JPH0133530B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing weather-resistant steel with excellent weldability and a strength of 50 kg/mm ² or more, in which the steel material after hot rolling is heated to an Ar ₃ transformation point or higher.
This method utilizes the principle that the strength of a steel plate is increased by accelerated cooling in a temperature range of 500 to 650°C at a cooling rate of 4 to 25°C/sec. More specifically, the alloy components, particularly the carbon content, can be reduced by the amount of increased strength achieved by accelerated cooling, thereby making it possible to manufacture high-strength steel with excellent weldability. It was discovered that there is a good correlation between the strength of the accelerated cooling agent as described above, its composition, and cooling rate, and by utilizing this relationship, it is possible to achieve a strength of 50 kg/mm ² or more with weldability and weather resistance. The object of the present invention is to provide a method for reliably producing a steel having a certain temperature (i.e., by setting a cooling rate corresponding to its composition or, conversely, a composition corresponding to this accelerated cooling rate). The technology of controlling the transformed structure and improving mechanical properties by accelerated cooling of thick steel plates after hot rolling is called controlled cooling.
Rolling (hereinafter abbreviated as CR) can be said to be a type of processing heat treatment. Until now, there has been little systematic research into this controlled cooling, but recently the so-called Interrupted Cooling method, which accelerates cooling only in a specific temperature range after finishing rolling, has been attracting attention. This method does not require tempering treatment like the direct quenching method after rolling, and can be applied to general non-tempered steel sheets. By combining it with CR, it can maintain high toughness and achieve even higher toughness. It can be expected to increase the tensile strength and improve weldability and other various steel material properties. However, although such an accelerated cooling method has already been operationally proven as run-out cooling in hot strip mills, it has not yet been put to practical use in plate rolling. The inventors' group has been actively promoting research on this accelerated cooling method, and has already made a number of proposals regarding the results. Also, other companies have products related to low-temperature toughness (for example, Japanese Patent Publication No. 35-4111
JP-A-51-59717), methods related to workability (e.g., JP-A-52-17319), and methods related to unstable ductile fracture (e.g., JP-A-54-14321) have been proposed. . However, even though proposals have been made regarding the material of such accelerated cooling materials, there are no proposals regarding weldability and weather resistance. One reason for this is that the effect of accelerated cooling on general low-alloy steels is to change the transformation structure from a ferrite + pearlite-based structure to a bainite + ferrite structure, thereby increasing the strength and preventing toughness deterioration. On the other hand, improvement in weldability can be obtained by utilizing this increase in strength and reducing alloy components, so the effect is at first glance indirect and may seem natural. This is so that you can be saved. It is believed that the biggest drawback of the conventional accelerated cooling method is that this technology has not been fully utilized due to lack of data accumulation and analysis thereof. In other words, when producing high-strength steel materials with excellent weldability by subsequent accelerated cooling after hot rolling, manufacturing conditions (steel It has not yet been elucidated how to set the composition and accelerated cooling rate. If weldability is emphasized, the alloying element content and carbon equivalent of the steel tend to be kept too low, and even if accelerated cooling is performed after hot rolling, the desired strength may not be obtained. On the other hand, if the components were designed with attention to the strength level, the desired strength could be obtained, but there was a problem in that poor weldability results were obtained. At the same time, in the case of steel products (mainly steel plates) manufactured by hot rolling followed by accelerated cooling, the strength changes depending on both the composition and cooling rate, so when cooled at a certain cooling rate, Even if the strength is insufficient, there is still a possibility that the target strength level can be obtained by cooling at a higher cooling rate. However, there are limits to the cooling rate that can be adopted due to problems with the equipment or distortion of the accelerated cooling material, and it is difficult to manufacture steel materials that have the desired properties such as weldability, strength, and weather resistance. Ta. The present invention was made based on the principle that the strength of a steel material obtained by accelerated cooling after hot rolling is determined by both its composition and cooling rate, and the influence of each element on weldability is also considered. Also, taking into consideration
The present invention relates to a method for producing a steel material that is resistant to welding cold cracking as typified by the Y-type cracking test, has excellent weather resistance, and has a strength of 50 Kg/mm ² or more (mainly tensile strength of 50 to 60 Kg/mm ² ). The main points are: C0.03~0.20%, Si0.6% or less, Mn0.5~2.5
%, Cr0.1~2.0%, Cu0.1~2.0%, sol.Al0.005~
The basic composition is 0.08%, and if necessary, Nb0.01~
0.1%, V0.01~0.15%, Ti0.01~0.15%, Ni2.0%
Below, it contains one or more types of Mo2.0% or less, the remainder consists of iron and unavoidable impurities, and P _CM
0.22% or less (However, P _CM = C + Si / 30 + Mn / 20 + Ni / 60 + Cu / 20 + Cr / 20 + Mo / 15 + V / 15) and OL
AC Ceq (%) 1/93 (34.6-0.36 V ) [However, OLAC Ceq (%) = C + Mn / 5 + Cu / 29 + Ni / 15.6 + Cr / 7.4 + Mo / 3.4 + Nb / 0.97 + V / 15, V is the cooling rate and 4 ~25℃/sec] After hot rolling, the temperature range from Ar ₃ points or more to 500 to 650℃ is cooled at the above cooling rate V.
This is a method for producing weather-resistant steel with weldability and a strength of 50 Kg/mm ² or more, which is characterized by accelerated cooling at a temperature of 50 kg/mm 2 or more, followed by cooling. The present invention will be explained in detail below. First, to explain the composition, in the present invention, the lower limit of C is set to 0.03% because if it is less than this, the target strength of 50 kg/mm ² or more cannot be obtained, and the upper limit is set to 0.20%. This is because if it exceeds this, weldability will be significantly impaired. Although Si is effective in increasing strength through solid solution strengthening, adding a large amount impairs weldability, so the upper limit was set at 0.6%. Mn is an element necessary to give strength to steel, but if it is less than 0.5%, its effect will be small, and if it exceeds 2.5%, weldability will be extremely poor, so it was set in the range of 0.5 to 2.5%. For sol.Al, minimum 0.005% for deoxidizing steel
is required, so this was set as the lower limit, and since this effect becomes saturated if it exceeds 0.08%, 0.08% was set as the upper limit. Regarding Cu and Cr, these are effective elements for improving corrosion resistance and strength, but 0.1
Since the effect is small if it is less than 0.1%, the lower limit was set at 0.1%, and the upper limit was set at 2.0% from the viewpoint of weldability and economical efficiency. Regarding Nb, V, and Ti, this was set as the lower limit because no effect on strength and toughness would be observed unless at least 0.01% of any one of these was dissolved in solid solution. In addition, it is necessary to regulate the upper limit for any of these substances because they impair weldability when added in large amounts.
The upper limit for Ti was set at 0.15% for the same reason as for Nb. Ni,
Although Mo is an effective element for improving strength, the upper limit was set at 2.0% from the viewpoint of weldability and economical efficiency. Although it is well known that the strength of steel is greatly influenced by its composition, and that the degree of contribution of the composition to strength varies greatly depending on manufacturing conditions, the inventors have not disclosed anything known so far. Regarding the influence of the composition on the strength of an accelerated cooling material that _has no 10℃/up to a temperature range of ~650℃
The experiment was repeated with accelerated cooling at a cooling rate of sec, and the contribution of each alloy component to the strength of the steel sheet was determined by multiple regression analysis. Next, we will discuss the effect of each alloy component on the steel sheet strength when accelerated cooling (cooling at 10°C/sec from the Ar ₃ transformation point or higher to the temperature range of 500 to 650°C after hot rolling) in the above experiment was performed. The relational expression obtained as a result of determining the contribution by multiple regression analysis is shown below. TS (Kg/mm ² ) = 19 + 93・OLAC Ceq ...(1) In the above formula (1), OLAC is On-Line Accelerated
OLAC Ceq is an abbreviation for Cooling, and OLAC Ceq is an accelerated coolant carbon equivalent formula, which is expressed by the following formula. OLAC Ceq (%) = C + Mn / 5 + Cu / 29 + Ni / 15.6 + Cr / 7.4 + Mo / 3.4 + Nb / 0.97 + V / 1.5 ... (2) In addition, the cooling rate is an important factor for accelerated cooling materials, and the influence on strength is Shown in Table 1 to find out
An experiment was conducted in which the cooling rate was varied over a wide range using three different types of OLAC Ceq steel.

[Table] As shown in Figure 1, the results show that an increase in accelerated cooling rate increases TS independently of OLAC Ceq, and in the accelerated cooling rate range of 4 to 25℃/sec, the amount of TS increase per 1℃/sec was found to be approximately 0.36Kg/mm ² (ΔTS shown in Figure 1 is the TS of the accelerated coolant).
TS of hot rolling under the same rolling conditions). From the above results, the accelerated coolant TS is OLAC.
If you set Ceq and cooling rate ( V °C/sec),
It can be seen from the following equation that it can be determined uniquely. TS = 19 + 93・OLAC Ceq + 0.36 ( V -10) ... (3) From this formula (3), in order to manufacture high tensile strength steel plates with TS50Kg/mm2 or ^more by accelerated cooling, OLAC Ceq and V
It turns out that it is sufficient if it satisfies the following formula. 5019+93・OLAC Ceq+0.36 ( V −10) ……(4) That is, OLAC Ceq (34.6−0.36 V )/93
...(5) Furthermore, alloy composition is basically a major factor in weldability, and it is thought that there is no difference if the accelerated cooling material and the as-hot rolled material have the same composition. Therefore, the low temperature cracking susceptibility composition P _CM value may be used to regulate it in the same way as conventional as-rolled materials. Preheating temperature 50
It has been confirmed that the P _CM value for preventing cold cracking as represented by the Y-type cracking test at ℃ is 0.22 or less when using a low-hydrogen welding rod with a steel plate thickness of 25 mm. This value was used as a judgment value for pass/fail. P _CM 0.22 ...(6) However, P _CM = C + Si / 30 + Mn / 20 + Ni / 60 + Cu / 20 + Cr / 20
+Mo/15 +V/15 As understood from the above explanation, in the present invention, the composition of the steel material is simply controlled by the content of each alloying element (for example, C: 0.03 to 0.20%),
This does not mean that it is sufficient to be within the lower limit.
In other words, by substituting the amount of each alloying element contained in the above formula (2), if the obtained OLAC Ceq does not satisfy the relationship (5) with the cooling rate V adopted at that time, 50O
Since a strength of less than Kg/mm ² can be obtained, the composition must be set so as to satisfy at least formula (5). However, if the amount of each alloying element is too large, equation (6) will not be satisfied. Therefore, it is no exaggeration to say that the upper limit of the composition of a steel plate subjected to accelerated cooling after hot rolling is regulated by equation (6), and the lower limit is regulated by equation (5). Furthermore, as can be seen from the shapes of both equations, the upper limit is affected by the accelerated cooling rate, and the lower limit is affected by the accelerated cooling rate; if the rate is set high, equation (5) will be satisfied even if the content of each alloying element in the steel material is somewhat small. You may get it. Next, to explain the manufacturing conditions, the reason why the accelerated cooling start temperature was set at Ar ₃ points or higher is that even if cooling starts from a temperature lower than Ar ₃ points, ferrite has already begun to form, so the transformation structure improvement effect is sufficient. This is because there is no hope for improvement, and therefore no improvement can be expected.
In addition, the upper limit of the cooling stop temperature was set at 650°C because if the stopping temperature is higher than this, bainite will not be sufficiently generated, and the effect of improving the transformed structure cannot be expected, so strengthening cannot be expected, so the lower limit was set at 500°C. This is because if the temperature is lower than this, the distortion of the steel plate becomes large, which is not desirable in terms of the manufacturing process. In addition, the lower limit of the cooling rate was set at 4℃/sec because sufficient strengthening cannot be expected as shown in Figure 1 below this rate, and the upper limit was set at 25℃/sec because the strain in the steel plate is large. This is because. Example All test steels having the chemical compositions shown in Table 2 were melted in a 250-ton converter. Steel 2 to 5 are P _MC
Values range from 0.16 to 0.18, OLAC Ceq values range from 0.30 to
It is in the range of 0.35. On the other hand, Steel 1 has a P _CM value of 0.22 or less and an extremely low OLAC Ceq value.
Steels 6 to 9 all have high alloy components, so the P _CM value is
It exceeds 0.22. Table 3 shows the manufacturing conditions and the results of mechanical property and weldability tests.

【table】

[Table] As is clear from Tables 2 and 3, for steels with low OLAC Ceq such as Steel 1, the accelerated cooling rate after hot rolling is increased to nearly 25℃/sec, as well as for as-rolled steels. (34.6−0.36 V )/93 value is OLAC
Since it exceeds the Ceq value, it cannot be made stronger than 50Kg/mm ² . For Steel 2 and Steel 3, the strength does not exceed 50Kg/ ^mm2 at low cooling rates, but by appropriately selecting the cooling rate, the OLAC Ceq value becomes larger than the (34.6-0.36V)/93 value, so the strength can be increased. is 50
It will exceed Kg/mm ² . However, even if the cooling rate and composition are appropriate as in Steel 2-E, if the cooling stop temperature is as high as 680℃, the strength will drop below 50Kg/ ^mm2 .
Also, as shown in Steel 3-H, the cooling start temperature is 770℃ and Ar ₃
If it falls below this point, the tension strength will also be insufficient.
Also, as shown in A, the strength of Steel 9 does not reach 50Kg/mm ² when hot rolled, but as shown in B, the strength is approximately 9Kg/mm2.
By performing accelerated cooling after hot rolling at a cooling rate of °C/sec, it is possible to obtain the desired strength of 50 Kg/mm ² or more. However, the Y-shaped cracking test results at 50°C do not prevent cracking. Next, the desired weldability obtained by accelerating cooling the steel plates shown in Table 3 and steel A and steel B in Table 1 at approximately 17°C/sec and 13°C/sec, respectively, after hot rolling,
Weather resistance tests were conducted on strong steel materials in the following manner. (1) Test piece dimensions and test method Test piece dimensions: 5 x 100 x 150 (mm) Surface finish accuracy vvv Test method: Atmospheric exposure test (ii) Test location, installation method and test period Test location: Rural residential area Installation method : Mounted on a test stand facing south with an inclination angle of 30° via an insulator. Test period: After 1.5 years, it was removed and corrosion weight loss was investigated. (iii) Test results

[Table] As shown in the table above, steels in Table 3 1 to 9
It can be seen that the coexistence of Cu and Cr is effective in improving weather resistance. As explained above, according to the present invention, it is possible to reliably manufacture a weathering steel material that is weldable and has a strength of 50 kg/mm ² or more without repeating trial and error as in the past. This is an industrially significant invention. Furthermore, for this purpose, conventional weathering steel must contain a certain amount or more of alloying elements such as Cr, which requires a preheating temperature of 100°C or higher to prevent Y-shaped cracking. However, according to the present invention, it is possible to set the preheating temperature to 50°C, which greatly reduces restrictions on welding work, thereby further promoting its adoption in welded structures.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between accelerated cooling rate of steel and ΔTS.

Claims (1)

[Claims] 1 C0.03~0.20%, Si0.6% or less, Mn0.5~2.5%,
Cr0.1~2.0%, Cu0.1~2.0%, solAl0.005~0.08%
The remainder consists of iron and unavoidable impurities, and P _CM is 0.22% or less (however, P _CM = C + Si / 30 +
Mn/20+Ni/60+Cu/20+Cr/20+Mo/15+
V/15) and OLAC Ceq (%) 1/93 (34.6
-0.36V) [However, OLAC Ceq (%) = C + Mn/5
+Cu/29+Ni/15.6+Cr/7.4+Mo/3.4+Nb/
0.97+V/15, V is the cooling rate, 4~25℃/
After hot rolling, the steel that satisfies the conditions of
Weldability characterized by accelerated cooling in the temperature range from ₃ points or more to 500 to 650℃ at the above cooling rate V, and then left to cool, and weather resistance with a strength of 50Kg/mm ^{2 or} more manufacturing method of steel. 2 C0.03~0.20%, Si0.6% or less, Mn0.5~2.5%,
Cr0.1~2.0%, Cu0.1~2.0%, solAl0.005~0.08%
The basic composition is Nb0.01~0.1%, V0.01~
0.15%, Ti0.01~0.15%, Ni2.0% or less, Mo2.0%
Contains one or more of the following, with the remainder consisting of iron and unavoidable impurities, and P _CM 0.22% or less (However, P _CM = C + Si / 30 + Mn / 20 + Ni / 60 + Cu /
20+Cr/20+Mo/15+V/15) and OLAC
Ceq (%) 1/93 (34.6−0.36V) [However, OLAC
Ceq (%)=C+Mn/5+Cu/29+Ni/15.6+
Cr／7.4＋Mo／3.4＋Nb／0.97＋V／15, V is the cooling rate of 4 to 25℃/sec], and after hot rolling, the steel was heated to 500℃ from a temperature of ₃ points or higher.
A method for producing weather-resistant steel having weldability and a strength of 50 Kg/mm ² or more, characterized by performing accelerated cooling at the above-mentioned cooling rate V in a temperature range of up to 650° C., and then allowing it to cool.