KR101133870B1 - Hot-pressed steel sheet member and process for production thereof - Google Patents

Abstract

The hot-press-formed steel sheet member having good toughness as it is quenched and a tensile strength of 1.8 Gpa or more has a microstructure including martensite which is automatically tempered at a spherical austenite particle diameter of 10 µm or less. The chemical composition of the steel sheet is C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 0.51%, Ti in an amount satisfying 3.42 N + 0.001 ≤ Ti ≤ 3.42 N + 0.5, and Si: 0.5% or less , Ni: 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less, one or two or more, in some cases, B: 0.01% or less, Nb: 0.51% or less, Mo: 1.0 % Or less, Ca: 0.001 to 0.005% of 1 type or 2 or more types are contained. After holding this steel plate at the temperature of Ac ₃ or more and (Ac ₃ +100 degreeC) or less, for 5 minutes or less, hot press molding is carried out, and then the cooling rate to Ms point is 150 degreeC above the upper critical cooling rate and from Ms point. Quenching is carried out by cooling of 10-500 degreeC / sec.

Description

Hot press-formed steel sheet member and its manufacturing method {HOT-PRESSED STEEL SHEET MEMBER AND PROCESS FOR PRODUCTION THEREOF}

TECHNICAL FIELD The present invention relates to a high strength steel sheet member produced by hot press molding and having a tensile strength of 1.8 GPa or more and a method for producing the same. This steel sheet member is very suitable as a mechanical structural component including an automobile body structural component, an underbody component and the like. The present invention further relates to a steel sheet for hot press forming used in the production of this steel sheet member and a method of manufacturing the same.

In recent years, in order to improve fuel efficiency of automobiles, efforts have been made to increase the strength of steel materials used and to reduce the weight of automobiles. As a result, in the production of a member by press molding of a thin steel sheet widely used in automobiles, it is difficult to produce a member having a complicated shape due to a decrease in press formability accompanying an increase in steel sheet strength. Specifically, due to the ductility reduction of the steel sheet, there arises a problem that spring back at breakage occurs at a site with high workability, or wall warpage is increased, resulting in deterioration in dimensional accuracy. For this reason, the manufacture of the component by press molding using the high strength steel plate which has the tensile strength of 780 Mpa or more especially comes with difficulty.

If roll forming is used instead of press forming, processing can be performed easily even on a high strength steel sheet. However, since roll forming can only be applied to the manufacture of parts having the same cross section in the longitudinal direction, it cannot be used for the manufacture of members having complicated shapes.

As proposed in GB 1,490,535, in the method called hot press forming for press forming a heated steel sheet, since the hot steel sheet is soft and has high ductility, it is possible to mold complex shapes with high dimensional accuracy. In addition, by heating the steel sheet to the austenite zone and then press-molding, and quenching and quenching the molded article in the mold used for press molding, at the same time as forming the steel sheet, the strength of the steel sheet due to martensite transformation can be achieved. Can be.

In Japanese Patent Laid-Open No. 10-96031, a steel sheet material is pre-molded in a predetermined shape at room temperature in advance, and then the molded article is heated in an austenite zone and rapidly cooled while being placed in a mold used for molding, thereby increasing the strength of the steel sheet. A preshaping press quenching method is disclosed which simultaneously performs press molding.

<Start of invention>

The above-mentioned hot press forming method or preforming press quenching method can simultaneously achieve the press molding of a steel plate and the high strength of a press-formed product.

By the way, when the tensile strength of the molded article after quenching is at a high strength of 1.8 GPa or more, in the conventional hot press molding method (including preformed press quenching method), the toughness of the press-formed product after quenching is insufficient, and the practical level is not reached. It turned out. Indeed, an example of producing a high-strength press-formed product having a tensile strength of 1.8 GPa or more with good toughness while hot pressing is not known until now.

Therefore, in the conventional hot press molding, in order to produce a practical press-formed product having a tensile strength of 1.8 GPa or more, it is necessary to temper the quenched press-formed product and increase its toughness. However, adding a tempering process in hot press molding leads to a remarkable increase in cost in terms of work efficiency and equipment, which is undesirable.

The present invention provides a technique capable of realizing the production of a hot press-formed press-formed product having excellent toughness and a tensile strength of 1.8 GPa or more without performing tempering after quenching.

According to the present invention, the chemical composition of the steel sheet is appropriately selected, and the hot rolling conditions at the time of steel sheet production, and optionally, subsequent heat rolling conditions such as cold rolling, annealing, alloyed hot dip galvanizing, and further, quenching conditions after hot press molding are appropriately selected. By controlling, the above object can be achieved.

In one aspect, the present invention is, by mass%, C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 1.0%, and a quantity of Ti, Si: 0 to 0 to satisfy the following formula (1): 0.5%, Ni: 0 to 2%, Cu: 0 to 1%, V: 0 to 1%, Al: 0 to 1%, B: 0 to 0.01%, Mo: 0 to 1.0%, Ca: 0 to 0.005 %, And having a chemical composition consisting essentially of the balance Fe and impurities, and having a microstructure including spherical austenite average particle diameter of 10 µm or less and including automatically tempered martensite, and also tensile strength Is a hot press formed steel sheet member having a diameter of 1.8 GPa or more:

3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5... (One)

Ti and N in a formula mean content (mass%) of the said element in steel, and N is contained in steel as an impurity.

In the present invention, hot press forming is performed at a lower temperature than the austenite region in addition to the hot press forming method in the sense of negotiating press forming after heating the steel sheet to austenite region (Ac ₃ or more) in advance. For example, a preform press quenching method is also included in which a molded article is heated to an austenite temperature and quenched after press molding at room temperature).

The said chemical composition is mass%, Si: 0.01 to 0.5%, Ni: 0.01 to 2%, Cu; 0.01 to 1%, V: 0.01 to 1%, Al: 0.01 to 1%, B: 0.001 to 0.01% , Mo: 0.01 to 1.0%, and Ca: 0.001 to 0.005% may contain one or two or more selected.

In the chemical composition, one or two or more of P, S, and N contained as impurities in steel satisfy P: 0.005% or less, S: 0.005% or less, and N: 0.002% or less by mass%. It is preferable that it is content.

In another aspect, the present invention is a steel sheet for hot press forming, which can produce a hot press-formed steel sheet member having a tensile strength of 1.8 GPa or more.

The invention also, and the steel sheet having the chemical composition, Ac ₃ point or higher, after a holding time of 5 minutes or less in a temperature range of less than (Ac ₃ point + 100 ℃), subjected to hot press molding to a steel sheet, hot press The hot press-formed steel sheet member comprising cooling the formed hot molded article such that the cooling rate to the Ms point is higher than the upper critical cooling rate and the average cooling rate from the Ms point to 150 ° C is 10 to 500 ° C / sec. It also provides a method for producing.

In one aspect, the hot press-formed steel sheet member may be produced by the above-mentioned preforming press quenching method. Manufacturing method of the steel sheet member according to this embodiment is a steel sheet having the chemical composition and using a die at a temperature lower than the Ac ₃ point press forming, while into the press-forming the steel sheet in the mold, Ac ₃ point or higher, (Ac ₃ point 5 minutes or less in the temperature range below +100 degreeC), and it cools so that the cooling rate to Ms point may become more than upper threshold cooling rate, and the average cooling rate from Ms point to 150 degreeC will be 10-500 degreeC / sec. It involves doing. The press molding temperature in this embodiment is typically room temperature.

This invention also provides the manufacturing method of the steel plate for hot press molding excellent in press formability especially suitable for use for a preform press hardening method. In the method for producing a steel sheet for hot press forming, the steel ingot or steel piece having the chemical composition is subjected to hot rolling after the temperature is 1050 to 1300 ° C, and the hot rolling at a temperature of 800 to 950 ° C. And winding the steel strip obtained by hot rolling to the temperature of 500-700 degreeC.

The manufacturing method of this steel plate for hot press molding may further contain the following process:

(1) Unwinding said coiled steel strip and performing a descaling process and cold rolling to this steel strip;

(2) After holding the cold rolled steel strip in said (1) for 10 second or more in the temperature range of (Ac ₁ point + 10 degreeC) or more and Ac ₃ points or less, 300 to 300 at an average cooling rate of 1-100 degreeC / sec. After cooling to the temperature range of 500 degreeC, this steel strip is hold | maintained in the temperature range of 300-500 degreeC for 30 second for 10 minutes, and then cooled to room temperature at the average cooling rate of 1-50 degreeC / sec;

(3) After holding the cold rolled steel strip in said (1) in the temperature range of (Ac ₁ point-100 degreeC) or more and (Ac ₁ point +30 degreeC) 1 to 24 hours, 1-100 degreeC / hour Cooling to room temperature at an average cooling rate;

(4) The coiled steel strip was unwound, the steel strip was subjected to descaling treatment and hot dip galvanizing, and then the steel sheet was subjected to alloy heat treatment at a temperature range of 500 ° C or higher and Ac ₁ point or lower;

(5) After annealing the cold rolled steel sheet in (1) at a temperature range of 700 to 900 ° C, the steel sheet is cooled to a temperature range of 500 ° C or less at an average cooling rate of 1 to 60 ° C / sec, and melted in the steel strip. Galvanizing and then alloying heat treatment at a temperature range of 500 ° C to Ac ₁ point; or

(6) The steel strip cooled to room temperature in the above (3) is subjected to hot dip galvanization and then alloy heat treatment at a temperature range of 500 ° C to Ac ₁ point.

As described above, the steel sheet for hot press forming of the present invention includes a hot rolled steel sheet, a cold rolled steel sheet, a heat treated cold rolled steel sheet, and an alloyed hot dip galvanized steel sheet based on a hot rolled steel sheet or a cold rolled steel sheet.

According to the present invention, it is possible to produce a high strength hot press-formed steel sheet member having excellent toughness and a tensile strength of 1.8 GPa or more without being hot-tempered and quenched at that time. As a result, the manufacturing cost of a high strength steel sheet member using hot press molding can be significantly reduced.

1: is explanatory drawing of the shape of the test piece for critical cooling rate measurement.

2 is a TEM photograph showing the microstructure of a hot press steel sheet member of the present invention.

3 is a schematic explanatory diagram of a hot molding method.

The present invention will be described in more detail below. In the following description, all "%" regarding the composition of the steel sheet and the plating represents "mass%".

In the present invention, the chemical composition of the steel sheet is as follows.

C: 0.26 ~ 0.45%

C is a very important element that increases the hardenability of the steel sheet and mainly determines post-quenchi mg strength. In order to achieve a high strength of 1.8 GPa or more after quenching, the C content is at least 0.26%. On the other hand, when C content exceeds 0.45%, the intensity | strength of the steel plate after hardening will become high too much, and the toughness deterioration will become remarkable. Preferable C content is 0.28 to 0.33%.

Mn + Cr ： 0.5 to 3.0%

Mn and Cr are elements that are very effective in increasing the hardenability of the steel sheet and stably obtaining the strength after high hardenability. If the total content of Mn and Cr (hereinafter referred to as "(Mn + Cr) content") is less than 0.5%, the effect is not sufficient. On the other hand, when (Mn + Cr) content exceeds 3.0%, the effect will be saturated and, on the contrary, it will become difficult to ensure stable strength. Preferable (Mn + Cr) content is 0.8 to 2.0%.

Nb ： 0.02 to 1.0%

Nb has the effect of suppressing recrystallization and forming fine carbide when the steel sheet is heated to Ac ₃ or more to make the austenite grains fine, thereby greatly improving the toughness of the steel sheet after quenching. There is. In order to secure this effect, 0.02% or more of Nb is contained. However, when the Nb content exceeds 1.0%, the above effect of Nb is saturated, resulting in an unnecessary cost increase. Preferable Nb content is 0.03 to 0.5%, More preferably, it is 0.04 to 0.15%.

Ti: Quantity satisfying the formula (1) (3.42N + 0.001≤Ti≤3.42N + 0.5)

Ti has the effect of suppressing recrystallization and forming fine carbide when the steel sheet is heated to Ac ₃ or more, thereby making the austenite grains fine, thereby greatly improving the toughness of the steel sheet after quenching. . In order to reliably exert this effect of Ti, Ti content (%) is made into (3.42N + 0.001) or more. N is contained as an impurity in steel. N may be substantially 0%. On the other hand, when the Ti content exceeds (3.42N + 0.5), the above effect of Ti is saturated, resulting in an unnecessary cost increase. Preferable Ti content is the quantity which satisfy | fills 3.42N + 0.02 <= Ti <3.42N + 0.08.

Si: 0 to 0.5%, Ni: 0 to 2%, Cu: 0 to 1%, V: 0 to 1%, Al: 0 to 1%

These elements are optional additive elements, and both are effective in increasing the hardenability of the steel sheet and stably attaining high strength after high quenching. Therefore, it is preferable to contain one or two or more of these elements. This effect of these elements becomes remarkable at Si: 0.01% or more, Ni: 0.01% or more, Cu: 0.01% or more, V: 0.01% or more, and Al: 0.01% or more. However, even if it contains each element more than the upper limit, since the said effect is small and wastelessly increases cost, content of each element shall be in the above-mentioned range. Preferable content in the case of adding these 1 type, or 2 or more types of elements is Si: 0.02-0.4%, Ni: 0.02-1%, Cu: 0.02-0.8%, V: 0.02-0.5%, Al: 0.01-0.1 %to be.

B ： 0% to 0.01%

B is an optional addition element, and is effective for increasing the hardenability of the steel sheet and stably obtaining strength after high hardening. In addition, B segregates at grain boundaries to increase grain strength, improves toughness of the steel sheet after quenching, and also has a high effect of inhibiting austenite grain growth during heating. These effects become remarkable when the B content is at least 0.001%. However, when the B content exceeds 0.01%, these effects are saturated and lead to an increase in cost. Preferable B content in the case of containing B is 0.001 to 0.01%, More preferably, it is 0.001 to 0.0030%.

Mo ： 0 ~ 1.0%

Mo is an optional additive element, and when the steel sheet is heated to Ac ₃ or more, fine carbides are formed to make the austenite particles into fine grains. Thus, Mo has an effect of greatly improving the toughness of the steel sheet after quenching. These effects become remarkable when Mo content is 0.01% or more. However, when the Mo content is more than 1.0%, the effect is saturated, resulting in an unnecessary cost increase. Preferable Mo content in the case of containing Mo is 0.01 to 1.0%, More preferably, it is 0.04 to 0.20%.

Ca ： 0∼0.005%

Ca is an optional additive element and has an effect of making the inclusions in the steel fine and improving the toughness of the steel sheet after quenching. These effects become remarkable when the Ca content is at least 0.001%. However, when Ca content exceeds 0.005%, the effect will be saturated. Therefore, preferable Ca content in the case of containing Ca is 0.001-0.005%, More preferably, it is 0.002-0.004%.

The balance of the chemical composition consists essentially of Fe and impurities. An impurity can contain nonmetallic elements, such as P, S, and N, and metal elements of that excepting the above. Among these, content of P, S, N is as follows.

P: 0.005% or less

P is an element which greatly degrades the toughness of the steel sheet after quenching, and therefore P is preferably 0.005% or less. More preferably, it is 0.003% or less.

S ： 0.005% or less

Since S is an element which greatly degrades the toughness of the steel sheet after quenching, S is preferably 0.005% or less. More preferably, it is 0.003% or less.

N: 0.002% or less

Since N is an element which forms an inclusion in steel and deteriorates the toughness of the steel plate after quenching, it is preferable to set it as 0.002% or less. More preferably, it is 0.001% or less.

It is preferable that content of at least 1 sort (s) among P, S, and N is the same as the above. Although content of remaining impurity element may exceed said upper limit, it is especially preferable that all content of P, S, and N is below the said upper limit.

The hot press-formed steel sheet member of the present invention has a high strength of at least 1.8 GPa. This tensile strength is achieved by quenching subsequent to press molding in the hot press molding step. Quenching is normally performed in the metal mold | die used for hot press molding, It is not limited to this.

As described above, such a high-strength hot press-formed steel sheet member has been significantly deteriorated in toughness, and thus could not be used for practical use. The hot press-formed steel sheet member of the present invention has a microstructure in which crystal grains have been refined such that the average austenite average particle diameter is 10 µm or less in order to achieve high strength of 1.8 GPa or more and good toughness. Old austenite average particle diameter becomes like this. Preferably it is 8 micrometers or less, More preferably, it is 4 micrometers or less. As described later, the former austenite average particle diameter changes depending on the heating conditions (holding temperature and holding time) before hot press forming.

According to this invention, hot press molding is performed with respect to the steel plate which has the said chemical composition, The heating conditions (holding temperature and holding time) before hot press molding at this time are as follows.

In order to obtain the desired strength and toughness by quenching in the hot press forming step, the steel sheet provided for hot press forming is subjected to a time of 5 minutes or less at a temperature range of Ac ₃ or more and (Ac ₃ points + 100 ° C) or less. Keep it. The maintenance temperature is set to Ac ₃ or more to obtain the desired strength by quenching the structure of the steel once as an austenite single phase. The upper limit of the holding temperature and the upper limit of the holding time are for suppressing the old austenite grain size after quenching to 10 µm or less, and achieving good toughness even when the tensile strength of the steel sheet is 1.8 GPa or more. If the holding temperature is higher than (Ac ₃ points + 100 ° C) or the holding time is more than 5 minutes, the old austenite grain size becomes 10 µm or more, and thus good toughness may not be obtained after quenching. More preferable holding temperature is Ac ₃ or more, (Ac ₃ +50 degreeC) or less, and more preferable holding time is 2 minutes or less. In addition, since the former austenite particle diameter is somewhat preferred as long as the grain size, the lower limit of the holding time is not particularly defined.

The hot press molding in the present invention is not particularly limited in itself, including the use mold. Although it is preferable to perform hot press molding, after heating a steel plate on the said conditions previously, it can also carry out according to the above-mentioned preforming press quenching method. In that case, what is necessary is just to heat the preformed molded article on the said conditions.

When employing the preformed press quenching method, performed at a low temperature to the press forming than Ac ₃ point, while the press-forming steel sheets into a mold, Ac ₃ point or higher, (Ac ₃ point + 100 ℃) 5 minutes in the temperature range of less than The following time is maintained, and it cools so that the cooling rate to Ms point may become more than upper threshold cooling rate, and the average cooling rate from Ms point to 150 degreeC will be 10-500 degreeC / sec. Although press molding in the preliminary press quenching method is usually performed at room temperature, it is also possible to press-form a steel sheet heated at a lower temperature than Ac ₃ point.

The cooling conditions and the cooling method for the quenching in the hot press molding process (including the pre-molding press quenching method) are as follows.

In order to obtain a strength with a tensile strength of 1.8 GPa or more and to improve the toughness even a little, the microstructure after quenching of the steel sheet member obtained by hot press molding is not made to be a fully martensite structure consisting substantially of martensite. It is important, however, to have a tissue containing automatically tempered martensite.

Automatically tempered martensite is a tempered martensite produced during cooling during quenching, not by heat treatment for tempering, for example, by Yano and Suzuki, `` Science of Steel Materials-Technology Condensed in Iron ''. Rokaku, Tokyo (2001). Tempered martensite can be distinguished from complete martensite by depositing fine cementite within the lath.

In the case of the steel sheet having the above chemical composition, the microstructure including the automatically tempered martensite has a cooling rate at the time of quenching to be above the upper critical cooling rate so that diffusion transformation does not occur to the Ms point, and thereafter, It can obtain by making the average cooling rate of the temperature range to 150 degreeC into 10-500 degreeC / sec. The preferable average cooling rate from Ms point to 150 degreeC is 15-200 degreeC / sec.

When the steel sheet temperature reaches the Ms point during cooling, transformation heat generation due to martensite transformation occurs, and the heat generation amount of this transformation is very large. As described above, the average cooling rate in the temperature range from the Ms point to 150 ° C. is slower than the cooling rate to the Ms point. If the cooling below the Ms point is performed by the same cooling method as before reaching the Ms point, Because of the large transformation exotherm, the required cooling rate may not be achieved. In that case, it is necessary to perform cooling from Ms point to 150 degreeC more strongly than cooling to Ms point, and it is preferable to specifically carry out as described below.

In the hot press molding method, cooling is usually achieved by a mold by press molding a hot steel sheet into a forced mold having a temperature of about normal temperature or tens of degrees Celsius. The cooling rate can be changed by changing the mold dimensions (for example, thickness) and changing the heat capacity of the mold. The cooling rate can also be changed by changing the mold material to a dissimilar metal (for example, copper). When the size and material of a metal mold | die cannot be changed, a cooling rate can be changed by changing the amount of cooling water using a water-cooling metal mold | die. In addition, the cooling rate can be changed by passing water through the grooves during press molding using a mold that has previously cut the grooves in advance. . Furthermore, a cooling rate can also be changed by changing a mold clearance and changing the contact area with a steel plate.

As means for changing the cooling rate before and after the Ms point, the following means can be considered, for example.

(1) Immediately after reaching the Ms point, the heat capacity is transferred to a mold having a different heat capacity or a mold at room temperature to change the cooling rate;

(2) in the case of the water-cooled mold, the cooling rate is changed by changing the amount of cooling water in the mold immediately after reaching the Ms point;

(3) Immediately after reaching the Ms point, the cooling rate is changed by allowing water to flow between the mold and the molded article, and changing the quantity thereof.

The form of the molding in the hot press molding method of the present invention is not particularly limited. Examples thereof include bending processing, drawing molding, overhanging molding, hole expanding molding, and flange molding. The press molding method can be suitably selected according to the kind of steel plate member made into the objective. As a representative example of the hot press-formed steel sheet member, a door guard bar, a bumper reinforcement, or the like, which is an automobile reinforcement part, may be mentioned. The manufacturing method of the steel plate member of this invention can also be applied to shaping | molding methods other than press molding, for example, roll forming, if it is equipped with a means to cool a steel plate simultaneously with or immediately after shaping | molding.

The steel plate member which concerns on this invention also has favorable toughness. As a level of toughness which can endure practically, it is preferable that the Charpy impact value in -120 degreeC is 30 J / cm <2> or more.

The hot press-formed steel sheet member is usually treated by a shot blast for the purpose of descaling. This shot blasting treatment has an effect of introducing compressive stress into the surface, and thus has an advantage that delayed fracture is suppressed and fatigue strength is improved.

In hot press molding, the steel sheet is heated to an austenite temperature range and subjected to austenite transformation. Therefore, apart from the preforming press quenching method having a low press molding temperature, the mechanical properties of the steel sheet at room temperature before heating are not important. Therefore, the type of steel sheet and the microstructure before heating are not particularly specified. That is, you may use any of a hot rolled sheet steel, a cold rolled sheet steel (full hard material, an annealing material), and a plated steel sheet as a hot press forming steel plate. Moreover, the manufacturing method is not specifically limited, either. As the plated steel sheet, an aluminum-based plated steel sheet (for example, a molten aluminum plated steel sheet, a molten 55% Al-Zn alloy plated steel sheet), a zinc-based plated steel sheet (for example, an electric or hot dip galvanized steel sheet, molten 5% Al-Zn plated steel sheet, alloyed hot dip galvanized steel sheet, electric Ni-Zn alloy plated steel sheet), etc. are mentioned.

On the other hand, in a hot press forming method such as a preforming press quenching method in which press molding is performed at a lower temperature than room temperature or austenite in advance, the steel sheet provided for hot press forming is preferably as soft as possible. For example, a tensile strength of 780 MPa or less for a hot rolled steel sheet or a cold rolled steel sheet subjected to continuous annealing, a tensile strength of 780 to 1180 MPa for a cold rolled steel sheet, and a cold rolled steel sheet subjected to a box annealing It is preferable that tensile strength is 590 Mpa or less. A very suitable manufacturing method for obtaining such a soft steel sheet is described next.

Hot rolling

The steel ingot or steel piece having the above-described chemical composition is subjected to hot rolling after the temperature is set at a temperature of 1050 to 1300 ° C to obtain a steel strip. Hot rolling is completed at the temperature of 800-950 degreeC, and the obtained steel strip is wound up to the temperature of 500-700 degreeC.

The ingot or slab is 1050-1300 degreeC in order to fully utilize the nonmetallic inclusion which degrades workability. Such an effect is recognized by setting it as 1050 degreeC or more with respect to the steel plate of the said composition. Even if it is 1300 degreeC or more, not only the effect will be saturated but a scale loss will increase. More preferably, this temperature is 1050-1250 degreeC, More preferably, it is 1050-11200 degreeC.

The method of setting the temperature of the ingot or slab to be provided for hot rolling at 1050 to 1300 ° C is not only a case where the ingot or steel piece which has been lower than 1050 ° C is heated to 1050 to 1300 ° C, but also the ingot or the fracture after continuous casting. ) It also includes the case where the steel piece after rolling is used for hot rolling, without falling below 1050 degreeC.

Hot rolling completion temperature shall not become lower than Ar <_3> point. When rolling is carried out at a temperature lower than the Ar ₃ point, the processed ferrite remains and the ductility deteriorates significantly. In the steel plate of the chemical composition mentioned above, these problems will not arise if hot rolling completion temperature is 800 degreeC or more. On the other hand, when hot rolling completion temperature becomes higher than 950 degreeC, surface defects, such as scale generation, may arise. Therefore, hot rolling completion temperature shall be 800-950 degreeC.

If the winding temperature is too low, many low-temperature transformation structures such as pearlite, bainite, and martensite are produced, and the ferrite structure decreases, so the steel sheet strength becomes too high. For this reason, the minimum of winding temperature shall be 500 degreeC. On the other hand, if the winding temperature is too high, the oxidation scale becomes thick and the descaling treatment becomes difficult, so the upper limit of the winding temperature is 700 ° C. Winding temperature becomes like this. More preferably, it is 550-650 degreeC.

The hot rolled steel strip thus produced has a structure containing 50% or more of ferrite by volume ratio in order to obtain good formability until hot rolling in a preform press quenching method, typically in press molding at room temperature. And it is preferable that tensile strength is 780 Mpa or less. The remainder of the tissue may include one or two or more of pearlite, bainite, martensite, and residual austenite. The ferrite may contain Fe-based carbides called cementite, Ti-based, Nb-based, Mo-based, Cr-based, V-based, and Mn-based carbides. From the viewpoint of formability, the strength of the steel strip is preferably lower, but from the viewpoint of cost and ease of adjustment of strength, the strength is preferably 590 MPa or more, and more preferably 690 MPa or more.

The steel strip wound and cooled after hot rolling is usually subjected to the removal (descheduling) treatment of the scale generated on the surface by one or two or more kinds such as acid washing, shot blasting, grinding, etc. after unwinding. .

Cold rolled

When cold rolling is performed on the hot rolled steel strip as described above and used for press molding as a cold rolled steel strip, in order to obtain good moldability in hot press molding by preforming press quenching, It is preferable to set it as the structure containing 50% or more of ferrite, and tensile strength is 1180 Mpa or less. The strength of the cold rolled steel strip is preferably lower from the viewpoint of formability, but is preferably 780 MPa or more in view of cost and ease of adjustment of strength. The tensile strength of the cold rolled steel strip is more preferably in the range of 780 to 1100 MPa, still more preferably in the range of 780 to 1050 MPa. It is preferable to make the reduction ratio at the time of cold rolling into 30 to 80%, More preferably, it is 40 to 70%.

Annealing Method

The annealing of the cold rolled steel strip as described above may be either a continuous annealing performed in an unwrapped state or a box annealing carried out in a coil shape.

In the case of continuous annealing of the cold rolled steel strip, after heating to (Ac ₁ point + 10 ° C) or more and Ac ₃ or less and holding for 10 seconds or more in the temperature range, 300 to 500 at an average cooling rate of 1 to 100 ° C / sec. Cooling is carried out to a temperature range of ° C, held for another 10 minutes or more in a temperature range of 300 to 500 ° C for 30 seconds, and then annealing is performed by cooling to room temperature at an average cooling rate of 1 to 50 ° C / sec.

If the heating temperature at this time is lower than (Ac ₁ point + 10 ° C), recrystallization does not sufficiently proceed, and the strength of the steel strip is likely to increase. On the other hand, when the heating temperature is higher than Ac ₃ point, the low temperature transformation phase is likely to be generated during cooling due to austenite single phase, and the strength of the steel strip is also likely to increase. When the holding time after heating becomes shorter than 10 second, segregation, such as Mn which is a substitution type element, will remain and the microstructure after annealing will become nonuniform. Since heating for a long time causes an unnecessary cost increase, the holding time after heating is preferably set to 300 seconds or less. The annealing atmosphere is preferably a non-oxidizing atmosphere (for example, 98% by volume N ₂ + ₂ % by volume H ₂ ).

If the average cooling rate at the time of annealing is too fast, a lot of low temperature transformation phase will generate | occur | produce, a ferrite will decrease, and a steel strip strength will increase. On the other hand, if the average cooling rate is too slow, the production efficiency is lowered. Preferable average cooling rate is 1-20 degreeC / sec, and 1-10 degreeC / sec is more preferable.

The cooling stop temperature range is set to 300 to 500 ° C in order to suppress the formation of low temperature transformation phase as much as possible. Cooling stop temperature range becomes like this. Preferably it is 350-500 degreeC, More preferably, it is 400-450 degreeC. Holding for 30 seconds to 10 minutes in a cooling stop temperature range is for promoting the ferrite transformation of unmodified austenite. This holding time becomes like this. Preferably it is for 30 second-5 minutes, More preferably, it is for 30 second-3 minutes. Thereafter, the steel strip is cooled to room temperature at an average cooling rate of 1 to 50 ° C / sec. If the average cooling rate at this time is faster than 50 degree-C / sec, many low temperature transformation phases will generate | occur | produce and a steel strip strength will become high. On the other hand, when an average cooling rate is slower than 1 degree-C / sec, productive efficiency will fall. Preferable average cooling rate is 1-10 degree-C / sec.

In the case of annealing the box after winding the cold rolled steel strip, it is maintained at a temperature range of (Ac ₁ point-100 ° C) or higher and (Ac ₁ point + 30 ° C) or lower for 1 to 24 hours, and then average cooling at 1 to 100 ° C / hr. Annealing is performed by cooling to room temperature at a rate. If the temperature is maintained lower than (Ac ₁ point -100 ℃), the tensile strength of the steel strip does not decrease sufficiently. On the other hand, if the holding temperature is higher than (Ac ₁ point + 30 ° C), the reusable-reverse transformation of cementite proceeds too much, resulting in a low temperature transformation phase in the subsequent cooling process, and the tensile strength of the steel strip becomes too high. If the holding time is less than 1 hour, the strength reduction of the steel strip is not sufficient, and even if the holding time exceeds 24 hours, the effect is saturated, causing waste of energy unnecessarily. In the cooling process after annealing, since a low-temperature transformation phase is produced when a cooling rate is fast, it is preferable to be as slow as possible. However, if it is too slow, the treatment efficiency is lowered, so the cooling rate is 1 to 100 ° C / hour, preferably 1 to 50 ° C / hour.

It is preferable that the atmosphere in a furnace of box annealing is a gas containing 95 volume% or more of hydrogen with little mixing of nitrogen gas and as low as possible in dew point.

The cold rolled steel strip subjected to the annealing after the cold rolling thus obtained is made of a structure containing 50% or more of ferrite in a volume ratio in order to obtain good formability in hot press molding by preforming press quenching. It is preferable that tensile strength is 780 Mpa or less in case of continuous annealing, and 590 Mpa or less in case of box annealing. The lower the tensile strength of the steel strip is, the more preferable, the lower the tensile strength is, the easier it is to adjust the strength.

Hot dip galvanized

Hot dip galvanizing can also be performed by any one of a hot rolled steel strip, a cold rolled steel strip, and the steel strip annealed after cold rolling. It is preferable to perform hot dip galvanizing by a continuous hot dip galvanizing line from a manufacturing cost viewpoint. In an ordinary continuous hot dip galvanizing line, a heating furnace, a cooling zone, a hot dip galvanizing bath, and an alloying furnace are arranged continuously. Here, suitable manufacturing conditions at each step, which affect the microstructure of the steel strip, will be described.

When performing annealing to a cold rolling steel strip as it is cold rolling, it is preferable to make annealing temperature into 700-900 degreeC. At a temperature lower than 700 ° C, recrystallization does not sufficiently proceed, and the strength of the steel strip tends to increase. On the other hand, at temperatures higher than 900 ° C, low-temperature transformation phases are likely to be generated during cooling due to austenite single phase formation, and the strength of the steel strip is also likely to increase. It is not necessary to perform annealing originally about the hot rolled steel strip and the cold rolled steel strip which performed the annealing. However, from the viewpoint of the operability of the continuous hot dip galvanizing line, it is difficult to make the heating temperature significantly lower, so it is preferable to perform heating within a normal operating range. In this case, it is preferable to make maximum heating temperature 900 degrees C or less for the said reason.

The steel strip is cooled to perform hot dip galvanizing from the annealing temperature or the highest heating temperature. It is preferable that the average cooling rate to the temperature range of 500 degrees C or less at the time of this cooling shall be 1-60 degreeC / sec. If the cooling is too fast, a lot of low temperature transformation phases are generated, the ferrite decreases and the strength of the steel strip becomes too high. On the other hand, if cooling is too slow, production efficiency will fall.

The hot dip galvanizing may be carried out by immersing the steel strip in a plating bath containing molten zinc or a zinc alloy (for example, a zinc alloy containing up to 5% Al) by a common method. The control of the coating amount is carried out by adjusting the pulling speed or the flow rate of the wiping gas blown out of the nozzle.

The steel strip subjected to the hot dip galvanizing is sent to an alloying furnace such as a gas, an induction heating furnace, and heated after leaving the plating bath. By this heating, alloying by metal diffusion between a plating layer and a base steel strip advances, and a plating layer becomes a zinc-iron alloy. It is preferable to make this heating temperature (alloyation temperature) 500 degreeC or more. If the alloying temperature is lower than 500 ° C., the alloying speed is slow. Therefore, it is necessary to deal with the equipment such as lowering the line speed or inhibiting productivity or lengthening the alloying furnace. The higher the alloying temperature, the faster the alloying speed. When the alloying temperature is higher than Ac ₁ point, the steel strip becomes high for the same reason as the upper limit of the annealing temperature or the maximum heating temperature. The preferable range of alloying temperature is 550-650 degreeC.

As mentioned above, you may perform temper rolling on the steel plate for hot press molding manufactured by the various manufacturing methods, in order to adjust flatness correction and surface roughness. The steel sheet for hot press forming may be an aluminum plated steel sheet such as another plated steel sheet, for example, a 55% Al-Zn alloy plated steel sheet.

Example

The following examples are illustrative of the invention and are not intended to limit the invention.

A cold rolled steel sheet (plate thickness: 1.6 mm) having the chemical composition shown in Table 1 was used as the base steel sheet. These steel sheets are steel sheets produced by hot rolling and cold rolling of slab that has been solvent in the laboratory.

Further, Al plating (plating amount per plate was 120 g / m 2), and hot dip galvanizing (plating amount per plate was 60 g / m 2) was applied to the steel sheet of steel grade No. 1 using the plating simulator. . Moreover, alloying heat processing (Fe content in a plating film is 15 mass%) was performed to the steel plate of No.2. The annealing temperature in a plating simulator was 800 degreeC, and the average cooling rate from 800 degreeC to Ms point was 5 degreeC / sec.

These steel sheets were cut to a dimension of 1.6t × 100w × 200L (mm), heated in the atmospheric furnace under the conditions shown in Table 2, and the steel plate immediately after being taken out of the furnace using a forced mold of a flat plate. Hot press molding was performed. The holding time refers to the time from when the steel sheet temperature reaches Ac ₃ point after being put into the furnace until it is taken out of the furnace. A thermocouple was attached to the steel sheet and the cooling rate was measured.

About the obtained hot press-formed steel plate member, it used for the former austenite particle size measurement by the cutting method, and the tensile test (JIS5 test piece).

Moreover, after laminating | stacking six steel pieces of thickness 1.6mm cut out from the hot press-formed steel plate member, and screwing them, the V notched test piece was produced and it provided for the Charpy impact test at -120 degreeC. Toughness is shown with (shock) the case where the impact value in -120 degreeC becomes 30 J / cm <2> or more as (circle) (pass), and the case where it is less than 30 J / cm <2> and an impact value.

Ac ₃ point, Ms point, and upper critical cooling rate of each steel grade were measured by the following method.

After cutting the column-shaped test piece (FIG. 1) of diameter 3.Omm and length 10mm from a hot rolled sheet steel, this test piece was heated to 900 degreeC in air | atmosphere at the temperature increase rate of 10 degree-C / sec, hold | maintained at that temperature for 5 minutes, Cooled to room temperature at various cooling rates. Ac ₃ point and Ms point were measured by measuring the thermal expansion change of the test piece during heating and cooling at this time. In addition, Vickers hardness measurement (load 49N, measurement number: 3) and structure observation of the obtained test piece were performed, and the upper critical cooling rate of the steel plate was evaluated from these results.

These results are shown in Table 3.

TABLE 1

TABLE 2

TABLE 3

In Nos. 1 to 12, which are inventive examples, the hot-pressed steel sheet member has a fine austenite grain size of 10 µm or less, a tensile strength of 1.8 GPa or more, and good toughness. On the other hand, in Nos. 13 to 15, which are Comparative Examples, the former austenite grain size exceeded 10 µm, and the toughness value was not good. In the comparative examples, Nos. 13 and 14 are examples in which the chemical composition of the steel is outside the range of the present invention. No. 15 is the chemical composition of the steel, but falls within the range of the present invention from the Ms point in hot press molding to 150 ° C. The average cooling rate of is outside the scope of the present invention.

When the microstructure of the steel of the hot-pressed steel sheet member was observed by transmission electron microscope (TEM), it was confirmed that the steel sheet members from No. 1 to No. 14 were structures containing auto-tempered martensite. On the other hand, the steel plate member of No. 15 was a complete martensite structure. Even if the hot press-formed steel sheet member has a structure containing martensite automatically tempered, good toughness cannot be obtained when the former austenite particle diameter exceeds 10 µm.

The TEM photograph at the other magnification of the hot press-formed steel plate member of No. 2 which is an invention example is shown in FIG. The top is 10,000 times and the bottom is 40,000 times. Most of the tissue is lath-like martensite, but in the large lath width, fine needle-like cementite is deposited inside the glass, and it is confirmed that it is made of automatically tempered martensite, especially from the bottom 40,000 times photograph. Can be. For example, the part shown by the arrow in the figure is fine cementite.

In addition, about the alloyed hot-dip galvanized steel sheet of steel grade No.2, after reaching | attaining 900 degreeC in the heating furnace of air | atmosphere using the blank of size 1.0t * 80w * 320L (mm), at this temperature, It hold | maintained for 1 minute, it was taken out of the heating furnace, and hot hot press molding of the hot type was immediately performed. 3 is a schematic explanatory diagram of a hot molding method. In the hot press molding conditions, the molding height was 70 mm, Rd (dice shoulder portion R) was 8 mm, Rp (punch shoulder portion R) was 8 mm, clearance was 1.0 mm, and wrinkle pressing force was 12.7 kN.

The low temperature impact test was done about this hot molded article. After cooling the member to −40 ° C., a weight of 2450 N (250 kgf) was collided with the member at a height of 1000 mm to investigate the presence of cracks. As a result, it was found that there was no cracking and had sufficient toughness.

Claims (19)

In mass%, C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 0.51%, Ti, Si: 0 to 0.5%, and Ni: 0 to 2% in amounts satisfying the following formula (1): Cu: 0 to 1%, V: 0 to 1%, Al: 0 to 1%, B: 0 to 0.01%, Mo: 0 to 1.0%, Ca: 0 to 0.005%, and the balance: Fe and impurities Hot-press-formed steel sheet member having a chemical composition comprising a fine austenite average particle diameter of 10 µm or less and having a microstructure including automatically tempered martensite and a tensile strength of 1.8 GPa or more: 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5... (One) Ti and N in a formula mean content (mass%) of each Ti and N element in steel, and N is contained as an impurity in steel. The method according to claim 1, The chemical composition is, in mass%, Si: 0.01% to 0.5%, Ni: 0.01% to 2%, Cu: 0.01% to 1%, V: 0.01% to 1%, Al: 0.01% to 1%, and B: 0.001% to 0.01% The hot press-formed steel sheet member containing 1 type (s) or 2 or more types chosen from Mo: 0.01-1.0% and Ca: 0.001-0.005%. The method according to claim 1 or 2, Hot press molding in which one, two or more kinds of P, S and N contained as impurities in the chemical composition satisfy P: 0.005% or less, S: 0.005% or less and N: 0.002% or less in mass%. Steel plate member. In mass%, C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 0.51%, Ti, Si: 0 to 0.5%, and Ni: 0 to 2% in amounts satisfying the following formula (1): , Cu: 0 to 1%, V: 0 to 1%, Al: 0 to 1%, B: 0 to 0.01%, Mo: 0 to 1.0%, Ca: 0 to 0.005%, and the balance Fe and impurities Steel sheet for hot press forming, which can produce a steel sheet member having a chemical composition, which is hot pressed by a hot press forming, having a tensile strength of 1.8 GPa or more: 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5... (One) Ti and N in a formula mean content (mass%) of each Ti and N element in steel, and N is contained as an impurity in steel. The method according to claim 4, Said chemical composition is in mass%, Si: 0.01 to 0.5%, Ni: 0.01 to 2%, Cu: 0.01 to 1%, V: 0.01 to 1%, Al: 0.01 to 1%, B: 0.001 to 0.01% The steel sheet for hot press molding containing 1 type (s) or 2 or more types selected from Mo: 0.01-1.0% and Ca: 0.001-0.005%. The method according to claim 4 or 5, Hot press molding in which the amount of one, two or more of P, S, and N contained in the chemical composition as impurities is satisfies P: 0.005% or less, S: 0.005% or less, and N: 0.002% or less by mass%. For grater. In mass%, C: 0.2 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 0.51%, Ti, Si: 0 to 0.5%, Ni: 0 to 0 to an amount satisfying the following formula (1): 2%, Cu: 0 to 1%, V: 0 to 1%, Al: 0 to 1%, B: 0 to 0.01%, Mo: 0 to 1.0%, Ca: 0 to 0.005%, and the balance Fe and impurities the subjected to hot press forming a steel sheet having formed the chemical composition, the Ac ₃ point or more, the mixture was kept for less than five minutes in a temperature range of less than (Ac ₃ point + 100 ℃), the steel sheet to, and hot press forming temperature The method for producing a hot press-formed steel sheet member comprising cooling the steel sheet to a Ms point such that the cooling rate to the Ms point is equal to or higher than the upper critical cooling rate and the average cooling rate from the Ms point to 150 ° C is 10 to 500 ° C / sec. ： 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5... (One) Ti and N in a formula mean content (mass%) of each Ti and N element in steel, and N is contained as an impurity in steel. In mass%, C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 0.51%, Ti, Si: 0 to 0.5%, and Ni: 0 to 2% in amounts satisfying the following formula (1): Cu: 0 to 1%, V: 0 to 1%, Al: 0 to 1%, B: 0 to 0.01%, Mo: 0 to 1.0%, Ca: O to 0.005%, and the balance Fe and impurities the steel sheet having a chemical composition, and press-formed using a mold at a temperature lower than the Ac ₃ point, press-hold the molded plate placed in the mold, Ac ₃ point or higher, (Ac ₃ point + 100 ℃) 5 minutes or less in a temperature range of less than Hot press-formed steel sheet which comprises cooling for a period of time, and then cooling such that the cooling rate up to the Ms point is equal to or higher than the upper critical cooling rate, and the average cooling rate from the Ms point up to 150 ° C is 10 to 500 ° C / sec. Manufacturing method of member: 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5... (One) Ti and N in a formula mean content (mass%) of each Ti and N element in steel, and N is contained as an impurity in steel. In mass%, C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 0.51%, Ti, Si: 0 to 0.5%, and Ni: 0 to 2% in amounts satisfying the following formula (1): , Cu: 0 to 1%, V: 0 to 1%, Al: 0 to 1%, B: 0 to 0.01%, Mo: 0 to 1.0%, Ca: 0 to 0.005%, and the balance Fe and impurities A steel strip obtained by hot rolling a steel ingot or steel piece having a chemical composition by hot rolling at a temperature of 1050 to 1300 ° C and completing the hot rolling at a temperature of 800 to 950 ° C. The manufacturing method of the steel plate for hot press molding which includes winding the iron to the temperature of 500-700 degreeC: 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5... (One) Ti and N in a formula mean content (mass%) of each Ti and N element in steel, and N is contained as an impurity in steel. The method according to claim 9, The method of manufacturing a steel sheet for hot press forming, further comprising unwinding the coiled steel strip and subjecting the steel sheet to descaling and cold rolling. The method according to claim 10, The cold rolled steel strip was kept at a temperature range of (Ac ₁ point + 10 ° C) or higher and Ac ₃ point or lower for 10 seconds or more, and then cooled to a temperature range of 300 to 500 ° C at an average cooling rate of 1 to 100 ° C / sec. The method of manufacturing a steel sheet for hot press forming, further comprising: cooling the steel strip to a temperature range of 300 to 500 ° C for 30 seconds to 10 minutes, and then cooling to room temperature at an average cooling rate of 1 to 50 ° C / sec. . The method according to claim 10, The cold rolled steel strip is kept at a temperature range of (Ac ₁ point-100 ° C) or higher and (Ac ₁ point + 30 ° C) or lower for 1 to 24 hours, and then cooled to room temperature at an average cooling rate of 1 to 100 ° C / hour. The manufacturing method of the steel plate for hot press molding which further includes making it carry out. The method according to claim 9, For hot press forming, further comprising unwinding the coiled steel strip, descaling and hot-dip galvanizing the steel sheet, and then performing alloying heat treatment at a temperature range of 500 ° C or higher and Ac ₁ point or lower. Method of manufacturing steel sheet. The method according to claim 10, The cold rolled steel strip is annealed at a temperature range of 700 to 900 ° C, and then cooled to a temperature range of 500 ° C or less at an average cooling rate of 1 to 60 ° C / sec. The method of manufacturing a steel sheet for hot press forming, further comprising, after the alloying heat treatment at a temperature range of 500 ° C or higher and Ac ₁ or lower. The method according to claim 12, A hot-rolled steel sheet for hot press forming, comprising hot-dip galvanizing and an alloy heat treatment at a temperature range of 500 ° C or higher and Ac ₁ point or lower after the steel sheet cooled to room temperature. The method according to claim 7 or 8, The chemical composition is, in mass%, Si: 0.01% to 0.5%, Ni: 0.01% to 2%, Cu: 0.01% to 1%, V: 0.01% to 1%, Al: 0.01% to 1%, and B: 0.001% to 0.01% , Mo: 0.01% to 1.0% and Ca: 0.001% to 0.005% A method for producing a hot press-formed steel sheet member containing one or two or more kinds. The method according to claim 7 or 8, Hot press molding in which one, two or more kinds of P, S and N contained as impurities in the chemical composition satisfy P: 0.005% or less, S: 0.005% or less, and N: 0.002% or less in mass%. Method of manufacturing a steel sheet member. The method according to any one of claims 9 to 15, The chemical composition is, in mass%, Si: 0.01% to 0.5%, Ni: 0.01% to 2%, Cu: 0.01% to 1%, V: 0.01% to 1%, Al: 0.01% to 1%, and B: 0.001% to 0.01% , Mo: 0.01% to 1.0% and Ca: 0.001% to 0.005% A method for producing a steel sheet for hot press forming, containing one or two or more kinds. The method according to any one of claims 9 to 15, Hot press molding in which one, two or more kinds of P, S and N contained as impurities in the chemical composition satisfy P: 0.005% or less, S: 0.005% or less, and N: 0.002% or less in mass%. Method for manufacturing steel plate for use.

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