JP4427521B2 - Method for producing high strength thick steel plate with tensile strength of 780 MPa excellent in weldability - Google Patents

Description

  The present invention does not use expensive Ni, Mo, V, Cu, high-temperature thick steel plates with a tensile strength of 780 MPa, which have excellent preheating-free weldability, and does not require reheating and tempering heat treatment after rolling. The present invention relates to a method of manufacturing with high productivity and low cost. The steel of the present invention is used in the form of a thick steel plate having a thickness of 12 mm or more and 40 mm or less as a structural member of a welded structure such as a construction machine, an industrial machine, a bridge, a building, or a shipbuilding.

  High tensile strength steel sheets with a tensile strength of 780MPa used as welded structural members for construction machinery, industrial machinery, bridges, architecture, shipbuilding, etc., in addition to the high strength and toughness of the base material, the high strength of the structural members Along with the increase in the demand for the production, a 780 MPa class thick steel plate that satisfies high pre-free weldability, is inexpensive, and can be manufactured in a short construction period has been required up to a thickness of about 40 mm. In other words, it is necessary to satisfy the high-strength and high toughness of the base metal and the preheating-free operation at the time of small heat input welding in the low-cost component system and the low-cost manufacturing process in addition to the short construction period.

  For example, as disclosed in Patent Documents 1 to 3, a conventional method of manufacturing a high-strength thick steel plate of 780 MPa class imparted with high weldability is directly quenched immediately after rolling of the steel plate, and then tempered. There are methods by direct quenching and tempering.

  Moreover, regarding the manufacturing method of the high strength thick steel plate of the 780 MPa class in the non-tempering which does not require the reheating tempering heat treatment after rolling, for example, patent documents 4 to 8 disclose the method. In terms of omission, the manufacturing method is excellent in terms of manufacturing period and productivity. Among these, the inventions described in Patent Documents 4 to 7 relate to a manufacturing method by an accelerated cooling-intermediate stop process in which accelerated cooling after rolling of a steel sheet is stopped halfway. The invention described in Patent Document 8 relates to a manufacturing method for cooling to room temperature by air cooling after rolling.

Japanese Patent Laid-Open No. 03-232923 JP 09-263828 A JP 2000-160281 A JP 2000-319726 A Japanese Patent Laid-Open No. 2005-015859 JP 2004-052063 A JP 2001-226740 A Japanese Patent Laid-Open No. 08-188823

  However, for example, in the inventions described in Patent Documents 1 to 3, since reheating and tempering heat treatment is required, there are problems in the manufacturing period, productivity, and manufacturing cost. For such a conventional technique, there is a strong demand for a so-called non-tempered manufacturing method in which reheating and tempering heat treatment can be omitted.

  As a non-tempered manufacturing method, the invention described in Patent Document 4 requires preheating at 50 ° C. or higher during welding as described in the examples, and satisfies high weldability without preheating. There is a problem that can not be. Furthermore, in the invention described in Patent Document 5, since 0.6% or more of Ni needs to be added, it becomes an expensive component system and there is a problem in manufacturing cost. In the invention described in Patent Document 6, it is possible to manufacture only the plate thickness of 15 mm described in the examples, and it is impossible to satisfy the plate thickness requirement of up to 40 mm. Furthermore, even when the plate thickness is 15 mm, there is a problem that the C content is small and the microstructure of the joint becomes coarse and sufficient joint low temperature toughness cannot be obtained. In the invention described in Patent Document 7, since it is necessary to add about 1.0% of Ni as described in the examples, it becomes an expensive component system and there is a problem in manufacturing cost. In the invention described in Patent Document 8, it is possible to manufacture only the plate thickness up to 12 mm described in the examples, and the plate thickness requirement up to 40 mm thickness cannot be satisfied. Furthermore, the rolling condition is characterized by rolling with a cumulative reduction ratio of 16 to 30% in the two-phase temperature range of ferrite and austenite, so that the ferrite grains are likely to be coarsened, and the strength and toughness are likely to deteriorate even in the production of 12 mm thickness. There is also.

  As described above, the high strength, high toughness, and high weldability of the base material can be obtained by adding no expensive alloy elements, such as Ni, Mo, V, and Cu, and omitting the reheating and tempering heat treatment after rolling cooling. A satisfactory method for producing a high-tensile steel plate having a thickness of up to 40 mm has not been invented yet, despite the strong demands of customers.

  In the case of a thick steel plate having a base material strength of 780 MPa, the influence of the plate thickness on the preheating free during welding is very large. If the plate thickness is less than 12 mm, preheating-free can be achieved relatively easily. This is because if the plate thickness is less than 12 mm, the cooling rate of the steel plate during water cooling can be physically increased to 100 ° C./sec or more even at the center of the plate thickness. In this case, a hard martensite or bainite structure can be obtained at a high cooling rate without adding a large amount of C or an alloy element, and a strength of 780 MPa can be obtained. And in the thin 780 MPa steel with a small amount of alloying elements added, the hardness of the weld heat affected zone can be kept low without preheating, and weld cracking can be prevented even without preheating. On the other hand, as the plate thickness increases, the cooling rate during water cooling decreases. For this reason, with the same component as the thin steel plate, the strength of the thick steel plate decreases due to insufficient quenching, and the strength of 780 MPa class cannot be satisfied. In particular, the strength reduction is remarkable at the center portion (1/2 t portion) where the cooling rate is the smallest. If the steel plate is thicker than 40 mm, and the cooling rate is less than 8 ° C./sec, it is essential to add a large amount of alloying elements to ensure the strength of the base material, making it difficult to make welding preheating free.

  Therefore, the present invention provides high strength, high toughness, and high weldability of the base material without adding the expensive alloy elements Ni, Mo, V, and Cu, and omitting reheating and tempering heat treatment after rolling cooling. Satisfactory, specifically, at the center of the thickness of the base metal, the tensile strength is 780 MPa or more, the yield stress is 685 MPa or more, the Charpy absorbed energy at −20 ° C. is 100 J or more, and the necessary preheating temperature for the y-cracking test Is intended to provide a method for producing a high-tensile steel plate having a tensile strength of 780 MPa that can satisfy 25 ° C. or less and has excellent weldability. Here, the plate | board thickness of the steel plate which this invention makes object is 12 mm or more and 40 mm or less.

  In order to solve the above-mentioned problems, the present inventors have made many studies on the base material and the welded joint on the premise of manufacturing by direct quenching after rolling in a component system containing no Ni, Mo, V, or Cu. . As a result of examining the additive component in order to realize preheating free at the time of small heat input welding for the additive component system of Ni, Mo, V and Cu additive-free, the amount of addition of C is 0.03% or more, and 0.0. By strictly regulating the weld cracking susceptibility index, which can be evaluated by the Pcm value, to 0.22% or less after strictly regulating to 055% or less, the required preheating temperature during the y-cracking test can be made 25 ° C or less. It was found that preheating can be made free. However, on the premise of a Pcm value of 0.22% or less, it has been difficult to achieve both the base material strength and toughness over the entire thickness in the thickness direction up to a thickness of 40 mm. On the other hand, as a result of various investigations regarding Nb, Mn, Cr addition amounts and rolling / cooling conditions in the B-added steel, the Nb addition amount was set to 0.02% or more and 0.05% or less, Immediately after adding 1.0% or more and adding 0.4% or more of Cr, adding Mn and Cr so as to satisfy 0.19% or more in Pcm value and rolling at 760 ° C. or more, 700 Achieves both strength and toughness of the base material over the entire thickness in the plate thickness direction up to 40 mm by accelerated cooling at a cooling rate of 8 ° C / sec to 80 ° C / sec from room temperature to 350 ° C. Specifically, it has been newly found that the Charpy absorbed energy at −20 ° C. with a tensile strength of 780 MPa or more, a yield stress of 685 MPa or more can be satisfied.

The present invention has been made on the basis of the above novel findings, and the gist thereof is as follows.
(1) By mass%, C: 0.03% or more, 0.055% or less, Mn: 1.0% or more, 2.3% or less, P: 0.02% or less, S: 0.0050% or less Cr: 0.4% or more, 1.5% or less, Nb: 0.02% or more, 0.05% or less, Ti: 0.005% or more, 0.015% or less, Al: 0.003% or more 0.08% or less, B: 0.0005% or more, 0.0020% or less, N: 0.0015% or more, 0.0060% or less, and the weld crack sensitivity index Pcm value shown below is 0 A steel slab or slab having a component composition of 19% or more and 0.22% or less and the balance Fe and inevitable impurities is heated to 1000 ° C. or more and 1200 ° C. or less and rolled at 760 ° C. or more. Subsequently, from 700 ° C. or higher, the cooling rate is 8 ° C./sec or higher, 80 ° C. / A method for producing a high-tensile steel plate having a tensile strength of 780 MPa class excellent in weldability, characterized by starting accelerated cooling at a sec. or less and stopping the accelerated cooling at room temperature or higher and 350 ° C. or lower.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are C, Si, Mn, Cu, and Ni, respectively. , Cr, Mo, V, B means the content expressed in mass%.
(2) Further, by mass%, Si: 0.10% or more and 0.40% or less, high tensile strength of 780 MPa class excellent in weldability according to (1) above Manufacturing method of thick steel plate.
(3) Further, it is characterized by containing one or two of Mg: 0.0005% or more and 0.01% or less and Ca: 0.0005% or more and 0.01% or less in mass%. The manufacturing method of the high strength thick steel plate of the tensile strength 780 MPa class which is excellent in the weldability as described in said (1) or (2).

  According to the present invention, a plate having a tensile strength of 780 MPa that is suitable as a structural member for welded structures such as construction machines, industrial machines, bridges, buildings, shipbuilding, and the like with strong needs for high strength and excellent in weldability without welding preheating. Produces high-tensile steel plates with a thickness of 12 mm to 40 mm with high productivity and low cost that do not use expensive Ni, Mo, V, Cu, and do not require reheating and tempering heat treatment after rolling. And the effect on the industry is extremely large.

  Below, the reason for limitation of manufacturing methods, such as each component composition and rolling conditions in this invention, is demonstrated.

  C needs to be added by 0.03% or more in order to satisfy the base material strength. If the addition amount exceeds 0.055%, the necessary preheating temperature during welding exceeds 25 ° C. and preheating free cannot be satisfied, so the upper limit is made 0.055%.

  Mn needs to be added in an amount of 1.0% or more in order to achieve both strength and toughness of the base material. If added over 2.3%, coarse MnS that is harmful to toughness tends to be generated in the center segregation portion, which may lead to a decrease in the base metal toughness in the center portion of the plate thickness, so the upper limit is 2.3%. And

  It is desirable not to contain P in order to lower the low temperature toughness of the base material and the joint. The allowable value as an impurity element inevitably mixed is 0.02% or less.

  It is desirable not to contain S in order to reduce the low temperature toughness of the base material and the joint. The allowable value as an impurity element inevitably mixed is 0.0050% or less.

  Cr needs to be added in an amount of 0.4% or more in order to achieve both the base material strength and toughness. If added over 1.5%, the base material toughness of the central portion of the plate thickness may be lowered, so the upper limit is made 1.5%.

  Nb needs to be added in an amount of 0.02% or more in order to achieve both strength and toughness of the base material. If added over 0.05%, the amount of precipitated C as Nb (C, N) increases, and the hardenability decreases due to the decrease in the amount of solid solution C. Since the base material strength may not be obtained, the upper limit is made 0.05%.

  Ti and N are effective for forming fine TiN particles and improving joint toughness through preventing austenite grain coarsening in the weld heat affected zone. Further, Ti has a certain effect on stabilizing the hardenability of the B-added steel. This is because solid solution N causes a decrease in hardenability by reducing the amount of solid solution B due to BN formation, but the effect of improving the hardenability of solid solution B is stable by adding Ti and precipitating solid solution N as TiN. It depends on what is obtained. In order to obtain both of these effects, it is necessary to add 0.005% or more of Ti and 0.0015% or more of N. In order to suppress both the joint low-temperature toughness drop due to coarse TiN particles and the hardenability drop due to excess N, the upper limit is 0.015% for Ti addition and 0.0060% for N addition.

  B is required to be added in an amount of 0.0005% or more in order to improve the hardenability and obtain high strength and toughness of the base material. On the other hand, if added over 0.0020%, the hardenability decreases, and good joint low temperature toughness and sufficient base metal high strength and high toughness may not be obtained, so the upper limit is made 0.0020%. .

  Al is contained in a range of 0.003% to 0.08% of the range usually added as a deoxidizing element.

  Si may be added to ensure the strength of the base material. In order to obtain this effect, addition of 0.10% or more is necessary. However, if added over 0.4%, the toughness of the base metal and joint decreases, so the upper limit is made 0.4%.

  By adding one or two of Mg and Ca, sulfides and oxides can be formed to increase the base metal toughness and joint toughness. In order to obtain this effect, Mg or Ca needs to be added in an amount of 0.0005% or more. However, if it is added in excess of 0.01%, coarse sulfides and oxides are produced, and the toughness may be lowered. Therefore, the addition amount is set to 0.0005% or more and 0.01% or less, respectively.

  Ni, Mo, V, and Cu may be contained in Ni, Mo: 0.05% or less, V: 0.01% or less, and Cu: 0.2% or less because of a slight increase in alloy cost. Good. Furthermore, when Ni, Mo, V, and Cu are inevitably mixed from scrap raw materials or the like, even if contained, it does not increase the cost. Within range.

  The weld cracking susceptibility index Pcm value must be 0.22% or less because preheating during welding cannot be made free, so the upper limit is made 0.22% or less. If the Pcm value is less than 0.19%, the high strength and high toughness of the base material cannot be satisfied, so the lower limit is made 0.19%.

  Here, Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B] [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] in the formula are C, Si, Mn, Cu, and Ni, respectively. , Cr, Mo, V, B means the content expressed in mass%.

  Next, manufacturing methods other than the component composition will be described.

  The heating temperature of the steel slab or slab needs to be 1000 ° C. or higher in order to sufficiently dissolve Nb (C, N), which is Nb carbonitride. If the temperature exceeds 1200 ° C, the austenite grains become coarse and cause toughness reduction, so the upper limit is set to 1200 ° C.

  If the rolling temperature is lower than 760 ° C., ferrite is locally generated due to accumulation of excessive rolling strain, and the high strength and high toughness of the base material may not be obtained. Therefore, the lower limit of the rolling temperature is restricted to 760 ° C.

  When the starting temperature of accelerated cooling after rolling is less than 700 ° C, ferrite is locally generated, and the high strength and high toughness of the base material may not be obtained. Therefore, the lower limit temperature is set to 700 ° C.

  When the cooling rate of accelerated cooling is less than 8 ° C./sec, ferrite is locally generated and the high strength and high toughness of the base material cannot be obtained. Therefore, the lower limit is set to 8 ° C./sec. The upper limit is 80 ° C./sec, which is a cooling rate that can be stably realized by water cooling.

  When the accelerated cooling stop temperature is higher than 350 ° C, the amount of ferrite, upper bainite, and island martensite generated due to insufficient quenching increases, especially in the thickness center of thick materials with a thickness of 30 mm or more. Since the strength cannot be obtained, the upper limit of the stop temperature is set to 350 ° C. The stop temperature at this time is the steel sheet surface temperature when the steel sheet is reheated after the cooling is completed. The lower limit of the stop temperature is room temperature, but the more preferable stop temperature is 100 ° C. or higher in terms of dehydrogenation of the steel sheet.

  Steel pieces obtained by melting steel having the component composition shown in Table 1 were made into steel plates having a thickness of 12 to 40 mm under the production conditions shown in Table 2. Among these, 1-A to 7-E in Table 2 are steels of the present invention, and 8-F to 19-B are comparative examples. In the table, underlined numbers and symbols indicate that manufacturing conditions such as components or rolling conditions deviate from the patent scope, or the characteristics do not satisfy the following target values. In addition, among Ni, Mo, V, and Cu in Table 1, the content that is not 0% is the content as an unavoidable impurity element.

  Table 2 shows the evaluation results of the base material strength, toughness, and weldability of these steel plates. The base material strength was measured by taking a 1A full thickness tensile test piece or a No. 4 round bar tensile test piece specified in JIS Z 2201 and measuring it according to JIS Z 2241. Tensile test specimens are sampled from No. 1A full-thickness tensile specimens with a thickness of 20 mm or less, and No. 4 round bar tensile specimens with thicknesses of more than 20 mm are 1/4 part (1/4 t part) of the thickness and central part of the thickness. (1/2 t part). The base material toughness is obtained by collecting an impact test piece specified in JIS Z 2202 in a direction perpendicular to the rolling direction from the center of the plate thickness, and by Charpy absorbed energy (vE-20) at −20 ° C. by the method specified in JIS Z 2242. ) Was evaluated. Weldability was evaluated by obtaining a preheating temperature necessary for preventing root cracking by performing coated arc welding with a heat input of 1.7 kJ / mm by the method prescribed in JIS Z 3158. The target values for each characteristic were a base material yield stress of 685 MPa or more, a base material tensile strength of 780 MPa or more, a base material vE-20 of 100 J or more, and a necessary preheating temperature of 25 ° C. or less.

  In each of Examples 1-A to 7-E, the base material yield stress is 685 MPa or more, the base material tensile strength is 780 MPa or more, the base material vE-20 is 100 J or more, and the necessary preheating temperature is 25 ° C. or less.

  On the other hand, the following comparative examples lack the yield stress and tensile strength of the base material. That is, since Comparative Example 8-F has a small C addition amount, 10-H has a small Mn addition amount, 12-J has a small Cr addition amount, and 15-M has a large Nb addition amount, 17- Since A has a rolling end temperature lower than 760 ° C., 18-B has a water cooling start temperature lower than 700 ° C., and 19-B has a cooling stop temperature higher than 350 ° C., the yield stress and tensile strength of the base material are low. Run short.

  Moreover, the following comparative examples lack base material toughness. That is, since Comparative Example 11-I has a large amount of Mn added, 13-K has a large amount of Cr added, 14-L has a small amount of Nb added, and 15-M has a large amount of Nb added. Since A has a high heating temperature, the base material toughness is insufficient.

  In addition, since Comparative Example 9-G has a large amount of C addition, 13-K has a high Pcm value, and thus the required preheating temperature exceeds 25 ° C., and the preheating is not satisfied.

Claims (3)

% By mass
C: 0.03% or more, 0.055% or less,
Mn: 1.0% or more, 2.3% or less,
P: 0.02% or less,
S: 0.0050% or less,
Cr: 0.4% or more, 1.5% or less,
Nb: 0.02% or more, 0.05% or less,
Ti: 0.005% or more, 0.015% or less,
Al: 0.003% or more, 0.08% or less,
B: 0.0005% or more, 0.0020% or less,
N: 0.0015% or more and 0.0060% or less, the weld cracking sensitivity index Pcm value shown below is 0.19% or more and 0.22% or less, and the balance is Fe and inevitable impurities. A steel slab or slab having a component composition is heated to 1000 ° C. or higher and 1200 ° C. or lower and rolled at 760 ° C. or higher. Subsequently, the cooling rate is increased from 700 ° C. or higher to 8 ° C./sec or higher, 80 ° C. / A method for producing a high-tensile steel plate having a tensile strength of 780 MPa class excellent in weldability, characterized by starting accelerated cooling at a sec. or less and stopping the accelerated cooling at room temperature or higher and 350 ° C. or lower.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are C, Si, Mn, Cu, and Ni, respectively. , Cr, Mo, V, B means the content expressed in mass%. Furthermore, in mass%,
The method for producing a high-tensile steel plate having a tensile strength of 780 MPa and excellent weldability according to claim 1, comprising Si: 0.10% or more and 0.40% or less. Furthermore, in mass%,
Mg: 0.0005% or more, 0.01% or less,
Ca: 0.0005% or more and 0.01% or less of 1 type or 2 types, The high strength thick steel plate of the tensile strength 780MPa class excellent in the weldability of Claim 1 or 2 characterized by the above-mentioned. Manufacturing method.