KR960005234B1 - Making method of hot rolling high strength steel sheet - Google Patents

Abstract

The aluminum killed steel slab having very low nitrogen, sulfur and carbon comprises, in weight percent, 0.002 to 0.04% carbon, 0.1 to 0.7% manganese, 0.03 to 0.08% phosphorus, 0.02 to 0.06% soluble aluminum, 0.0015 to 0.0025% nitrogen, sulfur not exceeding 0.009%, and 0.005 to 0.022% titanium or 0.014 to 0.025% niobium, Ti/C<1.0 (Ti = total Ti(48/12)N-(48/32)S) or Nb/C<0.8 (Nb = total Nb), and the balance of iron. The steel sheet is produced by the processes of homogenizing of the steel slab having the same chemical composition as mentioned above at over 1200deg.C, final hot rolling of the homogenized slab at 900 to 950deg.C, and then hot reeling of the hot rolled sheet at 700 to 750deg.C.

Description

Manufacturing method of hot rolled high tensile strength steel with excellent workability and hardening hardening

The present invention relates to a method for manufacturing a hot rolled steel sheet used in automobiles and the like, and more particularly, to a method for manufacturing a hot rolled high tensile strength steel sheet having excellent workability and bake hardenability.

Conventional hot rolled steel sheet has been used for applications requiring deep workability and elongated formability, and for this purpose, a steel sheet having excellent formability in the same strength class is required. Recently, the use of hot-rolled steel sheet as a material to replace the structural member of the vehicle, that is, the cold-rolled steel sheet is used to reduce the manufacturing cost is increasing day by day.

In general, steels exhibit characteristics in which strength and workability are mutually opposite. As steels that can satisfy both of these characteristics, there are a composite steel sheet and a hardened steel sheet. In general, composite tissue steel that can be easily manufactured has a tensile strength of 40kgf / ㎣ or more, which is a material used in automobiles, and is poor in formability compared to high tensile strength, and is manufactured due to excessive addition of alloying elements such as manganese and chromium. Results in higher costs. The hardened hardened steel has a yield strength close to a soft steel sheet during press molding in steels with a tensile strength of 40 kgf / ㎣ or less.It has excellent ductility. It is drawing attention as an ideal steel compared with the conventional high strength steel plate which worsens.

Baking hardening is based on the strain aging that occurs when carbon or nitrogen, an invasive element solid-solution dissolved in steel, adheres to the potential generated during the deformation process. Since aging causes deterioration of moldability, proper control of solid elements is very important. Recently, due to the development of steelmaking technology, the use of Al-Killed high tensile steel sheet which can control the appropriate amount of solid solution in steel and adds strong carbonitride forming elements such as Ti or Nb is increasing.

Hot rolled hardened hardened steel has been proposed so far, but it has its own problems and there is no sufficient explanation about it. A conventional low carbon Al-Killed soft steel sheet was appropriately controlled at each low temperature, and simply wound at a low temperature, that is, a hot rolled steel sheet having a hardening hardening amount of about 5 kgf / ㎣ was obtained by setting the hot rolled winding temperature in the range of 400-500 ° C. However, such a steel sheet is inevitable ductility deterioration in order to increase the hardening hardening amount of about 5kgf / ㎣. In addition, it is difficult to obtain a hot rolled steel sheet having good machinability and bake hardenability due to the increase in yield strength of the hot rolled structure.

Japanese Patent Laid-Open No. 61-272347 proposes a method of suppressing Al content and lowering the hot rolled coiling temperature so that N remains in solid solution. There are a lot of oxidation inclusions in the deterioration of machinability due to deterioration in mechanical properties, and also in the case of a large amount of solid carbon, there is a problem that aging degradation.

The present inventors have conducted research and experiments to solve the above-mentioned conventional problems, and based on the results, the present invention has been proposed, and the present invention provides carbon, nitrogen, and sulfur in the alloy components of ultra-low carbon Al-Killed steel. By adding Ti or Nb, which is a strictly controlled and strong carbonitride forming element, is suitable for manufacturing hot rolled high tensile strength steel with a tensile strength of 35kgf / ㎣ or more with excellent hardenability of 4.5kgf / ㎣ or more.

Hereinafter, the present invention will be described in detail.

In the present invention, C: 0.002-0.004%, Mn: 0.1-0.7%, P: 0.01-0.08%, S: 0.009% or less, Solution Al: 0.002-0.06%, N: 0.0015-0.0025%, Ti: 0.005-0.022% or Nb: 0.014-0.025%, wherein Ti is a formula of Ti * / C <1.0 (Ti * = total Ti- (48/12) N- (48/32) S) 1200 ° C or more of ultra low nitrogen, extreme resistance and ultra low carbon aluminum-kilted steel added to satisfy the following condition, or added to satisfy the relationship that Nb is Nb * / C <0.8 (Nb * = total Nb). In the temperature range of 900-950 ℃ after the homogenization treatment in the present invention relates to a method for producing a hot rolled high tensile strength steel sheet excellent in workability and baking hardening by winding in a temperature range of 700-750 ℃ after hot rolling.

Hereinafter, the alloy component and the manufacturing method of the present invention will be described in detail.

The carbon (C) is a component having a solid solution hardening and a hardening hardening effect. When the carbon content is 0.002% or less, the tensile strength is insufficient, and the sufficient hardening hardening property is not obtained. The moldability and ductility are lowered because the room temperature aging resistance is not secured. The manganese (Mn) is an element that refines particles and amplifies strength without damaging ductility, but in order to prevent hot brittleness due to FeS generation, addition of 0.1% or more is required. If necessary, the addition amount is more than 0.7%, the strength is sharply increased by solid solution, but the ductility deterioration is remarkable, so the addition amount is preferably limited to 0.1-0.7%.

The phosphorus (P) is a substitution type alloy element having the largest solid hardening effect, and serves to improve in-plane anisotropy and increase strength, and when the content is 0.01% or less, an addition effect is not obtained, and more than 0.08%. When added, the material is removed by segregation at the ductility and grain boundaries, so that the amount of P added is preferably limited to 0.01-0.08%.

Since the sulfur (S) is precipitated as a Ti-based sulfide at a high temperature, the narrower the amount of sulfur added in order to control the amount of Ti in order to obtain an appropriate amount of hardening hardening in the steelmaking process, the more stable the hardening of hardening of the part is obtained and the amount of Ti It is desirable to limit it to 0.009% or less in order to lower.

The aluminum (Al) is added for the deoxidation of the steel, but when added to 0.02% or less, there are many oxidation inclusions in the steel and deterioration in workability, which is detrimental to mechanical properties, and when excessively added to 0.06% or more, the hardening and manufacturing cost of the material Since the increase of, the content of Al is preferably limited to 0.02-0.06%.

Since nitrogen (N) is precipitated as Ti-based nitride at high temperature, it is suggested to be 0.0015-0.0025%, which is easy to manufacture practically in the steelmaking process by narrowly managing the component range of nitrogen for controlling the Ti component to obtain an appropriate baking hardening amount. It is desirable to.

Since Ti forms Ti-based sulfides and nitrides at high temperatures, if Ti is too small, nitrogen and sulfur in the steel cannot be fixed, leading to a large amount of hardened portion, but deterioration in moldability and aging. However, if the amount of Ti is too high, it will not be possible to obtain calcined hardenability by precipitating all dissolved carbon as well as nitrogen and sulfur in the steel. Therefore, it is necessary to control the amount of Ti in a narrow range together with the amounts of nitrogen and sulfur. In order to obtain stable bake hardenability, the amount of TI needs to be limited to TI * / C <1.0 (Ti * = Total Ti- (48/12) N- (48/32) S). In the present invention, it is preferable to limit the content of Ti component that satisfies the above regulatory range to 0.005-0.022%.

Since Nb forms a strong Nb-based carbide at high temperature, it is added for refining crystal grains and reinforcing materials in a range that does not form NbC. For this purpose, it is necessary to add Nb so as to satisfy the relationship of Nb * / C <0.8 (Nb * = Total Nb). However, in case of Nb * / C> 0.8, solid carbon precipitates completely as NbC precipitates, which contributes to a stable bake hardening. In the present invention, it is preferable to limit the Nb component content that satisfies the above regulatory range to 0.014-0.025%. Ti and Nb may be added in combination.

After dissolving in the converter in the above composition is made of slab (cast) continuously cast and heated at 1200 ℃ or more to finish the hot rolling in the temperature range of 900-950 ℃.

If the slab cracking temperature is less than 1200 ℃ because the structure of the steel does not become uniform austenite crystal grains are mixed, resulting in degradation of the material.

If the hot finishing temperature is 900 ℃ or less, the top, tail, and edges of the hot rolled coil become a single ferrite phase region, which increases the in-plane anisotropy and deteriorates the workability. After processing, it is easy to bond with orange peel on the surface.

In order to prevent deterioration of formability due to solid solution carbon remaining in the hot rolled sheet after the hot rolling, a high temperature winding of 700-750 ° C. is preferable, because the abnormal grain growth may occur when the winding temperature exceeds 750 ° C. This is because a good material cannot be obtained, and if the temperature is 700 ° C. or lower, the yield strength and the workability deteriorate due to the fine grain of the hot rolled structure. Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

Steel prepared as shown in Table 1 was hot rolled according to the hot rolling conditions shown in Table 2, and then mechanical properties were measured, and the measurement results are shown in Table 3 below.

TABLE 1

X * = Ti, Ti * Total Ti- (48/14) N- (48/32) S

X * = Nb, Nb * = Total Nb

TABLE 2

TABLE 3

As shown in Table 3, carbon: 0.0025-0.0035%, phosphorus: 0.07%, sulfur: 0.005-0.007%, nitrogen: 0.0017-0.0020%, Ti: 0.016%, or Nb: 0.017% and the amount of Ti is in the range of Ti The amount of Ti is controlled to satisfy the formula of * / C <1.0 (Ti * = Total Ti- (48/12) N- (48/32) S), or the range of Nb amount is Nb * / C <0.8 (NB According to the invention, the invention steel 1-3 in which the amount of Nb was controlled to satisfy * = Total Nb), according to the present invention, the hot rolled invention material (1-3) had a bake hardening amount of about 4.5-5.1kgf / ㎣, tensile strength 35-40kgf / ㎣, an elongation of 35-38% is shown. According to the present invention, it can be seen that a high tensile strength hot rolled steel sheet excellent in workability and bake hardenability can be produced.

On the other hand, while other elements satisfy the scope of the present invention, Comparative Material 4 having a Ti content of 0.05% and larger than Ti * / C <1.0, which is a Ti regulatory component range, can be seen that no amount of hardening of the baking is obtained.

In addition, if the Ti content is between 0.005-0.022% and satisfies the relationship of Ti * / C <1.0, which is an appropriate Ti component regulation range, the amount of carbon in the steel is 0.0016-0.0018%, and the lower carbon content is less than 0.002%. And Comparative material 5-6 in which the sulfur content is out of the range of the present invention, it can be seen that the baking hardening amount is very low as 0.4kgf / ㎣.

In addition, comparative material 7 having a hot finishing temperature lower than 900 ° C., which is a limit of the hot rolling finish temperature of the present invention, has a high in-plane anisotropy in steel, and also has a tensile strength of carbon, manganese, and sulfur, which is outside the scope of the present invention. Along with the decrease, it can be seen that the baking hardening amount is not obtained at all.

S amount out of the scope of the present invention, Nb amount 0.019%, which is larger than the Nb regulatory component range Nb * / C <0.8 can be seen that the baking hardening amount is 1.0kgf / 매우 very low value.

Claims (2)

By weight, C: 0.002-0.004%, Mn: 0.1-0.7%, P: 0.003-0.08%, S: 0.009% or less, Soluble Al: 0.02-0.06%, N: 0.0015-0.0025%, and Ti: 0.005 Ultra-low nitrogen, ultra-low sulfur and extreme, containing -0.022% and added such that Ti satisfies the formula of Ti * / C <1.0 (Ti * = total Ti (48/12) N− (48/32) S) Processability and quenching, characterized in that the slab of low carbon aluminum-killed (Al-Killed) steel is subjected to siliceous treatment at 1200 ° C. or higher to finish hot rolling at a temperature range of 900-950 ° C., and then wound in a temperature range of 700-750 ° C. Method for producing hot rolled high tensile strength steel sheet having excellent curability. By weight, C: 0.002-0.004%, Mn: 0.1-0.7%, P: 0.003-0.08%, S: 0.009% or less, Soluble Al: 0.02-0.06%, N: 0.0015-0.0025%, and Nb: 0.014 Homogenized ultra-low nitrogen, ultra-low sulfur and ultra-low carbon aluminum-killed slabs containing -0.025% and added such that Nb satisfies the formula of Nb * / C <0.8 (Nb * = total Nb). Process to produce hot rolled high tensile strength steel sheet excellent in workability and baking hardening, characterized in that after the treatment hot-rolled at a temperature range of 900-950 ℃ and wound in a temperature range of 700-750 ℃.