JPS5941424A - Manufacture of high-tensile strength hot-rolled steel sheet having low yield ratio - Google Patents

Abstract

PURPOSE:To obtain a steel sheet having a yield ratio below 70% and a composite structure as being hot-rolled regardless of performing low-temp. coiling, by adding P as a low cost component, and specifying a cooling condition. CONSTITUTION:A hot-rolled steel sheet having a composition containing, by wt., 0.03-0.15% C, Si<=1.5%, 0.3-0.6% Mn, 0.03-0.20% P, Al<=0.1% and S<=0.010% is finish rolled at a temp. above its Ar3 transformation point. Thereafter, said steel sheet is immediately air-cooled over 3-20sec, rapidly cooled and then coiled at a temp. below 300 deg.C. Before the air-cooling, it may be cooled down to 700 deg.C at a cooling speed above 30 deg.C/sec. Hence, the low-yield ratio high-tensile strength steel sheet having an excellent shape and a stabilized material property is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-tensile hot-rolled steel sheet with a low yield ratio, and in particular to advantageously achieve a yield ratio of 70 or less in the as-hot-rolled state, and to further improve weather resistance. The purpose is to achieve this goal.

In the automobile industry, the main aim is to make the car body lighter and more compact, and the development of alternative materials is progressing. Among these, the use of high-tensile steel plates to make the car lighter and lighter has been put into practical use in some cases.

However, due to its high strength, it has poor spring ring strength and elongation characteristics, and cannot withstand complex and severe machining that requires shape accuracy after machining, making it a practical problem.

To solve these problems, high-strength steel sheets with low yield ratios have been developed, and their manufacturing methods include hot rolling and then reheating;
There are two methods, one is to obtain the material as hot rolled, and the former requires a heat treatment process, so the latter method is attracting particular attention.

For information on how to obtain a flattened structure with a low yield ratio while hot-rolling,
There is a method in which the hot-rolled coil is wound in a two-phase state of α and γ, and then the untransformed γ phase is transformed into martensite at a low temperature during the subsequent slow cooling. There are two main methods: producing Si, Mn, (tr, M
The other disadvantage is that it requires a large number of alloying elements such as o, and is very expensive to manufacture.The latter requires relatively few alloying elements, but it requires extremely low temperature winding below 200°C, so the shape is unstable. Not only is it easy to change, but it also has the drawbacks of a narrow manufacturing range and unstable, non-uniform materials.

In order to overcome these drawbacks, the inventors added P, which is inexpensive as a component, and applied the cooling method described below to extremely reduce the amount of P alloying elements. We have found a method to obtain a low yield ratio, high tensile strength steel plate that has a good shape and stable material properties despite the above-mentioned conditions.

This invention provides a hot rolled steel sheet having a composition of C: o, oa ~ 0.15% (hereinafter referred to as single stroke), S: 0.010% or less, immediately after finish rolling at a transformation point of 2,/k r 3 or higher. ,
Or, following this, in a world where the temperature reaches 700℃, after cooling at a cooling rate of 30℃ or more per second, rapid cooling through air cooling for 8 to 20 seconds, and winding at a temperature of 800℃ or less, the low temperature is added to the structure. The object of the present invention is to obtain a composite structure steel sheet having a transformed phase and a yield ratio of 70 inches or less.

The hot rolled steel sheet has a composition containing REM equivalent to 2 to 5 times the S content or Oai equivalent to 1 to 3 times the S content,
Ou: a composition further containing 0.10 to 1.0%,
Also, Ni: 0.1-1.0%, Or: 0.1~
Practically preferred compositions further contain at least 1.5% of silane.

This invention is mainly based on new findings regarding the α-transformation accelerating effect of P, and is derived from the results of research and development of composite structure steel sheets by cryogenic winding involving controlled rolling and controlled cooling.

In this invention, a detailed explanation will be given in order for the reason for limiting the compositional components of hot rolled steel.

C: o, oa ~ 0.15% If C content is less than 0.03%, sufficient tensile strength cannot be obtained, and if it exceeds 0.15%, weldability and workability deteriorate, and alpha transformation occurs. This has the disadvantage that the second phase increases due to the suppression of bainite, and the shape and material become unstable and non-uniform due to the formation of bainite. Therefore, the sail is limited to a range of 03 to 0.15 inches.

Si: 1.5% or less The more 5iEtc there is, the more the α transformation is promoted and high ductility can be expected, but if it is too much, the toughness and weldability deteriorate. did.

Mn: 0.3 to 0.6% If the Mn pupil is less than 0.3%, a low-temperature transformed phase will not be obtained, so a higher Mn pupil of 0.3% or more is preferable. However, from an economic standpoint, it is preferable to have a small amount of Mn1i, and the upper limit was set to 0.6% or less. This is one of the major features of the invention.

p: o, oa ~ 0.20% In P mice, α metamorphosis is promoted at 0.03% or more, and C into γ
It was found that it was significantly concentrated, had an excellent balance of strength and ductility like $1, and had a low yield ratio. But 0.2
%, the toughness deteriorates significantly. Therefore 0.08
% to 0.2%.

At: 0.1% or less The amount of At is used as a deoxidizing element, and using more than 0.1% results in an increase in inclusions and is not economical. From this point of view, the content was set at 0.1% or less.

S: 0.010% or less S is preferably low from the viewpoint of stretch flange workability.
0.010% or less is required.

In addition to the above, REM and Oa make MnS spheroidal and improve processability, so they can be added as necessary.At this time, REM/S.

When Ca/'S is less than 2 and 1, respectively, the Ama9 effect is strong, and when it is more than 5 and 3, large inclusions are likely to be formed, and the amount of inclusions increases, resulting in deterioration of workability. It is preferable to set it as the range of 2-5.1-3.

Next, CU improves corrosion resistance, and the effect becomes even more remarkable when added in combination with P, but the effect is advantageous at 0.10% or more, but on the other hand, when it exceeds 1.0%, weldability deteriorates. is large, therefore it is desirable to set it to 0.10% to 1.0%.

Ni stabilizes γ and improves toughness, so its effect is effectively exhibited at a sail content of 1% or more. However, if it exceeds 1.0%, the α transformation is suppressed, resulting in an increase in the second phase, the yield ratio becomes low, and it becomes uneconomical. Therefore 0.1
% to 1.0%.

Like N1, Or also contributes to stabilizing γ, and its effect is exhibited at 0.10% or more, but if it exceeds 1.5%, it leads to deterioration of weldability and is not economical.

As described above, the greatest feature of the invention as a steel component is that P is added to obtain a high tensile strength steel plate with a low Hn range and a low yield ratio.

Next, hot rolling conditions will be explained.

There is no particular limit to the slab heating temperature, and the final finishing temperature is A.
r A low temperature of 1200° C. or lower is preferable in order to prevent the coarsening of γ grains, as long as the temperature does not drop below the B point. However, if the finishing temperature is below Ar 3, the processability will deteriorate due to the expanded α and the remaining rolling strain, the anisotropy will increase, and the yield ratio will also increase. It is necessary to maintain the finishing temperature at or above the A r B point.

Merely satisfying the above requirements will not satisfy the aim of the present invention unless the cooling method after rolling is taken into account. That is, in this cooling method, air cooling is performed for 8 seconds or more before starting rapid cooling after finish rolling so as to promote α transformation and C concentration in austenite. If this air cooling is carried out for less than 8 seconds, the α transformation will be insufficient and the second phase will become a bainite-based structure, making it impossible to obtain a yield ratio of 70% or less.

The air cooling time is 3 seconds or more, and the longer the better, but if it becomes excessive, pearlite will be generated and the material will deteriorate, so the air cooling time was set to 20 seconds or less. η The first half of the air cooling process is performed at a cooling rate of 30°C or more per second to 700°C, and then the air cooling is continued for 3 to 20°C.
It is particularly preferable to apply it for a few seconds because it quickly enters the α-transformation region and further promotes α-transformation. However, if the rapid cooling continues to a temperature lower than 700℃, α will become finely acicular, the amount of bainite will increase, and the yield ratio will increase.
It is preferable to only apply rapid cooling during the process.

After preferably carrying out α-transformation promoting cooling as described above,
Immediately, the untransformed γ is quenched at a temperature of 30° C./sea or higher and wound up at a temperature of 300° C. or lower to transform the untransformed γ at a low temperature. 80℃/
If the temperature is less than 880, a low-temperature transformed phase cannot be obtained, and even if the winding temperature exceeds 300° C., a low-temperature transformed phase cannot be obtained.

and the influence qI of the air cooling time t□, and it is shown that the desired characteristic values of yield ratio of 70% or less can be obtained when T□ is 700°C or more and tl is asec or more.

Figure 8 shows a comparison of the influence of the winding temperature on the yield ratio for different levels of P addition. When the P content is outside the invention range, the yield ratio is low even at a winding temperature of 800°C or less. This shows that even if the amount of P added is within the invention range, a low yield ratio can be obtained when the winding temperature exceeds 300°C.

Examples of this invention will be shown below.

Example After melting in a converter, the components were adjusted as shown in Table 1.
After forming the ingot into a 5 ton mold, it is rolled to 220ra by blooming.
A slab with a thickness of m and a width of 925 mm was cut. Each slab'i
After heating to 1200° C., it was finished to a thickness of 2.6 mm using a continuous hot rolling mill consisting of a 5-stand rough rolling mill and a 7-stand finishing mill under the hot rolling conditions shown in Table 1, and then wound into a coil. According to FIG. 1, the cooling rate in the quenching section QC is below 30-b°C/SeC. A JI No. 85 tensile test piece was taken from this coil in a direction perpendicular to the rolling direction, and a tensile test was conducted, and the results shown in Table 1 were obtained.

Mums 1 to 7 satisfy each requirement of the present invention and exhibit a low yield ratio of 70% or less. However, since the steels No. 8 to No. 14 do not satisfy all of the requirements of this invention, their yield ratios are as high as 70% or more.

The steel plate obtained by this invention has the same -TS compared to the comparative steel plate.
It can be seen that it shows a high elongation value even at a high level and has excellent ductility.

Comparative steel sheets Ya8 to AIO have low O, P, and Mn, and A11 to A14 do not satisfy the rolling conditions. Among these, A 11 is ArB at 800℃ and 76
FT at 0°C shows a high yield ratio due to the residual cold worked structure. A12 has a structure mainly composed of bainite due to excessive cooling in the first half, and has a high yield ratio and low elongation. Flat No. 18 was rapidly cooled from the first half without taking any air cooling time, so it has a structure mainly composed of bainite, which has a high yield ratio and low elongation. A14 has a high winding temperature, so some pearlite is observed and its strength is low.
The yield ratio is also high.

As is clear from the above examples, by adding P, 0.6
Even with low Mn steel of 0% or less, a composite structure steel sheet with a low yield ratio can be easily obtained by selecting rolling conditions and cooling conditions.

[Brief explanation of the drawings] Figure 1 is a graph showing the effect of cooling after rolling (turn diagram), Figure 2 is a graph showing the effect of air cooling start temperature and air cooling time on the yield ratio, Figure 3 is a graph showing the influence of the amount of P added and It is a graph showing the influence of winding temperature on yield ratio. Patent applicant: Kawasaki Steel Corporation (%) 1i, '1PJtd (%) 1ma, ηto

Claims (1)

[Claims] Finishing of a hot rolled steel sheet having a composition of LO: 0.03 to 0015% by weight, SL: 1.5, S: 0.010% by weight or less, at an A r 3 transformation point or higher Immediately after rolling, or subsequently cooling at a cooling rate of 30°C or more per second while reaching 700°C, and then rapidly cooling through air cooling for 3 to 20 seconds? A low yield ratio, high tensile strength hot rolled steel sheet characterized by obtaining a composite structure steel plate having a yield ratio of 70% or less and having all the low temperature deformation phases in the assemblage by winding at 1JA degrees of 300°C or less. Method of manufacturing steel plates. 2. The method according to claim 1, wherein a hot rolled steel sheet having a composition containing REM corresponding to 2 to 5 times the S content or Ga corresponding to 1 to 8 times the S content of the hot rolled steel sheet is used. & The method according to claim 1 or 2, in which a hot rolled steel sheet having a composition further containing (Eu: 0.10 to 1.0% by weight). Arc As a hot rolled steel sheet, Ni: O, ], O to 1.0 heavy weight, Qr: 0.10 to 1.5 weight is any one of claims 1 to 8 using a composition further containing at least 1'!M of Q. The method described in.