JP4742597B2 - Production method of non-tempered high strength steel - Google Patents

Description

  The present invention relates to a method for producing non-tempered high-tensile steel having a tensile strength of 570 MPa or more, and is particularly excellent in material stability and suitable for applications such as bridges, architecture, civil engineering, shipbuilding, marine structures, tanks and pressure vessels. About things.

Steel plates such as bridges, architecture, civil engineering, shipbuilding, offshore structures, tanks, pressure vessels, etc. have been improved in various properties, such as higher strength and higher toughness. Is required to be uniform in each part of the same steel material and to have small variation in characteristics among multiple steel materials.
This type of high-strength, high-tough steel plate is usually manufactured by the controlled rolling controlled cooling method, the so-called TMCP method. However, when steel is manufactured by this TMCP method, the steel structure changes due to the cooling rate in the cooling treatment after rolling being changed due to the position of the steel, the depth from the surface, or between each steel. Material variations are likely to occur between positions or between multiple steel materials.
In particular, in a high-strength thick steel plate having a tensile strength exceeding 570 MPa, it is common to add a large amount of alloy to ensure strength, or to enhance accelerated cooling. Material variations such as toughness increase. For this reason, there is a demand for a method for producing a high-tensile steel sheet with less variation in strength and low-temperature toughness.

  In response to such a demand, for example, Patent Document 1, Patent Document 2, and Patent Document 3 propose a method for producing a high-strength, high-tough steel plate. In Patent Document 1, the bainite lath is refined by combining optimization of Nb and Ti addition amounts and accelerated cooling at a high cooling rate after strong pressure in the austenite non-recrystallized region, and high strength and high toughness. Techniques for achieving both have been proposed.

  Patent Document 2 proposes a technology that refines austenite grains before rolling in the non-recrystallized zone, and then performs accelerated cooling after strong reduction in the non-recrystallized zone. Further, Patent Document 3 discloses that Ti nitride is finely dispersed to suppress the coarsening of the heated austenite grains and to add a combination of B, Mo, and Nb effective for expanding the austenite non-recrystallization temperature range. Therefore, a technique for effectively refining the austenite grain structure in austenite non-recrystallization zone rolling has been proposed.

However, in either technique, the microstructure becomes a mixed structure of ferrite and bainite, or a mixed structure of bainite and martensite, depending on the rolling conditions or cooling conditions, so that variations in the position of the steel material or between multiple steel materials can be avoided. Absent.
That is, even with the techniques described in Patent Document 1, Patent Document 2, and Patent Document 3, it was difficult to maintain high strength and low temperature toughness with a tensile strength of 570 MPa or more without variation.
JP-A-10-158778 JP 2001-123222 A JP 2000-256777 A

The present invention solves the above-mentioned problems of the prior art, has less variation in the position within the steel material or between a plurality of steel materials, and therefore has less restrictions on the cooling rate after rolling, and has a high tensile strength of 570 MPa or more. It aims at proposing the manufacturing method of the non-tempered high strength steel which is strong and excellent in material stability.

  Here, excellent material stability means that Charpy absorbed energy (vE-40) at −40 ° C .: 100 J or more can be stably obtained.

  In order to achieve the above-mentioned problems, the present inventors have intensively studied various factors affecting strength and low temperature toughness. As a result, in order to avoid variations in strength due to changes in the cooling rate, it is important to obtain a homogeneous structure in a wide range of cooling rates, and when appropriate amounts of Nb and B are added in the extremely low carbon component composition It was found that a bainite single-phase structure was obtained over a wide cooling rate range.

  Furthermore, in order to obtain excellent low-temperature toughness for the steel materials whose components are adjusted as described above, the primary rolling temperature range and the cumulative reduction in the austenite recrystallization region are used to refine the heated austenite grains. It was found that it is important to combine the optimization of the rate balance with the optimization of the secondary rolling temperature range and the cumulative reduction rate in the austenite non-recrystallized region for the purpose of refining the bainite grain structure.

The present invention has been completed based on the above-described findings and further studies. That is, the gist of the present invention is as follows.
1. In mass%, C: 0.005 to 0.03%, Si: 0.05 to 0.50%, Mn: 0.6 to 3.0%, P: 0.025% or less, S: 0.0050 % Or less, Al: 0.1% or less, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.03%, B: 0.0005 to 0.0030%, balance Fe and inevitable impurities After heating the steel consisting of 1050 to 1250 ° C., primary rolling is performed at a rolling temperature T (° C.) of each pass of 30 to 80% of the cumulative rolling reduction ratio: R _X1 (%). Non-tempered high excellent in material stability with a tensile strength (TS) of 570 MPa or more, characterized by air cooling after secondary rolling at 700 to 950 ° C. with cumulative rolling reduction: R _X2 (%) Tensile steel manufacturing method.
However, T (° C.) is according to the following formula (1), and R _X2 (%) is according to the following formula (2).
1040-0.05 (R _X1 -30) ² <T <1160-0.05 (R _X1 -30) ²
(1)
(80-R _X1 ) / (120-R _X1 ) <R _X2 / 100 <(92-R _X1 ) / (100-R _X1 ) (2)
2. Further steel composition: Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.5% or less, Mo: 0.7% or less, V: 0.2% or less, REM: 0.02% Hereinafter, one or two or more of Ca: 0.005% or less and Mg: 0.005% or less are added. The tensile strength (TS) according to 1, wherein the tensile strength (TS) is 570 MPa or more is excellent. Production method of non-tempered high-tensile steel.

  3. A method for producing a high-strength steel having a tensile strength (TS) of 570 MPa or more, characterized by tempering the steel by the production method according to 3.1 or 2 to 400 ° C. or more and the Ac1 transformation point or less.

  According to the present invention, in high-strength steel having a tensile strength of 570 MPa or more, there is little variation in the specimen sampling position in the steel material or between multiple steel materials, and stable tensile strength and excellent low temperature toughness can be stably achieved. In addition, it is possible to produce industrially stable non-tempered high-tensile steel with excellent material stability, which has a remarkable industrial effect.

[Ingredient composition]
The reason for limiting the composition of the steel material used in the present invention will be described in detail. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.005 to 0.03%
C is added to make a bainite single-phase structure independent of the cooling rate and to express the effect of Nb described later. However, if the content is less than 0.005%, the addition effect is poor. On the other hand, if it exceeds 0.03%, pearlite containing cementite appears in the structure. Is easily damaged. For this reason, C is limited to the range of 0.005 to 0.03%. In addition, Preferably it is 0.010 to 0.025%.

Si: 0.05 to 0.50%
Si acts as a deoxidizing material, and at least 0.05% is necessary for steelmaking, but if it exceeds 0.50%, the toughness of the base material deteriorates. For this reason, Si is limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.05 to 0.35%.

Mn: 0.6 to 3.0%
Mn has the effect of increasing the strength of the steel. In the present invention, Mn needs to be contained in an amount of 0.6% or more in order to ensure a tensile strength of 570 MPa or more. On the other hand, if the content exceeds 3.0%, the toughness of the base material is remarkably deteriorated. For this reason, Mn is limited to the range of 0.6 to 3.0%. In addition, Preferably it is 1.0 to 2.0%.

P: 0.025% or less P is an element that increases the strength of steel and deteriorates toughness, and particularly deteriorates the toughness of welds, so it is desirable to reduce it as much as possible. When P exceeds 0.025%, this tendency becomes remarkable, so the upper limit is set. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.005% or more.

S: 0.0050% or less S is an impurity element that deteriorates toughness, and is desirably reduced as much as possible. This tendency becomes significant when S exceeds 0.0050%, so S is limited to 0.0050% or less.

Al: 0.1% or less Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of high-strength steel. Moreover, N in steel is fixed as AlN, and it has the effect of ensuring the hardenability of B. However, when the content exceeds 0.1%, the toughness of the base material is lowered, and the toughness is deteriorated by being mixed in the weld metal part during welding. For this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.01 to 0.07%.

Nb: 0.005 to 0.1%
Nb expands the austenite non-recrystallization temperature range during rolling and contributes effectively to obtain a fine intragranular structure. It is also an element useful for obtaining a bainite structure having excellent toughness by lowering the bainite transformation temperature. In order to obtain such an effect, addition of 0.005% or more is necessary.

  However, if it exceeds 0.1%, the effect is saturated, and further, the austenite recrystallization temperature range during rolling is significantly reduced, thereby inhibiting the recrystallization refinement of the heated austenite grains and reducing the low temperature toughness. Invite. For this reason, Nb was limited to 0.005 to 0.1% of range. In addition, Preferably it is 0.02-0.05%.

Ti: 0.005-0.030%
Ti is a useful element that has a strong affinity for N and precipitates as TiN during solidification, and effectively exhibits the effect of B by fixing N in the steel. In addition, there is also an effect of suppressing the austenite grain growth at the time of heating the material and at the weld heat affected zone to refine the structure.

  In order to acquire such an effect, 0.005% or more needs to be contained. On the other hand, if the content exceeds 0.030%, the cleanliness and toughness of the steel deteriorate. For this reason, Ti is limited to 0.005 to 0.030% of range. In addition, Preferably it is 0.010 to 0.030%.

B: 0.0005 to 0.0030%
B contributes effectively to decrease the old γ grain boundary energy and suppress the nucleation of ferrite by adding a small amount. In order to exhibit this effect and make the steel structure a single phase of bainite, it is necessary to add 0.0005% or more.

  On the other hand, if the content exceeds 0.0030%, the hardenability is remarkably increased and the toughness and ductility of the base material are deteriorated. For this reason, B was limited to the range of 0.0005 to 0.0030%. In addition, Preferably it is 0.0005 to 0.0020%.

  In the present invention, when desired characteristics are further improved, in addition to the basic component system described above, one or two of Cu: 1.0% or less, Ni: 2.0% or less, and / or Cr: 1.5% or less, Mo: 0.7% or less, V: 0.2% or more, and / or REM: 0.02% or less, Ca: 0.005% or less, and Mg: 0 One or two or more of 0.005% or less may be contained.

One or two of Cu: 1.0% or less and Ni: 2.0% or less Cu and Ni are elements that can increase strength while maintaining high toughness, and have little influence on HAZ toughness. It is an element useful for increasing strength.

  When adding Cu, it is preferable to contain 0.1% or more. However, if the content exceeds 1.0%, hot brittleness is generated and the surface properties of the steel sheet are deteriorated. For this reason, Cu was limited to 1.0% or less. In addition, Preferably it is 0.2 to 0.7%.

  When adding Ni, it is preferable to contain 0.1% or more, but even if it contains more than 2.0%, the effect is saturated, the effect commensurate with the content can not be expected, economically disadvantageous Become. For this reason, Ni was limited to 2.0% or less. In addition, Preferably it is 0.2 to 1.7%.

One or two or more of Cr: 1.5% or less, Mo: 0.7% or less, V: 0.2% or less Cr, Mo, and V are all elements that contribute to improving the strength of steel. When adding Cr, it is preferable to contain 0.05% or more, but inclusion exceeding 1.5% degrades the base material and the HAZ toughness. For this reason, it is desirable to limit Cr to 1.5% or less.

  When adding Mo, it is preferable to contain 0.05% or more, but inclusion exceeding 0.7% adversely affects the base material toughness and the HAZ toughness. For this reason, it is desirable to limit Mo to 0.7% or less.

  When adding V, it is preferable to contain 0.01% or more, but the content exceeding 0.2% deteriorates HAZ toughness. For this reason, it is desirable to limit V to 0.2% or less.

One or more of REM: 0.02% or less, Ca: 0.005% or less, and Mg: 0.005% or less REM, Ca, and Mg are elements that contribute to toughness improvement. When adding REM, it is preferable to contain 0.002% or more, but even if it contains more than 0.02%, the effect is saturated, so 0.02% was made the upper limit.

  When adding Ca, it is preferable to contain 0.001% or more, but even if it contains exceeding 0.005%, the effect is saturated, so 0.005% was made the upper limit.

  Mg is a useful element that improves toughness through refinement of crystal grains. When adding Mg, it is preferable to contain 0.001% or more, but even if it exceeds 0.005%, the effect is saturated, so 0.005% was made the upper limit.

The balance other than the components described above is Fe and inevitable impurities.
[Production conditions]
The production method in the present invention will be described. The present invention optimizes the relationship between the rolling range and primary rolling temperature range in the austenite recrystallization temperature range, and optimizes the relationship between the rolling temperature range and cumulative reduction rate in secondary rolling in the austenite non-recrystallization temperature range. Combining is a feature in manufacturing conditions. The “° C.” display relating to the temperature means a temperature of 1/2 t part thickness unless otherwise specified.

  In the present invention, the molten steel having the above-described composition is made into a steel material (slab) after being melted and cast by a conventional method using a known apparatus such as a converter, an electric furnace, a vacuum melting furnace, and reheated to 1050 to 1250 ° C. To do.

  When the reheating temperature is less than 1050 ° C., the deformation resistance in hot rolling becomes high and the rolling reduction per pass cannot be increased, so the number of rolling passes increases, causing a reduction in rolling efficiency, and the predetermined rolling temperature It becomes difficult to ensure the range and the cumulative rolling reduction. Moreover, the case where the casting defect in steel materials (slab) cannot be crimped also arises.

  On the other hand, when the reheating temperature exceeds 1250 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. For this reason, it is preferable to make the reheating temperature of a steel raw material into the range of 1050-1250 degreeC.

In the present invention, after heating to 1050 to 1250 ° C., primary rolling consisting of a plurality of passes with a cumulative reduction ratio of 30 to 80% performed within a range of temperature T (austenite recrystallization temperature) defined by the following formula (1): And austenite before transformation by effectively using secondary rolling consisting of a plurality of passes with a cumulative reduction ratio R _X2 defined by the following formula (2) in the austenite non-recrystallization temperature range of 700 to 950 ° C. By controlling the state, it is possible to obtain an optimum bainite structure for reducing variations in strength and low temperature toughness.
1040-0.05 (R _X1 -30) ² <T <1160-0.05 (R _X1 -30) ²
(1)
Here, R _X1 : Cumulative rolling reduction (%) of primary rolling, T: Temperature (° C.)
(80-R _X1 ) / (120-R _X1 ) <R _X2 / 100 <(92-R _X1 ) / (100-R _X1 ) (2)
Here, R _X2 : Cumulative rolling reduction ratio of secondary rolling (%)
Hereinafter, the rolling conditions will be specifically described. After heating the steel material, primary rolling is performed at a cumulative reduction of 30 to 80% in the austenite recrystallization temperature range defined by the above formula (1), and the austenite grains are sufficiently refined by recrystallization.

  Furthermore, the secondary rolling of the cumulative reduction defined by the following formula (2) is performed in the subsequent austenite non-recrystallization temperature range of 700 to 950 ° C., and a large number of grains are contained in the austenite crystal grains refined by the primary rolling. By introducing strain, the bainite structure is refined.

  In the primary rolling, if the cumulative rolling reduction is 30% or less, the refining is insufficient, and if the cumulative rolling reduction is 80% or more, the cumulative rolling reduction of the secondary rolling is insufficient. Refinement in the austenite crystal grains becomes insufficient and the low temperature toughness deteriorates. For this reason, the cumulative rolling reduction of primary rolling is limited to the range of 30 to 80%.

  In the present invention, the balance between the cumulative rolling reduction and the rolling temperature range is important. When the rolling temperature becomes higher than the temperature defined by the formula (1), the austenite grains recrystallize, but they are not effectively refined, and the temperature is low. Toughness is disadvantageous.

  On the other hand, when the rolling temperature is lower than the temperature defined by the above formula (1), the austenite grains are not sufficiently recrystallized, and some coarse austenite grains remain, so that the low temperature toughness deteriorates. Variations occur. For this reason, primary rolling is limited to the temperature range of Formula (1).

  In secondary rolling, when the rolling start temperature is 950 ° C. or higher, introduction of strain into the austenite crystal grains is insufficient, the structure is not refined, and low temperature toughness is disadvantageous.

  On the other hand, when the rolling end temperature is less than 700 ° C., the deformation resistance becomes too high, the rolling load increases, the load on the rolling mill increases, and the anisotropy of the mechanical properties occurs. The generation of is promoted and the strength decreases.

  In addition, in order to lower the rolling end temperature of the thick material to less than 700 ° C., the time for waiting in the middle of rolling becomes remarkably long, which greatly hinders productivity. For this reason, the temperature range of secondary rolling was 700-950 degreeC.

  In addition, it is important to optimize the cumulative rolling reduction in secondary rolling in relation to the cumulative rolling reduction in primary rolling. When the cumulative rolling reduction in secondary rolling exceeds the range defined by equation (2) and becomes higher, anisotropy occurs in the mechanical properties, and further, the formation of ferrite is promoted and the strength is lowered. Become.

  On the other hand, if the cumulative rolling reduction in the secondary rolling exceeds the range defined by the formula (2) and becomes lower, refinement in the austenite crystal grains becomes insufficient, and the low temperature toughness deteriorates. For this reason, the cumulative rolling reduction of secondary rolling is limited to the range of Formula (2).

  The rolling reduction per pass is not specified, but if the rolling reduction per pass is less than 5%, the number of rolling passes increases and the rolling efficiency decreases, and the above rolling temperature range and cumulative rolling reduction are reduced. Since it becomes difficult to ensure, the rolling reduction per pass is preferably 5% or more.

  Further, in the case of an extremely thick steel plate having a thickness exceeding 60 mm, it is desirable to secure at least one or more rolling passes in which the rolling reduction per pass is 15% or more in the primary rolling for the Zaku pressure bonding.

  In the present invention, the cooling method after the end of rolling can obtain an almost uniform structure in a wide cooling rate range by the combination of the basic composition and rolling conditions described above. Not well specified. In addition, when performing accelerated cooling, it is preferable that cooling stop temperature shall be more than room temperature.

  When manufacturing a thick steel plate by this invention, you may give a tempering process. In the case of a thick steel plate, the toughness of the brittle hardened phase generated during cooling can be improved by tempering at 400 ° C. or higher and the Ac1 transformation point or lower. The tempering may be performed after cooling to room temperature in an off-line heat treatment facility or in-line heating facility after in-line cooling.

  In order to obtain such an effect, it is necessary to set the tempering temperature to 400 ° C. or higher. However, if it exceeds 600 ° C., the strength is lowered, so that the tempering treatment is preferably performed at 400 to 600 ° C.

  According to the present invention, which is characterized by subjecting a steel material having the above-described composition to hot rolling under the above-described conditions, in a high strength with a tensile strength of 570 MPa or more, a non-tempered high having both stable strength and low-temperature toughness. Tensile steel can be easily manufactured. Hereinafter, the effect of the present invention will be described with examples.

  A plurality of steel plates according to the present invention were manufactured under different manufacturing conditions, and the variation in material between the steel plates was investigated. Steel materials prepared in the composition shown in Table 1 by the converter-ladder refining-continuous casting method were made into thick steel plates under various production conditions (hot rolling process, cooling method). Table 2 shows the hot rolling conditions, the cooling conditions, and the thickness of the obtained thick steel sheet. Each of the thick steel plates is a steel according to the present invention in which the component composition is within the range of the present invention, and the hot rolling conditions and the cooling conditions are variously changed within the range of the present invention.

  JIS No. 4 tensile test specimens were collected from the thickness ¼ and ½ positions of each thick steel plate, and a tensile test was carried out in accordance with JISZ2241 defaults to investigate the tensile properties. In addition, V-notch Charpy impact test specimens were collected from the thickness 1/4 and 1/2 positions of each thick steel plate obtained in accordance with JISZ2202, and tested for Charpy impact test in accordance with JISZ2242. The test was carried out at a temperature of −40 ° C., Charpy impact absorption energy was determined, and the low temperature toughness of the base material was evaluated. In addition, the Charpy impact test was made into five each at the board thickness 1/4 and 1/2 position.

  Table 3 shows the tensile test results and Charpy impact test results. The steel according to the present invention has little variation in properties among steel materials even when the production conditions fluctuate, and has a high strength with a tensile strength of 570 MPa or more, a stable high strength, and a Charpy impact absorption energy at −40 ° C. It has a stable low temperature toughness of 100 J or more in individual values.

  Steel having the component composition shown in Table 4 was made into a steel material (slab) by a converter-ladder refining-continuous casting method, and was made into a thick steel plate by various production conditions (hot rolling process, cooling conditions). Some thick steel plates were tempered. Table 5 shows hot rolling conditions and cooling conditions.

  JIS No. 4 tensile test specimens were collected from the 1/4, 1/2, and 3/4 positions of the obtained steel plate thickness, and a tensile test was carried out in accordance with JISZ2241 defaults to investigate the tensile properties. In addition, V-notch Charpy impact test specimens were collected from the obtained thicknesses of 1/4, 1/2 and 3/4 of each thick steel plate in accordance with JISZ2202, and Charpy in accordance with JISZ2242. An impact test was performed at a test temperature of −40 ° C., Charpy impact absorption energy was determined, and the low temperature toughness of the base material was evaluated. In addition, the Charpy impact test was made into five each at each sampling position.

  Tables 6-1 and 6-2 show the tensile test results and Charpy impact test results. In the examples of the present invention, each of the thick steel plates has a stable high strength and a Charpy impact at −40 ° C. at a high strength of 570 MPa or more at each of the steel material thicknesses of 1/4, 1/2 and 3/4. It has stable low-temperature toughness base material characteristics with an absorbed energy of 100 J or more in individual values.

  On the other hand, in the comparative example in which a part of the component composition or the manufacturing condition is out of the scope of the present invention, the base material strength is at any or all of the positions in the steel materials of the thickness 1/4, 1/2, and 3/4. Charpy impact at −40 ° C. is less than 570 MPa (Comparative Examples No. 3-3, No. 6-2, No. 13, No. 15, No. 16) or a base material strength of 570 MPa or more. Stable low-temperature toughness cannot be obtained due to the occurrence of individual values with absorbed energy of less than 100 J (Comparative Examples No. 1-2, No. 1-3, No. 1-4, No. 3-2, No. 4-2, No. 8-2, No. 8-3, No. 11, No. 12, No. 14, No. 17, No. 18).

Claims (2)

In mass%, C: 0.005 to 0.03%, Si: 0.05 to 0.50%, Mn: 0.6 to 3.0%, P: 0.025% or less, S: 0.0050 % Or less, Al: 0.1% or less, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.03%, B: 0.0005 to 0.0030%, balance Fe and inevitable impurities After heating the steel consisting of 1050 to 1250 ° C., primary rolling is performed at a rolling temperature T (° C.) of each pass of 30 to 80% of the cumulative rolling reduction ratio: R _X1 (%). Non-tempered high excellent in material stability with a tensile strength (TS) of 570 MPa or more, characterized by air cooling after secondary rolling at 700 to 950 ° C. with cumulative rolling reduction: R _X2 (%) Tensile steel manufacturing method.
However, T (° C.) is according to the following formula (1), and R _X2 (%) is according to the following formula (2).
1040-0.05 (R _X1 -30) ² <T <1160-0.05 (R _X1 -30) ²
(1)
(80-R _X1 ) / (120-R _X1 ) <R _X2 / 100 <(92-R _X1 ) / (100-R _X1 ) (2) In steel composition, in mass%, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.5% or less, Mo: 0.7% or less, V: 0.2% or less, REM The tensile strength (TS) according to claim 1 is 570 MPa or more, characterized by adding one or more of: 0.02% or less, Ca: 0.005% or less, Mg: 0.005% or less A method for producing non-tempered high-tensile steel with excellent material stability.