The reason for using an external heating means as the above reheating means is that the reheating by the internal energy of the steel sheet may be difficult because reheating immediately after rolling, control of the reheating speed, and heating over the entire thickness may be difficult. Therefore, it should be reheated by external energy such as a heating furnace or induction heating equipment.

(Cooling conditions after reheating and holding)
When the reheating / holding is completed, it is desirable to enter the next cooling process as soon as possible in order to prevent the γ phase from becoming coarse due to grain growth. The time between the completion of reheating and the start of the next cooling is 90 seconds or less, preferably 30 seconds or less.

The cooling rate after reheating and holding is 3 ° C./second or more because the γ phase is prevented from coarsening due to grain growth. The faster the cooling rate, the more reliably the γ phase can be prevented from coarsening due to grain growth. On the other hand, when the cooling rate becomes excessively large, the martensite structure and the bainite structure become larger in size than the previous γ phase particle size. Therefore, the toughness is reduced at the same time as it does not contribute to the refinement. Therefore, a cooling rate is prescribed | regulated to 50 degrees C / sec or less. Further, if the thickness of the thick steel plate is 5 mm or more, there is a possibility that an extreme difference is caused in the overall thermal history. Therefore, the upper limit of the cooling rate is effective in order to prevent this.
The reason why the temperature at the end of the cooling process is 500 ° C. or less is that if the temperature at the end is too high, α transformation from the old γ grains is not completed and the old γ grains remain. Because. The cooling temperature is desirably 200 ° C. or higher. This is because it becomes difficult to obtain a complete ferrite-pearlite structure when rapidly cooled to a temperature lower than that. And after that, what is necessary is just to air-cool to normal temperature.

<Tempering treatment>
The steel plate obtained above is tempered at a temperature within the range of 480 to 520 ° C.
(About tempering treatment)
The reason for applying the tempering treatment is to erase martensite that may slightly remain, thereby further reducing the separation index SI value in the Charpy impact test.
In other words, it is possible to obtain a very important effect that the anisotropy of impact characteristics is improved (the separation index SI value is reduced). At this time, the reason why the processing temperature is set to 480 ° C. or more is that equiaxiality can be promoted by crystal rotation using the distortion of crystal grains as a driving force due to diffusion of Fe atoms in a relatively high temperature region. The separation index SI value decreases. Another reason is that if the impurity element phosphorus (P) is present in the material at a temperature lower than this, P is segregated at the prior austenite grain boundaries during the tempering treatment, and at a low temperature. Since there is a possibility of promoting grain boundary cracking, the meaning of avoiding this is also included.
On the other hand, the reason why the processing temperature is set to 520 ° C. or lower is that at a temperature exceeding this, it becomes difficult to ensure sufficient strength due to the coarsening of crystal grains due to grain growth.

<Chemical component composition of slab used in the present invention>
In the present invention, the chemical component composition under the ladle of the molten steel produced by the electric furnace is defined as described above. When the molten steel is cast into a steel ingot or slab, and then rolled into a slab shape by a normal ingot rolling method or a slab rolling method, at least the chemical component composition specified in the present invention of the slab is the rolling Since it does not change in the process, it can be considered the same as the chemical composition under the ladle. Furthermore, the chemical component composition does not change in the manufacturing process of the thick steel plate according to the present invention. Therefore, if the chemical component composition under the ladle is adjusted within a desired range, the chemical component composition of the final thick steel plate can be regarded as the same. Therefore, at any stage of the manufacturing process of the thick steel plate, the influence of the chemical composition of the steel on the rolling characteristics and material characteristics is governed by the chemical composition under the ladle, that is, the chemical composition of the slab.

(About the chemical composition of slabs)
In the present invention, the chemical component composition of the slab needs to be the following in mass%.
C: 0.10 to 0.20%, Si: 0.10 to 0.50%, Mn: 0.60 to 1.60%,
P: 0.025% or less, S: 0.015% or less, Al: 0.010 to 0.060%,
N: 0.0040 to 0.030%, the balance being Fe and inevitable impurities.

In order to obtain a component composition premised on application to a plate thickness of up to 200 mm, the specified range of the chemical composition composition of rolled steel for welded structures specified in JIS G3106 is satisfied, and within that range, for the following reason Limited.
(1) The reason why the C content is 0.10% by mass or more is to secure the strength of the steel at 500 MPa in consideration of the use being a thick steel plate for ship structures. The reason why the C content is 0.20% by mass or less is to prevent deterioration in welding performance.
(2) The reason why the Si content is 0.10% by mass or more is to ensure the performance as a deoxidizing element and as a steel strengthening element. The reason why the Si content is 0.50% by mass or less is to prevent the surface properties of the steel from being deteriorated and the welding performance from being deteriorated, and also to prevent the toughness from being lowered.
(3) The reason why the Mn content is 0.60% by mass or more is to increase the hardenability of the steel to increase the strength and at the same time to ensure the performance contributing to the improvement of toughness. The reason why the Mn content is 1.60% by mass or less is to prevent deterioration of weldability and workability.
(4) The reason why the Al content is 0.010% by mass or more is to ensure the performance as a deoxidizing element. The reason why the Al content is 0.060% by mass or less is to prevent the weldability of steel from being deteriorated.
(5) The P content and the S content both degrade the ductility and toughness of the steel and are desirably low, but excessively reducing it is not desirable in terms of cost. Therefore, considering both balances, the P content is 0.025% by mass or less, and the S content is 0.015% by mass or less.

  (6) In the case of a slab manufactured from molten steel that does not undergo any denitrification treatment in electric furnace melting using recycled iron (for example, steel material generated from a ship that has been dismantled) as the content of N When recycled iron is used, N is contained in an amount of about 0.004% by mass or more. Therefore, the lower limit of the N content is set to 0.0040% by mass in consideration of analysis accuracy. On the other hand, in order to further demonstrate the economic effect of using recycled iron, it is effective to use more recycled iron. From this point of view, the content is preferably 0.01% by mass or more. However, considering the toughness deterioration of the steel due to the increase in N content, the toughness level in the case of carrying out the present invention is also considered, and the upper limit content needs to be 0.03% by mass or less. Then, considering the analysis accuracy of N, it is set to 0.0300 mass% or less. However, according to the production method of the present invention, when the N content increases, the ferrite grains tend to become finer, the strength is further improved, but the ductility is reduced, and the increase in the N content increases the steel content. Considering that fracture toughness deteriorates, it is desirable that it is 0.0150 mass% or less. From the above, the N content is in the range of 0.0040 to 0.0300 mass%, and preferably in the range of 0.0040 to 0.0150 mass%.

<Selection of desirable manufacturing conditions>
Among the material properties of the thick steel plate obtained by the manufacturing conditions described above, (a) the separation index (SI value) in a V-notch Charpy impact test at an arbitrary temperature is 0.10 or less, and (b) tensile strength. The method of adjusting the manufacturing conditions of the thick steel plate so that (TS) has a material property of 500 MPa or more and (c) a ductile-brittle transition temperature of −50 ° C. or less includes the chemical component composition of the slab. Among the chemical composition described above, the C content is 0.15 to 0.16% by mass, the Si content is 0.19 to 0.25% by mass, the Mn content is 0.95 to 1.10% by mass, And after adjusting N content in the range of 0.0040-0.0150 mass%, the above-mentioned <1st rolling process>, <2nd rolling process>, <reheating and holding immediately after a 2nd rolling process , And cooling> and <tempering> Within the boss was condition, by performing the test using the actual manufacturing facility, it is to select a production condition the material characteristics of the (a) ~ (c) are provided.

<Material properties of thick steel plate>
Within the range of the embodiment of the manufacturing conditions of the first invention described above, the material property of the thick steel plate to be manufactured is a separation index (SI value) in a V-notch Charpy impact test at an arbitrary temperature of 0.10 or less. Thus, it is more desirable to adjust the production conditions so that the tensile strength (TS) is 500 MPa or more and the ductile-brittle transition temperature of the Charpy impact test is −50 ° C. or less. In order to adjust the manufacturing conditions so as to obtain the above material characteristics, in accordance with the equipment and process to be used, a test is performed in advance by a combination of various manufacturing conditions within the scope of the above-described manufacturing conditions of the first invention. By doing so, a condition for obtaining a desired material characteristic may be set.

About Separation Index (SI Value) of 0.10 or Less Separation refers to fracture of a peeled form that occurs in a cross section parallel to the rolling surface of a thick steel plate due to an impact load. Separation due to the rolling direction (= L direction) component and the width direction (= C direction) component of the steel plate of the impact load on the thick steel plate is difficult to occur, but is likely to occur due to the impact load of the plate thickness direction (Z direction) component. Since a steel plate for shipbuilding constitutes a ship by welding in multiple directions, it receives impact loads in the plate thickness direction (Z direction) component at numerous locations during voyages after the construction of the ship. Therefore, it will always be exposed to the environment where separation occurs, and the Z-direction impact resistance is extremely important for securing the strength of the ship.

  Since the V-notch Charpy impact test is usually performed using a C-direction specimen or a L-direction specimen of a thick steel plate, the V-notch is inserted in the thickness direction. Therefore, the separation is observed as a crack having a length parallel to the steel sheet rolling surface at the fracture surface of the V-notch Charpy impact test piece and opens at a right angle to the crack. FIG. 1 illustrates the separation observed on the fracture surface of a V-notch Charpy impact test piece at −80 ° C. of a steel plate obtained in Comparative Example 4 described later in the present specification.

The separation index (SI value) in the Charpy impact test is the sum of the lengths of cracks generated parallel to the rolling surface, which are observed on the specimen fracture surface after the Charpy impact test. The area divided by (excluding the area) is an index of anisotropy of Charpy impact properties. FIG. 1 shows the length of the crack in the measurement of the separation index. Numbers 1 to 6 in the figure indicate cracks, and the measured lengths of the cracks are also shown.
In a thick steel plate in which this separation occurs, it is usually difficult to ensure a high level of energy absorption for impact in the Z direction. In the thick steel plate for shipbuilding that constitutes the ship in the present invention, it is extremely important to secure a high level of absorbed energy against the Z-direction impact. Although the separation index (SI value) varies depending on the test temperature, it is important that the maximum value of the thick steel plate separation index (SI value) is small in consideration of the state used in the ship. Further, when the separation index (SI value) is 0.10 or less, it is extremely effective for suppressing the separation generated by the plate thickness direction (= Z direction) component of the impact load. Therefore, the separation index (SI value) is desirably defined as 0.10 or less.

Tensile strength (TS) is 500 MPa or more. Considering the need for high-strength steel for ship structural steel plates, the tensile strength (TS) is 500 MPa or more.
The ductile-brittle transition temperature is −50 ° C. or less. Since it is for thick steel plates for ship structures, the ductile-brittle transition temperature is desirably −50 ° C. or less in consideration of the use conditions in extreme cold.

In addition, considering that this invention is a manufacturing method of the thick steel plate used by a ship, as above-mentioned, it is a high level regarding a separation index (SI value), tensile strength (TS), and a ductile-brittle transition temperature. While securing the value, it is more desirable that the yield ratio = YS / TS is low. Therefore, as mechanical characteristic values, for example, in the JIS G3106 rolled steel for welded structure, SM490 grade, the conditions that are commonly satisfied for various plate thickness categories are yield point (YP) ≧ 365 MPa, total elongation (El) ≧ 21%. Is more desirable, and the Charpy impact property is in the range of C = 0.10 to 0.20% by mass in Appendix Table 1 (standard mechanical properties by carbon content classification) of JIS G4051 Carbon Steel for Machine Structures It is desirable that the absorbed energy satisfying all the carbon content categories is adopted and is 137 J / cm ² .

  Hereinafter, the effectiveness of the present invention will be described with reference to examples. However, the present invention is not limited by the following examples, and is appropriately within the scope described in the above-mentioned items for carrying out the invention. Modifications can be made and these are included within the scope of the present invention.

  Examples 1-5 and Comparative Examples 1-7 were tested as follows using laboratory-scale rolling equipment. The test materials used in the examples and comparative examples were prepared by melting a molten steel having chemical component compositions of component codes A and B shown in Table 1 using a 100 kg high-frequency vacuum melting furnace for experiments and casting it into a mold. After preparing the steel ingot, it was rolled into a steel bar having a cross-sectional size of 55 mm thickness x 65 mm width at 1200 ° C. using an experimental rolling mill, and from this, a test material of 50 mm thickness x 60 mm width x length 300 mm was prepared. 1 to 5 and Comparative Examples 1 to 7 were prepared by cutting out the required number by surface cutting and length cutting.

In addition, although the test material was melted using a 100 kg high-frequency vacuum melting furnace for experiments without using an electric furnace as described above, the chemical component composition was determined as follows. Within the scope of the present invention, Comparative Examples 1-7 can be considered outside the scope of the present invention. In the electric furnace operation, it is assumed that recycled iron is used as a raw material charging iron source, is melted under normal electric furnace operating conditions, and is not subjected to melting and refining for the purpose of denitrification gas. The N content range of the molten steel component under the ladle was set to 0.004 to 0.030 mass% in consideration of each N mass range of the N content and N absorption amount in the electric furnace melting process. The N content has the maximum value and the minimum value, and the other chemical component compositions were set to specific values satisfying SM490A and SM490B in the rolled steel for welded structure of JIS G3106. That is, N content was controlled to 0.0150 mass% for component code A, 0.0040 mass% for component code B, and the other components were as shown in Table 1.
The test material having the component code A or B of 50 mm thickness × 60 mm width × length 300 mm was subjected to Examples 1 to 2 by using a rolling mill for experiments with a roll diameter of 400 mm, a maximum load of 300 t, and a maximum speed of 30 m / min. 5 and Comparative Examples 1 to 7 were rolled under the conditions shown in Table 2.
Various conditions in each step are as shown in Table 2 to Table 4.

Examples 1 to 5 use a test material having a sheet thickness of 50 mm whose N content of the test material is 0.015% by mass or 0.0040% by mass, the first rolling step, the second rolling step, and reheating. The process, cooling and tempering were performed as follows.
In the first rolling step, rolling was performed with a reduction rate (sheet thickness reduction rate) of 20% per pass at 950 ° C., the sheet thickness was set to 40 mm, and then cooled to 500 ° C. at 30 ° C./second. Next, in the second rolling step, the first pass is heated at 5 ° C./second and the reduction rate is 38% (the thickness is reduced from 40 mm to 25 mm) at 750 ° C., and the reduction rate is 40% (the thickness is 24 mm). From 15 mm to 15 mm), the time between passes was 10 seconds, the rolling reduction was 62.5% at a strain rate of 1-2 / second to obtain a plate thickness of 15 mm, and then immediately as a reheating step, a heating furnace After reheating to 800 ° C., 825 ° C. or 850 ° C. at 5 ° C./second and holding for 10 seconds, as a cooling step, it was cooled to 200 ° C. at 30 ° C./second, and then cooled to room temperature. Finally, tempering was performed at 500 ° C. to complete the process. As can be seen from Table 2, the difference in the test conditions between Examples 1 to 5 is that at least one of the chemical component composition and the heating temperature in the reheating step is different.

  In Comparative Examples 1 and 2, test materials having a sheet thickness of 50 mm and N contents of 0.015% by mass and 0.0040% by mass, respectively, were used in the first rolling step (first reduction (20% reduction)) and second pass. (Passing rate 25%), 3rd pass (rolling rate 33%), 4th pass (rolling rate 25%), the time between passes is 10 seconds, and rolling is performed at a cumulative rolling reduction rate of 70%. After setting to 15 mm, air cooling to room temperature was completed. Therefore, it is outside the scope of the present invention.

  Comparative Examples 3 to 7 use a test material having a sheet thickness of 50 mm, where the N content of the test material is 0.015% by mass or 0.0040% by mass, the first rolling step, the second rolling step, and reheating. The process and the cooling process were performed in the same manner as in Examples 1-5. However, since the final tempering process was completed without being performed, it is out of the scope of the present invention. The difference in the test conditions between Comparative Examples 3 to 7 is that at least one of the chemical component composition and the heating temperature in the reheating step is different.

The following accuracy tests were performed on the steel plates having a thickness of 15 mm obtained in the examples and the comparative examples.
(1) Charpy impact test: Tested at 25 ° C., 0 ° C., −40 ° C., −80 ° C., −120 ° C., and −196 ° C. using a 2 mmV-notch and C direction test piece. As characteristic values, a separation index SI value (1 / mm), absorbed energy at 0 ° C. (J / cm ² ), and ductile-brittle fracture surface transition temperature (° C.) were obtained.
(2) Tensile test: The yield strength LYS (MPa), the tensile strength TS (MPa), the total elongation TEl (%), and the uniform elongation UEl (%) were determined using a C-direction test piece.
(3) Vickers hardness test: obtain an average Vickers hardness ^{_{H V (N / mm 2)}} .
(4) Microstructure observation: The average crystal grain size (μm) in the central part, ¼ part and surface vicinity of the plate thickness by SEM image was determined by the intercept method, and the crystal grain shape was observed.
Tables 5 and 6 show the characteristic test results obtained in each example and each comparative example.

  All of the thick steel plates obtained in Examples 1 to 5 have a separation index SI of 0.00 to 0.08 or less and a ductile-brittle fracture surface transition temperature of −72 ° C. or less (however, Examples 1 and 4 are No measured value), and the tensile strength is 528 MPa or more, which satisfies the material property level of the thick steel plate obtained under the more desirable conditions of the present invention. As for the mechanical properties other than the above, Charpy absorbed energy at 0 ° C., yield strength as defined in SM490 class among rolled steel materials for welded structure of JIS G3106, When the value of elongation is adopted, Examples 1 to 5 satisfy it. Moreover, it is obtained in Examples 1 and 2 in which the N content is 0.0150% of the higher content and in Examples 3 to 5 in which the N content is 0.0040% of the lower content. It can be seen that the material property levels of the thick steel plates are equivalent.

  On the other hand, Comparative Examples 1 and 2 are cases where the N content is 0.0150% by mass and 0.0040% by mass, respectively, and the rolling process is significantly different from the examples. A thick steel plate is rolled at a rolling temperature of 950 ° C. at a rolling temperature of 950 ° C. at a total rolling reduction rate of 70% and air-cooled. The obtained thick steel plate has an excellent separation index SI of 0.00, but the tensile strength TS is 489 MPa or less and does not clear the target value of 500 MPa or more, and the strength is insufficient. Also, the yield strength is considerably lower than in the examples. This is mainly because the average ferrite grain size is about 15 μm and is not refined.

  On the other hand, Comparative Examples 3 to 7 are different from Examples 1 to 5 in that the tempering treatment is not performed as the last step under the manufacturing conditions. Therefore, Comparative Examples 3 to 7 are excellent in strength and impact absorption energy and ductility-brittle transition temperature of the thick steel plate, which are equivalent to or higher than those in Examples 1 to 5, but ductility (here, total elongation) As for TE1, uniform elongation UEI), an improvement trend is seen in Examples 1-5.

An important characteristic of the thick steel plate produced in the present invention is that the anisotropy of the impact property, particularly the separation index SI, which is an index of the anisotropy in the thickness direction, is small. A thick steel plate with low toughness anisotropy is obtained.
A desirable target value of the separation index SI in the present invention is 0.10 or less. By performing the tempering treatment, the separation index SI is reduced and the anisotropy of the Charpy impact property is improved. The improvement situation is as follows.
Compared to 0.16 in Comparative Example 3, it is improved to 0.08 in Example 1.
Compared to 0.18 in Comparative Example 4, it is improved to 0.00 in Example 2.
Compared to 0.21 in Comparative Example 5, it is improved to 0.08 in Example 3.
Compared to 0.17 in Comparative Example 7, it is improved to 0.06 in Example 5.

  The level of the separation index SI is quite excellent in both the examples and comparative examples. Conventional target values are usually 0.50 or less. Thus, the reason why the separation index SI is good in both the examples and the comparative examples is that the ferrite crystal grain size is fine and the crystal grain shape is equiaxed. Furthermore, the reason why the examples are superior to the comparative examples is considered to be that tempering annealing limited to a range of 480 to 520 ° C. was performed in the final stage of the manufacturing process. That is, it was confirmed that residual martensite was erased by the tempering treatment under the specified conditions.

  In FIG. 2, the situation of the residual martensite recognized by the steel plate which has not been tempered in Comparative Example 4 is illustrated, whereas in FIG. 3, the tempering process is performed in Example 2. The SEM image microstructure in a state where the residual martensi has been erased is shown as an example of the steel plate. It is considered that anisotropy has been reduced by the tempering treatment. That is, the rotation of the Fe atoms in the high temperature region causes crystal rotation using the strain of the crystal grains as the driving force, and the equiaxedness is promoted, so that the occurrence of separation is reduced and the separation index SI is reduced. Presumed.

In recent years, an increase in the amount of steel used by melting steel scrap in an electric furnace has been predicted due to an increase in the reuse rate of steel and an increase in raw material costs for blast furnaces such as iron ore. In particular, in May 2009, the Ship Recycling Treaty entered into force, and it is expected that steel materials for dismantled hulls will increase. These scrap irons can be used in a process of melting them in an electric furnace and reclaiming them as new steel materials, and are effective as resource-saving steels because they do not add expensive various alloy elements. Moreover, in the production of thick steel plates that require high strength and high toughness even for such recycled steel materials, denitrification refining is possible even for high nitrogen content molten steel in which high concentration of nitrogen is dissolved in the electric furnace melting process. Since it is unnecessary, it can be said that it is effective for energy saving and carbon dioxide emission reduction. Furthermore, since it is excellent in toughness anisotropy with respect to impact which is important in thick steel plates for ship hulls, it is effective as a steel with higher added value.

JP 2001-234242 A JP 2007-32230 A JP 2007-046128 A JP 11-181546 A JP 2008-223081 A JP 2002-69571 A

Claims (2)

The chemical composition is mass%,
C: 0.10 to 0.20%,
Si: 0.10 to 0.50%,
Mn: 0.60 to 1.60%,
P: 0.025% or less,
S: 0.015% or less,
Al: 0.010 to 0.060%
N: 0.0040 to 0.0300%
And the balance is high strength steel consisting of Fe and inevitable impurities,
A high strength steel having a tensile strength (TS) of 500 MPa or more and a separation index (SI value) in a V-notch Charpy impact test of 0.10 or less. The high strength steel according to claim 1, wherein the ductile-brittle transition temperature is -50 ° C or lower.