JPS591632A - Manufacture of hot-rolled high-tension steel sheet with superior workability - Google Patents

Abstract

PURPOSE:To obtain a steel sheet having >=70kg/mm.<2> high tensile strength, superior workability, superior toughness at low temp. and superior bending crack resistance especially during edging by shearing by subjecting a Ti added steel having a specified composition to controlled rolling and by coiling the rolled sheet at a specified low temp. CONSTITUTION:A killed steel consisting of, by weight, 0.05-0.20% C, 0.04- 0.20% Ti, <=1.2% Si, 0.5-2.0% Mn, <=0.025% P, <=0.015% S, 0.005-0.15% sol. Al, <=0.008% N and the balance Fe or further contg. one or more among <=0.0100% Ca, <=0.0030% B and <=1.0% Cr is hot rolled at >=30% total draft in the temp. range of 900-800 deg.C. After finishing the rolling at >=800 deg.C, the rolled sheet is rapidly cooled at >=5 deg.C/sec cooling rate and coiled at 500-200 deg.C. By carrying out the coiling at said low temp., the precipitation of TiC is properly inhibited, and transformation hardening is mainly caused in place of precipitation hardening, so the purpose can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a T11A hot-rolled high-strength steel sheet that has a high tensile strength of near 0 g/- or more and has excellent workability and low-temperature toughness.

In recent years, there has been an increasing demand for steel materials with high strength and excellent workability as structural materials for various buildings and industrial machinery.
In order to cope with these problems, various steel materials have been developed and come into use. Examples include Nb-added steel, V-added steel, and T1-added steel. Among these, Ti is used because of its low manufacturing cost and high strength.
Additive steel is attracting attention, but it has a problem in that its toughness is inferior to Nb-added steel and V-added steel.

However, recently, due to the deterioration of the energy situation, resource development has become unavoidable under extremely harsh environments. If plastic deformation is applied and used in a cold region, there is a risk of brittle fracture occurring from the plastically deformed part, so from this point of view, in addition to the characteristics of high strength and easy workability, it is sufficient for use in cold regions. There was a strong demand for high-strength steel sheets that also had excellent low-temperature toughness and could withstand high temperatures.

Therefore, in order to provide a high-strength steel plate that satisfies these demands, a method of hot controlled rolling of Ti-added steel was proposed, as described in Japanese Patent Publication No. 55-4.5614. .

The feature of T1-added hot-rolled high-strength steel sheet is that TiC
At the same time, by utilizing the precipitation strengthening of
n S is replaced with TiS to form a C-based inclusion, thereby improving workability, and JIS uses a test material that has high strength and has a reduced machining allowance on the end face. In standard bending tests, it has been shown to have excellent cold workability, allowing even close bending. And, the method described in Japanese Patent Publication No. 55-45614 discloses that T1 having such characteristics is
By performing controlled rolling during the production of additive hot-rolled high-strength steel sheets, the low-temperature toughness of the steel sheets is further improved.

By the way, in the bending test according to the JIS standard, a material with a mechanically cut end face is used as a specimen, such as a double series, but in actual production of structural members, a material with a single end face is used. In most cases, materials (materials with unprocessed shear cut surfaces) are subjected to cold working as they are, and therefore, from a practical standpoint, good bending performance in specimens of shear end faces is determined by cold working. This will be required for industrial steel sheets.

However, T1-added hot-rolled steel sheets, including the steel sheets obtained by the method described in Japanese Patent Publication No. 55-45614, generally have poor bending test performance with one end facing. At the subsequent stage of practical application, it became clear that there was a serious problem in that the end face cracked during processing.

The present inventor has determined that the tensile strength is 70
We conducted extensive research to obtain a high-strength steel plate that not only has a high strength of 7" kg/rrn or more, excellent workability and low-temperature toughness, but also has good workability, especially for materials with one end face. As a result, if T1-added steel with a specific composition is controlled rolled and rolled at a low temperature of approximately 400°C, which is contrary to conventional wisdom, it exhibits a high tensile strength of 70 kg/mA or more, and has a high tensile strength of 70 kg/mA. It has been found that bending cracking of a steel plate with one end surface is improved, and a high-strength steel plate with excellent low-temperature toughness can be obtained.

Based on this, we continued further research, and the following (a)
~(We came to the knowledge shown in C1.

In other words, (a) When producing a T1-added hot rolled steel sheet with high strength, if it is coiled at a normal coiling temperature of about 600°C after hot rolling, it will certainly not adhere tightly within the range of JIS standard bending. Although it is possible to bend and has excellent workability, when bending a material with a single end surface, bending cracks due to the brittleness of the ferrite grain boundaries can be avoided. In addition, the precipitation of TiC also deteriorates the low temperature toughness. However, when the winding temperature was set to 5
When the temperature is controlled within the range of 00 to 200°C, the Ti
Precipitation of C is suppressed, transformation strengthening is achieved, embrittlement of ferrite grain boundaries is suppressed, and not only the bending performance of the single end face is significantly improved, but also the Charpy fracture transition temperature is improved. In particular, a total of 30
(1)) By combining this with controlled rolling in which rolling is continued at a temperature of 800°C or higher and the rolling is completed at a temperature of 800°C or higher, a high-strength steel plate with an extremely improved Charpy fracture transition temperature can be obtained. In addition, if the P content in the T1-added steel is reduced, each of the properties described in section (a) will be further improved. If a specific amount of the species or two or more species is contained,
Improved workability and toughness can be achieved.

In order to obtain these findings, the inventors of the present invention
Needless to say, a hot rolling simulation experiment method was established to investigate the effect of coiling temperature on the mechanical properties of i-added steel, and various experiments were repeated.

This hot rolling simulation experimental method is a method that involves
This is a method in which the rolled material is rapidly cooled to a predetermined temperature by water spray, and then the rolled material is heated to the predetermined temperature and then placed in a furnace for furnace cooling (cooling rate: 20°C/hr). be. It was also confirmed that if the "predetermined temperature" at this time was made to match the coiling temperature, a steel plate having the same structure and characteristics as in actual hot rolling and coiling operations could be obtained.

By such hot rolling simulation simulation method, 820
Figure 1 shows the results of an investigation into the effect of coiling temperature on the mechanical properties of T1-added steel under controlled rolling with ℃ finish.

Figure 1 shows 0.10%C-0,30%5i-1,65%
Mn-0,002%S-0,17%Ti-0,025%
Ae -0,0035%N steel (hereinafter, the ratio indicating the composition ratio is the highest) was subjected to a reduction of 50 at a temperature of 900°C or less, and a 6 mm thick hot rolled steel plate was produced at a finishing temperature of 820°C. After obtaining, 10 °C/see, which corresponds to the winding cooling rate.
FIG. 2 is a diagram showing the influence of coiling temperature on the mechanical properties of a steel sheet when cooled at a cooling rate of . From Figure 1, as the coiling temperature exceeds 400°C, the bendability of the plate with one end face and the Charpy fracture transition flow rate become noticeable, and especially when the coiling temperature exceeds 500°C, it becomes difficult to use it for practical purposes. Although the deterioration tendency becomes extreme to the extent that it is not a good thing,
It was found that workability and low-temperature toughness were extremely good when the coiling temperature was in the range of 500 to 200°C, and when the coiling temperature was further lowered to less than 200°C, there was a tendency for these properties to deteriorate again. It is clear that this will happen. It can also be seen that the P content in the steel also affects each of the properties listed in 1- above, and that good results can be obtained if the content is 0025 or less.

On the other hand, Fig. 2 shows a conventional 600°C rolled material (Fig. 2a) of a similar T1-added steel hot-rolled material, and a lower temperature 4
This shows the optical microscopic structure of the material rolled at 00°C (Fig. 2b). Comparing the two, it was found that when the material rolled at 600°C was subjected to nital corrosion, uneven ferrite intergranular corrosion occurred. Recognize.

From these experimental results, the following inferences were made. That is, (1) If T1-added steel is coiled at a normal coiling temperature of about 600° C., TiC will significantly precipitate in the ferrite ground during slow cooling after coiling, resulting in embrittlement.

In addition, uneven ferrite grain boundary corrosion due to nital corrosion,
This seems to be an expression of a grain boundary purification effect in which carbon present at grain boundaries decreases as TiC precipitates within ferrite grains. In steel materials that are prone to such uneven corrosion,
It is known that ferrite grain boundaries are weak, so due to both the embrittlement of the steel material and the embrittlement of the grain boundaries as described above, cracks already occur on the end face of the steel sheet at the time of shearing, and this cracks during subsequent bending. It has been determined that the book is subject to major cracks due to processing.

However, if winding is carried out at a low level, TiC precipitation is appropriately suppressed, and transformation strengthening becomes the main activity instead of precipitation strengthening, so it is possible to avoid the disadvantage described in 1. It is something.

■ In addition, when the material is rolled at a temperature lower than 200°C, the self-annealing effect due to slow cooling after winding is small, so the bendability and
It seems that both the Charpy properties and the Charpy properties were poor.

■ The reason why the bendability and Charpy properties of steel sheets with single end surfaces are improved by reducing the P content as much as possible is because the embrittlement of ferrite grain boundaries due to tempering embrittlement promoted by the presence of P. This is thought to be because it was suppressed by a decrease in P.

Therefore, the present invention has been made based on the findings described in section 1 with the aim of improving the workability and low-temperature toughness of a Ti-added hot-rolled steel material with a single end face, C: 0.05. ~020%, Ti:0.04~0.20%
.

Si: ], 22% or less Mn: 0.5 to 2.0%.

P: 0.025% or less, S: 0.015% or less.

sal, AQ: 0.005-0.15%.

N: 0.008% or less.

or in addition, Ca: 0.0100% or less.

r-1: 0.0030% or less.

Cr: ], 0% or less.

A killed steel containing one or more of the following, Fe and unavoidable impurities: the remainder is subjected to hot rolling such that the total rolling reduction in a temperature range of 90° to 800° C. is 30% or more,
To obtain a Ti-added strong hot-rolled high-strength steel sheet with excellent cold workability by completing rolling at 80° C. or higher and then winding at 500 to 200° C. while performing rapid cooling at 5° C./see or higher. It has the following characteristics.

Next, in the method for manufacturing a hot-rolled high-strength steel sheet of the present invention,
The reason why the composition range of the steel and the hot rolling winding conditions are limited as described in -1- will be explained.

■ C The C component has the effect of ensuring the strength of steel, and is an indispensable component to achieve tensile strength of near 0 kg, 4 mm or more, but its content is 0.05%.
If the content is less than 0.20%, the desired effect cannot be obtained; on the other hand, if the content exceeds 0.20%, a high carbon content bainite structure will be produced in low-temperature winding as employed in the present invention. The content should be reduced to 0.05 to 0.20% since it deteriorates bendability and low-temperature toughness.
It was determined that

■ Tl The Ti component not only strengthens steel through the precipitation of TiC, but also improves C bending performance by changing A-based inclusions such as MnS to C-based inclusions such as TiS. If the content is less than 0.04%, not only will the desired strength not be imparted to the steel material, but the control of the shape of inclusions will also be insufficient.
On the other hand, if the content exceeds 0.20 qb, the carbon-containing steel of the present invention (C: 0.05-0.20
%), significant precipitation hardening will adversely affect low temperature toughness, so the content was reduced to 0.04%.
It was set at ~0.20%.

■S] The Si component has a solid solution strengthening effect and a deoxidizing effect. Although it is preferable to contain about 0.05% or more in order to increase the strength, if the content exceeds 2%, toughness and weldability will deteriorate, so the content should be reduced to 1. It was set at 2'% or less.

■ Mn The Mn component has the effect of toughening steel and is an important component, but if its content is less than 0.5%, the desired effect cannot be obtained, and on the other hand, if the content is less than 0.5%, the desired effect cannot be obtained. If the content exceeds 0.5%, the A-based inclusions become brittle and the C bending performance deteriorates, so the content was set at 0.5 to 2.0.

■ P The P component segregates at the ferrite grain boundaries during slow cooling after winding and tends to cause grain boundary embrittlement. Therefore, it is better to minimize the bending performance of the material with one end surface, so it is better to reduce the bending performance as much as possible, but from an economic point of view,
The content was determined to be 0.025% or less. However, 0.010% or less is preferable.

■ S8 is an impurity element that tends to form A-based inclusions in steel, and even if it is Ti-added steel, its content is 0.
If the content exceeds 0.015%, it will combine with Mn to form A-based inclusions and deteriorate bending performance.
It has been set at 15% or less.

■ sot, Ae sot, Ae acid component has the effect of ensuring the effectiveness of added T1, but its content pso, o O5%
If it is less than 0.0, the effect of T1 addition will not be sufficiently exhibited;
If the content exceeds 15%, the amount of nonmetallic inclusions will increase and the steel will become brittle.
It was set as OO5-0.15%.

■ N The N component generates Ti and N in steel and is effective for precipitation hardening.
It is an impurity element that is better to be as small as possible because it reduces the amount of asTj, C or Tj, asTj, S that is effective for spheroidizing nonmetallic inclusions.
The upper limit of its content was set at 0.008% as an allowable range considering economic efficiency. However, 0
, OO50% is preferred.

■ Ca The Ca component combines with the B-based inclusions of the AQ 203 system,
This has the effect of improving workability as C-based inclusions.

In other words, since Ti can reduce A-based inclusions and Ca can also reduce B-based inclusions, adding Ca to Ti-added steel is very desirable for controlling the shape of inclusions, and particularly improves workability. When it is necessary to further improve O, OOO, it is desirable to contain 8% or more of O, OOO. However, if the content exceeds 0.0100%, the number of inclusions will increase beyond the allowable range, so the content is set at 0.0100% or less.

B The B component has the effect of improving the hardenability of steel and imparting toughness, especially under the condition that Ti-added steel is rolled up at a low flow rate, as in the high-strength steel sheet production method of this invention. So, B
The effects of the addition of small amounts of on the hardenability of steel are extremely large. Therefore, when higher toughness is required, it is desirable to contain O, OOO in an amount of 1% or more.

However, even if the content exceeds 30% of O, OO, no further improvement effect can be obtained, so the content is reduced to O, OO.
OO is set at 30% or less.

■ Cr The Cr component, like Mn, has the effect of toughening steel.
When it is necessary to further improve the toughness of steel, it is desirable to add 0.1% or more, but 1.0%
Even if the content exceeds 1.0%, no further anti-inflammatory effect can be obtained, so the content was set at 1.0% or less.

■ Hot-rolling/coiling conditions (1) Hot-rolling conditions In T1-added steel, precipitation hardening of T'iC and coarse TiN
As the low-temperature toughness deteriorates due to the presence of
It is necessary to carry out controlled rolling in which rolling is completed at 800°C or higher. In this case, if the rolling end temperature is higher than 900° C. or the rolling reduction is less than 30%, the desired sufficient fine grain structure cannot be obtained, and it becomes difficult to secure the low-temperature toughness required as a structural material. On the other hand, if rolling is finished at a temperature lower than 800°C, the texture will not only develop and cause anisotropy, but also the C-bending performance will deteriorate, so the total The reduction rate is 30%
The condition was that hot rolling be carried out such that the following conditions were met and the final temperature was 800°C or higher.

(11) Cooling rate control after hot rolling until coiling If the cooling rate is less than 5°C Aec after hot rolling and then coiling, the transformation strengthening effect hardly occurs and it is difficult to obtain the desired high strength. Therefore, it was decided to perform rapid cooling for 5 hours or more.

(iii) Winding temperature As mentioned above, if the winding temperature exceeds 500°C,
The bendability of the material with one end surface and the transition temperature of Charpy fracture surface are significantly deteriorated. The temperature was set at 500-200°C.

Next, the present invention will be explained by examples and in comparison with comparative examples.

Example First, a steel A having a chemical composition as shown in Table 1 and satisfying the chemical composition range of steel used in the method of the present invention is used.
−H and comparative steels ■ to S whose chemical composition ranges deviate from those of the steel subject to the present invention (components outside the composition range are marked with *) by normal melting. It was created using the casting method.

Next, these steel materials were hot-rolled under the conditions shown in Table 2 and wound up to produce hot-rolled steel plates having a thickness of 6 mm. Note that all * marks in Table 2 indicate conditions outside the range defined in the present invention.

Furthermore, the mechanical properties of the hot rolled steel sheet thus obtained were measured, and the obtained results are also listed in Table 2.

From the results shown in Table 2, the hot rolled steel sheets obtained by the methods of test numbers 1 to 8 in which the chemical composition range of the steel used and the hot rolling/coiling conditions are within the range specified by the present invention. It is clear that all of them have high strength, excellent low-temperature toughness, and shear end face orientation performance.On the other hand, the chemical composition of the steel material and the conditions of hot rolling and winding are Test number 9~ which is outside the range defined by the invention
It can be seen that the hot-rolled steel sheet obtained by method No. 22 is inferior in low-temperature toughness and shear end surface orientation performance.

In particular, when the comparative example of Test No. 9 (Apricot) is simply subjected to low-temperature coiling without controlled rolling, the structure of the resulting steel sheet is as shown in Figure 2 (bl), with a small amount of fine-grained ferrite. It was also found that the structure did not become a mixed structure of a fine bainite structure, but instead became a coarse bainite structure, resulting in a significant deterioration of toughness.The experimental results when the coiling temperature was outside the range of the present invention were as follows. As shown in FIG. 1 above, it goes without saying that the results were not good.

Furthermore, it was confirmed that the D steel, F steel, () steel, and H steel in Table 1 to which Ca was added had extremely poor bending performance.

In this way, by applying a combination of predetermined controlled rolling and low-temperature winding to T]-added steel with a specific chemical composition,
A mixed structure of a small amount of fine-grained ferrite and a fine bainite structure (the bainite structure is divided by fine-grained ferrite) is obtained, which ensures toughness and exhibits excellent workability properties. It is.

As described above, according to the present invention, it is possible to achieve high tensile strength of 70 kg/- or more without any special post-treatment, and to cool well without cracking even if the material has a single end face. Hot-rolled high-strength steel sheets that have excellent workability and extremely low-temperature toughness can be obtained by relatively simple means, making them ideal for buildings used in cold regions and other areas. By applying it to the structural materials of industrial machines, etc., we can expect to achieve better results than ever before, and it will bring useful effects to the industry.

[Brief explanation of drawings]

Figure 1 is a diagram showing the effect of coiling temperature on the mechanical properties of Ti-added steel, and Figure 2 is a graph showing the effects of nital corrosion on a steel plate rolled at 600°C after controlled rolling of T1-added steel. Microscopic structure diagram, Figure 2 (b) is an optical microscopic structure diagram due to nital corrosion of a similar steel plate rolled at 400°C.
j (0C n second da θot hiragitori

Claims (2)

[Claims] (1) C: 0.05 to 0.20 chi. 'I'l: 0.04-0.20%. Sl: 1.2% or less, Mn: 0.5-2.0%. P: 0.025% or less, s: o, 0.015% or less. snt, Al! :0.005-0.15%. N: 0.008% or less. Fe and unavoidable impurities: The remaining (weight%)
External hot rolling is performed so that the total rolling reduction force in the temperature range of 30% or more is achieved, and after finishing the rolling at 800°C or lower, quenching is performed at 5°C or higher. ~200
A T with excellent cold workability that can be rolled up at ℃.
A method for manufacturing a hot-rolled high-strength steel sheet with added toughness. (2) C: 0.05-〇, Section 20. Ti: 0.04-0.20. Si: 1.2% or less, Mn: 0.5 to 2.0%. P: 0.025% or less, S: 0.015% or less. soL, M: 0.005-0.15%. N: 0.008% or less. Contains 0.0100% or less of Ca2. B: 0.0030% or less. Cr: 1.0 or less. Contains one or more of the following: Fe and unavoidable impurities: the remainder. (900 to 800℃) to guild steel made of
After hot rolling with a total rolling reduction of 30% or more in the temperature range of 800℃ or higher, 55℃
1. A method for producing a Ti-added tough hot-rolled high-strength steel sheet with excellent cold workability, the method comprising performing rapid cooling at 500° C./sec or more and then winding at 500 to 200° C.