KR20070027733A - High tensile steel sheet of low acoustical anisotropy excelling in weldability, and process for producing the same - Google Patents

Abstract

A high tensile steel sheet of low acoustical anisotropy excelling in weldability, whose tensile strength is on the order of 570 MPa or greater; and a process for producing a steel sheet, based on an accelerated cooling-halfway stopping process realizing high productivity. There is provided a high tensile steel sheet comprising a steel structure of 0.055% <= Nb+2Ti <= 0.105%, value of A = (Nb+2Ti)Î(C+NÎ12/14) being in the range of 0.0025 to 0.0055, content of bainite being >= 30 vol% and content of pearlite plus island shaped martensite being = 1200‹C, and preliminarily rolled at >= 1020‹C. Thereafter, the preliminarily rolled steel is further rolled so that the cumulative draft at temperature of >920‹C but <1020‹C is <= 5% while the cumulative draft at temperature of 860‹ to 920‹C falls within the range of 20 to 50%, and an accelerated cooling of 2‹ to 30‹C/sec cooling rate is started from >= 800‹C. The accelerated cooling is stopped at 600‹ to 700‹C, and thereafter cooling is performed at a rate of <= 0.4‹C/sec. ® KIPO & WIPO 2007

Description

High tensile steel sheet with low acoustic anisotropy and excellent weldability and manufacturing method thereof {HIGH TENSILE STEEL SHEET OF LOW ACOUSTICAL ANISOTROPY EXCELLING IN WELDABILITY, AND PROCESS FOR PRODUCING THE SAME}

The present invention relates to a method for producing a high tensile strength steel sheet of 570 MPa or more, which has low acoustic anisotropy and excellent weldability, with high productivity without requiring an offline heat treatment. The steel of the present invention is used in the form of thick plates, steel pipes or section steels as structural members of welded structures such as bridges, ships, building structures, offshore structures, pressure vessels, penstocks, and line pipes.

Tensile strength of 570 MPa or higher, which is used as a welded structural member for bridges, ships, building structures, offshore structures, pressure vessels, hydraulic pipes, and line pipes, requires toughness and weldability in addition to strength. In many cases, weldability is required, and a lot of studies have been made on characteristics improvement in the past. As a composition and manufacturing conditions of a steel plate, Unexamined-Japanese-Patent No. 53-119219, Unexamined-Japanese-Patent No. 01-149923, etc. are disclosed, for example. These are related to the manufacturing method of rolling a steel plate, reheating quenching offline, and reheating tempering heat treatment again. For example, in Japanese Unexamined Patent Publication No. 52-081014, Japanese Unexamined Patent Publication No. 63-033521, Japanese Unexamined Patent Publication No. 02-205627 and the like, so-called direct quenching is performed online after rolling a steel sheet. Techniques for making by quenching are disclosed. Both of these reheat quenching and direct quenching require off-line tempering heat treatment. However, when the off-line heat treatment step is required, productivity is hindered. Thus, to increase the productivity, a so-called non-coarse manufacturing method that eliminates the tempering heat treatment and does not require off-line heat treatment is required. good.

Some techniques related to non-coarse manufacturing methods are disclosed, for example, Japanese Unexamined Patent Publication No. 54-021917, Japanese Unexamined Patent Publication No. 54-071714, Japanese Unexamined Patent Publication No. 2001-064723, and Japanese Unexamined Patent Publication Publication No. 2001-064728. These are related to the accelerated cooling-during stop process of stopping the accelerated cooling after rolling of the steel sheet in the middle. This is to achieve a quenching structure by quenching to below the transformation temperature by accelerated cooling, and to transfer to the quenching process by stopping the water cooling at a relatively high temperature after transformation, and to obtain a tempering effect in this quenching process so as to omit reheating tempering. . However, all of these manufacturing techniques require controlled rolling at a relatively low temperature in order to obtain toughness and strength, and since the temperature at which rolling is finished is around 800 ° C., cooling productivity is not required and productivity is not high. On the other hand, especially in applications such as bridges and constructions, the acoustic anisotropy is required to be small because it affects the precision of the ultrasonic rectangular flaw test of welds. Since the structure is formed, the acoustic anisotropy of the steel sheet is increased, which does not necessarily correspond to this use.

In addition, Japanese Laid-Open Patent Publication No. 2001-064728 discloses a technique for manufacturing a high tensile strength steel sheet of 570 MPa or more in tensile strength by an accelerated cooling-during stop process. In this patent, however, V contributes to precipitation strengthening even at the slow cooling stage after stopping the water cooling. According to the inventors, as described later, the precipitation rate at the slow cooling stage after stopping the water cooling is equal to Nb and Ti. Compared with this, it was found that it was slow and not very effective for reinforcement. However, it is thought that stable strength is not necessarily obtained with this component composition.

In addition, Japanese Unexamined Patent Publication No. 2002-053912 discloses a non-coarse process that does not perform water cooling after rolling, which does not increase acoustic anisotropy because no controlled rolling is carried out at low temperature, but in order to obtain strength instead. The addition amount of alloys, such as Cu, Ni, and Mn increases, and there exists a problem in economy.

Accordingly, in the present invention, a high tensile strength steel sheet of 570 MPa or more having low acoustic anisotropy and excellent weldability is obtained by a manufacturing method on the premise of an economical component composition with a small amount of alloying and a high acceleration cooling-during stop process with high productivity. The subject was made. The plate | board thickness of the target steel plate shall be 10 mm.

There are several means for reinforcing high tensile steel, and a method using precipitation strengthening such as carbides or nitrides of Nb, V, Ti, Mo, Cr, etc. can enable reinforcement with a relatively low alloying component. At this time, in order to obtain a large amount of precipitation strengthening, it is important to form precipitates consistent with body material.

In the accelerated cooling-during stop process, in the rolling step, the steel structure is austenite, transformed by the accelerated cooling after the end of rolling, and becomes the structure of the ferrite body such as bainite or ferrite. Precipitates precipitated in austenite during rolling lose their consistency with the ferrite body after transformation and the reinforcing effect is reduced. In addition, the precipitate precipitated at the initial stage of rolling may coarsen and become a factor of lowering toughness. Therefore, during rolling, it is important to suppress the precipitation of the rolled precipitate and to deposit as much as possible in the bainite or ferrite structure at the stage in the slow cooling after the water cooling stop. If the process of reheating and tempering heat treatment after water cooling can take sufficient temperature and time for precipitation, large precipitation strengthening can be obtained easily. On the other hand, in the case of an accelerated cooling-during stop process in which reheating and tempering is not performed, precipitation is expected during the slow cooling after the water-cooling stop. However, in order to obtain a quenching structure, the water-cooling stop temperature must be lowered to some extent, so that precipitation is performed. Both temperature and time are limited and are generally disadvantageous for precipitation strengthening. From this, as described above, the non-coarse process is high in productivity, but in order to obtain the same strength as that of the coarse process, a large amount of alloying elements are required, or control rolling at low temperature has to be performed.

Accordingly, the present inventors earnestly examine the method of utilizing the precipitation strengthening as much as possible in order to obtain high strength without adding a large amount of alloying elements or prematurely controlled rolling at a low temperature, on the premise of a highly productive accelerated cooling-during stop process. Repeated.

First, in order to clarify the precipitation behavior in the slow cooling process after stopping the water cooling, the precipitation rate and precipitation strengthening amount of the carbide, nitride, and carbonitride of each alloy element in the bainite or ferrite structure is detailed. Reviewed. As a result, in the bainite or ferrite structure or such a mixed structure, the precipitation rate of Nb carbonitride and Ti carbide is faster than that of other elements such as V, and since these are precipitates that match with the base, the strengthening amount is large. In particular, the deposition rate in the temperature range of 600 ° C to 700 ° C is fast and the amount of strengthening is large. In addition, when complex precipitation is performed by using Nb and Ti or Nb and Ti and Mo together, the precipitates matched with the base are finely dispersed even if kept for a short time due to the synergistic effect, thereby obtaining a large precipitation strengthening.

However, when the addition amount of Nb and Ti is too large, there exists a tendency for the precipitate formed to become coarse, and since the number of precipitates becomes rather small, precipitation strengthening quantity falls. The precipitation rate and the form of the precipitate in the carbides, nitrides and carbonitrides of Nb and Ti in austenite and in ferrite are greatly influenced by the amount of Nb, Ti added, and the amount of C and N. According to various experiments and interpretations, the inventors of the present invention predicted that the precipitation rates and precipitation forms of carbides, nitrides and carbonitrides of Nb and Ti were determined by the parameter A = ([Nb] + 2 × [Ti]) × ([C] + [N] X12 / 14), the knowledge was obtained that the precipitation in the slow cooling after stopping in the middle of water cooling can be sufficiently obtained while suppressing precipitation during rolling by setting this value within a predetermined range. In other words, as the amount of Nb and Ti added increases, the amount of added C and N needs to be reduced. If the value of A is too small, the precipitation rate in the ferrite becomes slow and sufficient precipitation strengthening cannot be obtained. On the contrary, if the value of A is too large, the precipitation rate of carbides, nitrides and carbonitrides in austenite becomes too fast, resulting in coarsening of precipitates, and the amount of coarsened precipitation in the slow cooling after the accelerated cooling stop is also insufficient. In addition, since Si also affects the formation rate of carbide, it is necessary to make the addition amount within a certain range.

 These precipitation strengthening effects also have a great influence on the tissues. The bainite structure is easier to maintain a processed structure such as dislocation density than ferrite. In order to promote fine registration precipitation, it is very effective that sufficient presence of precipitation sites such as dislocations and deformation zones included in the processed structure is present. According to the inventors' study, in order to obtain sufficient reinforcement, it is necessary to make bainite single phase into a mixed structure of bainite and ferrite having a volume ratio of bainite of 30% or more. In addition, since pearlite, island martensite, etc. precipitate at the phase interface, the reinforcing effect is decreased, and toughness is also reduced. Therefore, the sum of the volume fractions of pearlite and island martensite should be 3% or less.

The present inventors further examined specific manufacturing conditions for obtaining the maximum precipitation strengthening effect and obtained the following findings.

Since the precipitation of Nb and Ti in the rolling step is promoted by rolling deformation, the rolling conditions in the high temperature region of austenite, so-called rough rolling conditions, greatly influence the final precipitation strengthening effect. Specifically, rough rolling is completed in a temperature range of 1020 ° C or higher, and extremely rolling in a temperature range of 1020 ° C to 920 ° C is a requirement for suppressing precipitation during rolling. However, when all rolling is completed in the temperature range of 1020 degreeC or more, since the process structure hardly remains after accelerated cooling-stopping by recovery and recrystallization, precipitation sites, such as an electric potential and a deformation | transformation band, do not exist enough and sufficient Precipitation hardening cannot be obtained. Therefore, it becomes essential condition to perform necessary sufficient rolling in unrecrystallization temperature area | region, and to perform accelerated cooling rapidly after rolling. Specifically, relatively light rolling with a cumulative reduction of 20% to 50% in the range of 920 ° C to 860 ° C is performed. Under these conditions, it is possible to suppress the precipitation of Nb and Ti, which is unnecessary, and to maintain a suitable precipitation site even after the water cooling stops. In addition, if this condition does not form a strong aggregate, the acoustic anisotropy does not increase.

The water cooling stop temperature of the accelerated cooling-during stop process is 600 ° C to 700 ° C in the center of the sheet thickness so as to favor precipitation of Nb and Ti, but even at this stop temperature, steel structures having a volume ratio of bainite of 30% or more are obtained. In order to limit the component composition of the steel to a specific range described later, a cooling rate of 2 ° C./sec or more and 30 ° C./sec or less is required in accelerated cooling. In addition, in order to solidify Nb and Ti, it is essential to heat a steel sheet or cast steel at high temperature, and heating temperature of 1200 degreeC or more is required.

The knowledge gained here is a new way of controlling the precipitation of carbides or carbonitrides of Nb, Ti online during the rolling including the high temperature zone, during the accelerated cooling, and after the cooling stop, to the slow cooling process, which is more than the level of the conventional tempering process. Strengthening can be achieved with an accelerated cooling-during stop process that does not require offline heat treatment.

In addition, according to this manufacturing process, the weld crack susceptibility index Pcm of the steel composition (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] mean mass% of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B), respectively. It can provide excellent steels.

The gist of the present invention is as follows.

 (1) at mass%,

C: 0.03% or more, 0.07% or less,

Si: 0.1 to 0.6%,

Mn: 0.8-2.0%,

Al: 0.003% or more, 0.1% or less,

Nb: 0.025 to 0.1%,

Ti: 0.005 to 0.1%,

[Nb] + 2 x [Ti]: 0.045 to 0.105%

N: over 0.0025%, up to 0.008%

And containing Nb, Ti, C, and N in a range satisfying a relationship such that the value A represented by A of the following formula (1) is 0.0022 or more and 0.0055 or less, and is composed of residual Fe and unavoidable impurities: High tensile strength of 570 MPa or more with low acoustic anisotropy and excellent weldability, characterized by having a steel composition, and the steel structure having a volume ratio of bainite of 30% or more and a sum of volume ratios of pearlite and island-like martensite of less than 5%. Grater.

A = ([Nb] + 2 x [Ti]) x ([C] + [N] x 12/14)... (1) expression

Here, [Nb], [Ti], [C], and [N] mean mass% of Nb, Ti, C, and N, respectively.

 (2) in the steel sheet, also in mass%,

Mo: 0.05% or more, 0.3% or less,

Cu: 0.1% or more, 0.8% or less,

Ni: 0.1% or more, 1% or less,

Cr: 0.1% or more, 0.8% or less,

V: 0.01% or more, 0.03% or less,

W: 0.1% or more, 3% or less,

B: 0.0005% or more, 0.005% or less,

Mg: 0.000% or more, 0.01% or less,

Ca: 0.0005% or more, 0.01% or less

A high tensile steel sheet of 570 MPa or more having a tensile strength of 570 MPa or more, which is characterized by containing one kind or two or more kinds of the anisotropic properties as described in (1).

(3) After heating the steel slab or cast steel which has the component composition as described in (1) or (2) to 1200 degreeC or more and 1300 degrees C or less, and rough-rolling in the temperature range of 1020 degreeC or more, it is less than 1020 degreeC and more than 920 degreeC. Accelerates at a cooling rate of 2 ° C / sec or more and 30 ° C / sec or less after hot rolling at a cumulative reduction ratio of 15% or less in the temperature range, 920 ° C or less, or 860 ° C or more and 20% or more in the cumulative reduction rate in the temperature range. Cooling is started from a temperature of 800 ° C. or higher, and the accelerated cooling is stopped at a plate thickness center temperature of 700 ° C. or lower and 600 ° C. or higher, and then cooled at a cooling rate of 0.4 ° C./sec or lower. This small and weldable high tensile steel plate with a tensile strength of 570 MPa or more.

[The best embodiment for carrying out the invention;

Below, the reason for limitation of each component in this invention and a manufacturing method is demonstrated.

C is an important element which forms carbides and carbonitrides with Nb and Ti and which is a main element of the reinforcing mechanism of the present invention steel. If the amount of C is insufficient, the amount of precipitation in the slow cooling after the accelerated cooling stop is insufficient, and thus strength cannot be obtained. On the contrary, even if it is excessive, the precipitation rate in the austenite region during rolling is increased, and as a result, the amount of coincidence precipitation in the slow cooling after the accelerated cooling stop is insufficient and strength cannot be obtained. Therefore, the amount of C is limited to 0.03% or more and 0.07% or less of range.

Si is an element necessary as steelmaking deoxidation element and also affects the precipitation rate of carbides. By adding an appropriate amount of Si, there is an effect of suppressing carbide precipitation in the austenite region during rolling. For this purpose, Si is added at least 0.1%, preferably at least 0.3%. However, when the content exceeds 0.6%, the deposition rate becomes too slow and the toughness of the weld heat affected zone may be lowered. Therefore, the upper limit is 0.6%.

Mn is an element necessary to increase the quenchability and obtain a bainite single phase or a mixed structure of bainite and ferrite having a bainite fraction of 30% or more. Although 0.8% or more is required for this purpose, when it exceeds 2.0%, the toughness of a base material may fall, so let an upper limit be 2.0%.

Al is usually 0.003% or more and 0.1% or less, which is a range added as a deoxidation element.

Nb and Ti form an NbC, Nb (CN), TiC, TiN, Ti (CN) or a composite precipitate and also a composite precipitate of these and Mo, and are an important element which is the main element of the reinforcing mechanism of the present invention steel. In order to obtain sufficient composite precipitates in the accelerated cooling-during stop process, an appropriate addition considering the precipitation rate is necessary. That is, Nb is 0.025% or more, preferably 0.035% or more, Ti is 0.005% or more, and 0.045% ≦ ([Nb] + 2 × [Ti]) ≦ 0.105%, and A = ([Nb] When the value is set to + 2 × [Ti]) × ([C] + [N] × 12/14), the condition is that the value of A is 0.0022 or more and 0.0055 or less (where, [Nb], [Ti], [C] ] And [N] mean the mass% of Nb, Ti, C and N, respectively). In addition, the upper limit of Nb and Ti is good to set it as 0.1%, respectively.

Mo improves the quenchability, and also forms a complex precipitate with Nb and Ti, and contributes greatly to the strengthening, so it is added at least 0.05%. However, when added excessively, since it impairs the toughness of weld heat affected zone, it is added at 0.3% or less.

N combines with Ti to form TiN. When TiN is finely dispersed, the coarsening of the weld heat affected zone structure is suppressed by the pinning effect to improve the weld heat affected zone toughness. However, when N is insufficient, TiN becomes coarse to obtain a pinning effect. To finely disperse TiN, N is added more than 0.0025%, preferably more than 0.004%. In addition, when N is excessively contained, the toughness of the base metal may be lowered, so the upper limit is made 0.008%.

When Cu is added as a reinforcing element, it is necessary to add 0.1% or more to exhibit the effect. However, even if it is added in excess of 0.8%, the effect is not large compared to the added amount. Since it may inhibit, it may be 0.8% or less.

Ni is required to be 0.1% or more when added in order to increase the base metal toughness. However, when excessively added, Ni may be impaired in weldability. Since Ni is also an expensive element, the upper limit of addition is 1%.

Cr has the effect of increasing the quenchability like Mn and easily obtaining bainite structure. Although 0.1% or more is added for the purpose, when added excessively, the toughness of weld heat influence part will be inhibited, so an upper limit shall be 0.8%.

V has a smaller strengthening effect than Nb and Ti, but it has an effect of increasing precipitation strengthening and quenching to some extent. In order to obtain this effect, addition of 0.01% or more is required. However, excessive addition leads to a decrease in toughness of the weld heat affected zone.

When B is added in order to increase the quenchability and to obtain strength, the amount of B is required to be 0.0005% or more, but the effect is not changed even if it is added over 0.005%. Therefore, the amount is 0.0005% or more and 0.005% or less.

By adding one or two of Mg and Ca, sulfides and oxides can be formed to increase the base metal toughness and the weld heat affected zone toughness. To achieve this effect, Mg or Ca must be added at least 0.0005%, respectively. However, when excessively added exceeding 0.01%, coarse sulfides and oxides are produced, so the toughness may be lowered. Therefore, the addition amount is made 0.0005% or more and 0.01% or less, respectively.

In addition to the above components, P and S are harmful elements that lower the toughness of the base metal as inevitable impurities. Preferably, P is made 0.02% or less and S is made 0.02% or less.

Next, a manufacturing method is demonstrated.

In order to sufficiently solidify Nb and Ti, the heating temperature of the steel slab or cast steel at the time of rolling must be 1200 ° C or higher. However, even when the heating temperature exceeds 1300 ° C, the effect of solid solution does not change so much, and the energy cost increases. Therefore, the heating temperature of the steel or cast steel at the time of rolling is set to 1200 ° C or more and 1300 ° C or less.

In order to suppress precipitation of Nb and Ti in rolling as much as possible, rolling is carried out by rough rolling at an appropriate reduction ratio in the temperature range of 1020 degreeC or more, and rolling is 15% or less in the cumulative reduction rate below 1020 degreeC and more than 920 degreeC. Shall be. Further, in order to obtain a sufficient processing structure necessary as a precipitation site, rolling is performed at a cumulative reduction ratio of 20% or more and 50% or less in a range of 920 ° C or less and 860 ° C or more. Under these rolling conditions, the formation of the aggregate structure is controlled, so that the acoustic anisotropy does not increase.

In order to suppress the recovery of the processed structure and the precipitation after the processing, accelerated cooling is performed quickly after the end of rolling. Cooling is performed from 800 ° C or higher to water cooling under conditions such that the cooling rate is 2 ° C / sec or higher and 30 ° C / sec or lower in the sheet thickness center portion. In order to make the volume rate of bainite 30% or more, a cooling rate of 2 ° C / sec or more is required, and the cooling rate is 30 ° C / sec or less so that the sum of the volume ratios of pearlite and island-like martensite is less than 3%. Shall be. When the sheet thickness center temperature becomes 700 degrees C or less and 600 degrees C or more, water cooling is stopped midway, and cooling rate is made into 0.4 degrees C / sec or less by air cooling after that. This object is to ensure sufficient temperature and time for Nb, Ti and their composite precipitation and their composite precipitation with Mo. If the water cooling stop temperature is too high, it is difficult to obtain bainite structure. At low temperatures, precipitation is delayed and sufficient reinforcement cannot be obtained.

The steel of the present invention is used in the form of thick plates, steel pipes or section steels as structural members of welding structures such as bridges, ships, building structures, offshore structures, pressure vessels, hydraulic pipes, and line pipes.

The steel strip obtained by solvent-processing the steel of the component composition shown in Table 1 was made into the steel plate of 20-100 mm thickness on the manufacturing conditions shown in Table 2 and Table 3. Among them, 1-A to 14-N are steels of the present invention, and 15-O to 43-A are comparative examples. The underlined numbers in the table indicate that the components or manufacturing conditions are out of the patent range or the characteristics do not meet the following target values.

Table 2 shows the measurement results of tensile strength, weld heat affected zone toughness and acoustic anisotropy for these steel sheets. Tensile strength was taken by the method prescribed | regulated to JIS Z2241 by taking the 10 round bar tensile test piece prescribed | regulated to JIS Z2201. Base metal toughness was evaluated by extracting the impact test piece specified in JIS Z2202 from the center of the plate thickness in the direction perpendicular to the rolling direction, and obtaining the wavefront transition temperature (vTrs) by the method specified in JIS Z2242. The weld heat influence part toughness was evaluated by the absorption energy (vE- ₂₀ ) in _-20 degreeC of the impact test piece prescribed | regulated to JIS Z2202 which applied the heat cycle corresponded to submerged arc welding of 20 kJ / mm heat input. For steels up to 32 mm in thickness, prepare steel sheets with original thickness, and for steels over 32 mm in thickness, reduce the thickness to 32 mm, and heat input 20 kJ / mm Submerged arc welding was performed, the impact test piece prescribed | regulated to JISZ2202 was extract | collected so that a notch bottom might follow a melting line (fusion line), and it evaluated by the absorbed energy (vE- ₂₀ ) in _-20 degreeC. The acoustic anisotropy was evaluated as having low acoustic anisotropy when the sound velocity ratio was 1.02 or less according to the Japanese Non-Destructive Testing Association standard NDIS2413-86. The target value of each characteristic is yield strength of 450 MPa, tensile strength of 570 MPa or more, vTrs of -20 ° C or less, vE _-20 of 70 J or more, and sound velocity ratio of 1.02 or less.

All of Examples 1-A to 14-N had a yield strength of more than 450 MPa, a tensile strength of more than 570 MPa, a weld heat affected zone toughness vE- ₂₀ of more than 20OJ, and a sound velocity ratio of 1.02 or less, and thus small acoustic anisotropy.

On the other hand, since Comparative Example 15-O has low C and Comparative Example 16-P has high C, Comparative Example 17-Q has low Si, and Comparative Example 19-S has low Mn, and therefore Comparative Example 21 Since -U has a low Mo, Comparative Example 23-W has a low Nb. Since Comparative Example 25-Y has a low Ti, Comparative Example 27-AA shows the value of the parameter A (A = ([Nb] +2). Since X [Ti]) × ([C] + [N] × 12/14)) did not satisfy 0.0025, Comparative Example 37-A had a low heating temperature, and therefore, Comparative Example 40-A was 920 ° C. or less. Since the cumulative reduction ratio in the range of 860 ° C. or higher is high, Comparative Example 41-A has a small sheet thickness center cooling rate, so that Comparative Example 42-A has a high stop temperature for accelerated cooling, and therefore, Comparative Example 43-A is accelerated. Since the cooling stop temperature is low, the tensile strength is less than 570 MPa, respectively.

Since Comparative Example 18-R has high Si, Comparative Example 22-V has high Mo, and Comparative Example 24-X has high Nb and Nb + 2Ti exceeds 0.105%. Since Nb + 2Ti is higher than 0.105%, Comparative Example 29-AC has a low N, and Comparative Example 31-AE has a high V, so that Comparative Example 32-AF has a high Cu, Comparative Example 33-AG Since Ni is high, Comparative Example 34-AH has a high Cr, and Comparative Example 35-AI has a high Mg, and Comparative Example 36-AJ has a high Ca and therefore low weld heat affected zone toughness.

Since Comparative Example 20-T had a high Mn, Comparative Example 28-AB had a higher value of parameter A than 0.005. Thus, Comparative Example 30-AD had a low base metal toughness because N had a high value.

Since Comparative Example 38-A has a high cumulative reduction ratio in the range of less than 1020 ° C and more than 920 ° C, Comparative Example 39-A has a low cumulative reduction in the range of 920 ° C or less and 860 ° C or higher, so that the tensile strength is low. , Weld heat affected zone toughness is low.

Since Comparative Example 39-A has a high cumulative reduction ratio in the range of 920 ° C or less and 860 ° C or more, the tensile strength is low and the acoustic anisotropy is also large.

According to the present invention, a high tensile strength steel sheet of 570 MPa or more with a low acoustic anisotropy and a weldability of up to 10 mm in thickness, which is excellent in weldability, can be obtained by an economical component system having a small amount of alloy addition and a high-productivity manufacturing method. The effect on this industry is very large.

Claims (3)

In mass%, C: 0.03% or more, 0.07% or less, Si: 0.1 to 0.6%, Mn: 0.8-2.0%, Al: 0.003% or more, 0.1% or less, Nb: 0.025 to 0.1%, Ti: 0.005 to 0.1%, [Nb] + 2 x [Ti]: 0.045 to 0.105% N: over 0.0025%, up to 0.008% And containing Nb, Ti, C, and N in a range satisfying the relationship that the value A represented by A in the following formula (1) becomes 0.0022 or more and 0.0055 or less, and is composed of the balance Fe and unavoidable impurities: High tensile strength of 570 MPa or more with low acoustic anisotropy and excellent weldability, wherein the steel composition has a steel composition and the steel structure has a volume ratio of bainite of 30% or more and a sum of volume ratios of pearlite and island-like martensite of less than 5%. Grater. A = ([Nb] + 2 x [Ti]) x ([C] + [N] x 12/14)... (1) expression Here, [Nb], [Ti], [C], and [N] mean mass% of Nb, Ti, C, and N, respectively. The method of claim 1, To the steel sheet, and also in mass%, Mo: 0.05% or more, 0.3% or less, Cu: 0.1% or more, 0.8% or less, Ni: 0.1% or more, 1% or less, Cr: 0.1% or more, 0.8% or less, V: 0.01% or more, 0.03% or less, W: 0.1% or more, 3% or less, B: 0.0005% or more, 0.005% or less, Mg: 0.000% or more, 0.01% or less, Ca: 0.0005% or more, 0.01% or less A high tensile steel sheet having a tensile strength of 570 MPa or more with low acoustic anisotropy and excellent weldability, characterized by containing one or two or more thereof. The steel slab or cast steel which has the component composition of Claim 1 or 2 is heated to 1200 degreeC or more and 1300 degrees C or less, and after rough-rolling in the temperature range of 1020 degreeC or more, the temperature range of less than 1020 degreeC and more than 920 degreeC Accelerated cooling is performed at a cooling rate of 2 ° C / sec or more and 30 ° C / sec or less after hot rolling at a cumulative reduction ratio of 15% or less, 920 ° C or less, and 860 ° C or more in a temperature range of 20% or more. Starting from a temperature of 800 ° C. or higher, the accelerated cooling is stopped at a plate thickness center temperature of 700 ° C. or lower and 600 ° C. or higher, and then cooled at a cooling rate of 0.4 ° C./sec or lower. High tensile steel with a tensile strength of 570 MPa or more, small and weldable.