JP2010174332A - Non-heat-treated low yield ratio high tensile thick steel plate, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-heat-treated low yield ratio high tensile thick steel plate having a tensile strength of &ge;550 MPa, a yield ratio of &le;80% and a plate thickness of &le;30 mm. <P>SOLUTION: The steel plate has a composition comprising, by mass, 0.12 to 0.18% C, 0.05 to 0.45% Si and 1.2 to 1.8% Mn, and further comprising one or more selected from 0.05 to 0.20% Cu, 0.05 to 0.10% Ni, 0.1 to 0.3% Cr, 0.03 to 0.10% Mo, 0.010 to 0.080% V, 0.0003 to 0.0030% B and 0.005 to 0.030% Nb in such a manner that carbon equivalent Ceq is controlled to the range of 0.34 to 0.40%, and a structure in which surface back layer parts to a position of 1/8 of the plate thickness to a plate thickness direction from the surface and back face comprise martensite and/or bainite of &ge;80% by volume ratio, and the central part of the plate thickness of the thickness of 1/4 of the plate thickness, with the center of the plate thickness as the center, comprises ferrite with the average particle diameter of &ge;12 &mu;m in 50 to 85% by volume ratio. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a high-tensile steel plate suitable for use in bridges, shipbuilding, construction, line pipes, construction machinery, etc., especially for building structures that are subjected to large plastic deformation due to an earthquake and require earthquake resistance. The present invention relates to a high-tensile thick steel plate having a suitable low yield ratio.

In recent years, steel materials having excellent earthquake resistance are required for building structures and the like from the viewpoint of ensuring safety during an earthquake. Moreover, it has been clarified from the conventional research results that the steel material having a lower yield ratio is superior in earthquake resistance. Therefore, it is obliged to use steel materials with a yield ratio of 80% or less for building structures.
In addition, with an increase in the height and span of a building structure, the application of, for example, a 550 MPa class high-tensile steel material having a higher strength than before, to the building structure is increasing.

Conventionally, low yield ratio 550 MPa class high strength steel materials used for building structures were generally manufactured by performing multiple heat treatments such as two-phase region heat treatment and subsequent tempering heat treatment. . However, performing heat treatment a plurality of times left problems such as a rise in manufacturing cost and an increase in the manufacturing period.
In order to deal with such problems, studies have been made on omission of heat treatment such as two-phase region heat treatment and subsequent tempering heat treatment, that is, non-tempering.

  For example, in Patent Document 1, accelerated cooling is performed after rough rolling to supercool austenite (γ), then finish rolling to promote ferrite (α) transformation is performed, and accelerated cooling is further performed after finish rolling. Describes a method for producing high-tensile steel materials with low yield ratio, which achieves both high toughness and low yield ratio by appropriately controlling the ratio of soft phase and hard phase by making the soft phase α finer Has been. According to the technique described in Patent Document 1, it is possible to manufacture a high yield steel material having a low yield ratio without requiring addition of an expensive alloy element or complicated heat treatment with low productivity. However, in the technique described in Patent Document 1, since accelerated cooling is performed between rough rolling and finish rolling, temperature unevenness is likely to occur in the steel material, and therefore, variation in characteristics at each position of the steel material may increase. Concerned.

  Patent Document 2 contains C: 0.02 to 0.04%, solute B: 0.0002 to 0.002%, a composition having a formula CEN related to the alloy element content within a predetermined range, and mainly bainite. A 590 MPa class non-tempered type low yield ratio high tensile strength steel sheet composed of a structure in which 0.8 to 2.5% by volume of island martensite is dispersed is described. According to the technique described in Patent Document 2, it is said that a non-tempered high-tensile steel sheet having characteristics that do not require preheating and a low yield ratio can be manufactured over a wide plate thickness range. However, in the technique described in Patent Document 2, it is necessary to reduce the carbon content within a narrow range, and to ensure strength, it is necessary to contain a large amount of alloy elements, which increases the manufacturing cost. There was a problem of inviting.

In Patent Document 3, Patent Document 4, and Patent Document 5, in order to reduce the amount of alloying elements added, low cooling ratio non-refining high that achieves both high strength and low yield ratio by utilizing accelerated cooling. A method for producing tensile steel is described.
In the technique described in Patent Document 3, rolling is completed at least at the _Ar3 transformation point or higher, and then air-cooled until a predetermined temperature is reached to produce an appropriate amount of ferrite, and then 350 ° C from a predetermined temperature in the two-phase region. Accelerated cooling to a temperature between ˜600 ° C. to obtain a high-strength steel sheet that is untempered and has a low yield ratio. In the technique described in Patent Document 4, rolling is completed at least at the _Ar3 transformation point or more, and then air-cooled until a predetermined temperature is reached to produce an appropriate amount of ferrite, and then a predetermined temperature in the two-phase region. Accelerating and cooling to a temperature of 250 ° C. or lower, followed by tempering heat treatment. In the technique described in Patent Document 5, the billet containing Nb .003-.030%, subjected to at least A _r3 hot rolling to end rolled above the transformation point, air-cooled to then become a predetermined temperature Then, an appropriate amount of ferrite is produced, and then accelerated cooling from a predetermined temperature in the two-phase region to a temperature between 400 and 550 ° C. However, with the techniques described in Patent Document 3, Patent Document 4, and Patent Document 5, there is a concern that the amount of ferrite produced varies depending on a slight difference in cooling start temperature, resulting in large variations in material. Therefore, in actual work, it is necessary to strictly control the cooling start temperature, which makes it difficult to perform stable manufacturing and reduces productivity.

  Patent Document 6 discloses that a steel material containing 0.08 to 0.70% of Mo and W in terms of (Mo + W / 2) is hot-rolled at a temperature in the range of 800 to 950 ° C. A method for producing a non-tempered high-tensile steel plate is described, which is accelerated by cooling at a cooling rate and subjected to a cooling treatment that stops accelerated cooling at 500 to 670 ° C. to obtain a high-tensile steel plate having a low yield ratio. . According to the technique described in Patent Document 6, the central structure of the plate thickness is a composite structure including a soft phase of ferrite as a main phase and island-shaped martensite as a main phase and containing a hard phase of 20% by volume or less. In other words, it is possible to obtain a thick steel plate with a low yield ratio and less material variation in the thickness direction.

JP-A-10-306316 Japanese Unexamined Patent Publication No. 2000-219934 JP 63-219523 A JP 63-223123 A Japanese Unexamined Patent Publication No. 01-301819 JP 2007-177325 A

However, in the technique described in Patent Document 6, it is necessary to contain a large amount of expensive Mo and W, and there is a problem that the manufacturing cost increases.
In addition, when accelerated cooling as in the prior art described above is applied to a relatively thin thick steel plate, particularly a thick steel plate having a thickness of 30 mm or less, if the cooling stop temperature of accelerated cooling is lowered, the entire plate thickness is hardened. A phase is formed and the yield ratio tends to increase. On the other hand, when the cooling stop temperature of accelerated cooling becomes high, the entire plate thickness becomes a ferrite + pearlite structure, and a desired high strength cannot be secured. Therefore, in order to produce a relatively thin steel plate with a thickness of 30 mm or less that combines high strength with a tensile strength of 550 MPa or more and a low yield ratio of 80% or less with a conventional technique. Requires that the accelerated cooling process be performed at a very narrow cooling stop temperature, and depending on the prior art, it is not possible to stably produce a thick steel plate having the desired characteristics described above with high productivity. It was difficult.

  The present invention solves the above-mentioned problems of the prior art, is not tempered without heat treatment, is inexpensive without containing a large amount of alloying elements, has a high strength with a tensile strength of 550 MPa or more, and a yield ratio of 80. An object of the present invention is to provide a non-tempered low yield ratio high-tensile steel plate having a thickness of 30 mm or less and a method for producing the same. Another object of the present invention is to produce the above-described thick steel plate with high productivity without performing controlled rolling at a low temperature and without performing heat treatment. Note that the “thick steel plate” here refers to a steel plate having a thickness of 30 mm or less and 12 mm or more.

    In order to achieve the above-mentioned object, the present inventors diligently studied a method for ensuring a low yield ratio. As a result, by making a multilayer structure thick steel plate with a different structure in the plate thickness direction in which the surface layer portion of the steel plate has a hard structure and the central portion of the plate thickness is a relatively soft structure, the entire plate thickness direction is regarded as the soft phase. The inventors have conceived that a thick steel plate having high strength and a low yield ratio can be obtained without forming a micro two-phase structure in which a hard phase is dispersed. In other words, if the center of the plate thickness is a relatively soft structure and the hardness or strength differs greatly between the center of the plate thickness and the surface layer, a clear upper and lower Yield behavior where the yield point disappears was shown, and as a result, it was newly found that the yield ratio was lowered as in the case where the entire region in the thickness direction was made to have a micro two-phase structure. The “plate thickness central portion” as used herein refers to a region having a thickness of 1/4 of the plate thickness centered on the plate thickness center, and the “front and back layer portion” refers to the thickness direction from the front surface or the back surface. The area up to 1/8 position of the plate thickness.

The present invention has been completed based on such findings and further studies.
That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.12 to 0.18%, Si: 0.05 to 0.45%, Mn: 1.2 to 1.8%, P: 0.020% or less, S: 0.005% or less, Al: 0.08% or less, and Cu : 0.05 to 0.20%, Ni: 0.05 to 0.10%, Cr: 0.1 to 0.3%, Mo: 0.03 to 0.10%, V: 0.010 to 0.080%, B: 0.0003 to 0.0030%, Nb: 0.005 to 0.030% The selected one or two or more types are expressed by the following formula (1): Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Ni, Cu, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by is contained in a range of 0.34 to 0.40%, has a composition composed of the balance Fe and inevitable impurities, and from the front surface and the back surface to 1/8 position of the plate thickness in the plate thickness direction. The front and back layer portions have martensite and / or bainite with a volume ratio of 80% or more, have a structure with an average hardness of 230 to 290 HV, and are ¼ of the plate thickness centering on the plate thickness center. The thickness region has a structure in which ferrite having an average particle diameter of 12 μm or more includes a second phase having an average particle diameter of 50 to 85 μm or less and a mean hardness of 190 HV or less with a volume ratio of 50 to 85%. A non-refined low yield ratio high tensile steel plate with a thickness of 30 mm or less, characterized by a tensile strength of 550 MPa or more and a yield ratio of 80% or less.
(2) The non-tempered low yield ratio high-tensile thick steel plate according to (1), wherein, in addition to the above composition, the composition further contains, by mass%, Ti: 0.003 to 0.030%.
(3) In (1) or (2), in addition to the above composition, 1% selected from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050% in mass% A non-tempered low yield ratio high-tensile steel plate characterized by comprising a seed or a composition containing two or more.
(4) In the manufacturing method of the thick steel plate which gives a hot rolling process and the subsequent cooling process to a steel raw material, and makes the said steel raw material in mass%, C: 0.12-0.18%, Si : 0.05-0.45%, Mn: 1.2-1.8%, P: 0.020% or less, S: 0.005% or less, Al: 0.08% or less, further Cu: 0.05-0.20%, Ni: 0.05-0.10%, Cr: 0.1 to 0.3%, Mo: containing one or more selected from 0.03 to 0.10%, a steel material having a composition consisting of the balance Fe and inevitable impurities, the hot rolling step, The steel material is heated to 1050 to 1250 ° C., the rolling reduction at a surface temperature in the range of 900 to 1000 ° C. is 50% or more, and the rolling end temperature is a temperature in the range of 830 to 900 ° C. The cooling step starts cooling from a temperature of 770 ° C. or higher after the hot rolling is completed, and the average cooling rate to a temperature in the range of 550 to 670 ° C .: 10 to 80 A method of producing a non-tempered low yield ratio high strength thick steel sheet having a thickness of 30 mm or less, a tensile strength: 550 MPa or more, and a yield ratio: 80% or less, characterized in that it is a step of accelerated cooling at a temperature of ° C / s.
(5) In (4), in addition to the said composition, it is set as the composition which contains Ti: 0.003-0.030% by mass% further, The manufacturing method of the non-tempered low yield ratio high-tensile steel plate characterized by the above-mentioned.
(6) In (4) or (5), in addition to the above composition, in addition to mass, Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, Mg: 0.0010 to 0.0050% A method for producing a non-tempered low yield ratio high-tensile thick steel sheet, characterized by comprising a seed or a composition containing two or more.
(7) In the manufacturing method of the thick steel plate which gives a hot rolling process and the subsequent cooling process to a steel raw material, and makes the said steel raw material in mass%, C: 0.12-0.18%, Si : 0.05-0.45%, Mn: 1.2-1.8%, P: 0.020% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005-0.030%, the following (1) Formula Ceq = C + Mn / 6 + ( Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Ni, Cu, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by the above is contained in a range of 0.34 to 0.40%, and a steel material having a composition consisting of the balance Fe and unavoidable impurities is used. Heating to 1250 ° C, the cumulative rolling reduction in the range of 950 to 1050 ° C at the surface temperature is 50% or more, the rolling end temperature is a step of hot rolling at a temperature in the range of 880 to 950 ° C, The cooling step is a step of starting cooling from a temperature of 770 ° C. or higher after the hot rolling is completed and accelerating cooling at an average cooling rate of 10 to 80 ° C./s to a temperature in the range of 550 to 670 ° C. A method for producing a non-tempered low yield ratio high tensile steel plate having a sheet thickness of 30 mm or less, a tensile strength of 550 MPa or more, and a yield ratio of 80% or less.
(8) In (7), in addition to the above composition, Cu: 0.05 to 0.20%, Ni: 0.05 to 0.10%, Cr: 0.1 to 0.3%, Mo: 0.03 to 0.10%, V: 0.010 A method for producing a non-tempered, low yield ratio, high-tensile steel plate characterized by having a composition containing one or more selected from -0.080% and B: 0.0003-0.0030%.
(9) The non-tempered low yield ratio high-tensile thick steel plate according to (7) or (8), wherein, in addition to the above composition, the composition further contains Ti: 0.003 to 0.030% by mass%. Manufacturing method.
(10) In any one of (7) to (9), in addition to the above-mentioned composition, the mass is selected from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050%. A method for producing a non-tempered, low yield ratio, high-tensile thick steel plate, characterized in that the composition contains one or more types.

  According to the present invention, suitable for a building structure, high tensile strength of 550 MPa or more and low yield ratio of yield ratio of 80% or less. Thick steel plates can be manufactured without heat treatment without being tempered and with appropriate addition of alloying elements without containing a large amount of alloying elements, and they are inexpensive and excellent in productivity.

It is explanatory drawing which shows typically the point of the hardness measurement in an Example.

First, the manufacturing method of this invention thick steel plate is demonstrated.
The reason for limiting the composition of the steel material used in the present invention will be described first. Hereinafter, unless otherwise specified, mass% is simply referred to as%.
C: 0.12-0.18%
C is an element having an effect of increasing the strength of steel and increasing the content of hard phases other than ferrite, and is a useful element for ensuring high strength and a low yield ratio. In the present invention, the content of 0.12% or more is required in order to ensure the desired high strength and low yield ratio. On the other hand, if the content exceeds 0.18%, the weldability is significantly reduced. For this reason, C was limited to the range of 0.12 to 0.18%. In addition, Preferably it is 0.13-0.16%.

Si: 0.05-0.45%
Si is an element that acts as a deoxidizer, and such an effect is recognized when the content is 0.05% or more. On the other hand, if the content exceeds 0.45%, the toughness of the base material is lowered and the weld heat affected zone toughness is lowered. For this reason, Si was limited to the range of 0.05 to 0.45%. In addition, Preferably it is 0.05 to 0.35%.

Mn: 1.2-1.8%
Mn is an element having an effect of increasing the strength of steel, and in the present invention, in order to minimize the content of other alloy elements and ensure a high strength of a desired tensile strength of 550 MPa or more, 1.2% or more It is necessary to contain. On the other hand, if the content exceeds 1.8%, the toughness of the base material is lowered and the weld heat affected zone toughness is also significantly lowered. For this reason, Mn was limited to the range of 1.2 to 1.8%. In addition, Preferably it is 1.6% or less.

P: 0.020% or less P is an element that has the effect of increasing the strength of steel but lowering toughness, and further significantly lowering the toughness of welds. In the present invention, P is desirably reduced as much as possible. In particular, when the content exceeds 0.020%, the tendency becomes remarkable. Therefore, in the present invention, the content is limited to 0.020% or less. In addition, Preferably it is 0.015% or less. Moreover, since excessive reduction of P raises refining cost and becomes economically disadvantageous, it is more preferable to set it as 0.005% or more.

S: 0.005% or less S is an element that usually exists in steel as inclusions and has the effect of reducing ductility and toughness and causing hot brittleness. In the present invention, S is desirably reduced as much as possible. In particular, when the content exceeds 0.005%, a large amount of MnS is generated in the central segregation portion of the steel material, and defects such as casting defects are liable to occur, and the toughness of the base material and the welded portion is lowered. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.003% or less. Moreover, since excessive reduction of S raises refining cost and becomes economically disadvantageous, it is more preferable to set it as 0.001% or more.

Al: 0.08% or less
Al acts as a deoxidizer and is the most commonly used element in the melting of high-strength steels. To ensure this effect, 0.005% or more is desirable, but 0.08% Containing more than 10 lowers the toughness of the steel (base material). Moreover, in a welding part, it mixes with a weld metal and reduces the toughness of a weld metal. For this reason, Al was limited to 0.08% or less. In addition, Preferably it is 0.010 to 0.045%.

The steel material used in the present invention includes Cu: 0.05 to 0.20%, Ni: 0.05 to 0.10%, Cr: 0.1 to 0.3%, Mo: 0.03 to 0.10%, V: 0.010 to 0.080%, in addition to the components described above. It contains one or more selected from B: 0.0003 to 0.0030% and Nb: 0.005 to 0.030%.
Cu: 0.05-0.20%, Ni: 0.05-0.10%, Cr: 0.1-0.3%, Mo: 0.03-0.10%, V: 0.010-0.080%, B: 0.0003-0.0030%, Nb: 0.005-0.030% One or more selected from
Cu, Ni, Cr, Mo, V, B, and Nb are all elements that increase the strength of steel, and one or two or more are selected and contained for increasing the strength.

Cu is an element that has the effect of increasing the strength of the steel through solid solution strengthening and hardenability. In order to obtain such an effect, it is necessary to contain 0.05% or more, but 0.20 If the content exceeds 50%, hot brittleness becomes prominent and surface properties deteriorate. For this reason, when it contained, it limited to 0.05 to 0.20% of range.
Ni is an element that improves the toughness, increases the strength of the steel without causing a decrease in toughness, and has a small adverse effect on the weld heat affected zone toughness. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.10%, the production cost increases. For this reason, when it contained, Ni was limited to 0.05 to 0.10% of range.

Cr has an effect of increasing the strength of steel through improvement of hardenability, and is a useful element for increasing the strength. Such an effect is recognized at a content of 0.1% or more, but a content exceeding 0.3% causes an increase in production cost. For this reason, when contained, Cr is limited to a range of 0.1 to 0.3%.
Mo has an effect of increasing the strength of steel through improvement of hardenability and is a useful element for increasing the strength. In particular, the inclusion of Mo increases the hardness of the second phase, and can simultaneously lower the yield ratio and increase the strength. Such an effect is recognized when the content is 0.03% or more. On the other hand, a content exceeding 0.3% causes an increase in production cost. For this reason, when it contained, Mo was limited to 0.03 to 0.10% of range.

V has an effect of increasing the strength of steel through precipitation hardening, and is a useful element for increasing the strength. Such an effect is recognized when the content is 0.010% or more. On the other hand, if the content exceeds 0.080%, the toughness of the base metal decreases and the toughness of the weld heat affected zone decreases. For this reason, when contained, V is limited to a range of 0.010 to 0.080%.
B is an element having an action of increasing the strength of steel through hardenability, and such an effect can be obtained with a content of 0.0003% or more, while a content exceeding 0.0030% has a base metal toughness and welding heat. Reduces affected area toughness. For this reason, when contained, B is limited to the range of 0.0003 to 0.0030%. In addition, Preferably it is 0.0007 to 0.0020%.

  Nb is an element that has the action of improving the hardenability, further promoting the effect of controlled rolling, refining the microstructure, and increasing the strength of the steel. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, the content exceeding 0.030% lowers the base metal toughness and the weld heat affected zone toughness. For this reason, when it contained, Nb was limited to 0.005 to 0.030% of range. In addition, Preferably it is 0.007 to 0.020%.

Although the above-mentioned components are basic components, in addition to these basic compositions, the steel material used in the present invention further includes Ti: 0.003-0.030% and / or Ca: 0.0005-0.0050%, REM: One or more selected from 0.0010 to 0.0050% and Mg: 0.0010 to 0.0050% can be contained.
Ti: 0.003-0.030%
Ti has strong bonding strength with N and precipitates as TiN during solidification, suppresses the coarsening of austenite grains in the weld heat affected zone during welding, or acts as a ferrite transformation nucleus to increase the toughness of the weld heat affected zone. It is an element that contributes and can be contained if necessary. In order to obtain such an effect, the content of 0.003% or more is required. However, the content exceeding 0.030% promotes the coarsening of TiN grains, and the above effect cannot be expected. For this reason, when it contains, it is preferable to limit Ti to 0.003 to 0.030% of range. In addition, More preferably, it is 0.010 to 0.020%.

Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, Mg: One or more selected from 0.0010 to 0.0050%
Ca, REM, and Mg all contribute to improving the toughness and ductility of the base metal through the control of sulfide morphology, and when dispersed as fine sulfide particles in steel, It is an element which acts and contributes to increase the toughness of the weld heat affected zone, and can be selected as necessary and contained in one or more kinds. In order to obtain such an effect, it is necessary to contain at least Ca: 0.0005%, REM: 0.0010%, and Mg: 0.0010%. On the other hand, when the content exceeds Ca: 0.0050%, REM: 0.0050%, and Mg: 0.0050%, excessive inclusions may be generated, and the toughness may decrease. For this reason, when it contains, it is preferable to limit to Ca: 0.0005-0.0050%, REM: 0.0010-0.0050%, Mg: 0.0010-0.0050%.

In the present invention, the above-described components are contained within the above-described content range and the following formula (1): Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Ni, Cu, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq (%) defined in the above is contained so as to be in the range of 0.34 to 0.40%. In the present invention, the carbon equivalent Ceq defined by the formula (1) is adjusted within a range of 0.34 to 0.40% in order to have a good balance between strength and yield ratio and to ensure good weldability.

When Ceq is less than 0.34%, it is difficult to stably secure a desired high strength and a desired low yield ratio. On the other hand, when Ceq exceeds 0.40%, the weldability is significantly lowered. For this reason, the carbon equivalent Ceq (%) was limited to a range of 0.34 to 0.40%.
The balance other than the components described above consists of Fe and inevitable impurities.
As long as the steel material has the above-described composition, the manufacturing method is not particularly limited, but the molten steel having the above-described composition can be obtained by a conventional melting method such as a converter, an electric furnace, or a vacuum melting furnace. It is preferable to make a steel material such as a slab by a casting method such as a continuous casting method through melting and further deoxidation treatment or degassing process as necessary.

In the present invention, the steel material having the above composition is subjected to a hot rolling process and a subsequent cooling process to obtain a thick steel sheet having a desired shape and a thickness of 30 mm or less.
In the hot rolling process, the steel material is first heated to a temperature in the range of 1050 to 1250 ° C. in a heating furnace or the like.
When the heating temperature is less than 1050 ° C., the alloy elements are not sufficiently dissolved, the strength of the thick steel plate as a product is lowered, and a desired high strength cannot be ensured. On the other hand, when the heating temperature exceeds 1250 ° C., coarsening of austenite crystal grains becomes remarkable, and desired high toughness cannot be ensured. For this reason, the heating temperature of the steel material was limited to a temperature in the range of 1050 to 1250 ° C. In addition, it is preferably 1050 to 1150 ° C. from the viewpoint of adjusting the austenite grain size.

  Further, in the hot rolling process, when Nb is not contained, the cumulative rolling reduction in the range of 900 to 1000 ° C. at the surface temperature is 50% or more, and the rolling end temperature is in the range of 830 to 900 ° C. Hot rolling is performed. On the other hand, when Nb is contained, hot rolling is performed such that the cumulative rolling reduction in the range of 950 to 1050 ° C. at the surface temperature is 50% or more and the rolling end temperature is in the range of 880 to 950 ° C. The temperature in the hot rolling process is the surface temperature unless otherwise specified.

  In the present invention, in order to obtain a thick steel plate that simultaneously satisfies a low yield ratio and a high strength, it is important to make the central portion of the plate thickness a soft layer. Therefore, in the present invention, it is necessary to generate an appropriate amount of ferrite at the center of the plate thickness, and austenite is recrystallized at the time of hot rolling so as to refine the austenite. If austenite is refined, an appropriate amount of ferrite is generated, and the second phase other than ferrite is refined to prevent a decrease in toughness. For this reason, when it does not contain Nb, controlled rolling is performed so that the cumulative rolling reduction in the temperature range of 900 to 1000 ° C. is 50% or more. On the other hand, when Nb is contained, the rolling reduction is set to 50% or more in the temperature range of 950 to 1050 ° C. at the surface temperature. If Nb is not included, the cumulative reduction ratio in the temperature range of 900 to 1000 ° C. or Nb is not more than 50% in the temperature range of 950 to 1050 ° C., so that the cumulative reduction ratio is too small. Recrystallization of austenite does not proceed. For this reason, austenite refinement cannot be achieved, an appropriate amount of ferrite having an appropriate grain size cannot be generated, and a desired low yield ratio cannot be ensured. The cumulative rolling reduction in the temperature range is preferably 60% or more.

  In the hot rolling process, if the rolling end temperature is less than 830 ° C. (when Nb is not contained) or less than 880 ° C. (when Nb is contained), controlled rolling in the austenite non-recrystallization temperature region becomes excessive, At the central portion of the plate thickness, the ferrite grains obtained are excessively refined, the yield point rises, and the desired low yield ratio cannot be ensured. Further, in the front and back layer portions, the amount of ferrite produced increases, and it becomes impossible to ensure a desired hard phase structure even when rapidly cooled in the cooling step. On the other hand, when the rolling end temperature exceeds 900 ° C. (when Nb is not included) or exceeds 950 ° C. (when Nb is included), the recrystallized fine grains are coarsened and austenite can be refined. Disappear. For this reason, the rolling end temperature in the hot rolling process is limited to a temperature in the range of 830 to 900 ° C. when Nb is not contained, and limited to a temperature in the range of 880 to 950 ° C. when Nb is contained. .

A cooling process is performed following the hot rolling process. Note that the surface temperature is used as the temperature in the cooling step unless otherwise specified.
In the cooling step, after completion of hot rolling, cooling is started from a temperature of 770 ° C. or higher, and accelerated cooling is performed at an average cooling rate of 10 to 80 ° C./s to a temperature in the range of 550 to 670 ° C. (cooling stop temperature).
If the cooling start temperature of accelerated cooling is less than 770 ° C, ferrite will be generated in the front and back layer parts, so the hardness after rapid cooling will decrease, and the front and back layer parts shall be areas with a hard structure with the desired hardness. It becomes difficult. For this reason, the cooling start temperature in the cooling step is limited to a temperature of 770 ° C. or higher.

  In addition, when the cooling stop temperature exceeds 670 ° C., the strength is lowered and it becomes impossible to obtain a thick steel plate that secures a desired high strength. Further, when the cooling stop temperature is less than 550 ° C., particularly in a relatively thin steel plate having a thickness of 30 mm or less in the composition range of the present invention, the entire plate thickness direction becomes bainite and / or martensite, which is high strength but high yield. Ratio of thick steel plate. For this reason, the cooling stop temperature in the cooling step is limited to a temperature in the range of 550 to 670 ° C.

  Further, if the cooling rate of accelerated cooling is less than 10 ° C./s on average, ferrite transformation is induced in the middle of accelerated cooling even in the front and back layer portions, and a desired hard structure cannot be secured. On the other hand, when the cooling rate of accelerated cooling exceeds 80 ° C./s on average, warpage may occur, and it becomes difficult to keep the steel plate shape flat. For this reason, the cooling rate of accelerated cooling was limited to 10 to 80 ° C./s on average.

  The thick steel plate obtained by the manufacturing method described above has the above-described composition, and the front and back layer portions from the front surface and the back surface to the 1/8 position of the plate thickness in the plate thickness direction are martensite having a volume ratio of 80% or more and And / or a bainite-containing structure having an average hardness of 230 to 290 HV, and a central portion of the thickness of the region having a thickness of 1/4 of the thickness centered on the central thickness, Ferrite of 12μm or more contains 50 to 85% by volume, and the second phase has an average grain size of 50μm or less, and has a structure with an average hardness of 190HV or less, tensile strength: 550MPa or more, yield ratio : Non-tempered low yield ratio high-tensile steel plate with 80% or less.

  In the present invention, the structure of the front and back layer portions is a hard structure mainly composed of martensite and / or bainite having a volume ratio of 80% or more. By making the structure mainly composed of martensite and / or bainite, the yield point disappears and the yield ratio can be lowered. On the other hand, in a structure in which ferrite is generated and the volume ratio of martensite and / or bainite is less than 80%, the upper yield point appears and the yield ratio increases. If the hardness of the front and back layers is less than 230 HV in average hardness, a desired high strength (tensile strength of 550 MPa or more) cannot be secured, or a low yield ratio of 80% or less cannot be achieved. On the other hand, if the hardness of the front and back layer parts exceeds 290 HV on average, cracking of the steel sheet surface and reduction in toughness are caused. For this reason, the front and back layer portions were limited to a structure containing martensite and / or bainite having a volume ratio of 80% or more and having an average hardness of 230 to 290 HV. When present, the second phase other than martensite and / or bainite is ferrite having a volume ratio of less than 20%, or even pearlite.

In the present invention, the structure in the central portion of the plate thickness is made of a soft structure having a volume ratio of 50 to 85% ferrite and the remaining second phase and having an average hardness of 190 HV or less. The ferrite has an average particle size of 12 μm or more, and the second phase has an average particle size of 50 μm or less.
In the present invention, it is important to make the central portion of the plate thickness a soft structure in order to have both high strength and a low yield ratio. By making the central part of the plate thickness a soft structure with an average hardness of 190 HV or less, the difference in hardness between the front and back layers becomes large, and a low yield ratio and high strength can be combined. In order to make the center of the plate thickness a soft structure with an average hardness of 190 HV or less, it is necessary to make soft ferrite 50% or more by volume, but the ferrite exceeds 85% at the center of the plate thickness. If it is contained, the strength is remarkably lowered, so that the tensile strength at the entire plate thickness is lowered, and a tensile strength of 550 MPa or more cannot be achieved. On the other hand, if the ferrite at the center of the plate thickness is a fine grain having an average grain size of less than 12 μm, the yield strength increases and it is difficult to achieve a low yield ratio. In addition, when the average particle size of the second phase such as pearlite, bainite, martensite, etc. exceeds 50 μm, the toughness is significantly lowered. For this reason, the structure of the central portion of the plate thickness includes ferrite having an average particle diameter of 12 μm or more in volume ratio of 50 to 85%, and further includes a second phase having an average particle diameter of 50 μm or less, and an average hardness of 190 HV or less. Limited to organization.

  Hereinafter, the present invention will be further described in detail based on examples.

The molten steel having the composition shown in Table 1 was smelted in a converter and further subjected to ladle refining, and then made into a steel material (slab) by a continuous casting method. These steel materials were subjected to a hot rolling process and a cooling process under the conditions shown in Table 2 to obtain thick steel sheets having a thickness of 12 to 30 mm.
Test pieces were collected from the obtained thick steel plates and subjected to structure observation, tensile test, hardness test, and impact test. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation including the front and back portions of the obtained thick steel plate or the center portion of the plate thickness is collected, the cross section perpendicular to the rolling direction is polished, corroded with a nital liquid, and at each position. The tissue was observed. Observation of the tissue was performed by observing at least 5 fields of view with an optical microscope magnification: 400 times and imaging, and measuring the fraction and particle size of each phase. The fraction of each phase and the particle size were measured using an image analyzer. In addition, the average particle diameter measured the area of each grain, calculated | required the square root (nominal particle diameter) of the average area, and made it the average particle diameter.
(2) Tensile test JIS No. 5 full thickness tensile test specimens were collected from the obtained thick steel plate in accordance with JIS Z 2201 and subjected to tensile test in accordance with JIS Z 2241. YS (0.2% yield strength) and tensile strength TS were determined, and the yield ratio (= (YS / TS) × 100%) was calculated.
(3) Hardness test From the obtained thick steel plate, the region from the front or back surface to the 1/8 position of the plate thickness in the plate thickness direction (front and back layer part) and the plate thickness ¼ centered. Measure the Vickers hardness as shown in Fig. 1 using a Vickers hardness tester (test force: 98N). did. At the front and back layers, the pitch from the front or back to the 1/8 position or 7/8 position (1/8 t position or 7/8 t position) of the sheet thickness t is 0.5mm in the sheet thickness direction. The thickness from 1/4 position (1 / 4t position) to 3/4 position (3 / 4t position) of the thickness was measured at a pitch of 0.5 mm. The arithmetic average of the obtained values was defined as the average hardness at each position of the thick steel plate.
(4) Impact test V-notch impact test specimens were collected from 1 mm below the surface of the obtained thick steel plate so that the center of the specimen was 6 mm below the surface in accordance with the provisions of JIS Z 2242. An impact test was carried out to determine the fracture surface transition temperature Trs ₅₀ (° C.).

  The obtained results are shown in Table 3.

  Each of the examples of the present invention is a non-tempered low yield ratio high tension thick steel plate having a high tensile strength: 550 MPa or more and a low yield ratio of 80% or less and excellent toughness. On the other hand, in the comparative examples that are outside the scope of the present invention, the strength is insufficient, the toughness is lowered, the yield ratio is high, or the strength, toughness, and yield ratio all satisfy the predetermined values. Either was not or was.

Claims (10)

% By mass
C: 0.12-0.18%, Si: 0.05-0.45%,
Mn: 1.2 to 1.8%, P: 0.020% or less,
S: 0.005% or less, Al: 0.08% or less, further Cu: 0.05-0.20%, Ni: 0.05-0.10%, Cr: 0.1-0.3%, Mo: 0.03-0.10%, V: 0.010-0.080%, B: One or more selected from 0.0003 to 0.0030% and Nb: 0.005 to 0.030% so that the carbon equivalent Ceq defined by the following formula (1) is in the range of 0.34 to 0.40% Containing, having a composition consisting of the balance Fe and inevitable impurities,
The front and back layers from the front and back to the 1 / 8th position of the plate thickness in the plate thickness direction contain martensite and / or bainite with a volume ratio of 80% or more, and have an average hardness of 230 to 290 HV. ,
The region with a thickness of 1/4 of the plate thickness centered on the center of the plate thickness is 50 to 85% by volume of ferrite with an average particle size of 12 μm or more, and the second phase with an average particle size of 50 μm or less. Including a structure having an average hardness of 190 HV or less,
Non-refined low yield ratio high tensile steel plate with a thickness of 30 mm or less, characterized by tensile strength: 550 MPa or more and yield ratio: 80% or less.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Here, C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (mass%)   In addition to the said composition, it is set as the composition which contains Ti: 0.003-0.030% by the mass%, The non-tempered low yield ratio high-tensile thick steel plate of Claim 1 characterized by the above-mentioned.   In addition to the above composition, the composition further contains, by mass%, one or more selected from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050%. The non-tempered low yield ratio high tension thick steel plate according to claim 1 or 2. In the method of manufacturing a thick steel plate, the steel material is subjected to a hot rolling step and a subsequent cooling step to make a thick steel plate,
The steel material in mass%,
C: 0.12-0.18%, Si: 0.05-0.45%,
Mn: 1.2 to 1.8%, P: 0.020% or less,
S: 0.005% or less, Al: 0.08% or less, further Cu: 0.05-0.20%, Ni: 0.05-0.10%, Cr: 0.1-0.3%, Mo: 0.03-0.10%, V: 0.010-0.080%, B: A steel material containing one or more selected from 0.0003 to 0.0030% and having a composition comprising the balance Fe and inevitable impurities,
In the hot rolling step, the steel material is heated to 1050 to 1250 ° C., the cumulative rolling reduction in the surface temperature range of 900 to 1000 ° C. is 50% or more, and the rolling end temperature is in the range of 830 to 900 ° C. It is a process of hot rolling that is a temperature,
The cooling step is a step of starting cooling from a temperature of 770 ° C. or higher after the hot rolling is completed, and accelerating cooling at an average cooling rate of 10 to 80 ° C./s to a temperature in the range of 550 to 670 ° C.,
A method for producing a non-tempered low yield ratio high tensile steel plate having a sheet thickness of 30 mm or less, a tensile strength of 550 MPa or more, and a yield ratio of 80% or less.   In addition to the said composition, it is set as the composition which contains Ti: 0.003-0.030% by the mass%, The manufacturing method of the non-tempered low yield ratio high-tensile steel plate of Claim 4 characterized by the above-mentioned.   In addition to the above composition, the composition further contains, by mass%, one or more selected from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050%. The method for producing a non-tempered low yield ratio high-tensile thick steel plate according to claim 4 or 5. In the method of manufacturing a thick steel plate, the steel material is subjected to a hot rolling step and a subsequent cooling step to make a thick steel plate,
The steel material in mass%,
C: 0.12-0.18%, Si: 0.05-0.45%,
Mn: 1.2 to 1.8%, P: 0.020% or less,
S: 0.005% or less, Al: 0.08% or less,
Nb: 0.005-0.030%
Is a steel material having a composition comprising the balance Fe and inevitable impurities, so that the carbon equivalent Ceq defined by the following formula (1) is in the range of 0.34 to 0.40%,
In the hot rolling step, the steel material is heated to 1050 to 1250 ° C., the cumulative rolling reduction in the range of 950 to 1050 ° C. at the surface temperature is 50% or more, and the rolling end temperature is in the range of 880 to 950 ° C. It is a step of performing hot rolling that is a temperature, and after the hot rolling is finished, the cooling starts from a temperature of 770 ° C. or higher, and an average cooling rate to a temperature in the range of 550 to 670 ° C .: 10 to A method for producing a non-tempered low yield ratio high strength thick steel plate having a thickness of 30 mm or less, a tensile strength of 550 MPa or more, and a yield ratio of 80% or less, characterized by accelerated cooling at 80 ° C./s .
Record
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Here, C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (mass%)   In addition to the above composition, Cu: 0.05 to 0.20%, Ni: 0.05 to 0.10%, Cr: 0.1 to 0.3%, Mo: 0.03 to 0.10%, V: 0.010 to 0.080%, B: 0.0003 to The method for producing a non-tempered low yield ratio high-tensile thick steel plate according to claim 7, wherein the composition contains one or more selected from 0.0030%.   In addition to the said composition, it is set as the composition which contains Ti: 0.003-0.030% by the mass%, The manufacturing method of the non-tempered low yield ratio high-tensile steel plate of Claim 7 or 8 characterized by the above-mentioned.   In addition to the above composition, the composition further contains, by mass%, one or more selected from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050%. A method for producing a non-tempered low yield ratio high-tensile thick steel plate according to any one of claims 7 to 9.

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