JP2001355040A - High strength and high toughness steel sheet excellent in weldability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength and high toughness steel sheet excellent in weldability even in the case Ti is added more largely than the addition amount of Ti allowable in the ordinary rolling condition and to provide a method for efficiently producing the same steel sheet. SOLUTION: This high strength and high toughness steel sheet excellent in weldability has a composition satisfying, by mass, 0.01 to 0.15% C, 0.03 to 0.2% Ti, 0.8 to 3.0% Mn and <=0.6% Si, and in which Ti-containing carbide and/or Ti-containing carbonitride with a particle size of 7 to 50 nm is present by >=0.1 piece on the average per the area μm2 to be inspected. Further, the addition of prescribed amounts of B, Cu, Ni, Cr, Al, Ca, rare earth metals, Mo, Nb, V, Zr or the like is also effective. Moreover, in the production, rolling reduction of >=60% is performed in an unrecrystallized γregion.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength and high-toughness steel sheet excellent in weldability, and more particularly, to a steel sheet having a strength of 49.
0 MPa or more, base material toughness vTrs ≦ −30 ° C., HAZ
(Weld heat affected zone) A high strength and high toughness steel sheet satisfying toughness vE _o ≧ 100J.

[0002]

2. Description of the Related Art For high-strength steel sheets of 490 MPa class or higher,
Components such as Nb, V, and Cr are added in large amounts from the viewpoint of securing base metal strength.
There has been a problem that HAZ (welding heat affected zone) is hardened to cause welding cracks (low-temperature cracking), resulting in poor HAZ toughness. Therefore, Ti is added to improve such HAZ toughness deterioration. However, when the amount of Ti added is large, under normal heating conditions [heating temperature: 1100 to 1250 ° C, rolling end temperature (steel sheet surface): 850 to 1000 ° C, cooling: air cooling]
In the case of fine TiC (Ti-containing carbide) and TiC / N (T
(i-containing carbonitride) is precipitated, and a new problem that the base material toughness is deteriorated arises.
Usually, only 0.01 to 0.02% is added without adding much, and the improvement of the base metal strength is compensated by adding other quenching improving elements (Nb, Mo, etc.). Was the actual situation.

[0003]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to provide a method for adding a larger amount of Ti than is permissible under normal rolling conditions. An object of the present invention is to provide a high-strength and high-toughness steel sheet excellent in weldability, and to provide a method capable of efficiently manufacturing the steel sheet.

[0004]

The high-strength and high-toughness steel sheet excellent in weldability according to the present invention which can solve the above-mentioned problems includes:
: 0.01 to 0.15% (meaning by mass%, the same applies hereinafter), Ti: 0.03 to 0.2%, Mn: 0.8 to 3.0.
0%, Si: 0.6% or less, and the test area 1
The gist lies in that at least 1.0 or more Ti-containing carbides and / or Ti-containing carbonitrides having a particle diameter of 7 to 50 nm exist per μm ² .

[0005] In the present invention, the average grain size of a crystal surrounded by grain boundaries in which all adjacent crystals have a misorientation of 15 ° or more is 30 °.
The one controlled to μm or less is a preferable embodiment because the toughness is further improved.

[0006] Further, in the steel component, B: 0.0
003 to 0.005%; Cu: 3% or less (excluding 0%), Ni: 6% or less (excluding 0%), and Cr: 0.5% or less (including 0%) Not containing at least one member selected from the group consisting of:
Al: 0.5% or less (excluding 0%), Ca: 0.0
5% or less (excluding 0%), and REM: 0.05%
Mo: 0.50% or less (0% or less) containing at least one selected from the group consisting of:
%), Nb: 0.05% or less (excluding 0%), V: 0.10% or less (excluding 0%), and Z
r: preferably contains at least one selected from the group consisting of 0.05% or less (excluding 0%).

[0007] Further, the method for producing the steel sheet of the present invention which can solve the above-mentioned problem has a gist in controlling the rolling reduction in the unrecrystallized γ region to 60% or more using the above steel. .

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the conventional high strength and high toughness steel sheet having excellent weldability, the present inventors have found that "the base metal toughness decreases with the precipitation of fine TiC by adding a large amount of Ti." From the viewpoint of prevention, at most T
i was added only in an amount of about 0.01 to 0.02%, and attention was paid to the fact that the advantages of improving the base material strength and HAZ toughness by adding Ti were not utilized. Therefore, the inventors of the present invention did not use the conventional technique of “adding a large amount of Ti, instead of reducing the amount of added Ti as much as possible, thereby suppressing the precipitation of fine TiC or the like that adversely affects the toughness of the base material”. In addition, by precipitating coarse TiC and the like as much as possible, the excellent effect of adding Ti can be effectively exhibited without deteriorating the base material toughness. " As a result, it has been found that the intended purpose can be achieved by specifying the above requirements, and the present invention has been completed.

As described above, the present invention has a technical idea for promoting the deposition of coarse TiC and the like, and is therefore different from the conventional technical idea for suppressing the deposition of fine TiC and the like. In the present invention, the weldability is excellent while effectively exhibiting the merits (increase the base metal strength without impairing the HAZ toughness) by adding Ti, and also ensuring a predetermined base metal toughness. The technical significance exists where a high-strength, high-toughness steel sheet is obtained.

Hereinafter, each requirement constituting the present invention will be described.

First, the chemical components in steel are C: 0.01 to
0.15%, Ti: 0.03 to 0.2%, Mn: 0.8
３3.0%, Si: 0.6% or less. The reasons for limiting each component are as follows.

C: 0.01 to 0.15% C is an element useful for securing strength, and its lower limit is set to 0.01% in order to effectively exert such an effect. Preferably it is 0.02% or more. However, the amount of C added is 0.1
If it exceeds 5%, the toughness of the weld decreases. Preferably it is 0.07% or less.

Ti: 0.03 to 0.2% Ti is an effective element in that the strength of the base material can be improved without impairing the HAZ toughness. 0.03%. Preferably it is 0.035% or more. However, when the added amount of Ti exceeds 0.2%, the above-mentioned effect is saturated. Therefore, the upper limit is set to 0.2%. Preferably it is 0.08% or less.

Mn: 0.8-3.0% Mn is not only useful as a hardenability improving element,
As in the present invention, by making the C extremely low and preferably further adding a predetermined amount of B, it is useful in that both HAZ toughness and base material strength can be ensured. The lower limit is set to 0.8% in order to effectively exert such an effect. It is preferably at least 1.0%. However, the amount of Mn added was 3.0.
%, The toughness deteriorates, so the upper limit was set to 3.0%. Preferably it is 2.5% or less.

Si: 0.6% or less (including 0%) Si is an element effective for improving the strength.
% Is preferably added. However, if the addition exceeds 0.6%, the weldability is impaired.
%.

Further, in the steel sheet of the present invention, Ti-containing carbide and / or Ti-containing carbonitride having a particle diameter of 7 to 50 nm (hereinafter, may be represented by "TiC or the like" in some cases) per 1 μm ² of the test area on average. It is necessary that there be 1.0 or more. As described above, the steel sheet of the present invention has a technical idea in that the base metal toughness is improved by positively precipitating coarse TiC and the like, and that such coarse TiC and the like is precipitated. Fine T when turned over
This is because the number of precipitated iC or the like can be reduced, which is extremely effective in improving the base material toughness. According to the present invention, the test area of 1 μm ²
As a result, it was found that the intended purpose can be achieved if 1.0 or more exist on average.

The particle size of TiC or the like is determined as follows.
First, an arbitrary longitudinal section near t / 4 (t: plate thickness) in the thickness direction of a steel plate (TEM: magnification is 50,000 times or more, preferably observed at around 100,000 times).
The number of precipitates having a particle diameter of 7 to 50 nm among the precipitates such as TiC per 1 μm ² is visually observed. The same operation was performed in arbitrary 10 visual fields, and the average number was calculated to determine the “number of TiC or the like” in the present invention.

In the present invention, TiC satisfying the above particle diameter is used.
The more the number is, the more desired characteristics are exhibited. However, in consideration of the upper limit of the Ti amount and the C amount used in the present invention, the upper limit is set to about 1,000.

In the present invention, the particle diameter of the TiC or the like is set in the range of 7 to 50 nm. This is because TiC or the like having a particle diameter of less than 7 nm adversely affects the base material toughness.
Note that the upper limit is set to 50 nm because the T over 50 nm is used.
iC and the like are not dissolved but remain dissolved during the heating during rolling, and such TiC and the like are different from those precipitated during the rolling and do not particularly adversely affect the properties of the steel sheet.

Further, in the steel sheet of the present invention, it is recommended to control the average grain size of crystals surrounded by grain boundaries in which all adjacent crystals have an orientation difference of 15 ° or more to 30 μm or less.
Further, improvement in toughness is promoted. Here, the reason why all the orientation differences of the crystal grain boundaries are set to 15 ° or more is that if they are less than 15 °, they do not function as crystal grain boundaries that contribute to improvement in toughness. Further, the crystal having a crystal orientation difference satisfying the above requirements and surrounded by a large-angle grain boundary, and the average grain size of the crystal is set to 30 μm or less (more preferably 15 μm or less) is because of such a fine crystal. This contributes to improvement in toughness. In general, it is considered that the toughness is improved when individual crystal grains are refined, but the correlation with the crystal orientation difference was not known as in the present invention. According to the study results of the present inventors, even if each crystal grain becomes fine, if the orientation difference between the crystal grains is small, an aggregate of several crystal grains acts as if it were one crystal grain. Therefore, it was clarified that the relationship of “fine grain refinement → improvement of toughness” was not always satisfied. Therefore, in the present invention, not only the average grain size of the crystal but also the orientation difference of the crystal grain boundary has been specified.

Here, a method for measuring the orientation difference of the crystal grain boundaries and a method for measuring the average grain size of the crystals surrounded by the grain boundaries will be described. First, in measuring the misorientation of crystal grains, SEM-EBSP [Scanning Electron Micros
cope-Electron Back Scattering (Scattered) Patern,
Alternatively, the crystal orientation is measured by EBSD (Diffraction)], and by analyzing the crystal orientation, it is possible to display only the grain boundaries where the orientation difference between adjacent crystals is 15 ° or more. Next, by measuring the average particle size by a cutting method, the average particle size surrounded by large-angle grain boundaries having adjacent angles of 15 ° or more can be measured. Regarding the measurement by EBSP, three or more points in a range of 0.1 to 0.2 mm square may be measured in the longitudinal section near t / 4 in the plate thickness direction, and the average value thereof may be calculated.

More specifically, a sample is cut out from the above position, and the measurement surface is subjected to an electrolytic polishing treatment (controlling to 0.1 to 0.3 angstroms with 10% ethanol perchlorate), and then the TEX SEM Laboratory SEM-EBS
P is measured at three points in a range of about 0.12 mm square by P, and a diagram of only large-angle grain boundaries having adjacent angles of 15 ° or more is printed out. Next, the average particle size is determined by a cutting method.
The average length of the section where these straight lines were separated by the grain boundary was defined as the average particle size.

As described above, the components in steel used in the present invention contain C, Ti, Mn, and Si, and the balance is substantially iron, but is usually used as long as the action of the present invention is not impaired. Allowable components and impurities can be added. Specifically, in the present invention, it is recommended to actively add the following components to steel.

B: 0.0003 to 0.005% B is an element effective for suppressing the ferrite nucleation by reducing the grain boundary energy of the old γ grain boundary by adding a small amount. In order to exert such an effect effectively, 0.0003
% Or more, more preferably 0.0% or more.
007% or more. However, if added excessively, a B compound such as BN is formed and the toughness is deteriorated. Therefore, it is recommended to control the content to 0.005% or less, more preferably 0.003% or less.

Cu: 3% or less (excluding 0%), N
i: 6% or less (excluding 0%), and Cr: 0.5%
At least one element selected from the group consisting of the following (not including 0%): These elements are effective elements for improving strength, and in order to exert such an effect effectively, Cu: 0.2% or more (more preferably). Is 0.4% or more), Ni: 0.1% or more (more preferably 0.2% or more), and Cr: 0.1% or more (more preferably 0.2% or more) are preferably added. However, excessive addition of Cu and Ni saturates the effect and is economically useless, and excessive addition of Cr lowers the weldability and HAZ toughness. : 3% (more preferably 2%), Ni: 6%
(More preferably 3%), it is preferable to control Cr: 0.5%. The above elements may be used alone or in combination of two or more.

Al: 0.5% or less (excluding 0%),
Ca: 0.05% or less (excluding 0%), and RE
M: at least one element selected from the group consisting of 0.05% or less (not including 0%) These elements are effective elements for improving HAZ toughness,
In order to exhibit such an effect effectively, Al: 0.01%
As described above, it is preferable to add Ca: 0.01% or more and REM: 0.01% or more. Here, REM means a rare earth element, and Sc, Y, lanthanoids (such as La, Ce, etc.)
Element). Of these, lanthanoid elements are preferable, and the use of La and Ce is particularly recommended. However, since excessive addition of Al lowers the cleanliness of the steel, it is 0.5% or less (more preferably 0.2% or less).
It is recommended to control Further, if Ca is added excessively, coarse inclusions in the steel are formed and the properties of the steel deteriorate, so that 0.05% or less (more preferably 0.04% or less) is used.
It is recommended to control Further, if REM is added excessively, the cleanliness of the steel is impaired. Therefore, the content is preferably set to 0.05% or less (more preferably 0.04% or less). still,
The above elements may be used alone or in combination of two or more.

Mo: 0.50% or less (excluding 0%), Nb: 0.05% or less (excluding 0%), V:
0.10% or less (excluding 0%), and Zr: 0.0
At least one element selected from the group consisting of 5% or less (excluding 0%). These elements are effective elements for improving strength, and Mo: 0.05% or more ( It is more preferable to add Nb: 0.01% or more (more preferably 0.02% or more), V: 0.02% or more, and Zr: 0.01% or more. However, if Mo is added excessively, the weldability is reduced.
It is recommended to control to 50% or less (more preferably 0.4% or less). Further, if Nb, V, and Zr are excessively added, the HAZ toughness is deteriorated. 07% or less), Zr: 0.05
% Or less (more preferably 0.04% or less). The above elements may be used alone or 2
More than one species may be used in combination.

Next, a method for producing the steel sheet of the present invention will be described.

As described above, in the present invention, since the most important point exists where a predetermined coarse TiC or the like is positively precipitated, the form of the TiC or the like is controlled to a predetermined size. is important. Therefore, in the present invention, the rolling reduction in the unrecrystallized γ region is set to 60% or more (more preferably, 66% or more). If the rolling amount is increased in this manner, precipitation of fine TiC or the like that adversely affects toughness is suppressed, and desired coarse TiC or the like can be efficiently obtained. It is feared that a load is applied to the rolling mill when the rolling reduction is increased. However, in the present invention, the rolling reduction is not increased in the pass but is controlled so as to increase the cumulative rolling reduction in the multiple passes. Therefore, it is considered that there is no particular problem.

Further, in rolling, it is preferable to appropriately adjust the heating temperature, the rolling finishing temperature, and the cooling conditions after rolling. These conditions are interrelated, and if the type of steel is different, it will have a subtle effect on the TiC and the like, the form of crystal grains, and the like, so it is difficult to determine each condition uniquely. As shown in the examples described later, if the steel type, heating temperature, finishing temperature, and cooling conditions are not properly controlled in a well-balanced manner, even if a reduction of 60% or more is applied in the unrecrystallized γ region, desired characteristics are obtained. May not be obtained in some cases. Therefore, when manufacturing the steel sheet of the present invention with an actual machine, a heating temperature suitable for the target steel type is determined in advance by preliminary experiments and the like.
It is preferable to investigate the rolling finish temperature and the cooling conditions after rolling, and to grasp the conditions for obtaining a desired structure. As a condition for obtaining a predetermined steel sheet, as a general tendency, it is preferable to set a lower rolling finish temperature when the heating temperature is relatively high, and on the other hand, when the finishing temperature is higher, Early cooling is recommended. Of course, even with these tendencies, the range varies depending on the type of steel or the like.

Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention, and all changes and implementations without departing from the spirit of the preceding and the following are included in the technical scope of the present invention.

[0032]

EXAMPLE Steel having the composition shown in Table 1 was melted by a conventional melting method to form a slab, and a steel sheet having a thickness of 25 mm was manufactured according to the conditions shown in Tables 2 and 3.

The properties of the base material [strength and toughness (vTrs)] of each steel sheet obtained in this manner are evaluated in the following manner, and the number of precipitates such as TiC having a predetermined size is determined according to the method described above. Was measured, and the average grain size of crystals surrounded by grain boundaries in which all adjacent crystals had an orientation difference of 15 ° or more was measured. Further, the HAZ toughness (vE ₀ ) was also measured in the following manner.

[Base Material Property Test] Tensile test: A JIS No. 4 test piece was sampled from a quarter of the thickness of each steel sheet and subjected to a tensile test to determine the tensile strength (T
S) was measured. In the present invention, tensile strength ≧ 490 MPa
Was passed.

Impact test: A JIS No. 4 test piece was sampled from a quarter of the thickness of each steel sheet and subjected to a Charpy impact test to obtain vTrs. In the present invention, vTrs ≦ −
30 ° C. was accepted.

[Weldability test] HAZ toughness: Welding was performed at a heat input of 21 to 112 kJ / cm (submerged welding method), a JIS No. 4 test piece was sampled from the site shown in FIG. Absorbed energy (vE ₀ ) was determined. In the present invention, vE ₀ ≧ 1
00J was accepted.

The results are shown in Tables 2 and 3.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

From these tables, the following can be considered.

No. 1-5, 8-11, 14, 16-1
Nos. 7, 20 to 22, 25, and 27 to 28 are examples in which steel sheets satisfying the requirements of the present invention were used and steel sheets were produced under predetermined rolling conditions. , A target level of base material properties and HAZ toughness are obtained.

On the other hand, the following steel sheets which do not satisfy the requirements of the present invention have the following disadvantages.

No. Nos. 6, 15, 18 and 26 are examples in which the amount of reduction was small, the desired TiC or the like was not obtained, and the base material toughness was reduced. No. HAZ at 15, 18 and 26
The toughness also decreased.

No. Nos. 7 and 19 have the desired TiC because the heating temperature and the finishing temperature are low for this steel type.
Etc. were not obtained, and the base material strength was reduced.

No. Nos. 12 and 23 have the desired TiC because the heating temperature and the finishing temperature are high for this steel type.
And the like were not obtained, and the base material toughness and the HAZ toughness were lowered.

No. In Nos. 13 and 24, the balance of the heating temperature, the finishing temperature, and the cooling rate was poor for this steel type, so that the effect of improving the base material strength by Ti was not exhibited, and the base material strength was reduced.

No. Nos. 29 to 32 are examples in which components in steel deviate from the requirements of the present invention. No. In the cases of Nos. 29 and 31, a steel type having a small amount of Ti was used, so that the base metal strength was reduced. In addition, No. In No. 30, the base material toughness was deteriorated due to the large amount of Ti. No. 32 is an example in which the amount of C is large, and the base material toughness and the HAZ toughness are reduced.

[0049]

Since the present invention is constituted as described above, it is possible to effectively exhibit high strength and improved HAZ toughness by adding a large amount of Ti, and improved base metal toughness by depositing coarse TiC and the like. In addition, as a result of further improving the toughness by controlling the microstructure, it was possible to efficiently provide an excellent high-strength and toughness steel sheet having excellent weldability.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic explanatory view showing a test specimen collection position of bond toughness during submerged arc welding.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA15 AA16 AA17 AA19 AA22 AA23 AA24 AA31 AA35 AA36 AA39 AA40 BA01 CA01 CA02 CB02 CC02 CC03 CD06

Claims (7)

[Claims] 1. C: 0.01 to 0.15% (mean% by mass, the same applies hereinafter), Ti: 0.03 to 0.2%, Mn:
0.8 to 3.0%, Si: 0.6% or less, and Ti having a particle size of 7 to 50 nm per 1 μm ^{2 of} the test area.
Containing carbide and / or Ti-containing carbonitride is 1.0 on average
High strength and high toughness steel sheet with excellent weldability characterized by the presence of more than one piece. 2. The high-strength and high-toughness steel sheet according to claim 1, wherein the average grain size of the crystals surrounded by the grain boundaries in which all adjacent crystals have an orientation difference of 15 ° or more is 30 μm or less. 3. The high-strength, high-toughness steel sheet according to claim 1, further comprising B: 0.0003 to 0.005%. 4. Cu: 3% or less (excluding 0%), Ni: 6% or less (excluding 0%), and C:
r: At least one selected from the group consisting of 0.5% or less (excluding 0%).
4. The high-strength and high-toughness steel sheet according to any one of items 1 to 3. 5. Al: 0.5% or less (excluding 0%), Ca: 0.05% or less (excluding 0%), and REM: 0.05% or less (excluding 0%) ) And at least one selected from the group consisting of: a high-strength and high-toughness steel sheet according to any one of claims 1 to 4. 6. Mo: 0.50% or less (excluding 0%), Nb: 0.05% or less (excluding 0%),
V: 0.10% or less (excluding 0%), and Zr:
The composition contains at least one selected from the group consisting of 0.05% or less (excluding 0%).
5. The high-strength and high-toughness steel sheet according to any one of 5. 7. A method for producing a steel sheet according to any one of claims 1 to 6, wherein the steel according to any one of claims 1 to 6 is used, and the steel sheet according to claim 1 is not recrystallized γ.
A method for producing a high-strength steel sheet excellent in weldability, characterized in that the amount of reduction in a region is controlled to 60% or more.