JP2001247916A - Method for producing low yield ratio high titanium base steel plate excellent in bare atmosphere corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a useful method for producing such a slow yield ratio high Ti base steel plate, as which a good atmosphere corrosion resistance is achieved by adding Ti in comparatively high content and in spite of addition of Ti, the low yield ratio can be realized by restraining the precipitation of TiC after transformation and if necessary, the excellent weldability can be exhibited. SOLUTION: A steel material containing <=0.2% C, 0.1-1% Si, <=2.5% (excluding 0%) Mn, 0.025-0.5% Ti and <=0.01% (excluding 0%) N, is used. Then, a rolling reduction is applied to this steel material at >=30% the accumulated value so that the surface temperature is in the temperature range of ±50 deg.C of a temperature T( deg.C) regulated by the following equation (1). T( deg.C)=1500+([Ti]-[N]×3.4)+800-the plate thickness (mm) ...(1) Wherein, [Ti] and [N] are contents (mass%) of Ti and N, respectively and when [Ti] is >0.1%, [Ti]=0.1%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a steel sheet applied to a steel structure, such as a bridge or a steel tower, in which maintenance work including repainting is difficult to perform on a daily basis. In particular, by adding a relatively large amount of Ti, good weather resistance can be exhibited, and despite the addition of Ti, precipitation of TiC after transformation can be suppressed to realize a low yield ratio. The present invention relates to a useful method for producing a Ti-based steel sheet having a low yield ratio and high yield.

[0002]

2. Description of the Related Art Steel materials used for bridge structures such as road bridges in a salt corrosive environment where salt water and snow-melting salt fly, such as in mountainous areas and coastal areas, are conventionally coated to improve corrosion resistance. Has been used. However, since this coating film always deteriorates with time, in order to maintain corrosion resistance,
There is a need for maintenance to repaint in a certain period.

On the other hand, in recent years, a small number of main girder bridges having a small number of main girder bridges, such as two main girder bridges, have been increasingly used for these bridges in place of conventional multiple girder bridges. . Compared with a multi-girder bridge, this minor girder bridge has advantages in that the amount of steel used (steel amount) and the number of bridge pieces can be reduced, the workability is good, the environment can be protected, and the construction period can be shortened. Have. For such a small number of main girder bridges, there is a strong demand for minimizing the load and cost of maintenance and maintenance after the bridge is installed and for extending the life of the bridge itself. For this reason, the steel material used for the structural material of such a small number of main girder bridges is a steel material having excellent so-called bare weather resistance, which can be used without painting (bare use) even under the salt corrosion environment. Is strongly required.

[0004] From the viewpoint of workability and shortening of the construction period, these structural materials such as a few main girder bridges have a heat input of 5 KJ / mm or more by carbon dioxide arc welding or electrogas arc welding.
In some cases, large heat input welding with a heat input of 100 to 300 KJ / mm or more is performed. Therefore, as a steel material used for such a structural material, a steel material which does not require preheating and which can perform high-efficiency welding such as large heat input welding and has excellent weldability is required.

[0005] As described above, steel materials used for the above-mentioned bridges include not only mechanical properties such as strength required for bridge steel materials but also steel materials having both excellent bare weather resistance and weldability. Has been requested. Conventionally, as this type of steel material, P: 0.15% or less, Cu: 0.2 to 0.6%, C:
A highly weather-resistant rolled steel material (JIS G 3125) is specified, in which r: 0.3 to 1.25% and Ni: 0.65% or less. In addition, from the same viewpoint, a weather-resistant hot-rolled steel material for a welded structure (JIS G31
14) is also known. These weather-resistant steel materials are characterized by the rust generated on the steel surface by the action of the trace elements, and a dense “stable rust layer” having high weather resistance represented by bare weather resistance.
It has a self-corrosion protection function. Due to such properties, the weather-resistant steel has been basically used as a maintenance-free structural material for various structures without painting.

[0006] However, such a steel material exhibits good weather resistance in a normal corrosive environment, but in a salt corrosion environment as described above, the "stable rust layer," which is a characteristic of weather resistant steel due to the effect of salt. Is not easily formed, and the desired weather resistance is not exhibited.

The reason why such a phenomenon occurs can be considered as follows. It can be considered that in a corrosive environment with a large amount of salt, the pH in the rust film is particularly lowered with the corrosion of steel. Then, when the corrosion of the normal steel starts even slightly, first, Fe → Fe ²⁺ +
2 ^e− followed by Fe ²⁺ + 2H ₂ O → Fe (OH) ₂ +
Due to the reaction of 2H ⁺ , the pH of the steel surface decreases, and the pH of the rust film or the interface between the rust film and the steel also decreases. These p
Once H decreases, transport of chloride ions in the rust film increases to maintain electrical neutrality, and chloride ion concentration occurs at the interface between the rust film and steel. As a result, a hydrochloric acid atmosphere is formed at the interface, and the corrosion of the steel is promoted. At the same time, a decrease in the pH in the rust film increases the solubility of iron ions, and inhibits the formation of the "stable rust layer", which is a key to the corrosion prevention mechanism in corrosion-resistant low-alloy steel such as weathering steel. It can be considered that a corrosion accelerated situation is formed.

The present inventors have been conducting research for the purpose of realizing a steel material exhibiting good weather resistance even in the above-described salt corrosion environment and also having excellent weldability. As part of the research, for example,
No. 71632 is proposed. In this technique, Cr, which has conventionally been regarded as an element for promoting the formation of a “stable rust layer”, is an element that lowers the pH instead.
The new knowledge of reducing the content of Ti as much as possible and allowing a relatively large amount of Ti to be contained in place of it makes it possible to exhibit excellent bare weather resistance even in a salt-corrosion environment and to obtain a carbon equivalent Ceq represented by a predetermined relational expression. Is defined in an appropriate range, thereby improving the weldability.

[0009]

SUMMARY OF THE INVENTION The development of the above-mentioned steel material has enabled the steel material to exhibit good bare weather resistance even in a salt-corrosion environment and to have excellent weldability. There were still some problems to be solved.

In the present invention, the steel material includes, for example, 49
Although a strength of 0 MPa or more is required, in order to exhibit good workability, the yield ratio (yield stress / tensile strength) must be low (for example, 85% or less in the case of a high Ti system). It is also important.

On the other hand, such a steel material achieves a predetermined strength by heating and rolling. The heating and rolling conditions include a heating temperature of 1100 to 1250.
° C, rolling end temperature (steel plate surface): 850-1000 ° C
(Then it is generally air-cooled).

However, when the steel is manufactured under the above-described heating and rolling conditions, there is no problem because the yield ratio is basically low in a steel material having a relatively small Ti content. In a high Ti-based steel material having a relatively high content, TiC is consistently precipitated in the Fe matrix after transformation to cause precipitation hardening, yield stress increases, yield ratio increases, and desired workability cannot be obtained. Problems arise.

Incidentally, high-strength steels having a tensile strength of 570 MPa or more (including steels having a high Ti content) generally have a high yield ratio. It is the fact that it is being converted. However, there is a problem that the formation of a dual phase structure of a hard phase and a soft phase causes deterioration of toughness, and conversely, it is difficult to produce a material having a not so high strength of 400 MPa class or 490 MPa class.

The present invention has been made under such a circumstance, and an object of the present invention is to achieve good bare weather resistance by adding a relatively large amount of Ti, and to achieve a good post-transformation despite the addition of Ti. It is an object of the present invention to provide a useful method for producing a low-yield-ratio high-Ti steel sheet which can suppress the precipitation of TiC and realize a low yield ratio and can also exhibit excellent weldability if necessary. .

[0015]

Means for Solving the Problems The method of the present invention which can solve the above-mentioned problems is as follows: C: 0.2% or less, Si:
0.1-1%, Mn: 2.5% or less (excluding 0%), Ti: 0.025-0.5%, and N: 0.01
% Or less (excluding 0%),
The surface temperature is the temperature T defined by the following equation (1).
In the temperature range of ± 50 ° C. of (° C.), the point is to apply a cumulative reduction of 30% or more. T (° C.) = 1500 + ([Ti]-[N] × 3.4) + 800-plate thickness (mm) (1) where [Ti] and [N] are the contents (mass) of Ti and N, respectively. %), And when [Ti]> 0.1%, [Ti]
= 0.1%.

In the method of the present invention, Cu: 0.0
5-3% and 0.05-6% Ni, P: less than 0.03% (including 0%), Cr: 0.0
Each is suppressed to less than 5% (including 0%), and the following (2)
It is also preferable to use a steel sheet having a carbon equivalent Ceq (%) specified by the formula of 0.41% or less. By satisfying such a chemical composition, excellent weldability can be exhibited. Ceq (%) = [C] + [Si] / 22 + [Mn] / 6 + [P]-[Cu] / 20- [Ni] / 24 + [Cr] / 2 ... (2) where [C], [ [Si], [Mn], [P], [Cu], [Ni] and [Cr] indicate the contents (% by mass) of C, Si, Mn, P, Cu, Ni and Cr, respectively.

Further, as steel materials used in the production method of the present invention, (a) Ca: 0.01% or less (excluding 0%), (b) Al: 0.5% (excluding 0%),
(C) B: 0.005% or less (excluding 0%),
(D) La: 0.05% or less (excluding 0%), C
e: 0.05% (excluding 0%) and Mg: 0.0
It is also useful to contain one or more selected from the group consisting of 5% (not including 0%), and the properties of the steel sheet can be further improved according to the type of the component to be contained.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION In a steel material having a relatively high Ti content, the yield stress increases under the ordinary heating and rolling conditions as described above. The cause of such a phenomenon is considered as follows. Was completed. That is, under normal heating and rolling conditions, after transformation (after hot rolling), TiC
Coherently precipitates to cause precipitation hardening, which causes an increase in yield stress (ie, yield ratio).

The present inventors have studied various means from various angles from the viewpoint that the suppression of the coherent precipitation after the transformation as described above may suppress the rise of the yield ratio. . As a result, the surface temperature of the steel sheet whose chemical composition has been appropriately adjusted is ± 5 of the temperature T (° C.) expressed by the above equation (1) using [Ti], [N] and the sheet thickness as parameters.
In the temperature range of 0 ° C, if a cumulative reduction of 30% or more is applied,
The present inventors have found that the coherent precipitation of TiC in the Fe matrix after the transformation is prevented, and as a result, an increase in the yield ratio is suppressed, and the present invention has been completed.

The operation of the above equation (1) will be described in more detail. This equation (1) is a regression equation of the steel sheet surface temperature,
In a temperature range of ± 50 ° C. of the temperature T (° C.) defined by the equation (1), it means that TiC is easily precipitated in the austenite phase (in the γ phase). This temperature range (hereinafter, “T
By rolling at a predetermined reduction amount in the "iC precipitation easy temperature range", TiC can be positively precipitated before transformation. In this way, T
By positively depositing iC, Ti
Precipitation of C in the Fe matrix was suppressed, and an increase in yield stress (that is, yield ratio) could be suppressed.

Since the above-mentioned temperature range for easy precipitation of TiC varies depending on the composition and thickness of the steel sheet, [Ti], [N] and the thickness of the steel sheet are defined as parameters. Further, in order to positively precipitate TiC in this temperature range, it is necessary to perform rolling at a cumulative reduction of 30% or more in this temperature range.

In the above equation (1), the content of Ti
The difference between the [Ti] and the Ti equivalent of the N content [N] is T
It is not Ti that combines with N and precipitates as TiN that directly contributes to the increase in the yield ratio due to iC precipitation, and therefore excludes that amount (the amount of Ti that combines with N). However,
As will be described later, if N is contained in an excessively large amount, the inconvenience of toughness deterioration due to coarsening of TiN occurs, so that it can be contained in an excessively large amount. On the other hand, from the viewpoint that Ti is added, the weather resistance is exhibited. And the N content is T
It does not exceed the content of i. In addition, Ti
Regarding the content [Ti], when [Ti]> 0.1, [Ti] =
The reason why they are unified to 0.1 is that when [Ti]> 0.1, even if [Ti] changes, the “TiC easy precipitation temperature range” does not change.

It is necessary to appropriately adjust the chemical composition of the steel sheet targeted by the method of the present invention. The reasons for limiting the ranges of the basic elements such as C, Si, Mn, Ti and N are as follows. It is on the street.

C: 0.2% or less (excluding 0%) C is 390 to 630 MPa as a steel structural material application
Grade, or an element necessary to secure the required strength of higher than 0.2%.
If it exceeds, the weldability and corrosion resistance of the steel will be degraded. Therefore,
The C content is set to 0.2% or less so that the required strength can be secured. From the viewpoint of securing the strength, a preferable lower limit of the C content is 0.03%.

Si: 0.1-1% Si is an essential element for deoxidizing molten steel and strengthening solid solution, and promotes formation of a dense “stable rust layer” and improves bare weather resistance. Also has the effect of causing. It also contributes to the improvement of weldability. In order to exert such effects, the Si content needs to be 0.1% or more. However, if the content is excessive and exceeds 1%, the weldability will be reduced. Note that a preferable upper limit of the Si content is 0.40%.

Mn: 2.5% or less Mn is an element necessary to secure the required strength of 390 to 630 MPa class or more like C, but its content becomes excessive. M exceeds 2.5%
There is a possibility that nS may be generated in a large amount in steel to deteriorate bare weather resistance. Therefore, the Mn content needs to be 2.5% or less. The preferred lower limit of the Mn content is 0.4%, and the preferred upper limit is 2.0%.

Ti: 0.025 to 0.5% Ti is an important element as an element promoting the formation of the “stable rust layer” in place of Cr in the steel sheet targeted in the present invention. There is no adverse effect on weather resistance that would cause a drop in pH. Further, Ti also has the effect of reducing the rust formed by the refinement of the crystal grains in the steel material structure, or improving the toughness and weldability. That is, TiC, which becomes a strong ferrite transformation nucleus in the process of cooling the weld due to the inclusion of Ti,
And TiN and the like are dispersed and precipitated in the steel, which greatly contributes to the refinement of ferrite in the structure of the weld heat affected zone. In order to exhibit such an effect, it is necessary to contain 0.025% or more, but if it exceeds 0.5%, the effect is saturated and is not economical. Therefore, the content of Ti is 0.025 to 0.5%.
It was specified.

However, the rust formed on the surface of the steel sheet must be reduced in order to ensure that the rust generated on the surface of the steel sheet is densified and that the steel sheet can be used without paint even under a salt corrosion environment, so that it can be used without coating. It is preferable that the fraction of the amorphous component obtained by the X-ray diffraction method is 30% or more, and the fraction of the β-FeOOH component is 20% or less. 0.05%. If the content of Ti exceeds 0.2%, embrittlement of steel may become a problem, and as described above, it is not economical. From this perspective,
The preferable upper limit of the Ti content is about 0.2%.

N: 0.01% or less (excluding 0%) N combines with Ti and is finely dispersed and precipitated as TiN in the steel, thereby suppressing “growth of crystal grains” and “thermal influence of a welded portion”. In this, the toughness is improved by becoming a ferrite forming nucleus. "
Etc., but if it exceeds 0.01%, T
iN coarsens and deteriorates toughness. From such a viewpoint, the N content needs to be 0.01% or less.

The basic components of the steel sheet used in the method of the present invention are as described above.
0.05-3% and Ni: 0.05-6%, respectively, and P: less than 0.03% (including 0%) and C
r: each less than 0.05% (not including 0%),
And the carbon equivalent Ceq (%) defined by the above formula (2) is 0.4
It is also preferable to use a steel sheet of 1% or less, and by satisfying such a chemical component composition, bare weather resistance can be further enhanced and excellent weldability can be exhibited. In this chemical component composition, the preferable range of each element and the reason for defining the above formula (2) are as follows.

Cu: 0.05 to 3% Cu is an element that is electrochemically nobler than iron, and densifies rust generated on the surface of iron, promotes the formation of a “stable rust layer”, and It has the effect of improving weather resistance. It also contributes to the improvement of weldability. In order to exhibit such effects, it is preferable that the Cu content is 0.05% or more. However, if the Cu content exceeds 3%, the effect is not only saturated, but also the processing such as hot rolling is performed. In this case, the material may be embrittled. For this reason, the Cu content is preferably set to 0.05 to 3%, but the more preferable lower limit is 0.3% and the more preferable upper limit is 2.0%.

Ni: 0.05 to 6% Ni is an element having the same properties as Cu to improve bare weather resistance and weldability. It also has the effect of suppressing hot working brittleness due to the Cu content. Therefore, when it is contained together with Cu, a synergistic effect of an effect of improving corrosion resistance and an effect of suppressing hot working brittleness can be expected. To achieve these effects, N
The i content is preferably 0.05% or more. However, when the Ni content is excessive, the solid-liquid solidification temperature range in the complete austenite structure is widened to promote segregation of the low melting point impurity element to the dendrite interface, and to react with S to reduce the low temperature at the grain boundary of the weld metal. A NiS compound having a melting point is precipitated, and the ductility of the grain boundary of the solidified metal is deteriorated. For this reason, an excessive content of Ni has an adverse effect on welding hot cracking resistance, so the upper limit of the content is preferably 6%. Note that a more preferred lower limit of the Ni content is 0.2%, and a more preferred upper limit is 4%.

P: less than 0.03% (including 0%) P prevents the rust chloride ions generated on the steel surface from entering the weather-resistant steel and forms a dense “stable rust layer”. And is a characteristic element having an effect of improving weather resistance. And, in order to exhibit this effect, the conventional weather-resistant steel requires a content of about 0.05% or more and about 0.15% or less. However, in the steel sheet targeted in the present invention, if a relatively large amount of P is contained, the weldability is remarkably impaired, which is important for the construction of the small-numbered girder bridge, and has a high efficiency without preheating (preheating free). It becomes impossible to satisfy the required characteristics of weldability that enables heat welding.

In the steel sheet targeted by the present invention, the formation of a dense “stable rust layer” can be achieved by the inclusion of Ti or the like, so that excessive P is not required. Therefore, in the steel sheet targeted by the present invention, the content of P is preferably reduced as much as possible, and it is preferable to suppress the content of P to less than 0.03% in consideration of the economics of reducing the P content. Also, by reducing the P content in this way, it will greatly contribute to improving weldability. Incidentally, the P content is more preferably 0.
It is better to reduce it to 015% or less.

Cr: less than 0.05% (including 0%) Cr, as described above, causes micro surface defects of steel to cause a decrease in pH in the part and promotes oxidization of condensed moisture in the defects. It has the effect of inducing corrosion and degrades the bare weather resistance of steel. Therefore, the steel sheet used in the present invention preferably has a Cr content of less than 0.05%.

Carbon equivalent Ceq (%) ≦ 0.41% In the steel sheet used in the present invention, the carbon equivalent Ceq (%) represented by the above formula (2) is preferably specified as low as 0.41% or less. This is because the steel material for a structure such as a small number of girder bridges exhibits excellent weather resistance and secures good weldability even when the plate thickness is large. Specifically, 50
Even with a thick plate having a thickness of 80 mm or more, and even a thickness of 80 mm or more, a heat input of 5 KJ / mm or more, and in some cases 100 to 30 without preheating and without causing welding defects such as welding cracks.
This is to ensure weldability that enables high-efficiency welding such as large heat input welding of 0 KJ / mm or more. Reducing the carbon equivalent of this steel lowers the hardenability of the steel matrix and is also effective in refining the ferrite in the weld heat affected zone structure during welding. Therefore, when the carbon equivalent Ceq (%) of the steel material represented by the above formula (2) exceeds 0.41%,
The weldability deteriorates, making it impossible to perform high-efficiency welding such as large heat input welding with a heat input of 5 KJ / mm or more without preheating with a thick plate of 50 mm or more. It cannot be used for girder bridge applications.

The steel material used in the method of the present invention may be (a) Ca: 0.01% or less (not including 0%) and (b) Al: 0.5% or less (not including 0%), if necessary. ,
(C) B: 0.005% or less (excluding 0%),
(D) La: 0.05% or less (excluding 0%), C
e: 0.05% or less (excluding 0%) and Mg:
It is also useful to contain one or more selected from the group consisting of 0.05% or less (not including 0%), and the properties of the steel sheet can be further improved according to the type of the component to be contained. Also, suppressing the content of S to 0.02% or less is effective from the viewpoint of improving corrosion resistance. The reasons for defining the ranges of these components are as follows.

Ca: 0.01% or less (excluding 0%) Ca is an element for further improving bare weather resistance and also has an effect of improving weldability. One of the effects of Ca to improve weather resistance is to fix S harmful to weather resistance and to clean the steel matrix. Another effect is that, in the course of the progress of corrosion on the steel surface and microscopic defects, a small amount of Ca dissolved in the steel in a minute amount dissolves with the corrosion reaction of iron to exhibit alkalinity. Therefore, it is an element having a solution pH buffering effect at the tip of the corrosion (anode) and an effect of suppressing corrosion at the tip of the corrosion. These have an effect completely opposite to that of the element that lowers the pH during dissolution, such as Cr.

When such Ca is used in combination with Ti, a synergistic effect of improving corrosion resistance such as bare weather resistance occurs together with the effect of reducing Cr and the effect of promoting the formation of a "stable rust layer" of Ti and the like.
This synergistic effect increases as the content of Ca increases, but when the content is excessive, the effect saturates and is not economical. In particular, when Ca is contained excessively, the cleanliness of the steel is deteriorated, and there is a possibility that the furnace wall during the production of the weather-resistant steel, particularly during the steel making, may be damaged. Therefore, when Ca is contained, the content is preferably up to about 0.01%. In order to effectively exhibit the above-mentioned effects due to the Ca content, it is preferable to contain 0.0001% or more.

Al: 0.5% or less (excluding 0%) Al makes the rust generated on the steel surface finer and “stable rust layer”
Has the effect of promoting the formation of, and improving bare weather resistance.
It also has the effect of improving weldability. The effect of improving the weldability is further enhanced by the addition of a composite with Ti. Al is a deoxidizing element for molten steel, which captures solid solution oxygen, prevents blowholes, and is also an effective element for improving the toughness of steel.

These effects increase as the Al content increases. However, even when the Al content is excessive, the above effects are not only saturated, but rather deteriorate the weldability and increase the amount of alumina-based inclusions. Since the toughness is reduced, the content is preferably 0.5% or less. In order to effectively exhibit these effects, the content is preferably 0.05% or more.

B: 0.005% or less (excluding 0%) B exerts an effect of remarkably enhancing the hardenability of a steel material when added in a small amount. Such effects increase as the content increases, but if the content is excessive, the B compound is generated and the toughness is reduced. Therefore, the content is preferably 0.005% or less. In order to effectively exhibit such effects, it is preferable to contain 0.0003% or more.

La: 0.05% or less (excluding 0%)
Ce): 0.05% or less (not including 0%) and
Mg: group consisting of 0.05% or less (excluding 0%)
At least one selected from the group consisting of La, Ce and Mg is selectively contained as necessary from the viewpoint of further improving corrosion resistance such as bare weather resistance.
In the process of progressing corrosion of the steel surface and microscopic defects, these elements are dissolved in a small amount along with the corrosion reaction of iron and exhibit alkalinity. Therefore, it is an element having a solution pH buffering effect at the tip of the corrosion (anode) and an effect of suppressing corrosion at the tip of the corrosion. These have an action completely opposite to the action of an element such as Cr that lowers the pH during dissolution. From these facts, by using the effect of reducing Cr and the effect of promoting the formation of a “stable rust layer” such as Ti,
A synergistic effect that further improves the corrosion resistance such as bare weather resistance can be expected. Such effects are exhibited by containing at least one of La, Ce and Mg.
Even if it is contained excessively, the effect is saturated and it is not economical,
Also, the mechanical properties may be deteriorated. For these reasons, it is preferable that each of the above elements be contained at a maximum of 0.5%. In order to effectively exhibit the above effects, the content of each element is preferably set to 0.0001% or more.

S: 0.02% or less (including 0%) When S is contained in excess of 0.02%, a large amount of FeS and MnS, which are the starting points of corrosion, are formed in steel, and bare weather resistance is obtained. There is a possibility that the corrosion resistance such as the property may decrease. As described above, N
When i is excessively contained, a low melting point NiS compound is precipitated at the grain boundary of the weld metal by reaction with S, and the ductility of the grain boundary of the solidified metal is easily deteriorated.

In this regard, if the S content is suppressed to 0.02% or less, the NiS compound having a low melting point is not deposited,
Can be contained in a larger amount. For example, S
When the content exceeds 0.02%, the Ni content becomes 3
%, But by setting the S content to 0.02% or less, the Ni content can be increased to 6%. The S content is more preferably 0.01% or less.

The chemical composition of the steel sheet targeted in the present invention is as described above, and the balance is substantially composed of Fe. Here, “substantially Fe” means that the steel sheet targeted in the present invention may contain, in addition to Fe, a trace component (permissible component) to the extent that its characteristics are not impaired. , Zr, V, Nb, Mo, W and the like, and unavoidable impurities such as Sn, Sb, As, O, H and the like.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and any change in design based on the above and following points will be described below. It is included in the technical range of.

[0048]

EXAMPLES Steel sheet A having the chemical composition shown in Table 1 below
Using D, the rolling temperature was variously changed in the pass schedule in which the plate thickness was rolled from 120 mm to 20 mm (that is, the cumulative draft at T (° C.) ± 50 ° C. was changed).
Rolling was performed under manufacturing conditions, and the effects of the manufacturing conditions on the mechanical properties (yield stress YS, tensile strength TS, and yield ratio YR) of the steel sheet were investigated.

[0049]

[Table 1]

The results were compared with the values of T (° C.) ± 50 ° C. in each pass for each manufacturing condition, each steel type, and the T (° C.)
The results are shown in the following 2 and 3, together with the cumulative rolling reduction at ± 50 ° C. For example, in manufacturing condition 1 of steel type A, T (° C.)
Temperature range of ± 50 ° C (53 mm in pass schedule → 2
5 mm), the reduction amount is 53% [[(53-25) /
53] × 100]. still,
Tables 2 and 3 also show the evaluation results of the weldability and the bare weather resistance evaluated by the following methods.

(Weldability) A steel sheet as a test material was subjected to high heat input welding without preheating, and the welded portion was evaluated for hot cracking, tensile strength and toughness. The preferred weldability in the present invention is hot cracking:
None, prevention of low-temperature cracking Preheating temperature: 25 ° C. or less, tensile strength: the same class as the base material, toughness vE ₋₄₀ ≧ 100J.
For welding, submerged arc welding with a heat input of 35 kJ / cm was performed. The hot cracking rate was determined by a C-type jig restrained butt welding cracking test (hot cracking test) established by JIS. The low-temperature cracking of the test material was evaluated by the preheating temperature of the test material which can prevent the occurrence of cracks in an oblique Y-type restraint butt welding crack test (low-temperature cracking test) established by JIS standards. Also, the toughness of the weld joint
It is evaluated by the absorption energy vE _-40 [J] at 40 ° C.

(Nude weather resistance) As preferable conditions, the rust that grew on the surface of the steel material is a dense “stable rust layer”, and the steel material is exposed to the atmosphere for one year (including once a week spraying of 5% salt water). It is an indication of whether or not the rust has a thickness reduction of 0.8 mm or less, preferably 0.5 mm or less. The standard of whether or not a dense "stable rust layer" exists is the degree of amorphousness of rust (degree of amorphousness) and the fraction of β-FeOOH in rust. That is, the main components of iron rust generated on the steel surface are α-FeOOH, β
-FeOOH, crystalline and rust gamma-FeOOH and Fe ₃ O _4, consists of five types of amorphous rust. Among them, the amorphous rust forms a dense “stable rust layer” and guarantees long-term bare weather resistance of the steel material. Therefore, the higher the ratio of amorphous rust (degree of amorphousness) of iron rust, and the smaller the ratio of β-FeOOH, which is particularly apt to promote corrosion among the components of crystalline rust, the denser the “stability”. Rust layer ". In the present invention, as a dense and dense “stable rust layer”, the fraction of the amorphous component of rust is 3%.
It is specified that the content of the β-FeOOH component is 0% by mass or more and 20% by mass or less. In addition, we improve bare weather resistance more,
In order to more reliably compensate for the bare use of steel, it is more preferable that the fraction of the rust amorphous component be 40% by mass or more and the ratio of the β-FeOOH component be 10% by mass or less.

The average thickness reduction was measured by measuring the average thickness of the test material before and after the air exposure test with a micrometer, and taking the density into consideration to determine the average thickness reduction [mm]. As means for measuring the degree of amorphousness, “corrosion protection 95C-306 (3
X-ray powder diffractometry disclosed in "Quantification of Iron Rust Components by Powder X-Ray Diffraction and Its Application" on page 04-344) is effective. In this document, an attempt was made to quantify the iron rust component on the surface of a steel material by a powder X-ray diffraction method for a weather-resistant steel material, and the higher the ratio of amorphous rust (degree of amorphousness) in the iron rust, It supports a corrosion-resistant improvement model that provides a dense “stable rust layer”.

Further, as a more specific powder X-ray diffraction method, in the literature, a constant mass ratio of CaF ₂ was used as an internal standard.
Alternatively, a powder obtained by mixing ZnO or the like with a rust sample collected from a steel material and identifying the powder by a normal X-ray diffraction method is used. , Each crystalline rust component is quantified from the calibration curve of the rust component, and the ratio of the amorphous component is calculated by subtracting the amount of each crystalline rust component from the total amount of rust. This is because the integrated intensity ratio of the diffraction peak of the amorphous component itself is difficult to obtain and it is difficult to quantify. Incidentally, as disclosed in the same document, qualitative analysis of rust components is possible with other analysis methods such as infrared spectroscopy other than X-ray diffraction, but quantitative analysis is not possible. Difficult and there is no established method for the determination of rust components. Therefore, the term "amorphous degree of rust on the surface of steel" referred to in the present invention means a value quantitatively measured by this X-ray diffraction method, particularly the powder X-ray diffraction method disclosed in the above-mentioned literature.

As for the exposure test, a one-year air exposure test including once weekly spraying with salt water was performed to evaluate the long-term durability. The condition of the one year air exposure test is 1 week
5% salt water spraying was carried out twice, and the test material was placed facing south and inclined at an angle of 30 ° to the horizontal.

[0056]

[Table 2]

[0057]

[Table 3]

As is apparent from these results, in the steel sheet manufactured while satisfying the requirements specified in the present invention, the precipitation of TiC was prevented and the low yield ratio was achieved despite the addition of a large amount of Ti. You can see that.

[0059]

The present invention is configured as described above,
In addition to achieving good bare weather resistance by adding a relatively large amount of i, the Ti after transformation is notwithstanding the addition of Ti.
It is possible to realize a low yield ratio by suppressing the precipitation of C,
Low yield ratio high T that can also exhibit excellent weldability if necessary
An i-type steel sheet was manufactured.

Continued on the front page (72) Inventor Toshiaki Suga 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Kobe Steel, Ltd. Kakogawa Works F term (reference) 4K032 AA00 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA15 AA16 AA17 AA21 AA23 AA29 AA AA31 AA35 AA40 BA01 CB02 CC03 CC04

Claims (6)

[Claims] 1. C: 0.2% or less (meaning by mass%, the same applies hereinafter), Si: 0.1 to 1%, Mn: 2.5% or less (0%
%), Ti: 0.025 to 0.5%, and N: 0.01% or less (excluding 0%), and the surface temperature is determined by the following equation (1). In a temperature range of ± 50 ° C. of the specified temperature T (° C.), 3
A method for producing a Ti-based steel sheet having a low yield ratio and excellent weatherability, wherein a reduction of 0% or more is applied. T (° C.) = 1500 + ([Ti]-[N] × 3.4) + 800-plate thickness (mm) (1) where [Ti] and [N] are the contents (mass) of Ti and N, respectively. %), And when [Ti]> 0.1%, [Ti]
= 0.1%. 2. Cu: 0.05-3%, Ni 0.05-
6%, P: less than 0.03% (including 0%), Cr: less than 0.05% (including 0%), respectively, and carbon defined by the following formula (2): Equivalent Ceq
The production method according to claim 1, wherein a steel sheet whose (%) is 0.41% or less is used. Ceq (%) = [C] + [Si] / 22 + [Mn] / 6 + [P] / 10- [Cu] / 20- [Ni] / 24 + [Cr] / 2 ... (2) where [C] , [Si], [Mn], [P], [Cu], [Ni] and [Cr] indicate the contents (% by mass) of C, Si, Mn, P, Cu, Ni and Cr, respectively. 3. The method according to claim 1, wherein a steel sheet containing Ca: 0.01% or less (excluding 0%) is used. 4. Al: 0.5% or less (excluding 0%)
The production method according to any one of claims 1 to 3, wherein a steel sheet containing is used. 5. The production method according to claim 1, wherein a steel sheet containing B: 0.005% or less (excluding 0%) is used. 6. La: 0.05% or less (excluding 0%), Ce: 0.05% or less (excluding 0%) and Mg: 0.05% or less (excluding 0%). The production method according to any one of claims 1 to 5, wherein a steel sheet containing at least one selected from the group consisting of: